The authors would like to thank the Ontario Institute for Cancer Research, Imaging Translation Program and the Natural Sciences and Engineering Research Council of Canada for funding this research. We also like to acknowledge useful discussions with Albert Chen, GE Healthcare, Toronto, Canada, Gianni Ferrante, Stelar s.r.l., Italy, and William Mander, Oxford Instruments, UK.

1. Sample Preparation
NOTE: Steps 1.1-1.8 are performed just once
<ol>
	<li>Prepare 1 mL of stock 13C-enriched pyruvic acid solution, widely used for in vivo research1,2,5,6, consisting of 15-mmol/L of triarylmethyl radical dissolved in [1-13C]pyruvic acid (see Table of Materials). Aliquots from this stock solution will be used for the samples that will be individually polarized and subsequently undergo relaxometry at different magnetic fields. A representation of the [1-13C]pyruvic acid molecule is shown in Figure 1.</li>
	<li>On the dynamic nuclear polarizer software interface (see Table of Materials), click on the Cooldown button to lower the temperature of the variable temperature insert (VTI) to 1.4 K.</li>
	<li>Once the DNP has reached the desired temperature, load 10 &#181;L of the stock solution in a sample cup, open the turret doors and insert the cup into the VTI using an insertion wand specifically designed for this task.</li>
	<li>After that, quickly extract the wand and make sure the cup is released. Then close the turret doors and continue with the following steps while the temperature of the VTI goes back to 1.4 K.</li>
	<li>Prepare the DNP to run a microwave sweep in order to find the optimal RF frequency for hyperpolarization of the stock solution.
	<ol>
		<li>On the computer controlling the spectrometer (part of the DNP system), establish the communication between the spectrometer and the DNP control software by double-clicking on HyperTerminal icon, previously configured with the appropriate serial communication parameters.</li>
		<li>Once the communication is established, launch the RINMR software, type in its command line&#160;.HYPERSENSENMR, and then press enter.</li>
		<li>After that, a new window will be shown on the screen and on it type the number one (1) in the Configuration Number field. Then, click on the Select Configuration button.</li>
		<li>Click on the button Do microwave sweep. A small window with a descending counter of seconds will be launched indicating that the spectrometer is ready and it will be waiting for periodic trigger signals, coming from the DNP control software, to sample the polarization.</li>
		<li>On the DNP control software, select the Calibrate tab and click on the Generate button.</li>
		<li>Using the calibration setup window, enter the following information: Start Frequency = 94.117 GHz, End Frequency = 94.137 GHz, Step Size = 1 MHz, Step Duration = 300 s, Power = 50 mW, Liquid Helium Level = 65%, and Temperature = 1.4 K.</li>
		<li>Click on the button Generate, which will close the setup window and return to the Calibrate tab that will display the number of steps and the time required to perform the desired microwave sweep.</li>
		<li>Once the desired VTI temperature is achieved, click the Enable button and then Start to initialize the microwave sweep process.&#160;&#160;&#160;&#160;&#160;</li>
	</ol>
	</li>
	<li>At the end of the microwave sweep, recover the sample and record the optimal frequency where the maximum polarization is achieved. This optimal frequency is defined as the polarization frequency that provides the maximum polarization as shown in Figure 2. This frequency will be used for hyperpolarizing all the aliquots obtained from that specific stock solution of pyruvic acid.</li>
	<li>Prepare 250 mL of stock dissolution medium using a solution of 40-mmol/L Tris base, 50 mmol/L of sodium chloride, and 80-mmol/L sodium hydroxide in de-ionized water. Add ethylenediaminetetraacetic acid (EDTA) at a concentration of 100 mg/L to sequester any metal ion contamination. Similarly to the pyruvic acid stock solution, this dissolution medium will be used for all the different samples that will be polarized. Refer to the Table of Materials for more specific details regarding the chemicals used.</li>
	<li>Also, prepare 500 mL of stock cleaning solution consisting of 100 mg/L EDTA dissolved in deionized water. Approximately 10 mL of this cleaning solution is used after each polarization to clean the dissolution path of the DNP. 
	NOTE: Steps 1.9-1.27 are performed for each individual sample.</li>
	<li>Cool the DNP apparatus to 1.4 K in preparation of hyperpolarizing a [1-13C]pyruvic acid sample by pressing the Cooldown button in the DNP main window.</li>
	<li>If the software used for the spectrometer is already active with configuration 1 selected, proceed with the following steps. Otherwise, perform steps 1.5.1 to 1.5.3 and then continue with the following steps.</li>
	<li>After verifying that configuration 1 is selected in the window controlling the DNP&#39;s spectrometer, click on the Solid Build Up button.</li>
	<li>Enter the file name SSBuilupXXX, where &#34;XXX&#34;&#160;is a number in the sequence of files stored with build-up data. This number is automatically incremented by the software. Then click OK. Similarly to the microwave sweep case, a small window with a descending counter of seconds will be launched indicating that the spectrometer is ready and it will be waiting for periodic trigger signals, coming from the DNP control software, to sample the polarization.</li>
	<li>Using the pyruvic acid - OX063 stock solution prepared in step 1.1, weigh out 30 mg in a sample cup.</li>
	<li>When the desired VTI temperature is achieved (1.4 K) click on Insert Sample, then select Normal Sample and then click on Next. Following the safety precautions displayed on the screen, insert the cup in the cold DNP apparatus, using a long wand specifically designed for this task.</li>
	<li>Once the cup is inserted, the wand removed, and the DNP doors closed, click Next and then Finish. At that point the hyperpolarizer system lowers the sample cup to the irradiation chamber partially filled (65%) with liquid helium.</li>
	<li>Wait until the temperature has returned to 1.4K and then click on the Polarize Sample button.</li>
	<li>In the new pop up window, set the frequency value to that obtained from the microwave sweep in step 1.6. In the same window, also set the power to 50 mW and the sampling time to 300 s. Click on Next, check the Enable Build-up Monitoring box, and then click on Finish. 
	NOTE: Once the polarization is started, the DNP control software generates trigger signals every 300 s to instruct the spectrometer to sample the polarization using a small tip angle. That way, the spectrometer software adds a sample point to a solid-state magnetization curve, now displayed in both the spectrometer software and in the DNP control software under the tab Polarization Build-Up. After the 4th sample and every sample after that, the spectrometer software fits the curve to an exponential growth function of the form: 
	 
