Abstract
Signal acquisition in most MRS experiments requires a correction for partial saturation that is commonly based on a single exponential model for T 1 that ignores effects of chemical exchange. We evaluated the errors in 31 P MRS measurements introduced by this approximation in two-, three-, and four-site chemical exchange models under a range of flip-angles and pulse sequence repetition times (T R ) that provide near-optimum signal-to-noise ratio (SNR). In two-site exchange, such as the creatine-kinase reaction involving phosphocreatine (PCr) and γ-ATP in human skeletal and cardiac muscle, errors in saturation factors were determined for the progressive saturation method and the dual-angle method of measuring T 1 . The analysis shows that these errors are negligible for the progressive saturation method if the observed T 1 is derived from a three-parameter fit of the data. When T 1 is measured with the dual-angle method, errors in saturation factors are less than 5% for all conceivable values of the chemical exchange rate and flip-angles that deliver useful SNR per unit time over the range T 1 /5 ≤ T R ≤ 2T 1 . Errors are also less than 5% for three- and four-site exchange when T R ≥ T 1 */2, the so-called "intrinsic" T 1 's of the metabolites. The effect of changing metabolite concentrations and chemical exchange rates on observed T 1 's and saturation corrections was also examined with a three-site chemical exchange model involving ATP, PCr, and inorganic phosphate in skeletal muscle undergoing up to 95% PCr depletion. Although the observed T 1 's were dependent on metabolite concentrations, errors in saturation corrections for T R = 2 s could be kept within 5% for all exchanging metabolites using a simple interpolation of two dual-angle T 1 measurements performed at the start and end of the experiment. Thus, the single-exponential model appears to be reasonably accurate for correcting 31 P MRS data for partial saturation in the presence of chemical exchange. Even in systems where metabolite concentrations change, accurate saturation corrections are possible without much loss in SNR.
Original language | English (US) |
---|---|
Pages (from-to) | 425-435 |
Number of pages | 11 |
Journal | Journal of Magnetic Resonance |
Volume | 148 |
Issue number | 2 |
DOIs | |
State | Published - 2001 |
ASJC Scopus subject areas
- Biophysics
- Biochemistry
- Nuclear and High Energy Physics
- Condensed Matter Physics