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Velocity Sensitivity of Inner-Volume Cardiac Echo Planar Imaging

Ahsan Javed¹, Terrence Jao², and Krishna S. Nayak^1,2

¹Electrical Engineering, University of Southern California, Los Angeles, CA, United States, ²Biomedical Engineering, University of Southern California, Los Angeles, CA, United States

Synopsis

Inner volume echo planar imaging (IV-EPI) is feasible and useful for cardiac diffusion tensor imaging and arterial spin labelling . IV-EPI inherently provides velocity-selective saturation caused by the crusher gradients around the 180º refocusing-pulse. In this study, we characterize this velocity-dependent effect on myocardial signal in systolic and diastolic cardiac phases. We find that a simple velocity distribution model fits the data well, and that the data from our experiments allows us to make recommendation for designing gradients to avoid myocardial signal loss for diastolic and systolic IV-EPI imaging.

Introduction

Myocardial tissue characterization often requires fast (a.k.a. “snapshot”) imaging of the heart. Candidate techniques include transient bSSFP or GRE, and single-shot spiral or echo-planar imaging. Recently, inner volume single-shot echo planar imaging (IV-EPI) has been demonstrated as feasible and useful for cardiac diffusion tensor imaging¹ (DTI) and cardiac arterial spin labelling² (ASL). It is capable of acquiring a single image in <70ms (well within a stable cardiac phase) and efficiently measures the prepared longitudinal magnetization or a stimulated echo. This study examines the velocity-selective saturation effects in IV-EPI caused by the crusher gradients around the 180º refocusing-pulse (see Fig. 2A). These provide the benefit of suppressing undesirable LV-blood pool signal that may cause partial-volume artifacts, but they can also reduce desirable myocardial signal. We experimentally characterize the myocardial signal loss in systolic and diastolic cardiac phases in human adult volunteers.

Methods

Imaging: Three healthy subjects (2M/1F,Age: 23-26) were scanned on a GE 3T Signa HDxt scanner using single-shot partial-Fourier IV-EPI , shown in Fig. 2A; one subject was scanned twice on separate days, resulting in four datasets. The imaging sequence consisted of a 90º spectral-spatial excitation³, zonal refocusing to reduce FOVy⁴ and partial Fourier EPI (5/8^th) to reduce the echo-time and readout duration. TE/TR/FA=51.3/95ms/90, matrix size=128x64, partial-fourier=5/8^th , and readout time=40ms. TE was fixed at 51.3ms based on the largest crusher gradients shown in Fig. 2A.

Experiment: A plethysmograph (PG) gated cine bSSFP scan was used to visually determine stable systolic and diastolic phases shown in Fig 1. Trigger-delays were set as ratios of the RR duration to place the imaging at the center of these periods. Eleven images were acquired for each of 14 k_v values ( between 0.0125-0.5s/cm) during both systole and diastole, where k_v= γAT, γ is the gyromagnetic-ratio, A is the area of each crusher gradient in Fig. 2A, and T is the separation between them.

Reconstruction: Ramp-sampled partial-Fourier EPI data was re-gridded and phase corrected using GE Orchestra then reconstructed using projection over convex sets⁵ (POCS). Data was then converted to SNR units for analysis⁶. Images for all kv were aligned using advanced normalization tools⁷ (ANTs). One image for each subject was manually segmented, with the ROI propagated to all aligned images. The myocardium was divided into 6 segments⁸ as shown in Fig. 2B and the data for each segment was fitted to the model of velocity-selective modulation of image signal⁹.

