RARE imaging of PCr in human forearm muscles

Assessing tissue metabolism by phosphorous-31 magnetic resonance spectroscopy and imaging: a methodology review

Many human diseases are caused by an imbalance between energy production and demand. Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) provide the unique opportunity for in vivo assessment of several fundamental events in tissue metabolism without the use of ionizing radiation. Of particular interest, phosphate metabolites that are involved in ATP generation and utilization can be quantified noninvasively by phosphorous-31 (31P) MRS/MRI. Furthermore, 31P magnetization transfer (MT) techniques allow in vivo measurement of metabolic fluxes via creatine kinase (CK) and ATP synthase. However, a major impediment for the clinical applications of 31P-MRS/MRI is the prohibitively long acquisition time and/or the low spatial resolution that are necessary to achieve adequate signal-to-noise ratio. In this review, current 31P-MRS/MRI techniques used in basic science and clinical research are presented. Recent advances in the development of fast 31P-MRS/MRI methods are also discussed.

MRI assessment of regional differences in phosphorus-31 metabolism and morphological abnormalities of the foot muscles in diabetes

PURPOSE

To assess differences in the phosphorus-31 (³¹ P) metabolism and morphology in multiple muscle regions in the forefoot of diabetic patients and normal subjects.

MATERIALS AND METHODS

Fifteen diabetic patients and 15 normal subjects were assessed for muscle atrophy by ¹ H magnetic resonance imaging (MRI) at 3T to grade the flexor hallucis, adductor hallucis, interosseous regions, and entire foot cross-section. Each region and the entire foot were also quantitatively evaluated for metabolic function using ³¹ P imaging for spatial mapping of the inorganic phosphate (Pi) to phosphocreatine (PCr) ratio (Pi/PCr). The ratio of viable muscle area to the predefined region areas (³¹ P/¹ H) was calculated. The variability of each method was assessed by its coefficient of variation (CV).

RESULTS

Muscle atrophy was significantly more severe in diabetic compared to normal subjects in all regions (P < 0.01). The ³¹ P/¹ H area ratio was significantly larger in the adductor hallucis than in the other two regions (P < 0.05). The Pi/PCr ratio was significantly different between the two groups in the flexor hallucis and interosseous regions (P < 0.05) but not adductor hallucis region. The CV for Pi/PCr ranged from 10.13 to 55.84, while it ranged from 73.40 to 263.90 for qualitative grading.

CONCLUSION

Changes in atrophy and metabolism appear to occur unequally between different regions of the forefoot in diabetes. The adductor hallucis region appears more capable of maintaining structural and metabolic integrity than the flexor hallucis or interosseous regions. The CV analysis suggests that the quantitative ³¹ P methods have less variability than the qualitative grading. J. Magn. Reson. Imaging 2016;44:1132-1142.

Dynamic PCr and pH imaging of human calf muscles during exercise and recovery using (31) P gradient-Echo MRI at 7 Tesla

PURPOSE

Simultaneous acquisition of spatially resolved (31) P-MRI data for evaluation of muscle specific energy metabolism, i.e., PCr and pH kinetics.

METHODS

A three-dimensional (3D) gradient-echo sequence for multiple frequency-selective excitations of the PCr and Pi signals in an interleaved sampling scheme was developed and tested at 7 Tesla (T). The pH values were derived from the chemical shift-induced phase difference between the resonances. The achieved spatial resolution was ∼2 mL with image acquisition time below 6 s. Ten healthy volunteers were studied performing plantar flexions during the delay between (31) P-MRI acquisitions, yielding a temporal resolution of 9-10 s.

RESULTS

Signal from anatomically matched regions of interest had sufficient signal-to-noise ratio to allow single-acquisition PCr and pH quantification. The Pi signal was clearly detected in voxels of actively exercising muscles. The PCr depletions were in gastrocnemius 42 ± 14% (medialis), 48 ± 17% (lateralis) and in soleus 20 ± 11%. The end exercise pH values were 6.74 ± 0.18 and 6.65 ± 0.27 for gastrocnemius medialis and lateralis, respectively, and 6.96 ± 0.12 for soleus muscle.

CONCLUSION

Simultaneous acquisition of PCr and Pi images with high temporal resolution, suitable for measuring PCr and pH kinetics in exercise-recovery experiments, was demonstrated at 7T. This study presents a fast alternative to MRS for quantifying energy metabolism of posterior muscle groups of the lower leg. Magn Reson Med 75:2324-2331, 2016. © 2015 Wiley Periodicals, Inc.

