Measurement of spin‐lattice relaxation times and chemical exchange rates in multiple‐site systems using progressive saturation

Quantitative information about concentrations of several metabolites in human skeletal muscle can be obtained through localized 31P-MRS methods. However, these methods have shortcomings: long acquisition times, limited volume coverage, and coarse resolution. Significantly higher spatial and temporal resolution of imaging of single metabolites can be achieved through spectrally selective 3D-imaging methods. This study reports the implementation of a 3D spectrally selective turbo spin echo (TSE) sequence, on a 3T clinical system, to map the concentration of phosphocreatine (PCr) in the human calf muscle with significantly increased spatial resolution and in a clinically feasible scan time. Absolute PCr quantification was performed with the use of external phantoms after relaxation and flip angle correction of the TSE voxel signal. The mean ± standard deviation of the PCr concentration measured in five healthy volunteers was 29.4 ± 2.5 mM, with signal-to-noise ratio of 14:1 and voxel size of 0.52 ml.

Section: Discussionmentioning

confidence: 91%

Spectrally selective 3D TSE imaging of phosphocreatine in the human calf muscle at 3 T

Parasoglou

Xia

Regatte

2012

“…In contrast, our results demonstrate no significant change in T 1 of Pi, but the T 1 decrease in PCr (39%) was comparable to that of all other metabolites (30%–62%). This strong decrease in PCr T 1 could be partly attributable to changes in T 1 of γ‐NTP and rapid chemical exchange (29), which cannot be observed in phantom solutions (22), and partly to the lower symmetry of PCr compared to Pi (21).…”

Section: Discussionmentioning

confidence: 99%

“…It is important to mention that the listed T 1 times of PCr and γ‐NTP are not the “true” longitudinal relaxation times, but rather, apparent T 1 relaxation times, as both undergo chemical exchange (29). However, for parameter optimization, knowledge of apparent T 1 and not “true” T 1 is more useful.…”

Section: Discussionmentioning

confidence: 99%

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

Bogner

Chmelík

Schmid

et al. 2009

123

162

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 (...

“…Transport processes within the Fig. 1 model voxel can be mathematically described using the framework of a three‐site exchange model (3SX) of nuclear magnetization (6, 19–22). This was used to account for the equilibrium water exchange processes between the extracellular, the cytoplasmic, and the vesicular compartments.…”

Section: Methodsmentioning

confidence: 99%

Three‐compartment T₁ relaxation model for intracellular paramagnetic contrast agents

Strijkers

Hak

Kok

et al. 2009

The goal of this work was to elaborate a model describing the effective longitudinal relaxation rate constant R 1 for 1 H 2 O in three cellular compartments experiencing possible equilibrium water exchange, and to apply this model to explain the effective R 1 dependence on the overall concentration of a cell-internalized Gd 3؉ -based contrast agent (CA). The model voxel comprises three compartments representing extracellular, cytoplasmic, and vesicular (e.g., endosomal, lysosomal) subcellular spaces. Relaxation parameters were simulated using a modified Bloch-McConnell equation including magnetization exchange between the three compartments. With the model, several possible scenarios for internalized CA distribution were evaluated. Relaxation parameters were calculated for contrast agent restricted to the cytoplasmic or vesicular compartments. The size or the number of CA-loaded vesicles was varied. The simulated data were then separately fitted with empirical monoand biexponential inversion recovery expressions. The voxel CA-concentration dependencies of R 1 can be used to qualitatively and quantitatively understand a number of different experimental observations reported in the literature. Most important, the simulations reproduced the relaxivity "quenching" for cell-internalized contrast agent that has been observed.