1357579 - Simultaneous multi-slice (SMS) real-time cardiac MRI at 0.55T

Cine cardiac MRI (CMR) is routinely used to evaluate ventricular function, wall motion, and regional wall thickening. It typically uses electrocardiogram (ECG) gating and breath-holds, which can result in compromised quality for patients with arrythmias or inability to comply with breath-hold. Real-time CMR addresses these issues as no ECG-synchronization or breath-holding is required. A standard CMR scan requires a stack of short-axis slices (~9-12slices, slice-by-slice) to evaluate the left ventricular (LV) myocardium. Simultaneous multi-slice (SMS) technique can accelerate acquisition by a factor of 2-4. If >3 slices are simultaneously acquired this can provide simultaneous evaluation of wall thickening in 16 out of 17 cardiac segments.

Here we demonstrate the feasibility of SMS real-time CMR at 0.55T using a SMS factor of 3 and compared SMS to single band (SB) imaging.



Methods:

Real-time CMR was performed using a free-breathing blipped-CAIPI SMS bSSFP sequence combined with golden-angle spiral readout^[1,2]. We used SMS factor of 3 and a slice gap of 24mm to capture the basal, mid, and apical sections of LV myocardium in the short-axis. For comparison SB images at the same slice positions were acquired.

Experiments were performed on a 0.55T whole-body system (prototype MAGNETOM Aera, Siemens Healthineers, Germany) equipped with high-performance shielded gradients (45mT/m amplitude, 200T/m/s slew rate)^[3]. Imaging was performed using the RTHawk system (HeartVista Inc., CA).  3 healthy volunteers (2males, 1female) were scanned with: TE/TR = 1.3/6.77ms for SMS, TE/TR = 0.71/5.32ms for SB; FOV = 24cm; slice thickness = 8mm; in-plane resolution of 1.2;1.5;1.8, and 2.2mm were tested. Flip angle of 100^o was experimentally chosen to maximize blood-myocardium contrast^[4].

Reconstruction used gradient impulse response function corrected trajectory and temporally constrained reconstruction modified for SMS, set to 45ms/frame^[5]. Parameters were determined through a sweep and qualitative assessment by a board-certified cardiologist.



Results:

Fig.1 shows the parameter sweep on the SMS images. One slice is shown in diastolic and systolic phases together with the line intensity profiles. Parameter λ_t = 0.1 is chosen as it provides images with clear anatomy and dynamics.

Fig.2 shows different resolutions reconstructed at λ_t = 0.1. In-plane resolution 2.2mm provides good quality, however, 1.8, 1.5 and 1.2mm results in relatively poor SNR and the last two are not capturing the cardiac motion.

Fig.3 shows the comparison of SMS and the corresponding SB images for 2 subjects. SMS provides sufficient blood-myocardium contrast and regional wall motion evaluation, however, there is relatively low distinction between the lateral wall and the lung cavity compared to SB. 



Conclusion:

Real-time SMS cardiac imaging at 0.55T is feasible and provides adequate image quality compared to SB images with an acceleration factor of three. However, to achieve diagnostic confidence further investigation is required.