In vivo application of lactate chemical exchange saturation transfer imaging: human exercise study
Catherine DeBrosse1, Ravi Nanga1, Puneet Bagga1, Mohammad Haris2, Hari Hariharan1, and Ravinder Reddy1

1Center for Magnetic Resonance and Optical Imaging, University of Pennsylvania, Philadelphia, PA, United States, 2Research Branch, Sidra Medical and Research Center, Doha, Qatar

Synopsis

Metabolic regulation is disrupted in many diseases. As a result, the levels of lactate present in the body are often affected and implicated in disease progression and clinical outcome. To better understand lactate metabolism, an imaging technique with high sensitivity and spatial resolution is required. In this study, a chemical exchange saturation transfer (CEST) magnetic resonance imaging method, based on the exchange between lactate hydroxyl proton and bulk water protons was used to image lactate. As proof-of-principle, LATEST was implemented in vivo in exercising human skeletal muscle to image the increased lactate that results from intense exercise.

Introduction

The ability to measure lactate in vivo is valuable in the study of many pathologies where metabolic regulation is compromised, including cancer, cardiac failure, diabetes, and other metabolic disorders. Traditional magnetic resonance spectroscopy (1H and 13C MRS)1 of lactate is limited in vivo by spectral overlap with lipids or inadequate sensitivity and/or poor spatial resolution. Fast kinetics (<1min) of lactate metabolism can be studied with high sensitivity using dynamic nuclear polarization (DNP)2 but requires special polarizing equipment and complex modeling for data analysis. In this study, we used chemical exchange saturation transfer (CEST)3 to exploit the exchange of –OH protons on lactate with bulk water (LATEST). This method was validated in vivo with a human exercise study where an increase in lactate CEST was measured after intense exercise. With CEST, lactate can be imaged with high sensitivity and spatial resolution, and is not limited by time constraints of hyperpolarized imaging.

Methods

All human studies were approved by the Institutional Review Board protocol of the University of Pennsylvania. Informed consent was obtained from all subjects prior to imaging. Human skeletal muscle studies were performed on a whole-body 7T whole-body MRI scanner (Siemens, Erlangen, Germany) with a 28-channel QED knee RF coil. LATEST imaging was performed on the right calf muscle of 5 healthy male subjects (ages 21-35). CEST images and WASSR4 B0 and B1 field maps were acquired while the subjects rested. CEST images were collected using a saturation pulse consisting of a 3s long saturation pulse train of 30 100ms long Hanning windowed saturation pulses (B1rms = 0.73μT). Imaging parameters: slice thickness =10mm, SHOT TR=6s, GRETR=6.1 ms, TE=2.9ms, FOV = 140x140 mm2, matrix size 128x128. CEST images were acquired from 0 - ±0.8 ppm, in steps of 0.1ppm. After baseline image acquisition, subjects were instructed to perform plantar flexion exercise in-magnet with an MR-compatible, pneumatically-controlled foot pedal, until exhaustion. Immediately following exercise, CEST images were acquired with a time resolution of 1.8 minutes. B1 and WASSR maps were acquired again after post-exercise CEST images. Lactate-edited spectroscopy was also performed the right calf muscle of three healthy male subjects (ages 24-65). In order to select a voxel in the desired muscle region, an inversion spectroscopy (ISIS) localization5 was implemented before a selective multiple quantum coherence (MCQ) editing sequence6. For all three subjects, a voxel of 30x80x40mm was placed over the calf muscle including the gastrocnemius muscles and a portion of the soleus muscle. Spectra were first acquired for 96s while the subject rested. The same exercise paradigm was repeated as described above. After exercise, spectra were acquired for a total of 20 minutes. For quantification, water spectra and lactate spectra were acquired from the same voxel. Spectroscopy parameters: TR=3s, TE=165.6ms, dummy scans=4, water averages=8, lactate edited spectra averages=8*n, n=2 for pre-exercise, n=50 for post-exercise.

Results and Discussion

During resting state imaging, minimal LATEST signal was observed in human skeletal muscle (Figure 1a-b). After intense anaerobic exercise, when glycolysis leads to production of lactate, an increase of ~8% CESTasym was measured in the gastrocnemius muscle (Figure 1c). After ~18 minutes minimal LATEST signal is detected as the lactate recovers to baseline (Figure 1d). Immediately after exercise, lactate asymmetry peaked at ~0.4ppm downfield from bulk water, with a B1rms of 0.73uT and a saturation duration of 3s as shown in the medial gastrocnemius (Figure 2). Recovery of lactate levels to baseline over ~18 minutes (Figure 3) from 5 subjects’ medial gastrocnemius muscle corresponds with recovery measured by lactate-edited spectroscopy (Figure 4). Correlation between spectroscopy-derived lactate and CEST-derived lactate concentration (Figure 5) gives a slope of ~0.56% per mM of lactate from spectroscopy. These results are consistent with reported lactate concentration increase measured in muscle biopsy after intense exercise7. We have demonstrated that LATEST can be used to measure in vivo, dynamic changes in lactate with high sensitivity and spatial resolution. Lactate levels are often increased as a result of metabolic impairment. This technique can be applied to monitor therapeutic response in a wide range of disorders where lactate metabolism is implicated, including genetic diseases with inborn errors of metabolism, and cancer.

Acknowledgements

This work was supported by NIH P41 grant EB015893

References

1. DeBerardinis, R.J., et al. Beyond aerobic glycolysis: transformed cells can engage in glutamine metabolism that exceeds the requirement for protein and nucleotide synthesis PNAS 2007; 104(49):19345-19350.

2. Golman, K., et al., Real-time metabolic imaging. Proc Natl Acad Sci. 2006;103(30): 11270-5.

3. Ward, K.M., Aletras, A.H., and Balaban, R.S., A new class of contrast agents for MRI based on proton chemical exchange dependent saturation transfer (CEST). J Magn Reson. 2000; 143(1): 79-87.

4. Kim, M., et al., Water saturation shift referencing (WASSR) for chemical exchange saturation transfer (CEST) experiments. Magn Reson Med. 2009;61(6):1441-50.

5. Odidge, R.J., Connelly, A., and Lohman, J.A.B., Image-selected in vivo spectroscopy (ISIS). A new technique for spatially selective NMR spectroscopy. J Magn Reson. 1985; 66:283-294.

6. He, Q., Shungu, D.C., et al. Single-scan in vivo lactate editing with complete lipid and water suppression by selective multiple-quantum-coherence transfer (Sel-MQC) with application to tumors. J Magn Reson. 1993; 106(3): 203-211.

7. Bangsbo, J. et al. Lactate and H+ effluxes from human skeletal muscles during intense, dynamic exercise. J Physiol. 1993. 462:115-133

Figures

a. anatomical image of human calf muscle: lateral and medial heads of the gastrocnemius muscles indicated; b. resting-state LATEST image with no discernible signal; c. increase in the gastrocnemius muscles measured with lactate LATEST following ~3min of intense plantar flexion exercise; d. after ~18min, LATEST signal has almost completely recovered.

Asymmetry map from the medial gastrocnemius before exercise (blue) with no discernible CEST effect, and the post-exercise increase after ~3min of intense exercise.

Pre-exercise lactate and recovery post-exercise from the medial gastrocnemius muscles of 5 subjects.

Lactate recovery measured by lactate-edited spectroscopy after exercise in three subjects, from a voxel placed over the medial gastrocnemius muscle. Recovery time is comparable to that measured by LATEST.

Correlation between lactate concentration, derived through lactate-edited spectroscopy, and LATEST.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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