High-resolution, three-dimensional diffusion-weighted breast imaging using DESS

Abstract

Purpose: To evaluate the use of the double-echo steady-state (DESS) sequence for acquiring high-resolution breast images with diffusion and T2 weighting.

Materials and methods: Phantom scans were used to verify the T2 and diffusion weighting of the DESS sequence. Image distortion was evaluated in volunteers by comparing DESS images and conventional diffusion-weighted images (DWI) to spoiled gradient-echo images. The DESS sequence was added to a standard clinical protocol, and the resulting patient images were used to evaluate overall image quality and image contrast in lesions.

Results: The diffusion weighting of the DESS sequence can be easily modulated by changing the spoiler gradient area and flip angle. Radiologists rated DESS images as having higher resolution and less distortion than conventional DWI. Lesion-to-tissue contrast ratios are strongly correlated between DWI and DESS images (R=0.83) and between T2-weighted fast spin-echo and DESS images (R=0.80).

Conclusion: The DESS sequence is able to acquire high-resolution 3D diffusion- and T2-weighted images in short scan times, with image quality that facilitates morphological assessment of lesions.

Keywords: 3D; Breast; DESS; DWI; Diffusion; T2.

Fig. 1

DESS sequence diagram showing two repetitions. Two echoes are acquired per TR with all gradients rewound except a non-rewound spoiler component (shaded), which is combined with dephaser/rephaser gradients if played on Gx. The spoiler gradients provide diffusion weighting because moving spins will experience different gradients before and after the refocusing RF pulse. The differences in gradient history will cause imperfect rephasing, resulting in signal loss.

Fig. 2

Effect of sequence parameters on image contrast. Phantoms with different T2 values and diffusivities show different contrast in the two echoes and for different diffusion attenuations. The plots show signal ratios to account for differences in proton density. Each acquisition produces an image (Echo 1) with a shorter TE (TE₁) and an image (Echo 2) with a longer effective TE (TE₂). (In regions of high diffusivity (egg white, water), the signal loss from Echo 1 to Echo 2 increases as the diffusion attenuation increases. This change in signal ratios reflects the mixed contrast of the DESS sequence: the images have both T2 and diffusion weighting, so multiple acquisitions are necessary to separate out the effects of each.)

Fig. 3

Effect of T1 and flip angle on signal ratios. Signal ratios were simulated using the extended phase graph model for a low diffusivity species (T1/T2 = 350/50 ms, D = 0.01 × 10⁻⁹ m²/s). As expected, the signal ratio does not change much with spoiler area for a low-diffusivity species (a). The signal ratio is independent of T1 when the flip angle is consistent (a), but when the flip angle changes between acquisitions, there is greater attenuation of species with shorter T1 due to T1 decay while magnetization is along the longitudinal axis (b). Tissues of interest have a longer T1 than fat, and have a signal ratio close to 1 even when the flip angle changes. The signal attenuation versus gradient area is shown for oil, and experimental results match very well with simulation results (c). There is very little signal loss as the spoiler area increases for either combination of flip angles, but there is signal attenuation due to T1 when the flip angle changes between acquisitions (gray line and points). Simulation results are also shown for fibroglandular tissue (T1/T2 = 1400/55 ms, D = 1.6 × 10⁻⁹ m²/s). As expected, the signal ratio decreases with increasing spoiler area for a high-diffusivity species (d). The signal ratios are largely independent of T1 when the flip angles are consistent (d), but when the flip angle changes between acquisitions, there is again greater signal attenuation for species with shorter T1 (e). The signal attenuation versus gradient area is shown for fibroglandular tissue, with increased diffusion attenuation when the flip angle is decreased. Vertical lines indicate the T1 of oil (a, b) and fibroglandular tissue (d, e).

Fig. 4

Diffusion attenuation contrast comparison. Contrast ratios were calculated as the signal ratio between ROIs in a variety of phantoms and an ROI in a reference phantom with T2 and ADC values similar to those of breast tissue. The contrast ratios of DESS Echo 2, high DW (a) are plotted against the contrast ratios of diffusion-weighted EPI. The contrast ratios are also shown on a Bland–Altman plot with 96% confidence intervals (b).

