Assessment of (31)P relaxation times in the human calf muscle: a comparison between 3 T and 7 T in vivo.

Phosphorus ((31)P) T(1) and T(2) relaxation times in the resting human calf muscle were assessed by interleaved, surface coil localized inversion recovery and frequency-selective spin-echo at 3 and 7 T. The obtained T(1) (mean +/- SD) decreased significantly (P < 0.05) from 3 to 7 T for phosphomonoesters (PME) (8.1 +/- 1.7 s to 3.1 +/- 0.9 s), phosphodiesters (PDE) (8.6 +/- 1.2 s to 6.0 +/- 1.1 s), phosphocreatine (PCr) (6.7 +/- 0.4 s to 4.0 +/- 0.2 s), gamma-NTP (nucleotide triphosphate) (5.5 +/- 0.4 s to 3.3 +/- 0.2 s), alpha-NTP (3.4 +/- 0.3 s to 1.8 +/- 0.1 s), and beta-NTP (3.9 +/- 0.4 s to 1.8 +/- 0.1 s), but not for inorganic phosphate (Pi) (6.9 +/- 0.6 s to 6.3 +/- 1.0 s). The decrease in T(2) was significant for Pi (153 +/- 9 ms to 109 +/- 17 ms), PDE (414 +/- 128 ms to 314 +/- 35 ms), PCr (354 +/- 16 ms to 217 +/- 14 ms), and gamma-NTP (61.9 +/- 8.6 ms to 29.0 +/- 3.3 ms). This decrease in T(1) with increasing field strength of up to 62% can be explained by the increasing influence of chemical shift anisotropy on relaxation mechanisms and may allow shorter measurements at higher field strengths or up to 62% additional signal-to-noise ratio (SNR) per unit time. The fully relaxed SNR increased by +96%, while the linewidth increased from 6.5 +/- 1.2 Hz to 11.2 +/- 1.9 Hz or +72%. At 7 T (31)P-MRS in the human calf muscle offers more than twice as much SNR per unit time in reduced measurement time compared to 3 T. This will facilitate in vivo (31)P-MRS of the human muscle at 7 T.