	S = A *exp(-t/Tp) + y0 
	 
	where A is the polarization amplitude, in arbitrary units, t is the sampling time, Tp is the polarization time constant (both in seconds), and y0 is an offset. Based on the fitted parameters, the software also calculates the percentage polarization achieved up to that point in time, which is also displayed in the DNP&#39;s Polarization Status tab.</li>
	<li>Polarize until the build-up of the solid-state magnetization reaches at least 95% of maximum (approximately one hour).</li>
	<li>While the sample is polarizing, prepare the Fast-Field-Cycling Relaxometer as explained in Section 2 below.</li>
	<li>When the desired polarization is achieved, click on Run Dissolution and under Method, select Pyruvic Acid Test. Then, click on Next.</li>
	<li>Following the instructions on the screen, open the DNP turret doors and load the heating and pressurizing chamber at the top of the apparatus with ~ 4.55 mL of the dissolution medium prepared in section 1.5 to produce a concentration of 80-mmol/L pyruvate upon dissolution at a pH of ~7.75 and temperature of ~37 &#176;C.</li>
	<li>Position the recovering wand in the right position, close the turret doors, and at the computer click on Next and then on Finish. At that point the dissolution media will be superheated until the pressure reaches 10 bar.</li>
	<li>Once the 10 bar pressure is attained, the frozen and hyperpolarized pyruvate is automatically lifted from the liquid helium bath, quickly mixed, and thawed with the superheated dissolution media and ejected through a capillary tubing into a pear-shaped flask. While the hyperpolarized pyruvate/dissolution media mixture is ejected, constantly swirl the flask to ensure a homogeneous mixture.</li>
	<li>When all the mixture has been ejected, quickly draw 1.1 mL of the liquid into a syringe, transfer to a pre-warmed (37 &#176;C) 10-mm-diameter NMR tube, and rapidly transport to the field-cycling relaxometer (See step 2.2.12).</li>
	<li>Dispense the remaining aliquot of every pyruvate dissolution into a 0.55-T benchtop NMR spectrometer (see Table of Materials) to check for possible systematic experimental effects.</li>
	<li>Immediately clean DNP fluid path using clean dissolution medium followed by ethanol. Blow helium gas through fluid path to remove remaining cleaning fluids and purge path of oxygen. Clean all glassware.</li>
	<li>After each measurement, record the pH of samples from both the bench top spectrometer and the field-cycling relaxometer. 
	NOTE: Each T1 measurement is a separate hyperpolarized dissolution from the DNP apparatus, so care is required to assure measurement-to-measurement reproducibility of the sample composition. This is accomplished by weighing all agents and solvents with a precision of 0.1 mg to assure accurate and reproducible preparation of the final hyperpolarized solutions.</li>
</ol>
2. Relaxometry
NOTE Please refer to Table 1 for a better understanding of the selection and use of the different parameters described in the following steps. Prior to dissolution, the relaxometer flip angle must be calculate and the relaxometer must be setup and ready for measurement of the hyperpolarized solution (see below).
<ol>
	<li>Flip-angle calibration 
	<ol>
		<li>Prepare 1 mL of neat [1-13C]pyruvic acid in an NMR tube and add a gadolinium contrast agent to reduce the T1 of the 13C nuclei to a value of less than 200 ms but more than 50 ms.</li>
		<li>Seal the NMR tube so it can be used multiple times as a calibration standard.</li>
		<li>Using the depth gauge of the relaxometer, set the depth of insertion of the NMR tube to the proper height to ensure the sample will be located at the center of the relaxometer RF coil.</li>
		<li>Mark the insertion depth of the 13C pyruvate calibration standard with adhesive tape to ensure repeatability.</li>
		<li>Place the depth stopper on the NMR tube to the position indicated by the tape and insert this calibration standard into the bore of the field-cycling relaxometer. Use a weight to keep the NMR tube in position.</li>
		<li>Open the instrument air valve and from the relaxometer front panel set the temperature controller to 37 &#176;C. That will maintain the temperature of the sample at 37&#176;C (&#177; 0.5&#176;C) using heated air during the experiment.</li>
		<li>Setup the field-cycling relaxometer hardware to acquire 13C nuclei signals. That includes installing and energizing the external shim coil (see Table of Materials), tuning and matching the RF coil to 8 MHz (~0.75 T for 13C nuclei), and using the appropriate &#955;/4 cable.</li>
		<li>In the instrument software, perform the following steps:
		<ol>
			<li>Select the Main par tab</li>
			<li>Click on the cell next to the label Experiment and scroll down in the pop-up window to select the pulse sequence &#34;13CANGLE.FFC &#34;.</li>
			<li>Set the following acquisition parameters: RFA = 5; SWT = 0.005, RD = 0.5, BPOL = 30 MHz, TPOL = 0.5.</li>
			<li>Select the Acq. par tab and then select the Basic sub-tab.</li>
			<li>Click on the cell next to the label Nucleus and scroll down in the pop-up window to select 13C.</li>
			<li>Then, set the following parameters: SF = 8 MHz, SW = 1000000, BS = 652, FLTR = 100000, MS = 32.</li>
			<li>Select the Conf sub-tab.</li>
			<li>Set the following parameters: RINH = 25, ACQD = 25.</li>
			<li>Select the nDim sub-tab</li>
			<li>Set NBLK = 32, BINI = 2, BEND = 62.</li>
			<li>Select the Evaluation tab and then the Parameters sub-tab.</li>
			<li>Set the following parameters: EWIP = 10, EWEP = 128, EWIB = 1, EWEB = 32.</li>
			<li>Then, click the Start Acquisition icon to run the pulse sequence.</li>
		</ol>
		</li>
		<li>Once the acquisition is finished, save the data, select the Evaluation dialog icon and from the analysis menu select WAM Window: Absolute Magnitude. Then select Report Sheet, Graphs and Export File and finally click on Execute.</li>
		<li>In the Report window find the RF pulse width that provides the maximum amplitude and fine-tune the value with the help of the cursor in the displayed graph, which is similar to the plots shown at the bottom row of Figure 3. This pulse width will be used for the parameter PW90 of the following experiments.</li>
		<li>Click the F1 icon to adjust the frequency shift of the relaxometer. 
		NOTE: WAM Window: Absolute Magnitude is a procedure to integrate the magnitude of a single or a sequence of free-induction decay acquisitions (FIDs) from the point defined by EWIP to the point specified by EWEP and from the block defined by EWIB to the block specified by EWEB.</li>
	</ol>
	</li>
	<li>T1-Measurements 
	<ol>
		<li>Make sure the external shim coil is installed and energized.</li>
		<li>In the instrument software perform the following steps:
		<ol>
			<li>Select the Main par tab</li>
			<li>Click on the cell next to the label Experiment and scroll down in the pop-up window to select the pulse sequence HPUB/S, which is shown in Figure 4.</li>
			<li>Set the following acquisition parameters: RFA = 25, T1MX = values between 3 and 5; SWT = 0.2, RD = 0, BRLX = Desired relaxation field in MHz (proton Larmor frequency).</li>
			<li>Select the Acq. par tab and then select the Basic sub-tab.</li>
			<li>Click on the cell next to the label Nucleus and scroll down in the pop-up window to select 13C.</li>
			<li>Then, set the following parameters: SF = 8 MHz, SW = 1000000, BS = 652, FLTR = 50000.</li>
			<li>Select the Conf sub-tab.</li>
			<li>Set the following parameters: PW90 equal to the value found in step 2.1.10, RINH = 25, ACQD = 25.</li>
			<li>Select the Puls sub-tab and set PW = 5.</li>
			<li>Select the nDim sub-tab and set NBLK = 100.</li>
			<li>Wait and get ready to receive the hyperpolarized solution to initiate the data acquisition.</li>
			<li>Immediately before inserting the sample into the relaxometer, manually start the pulse sequence from the console, to avoid inserting the sample into a null magnetic field. For this reason, it is important to ignore the first Free Induction Decay (FID) during the data analysis.</li>
			<li>Once the acquisition is done, save the data by clicking the Save button.</li>
		</ol>
		</li>
		<li>Using the analysis software, integrate the magnitude of each FID signal to produce a data series comprised of sample magnetization as a function of time.</li>
		<li>Extract the spin-lattice relaxation time from a three-parameter exponential model using a standard non-linear least-squares fitting algorithm implemented in a commercial analytical software (see Table of Materials) assuming even weighting for all data: 
		<img alt="Equation 1" src="/files/ftp_upload/59399/59399eq1.jpg" /> 
		where A is the initial signal amplitude (y-intercept), T1 is the spin-lattice relaxation time, TR is the repetition time, which is a known value, y0 is the signal offset, and cos(n-1)(&#945;) is a correction for loss of longitudinal magnetization at the nth measurement for a flip angle, &#945;.</li>
	</ol>
	</li>
</ol>

