Breast ultrasound tomography versus MRI for clinical display of anatomy and tumor rendering: preliminary results

Abstract

Objective: The objective of our study was to determine the clinical display thresholds of an ultrasound tomography prototype relative to MRI for comparable visualization of breast anatomy and tumor rendering.

Subjects and methods: Thirty-six women were imaged with MRI and our ultrasound tomography prototype. The ultrasound tomography scan generated reflection, sound-speed, and attenuation images. The reflection images were fused with the components of the sound-speed and attenuation images that achieved thresholds to represent parenchyma or solid masses using an image arithmetic process. Qualitative and quantitative comparisons of MRI and ultrasound tomography clinical images were used to identify anatomic similarities and optimized thresholds for tumor shapes and volumes.

Results: Thresholding techniques generated ultrasound tomography images comparable to MR images for visualizing fibrous stroma, parenchyma, fatty tissues, and tumors. In 25 patients, tumors were cancerous and in 11, benign. Optimized sound-speed thresholds of 1.46±0.1 and 1.52±0.03 km/s were identified to best represent the extent of fibroglandular tissue and solid masses, respectively. An arithmetic combination of attenuation images using a threshold of 0.16±0.04 dB/cm (mean±SD) further characterized benign from malignant masses. No significant difference in tumor volume was noted between benign or malignant masses by ultrasound tomography or MRI (p>0.1) using these universal thresholds.

Conclusion: Ultrasound tomography is able to image and render breast tissues in a manner comparable to MRI. Using universal ultrasound tomography threshold values for rendering the size and distribution of benign and malignant tissues appears feasible without IV contrast material.

Figures

Fig. 1

Fusion method. First, reflection, sound speed, and attenuation images are obtained from the UST scanner. The reflection image depicts fibrous architecture and is used as the background. The sound speed image is thresholded at two separate instances, once at 1.46±0.01 to show parenchyma and 1.52±0.03 to depict a solid mass. The attenuation image, thresholded at 0.16±0.04, is then added using the logical .AND. and .NOT. operator. The circled numbers represent the steps in image fusion. The final fused image then shows a benign solid mass as yellow, or a cancer as red. Underlines represent tissue estimates.

Fig. 2

Coronal T1, fat-saturated enhanced MR image (A) of a 45-year-old woman with scattered breast density. Fused UST image (B) shows similar anatomic distribution of fibrous bands and overlying fibroglandular tissue, similar to the MR image. The dark gray corresponds to fat, while the semi-transparent lighter gray represents denser fibroglandular tissue with underlying thin white fibrous bands.

Fig. 3

Coronal T1 fat-saturated enhanced MR image (A) of a 52-year-old woman with heterogeneously dense breasts and two simple cysts in the 1 and 7 o'clock positions, which have much better contrast in the T2 image (B). Reflection UST image (C) shows these simple cysts which did not reach sound speed or attenuation thresholds for solid masses.

Fig. 4

Coronal fat-saturated gadolinium-enhanced MR image (A) of a 43-year-old woman with a 1 cm fibroadenoma in the five o'clock position (arrow). UST image (B) after step #2 of the fusion process caused the fibroadenoma to be obscured by adjacent fibroglandular tissue since that entire region surpassed the sound speed threshold of 1.48 km/s. However, the final fused UST image (C) now shows the benign yellow overlay color from the .NOT. operator function, whereby the sound speed threshold of 1.52 km/s was surpassed but not the attenuation threshold.

Fig. 5

Coronal T1 fat-saturated enhanced MR image (A) of a 56-year-old woman with a 1.7 cm invasive ductal carcinoma in the 1–2 o'clock position showing bright enhancement. Initial fused UST image after step #2 only shows 2 semi-transparent lighter gray regions (arrows) that achieved the first fibroglandular threshold of 1.48 kilometers per second. Final fused UST image (C) using the .AND. operator now produces a red overlay for the mass in the 1–2 o'clock position since it had both high sound speed and attenuation, whereas only a tiny a regular portion of the nine o'clock region surpassed both thresholds (arrow). Some parenchyma and/or fibrous band junctions can incidentally reach threshold (9–10:00 position) but were easily excluded as not having mass effect on several slices. Magnified reflection image using RF component (D) shows distinct mass effect with prominent architectural distortion corresponding to the region around the cancer seen in C.

Fig. 5

Coronal T1 fat-saturated enhanced MR image (A) of a 56-year-old woman with a 1.7 cm invasive ductal carcinoma in the 1–2 o'clock position showing bright enhancement. Initial fused UST image after step #2 only shows 2 semi-transparent lighter gray regions (arrows) that achieved the first fibroglandular threshold of 1.48 kilometers per second. Final fused UST image (C) using the .AND. operator now produces a red overlay for the mass in the 1–2 o'clock position since it had both high sound speed and attenuation, whereas only a tiny a regular portion of the nine o'clock region surpassed both thresholds (arrow). Some parenchyma and/or fibrous band junctions can incidentally reach threshold (9–10:00 position) but were easily excluded as not having mass effect on several slices. Magnified reflection image using RF component (D) shows distinct mass effect with prominent architectural distortion corresponding to the region around the cancer seen in C.

Fig. 6

Coronal T1 fat-saturated enhanced MR image (A) showing a centrally necrotic 6.0 × 4.0 cm invasive ductal carcinoma in a 50-year-old African American woman with heterogeneously dense breasts. UST image after step 2–4 of the fusion process (B) highlights the extent of mass margins similar to MR, including both the exophytic nodular portion (white arrows) and central necrosis (black arrows), assuming slight position differences between scanning in water (UST) and air (MR). Note the dense parenchyma displayed as semi-transparent gray surrounding the red tumor but not obscuring it.