Clinical Feasibility of Ultrafast Contrast-Enhanced T1-Weighted 3D-EPI for Evaluating Intracranial Enhancing Lesions in Oncology Patients: Comparison with Standard 3D MPRAGE Sequence

DISCUSSION

In this study, we determined that the ultrafast 3D-EPI T1WI showed sufficient diagnostic image quality and comparable diagnostic performance for detecting enhancing intracranial lesions in oncology patients with fewer motion artifacts and a 7-fold reduction in scan time, compared with the standard MPRAGE sequence. Conversely, the ultrafast 3D-EPI T1WI had overall inferior image quality with more susceptibility artifacts and lower CNR_GM/WM than the standard MPRAGE sequence.

For the assessment of brain metastasis, a 3D sequence has the advantage in detecting small, enhancing lesions due to the higher spatial resolution, which reduces the partial volume effects compared with 2D sequences.¹⁰ Additionally, a multiplanar reformation is another advantage of 3D sequences for assessing brain tumors within the complex brain anatomy.¹⁷ However, the relatively long scan time required for 3D sequences is a major drawback to applying this sequence in oncology patients presenting with a poor general condition and who cannot tolerate long scan times due to restlessness, which may contribute to increasing motion artifacts and patient anxiety.^18,19 To date, a few studies have focused on reducing scan times without the loss of important clinical information from CE 3D T1 sequences for brain MR imaging protocols. In a recent study, ultrafast 3D-EPI T1WI was developed using a fat-suppressed multishot 3D-EPI to obtain isotropic T1-weighted volumes, revealing the possibility of clinical application of this novel sequence for brain imaging.¹¹ However, to the best of our knowledge, there have been no studies comparing ultrafast 3D-EPI T1WI and conventional 3D T1 sequences to evaluate intracranial lesions by assessing the diagnostic performance and overall image quality from the perspective of the clinical application.

In this study, ultrafast 3D-EPI T1WI showed diagnostic performance comparable with that of the standard MPRAGE sequence. This result was similar to that of a recent study that showed equivalent diagnostic performance with only marginally higher background noise using a highly accelerated Wave-Controlled Aliasing in Parallel Imaging (Wave-CAIPI; Siemens Healthineers) 3D T1 sampling perfection with application-optimized contrasts by using different flip angle evolutions (SPACE) sequence for detecting brain metastasis at 3T.²⁰ However, the total scan time of the accelerated Wave-CAIPI T1 SPACE sequence was 1 minute 40 seconds, which is >3 times longer than ultrafast 3D-EPI T1WI and is required at the expense of additional calibration and reconstruction effort. In contrast, the ultrafast 3D-EPI T1WI can provide comparable diagnostic confidence and CNR_{lesion/parenchyma} in 30 seconds using the Cartesian acquisition scheme and the conventional parallel imaging to reduce anxiety and discomfort more efficiently for oncology patients in real-world practice. In particular, our results showed that the CNR_{lesion/parenchyma} of the ultrafast 3D-EPI T1WI was not significantly different from that of the standard MPRAGE. This finding was consistent with the previous studies that proposed the clinical implication of CNR_{lesion/parenchyma} to detect enhancing metastatic lesions.^21⇓-23 We believe that this finding was valuable because CNR_{lesion/parenchyma} is known as a key factor for contributing the higher detectability of enhancing lesions.^22,23

In contrast, the overall image quality of ultrafast 3D-EPI T1WI in our study was considered inferior, compared with representative cases from the previous study,¹¹ even though it is difficult to perform a direct comparison of our image quality with that of the original work at this time. The exact reasons for these differences are unclear, though these may be related to the intrinsic difference in the scanning environment, including the scanner and the number of shots of EPI between the 2 institutions. In general, any newly developed sequence should be validated in various ways to establish its clinical utility. From this perspective, we believe that our results are meaningful in that they can provide additional information regarding the acquisition of this novel sequence in different scan environments.

