Diffusion-Weighted MRI of Breast Cancer: Improved Lesion Visibility and Image Quality Using Synthetic b-Values

Abstract

Background: Diffusion-weighted imaging (DWI) is an MRI technique with the potential to serve as an unenhanced breast cancer detection tool. Synthetic b-values produce images with high diffusion weighting to suppress residual background signal, while avoiding additional measurement times and reducing artifacts.

Purpose: To compare acquired DWI images (at b = 850 s/mm² ) and different synthetic b-values (at b = 1000-2000 s/mm² ) in terms of lesion visibility, image quality, and tumor-to-tissue contrast in patients with malignant breast tumors.

Study type: Retrospective.

Population: Fifty-three females with malignant breast lesions.

Field strength/sequence: T₂ w, DWI EPI with STIR fat-suppression, and dynamic contrast-enhanced T₁ w at 3T.

Assessment: From acquired images using b-values of 50 and 850 s/mm² , synthetic images were calculated at b = 1000, 1200, 1400, 1600, 1800, and 2000 s/mm² . Four readers independently rated image quality, lesion visibility, preferred b-value, as well as the lowest and highest b-value, over the range of b-values tested, to provide a diagnostic image.

Statistical tests: Medians and mean ranks were calculated and compared using the Friedman test and Wilcoxon signed-rank test. Reproducibility was analyzed by intraclass correlation (ICC), Fleiss, and Cohen's κ.

Results: Relative signal-to-noise and contrast-to-noise ratios decreased with increasing b-values, while the signal-intensity ratio between tumor and tissue increased significantly (P < 0.001). Intermediate b-values (1200-1800 s/mm² ) were rated best concerning image quality and lesion visibility; the preferred b-value mostly lay at 1200-1600 s/mm² . Lowest and highest acceptable b-values were 850 s/mm² and 2000 s/mm² . Interreader agreement was moderate to high concerning image quality (ICC: 0.50-0.67) and lesion visibility (0.70-0.93), but poor concerning preferred and acceptable b-values (κ = 0.032-0.446).

Data conclusion: Synthetically increased b-values may be a way to improve tumor-to-tissue contrast, lesion visibility, and image quality of breast DWI, while avoiding the disadvantages of performing DWI at very high b-values.

Level of evidence: 3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1754-1761.

Keywords: Breast neoplasms; computer-assisted image processing; diffusion magnetic resonance imaging; magnetic resonance imaging.

Figure 1

Contrast‐enhanced T₁w (top image) and diffusion‐weighted images of the same patient (55‐year‐old female with FGT type D and grade 2 invasive lobular cancer in the right breast [white arrow]) at different calculated b‐values as displayed next to the image, and the corresponding relative SNR, CNR, and SIR. With increasing b‐values, the nondiffusion‐restricted background tissue is suppressed, while the restricted tumor retains its high signal intensity, increasing visibility.

Figure 2

A 39‐year‐old female with grade 3 invasive ductal cancer in the right breast (arrow). Diffusion‐weighted image at b = 850 s/mm² without (a) and with (b) the regions of interest (ROIs) used to calculate relative SNR, CNR, and SIR. Round/oval ROIs were placed inside the lesion of interest (right ROI), the air between the breasts (middle ROI), and the healthy breast tissue of the contralateral breast (left ROI). (c) The corresponding contrast‐enhanced T₁w image. The lesion in the left breast (arrowhead) is a large fibroadenoma).

Figure 3

Line graph displaying mean relative SNR, CNR, and SIR between the lesions, normal breast tissue, and the air between the breasts at different b‐values. While relative SNR and CNR are shown to decrease as b‐values increase, relative SIR increases, reflecting the faster signal decrease of nondiffusion‐restricted background tissue compared with the diffusion‐restricted tumor. The error bars represent the 95% confidence intervals. B‐values are displayed in s/mm².

Figure 4

Contrast‐enhanced T₁w (top image) and diffusion‐weighted images of the same patient (27‐year‐old female with FGT type C and grade 3 invasive ductal cancer in the right breast [white arrow]) at different calculated b‐values as displayed next to the image, and the corresponding mean scores from all readers for image quality (IQ) and lesion visibility (LV). The lesion is hardly visible on the contrast‐enhanced image due to strong background parenchymal enhancement. IQ and LV increase with increasing b‐values up to 1400–1800 s/mm², but are lower at b = 2000 s/mm².

Figure 5

Radar charts displaying the number of mentions for preferred (a), lowest acceptable (b), and highest acceptable b‐value (c) by readers 1–4 (R1‐4). Images obtained with moderately increased b‐values were most commonly chosen as the preferred b‐value. B‐values are displayed in s/mm².

Figure 6

Line graph displaying the mean ranks for lesion visibility by b‐value and FGT type according to the Friedman test. FGT type A was excluded from the analysis since only two cases with this FGT type were found in the study population. A shift to better lesion visibility at higher b‐values can be seen in patients with FGT type D.