Simultaneous multi-slice readout-segmented echo planar imaging for accelerated diffusion-weighted imaging of the breast

https://doi.org/10.1016/j.ejrad.2015.10.009Get rights and content

Highlights

  • Conventional rs-EPI is limited by its long scan time.

  • SMS acquisition can compensate for this limitation.

  • SMS rs-EPI is feasible for DWI of the breast.

Abstract

Objectives

Readout-segmented echo planar imaging (rs-EPI) significantly reduces susceptibility artifacts in diffusion-weighted imaging (DWI) of the breast compared to single-shot EPI but is limited by longer scan times. To compensate for this, we tested a new simultaneous multi-slice (SMS) acquisition for accelerated rs-EPI.

Materials and methods

After approval by the local ethics committee, eight healthy female volunteers (age, 38.9 ± 13.1 years) underwent breast MRI at 3T. Conventional as well as two-fold (2× SMS) and three-fold (3× SMS) slice-accelerated rs-EPI sequences were acquired at b-values of 50 and 800 s/mm2. Two independent readers analyzed the apparent diffusion coefficient (ADC) in fibroglandular breast parenchyma. The signal-to-noise ratio (SNR) was estimated based on the subtraction method. ADC and SNR were compared between sequences by using the Friedman test.

Results

The acquisition time was 4:21 min for conventional rs-EPI, 2:35 min for 2× SMS rs-EPI and 1:44 min for 3× SMS rs-EPI. ADC values were similar in all sequences (mean values 1.62 × 10−3 mm2/s, p = 0.99). Mean SNR was 27.7–29.6, and no significant differences were found among the sequences (p = 0.83).

Conclusion

SMS rs-EPI yields similar ADC values and SNR compared to conventional rs-EPI at markedly reduced scan time. Thus, SMS excitation increases the clinical applicability of rs-EPI for DWI of the breast.

Introduction

Diffusion-weighted imaging (DWI) is a well-established method for the characterization of breast lesions. DWI improves diagnostic accuracy of breast MRI [1], [2], [3], [4], [5] and has the potential for predicting and monitoring treatment outcome [6], [7], [8]. Because of their higher cellularity, malignant lesions are associated with a lower apparent diffusion coefficient (ADC) compared to benign lesions and normal breast parenchyma [9], [10], [11].

DWI is most commonly based on single-shot echo planar imaging (ss-EPI), where the k-space is filled completely during a single T2* decay. However, this technique is prone to susceptibility artifacts leading to geometric distortions, which in DWI of the breast mainly occur in the prepectoral region [12]. Furthermore, the spatial resolution is limited by signal blurring in the phase-encoding direction. Both of these effects (blurring and distortions) increase at higher field strength due to the shorter T2* relaxation time [13], and can only partially be reduced with parallel imaging techniques [14]. A relatively new approach to overcome these limitations in DWI is readout-segmented echo planar imaging (rs-EPI), which reduces the echo-spacing compared to ss-EPI by dividing the k-space into separate segments in the readout direction [13]. When combined with parallel imaging, susceptibility artifacts and image blurring are reduced even further. In recent studies on breast lesions, rs-EPI showed significantly better image quality and higher diagnostic performance compared to ss-EPI [12].

The main disadvantage of rs-EPI is the longer acquisition time compared to ss-EPI, because each readout segment in the k-space requires a separate radiofrequency pulse with subsequent time for recovery of longitudinal magnetization (repetition time TR). In order to reduce the acquisition time, recently simultaneous multi-slice (SMS) acquisition based on the blipped ‘Controlled Aliasing In Parallel Imaging Results In Higher Acceleration’ (blipped CAIPIRINHA) technique has been proposed. Initially developed for ss-EPI [15], blipped CAIPIRINHA has meanwhile also been implemented to rs-EPI of the brain [16]. In the present work, we tested the feasibility of SMS rs-EPI for DWI of the breast. The hypothesis was that SMS rs-EPI would yield similar ADC quantification accuracy and image quality compared to conventional rs-EPI despite a much shorter acquisition time.

Section snippets

Study population

This prospective study was approved by the local ethics committee, and written informed consent was obtained from all subjects. In March 2015, 8 healthy female volunteers (mean age, 38.9 years; age range, 26–62 years; 6 premenopausal, 2 postmenopausal) underwent breast MRI. Inclusion criteria were: age >18 years, no pregnancy or breastfeeding, no prior breast surgery or biopsy, and no contraindication for breast MRI [12], [17].

Imaging protocol

All images were acquired with a 3T scanner (MAGNETOM Skyra with

Results

All images were successfully acquired, and the specific absorption rate (SAR) remained below individual limits in all sequences without the need for switching to first-level mode. The acquisition time was 4:21 min for conventional rs-EPI, 2:35 min for two-fold (2×) SMS rs-EPI and 1:44 min for three-fold (3×) SMS rs-EPI. Visual impression suggested that the diffusion-weighted images and ADC maps were quite similar in all sequences (examples shown in Fig. 1).

The inter-observer agreement was good for

Discussion

In the present study, SMS rs-EPI was compared with conventional rs-EPI for DWI of breast parenchyma in healthy volunteers. We demonstrated that SMS rs-EPI exhibits a markedly improved signal yield per measurement time (SNR efficiency) compared to conventional rs-EPI, allowing for a substantial reduction of acquisition time while maintaining similar image quality and ADC accuracy if a moderate acceleration factor is applied. SMS rs-EPI was already shown to yield high-resolution images within

Acknowledgements

The authors kindly thank Berthold Kiefer, Ph.D, and Markus Klarhöfer, Ph.D (both Siemens Healthcare) for providing us with the software for SMS rs-EPI.

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