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The Feasibility of Measuring Phosphocreatine Recovery Kinetics in Muscle Using a Single-shot31 P RARE MRI Sequence

Posted Jun 30, 2011 Updated Apr 10, 2023
By Robert L. Greenman PhD , Howard A. Smithline MD
8 min read

Rationale and Objectives

Heterogeneity of skeletal muscle structure, composition, and perfusion results in spatial differences in oxidative function between muscles and muscle regions. The simultaneous measurement of the postexercise phosphocreatine (PCr) recovery rate across all muscles of a human limb cross-section may provide new insights into normal physiology and disease states. The objective of this work was to assess the feasibility of acquiring PCr rapid acquisition with relaxation enhancement (RARE) images with sufficient temporal and spatial resolution to accurately measure PCr recovery kinetics in a cross-section of a human limb.

Materials and Methods

One normal subject performed a finger exercise until fatigued. At cessation of exercise surface coil localized pulse-and-acquire phosphorus-31 MR spectra ( 31 P- magnetic resonance spectroscopy [MRS]) of the forearm were acquired at 6 S intervals for 4 minutes. The exercise protocol was repeated 7 days later and axial PCr RARE images of the forearm were acquired following exercise with 5.6 cm 3 voxels at 6-second intervals for 4 minutes.

Results

The PCr recovery time constants for the PCr RARE and 31 P-MRS measurements were 91.0 and 91.1 seconds, respectively, based on a monoexponential fit. A Pearson correlation test showed that the PCr recovery data that resulted from the RARE PCr imaging were highly correlated with the data resulting from the 31 P-MRS ( r = 0.91, P < .0001).

Discussion

Data from selected regions of RARE PCr images acquired at 6-second intervals compare well to those acquired using surface coil 31 P MR spectroscopy and can provide an accurate assessment of PCr recovery kinetics.

Substantial variation in the biochemical properties, vascular supplies, and composition (eg, fiber type) among human skeletal muscles has been well-documented . These characteristics influence the mitochondrial capacity to varying degrees in different muscles in normal and athletically trained individuals . Further changes in these characteristics occur because of normal aging and some disease states, which may result in a heterogeneous pattern of altered metabolic function .

The rate of resynthesis of phosphocreatine (PCr) in skeletal muscle following exercise is an index of the capacity of the mitochondria to carry out oxidative metabolism . Phosphorus-31 magnetic resonance spectroscopy ( 31 P-MRS) with surface coil localization is an accepted method for measuring the postexercise recovery rate of skeletal muscle PCr and can provide insights into normal physiology and pathophysiology in disease states . A limitation of surface coil 31 P-MRS is that it does not provide precise spatial information and is limited to superficial muscle regions. It is also uncertain whether the acquired signal is from only a single muscle or from multiple muscles with different characteristics within the sensitive region of the surface coil . Current 31 P-MRS localization methods require times that are too long for the precise assessment of postexercise PCr recovery kinetics .

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Materials and methods

Study Subject

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Exercise Protocol

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MRS

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Figure 1, (a) A phosphorus-31 ( 31 P) B1 map of the surface coil that was used for the unlocalized magnetic resonance spectroscopy phosphocreatine (PCr) recovery study. (b) The B1 map is shown superimposed onto a 1 H image of the subject's arm that was acquired at the same level as the 31 P rapid acquisition with relaxation enhancement (RARE) images. The position of the surface coil during the in vivo PCr recovery and phantom B1 map acquisitions was used to align the B1 map with the subject's anatomy to determine which portion of the sensitive region of the surface coil contributed to in vivo signal reception ( pixels blocked out with dark lines ). A pre-exercise 31 P RARE image is similarly overlaid on the 1 H image and the corresponding target voxels that were combined to measure the relative PCr signal during recovery are outlined with white lines .

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Mapping the region of sensitivity of the 31 P surface coil

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MR Imaging

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Measurement of RF excitation pulse width

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Mapping of the static field

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Proton imaging

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PCr imaging

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Data Analysis

Spectroscopy

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Imaging

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Calculation of recovery time constants

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SNR measurements

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Results

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Figure 2, Phosphorus-31 spectra acquired before the start of exercise (a) , at the cessation of exercise (b) and at the end of the measured recovery period (c) . All postexercise spectral amplitudes are shown normalized to the pre-exercise phosphocreatine peak amplitude. PPM: parts per million.

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Figure 3, The bandwidth profile of the 15 ms chemical-shift selective radiofrequency excitation pulse. The bandwidth of the pulse is narrow enough to avoid exciting the spins of the Pi resonance while being wide enough to excite all of the spins at the phosphocreatine resonance based on the forearm B0 map information.

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Figure 4, A B0 map of the subject's forearm acquired immediately before the exercise protocol began (a) . A histogram of the static field across the forearm in units of Hz (b) . The large peak in the center of the histogram represents water tissues and the small peak that extends from approximately -10.0 to -20.0 Hz represents the fat/lipid tissues. The imaging parameters were not selected to account for the fat/water chemical shift.

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Figure 5, A representative subset of the phosphorus-31 RARE images obtained during the 4-minute postexercise recovery period demonstrating the recovery of the phosphocreatine concentration in the region contained by the 8 target voxels.

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Figure 6, The phosphocreatine (PCr) recovery data for both the phosphorus-31 ( 31 P) spectroscopic and rapid acquisition with relaxation enhancement (RARE) PCr image acquisitions are plotted together with the monoexponential fit results. The correlation between the 31 P- magnetic resonance spectroscopy and PCr RARE recovery data was significant ( P < .0001).

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Discussion

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Academic Radiology, Volume 18, Issue 7
Journals General Radiology
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