Three-dimensional mapping of the creatine kinase enzyme reaction rate in muscles of the lower leg

Phosphorus ((31) P) magnetization transfer (MT) techniques enable the non-invasive measurement of metabolic turnover rates of important enzyme-catalyzed reactions, such as the creatine kinase reaction (CK), a major transducing reaction involving adenosine triphosphate and phosphocreatine. Alteration in the kinetics of the CK reaction rate appears to play a central role in many disease states. In this study, we developed and implemented at ultra-high field (7T) a novel three-dimensional (31) P-MT imaging sequence that maps the kinetics of CK in the entire volume of the lower leg at relatively high resolution (0.52 mL voxel size), and within acquisition times that can be tolerated by patients (below 60 min). We tested the sequence on five healthy and two clinically diagnosed type 2 diabetic subjects. Overall, we obtained measurements that are in close agreement with measurements reported previously using spectroscopic methods. Importantly, our spatially resolved method allowed us to measure local CK reaction rate constants and metabolic fluxes in individual muscles in a non-invasive manner. Furthermore, it allowed us to detect variations of the CK rates of different muscles, which would not have been possible using unlocalized MRS methods. The results of this work suggest that 3D mapping of the CK reaction rates and metabolic fluxes can be achieved in the skeletal muscle in vivo at relatively high spatial resolution and with acquisition times well tolerated by patients. The ability to measure bioenergetics simultaneously in large areas of muscles will bring new insights into possible heterogeneous patterns of muscle metabolism associated with several diseases and serve as a valuable tool for monitoring the efficacy of interventions.

Postexercise phosphocreatine recovery, an index of mitochondrial oxidative phosphorylation, is reduced in diabetic patients with lower extremity complications

OBJECTIVE

To identify differences in postexercise phosphocreatine (PCr) recovery, an index of mitochondrial function, in diabetic patients with and without lower extremity complications.

METHODS

We enrolled healthy control subjects and three groups of patients with type 2 diabetes mellitus: without complications, with peripheral neuropathy, and with both peripheral neuropathy and peripheral arterial disease. We used magnetic resonance spectroscopic measurements to perform continuous measurements of phosphorous metabolites (PCr and inorganic phosphate [Pi]) during a 3-minute graded exercise at the level of the posterior calf muscles (gastrocnemius and soleus muscles). Micro- and macrovascular reactivity measurements also were performed.

RESULTS

The resting Pi/PCr ratio and PCr at baseline and the maximum reached during exercise were similar in all groups. The postexercise time required for recovery of Pi/PCr ratio and PCr levels to resting levels, an assessment of mitochondrial oxidative phosphorylation, was significantly higher in diabetic patients with neuropathy and those with both neuropathy and peripheral arterial disease (P < .01 for both measurements). These two groups also had higher levels of tumor necrosis factor-α (P < .01) and granulocyte colony-stimulating factor (P < .05). Multiple regression analysis showed that only granulocyte colony-stimulating factor, osteoprotegerin, and tumor necrosis factor-α were significant contributing factors in the variation of the Pi/PCr ratio recovery time. No associations were observed between micro- and macrovascular reactivity measurements and Pi/PCr ratio or PCr recovery time.

CONCLUSIONS

Mitochondrial oxidative phosphorylation is impaired only in type 2 diabetes mellitus patients with neuropathy whether or not peripheral arterial disease is present and is associated with the increased proinflammatory state observed in these groups.

Dynamic three-dimensional imaging of phosphocreatine recovery kinetics in the human lower leg muscles at 3T and 7T: a preliminary study

The rate of phosphocreatine (PCr) resynthesis after physical exercise has been extensively studied with phosphorus (³¹P)-MRS. Previous studies have used small surface coils that were limited to measuring one superficial muscle per experiment. This study focuses on the development and implementation of a spectrally selective three-dimensional turbo spin echo (3D-TSE) sequence at 3T and 7T with temporal resolution of 24 s, using two geometrically identical volume coils. We acquired imaging data of PCr recovery from four healthy volunteers and one diabetic patient, who performed plantar flexions using resistance bands. We segmented the anatomical regions of six different muscles from the lower leg, namely the gastrocnemius [lateral (GL) and medial (GM)], the tibialis [anterior (TA) and posterior (TP)], the soleus (S) and the peroneus (P) and measured the local PCr resynthesis rate constants. During the same examination, we also acquired unlocalized (³¹P-MRS data at a temporal resolution of 6 s. At 3T, the PCr resynthesis rate constants were measured at 25.4 ± 3.7 s [n = 4, mean ± standard deviation (SD)] using the MRS method and 25.6 ± 4.4 s using the MRI method. At 7T, the measured rates were 26.4 ± 3.2 s and 26.2 ± 4.7 s for MRS and MRI. Using our imaging method, we measured the local PCr resynthesis rate constants in six individual muscles of the lower leg (min/max 20.2/31.7 ). The recovery rate constants measured for the diabetic patient were 55.5 s (MRS) and 52.7 s (MRI). The successful implementation of our 3D-method suggests that imaging is possible at both fields with a relatively high spatial resolution (voxel size: 4.2 mL at 3T and 1.6 mL at 7T) using volume coils and that local PCr resynthesis rates can be obtained in a single measurement. The advantage of the imaging method is that it can highlight differences in PCr resynthesis rates between different muscles in a single measurement in order to study spatial gradients of metabolic properties of diseased states for which very little is currently known.

