3D-mapping of phosphocreatine concentration in the human calf muscle at 7 T: comparison to 3 T

Abstract

Purpose: The development and implementation of a spectrally selective 3D-Turbo Spin Echo sequence for quantitative mapping of phosphocreatine (PCr) concentration in different muscles of the lower leg of healthy volunteers both at 3 T and 7 T.

Methods: Nine healthy volunteers were recruited, all of whom where scanned at 3 T and 7 T. Three dimensional PCr concentration maps were obtained after images were corrected for B1 inhomogeneities, T1 relaxation weighting, and partial volume of fatty tissue in the muscles. Two volunteers performed plantar flexions inside the magnet, and the oxidative capacity of their muscles was estimated.

Results: Three dimensional PCr concentration maps were obtained, with full muscle coverage and nominal voxel size of 0.52 mL at both fields. At 7 T a 2.7-fold increase of signal-to-noise ratio was achieved compared to 3 T.

Conclusion: Imaging (31) P metabolites at 7 T allowed for significant increase in signal to noise ratio compared to imaging at 3 T, while quantification of the PCr concentration remained unaffected. The importance of such an increase in signal-to-noise ratio is 2-fold, first higher resolution images with reduced partial volume effects can be acquired, and second multiple measurements such as dynamic imaging of PCr post-exercise, (31) P magnetization transfer, or other (1) H measurements, can be acquired in a single imaging session.

Keywords: 31P imaging; TSE imaging; calf muscle; high field 31P-MRI; phosphocreatine.

Fig. 1

Flow chart of the data acquisition, image reconstruction and quantification processes for measuring PCr concentration in muscles of the lower leg. Inorganic phosphate phantoms are used to derive a calibration curve of PCr concentration as a function of signal intensity (top). Imaging of phantomes is performed under fully relaxed conditions (TR = 60 s), and therefore there is no T₁ correction step in the processing. In-vivo PCr images (bottom) are corrected (B₁, T₁) and compared voxelwise against the calibration curve. In cases where significant fat infiltration is present in the muscles, ¹H images can be used to estimate the fraction of fat/muscle in the tissue.

Fig. 2

Actual to nominal flip angle maps (r) of the same volunteer at 3T (right) and 7T (middle) together with an anatomical ¹H image acquired at 7T (left).

Fig. 3

Apparent T₁ measurement of the same volunteer at 3T and 7T. a) At 3T seven images were acquired at different inversion times TI: 200, 1500, 3000, 6000, 10000 15000, 20000 ms. b) At 7T eight images were acquired at different TI: 200, 700, 1000, 1800, 3000, 5000, 8000, 15000 ms. c) Anatomical image acquired at 7T. d) T₁ fitting in the tibialis anterior (TA). At 3T apparent T₁ was 5.82 s, which reduced to 3.61 s at 7T.

Fig. 4

PCr concentrations maps and respective anatomical ¹H images at 3T (top) and 7T (bottom) from the same female volunteer after T₁ and B₁ correction. At 3T five averages where acquired (SNR = 11:1), while three signal averages at 7T yielded SNR of 25:1.

Fig. 5

Segmentation of fat in the muscle tissue. High resolution ¹H datasets can be used to identify adipose tissue in the muscle (top left), that can be segmented (top right). [PCr] maps extracted without correction of fat content exhibit fluctuations (bottom left), in areas where voxels contain signals both from fat and muscle as can be seen by overlaying the ¹H and ³¹P images (middle). Fat segmentations can be inverted and downsampled to the size of the PCr image, in order to estimate the muscle fraction in the voxels (n_m). By selecting a VOI, [PCr] can be extracted by correcting for partial volume effects of fat infiltration.

Fig. 6

Measurement of oxidative capacity in the muscle of a 60 yr old female volunteer. a) [PCr] map at rest b) Dynamic imaging of PCr resynthesis post exercise at 12 s temporal resolution. PCr levels in the TA are depleted immediately after exercise (t = 12 s), and fully recover after sufficient time (t = 360 s). c) Evolution of [PCr] in the TA muscle post-exercise, together with the fit of Eq. 3 to the data in order to estimate k_PCr. Oxidative capacity of the TA muscle (Q_max ) was measured at 0.69 mM ATP s⁻¹.