Oral Abstract Session
SCMR 22nd Annual Scientific Sessions
Main field (B0) inhomogeneity has been a long-standing challenge in high field(>=3T) cardiac MRI. The heart and lung interface induce strong local high-order B0 variation, which is proportional to the field strength. This effect degrades capabilities in many important CMR sequences that are sensitive to off resonance, such as bSSFP readout, inversion pulses and fat saturation preparations. It also diminishes the SNR and spectral benefits high field strength offers and restricts CMR applications in high field scanners. Recently, an integrated Parallel Reception, Excitation, and Shimming (iPRES) (Fig.1A) technique was developed to combine shim into an RF array 1-3. However, iPRES shim loops have several inherent physical limitations in their size, shape, position, total number, and loops turns (1-turn) in addition to significant RF sensitivity loss, which leads to unsatisfactory shimming as for cardiac imaging. Here, we propose a novel approach, Unified Coils (UNIC) 4-6, which overcomes the aforementioned limitations by introducing innovative decoupling methods (Figure 1B). The method was tested in healthy human volunteers at a 3T clinical scanner.
Following the circuit design, a coil was constructed with 12 RF-receive and 42 shimming loops (each loop 2-turns, 5A maximum current), which was placed on top of the subject with close proximity for maximized SNR and shimming efficiency. Scans were performed in healthy human subjects (N=2) under breath holds and ECG gating. B0 field maps and bSSFP cine images with extended TR (TR= 6 ms) were acquired and compared under scanner shim and UNIC shim.
Figure 2 shows the comparison of B0 field maps and bSSFP cine images between scanner and UNIC shimming. The mean and standard deviation of the off resonance frequency were both significantly reduced in all slices by UNIC shimming (62.9 ± 57.3Hz vs 30.3± 40.5Hz; p<0.05)(Fig. 2A). A set of representative short axis images with scanner and UNIC shimming are presented in Figure 2B. Under scanner shim, strong myocardial B0 inhomogeneity was observed with severe banding artifacts in the corresponding cine images. In comparison, field inhomogeneity was remarkably reduced, and cine image quality was significantly improved with UNIC shimming.
To the best of our knowledge, this is the first study to enable cardiac shimming using local coils in a high field clinical scanner. Our findings here support the notion that UNIC coils have the promise to markedly reduce off-resonance frequency shifts, which has been a major difficulty for cardiac imaging at 3T. This advancement opens door for multiple new opportunities in CMR that were previously out of reach due to limitations in B0 homogeneity.