Rapid 3D-imaging of phosphocreatine recovery kinetics in the human lower leg muscles with compressed sensing

The rate of phosphocreatine (PCr) resynthesis following physical exercise is an accepted index of mitochondrial oxidative metabolism and has been studied extensively with unlocalized (31)P-MRS methods and small surface coils. Imaging experiments using volume coils that measure several muscles simultaneously can provide new insights into the variability of muscle function in healthy and diseased states. However, they are limited by long acquisition times relative to the dynamics of PCr recovery. This work focuses on the implementation of a compressed sensing technique to accelerate imaging of PCr resynthesis following physical exercise, using a modified three-dimensional turbo-spin-echo sequence and principal component analysis as sparsifying transform. The compressed sensing technique was initially validated using 2-fold retrospective undersampling of fully sampled data from four volunteers acquired on a 7T MRI system (voxel size: 1.6 mL, temporal resolution: 24 s), which led to an accurate estimation of the mono-exponential PCr resynthesis rate constant (mean error <6.4%). Acquisitions with prospective 2-fold acceleration (temporal resolution: 12 s) demonstrated that three-dimensional mapping of PCr resynthesis is possible at a temporal resolution that is sufficiently high for characterizing the recovery curve of several muscles in a single measurement.

Spectrally selective 3D TSE imaging of phosphocreatine in the human calf muscle at 3 T

Quantitative information about concentrations of several metabolites in human skeletal muscle can be obtained through localized (31)P magnetic resonance spectroscopy methods. However, these methods have shortcomings: long acquisition times, limited volume coverage, and coarse resolution. Significantly higher spatial and temporal resolution of imaging of single metabolites can be achieved through spectrally selective three-dimensional imaging methods. This study reports the implementation of a three-dimensional spectrally selective turbo spin-echo sequence, on a 3T clinical system, to map the concentration of phosphocreatine in the human calf muscle with significantly increased spatial resolution and in a clinically feasible scan time. Absolute phosphocreatine quantification was performed with the use of external phantoms after relaxation and flip angle correction of the turbo spin-echo voxel signal. The mean ± standard deviation of the phosphocreatine concentration measured in five healthy volunteers was 29.4 ± 2.5 mM with signal-to-noise ratio of 14:1 and voxel size of 0.52 mL.

Simultaneous acquisition of phosphocreatine and inorganic phosphate images for Pi:PCr ratio mapping using a RARE sequence with chemically selective interleaving

The ratio of inorganic phosphate to phosphocreatine (Pi:PCr) is a validated marker of mitochondrial function in human muscle. The magnetic resonance imaging rapid acquisition with relaxation enhancement (RARE) pulse sequence can acquire phosphorus-31 ((31)P) images with higher spatial and temporal resolution than traditional spectroscopic methods, which can then be used to create Pi:PCr ratio maps of muscle regions. While the (31)P RARE method produces images that reflect the content of the (31)P metabolites, it has been limited to producing an image of only one chemical shift in a scan. This increases the scan time required to acquire images of multiple chemical shifts as well as the likelihood of generating inaccurate Pi:PCr maps due to gross motion. This work is a preliminary study to demonstrate the feasibility of acquiring Pi and PCr images in a single scan by interleaving Pi and PCr chemical shift acquisitions using a chemically selective radiofrequency excitation pulse. The chemical selectivity of the excitation pulse evaluated and the Pi:PCr maps generated using the interleaved Pi and PCr acquisition method with the subject at rest and during exercise are compared to those generated using separate Pi and PCr acquisition scans. A paired t test indicated that the resulting Pi:PCr ratios for the exercised forearm muscle regions were not significantly different between the separate Pi and PCr acquisition method (3.18±1.53) (mean±standard deviation) and the interleaved acquisition method (3.41±1.66). This work demonstrates the feasibility of creating Pi:PCr ratio maps in human muscle with Pi and PCr images acquired simultaneously by interleaving between the Pi and PCr resonances in a single scan.

