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

Abstract

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), ␥-NTP (nucleotide triphosphate) (5.5 ؎ 0.4 s to 3.3 ؎ 0.2 s), ␣-NTP (3.4 ؎ 0.3 s to 1.8 ؎ 0.1 s), and ␤-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 ␥-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%. Key words: phosphor; spectroscopy; human muscle; relaxation times; 7 Tesla; high field; chemical shift anistrophy Phosphorus ( 31 P) MR spectroscopy (MRS) is a powerful tool for the noninvasive investigation of human muscle metabolism under various physiological and pathological conditions (1). High-field MR systems, i.e., 7 T, offer advantages to 31 P-MRS in terms of sensitivity and spectral resolution, which has recently been shown in the brain (2).To optimize the measurement parameters of clinical spectroscopy protocols, such as echo time (TE) and repetition time (TR), an accurate knowledge of T 1 and T 2 relaxation times is essential. With TR chosen on the order of T 1 , severe saturation effects must be taken into account, in addition to T 2 decay. For absolute quantification of metabolites, it is therefore essential to accurately determine relaxation times to allow subsequent corrections for T 1 and T 2 decay (3,4).Relaxation times vary not only between different metabolites, but also with B 0 . The T 1 and T 2 of 31 P metabolites in the human leg were previously determined only at lower field strengths, i.e., at 1.5 T (5-13), 2.0 T (14), 2.35 T (15), and 3 T (16), while in vivo data at higher fields have been obtained only from animal studies (17,18).Relaxation in 1 H-MRS is dominated by magnetic dipoledipole interactions according to the Bloembergen-Purcell-Pound (BPP) theory (19). Therefore, 1 H-MRS T 1 relaxation times are increasing with B 0 (20). However, for 31 P-MRS, both dipolar relaxation and chemical shift anisotropy (CSA) are the two major, competing relaxation mechanisms (17,(21)(22)(23). In contrast to dipolar interaction, the contributions of CSA to 1/T 1 and 1/T 2 relaxation rates are proportional to the gyromagnetic ratio (␥), B 0 2 , the asymmetry of the magnetic shielding (...

In vivo 31P spectroscopy by fully adiabatic extended image selected in vivo spectroscopy: A comparison between 3 T and 7 T

“…This leads to additional signalto-noise ratio per unit time 2 (SNR t ). B 1 inhomogeneities in 31 P-MRS at 7 T are similar to those of 1 H at only 3 T. A strong decrease in T 2 was observed for 31 P metabolites with increasing magnetic field strength (B 0 ) (10). This favors the application of pulse-acquire MRS sequences at high B 0, compared with spin echo methods.…”

“…Cramer-Rao lower bounds were calculated. The results were corrected for relaxation time differences (10).…”

“…The spectral resolution and signal-to-noise ratio (SNR) was shown to improve significantly from 3 T to 7 T (10). In contrast to 1 H, T 1 relaxation times for 31 P metabolites decrease significantly with increasing B 0 (10). This leads to additional signalto-noise ratio per unit time 2 (SNR t ).…”

“…Nevertheless, some groups have proved the clinical value of 31 P-MRS for the assessment of metabolic disorders (7)(8)(9) or oncology (2). 31 P-MRS significantly benefits from higher magnetic field strengths (i.e., 7 T) (10)(11)(12). The spectral resolution and signal-to-noise ratio (SNR) was shown to improve significantly from 3 T to 7 T (10).…”

Dynamic 31P–MRSI using spiral spectroscopic imaging can map mitochondrial capacity in muscles of the human calf during plantar flexion exercise at 7 T

“…To improve the SNR of the γ‐ATP peak for quantification purposes, the γ‐ATP signal intensity was averaged over the last six measurements (1 min) of the recovery period. For the saturation correction, the actual flip angle was calculated for each muscle group as the average of the measured datasets, and the previously measured T 1 values41 were used. The chemical shift of P i relative to PCr was used to calculate the intracellular pH,42 according to the modified Henderson‐Hasselbalch equation 40…”