Quantitative cardiac ³¹P spectroscopy at 3 Tesla using adiabatic pulses

Cardiac phosphorus magnetic resonance spectroscopy (MRS) with surface coils promises better quantification at 3 Tesla (T) from improed signal-to-noise ratios and spectral resolution compared with 1.5T. Howewer, Bloch equation and field analyses at 3T show that for efficient quantitative MRS protocols using small-angle adiabatic (BIR4/BIRP) pulses the excitationfield is limited by radiofrequency (RF) power requirements and power deposition. When BIR4/BIRP pulse duration is increased to reduce power levels, T2-decay can introduce flip-angle dependent errors in the steady-state magnetization, causing errors in saturation corrections for metabolite quantification and in T1s measured by varying the flip-angle. A new dual-repetition-time (2TR) T1 method using frequency-sign-cycled adiabatic-half-passage pulses is introduced to alleviate power requirements, and avoid the problem related to T2 relaxation during the RF pulse. The 2TR method is validated against inversion-recovery in phantoms using a practical transmit/receive coil set designed for phosphorus MRS of the heart at depths of 9-10 cm with 4 kW of pulse power. The T1s of phosphocreatine (PCr) and adenosine triphosphate (γ-ATP) in the calf-muscle (n = 9) at 3T are 6.8 ± 0.3 s and 5.4 ± 0.6 s, respectively. For heart (n = 10) the values are 5.8 ± 0.5 s (PCr) and 3.1 ± 0.6 s (γ-ATP). The 2TR protocol measurements agreed with those obtained by conventional methods to within 10%.