### 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) |
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Pages (from-to) | 425-435 |

Number of pages | 11 |

Journal | Journal of Magnetic Resonance |

Volume | 148 |

Issue number | 2 |

DOIs | |

State | Published - Jan 1 2001 |

### ASJC Scopus subject areas

- Biophysics
- Biochemistry
- Nuclear and High Energy Physics
- Condensed Matter Physics