With regard to the technical aspects of ultrafast 3D-EPI T1WI, the inherently unavoidable geometric distortion of EPI-derived sequences cannot be completely removed at the air-tissue interface, even though this novel ultrafast 3D-EPI T1WI can further reduce the geometric distortion using the higher number of shots (here equal to 12) with the parallel imaging factor (here equal to 2) in contrast to the 2D-EPI sequence, which typically uses a single shot with parallel imaging.¹¹ While the parallel imaging factor is coil-limited and may not be increased beyond 3 to avoid SNR loss, increasing the number of EPI shots reduces the geometric distortions at the expense of longer scan times. Ultimately, with the number of shots equal to the number of acquired lines, the 3D-EPI sequence will be distortion-free with the same scan time. We found that the current setting of 12 EPI shots gives a reasonable trade-off between scan time and geometric distortions because there was a minimal difference in the overall diagnostic accuracy. However, this sequence still presents the potential limitation relative to overall image quality, which can be degraded by susceptibility artifacts and results in insufficient detection of enhancing lesions near the skull base and brain stem. In addition to the lower SNR, this issue may also contribute to the concerns about the ultrafast 3D-EPI T1WI, such as inferior overall image quality and the lower mean value of the CNR_WM/GM. Therefore, for the patient in whom the expected lesion is in the vicinity of a tissue-air interface (such as the pituitary gland), where the field inhomogeneity is high, one could increase the number of shots to obtain higher geometric accuracy at the expense of a longer scan time.

Furthermore, we observed hypointense or hyperintense dots in the pons, induced by pulsation artifacts of the basilar artery, which mimicked the small, enhancing lesions at first glance. However, the readers could easily distinguish these characteristic artifacts from pathologic conditions without any remarkable impact on decision-making in this study. It may be helpful to apply a sagittal scan plane with frequency-encoding in the superior-inferior direction to reduce the pulsation artifacts.^7,11 In addition, the use of inferior saturation bands for the axial image acquisition is another possible option, but this use may significantly decrease the CNR_WM/GM due to magnetization transfer effects.¹¹ However, in the current study, we obtained the ultrafast 3D-EPI T1WI with a sagittal scan plane; thus, we did not compare the degree of pulsation artifacts directly between the sagittal and axial scan planes. In the present study, we also observed that accompanying artifacts in and around the hemorrhagic lesion may lead to difficulties in evaluating the enhancing lesion; in particular, susceptibility artifacts of hemorrhagic metastasis may result in the underestimation of the enhancing portion, and signal pileup in the vicinity of hemorrhagic lesions may mimic the enhancing component.¹¹ Therefore, further technical advances are needed to correct the aforementioned issues before expanding the diagnostic use of ultrafast 3D-EPI T1WI in clinical practice.

Despite these shortcomings, ultrafast 3D-EPI T1WI can obtain CE 3D T1WI with a resolution of 1.2 × 1.2 × 1.2 mm³ with a short scan time of 30 seconds, which is approximately 7 times shorter than the standard MPRAGE sequence. In addition, the diagnostic confidence and CNR_{lesion/parenchyma} of ultrafast 3D-EPI T1WI were comparable with those of standard MPRAGE. Therefore, ultrafast 3D-EPI T1WI can be used as a feasible alternative in certain clinical situations such as motion-prone patients or for unexpected termination of scans while obtaining conventional 3D T1WI.

This study has several limitations. First, there was an unavoidable selection bias because the data from all patients were evaluated retrospectively, the sample size was small, and the study was conducted in a single center. Second, we could not handle the acquisition order of the 2 different 3D T1WI sequences because of the retrospective study design. Thus, we did not consider the potential differences related to the timing bias between contrast injection and image acquisition, which can increase contrast agent uptake due to the delay. A future prospective study with a large study population is needed to validate the effect of the differences in postcontrast time delay. Third, we did not conduct the study including the patient groups with homogeneous types of brain tumors. In contrast, we took the pragmatic approach to obtain real-time data in daily clinical practice because this study was a feasibility study of the ultrafast 3D-EPI T1WI for identifying lesions in oncology patients. Therefore, it would not be necessary to provide additional analysis according to the specific pathologic diagnosis. Our results showed a clinically acceptable diagnostic image quality and lesion detectability with the benefit of a shorter scan time. Therefore, this broad study population can be helpful to generalize the clinical utility of ultrafast 3D-EPI T1WI to various types of brain tumors. Fourth, pathologic confirmation was not obtained for all brain tumors because patients with multiple brain tumors such as metastases usually do not undergo surgical intervention. Last, we did not apply the recommended time delay for obtaining the enhanced T1WI in brain tumor imaging.^8,9 Even though previous studies have provided a recommendation for a brain MR imaging protocol in oncology patients,^8,9 there is often a gap between real-world clinical practice and ideals. For practical reasons, it is believed that many previous studies did not accurately specify the delayed time or apply a delay time of <4 minutes. In this study, it was also difficult to apply the recommended protocol for enhanced T1WI in the study patients because actual clinical situations such as the number of MR imaging systems or the time table of the MR imaging room were inevitably considered.