Foot muscle energy reserves in diabetic patients without and with clinical peripheral neuropathy

OBJECTIVE To investigate changes in the foot muscle energy reserves in diabetic non-neuropathic and neuropathic patients. RESEARCH DESIGN AND METHODS We measured the phosphocreatinine (PCr)/inorganic phosphate (Pi) ratio, total (31)P concentration, and the lipid/water ratio in the muscles in the metatarsal head region using MRI spectroscopy in healthy control subjects and non-neuropathic and neuropathic diabetic patients. RESULTS The PCr/Pi ratio was higher in the control subjects (3.23 +/- 0.43) followed by the non-neuropathic group (2.61 +/- 0.36), whereas it was lowest in the neuropathic group (0.60 +/- 1.02) (P < 0.0001). There were no differences in total (31)P concentration and lipid/water ratio between the control and non-neuropathic groups, but both measurements were different in the neuropathic group (P < 0.0001). CONCLUSIONS Resting foot muscle energy reserves are affected before the development of peripheral diabetic neuropathy and are associated with the endothelial dysfunction and inflammation.

Musculoskeletal spectroscopy

Magnetic resonance spectroscopy (MRS) of skeletal muscle has been successfully applied by physiologists over several decades, particularly for studies of high-energy phosphates (by (31)P-MRS) and glycogen (by (13)C-MRS). Unfortunately, the observation of these heteronuclei requires equipment that is typically not available on clinical MR scanners, such as broadband capability and a second channel for decoupling and nuclear Overhauser enhancement (NOE). On the other hand, (1)H-MR spectra of skeletal muscle can be acquired on many routine MR systems and also provide a wealth of physiological information. In particular, studies of intramyocellular lipids (IMCL) attract physiologists and endocrinologists because IMCL levels are related to insulin resistance and thus can lead to a better understanding of major health problems in industrial countries. The combination of (1)H-, (13)C-, and (31)P-MRS gives access to the major long- and short-term energy sources of skeletal muscle. This review summarizes the technical aspects and unique MR-methodological features of the different nuclei. It reviews clinical studies that employed MRS of one or more nuclei, or combinations of MRS with other MR modalities. It also illustrates that MR spectra contain additional physiological information that is not yet used in routine clinical applications.

Acquisition time reduction in magnetic resonance spectroscopic imaging using discrete wavelet encoding

This paper describes a new magnetic resonance spectroscopic imaging (MRSI) technique based upon the discrete wavelet transform to reduce acquisition time and cross voxel contamination. Prototype functions called wavelets are used in wavelet encoding to localize defined regions in localized space by dilations and translations. Wavelet encoding in MRSI is achieved by matching the slice selective RF pulse profiles to a set of dilated and translated wavelets. Single and dual band slice selective excitation and refocusing pulses, with profiles resembling Haar wavelets, are used in a spin-echo sequence to acquire 2D-MRSI wavelet encoding data. The 2D space region is spanned up to the desired resolution by a proportional number of dilations (increases in the localization gradients) and translations (frequency shift) of the Haar wavelets (RF pulses). Acquisition time is reduced by acquiring successive MR signals from regions of space with variable size and different locations with no requirement for a TR waiting time between acquisitions. An inverse wavelet transform is performed on the data to produce the correct spatial MR signal distribution.

Foot small muscle atrophy is present before the detection of clinical neuropathy

OBJECTIVE

To characterize structural changes and the metabolic profile of foot muscles and correlate them with diabetic neuropathy measurements using phosphorus-31 ((31)P) rapid acquisition with relaxation enhancement (RARE) magnetic resonance imaging (MRI).

RESEARCH DESIGN AND METHODS

We studied 12 control subjects, 9 non-neuropathic diabetic patients, and 12 neuropathic diabetic patients using (31)P RARE and proton ((1)H) MRI at 3 Tesla. The ratio of the total cross-sectional area of the foot to that of the muscle tissue was calculated from transaxial (1)H and (31)P images. The average (31)P concentration across the metatarsal head region was measured from the (31)P images.