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H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Spin dynamics: basics of nuclear magnetic resonance. , (2001).</li><li>Mieville, P., Jannin, S., Bodenhausen, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Relaxometry+of+insensitive+nuclei:+optimizing+dissolution+dynamic+nuclear+polarization.">Relaxometry of insensitive nuclei: optimizing dissolution dynamic nuclear polarization.</a> Journal of Magnetic Resonance. 210 (1), 137-140 (2011).</li><li>Redfield, A. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shuttling+device+for+high-resolution+measurements+of+relaxation+and+related+phenomena+in+solution+at+low+field,+using+a+shared+commercial+500+MHz+NMR+instrument.">Shuttling device for high-resolution measurements of relaxation and related phenomena in solution at low field, using a shared commercial 500 MHz NMR instrument.</a> Magnetic Resonance in Chemistry. 41 (10), 753-768 (2003).</li><li>Grosse, S., Gubaydullin, F., Scheelken, H., Vieth, H. -. M., Yurkovskaya, A. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Field+cycling+by+fast+NMR+probe+transfer:+Design+and+application+in+field-dependent+CIDNP+experiments.">Field cycling by fast NMR probe transfer: Design and application in field-dependent CIDNP experiments.</a> Applied Magnetic Resonance. 17 (2), 211-225 (1999).</li><li>Kimmich, R., Anoardo, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Field-cycling+NMR+relaxometry.">Field-cycling NMR relaxometry.</a> Progress in Nuclear Magnetic Resonance Spectroscopy. 44 (3-4), 257-320 (2004).</li><li>Gu&#240;j&#243;nsd&#243;ttir, M., Belton, P., Webb, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Magnetic Resonance in Food Science: Challenges in a Changing World. , 65-72 (2009).</li><li>Anoardo, E., Galli, G., Ferrante, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fast-field-cycling+NMR:+Applications+and+instrumentation.">Fast-field-cycling NMR: Applications and instrumentation.</a> Applied Magnetic Resonance. 20 (3), 365-404 (2001).</li><li>Chattergoon, N., Martinez-Santiesteban, F., Handler, W. B., Ardenkjaer-Larsen, J. H., Scholl, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Field+dependence+of+T1+for+hyperpolarized+[1-13C]pyruvate.">Field dependence of T1 for hyperpolarized [1-13C]pyruvate.</a> Contrast Media &amp; Molecular Imaging. 8 (1), 57-62 (2013).</li><li>Mart&#237;nez-Santiesteban, F. M., Dang, T. P., Lim, H., Chen, A. P., Scholl, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=T1+nuclear+magnetic+relaxation+dispersion+of+hyperpolarized+sodium+and+cesium+hydrogencarbonate-13C.">T1 nuclear magnetic relaxation dispersion of hyperpolarized sodium and cesium hydrogencarbonate-13C.</a> NMR in Biomedicine. 30 (9), 3749 (2017).</li></ol>

The authors have no disclosures.

The use of DNP to enhance signal acquisition is a technical solution to insufficient magnetic resonance signal available from 13C nuclei at limited concentrations, as those used in animal injections, but presents other experimental challenges. Each relaxation measurement shown in Figure 7 represents a measurement of a uniquely prepared sample because it cannot be repolarized after dissolution for remeasurement. This inevitably leads to experimental variability due to minor differences in sample preparation during weighing of the sample and dissolution media or variations in the dissolution process itself, such as incomplete extraction and thorough mixing of the sample with the dissolution media. This variability may be partially assessed by measuring the pH of each pyruvate solution after relaxometry. Regardless of careful weighing of stock pyruvate/radical mixture and dissolution medium before insertion in the DNP apparatus to better than a milligram, in our experiments the pHs ranged from 5.5 to 8.3. We have chosen to reject any T1 data outside the pH range 7.6 to 8.0.
As mentioned above, the solid-state polarization level for each sample was at least 95%, which was obtained in about one hour. The liquid-state polarization was not estimated for each sample; however, periodic quality assurance of the DNP system, using the same sample preparation, resulted in liquid state polarization levels of about 15%.
During sample preparation, metal ion contamination may occur from contact between the dissolution medium and the DNP dissolution fluid path. This possibility required the addition of disodium ethylenediaminetetraacetic acid (EDTA) to sequester any of the metal ion contamination and preserve spin-lattice relaxation.
Comparing the shuttling method used in reference28 and the fast field cycling presented in this protocol, we can say that the shuttling method is only possible when the shuttle time is small in comparison to the relaxation time; otherwise, the average magnetic fields experienced during the shuttling time may have a significant effect. With the fast field cycling relaxometer we used, the user is in complete control of the switching time, which can go as low as 3 ms. However, for hyperpolarized substrates, a slow switching time is required to keep adiabaticity and not to destroy the polarization of the sample during filed transitions. In our experience, for hyperpolarized 13C-pyruvic acid, a switching time as low as 50 ms does maintain the polarization, but we observed more consistent results using a switching time of 100 or 200 ms. This small transition time from relaxation to acquisition and back to relaxation fields is negligible in comparison to the measured T1 times and has no systematic effect on these measurements. We consider that further research is required to establish the boundaries of adiabaticity of different hyperpolarized substrates at different magnetic fields.
Another important difference between the two methods is the range of magnetic fields, which is 2 mT to 18.8 T for the shuttling method and 0.237 mT to 0.705 T for the field cycling relaxometer. In this regard we can see the two methods as complementary to each other. However, for in vivo studies with hyperpolarized compounds, magnetic fields of up to 3 T are more common.
At field strengths of less than 1 mT, stray magnetic fields from surrounding objects were observed to have a systematic effect on our relaxation measurements. To eliminate these fields, we designed and added a custom magnetic shim around the field-cycling magnet. In comparison, the shuttling method uses &#181;-metal cylindrical shielding that produces an abrupt change of magnetic field from about 2 mT to 0.2 mT.
Temperature control of the sample was important due to the relatively long acquisition times requiring 300 to 510 s to capture the entire decay curve. We pre-warmed the NMR tubes prior to dispensing the hyperpolarized solution and then maintained the sample temperature by blowing warmed, temperature-regulated (37 &#176;C) air over the tubes during relaxometry. This is an important advantage of the field-cycling relaxometer over the shuttling method because the temperature of the sample can be precisely controlled since the sample is stationary during measurements.
In addition, it was not practical to control the sample exposure to ambient temperature and magnetic field during the brief transfer time between polarizer and relaxometer. The T1 of samples were measured at known magnetic fields and temperature controlled by the relaxometer, so transportation had limited influence. Conditions during transportation can only affect the amount of hyperpolarization that survives for measurement at the relaxometer. A portable holding field magnet (10 mT) was developed for transferring the hyperpolarized solution to the imaging magnet or relaxometer; however, its use was not worthwhile in this experiment given the brief transfer time but may be useful for other hyperpolarized liquids with greater T1-dispersion at lower magnetic fields. A holding field of 0.01 T would increase the T1 of the pyruvate solution by nearly 18% during transportation; however, with our relatively short transfer time of 8 s, these measurements suggest that only a 2.3% increase in signal would be observed.

