Assessment of 31P relaxation times in the human calf muscle: A comparison between 3 T and 7 T in vivo

Phosphorus (³¹P) T₁ and T₂ 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 T₁ (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), γ‐NTP (nucleotide triphosphate) (5.5 ± 0.4 s to 3.3 ± 0.2 s), α‐NTP (3.4 ± 0.3 s to 1.8 ± 0.1 s), and β‐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 T₂ 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 γ‐NTP (61.9 ± 8.6 ms to 29.0 ± 3.3 ms). This decrease in T₁ 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 ³¹P‐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 ³¹P‐MRS of the human muscle at 7 T. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.