Fig. 5

T2 weighting contrast comparison. Contrast ratios were calculated as the signal ratio between ROIs in a variety of phantoms and an ROI in a reference phantom with T2 and ADC values similar to those of breast tissue. The contrast ratios of DESS Echo 2, low DW (a) and DESS T2 maps (b) are plotted against the contrast ratios of T2-weighted FSE. The contrast ratios are also shown on Bland–Altman plots with 96% confidence intervals (c, d).

Fig. 6

Distortion comparison. The green contours show the skin and the interfaces between fat and glandular tissue on the SPGR images (a, e) superimposed on the remaining images. Severe distortion in the spin-echo EPI image (b) can cause some of the signal from the glandular tissue to appear outside of the breast (arrow) when compared to the anatomic reference image (a), but the structures in the DESS images (c, d) correspond well to those in the reference image (a). Even for a high-quality spin-echo EPI image (f), there is still some distortion, and signal from the glandular tissue appears in a fat region (arrow); however, there is no distortion in the DESS images (g, h) when compared to the reference image (e).

Fig. 7

Reformatted images. The images acquired with the DESS sequence (b, d, f) have higher resolution than those acquired with the EPI DWI sequence (a, c, e), which is particularly evident in the images reformatted in the sagittal plane (c, d). The higher resolution allows for better depiction of fine features (arrows). Hash marks indicate the locations of the reformatted planes. Detail shows a grade 2 invasive ducal carcinoma.

Fig. 8

Diffusion attenuation contrast comparison. Contrast ratios were calculated as the signal ratio between a lesion ROI and a fibroglandular ROI for a number of benign and malignant lesions. The contrast ratios of the DESS images with high diffusion attenuation (a) and low diffusion attenuation (c) are plotted against the contrast ratios of the EPI diffusion-weighted images. The contrast ratios are also shown on Bland–Altman plots with 96% confidence intervals (b, d).

Fig. 9

In vivo diffusion attenuation comparison. Typical patient images depicting an invasive ductal carcinoma (IDC), a benign cyst, a high-grade ductal carcinoma in situ (DCIS), and an invasive lobular carcinoma (ILC) acquired with contrast-enhanced T1 weighting (Post contrast) and diffusion weighting. The DESS images show higher resolution and less distortion than the conventional spin-echo EPI diffusion-weighted images (DWI), but with comparable image contrast.

Fig. 10

T2 weighting contrast comparison. Contrast ratios were calculated as the signal ratio between a lesion ROI and a fibroglandular ROI for a number of benign and malignant lesions. The T2 weighting was evaluated in DESS images (left column) and in T2 maps (right column). The contrast was evaluated in DESS images with high diffusion attenuation (a–d) and low diffusion attenuation (e–h). The contrast ratios for the DESS data are plotted against the contrast ratios for the FSE T2-weighted images (a, b, e, f) and are also shown on Bland–Altman plots with 96% confidence intervals (c, d, g, h).

Fig. 11

In vivo T2 weighting comparison. Typical patient images depicting an invasive ductal carcinoma (IDC), a benign cyst, a high-grade ductal carcinoma in situ (DCIS), and an invasive lobular carcinoma (ILC) acquired with fast-spin echo (FSE) and DESS. The DESS Echo 2, low DW images show similar image contrast to the T2-weighted FSE images, and the DESS T2 maps show T2 values that correspond to the literature values for glandular tissue.

Fig. 12

In vivo challenges. Poor fat suppression can reduce the conspicuity of lesions. In the case of incomplete fat suppression (a) the image contrast is diminished; the arrows show a grade 3 invasive ductal carcinoma, also shown in a post-contrast image for reference (b). If the field variation is severe enough, the signal from the tumor can be suppressed (c). The arrows indicate the location of a grade 1 invasive ductal carcinoma, also shown in a post-contrast image for reference (d). Motion can cause image artifacts, such as signal appearing outside of the breasts (e); the green contour shows the skin and the arrow shows artifact signal.