Impact of temporal resolution on cardiac phase-resolved oxygen-sensitive myocardial steady-state free precession imaging
Journal of Cardiovascular Magnetic Resonance volume 11, Article number: P178 (2009)
Cardiac phase-resolved imaging studies that are used in the assessment of cardiac function are performed with a temporal resolution (TRES) of approximately 50 ms to mitigate the effects from cardiac motion and flow. To date, there has been minimal interest on the characterization of myocardial signal intensities from cine images. Steady-state free precession based cardiac phase-resolved blood-oxygen-level-dependent (CP-SSFP BOLD) imaging is a relatively new method for identifying myocardial oxygen abnormalities on the basis of regional signal differences. For reliable assessment of oxygenation changes, it is imperative to ensure that acquisitions enable robust image quality. We hypothesize that TRES plays a significant role on CP-SSFP image quality and that, in particular, myocardial signal characteristics disintegrate with elevations in TRES.
Dogs were used to test the hypothesis under controlled conditions. Animals (n = 4) were anesthetized and their heart rate was monitored with ECG (R-R interval = 710 ms–780 ms). Multiple breath-held acquisitions (20–40 s) were performed in each animal, interrupted by 2–3 minute rest, ensuring that the heart rate remained relatively constant between acquisitions. 2D balanced-SSFP imaging was prescribed in the cine mode to study the effects of TRES on short-axis mid-left-ventricular images of the myocardium in a clinical 1.5 T scanner. The scan parameters were: in-plane resolution = 1.2 × 1.2 mm2, TR/TE = 3.5 ms/1.75 ms (conventional cine SSFP) and 6.0 ms/3.0 ms (CP-SSFP BOLD), flip angle = 70°, NEX = 1, segments/cardiac phase were changed to obtain different TRES (10 ms to 200 ms). This study was repeated 2–3 times with a two-day interval for each animal. In total 10 studies were performed. Two indices were used to quantify the myocardial signal characteristics obtained with cine SSFP images at different TRES: (1) Myocardial Signal Inhomogeneity Index (MSI), defined as the standard deviation of the LV myocardial signal intensity; and (2): Transmural Heterogeneity Index (THI), defined as the minimum pixel intensity difference between pixels along a line perpendicular to the blood-muscle interface in the LV chamber. The global THI was calculated by sweeping the line along the interface for 360° and averaging the THI for each 1° increment. Results were averaged across all studies for late systole(LS) and late diastole(LD). Note that MSI measures the signal variation throughout the myocardium and THI measures the image quality permitting reliable delineation of the endocardial border from blood.
Figure 1 shows short-axis SSFP images with different TRES (10 ms, 42 ms, 80 ms and 202 ms from left to right obtained at TR = 6.0 ms). The upper row images are from LS and the lower row images are from LD. Numerical results from MSI (upper plot) and THI (lower plot) computations from the LS and LD images as a function of TRES are shown in Figure 2. On average, MSI and THI are directly related to TRES at LS and LD (for TR = 3.5 and 6.0 ms), albeit the rate of change of the indices at LS is much greater than at LD. In particular, results showed that, at TR = 6.0 ms, for TRES>42 ms, MSI and THI are significantly greater than with TRES = 18 ms (t-test, p < 0.01).
Reliable image quality is critical for accurate detection of changes in myocardial oxygenation. This study investigated the impact of TRES on two important features of CP-SSFP BOLD images: (1) myocardial signal variations, (2) endocardial blur. Findings show that MSI and THI are strongly influenced by TRES. In particular, with both conventional cine SSFP (TR = 3.5 ms) and CP-BOLD SSFP (TR = 6 ms) imaging, the image quality diminishes with increasing TRES. Also, for any given TRES, the reduction in image quality is significantly greater at systole than at diastole. We conclude that for reliable detection of myocardial oxygenation on the basis of CP-SSFP BOLD imaging, it is necessary to keep TRES as short as possible. These findings remain to be validated in humans.
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Zhou, X., Tang, R., Klein, R. et al. Impact of temporal resolution on cardiac phase-resolved oxygen-sensitive myocardial steady-state free precession imaging. J Cardiovasc Magn Reson 11 (Suppl 1), P178 (2009). https://doi.org/10.1186/1532-429X-11-S1-P178