Validation of Zero TE–MRA in the Characterization of Cerebrovascular Diseases: A Feasibility Study

Discussion

In this study, by comparing zTE-MRA with TOF-MRA, we demonstrate that zTE-MRA was able to depict cerebrovascular diseases (steno-occlusive arterial disease, cerebral aneurysm, AVM, and Moyamoya disease) with dramatically reduced acoustic noise, higher signal homogeneity, and higher quality of venous signal suppression. Interobserver and intermodality agreement was higher for zTE-MRA than for TOF-MRA for stenosis grading, indicating that zTE-MRA is a robust method for stenosis assessment. In addition, zTE-MRA provided more accuracy with regard to aneurysm-diameter measurement and morphologic assessment, especially in smaller aneurysms. zTE-MRA was superior to TOF-MRA in the identification of lesion details in both AVM and Moyamoya disease, despite the relatively small sample size.

Currently, TOF-MRA is the MR imaging technique conventionally used to depict intracranial arteries. However, there are known major limitations to the use of TOF-MRA for the delineation of cerebrovascular disorders, such as pseudo-occlusions,¹⁵ the inaccurate detection of aneurysms¹⁶ and the nidus and draining veins in AVMs,¹⁷ and collateral vessels in Moyamoya disease.¹⁸ The resultant signal losses are mainly due to saturation secondary to slow flow or phase-dispersion effects caused by turbulent flow,¹⁹ which account for the overestimation or inaccuracy of TOF-MRA observed in our study. In contrast to TOF-MRA, zTE-MRA showed some encouraging characteristics as a nonenhanced MRA approach. Continuous ASL-based angiography was introduced as an endogenous contrast tracer in zTE-MRA to magnetically label inflowing blood, and this placed no constraint on the direction of blood flow.⁶ Thus, zTE-MRA has the potential to visualize slow flow and is independent of flow direction, features especially important for evaluating the flow in severe stenosis (grade 3), microaneurysms, and collateral vessels.

Acoustic noise induced by rapid switching of gradient coils during long scan times is still the main drawback that causes patient discomfort and movement.^20,–,22 The zTE technique, which uses a sequence solution, attenuates acoustic noise by reducing the slew rate and enabling minimal gradient variations.^23,24 Alibek et al¹² assessed the noise level and image quality of silent T1-weighted sequences on a clinical 3T MR imaging scanner. However, whether a similar amount of acoustic noise reduction could be achieved using zTE-MRA remains unclear because an extra ASL-based angiography was integrated into this approach. In this study, patients were much more comfortable during zTE-MRA than TOF-MRA, and the reduction in sound intensity between the 2 was comparable with that of a silent T1-weighted sequence. Thus, zTE-MRA would be an acceptable alternative angiography method for these patients.

In addition to acoustic noise attenuation, a zTE inside-out radial acquisition could also minimize the phase dispersion of the labeled blood flow signal in the voxel space, thereby making zTE-MRA insensitive to turbulent flow.^9,25 The flow signal inside the large aneurysms was more homogeneous on zTE-MRA than on TOF-MRA in our study, though the correlations with CTA were both high and the consistency with CTA was excellent for the large group.

Susceptibility artifacts, which occur mostly in regions adjacent to the skull base or cavernous sinus, always induce signal loss as a result of air–soft tissue or bone–soft tissue boundary interference.²⁶ Because of its use of a constant gradient and a long readout duration, zTE-MRA is less susceptible to field inhomogeneity and eddy current and can acquire more homogeneous flow signals in vascular lesions, such as stenosis or aneurysms in the ICA siphon segment. Moreover, zTE-MRA is less sensitive than TOF-MRA to motion effects,²⁷ and it therefore minimizes the misregistration and degradation of image quality and achieves improved diagnostic performance.

According to the principles of the ASL strategy used by zTE-MRA, images are acquired via subtraction to yield an angiographic image with nearly zero background.⁷ The efficiency of postprocessing (ie, VR or MIP reconstructions) can be improved with automatic subtraction, whereas the artifacts caused by skull and brain tissue on TOF-MRA must be manually removed. Furthermore, zTE-MRA demonstrated effective background suppression while a search for aneurysms was performed in the setting of subarachnoid hemorrhage in the presence of residual T1-bright blood. An optimal postlabel delay was set,^6,28 ensuring that the labeled bolus flowed into the target tissue in the imaging region and that the labeled protons lost their signal before entering the veins. Thus, better diagnostic image quality was achieved due to an acceptable signal-to-noise ratio and the elimination of vein contamination. Additionally, zTE-MRA can detect arteriovenous shunting on the basis of this technical feature. In this study, we identified a huge AVM and found that zTE-MRA was capable of revealing its tubular structures, nidus, and draining veins and even a venous sinus.

Contrast-enhanced MRA is another commonly used noninvasive MRA approach performed by administrating a T1-shortening paramagnetic contrast medium. Despite the controversy related to its diagnostic accuracy,^15,16 contrast-enhanced MRA has demonstrated some superiorities over TOF-MRA, such as it is less vulnerable to blood flow disturbances, has a reduced risk of movement artifacts, and provides large coverage for additional findings, while TOF-MRA also has some advantages over contrast-enhanced MRA in that it requires no exogenous contrast agent and is repeatable and easy to manipulate. Although our results indicate that zTE-MRA integrates these potential aspects beyond the reduction of venous artifacts, these characteristics need to be verified by further research.

In the present study, we demonstrated that zTE-MRA is a robust sequence that maintains both zTE-like and ASL-like characteristics. Several studies^7,–,10 have focused on zTE-MRA in the follow-up imaging of various aneurysm embolizations and have revealed that this method achieves higher image quality than is obtained by TOF-MRA. Our study enriches our knowledge of zTE-MRA and highlights the clinical usefulness in evaluating cerebrovascular diseases.

Holdsworth et al²⁴ noted that zTE-MRA is more prone to blurring of the vessel edge because of center-out radial sampling, as shown in Fig 1 and our previous study.¹⁰ Qu et al²⁹ implemented a hybrid ASL strategy in zTE-MRA that led to a clearer depiction of vessels. Therefore, image quality could be further improved by the development of new techniques.

However, some limitations of this study should be recognized. First, because of the high accuracy of CTA and the disadvantages of DSA, CTA was used as the reference in this study. Furthermore, DSA is not generally routinely used for screening cerebrovascular diseases, and it would be very difficult to obtain approval of the ethics committee to evaluate this issue in our patient population. However, DSA would be more accurate if used as the reference. Second, contrast-enhanced MRA was not performed as a comparison method because there is some concern regarding the combined adverse effects of MR imaging and CTA contrast media on renal function. In addition, we aimed to investigate the validation of zTE-MRA in noncontrast MRA sequences. Third, when scan time was compromised, the resolution of peripheral vessels by zTE-MRA seemed to be lower than that achieved by TOF-MRA, though larger coverage was obtained, and cerebrovascular lesions were visualized much better on zTE-MRA. The modification of some parameters would improve the resolution of zTE-MRA in future studies. Fourth, the data obtained for objective sound-level measurements do not reflect the real situation inside the coil, though noise levels were statistically lower during the zTE-MRA scan. More advanced and accurate measurement techniques must be used in future studies.