Synopsis
The aim of this study was to compare a prototype simultaneous multi-slice single-shot echo planar imaging (SMS-ss-DWI-EPI) sequence with conventional readout-segmented echo-planar imaging (rs-DWI-EPI) for diffusion-weighted imaging of the breast at 3T magnetic resonance imaging (MRI). A reader study was conducted to evaluate image quality, lesion conspicuity and BI-RADS score. Our results show that although the image quality with the conventional rs-DWI-EPI is superior, malignant lesions have improved visibility with the SMS-ss-DWI-EPI sequence.
Introduction
The addition of diffusion-weighted imaging (DWI) to contrast-enhanced breast MRI improves the classification of breast lesions, which leads in turn to an increased positive predictive value of biopsies. Consequently, DWI with evaluation of the corresponding apparent diffusion coefficient (ADC) is included in most state-of-the-art breast MRI protocols. The echo train of the readout-segmented echo-planar imaging-based DWI sequence (rs-DWI-EPI) was shortened to reduce distortion and improve the resulting image quality. However, this sequence results in a lower signal-to-noise ratio (SNR) than single-shot echo planar imaging (ss-EPI). In practice, detection of lesions on DWI is often problematic due to a relatively low lesion conspicuity. To improve the detectability of lesions and the speed of acquisition, a prototype DWI sequence, the simultaneous multi-slice single-shot DWI-EPI (SMS-ss-DWI-EPI), was developed. In this study, we compare this prototype sequence with rs-DWI-EPI at 3T, in terms of image quality (IQ), lesion conspicuity, and the presence of artifacts.
Methods
From September 2017 to December 2018, 25 women with known breast cancer or suspicious breast lesions were included, after providing signed informed consent. Women were scanned with the conventional rs-DWI-EPI and the SMS-ss-DWI-EPI during the same clinical examination on a 3T MAGNETOM Skyra system (Siemens Healthcare, Erlangen, Germany) using a 16-channel bilateral breast coil. Parameters of the rs-DWI-EPI sequence were: TR: 5450 ms, TE: 57 ms, FoV: 340 mm, voxel size: 1.2x1.2x5 mm , acquisition time: 4:23 min, b-values: 50, 850 s/mm , SPAIR fat suppression. Parameters of the SMS-ss-DWI-EPI sequence were: TR: 4000 ms, TE: 70 ms, FoV: 360 mm, voxel size: 0.9(i)x0.9(i)x4 mm, acquisition time: 2:45 min, b-values: 50, 400, 800 s/mm , SPAIR fat suppression. In addition, the clinical protocol included one pre- and five post-contrast regular T1-weighted Dixon acquisitions, ultrafast T1-weighted TWIST acquisitions during the inflow of contrast, and a T2 weighted Dixon acquisition. In total, 42 malignant (32 invasive ductal carcinomas, 4 invasive lobular carcinomas, 1 ductal carcinoma in situ and 5 other malignant lesions) and 12 benign lesions were detected on the contrast-enhanced series. Malignant lesions had a mean MRI size of 18.7 mm +- 15.1 mm (range: 3 - 92 mm) and benign lesions had a mean size of 5.9 mm +- 3.8 mm (range: 3 - 15 mm). Four dedicated breast radiologists (4 to 15 years of experience with breast MRI) independently scored both sequences for overall IQ (1: extremely poor to 9: excellent). All lesions were also independently evaluated for conspicuity (1: not visible, 2: visible if location is given, 3: visible). Statistical analysis was performed in SPSS using Generalized Linear Models and the Wilcoxon signed-rank test.
Results
Overall IQ was significantly higher for the conventional rs-DWI-EPI (Mean +- SD: 5.5 +- 1.9) than for the SMS-ss-DWI-EPI (Mean +- SD: 4.2 +- 2.0) (p=0.002). Lesion conspicuity scores were significantly higher for SMS-ss-DWI-EPI (p=0.009). Benign lesions had similar conspicuity with both sequences while malignant lesions had significantly higher conspicuity with SMS-ss-DWI-EPI (p=0.041) (for example, see Figure 1).
Infolding and ghosting artifacts were scored as disturbing or worse by 2 or more radiologists in 6 and 15 cases, for Resolve and SMS respectively. Distortion artifacts were scored as disturbing or worse in 4 and 17 cases, respectively.
Discussion: Although the conventional rs-DWI-EPI sequence results in better IQ, in general ss-EPI results in a higher SNR, which may lead to better visibility of malignant lesions with SMS-ss-DWI-EPI. This might eventually improve the clinical value of DWI in addition to contrast enhanced breast MRI. Simultaneous Multi-Slice (SMS) ensures that slices are excited simultaneously with a multiband pulse, which leads to a reduced acquisition time. In our protocol, the combination of ss-EPI and SMS results in a higher spatial resolution while still having a shorter acquisition time than the conventional sequence. The higher achievable spatial resolution may be an important factor for the improved lesion visibility, and conspicuity of malignant lesions. This may make the SMS approach suitable for fast screening and diagnosis of breast cancer. Still, further development of the SMS-ss-DWI-EPI sequence is needed for improved IQ, decreased presence of artifacts and even better lesion conspicuity.
Conclusion
Despite the perceived poorer image quality and the more disturbing presence of artifacts in the SMS-ss-DWI-EPI sequence, malignant lesions are better visualized using this sequence. When image quality and conspicuity are further improved, this technique might enable improved lesion detection on unenhanced diffusion weighted breast MRI.