Magnetic resonance spectroscopic imaging (MRSI) can produce spatial maps of metabolites detected by spectroscopic imaging, but its practical use is often limited by its relatively low sensitivity. This low sensitivity of in vivo magnetic resonance imaging and spectroscopy methods stems from the small degree of nuclear magnetization achievable at body temperatures and reasonable magnetic field strengths. However, this limitation can be overcome by the use of dynamic nuclear polarization (DNP) to greatly enhance the in vitro magnetization of liquid substrates, which are subsequently injected to probe in vivo metabolism using MRSI1,2,3,4. DNP is capable of enhancing the magnetization of most nuclei with non-zero nuclear spin and has been used to increase in vivo MRSI sensitivity of 13C-enriched compounds such as pyruvate5,6, bicarbonate7,8, fumarate9, lactate10, glutamine11, and others by more than four orders of magnitude12. Its applications include imaging of vascular disease13,14,15, organ perfusion13,16,17,18, cancer detection1,19,20,21,22, tumor staging23,24, and quantification of therapeutic response2,6,23,24,25,26.
Slow spin-lattice relaxation is essential for in vivo detection with MRSI. Spin-lattice relaxation times (T1s) on the order of tens of seconds are possible for nuclei with low gyromagnetic ratios within small molecules in solution. Several physical factors influence the transfer of energy between a nuclear spin transition and its environment (lattice) leading to relaxation, including the magnetic field strength, temperature, and molecular conformation27. Dipolar relaxation is reduced in molecules for carbon positions with no protons directly attached, and deuteration of dissolution media can further reduce intermolecular dipolar relaxation. Unfortunately, deuterated solvents have limited abilities to extend in vivo relaxation. Increased relaxation of carbonyls or carboxylic acids (such as pyruvate) can occur at high magnetic field strengths due to chemical shift anisotropy. The presence of paramagnetic impurities from the fluid path during dissolution after polarization can cause rapid relaxation and need to be avoided or eliminated using chelators.
Very little data exist for the relaxation of 13C-containing compounds at low fields, where spin-lattice relaxation could be significantly faster. However, it is important to measure T1 at low fields to understand relaxation during preparation of the agent used for in vivo imaging, since the hyperpolarized contrast agents are usually dispensed from the DNP apparatus near or at the earth&#8217;s field. Additional physical factors such as 13C-enriched substrate concentration, solution pH, buffers and temperature also influence relaxation, and consequently have an effect on the formulation of the agent. All these factors are essential in the determination of key parameters in optimizing the DNP dissolution process, and the calculation of the magnitude of signal loss that occurs in transportation of the sample from the DNP apparatus to the imaging magnet.
Nuclear magnetic resonance dispersion (NMRD) measurements, i.e., T1 measurements, as a function of magnetic field are typically acquired using an NMR spectrometer. To acquire these measurements, a shuttling method could be used where the sample is first shuttled out of the spectrometer to relax at some field determined by its position in the fringe field of the magnet28,29,30 and then rapidly transferred back into the NMR magnet to measure its remaining magnetization. By repeating this process at the same point in the magnetic field but with increasing periods of relaxation, a relaxation curve can be obtained, which can then be analyzed to estimate T1.
We use an alternative technique known as fast field-cycling relaxometry31,32,33 to acquire our NMRD data. We have modified a commercial field-cycling relaxometer (see Table of Materials), for T1 measurements of solutions containing hyperpolarized 13C nuclei. Compared with the shuttle method, field-cycling enables this relaxometer to systematically acquire NMRD data over a smaller range of magnetic fields (0.25 mT to 1 T). This is accomplished by rapidly changing the magnetic field itself, not the sample location in the magnetic field. Therefore, a sample can be magnetized at a high field strength, &#34;relaxed&#34;&#160;at a lower field strength, and then measured by acquisition of a free-induction-decay at a fixed field (and Larmor frequency) to maximize signal. This means that the sample temperature can be controlled during the measurement, and the NMR probe does not need to be tuned at each relaxation field promoting automatic acquisition over the entire magnetic field range.
Focusing our efforts to the effects of dispensing and transporting the hyperpolarized solutions at low magnetic fields, this work presents a detailed methodology to measure the spin-lattice relaxation time of hyperpolarized 13C-pyruvate using fast field-cycling relaxometry for magnetic fields in the range of 0.237 mT to 0.705 T. The main results of using this methodology have been previously presented for [1-13C]pyruvate34 and 13C-enriched sodium and cesium bicarbonate35 where other factors such as radical concentration and dissolution pH have also been studied.

<table><tbody><tr><td>[1-&lt;sup&gt;13&lt;/sup&gt;C]Pyruvic Acid</td><td>Sigma-Aldrich, St. Louis, MO, USA</td><td>677175</td><td></td></tr><tr><td>10mm NMR Tube</td><td>Norell, Inc., Morganton NC, USA</td><td>1001-8</td><td></td></tr><tr><td>De-ionized water</td><td></td><td></td><td></td></tr><tr><td>Ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA)</td><td>Sigma-Aldrich, St. Louis, MO, USA</td><td>E5134</td><td></td></tr><tr><td>HyperSense Dynamic Nuclear Polarizer</td><td>Oxford Instruments, Abingdon, UK</td><td></td><td>Includes the following: &quot;DNP-NMR Polarizer&quot; software used to control and monitor the whole DNP polarizer; &quot;RINMR&quot; used to monitor the solid state polarization levels; &quot;HyperTerminal&quot; used to communicate the DNP software with the RINMR software that monitors the solid state polarization level. Also includes the MQC bench top spectrometer to monitor the liquid state polarization in conjunction with it own RINMR software</td></tr><tr><td>MATLAB R2017b</td><td>MathWorks, Natick, MA</td><td></td><td>Include scripts for non-linear fitting of magnetization decay over time and T1 NMRD analysis of hyperpolarized pyruvic acid.</td></tr><tr><td>OX063 Triarylmethyl radical</td><td>Oxford Instruments, Abingdon, UK</td><td></td><td></td></tr><tr><td>pH meter - SympHony</td><td>VWR International, Mississauga, ON., Canada</td><td>SB70P</td><td></td></tr><tr><td>ProHance</td><td>Bracco Diagnostics Inc.</td><td></td><td>Gadoteridol, Gd-HP-DO3A</td></tr><tr><td>Pure Ethanol (100% pure)</td><td>Commercial Alcohols, Toronto, ON, Canada</td><td>P016EAAN</td><td></td></tr><tr><td>Shim Coil</td><td></td><td></td><td>Developed in-house</td></tr><tr><td>Sodium Chloride</td><td>Sigma-Aldrich, St. Louis, MO, USA</td><td>S7653</td><td></td></tr><tr><td>Sodium Hydroxide</td><td>Sigma-Aldrich, St. Louis, MO, USA</td><td>S8045&amp;nbsp;</td><td></td></tr><tr><td>SpinMaster FFC2000 1T C/DC</td><td>Stelar s.r.l., Mede (PV) Italy</td><td></td><td>Includes the software &quot;AcqNMR&quot; that is used to set experimental parameters, monitor the tuning and matching of the RF coil, loading different pulse sequences, calibrate flip angle, data acquisition and curve fitting, among other functions. Also includes a depth gauge, some weights and a depth stopper.</td></tr><tr><td>Trizma Pre-Set Crystals (pH 7.6)</td><td>Sigma-Aldrich, St. Louis, MO, USA</td><td>T7943</td><td></td></tr></tbody></table>

measuring the spin-lattice relaxation magnetic field dependence of hyperpolarized [1-13c]pyruvate

The fundamental limit to in vivo imaging applications of hyperpolarized 13C-enriched compounds is their finite spin-lattice relaxation times. Various factors affect the relaxation rates, such as buffer composition, solution pH, temperature, and magnetic field. In this last regard, the spin-lattice relaxation time can be measured at clinical field strengths, but at lower fields, where these compounds are dispensed from the polarizer and transported to the MRI, the relaxation is even faster and difficult to measure. To have a better understanding of the amount of magnetization lost during transport, we used fast field-cycling relaxometry, with magnetic resonance detection of 13C nuclei at ~0.75 T, to measure the nuclear magnetic resonance dispersion of the spin-lattice relaxation time of hyperpolarized [1-13C]pyruvate. Dissolution dynamic nuclear polarization was used to produce hyperpolarized samples of pyruvate at a concentration of 80 mmol/L and physiological pH (~7.8). These solutions were rapidly transferred to a fast field-cycling relaxometer so that relaxation of the sample magnetization could be measured as a function of time using a calibrated small flip angle (3&#176;-5&#176;). To map the T1 dispersion of the C-1 of pyruvate, we recorded data for different relaxation fields ranging between 0.237 mT and 0.705 T. With this information, we determined an empirical equation to estimate the spin-lattice relaxation of the hyperpolarized substrate within the mentioned range of magnetic fields. These results can be used to predict the amount of magnetization lost during transport and to improve experimental designs to minimize signal loss.

