Introduction

Various sequences have been proposed for whole heart coronary magnetic resonance angiography (WHC-MRA)1-4. In conventional WHC-MRA, it is typically performed during free-breathing, and respiratory motion is prospectively compensated by using a 1D-right-diaphragmatic-navigator (NAV)5. However, an important limitation of the NAV method is its relatively long acquisition time6. Single-breath-hold technique using Turbo-field-echo-planar-imaging (TFEPI) could significantly reduce the acquisition time of WHC-MRA while providing image quality similar to that of conventional free-breathing gradient echo sequence7. TFEPI sequence is a hybrid technique that combines TFE and EPI scan7 (Fig.1). It enables high-speed imaging with EPI readout, resulting in shorter scan time by allowing more echoes to be acquired during one heartbeat. However, there is a problem that scan time is extended depending on the gradient-spec of MRI-system. On the other hand, even if SENSE-factor is increased to shorten scan time, image quality is expected to degrade due to increased noise. 3D TFEPI with Compressed SENSE (C-SENSE) has the possibility to solve these problems. C-SENSE has recently been introduced as acceleration technique. By combining TFEPI with C-SENSE (CS-TFEPI), we hypothesized that scan time of single-breath-hold WHC-MRA can be further accelerated without degrading the image quality by denoising. In this study, we investigated the feasibility of single-breath-hold CS-TFEPI for WHC-MRA by comparing with single-breath-hold TFEPI with SENSE and conventional free-breathing TFE.

Methods

A total of six volunteers (5 males and 1 female; age range: 23~44) were examined on a 3.0T-MRI (Ingenia, Philips Healthcare). The study was approved by the local IRB, and written informed consent was obtained from all subjects. We compared image quality and scan time among WHC-MRA using CS-TFEPI or SENSE, and TFE. Four sequences were acquired with cardiac triggering. For respiratory compensation, TFEPI was acquired with single-breath-hold and TFE was acquired with NAV. Imaging-parameters; TFEPI: FOV=300×282mm2, voxel-size=1.56×1.93×3.6mm3, TR/TE/FA=9.3/4.3/20, TFE-factor=17, EPI-factor=7, shot-duration=158.7ms, SENSE/C-SENSE factor=SENSE:2.5×1.5 (TFEPI-S2.5×1.5) /SENSE:3.6×2.0 (TFEPI-S3.6×2.0) /C-SENSE:3.6×2.0 (TFEPI-CS3.6×2.0); TFE: FOV=300×288mm2, voxel-size=1.6×1.6×1.8mm3, TR/TE/FA=3.2/1.39/12, TFE-factor=31, shot-duration=100ms, SENSE-factor=2.0×1.2 (T1TFE-S2.0×1.2). Each actual scan time was recorded. Curved Planer Reconstructions (CPRs) were performed using Ziostation2 (Ziosoft Co, Tokyo) and image quality was evaluated by visual score at the 10-points (RCA: #1/2/3/4 LAD: #5/6/7/8 CX: #11/13) based on American-Heart-Association classification. For overall image quality, sharpness and noise and artifacts, we evaluated them as 4-point grades (grade “4” was excellent, “1” was severe) by two blinded readers. Visual evaluation was assessed by Steel-Dwass test. For quantitative comparison, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured. The SNR was assessed in the blood and myocardium. To allow quantitative SNR measurements, we used a noise-measurement-method proposed by Zwanenburg et al8. The standard-deviation of a region of interest of the corresponding area in the noise image was used as metric for the noise. SNRblood and SNRmyocardium were then calculated as follows: SNRA = SI(A) / SDnoise(A) Where SI are the mean average signal intensity of the blood and myocardium respectively, and the corresponding SDnoise is the standard-deviation at the same location on the noise images. Subsequently, we measured the CNR for comparing image contrast quantitatively. The CNR was estimated for blood and myocardium (CNRblood-myocardium). The CNRblood-myocardium was calculated by the following equations: CNRA-B = [SI(A) - SI(B)] / 0.5 [SDnoise(A) + SDnoise(B)] The SNR and CNR were assessed by one-way repeated measures analysis of variance (ANOVA) and the post-hoc Tukey-Kramer test.

Results

Figure 2, 3 shows the results of visual evaluation. Regarding proximal coronary arteries, for overall image quality and noise and artifacts, TFEPI-CS3.6×2.0 and T1TFE-S2.0×1.2 were showed significantly higher than TFEPI-S3.6×2.0 and TFEPI-S2.5×1.5 (p<0.0001). In addition, there was no significant difference between TFEPI-CS3.6×2.0 and T1TFE-S2.0×1.2. For the sharpness, TFEPI-CS3.6×2.0 was showed significantly lower than T1TFE-S2.0×1.2 (p<0.0001), but significantly higher than TFEPI-S3.6×2.0 and TFEPI-S2.5×1.5 (p<0.0001). Regarding distal coronary arteries, for overall image quality, sharpness and noise and artifacts, TFEPI-CS3.6×2.0 was showed significantly lower than T1TFE-S2.0×1.2 (p<0.0001), but significantly higher than TFEPI-S3.6×2.0 and TFEPI-S2.5×1.5 (p<0.001). Figure 4 shows SNR and CNR comparison among four sequences. For the SNRblood, SNRmyocardium and CNRblood-myocardium, TFEPI-CS3.6×2.0 was showed significantly higher than T1TFE-S2.0×1.2 and TFEPI-S3.6×2.0 (p<0.0001), except for the comparison with TFEPI-S2.5×1.5. Figure 5 shows the representative images and average scan time using TFEPI-S2.5×1.5, TFEPI-S3.6×2.0, TFEPI-CS3.6×2.0 and T1TFE-S2.0×1.2.

Discussion & Conclusion

TFEPI-S2.5×1.5 had a low visual score due to artifacts caused by relatively long breath holding time, and TFEPI-S3.6×2.0 had a low visual score due to increased noise with high SENSE factor. On the other hand, TFEPI-CS3.6×2.0 had high visual score regarding proximal coronary arteries as a result of high SNR and CNR due to the effect of denoising. Regarding distal coronary arteries, TFEPI-CS3.6×2.0 had low visual score due to its low spatial resolution and relatively long shot-duration time. Nevertheless, coronary MRA is often acquired after contrast-enhancement. By using contrast agent, the diagnostic ability of the distal coronary arteries can be improved by increasing the blood SNR. Thus, CS-TFEPI has clinically usefulness because it provides useful information with short breath holding time of about 20 seconds and also reduces the burden on the patient. Furthermore, 4D-coronary-MRA using 3D TFEPI has been introduced9, but it takes a long scan time for whole heart acquisition. This study suggests that CS-TFEPI can be extended to whole-heart 4D-coronary-MRA.

Acknowledgements

No acknowledgements found.

References

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