Introduction

In several pathophysiologic conditions (myocardial infarction, heart failure, valvular heart disease, etc), myocardial oxygen supply and demand (oxygen consumption) become uncoupled. To compensate, the heart increases its myocardial oxygen extraction fraction (mOEF), which usually precedes ventricular mechanical dysfunction. A noninvasive, robust, and early means of assessment of mOEF would be highly desirable. We recently developed a noncontrast cardiovascular magnetic resonance (CMR) imaging technique to measure absolute mOEF.¹ However, the current mOEF imaging technique suffers from image distortion artifacts and relatively low spatial resolution. The objective of this study was to develop a new imaging acquisition technique for the quantification of mOEF which would minimize artifacts and improve image quality.

Methods

The previous CMR mOEF sequence is a 2D Asymmetric-spin-echo (ASE) echo-planar readout sequence (ASE-EPI, dark blood), with an inplane spatial resolution of 3.4 x 3.4 mm2.¹ The new CMR mOEF sequence is a 2D Asymmetric-spin-echo (ASE) prepared balanced steady state free precession (SSFP) readout sequence with TE of 50-60 msec (ASEprep, bright blood). Figure 1 shows the new sequence diagram with one TE. The ASE preparation module consists of 90^o-180^o-180^o-90^o RF pulses with alternate 0 and 180^o phase to minimize B₀ inhomogeneity. For a time shift of τ towards opposite directions in the timing of the two 180^o pulses, the signal intensity acquired represents echo shift of dTE = 4τ. To accommodate short breath-hold time, the sequence collects 14-16 images when the time shift τ changes from 0 to 3.9 – 4.5 msec. Other imaging parameters are: TE = 50 msec, inplane resolution = 1.7 x 1.7 mm2. A trigger delay (TD) ensures the data acquisition is within the quiescent period of the heart. Six healthy volunteers (24 – 30 years old, 3F) were recruited for the initial evaluation of the sequence and reproducibility study at rest, i.e., same imaging scans at two different days (D1 and D2). The CMR mOEF measurements was performed along short-axis using both ASE-EPI and ASEprep sequences. For EPI-ASE, a total of 17 images per slice with different dTE were acquired for a total acquisition time of 17 RR-intervals within one breath-hold. For ASEprep, a total of 16 images per slice were acquired for a total acquisition time of 16 RR-intervals. One mid slice each for the first two subjects and three slices each for the last 4 subjects were prescribed: basal, mid, and apex. The mOEF maps were derived with a custom-made software program and a region-of-interest (ROI) was drawn in the septum area for all mOEF measurements to compare both sequences. The study was performed at a 3T Prisma Siemens MR system (Siemens Healthcare, Malvern, PA). The reproducibility was represented by the coefficient of variation (CV) between D1 and D2.

Results

Two apex slices were excluded for analysis due to severe cardiac motion in two subjects, otherwise a total of 12 slices were included for each method in each patient. Table 1 shows the comparison of the resting mOEF measured in septum by both mOEF sequences. ASEprep and EPI-ASE measurements showed no significant difference and comparable reproducibility between the two sequences. Correlation between two sequence measurements for septal mOEF were r = 0.8 and 0.79, slope = 1.02 and 0.98, in D1 and D2, respectively. Figure 2 shows examples of mOEF images and maps obtained by the two sequences, demonstrating improved image quality by using the new ASEprep sequence. In these limited number of subjects, no significant difference in mOEF was shown between male and female subjects.

Discussion & Conclusion

We have successfully implemented a new CMR sequence to quantify mOEF, without image distortion and with doubled in-plane resolution. Comparable resting mOEF and reproducibility are demonstrated in this initial study. Rigorous animal and/or human validation studies are warranted to study its translational potential in the assessment of patients with myocardial metabolic dysfunction.

Acknowledgements

No acknowledgement found.

References

1. Lu L, Eldeniz C, An H, et al. Quantification of myocardial oxygen extraction fraction: A proof-of-concept study. Magn Reson Med, 2021;85:3318-3325.