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High-Performance 0.55T Supports Contrast-Optimal SMS bSSFP Cardiac Imaging

Ye Tian¹, Sophia X. Cui², Yongwan Lim¹, Nam G. Lee³, Ziwei Zhao¹, and Krishna S. Nayak¹
¹Ming Hsieh Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, CA, United States, ²Siemens Medical Solutions USA, Inc., Los Angeles, CA, United States, ³Biomedical Engineering, University of Southern California, Los Angeles, CA, United States

Synopsis

Balanced steady-state free precession (bSSFP) cardiac cine MRI at 1.5T and 3T is routinely used for cardiac function assessment. Simultaneous multi-slice (SMS) imaging significantly reduces the number of required breath holds, but is typically performed with suboptimal flip angles (FA) due to SAR constraints and banding artifacts, both of which are significantly relaxed at 0.55T. In this work, we demonstrate blipped-CAIPI bSSFP cine imaging combined with spiral sampling for ventricular function at 0.55T with optimal FA for blood-myocardium contrast (100^o-120^o) at SMS factors of 2 and 3.

Introduction

Cardiac cine MRI is routinely used to evaluate ventricular function, wall motion, and regional wall thickening. bSSFP imaging is typically used because it offers excellent SNR efficiency and blood-myocardium contrast. Standardized protocols¹ acquire a stack of short-axis slices that cover the whole heart with multiple breath-holds (one per slice, 10-12 in total). Recently, SMS bSSFP techniques^2-4 have been utilized to reduce the number of breath holds. However, contrast-optimal FA could not be used due to SAR constraints and banding artifacts, resulting in compromised image quality. Our hypothesis in this study is that contemporary 0.55T scanners⁵ offer improved B₀homogeneity and relaxed SAR constraints, allowing contrast-optimal FA being used for SMS bSSFP.

In-vivo studies have suggested contrast-optimal FA of 130^o for 0.35T⁶ and 105^o for 1.5T⁷. Note that blood inflow contributes to the contrast between left ventricle (LV) blood and myocardium⁷. 3D bSSFP excites a thick slab and typically has reduced contrast compared to 2D bSSFP⁸. To our knowledge, the contrast-optimal FA has not been documented at 0.55T and effects of blood saturation have not been characterized for 2D SMS bSSFP.

In this study, we demonstrate a blipped-CAIPI SMS bSSFP³ combined with spiral sampling for ventricular function assessment. SMS image quality is comparable with single band acquisition at SMS factor of 2 and 3. Contrast-optimal FA has been experimentally determined to be 120^o-140^o for single band and 100^o-120^o for SMS from 6 healthy volunteers.

Methods

Pulse Sequence
We implemented a blipped-CAIPI SMS bSSFP sequence³ combined with spiral readout for cardiac cine as shown in Figure 1. Slice encoding gradient blips are incorporated into the pre-winder and re-winder of the slice-selective gradients (Figure 1(b-c)) to minimize TR. Single slice RF excitation pulse was designed with the SLR algorithm⁹ and then superimposed to form an SMS RF pulse with optimized slice phases¹⁰ to reduce the peak B₁. Spiral trajectory with M₁ nulling (Figure 1(d)), fully sampled with 68 spirals for FOV=48cm, and a readout duration of 2.7 ms was used to acquire k-space data. Sampling order for ECG-triggered cine is illustrated in Figure 2.

Acquisition
Experiments were performed using a whole body 0.55T system (prototype Magnetom Aera, Siemens Healthineers, Erlangen, Germany) equipped with high-performance shielded gradients (45 mT/m amplitude, 200 T/m/s slew rate). Imaging was performed using the RTHawk system (HeartVista Inc., Menlo Park, CA). Six healthy volunteers (three males, three females, age 27±4) were scanned with parameters: voxel size=1.5x1.5x8 mm³, TR and TE varied from 5.8ms-7.1ms and 0.9ms-1.4ms due to RF duration differences. FA=60^o-140^o (20^o increment) were acquired for all SMS factors, and single band cine acquired additional FA=160^o. A short-axis slice in the middle of the LV myocardium was prescribed for single band. SMS cines simultaneously acquired two slices at 0 and -2.4cm (SMS=2) or three slices at 0, ±2.4cm (SMS=3) away from the middle slice.

Reconstruction
Image reconstruction used gradient impulse response function¹¹ corrected spiral trajectory and spatiotemporal constrained reconstruction¹² with modification for SMS¹³. K-space data was sorted to have six TR from each R-R for each cardiac phase (~40 ms/cardiac phase). The number of cardiac phases was determined form the minimal duration R-R.

Analysis
One systolic and one diastolic phase were selected in the middle slice of each acquisition. Manual ROIs for LV blood and myocardium were drawn conservatively on an averaged image after translation-only registration, and were applied to each image individually. Averaged SI within LV and myocardial ROIs were reported, and their difference was reported as blood-myocardial contrast.