Modeling: The model used here was borrowed from velocity-selective arterial spin labeling literature⁹, originally developed for capillary blood flow. This assumes that the velocity of spins within a voxel is uniformly distributed from 0 to 2v_z , where v_zis the average longitudinal-velocity in cm/s . Using this model the velocity-selective modulation of signal amplitude in IV-EPI is given by: $S=\vert{M_osinc(2k_v v_z)}\vert$ , where S is the measured signal, M_o is the scaling term in SNR units . The first zero-crossing of sinc is $V_{cut}=1/2k_v$ in cm/s and spins moving at velocities faster than V_cut are saturated. 14 V_cut values of 1.0-40cm/s (that correspond to k_v values of 0.0125–0.5s/cm) were used for this experiment.

Results

Results are shown in Figs. 3,4 and 5. The estimated average v_z of the myocardium was 0.3-1.6cm/s during stable-diastole and 0.9–4.3cm/s during stable-systole. The residue for the fitting (not shown here) was uniform and low for both systole and diastole. Subject 1 had a higher HR of 70-85 (vs subject 2 and 3 with HR of 50-55 and 60-65) and had higher estimated diastolic and systolic velocities. The segment-by-segment variation in SNR was consistent between subjects and the average is shown in Fig. 2C.

Fig. 4 contains representative images from Subject 2 and shows that the model fits the data well for both diastolic and systolic images. Fig. 5. shows a scatter plot of normalized signal from all segments and all subjects plotted. This can be used to infer the choice of V_cut that minimizes the myocardial velocity-saturation.

Conclusion

In order to minimize myocardial velocity saturation effects in IV-EPI and to suppress blood pool signal as much as possible, it is appropriate to design crusher gradients with a V_cut=15cm/s at stable-diastole or V_cut=25cm/s at stable-systole. This is because there was no detectable saturation of myocardium using the IV-EPI sequence for these and larger V_cutvalues based on Fig.5. This study also demonstrates the ability to experimentally characterize velocity saturation effects of IV-EPI, and supports use of a simple model originally developed for capillary blood flow. If the range of candidate V_cut were sampled much more finely, this experiment could inform the development of better models for myocardial longitudinal-velocity distribution.

Acknowledgements

Funding NIH R01-HL130494

References

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Figures

Figure 1: Cardiac gating signal ECG (orange) and PG (light-blue) with changes in LV volume (deep-blue) during the cardiac cycle. The figure shows periods identified as stable-systole(pink) and stable-diastole(green) based on a PG-triggered cine. These periods were visually identified as periods of stasis from a short axis PG triggered cine.

Figure 2: A) EPI image acquisition, which is comprised of a 90º spectral-spatial excitation pulse, 180º spin echo pulse that is spatially selective y with z crusher gradients that suppress flowing blood above a cut off velocity V_cut, and a single-shot partial Fourier EPI readout. B) Labelling convention of myocardium that corresponds to the bulls eye plots. C) Asymptotically approached M0 which includes proton density, coil sensitivity, T2* weighting, and B1+ inhomogeneity. averaged over all subjects showing the distribution of SNR around the myocardium.

Figure 3: Bullseye plots showing estimated velocities for 3 subjects. The average velocity during stable diastole is estimated to be 0.3-1.6 cm/s across subjects. The average velocity during stable systole is estimated to be 0.9 – 4.3 cm/s. In general, the residue was low and about the same throughout the myocardium, indicating goodness of fit for this simple model. Additionally, myocardial velocity during systole was slightly higher in the lateral wall compared to the septal wall, consistent with the literature^10,11.

Figure 4: Six segment measured data (dots) and fitted data (solid line) for a representative subject during diastole and systole. Increasing velocity cutoff increased the signal in diastole and systole. Also shown are representative images demonstrating the change in SNR with V_cut. Qualitatively, this also helps verify the difference in cardiac wall motion as predicted by the velocity maps shown in the Fig. 3 ( i.e. the cardiac velocity is slightly higher in the lateral wall than the septal wall).

Figure 5: Scatter plot of normalized signal from all segments and all subjects plotted. There was no detectable saturation of myocardium using the IV-EPI sequence, using V_cut ≥15cm/s for diastolic and ≥25 cm/s for systolic imaging.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)

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