RESULTS

The muscle area-to-total area ratio differed among all three groups (means +/- SD): 0.55 +/- 0.04 vs. 0.44 +/- 0.05 vs. 0.06 +/- 0.06 for control, non-neuropathic, and neuropathic subjects, respectively (P < 0.0001). The average (31)P concentration also differed among all groups: 27.7 +/- 3.8 vs. 21.7 +/- 4.8 vs. 7.9 +/- 8.8 mmol/l for control, non-neuropathic, and neuropathic subjects (P < 0.0001). The muscle area-to-total area ratio strongly correlated with clinical measurements: Neuropathy Disability Score, r = -0.83, P < 0.0001; vibration perception threshold, r = -0.79, P < 0.0001; and Semmes-Weinstein monofilaments, r = -0.87, P < 0.0001.

CONCLUSIONS

Small muscle atrophy is present in diabetes before clinical peripheral neuropathy can be detected using standard clinical techniques. The (31)P RARE MRI method evaluates the severity of muscle atrophy, even in the early stages when neuropathy is absent. This technique may prove to be a useful diagnostic tool in identifying early-stage diabetic foot problems.

Quantification of the31P metabolite concentration in human skeletal muscle from RARE image intensity

A method is described for quantifying the cellular phosphorus-31 (31P) concentration in human skeletal muscle based on RARE (rapid acquisition with relaxation enhancement) image intensities. The 31P concentrations were calculated using relaxation rates, RF coil spatial characteristics, and RARE signal intensities from foot muscle and an external 31P standard. 31P RARE and 1H T2-weighted images of the foot muscles in 11 normal subjects were acquired at 3.0 T using a double-tuned (31P/1H) birdcage coil. 31P PRESS (point-resolved spectroscopy) spectra were acquired to verify the measurable 31P concentrations in a multiecho acquisition. The mean measured concentration was 26.4 +/- 3.1 mM (mean +/- SD) from RARE signal intensities averaged over the entire imaged foot anatomy and 27.6 +/- 4.1 mM for a 3 x 3 pixel region-of-interest measurement. The 31P RARE image acquisition time was 4 min with a 0.55 cm3 voxel size. These results demonstrate that the 31P concentration can be accurately measured noninvasively in human muscle from RARE images acquired in short scan times with relatively high spatial resolution.

Fast 31P chemical shift imaging using SSFP methods

Steady-state free precession (SSFP) methods have been very successful due to their high signal and short imaging times. These properties make them good candidates for applications that intrinsically suffer from low signal such as low gamma nuclei imaging. A new chemical shift imaging (CSI) technique based on the SSFP signal formation has been implemented and applied to (31)P. The signal properties of the SSFP CSI method have been evaluated and the steady-state signal of (31)P has been measured in human muscles. Due to the T(2) and T(1) signal dependence of SSFP, the steady-state signal mainly consists of phosphocreatine (PCr). The technique allows fast CSI acquisitions with high SNR of the PCr signal. The SNR gain for PCr over a FLASH-based CSI method is approx. 4-5. Fast in vivo CSI of human muscle with subcentimeter resolution and high SNR is demonstrated at 2 T.

Fast imaging of phosphocreatine in the normal human myocardium using a three-dimensional RARE pulse sequence at 4 Tesla

PURPOSE

To investigate the use of a three-dimensional rapid acquisition with relaxation enhancement (RARE) pulse sequence for direct acquisition of phosphocreatine (PCr) images of the human myocardium.

MATERIALS AND METHODS

A short elliptical birdcage radiofrequency (RF) body coil was constructed to produce a uniform flip angle throughout the chest cavity. In vivo images using a spectrally-selective RARE sequence with a spatial resolution of 1.2 cm x 1.2 cm x 2.5 cm (4 cm(3)) were acquired in nine minutes and 40 seconds.

RESULTS

Scans of phantoms demonstrated excellent spectral selectivity. The signal-to-noise ratio in the myocardium ranged from 12.6 in the anterior wall to 5.3 in the mid septum.

CONCLUSION

This study demonstrates that PCr data can be acquired using a three-dimensional RARE sequence with greater spatial and temporal resolution than spectroscopic techniques.

An image processing strategy for the quantification and visualization of exercise-induced muscle MRI signal enhancement

Exercise increases the skeletal muscle water signal in T2-weighted images. Potential medical applications of MR studies of exercise-induced muscle signal intensity changes are the assessment of myopathies, sport training regimens, and physical therapy approaches following surgeries. We developed an automated image processing technique that provides volumetric analysis and visualization of exercise-related T2-weighted image intensity changes. The image processing was applied to the segmentation and quantification of activated muscle volumes. Qualitative assessment of muscle activation is demonstrated with three-dimensional surface rendering. Quantitative determination of active muscle volume, signal intensity, and change over time is demonstrated. Visualization of the activated muscles allows functional anatomical assessment of exercise, which in turn allows detection of muscle utilization.