Results

Figures 3 compares the image quality between single band and SMS cines at FA=120^o. The flow effects appear differently since SMS used additional slice encoding blips and RF durations were different.

Figure 4 compares the mid-slice cine movies acquired with different FAs and SMS factors for one volunteer. Figure 5 shows the quantitative blood-myocardium contrasts and signal intensities averaged across 6 volunteers. For single band, blood-myocardium contrast is higher as FA increases. For SMS=2 and 3, contrast drops at FA=100^o-120^oand higher. SMS generally have lower blood signal and lower contrast due to the blood suppression effects.

Discussions

In this study, we have demonstrated a blipped-CAIPI SMS bSSFP spiral sequence for cardiac function assessment at 0.55T. The reduced SAR and B₀ inhomogeneity at 0.55T allow for contrast-optimal FA being used for SMS (100^o-120^o), which is not possible at higher field strengths. The SMS image quality is comparable with single band and has slightly lower blood-myocardium contrast at the same FA, due to inflow blood being saturated by SMS excitations.

For cardiac cine, a large portion of the contrast is contributed by the inflow of blood that has equilibrium magnetization. This has been simulated and studied at 1.5T⁷, and the authors also showed the contrast-optimal FA is lower with reduced ejection fraction. Volunteers involved in this study were all healthy (ejection fraction ~60%). We expect that the contrast-optimal FA is changing with blood flow speed at 0.55T.

Conclusion

We demonstrate that cardiac function imaging at 0.55 Tesla can be expedited using SMS techniques (rate 2 or rate 3) without detected compromised image quality. Contrast-optimal FA at 0.55T was found to be 140^o-160^o for single band and 100^o-120^o for SMS = 2 or 3.

Acknowledgements

We acknowledge grant support from the National Science Foundation (#1828736) and National Institute of Health (R01-HL130494) and research support from Siemens Healthineers.

References

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Figures

Figure 1. Pulse sequence diagrams. Pulse sequences for (a) single band, (b) SMS = 2 and (c) SMS = 3 with FA = 120^o are shown. The RF phase for each slice is optimized to minimize peak B₁. For SMS = 2 and 3, RF peak value is 1.89 and 2.17 times of single band RF, respectively. RF duration is minimized under the peak B₁ limitation. Blipped-CAIPI phase modulation gradients are incorporated into the pre-winder and re-winder of the slice-selective gradient to reduce the TR. (d) A M₁-nulled spiral-out trajectory is used, where the data acquisition duration is 2.71 ms and the M₁ nulling duration is 1.74 ms.

Figure 2. Sampling scheme for SMS = 2 bSSFP cine. Data acquisition duration is 12 R-R (N_hb) for single band and SMS = 2, and 15 R-R for SMS = 3. Each R-R acquires one segment of k-space with a total number of N_hbsegments. Within each R-R, each group of 6 spirals are equally spaced, and a rotation is added between groups that maximizes the sampling angle (θ) between adjacent groups. The sampling order of 360/N_hb spirals is repeated until a trigger is received and each group of 360/N_hb spirals have a slice encoding shift (color coded). Six spirals from each R-R are reconstruct to one cardiac phase.

Figure 3. Representative comparison of mid short-axis cine movies acquired with single band, SMS = 2, and SMS = 3 at flip angle of 120^o. Images between single band and SMS = 2 and 3 have no visual image quality difference and slightly different flow effects. This is because the images were acquired with different RF durations and slice encoding gradients. LV blood-myocardium contrast is slightly different as expected, with the SMS excitation saturating the outer-slice blood which decreases the signal intensity contributed from inflowing blood.

Figure 4. Representative comparison of mid short-axis cine movies acquired with single band, SMS = 2, and SMS = 3 at different flip angles. For single band image, the apparent contrast is higher with a higher flip angle. For SMS, the contrast increment with FA is present but reduced compared to single band. The overall image contrast is slightly less in SMS than single band. As FA goes up, the RF duration has to be elongated to reduce the peak B₁, which increases the flow artifacts. No other visual quality difference can be seen between single band, SMS = 2 and 3.

Figure 5. Blood-myocardium signal intensities and contrast. For single band acquisition, the contrast increases with the FA, and is largely due to the suppressed myocardial signal. The incremental improvement in blood SI is small for FA ≥ 120^o. For SMS acquisitions, the blood-myocardium contrast starts to decrease when FA ≥ 100^o-120^o, largely due to reduced blood signal. The contrast-optimal flip angle is 140^o-160^ofor singe band and is 100^o-120^ofor SMS. The lower values of these flip angles (140^o for single band and 100^o for SMS = 2 and 3) maybe used for a higher SI in the blood.

Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)

4448

DOI: https://doi.org/10.58530/2022/4448