Figure 2 presents an example of a high-resolution full-range microwave sweep for pyruvic acid. For the presented case, that optimal microwave frequency corresponds to 94.128 GHz, highlighted in the figure insert. Our DNP system can normally work in the range of 93.750 GHz to 94.241 GHz with step size of 1 MHz, polarization time of up to 600 s, and power of up to 100 mW. A full range of frequencies is investigated only for novel substrates. However, based on previous experience with 13C-pyruvic acid, we expect the optimal frequency to be around 94.127 GHz. Therefore, a scan range between 94.117 GHz to 94.137 GHz, with a step size of 1 MHz and a sampling time of 300 s with 50 mW of power, are typically used.
The left column of Figure 3 presents the results for the tip angle calibration for [1-13C]pyruvic acid, which involves acquisition of a series of signal measurements as a function of a linearly varying RF pulse durations to determine the pulse width corresponding to a flip angle of 90&#176; and 180&#176; for 13C nuclei. The pulse width that provides the maximum amplitude corresponds to a flip angle of 90&#176; and the zero crossing corresponds to a flip angle of 180&#176;. The relationship between the two pulse widths should be a factor of two.
The acquisition parameters for the 13C tip angle calibration shown above may require some adjustments depending on the transmit power of the field-cycle relaxometer, the T1 of the sample, and the noise characteristic of the system. Some trial and error may be needed as well to properly find the 90&#176; and 180&#176; without the effects of stimulated echoes, amplifier saturation, and poor SNR.
This procedure, although accurate, is normally time consuming because the poor SNR of thermally polarized 13C compounds requires many averages. An alternative and faster method involves calibrating the flip angle with a gadolinium-doped 1H phantom and scaling the duration of the 90&#176; RF pulse for 13C by multiplying the duration of the 90&#176;-1H RF pulse by the ratio of the gyromagnetic ratios of 1H/13C, which corresponds to a factor of 3.976. For this case, the standard acquisition parameters should be: EXP = ANGLE.FFC, NUC = 1H, TPOL = 0.1 s, BPOL = 30 MHz, SWT = 0.005, BINI = 0 &#181;s, BEND = 15.5 &#181;s, NBLK = 32, MS = 1, RFA = 25, RD = 0.1 s, BS = 652, SW = 1 MHz, FLTR = 100 KHz, SF = 8, RINH = 25, ACQD = 25, EWIP = 10, EWEP = 512, EWIB = 1, and EWEB = 32. The results for this alternative method are shown in the right column of Figure 3. As a comparison, for the presented cases, the total acquisition time for tip angle calibration for 13C was 13.5 minutes whereas for 1H was 7.1 seconds.
Figure 5 illustrates the typical series of decaying FIDs as the hyperpolarized magnetization is sampled. Each T1 measurement at a given BRLX is a separate hyperpolarized dissolution from the DNP apparatus. For this particular case, the relaxation field (BRelax) was 0.2916 mT, with a repetition time of 3.4 s and a flip angle of 5&#176;. All sample temperatures were controlled to 37 &#176;C (&#177;0.5 &#176;C).
Figure 6 presents the relaxation curve for hyperpolarized [1-13C]pyruvate obtained from the data of the previous figure. Each blue point on the curve represents the area under an FID. The T1 value (53.9 &#177; 0.6s) was obtained by a non-linear least-squares fit of the signal equation to the decay curve data, which included the effects of the flip angle used for excitation. The goodness of fit was assessed by computing the R2 value (0.9995), assuming even weighting of the data points. Fitting residuals (data-fit) are shown as open triangles.
Figure 7 presents the T1 results for all 26 measurements over a range of 0.237 mT and 0.705 T at 37 &#176;C (&#177;0.5 &#176;C). The T1 had an average fitting uncertainty of &#177;0.33 s for all the results. Analysis of the scatter of measurements repeated at a particular relaxation field yielded an experimental reproducibility several times larger than the statistical uncertainty quoted above, with a T1 of 1.91 s. An uncertainty of 2.24 s was conservatively assigned for all T1 measurements calculated as the sum of the two uncertainties quoted above. The T1-dispersion data are well characterized by the empirical formula T1 = (3.74 &#177; 0.52) x log10(BRelax) + (63.0 &#177; 1.2) s; where BRelax is the relaxation field measured in Tesla. The uncertainties for the fitted parameters represent one standard deviation. The solid line on Figure 7 represents the formula along with the dashed lines representing the 95% confidence bands. pHs for these samples ranged from 7.63 to 7.93, with an average pH of 7.75 and a standard deviation of 0.09. Analysis of the results showed that the relaxation time for the C-1 nucleus is ~ 46.9 s at earth&#8217;s magnetic field (0.05 mT) compared with ~ 65 s at 3 T, which represents a decrease of 28%.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59399/59399fig1.jpg" /> 
Figure 1: [1-13C]pyruvic acid molecule. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/59399/59399fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59399/59399fig2.jpg" /> 
Figure 2: Full-range microwave sweep and zoom-in section showing the optimal polarization frequency. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/59399/59399fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59399/59399fig3.jpg" /> 
Figure 3: Tip angle calibration for 13C (left) and 1H (right) samples. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/59399/59399fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59399/59399fig4.jpg" /> 
Figure 4: Field-cycled pulse sequence (HPUB/S) to measure the T1-relaxation time of a hyperpolarized sample at a particular relaxation field (BRLX). <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/59399/59399fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/59399/59399fig5.jpg" /> 
Figure 5: Sequence of FIDs obtained with the HPUB/S pulse sequence. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/59399/59399fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/59399/59399fig6.jpg" /> 
Figure 6: Relaxation signal (blue dots), curve fitting (red line), and fitting error (open triangles) obtained from the sequence of FIDs presented in Figure 5. This figure has been modified with permission from Chattergoon et al. 201334. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/59399/59399fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/59399/59399fig7.jpg" /> 
Figure 7: NMRD profile of hyperpolarized [1-13C]pyruvic acid at low magnetic fields. This figure has been modified with permission from Chattergoon et al. 201334. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/59399/59399fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table>
	<tbody>
		<tr>
			<td>Parameter</td>
			<td>Short Description</td>
			<td>Comments</td>
			<td>Units</td>
		</tr>
		<tr>
			<td>ACQD</td>
			<td>Acquisition delay</td>
			<td>Delay required to allow magnetic field to reach steady state after transition and before data acquisition</td>
			<td>&#181;s</td>
		</tr>
		<tr>
			<td>BACQ</td>
			<td>Acquisition Field</td>
			<td>Specified by means of 1H Larmor frequency</td>
			<td>MHz</td>
		</tr>
		<tr>
			<td>BEND</td>
			<td>End value</td>
			<td>Final value of the arrayed parameter</td>
			<td></td>
		</tr>
		<tr>
			<td>BINI</td>
			<td>Initial value</td>
			<td>First value of the arrayed parameter</td>
			<td></td>
		</tr>
		<tr>
			<td>BPOL</td>
			<td>Polarization Field</td>
			<td>Specified by means of 1H Larmor frequency</td>
			<td>MHz</td>
		</tr>
		<tr>
			<td>BRLX</td>
			<td>Relaxation Field</td>
			<td>Specified by means of 1H Larmor frequency</td>
			<td>MHz</td>
		</tr>
		<tr>
			<td>BS</td>
			<td>Block size</td>
			<td>Number of data points in a single block</td>
			<td></td>
		</tr>
		<tr>
			<td>EWEB</td>
			<td>End block</td>
			<td>Any integer number in the range of Number of Blocks (NBLK). 0 means &#34;all&#34;</td>
			<td></td>
		</tr>
		<tr>
			<td>EWEP</td>
			<td>End point</td>
			<td>Any integer number in the range of Block size (BS). 0 means &#34;all&#34;</td>
			<td></td>
		</tr>
		<tr>
			<td>EWIB</td>
			<td>Initial block</td>
			<td>From 1 to number of blocks (NBLK)</td>
			<td></td>
		</tr>
		<tr>
			<td>EWEP</td>
			<td>Initial point</td>
			<td>From 1 to block size (BS)</td>
			<td></td>
		</tr>
		<tr>
			<td>EXP</td>
			<td>Experiment</td>
			<td>Name of pulse sequence to be used</td>
			<td></td>
		</tr>
		<tr>
			<td>FLTR</td>
			<td>Observe filter</td>
			<td>Cutoff frequency of the audio signal filters</td>
			<td>Hz</td>
		</tr>
		<tr>
			<td>MS</td>
			<td>Maximum scans</td>
			<td>Desired number of averages</td>
			<td></td>
		</tr>
		<tr>
			<td>NBLK</td>
			<td>Number of blocks</td>
			<td>Number of sections for the arrayed parameter. The arrayed parameter is &#34;PW90&#34; for &#34;13CANGLE&#34; and &#34;ANGLE&#34; pulse sequences and &#34;T1MX&#34; for &#34;HPUB/S&#34; pulse sequence. PW90 changes after each repetition but T1MX remains constant.</td>
			<td></td>
		</tr>
		<tr>
			<td>NUC</td>
			<td>Nucleus</td>
			<td>For this protocol 13C or 1H</td>
			<td></td>
		</tr>
		<tr>
			<td>PW</td>
			<td>Main RF pulse</td>
			<td>Tip angle</td>
			<td>Degrees (&#176;)</td>
		</tr>
		<tr>
			<td>PW90</td>
			<td>90deg pulse</td>
			<td>Duration of the 90-degrees pulse</td>
			<td>&#181;s</td>
		</tr>
		<tr>
			<td>RD</td>
			<td>Recycle delay</td>
			<td>Pre-scan magnet-cooling interval</td>
			<td>s</td>
		</tr>
		<tr>
			<td>RFA</td>
			<td>RF attenuation</td>
			<td>RF receiver attenuation</td>
			<td>dB</td>
		</tr>
		<tr>
			<td>RINH</td>
			<td>Receiver inhibit</td>
			<td>Delay required to allow the decay of RF-coil ringing</td>
			<td>&#181;s</td>
		</tr>
		<tr>
			<td>SF</td>
			<td>System Frequency</td>
			<td>Larmor frequency used during acquisition</td>
			<td>MHz</td>
		</tr>
		<tr>
			<td>SW</td>
			<td>Sweep Width</td>
			<td>Spectral window width (Nyquist Frequency)</td>
			<td>Hz</td>
		</tr>
		<tr>
			<td>SWT</td>
			<td>Switching time</td>
			<td>Global magnet-switching time</td>
			<td>s</td>
		</tr>
		<tr>
			<td>T1MX</td>
			<td>Maximum T1</td>
			<td>Parameter used by the HPUB/S pulse sequence to define the polarization time during each repetition</td>
			<td>s</td>
		</tr>
		<tr>
			<td>TPOL</td>
			<td>Polarization time</td>
			<td>Parameter used by the &#34;ANGLE&#34; and &#34;13CANGLE&#34; pulse sequence to define the polarization time during each repetition</td>
			<td>s</td>
		</tr>
	</tbody>
</table>
Table 1: Description of parameters used by the field-cycling relaxometer.

