1343531 - Spatial-Spectral Selective Pulses for B1 Compensation in Cardiac CEST

Chemical exchange saturation transfer (CEST) has been used to map myocardial creatine kinase metabolism both in large animal models¹ and in humans²; however, CEST is highly sensitive to B1-field inhomogeneities. A robust method to improve B1 homogeneity in cardiac CEST is crucial for its application in cardiac metabolism. Here, we evaluate the use of tailored spatial-spectral pulses for B1 compensation across the myocardium.

Methods:

All in vivo studies were performed on a Siemens 3T Tim Trio scanner using a body matrix coil and a spine matrix coil. Nine healthy volunteers have been recruited for this study. A B1 map was acquired in one midventricular short axis slice using a Turbo FLASH sequence with a prescribed saturation flip angle of 50⁰. Additional parameters included a slice thickness of 5mm, TE of 2.12ms, and TR of 2.73s. Using the measured B1 map for each subject, a tailored spatial-spectral pulse was designed and used to simulate a B1 map following previously described methods⁴ via MATLAB version 2021b. The imaging sequence utilized 23 tailored saturation pulses, each with a duration of 36 ms,1.2 µT peak B1 power, saturation offsets ranging from -10ppm to +10ppm, and were followed by a GRE readout. A reference scan was taken at 10000 Hz. Other scanning parameters included a FOV of 26 cm × 30 cm, TR of 1.45 s, and TE of 2.59 ms. ECG triggering was used to time readout during diastole. Every scanning subject required two sets of frequency offset acquisitions consisting of either a conventional Gaussian or tailored saturation. For each image, the myocardium was segmented into 6 regions consistent with AHA standards. The mean signal intensity within each segment was normalized and the resulting z-spectrum was corrected for B0 shift using a WASSR correction⁵. CEST contrast was quantified by performing a 5-pool Lorentzian fitting of direct saturation, MT, creatine, amide, and NOE.

Results:

Simulations showed the spatial-spectral pulses reduced flip angle variation across the myocardium from an average range of 23.7˚ with a Gaussian pulse to 14.6˚. The subsequent impact upon Z-spectra derived from different myocardial regions is shown in Figure 1. Whereas the use of Gaussian saturation pulse resulted in significant regional variation in derived Z-spectra, spatial-spectral saturation pulse generated consistent Z-spectra across the heart. The impact on highly B1-sensitive parameters of direct saturation, MT, creatine, amide, and NOE is shown in Figure 2, which shows a significantly lower spatial variation of creatine with spatial-spectral saturation.

Conclusion:

Through simulations, the tailored pulse resulted in an improved flip angle homogeneity across the myocardium when compared to the conventional Gaussian pulse. To validate these results, in vivo cardiac scans compared the CEST contrast generated with each pulse shape. Importantly, the use of the tailored RF pulse resulted in an improved spatial homogeneity of creatine with similar trends for NOE and amide.