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147 31P Cardiac spectroscopy at 3 T: T1 quantification
Journal of Cardiovascular Magnetic Resonance volume 10, Article number: A48 (2008)
Phosphorus (31P) MRS provides measures of the high energy metabolites, phosphocreatine (PCr) and adenosine triphosphate (ATP), in the heart. It permits the evaluation of ischemic changes during myocardial stress , and ATP turnover through the creatine-kinase reaction in the normal and failing human heart[2, 3]. Recent cardiac 31P MRS studies suggest higher signal-to-noise atio (SNR) at 3 T compared to 1.5 T in healthy subjects. For accurate metabolite quantification, the longitudinal relaxation times (T1) are needed, and measuring these at 3 T is confounded by the combined effects of: (i) RF field uniformity with surface coil use; (ii) the available RF pulse power and its decrease with depth; and (iii) RF power deposition limits. While prior studies at 1.5 T used low-angle adiabatic (BIR4) pulses [2, 3], at 3 T these are limited by low bandwidth and high power requirements. We show, using a Bloch equation analysis that such effects can significantly reduce the accuracy of T1 measurements at long adiabatic pulse lengths (≥ 10 ms) for 31P MRS, but that the problems are ameliorated by use of adiabatic half passage 90° (AHP) pulses.
The first aim of this work was to construct a high-SNR surface coil set for 3 T cardiac 31P MRS that provides adequate adiabatic pulse power at the depth of the myocardium, while avoiding local power deposition problems. The second aim was to determine the T1 of PCr and γ-ATP in the human heart using a new, efficient dual repetition time (2TR) approach that minimizes T1 estimation errors at 3 T. The method is validated against the conventional saturation-recovery (SR) method.
A dual 31P coil with 17-cm transmitter and 8-cm receiver set was designed and built to optimize the transmit RF field at a 10 cm depth with 4 kW transmit power. Coils were interfaced to a 3 T Achieva (Philips) broadband scanner. RF power deposition was computed and measured calorimetrically in phantoms to ensure safe performance. AHP pulses (10 ms) were tailored to achieve an excitation bandwidth ≥ 200 Hz for depths ≤ 10 cm. Six healthy volunteers (4 M/2 F, 28 ± 6 years) were positioned prone with the heart centered over the surface coils, as verified by scout-MRI. Localized second-order shimming was performed, followed by cine-MRI to determine the period of least cardiac motion. The 31P frequency was set between PCr and γ-ATP. Cardiac-gated one-dimensional chemical shift imaging was performed with TR = 2, 4, 12, 32 s with 24, 12, 4, and 2 averages, respectively (16 slices; 10 mm slice thickness; 2.5 kHz bandwidth).
T1 values for the human heart were determined from the signal S(TR) = M0(1-exp(-TR/T1)), where M0 is the fully-relaxed magnetization, in three ways:
1) Conventional SR with a two-parameter least-squares fit;
2) Point estimation (PE) with M0 = S(TR = 32 s) as prior knowledge;
3) The new 2 TR method (with TRs of 2/12 s and 4/12 s)
In addition M0 was predicted (M0p) from the measured signal at the shorter TR and the estimated T1 from both of the 2 TR methods. Percentage errors were calculated as (M0p-S(TR = 32 s))/S(TR = 32 s)*100%.
Cardiac spectra with PCr SNR ≥ 30 were acquired in all 6 volunteers (Figure 1). Conventional SR gave mean T1 values of 5.9 s for PCr and 3.1 s for γ-ATP (Table in Figure 2). Dual TR gave similar T1s in scan-times of 26 minutes, just 46% of the SR scan-time of 56 minutes, albeit with slightly larger errors.
Bandwidth and RF power limitations at 3 T necessitate significant modifications to 31P MRS protocols as compared with 1.5 T. Our new dual-TR method provides fast cardiac 31P spectra acquisition at 3 T, predicting the fully-relaxed magnetization within a 10% error compared with actual fully-relaxed values.
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El-Sharkawy, AM.M., Schär, M., Ouwerkerk, R. et al. 147 31P Cardiac spectroscopy at 3 T: T1 quantification. J Cardiovasc Magn Reson 10, A48 (2008). https://doi.org/10.1186/1532-429X-10-S1-A48
- Surface Coil
- Adiabatic Pulse
- High Power Requirement
- Adiabatic Half Passage
- Adiabatic Half Passage Pulse