Watch this Scientific Journal Video about Measuring the Spin-Lattice Relaxation Magnetic Field Dependence of Hyperpolarized [1-13C]pyruvate at JoVE.com

Measuring the Spin-Lattice Relaxation Magnetic Field Dependence of Hyperpolarized [1-13C]pyruvate

We present a protocol to measure the magnetic field dependence of the spin-lattice relaxation time of 13C-enriched compounds, hyperpolarized by means of dynamic nuclear polarization, using fast field-cycled relaxometry. Specifically, we have demonstrated this with [1-13C]pyruvate, but the protocol could be extended to other hyperpolarized substrates.

Measuring the Spin-Lattice Relaxation Magnetic Field Dependence of Hyperpolarized [1-13C]pyruvate

The fundamental limit to in vivo imaging applications of hyperpolarized 13C-enriched compounds is their finite spin-lattice relaxation times. Various ...

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Nanyang Technological University

Research

JoVE Journal

Bioengineering

6.5K Views.  Western University. We present a protocol to measure the magnetic field dependence of the spin-lattice relaxation time of 13C-enriched compounds, hyperpolarized by means of dynamic nuclear polarization, using fast field-cycled relaxometry. Specifically, we have demonstrated this with [1-13C]pyruvate, but the protocol could be extended to other hyperpolarized substrates.

Video: Measuring the Spin-Lattice Relaxation Magnetic Field Dependence of Hyperpolarized [1-13C]pyruvate

Hyperpolarized 13C Metabolic Magnetic Resonance Spectroscopy and Imaging

In the past decades, new methods for tumor staging, restaging, treatment response monitoring, and recurrence detection of a variety of cancers have emerged in conjunction with the state-of-the-art positron emission tomography with 18F-fluorodeoxyglucose ([18F]-FDG PET). 13C magnetic resonance spectroscopic imaging (13CMRSI) is a minimally invasive imaging method that enables the monitoring of metabolism in vivo and in real time. As with any other method based on 13C nuclear magnetic resonance (NMR), it faces the challenge of low thermal polarization and a subsequent low signal-to-noise ratio due to the relatively low gyromagnetic ratio of 13C and its low natural abundance in biological samples. By overcoming these limitations, dynamic nuclear polarization (DNP) with subsequent sample dissolution has recently enabled commonly used NMR and magnetic resonance imaging (MRI) systems to measure, study, and image key metabolic pathways in various biological systems. A particularly interesting and promising molecule used in 13CMRSI is [1-13C]pyruvate, which, in the last ten years, has been widely used for in vitro, preclinical, and, more recently, clinical studies to investigate the cellular energy metabolism in cancer and other diseases. In this article, we outline the technique of dissolution DNP using a 3.35 T preclinical DNP hyperpolarizer and demonstrate its usage in in vitro studies. A similar protocol for hyperpolarization may be applied for the most part in in vivo studies as well. To do so, we used lactate dehydrogenase (LDH) and catalyzed the metabolic reaction of [1-13C]pyruvate to [1-13C]lactate in a prostate carcinoma cell line, PC3, in vitro using 13CMRSI.

Hyperpolarized 13C Metabolic Magnetic Resonance Spectroscopy and Imaging

Dynamic nuclear polarization with subsequent sample dissolution has enabled real-time studies of metabolism in biological systems. Hyperpolarized [1-13C]pyruvate was used to study lactate dehydrogenase activity in a prostate carcinoma cell line in vitro.

In the past decades, new methods for tumor staging, restaging, treatment response monitoring, and recurrence detection of a variety of cancers have ...

Cancer Research

Magnetic Resonance Elastography Methodology for the Evaluation of Tissue Engineered Construct Growth

Traditional mechanical testing often results in the destruction of the sample, and in the case of long term tissue engineered construct studies, the use of destructive assessment is not acceptable. A proposed alternative is the use of an imaging process called magnetic resonance elastography. Elastography is a nondestructive method for determining the engineered outcome by measuring local mechanical property values (i.e., complex shear modulus), which are essential markers for identifying the structure and functionality of a tissue. As a noninvasive means for evaluation, the monitoring of engineered constructs with imaging modalities such as magnetic resonance imaging (MRI) has seen increasing interest in the past decade1. For example, the magnetic resonance (MR) techniques of diffusion and relaxometry have been able to characterize the changes in chemical and physical properties during engineered tissue development2. The method proposed in the following protocol uses microscopic magnetic resonance elastography (&mu;MRE) as a noninvasive MR based technique for measuring the mechanical properties of small soft tissues3. MRE is achieved by coupling a sonic mechanical actuator with the tissue of interest and recording the shear wave propagation with an MR scanner4. Recently, &mu;MRE has been applied in tissue engineering to acquire essential growth information that is traditionally measured using destructive mechanical macroscopic techniques5. In the following procedure, elastography is achieved through the imaging of engineered constructs with a modified Hahn spin-echo sequence coupled with a mechanical actuator. As shown in Figure 1, the modified sequence synchronizes image acquisition with the transmission of external shear waves; subsequently, the motion is sensitized through the use of oscillating bipolar pairs. Following collection of images with positive and negative motion sensitization, complex division of the data produce a shear wave image. Then, the image is assessed using an inversion algorithm to generate a shear stiffness map6. The resulting measurements at each voxel have been shown to strongly correlate (R2&gt;0.9914) with data collected using dynamic mechanical analysis7. In this study, elastography is integrated into the tissue development process for monitoring human mesenchymal stem cell (hMSC) differentiation into adipogenic and osteogenic constructs as shown in Figure 2.

The procedure demonstrates the methodology of magnetic resonance elastography for monitoring the engineered outcome of adipose and osteogenic tissue engineered constructs through noninvasive local assessment of the mechanical properties using microscopic magnetic resonance elastography (&mu;MRE).

Traditional mechanical testing often results in the destruction of the sample, and in the case of long term tissue engineered construct studies, the use ...

Dissolution Dynamic Nuclear Polarization Instrumentation for Real-time Enzymatic Reaction Rate Measurements by NMR

The main limitation of NMR-based investigations is low sensitivity. This prompts for long acquisition times, thus preventing real-time NMR measurements of metabolic transformations. Hyperpolarization via dissolution DNP circumvents part of the sensitivity issues thanks to the large out-of-equilibrium nuclear magnetization stemming from the electron-to-nucleus spin polarization transfer. The high NMR signal obtained can be used to monitor chemical reactions in real time. The downside of hyperpolarized NMR resides in the limited time window available for signal acquisition, which is usually on the order of the nuclear spin longitudinal relaxation time constant, T1, or, in favorable cases, on the order of the relaxation time constant associated with the singlet-state of coupled nuclei, TLLS. Cellular uptake of endogenous molecules and metabolic rates can provide essential information on tumor development and drug response. Numerous previous hyperpolarized NMR studies have demonstrated the relevancy of pyruvate as a metabolic substrate for monitoring enzymatic activity in vivo. This work provides a detailed description of the experimental setup and methods required for the study of enzymatic reactions, in particular the pyruvate-to-lactate conversion rate in presence of lactate dehydrogenase (LDH), by hyperpolarized NMR.

The sensitivity enhancement provided by dissolution dynamic nuclear polarization (DNP) enables following metabolic processes in real time by NMR and MRI. The characteristics and performances of a dedicated dissolution DNP setup designed for study enzymatic reactions are discussed.

The main limitation of NMR-based investigations is low sensitivity. This prompts for long acquisition times, thus preventing real-time NMR measurements of ...

Chemistry

Use of a Multi-compartment Dynamic Single Enzyme Phantom for Studies of Hyperpolarized Magnetic Resonance Agents

Imaging of hyperpolarized substrates by magnetic resonance shows great clinical promise for assessment of critical biochemical processes in real time. Due to fundamental constraints imposed by the hyperpolarized state, exotic imaging and reconstruction techniques are commonly used. A practical system for characterization of dynamic, multi-spectral imaging methods is critically needed. Such a system must reproducibly recapitulate the relevant chemical dynamics of normal and pathological tissues. The most widely utilized substrate to date is hyperpolarized [1-13C]-pyruvate for assessment of cancer metabolism. We describe an enzyme-based phantom system that mediates the conversion of pyruvate to lactate. The reaction is initiated by injection of the hyperpolarized agent into multiple chambers within the phantom, each of which contains varying concentrations of reagents that control the reaction rate. Multiple compartments are necessary to ensure that imaging sequences faithfully capture the spatial and metabolic heterogeneity of tissue. This system will aid the development and validation of advanced imaging strategies by providing chemical dynamics that are not available from conventional phantoms, as well as control and reproducibility that is not possible in vivo.

A multi-compartment dynamic phantom is used to simulate some biology of interest for metabolic studies using hyperpolarized magnet resonance agents.

Imaging of hyperpolarized substrates by magnetic resonance shows great clinical promise for assessment of critical biochemical processes in real time. Due ...

Engineering

Site Directed Spin Labeling and EPR Spectroscopic Studies of Pentameric Ligand-Gated Ion Channels

Ion channel gating is a stimulus-driven orchestration of protein motions that leads to transitions between closed, open, and desensitized states. Fundamental to these transitions is the intrinsic flexibility of the protein, which is critically modulated by membrane lipid-composition. To better understand the structural basis of channel function, it is necessary to study protein dynamics in a physiological membrane environment. Electron Paramagnetic Resonance (EPR) spectroscopy is an important tool to characterize conformational transitions between functional states. In comparison to NMR and X-ray crystallography, the information obtained from EPR is intrinsically of lower resolution. However, unlike in other techniques, in EPR there is no upper-limit to the molecular weight of the protein, the sample requirements are significantly lower, and more importantly the protein is not constrained by the crystal lattice forces. Therefore, EPR is uniquely suited for studying large protein complexes and proteins in reconstituted systems. In this article, we will discuss general protocols for site-directed spin labeling and membrane reconstitution using a prokaryotic proton-gated pentameric Ligand-Gated Ion Channel (pLGIC) from Gloeobacter violaceus (GLIC) as an example. A combination of steady-state Continuous Wave (CW) and Pulsed (Double Electron Electron Resonance-DEER) EPR approaches will be described that will enable a complete quantitative characterization of channel dynamics.

This article describes methods for site-directed spin labeling and reconstitution of pentameric ligand-gated channels for Electron Paramagnetic Resonance studies. This protocol can be adapted for any membrane protein. The reconstitution method described here can also be used for patch-clamp measurements of macroscopic and single-channel currents in a defined lipid system.

Ion channel gating is a stimulus-driven orchestration of protein motions that leads to transitions between closed, open, and desensitized states. ...

Measuring the Mechanical Properties of Living Cells Using Atomic Force Microscopy

Mechanical properties of cells and extracellular matrix (ECM) play important roles in many biological processes including stem cell differentiation, tumor formation, and wound healing. Changes in stiffness of cells and ECM are often signs of changes in cell physiology or diseases in tissues. Hence, cell stiffness is an index to evaluate the status of cell cultures. Among the multitude of methods applied to measure the stiffness of cells and tissues, micro-indentation using an Atomic Force Microscope (AFM) provides a way to reliably measure the stiffness of living cells. This method has been widely applied to characterize the micro-scale stiffness for a variety of materials ranging from metal surfaces to soft biological tissues and cells. The basic principle of this method is to indent a cell with an AFM tip of selected geometry and measure the applied force from the bending of the AFM cantilever. Fitting the force-indentation curve to the Hertz model for the corresponding tip geometry can give quantitative measurements of material stiffness. This paper demonstrates the procedure to characterize the stiffness of living cells using AFM. Key steps including the process of AFM calibration, force-curve acquisition, and data analysis using a MATLAB routine are demonstrated. Limitations of this method are also discussed.

This paper demonstrates a protocol to characterize the mechanical properties of living cells by means of microindentation using an Atomic Force Microscope (AFM).

Mechanical  properties of cells and extracellular matrix (ECM) play important roles in many  biological processes including stem cell differentiation, ...

Microfluidic Platform for Measuring Neutrophil Chemotaxis from Unprocessed Whole Blood

Neutrophils play an essential role in protection against infections and their numbers in the blood are frequently measured in the clinic. Higher neutrophil counts in the blood are usually an indicator of ongoing infections, while low neutrophil counts are a warning sign for higher risks for infections. To accomplish their functions, neutrophils also have to be able to move effectively from the blood where they spend most of their life, into tissues, where infections occur. Consequently, any defects in the ability of neutrophils to migrate can increase the risks for infections, even when neutrophils are present in appropriate numbers in the blood. However, measuring neutrophil migration ability in the clinic is a challenging task, which is time consuming, requires large volume of blood, and expert knowledge. To address these limitations, we designed a robust microfluidic assays for neutrophil migration, which requires a single droplet of unprocessed blood, circumvents the need for neutrophil separation, and is easy to quantify on a simple microscope. In this assay, neutrophils migrate directly from the blood droplet, through small channels, towards the source of chemoattractant. To prevent the granular flow of red blood cells through the same channels, we implemented mechanical filters with right angle turns that selectively block the advance of red blood cells. We validated the assay by comparing neutrophil migration from blood droplets collected from finger prick and venous blood. We also compared these whole blood (WB) sources with neutrophil migration from samples of purified neutrophils and found consistent speed and directionality between the three sources. This microfluidic platform will enable the study of human neutrophil migration in the clinic and the research setting to help advance our understanding of neutrophil functions in health and disease.

This protocol details an assay designed to measure human neutrophil chemotaxis from one droplet of whole blood with robust reproducibility. This approach circumvents the need for neutrophil separation and requires only a few minutes of assay preparation time. The microfluidic chip enables the repeated measure of neutrophil chemotaxis over time in infants or small mammals, where sample volume is limited.

Neutrophils play an essential role in protection against infections and their numbers in the blood are frequently measured in the clinic. Higher ...

Magnetic Tweezers for the Measurement of Twist and Torque

Single-molecule techniques make it possible to investigate the behavior of individual biological molecules in solution in real time. These techniques include so-called force spectroscopy approaches such as atomic force microscopy, optical tweezers, flow stretching, and magnetic tweezers. Amongst these approaches, magnetic tweezers have distinguished themselves by their ability to apply torque while maintaining a constant stretching force. Here, it is illustrated how such a &#8220;conventional&#8221; magnetic tweezers experimental configuration can, through a straightforward modification of its field configuration to minimize the magnitude of the transverse field, be adapted to measure the degree of twist in a biological molecule. The resulting configuration is termed the freely-orbiting magnetic tweezers. Additionally, it is shown how further modification of the field configuration can yield a transverse field with a magnitude intermediate between that of the &#8220;conventional&#8221; magnetic tweezers and the freely-orbiting magnetic tweezers, which makes it possible to directly measure the torque stored in a biological molecule. This configuration is termed the magnetic torque tweezers. The accompanying video explains in detail how the conversion of conventional magnetic tweezers into freely-orbiting magnetic tweezers and magnetic torque tweezers can be accomplished, and demonstrates the use of these techniques. These adaptations maintain all the strengths of conventional magnetic tweezers while greatly expanding the versatility of this powerful instrument.

Magnetic tweezers, a powerful single-molecule manipulation technique, can be adapted for the direct measurements of the twist (using a configuration called freely-orbiting magnetic tweezers) and torque (using a configuration termed magnetic torque tweezers) in biological macromolecules. Guidelines for performing such measurements are given, including applications to the study of DNA and associated nucleo-protein filaments.

Single-molecule techniques make it possible to investigate the behavior of individual biological molecules in solution in real time. These techniques ...

Alternating Magnetic Field-Responsive Hybrid Gelatin Microgels for Controlled Drug Release

Magnetically-responsive nano/micro-engineered biomaterials that enable a tightly controlled, on-demand drug delivery have been developed as new types of smart soft devices for biomedical applications. Although a number of magnetically-responsive drug delivery systems have demonstrated efficacies through either in vitro proof of concept studies or in vivo preclinical applications, their use in clinical settings is still limited by their insufficient biocompatibility or biodegradability. Additionally, many of the existing platforms rely on sophisticated techniques for their fabrications. We recently demonstrated the fabrication of biodegradable, gelatin-based thermo-responsive microgel by physically entrapping poly(N-isopropylacrylamide-co-acrylamide) chains as a minor component within a three-dimensional gelatin network. In this study, we present a facile method to fabricate a biodegradable drug release platform that enables a magneto-thermally triggered drug release. This was achieved by incorporating superparamagnetic iron oxide nanoparticles and thermo-responsive polymers within gelatin-based colloidal microgels, in conjunction with an alternating magnetic field application system.

We present a facile method to fabricate a biodegradable gelatin-based drug release platform that is magneto-thermally responsive. This was achieved by incorporating superparamagnetic iron oxide nanoparticles and poly(N-isopropylacrylamide-co-acrylamide) within a spherical gelatin micro-network crosslinked by genipin, in conjunction with an alternating magnetic field application system.

Magnetically-responsive nano/micro-engineered biomaterials that enable a tightly controlled, on-demand drug delivery have been developed as new types of ...

Investigating Cardiac Metabolism in the Isolated Perfused Mouse Heart with Hyperpolarized [1-13C]Pyruvate and 13C/31P NMR Spectroscopy

Metabolism is the basis of important processes in cellular life. Characterizing how metabolic networks function in living tissues provides crucial information for understanding the mechanism of diseases and designing treatments. In this work, we describe procedures and methodologies for studying in-cell metabolic activity in a retrogradely perfused mouse heart in real-time. The heart was isolated in situ,&#160;in conjunction with cardiac arrest to minimize the myocardial ischemia and was perfused inside a nuclear magnetic resonance (NMR) spectrometer. While in the spectrometer and under continuous perfusion, hyperpolarized [1-13C]pyruvate was administered to the heart, and the subsequent hyperpolarized [1-13C]lactate and [13C]bicarbonate production rates served to determine, in real-time, the rates of lactate dehydrogenase and pyruvate dehydrogenase production. This metabolic activity of hyperpolarized [1-13C]pyruvate was quantified with NMR spectroscopy in a model free-manner using the product selective saturating-excitations acquisition approach. 31P spectroscopy was applied in between the hyperpolarized acquisitions to monitor the cardiac energetics and pH. This system is uniquely useful for studying metabolic activity in the healthy and diseased mouse heart.

Investigating Cardiac Metabolism in the Isolated Perfused Mouse Heart with Hyperpolarized [1-13C]Pyruvate and 13C/31P NMR Spectroscopy

We describe an experimental setup for administrating hyperpolarized 13C-labeled metabolites in continuous perfusion mode to an isolated perfused mouse heart. A dedicated 13C-NMR acquisition approach enabled the quantification of metabolic enzyme activity in real-time, and a multiparametric 31P-NMR analysis enabled the determination of the tissue ATP content and pH.

Measuring the Spin-Lattice Relaxation Magnetic Field Dependence of Hyperpolarized [1-¹³C]pyruvate

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Hyperpolarized ¹³C Metabolic Magnetic Resonance Spectroscopy and Imaging