The study protocol was approved by the local ethics committee (Medical Faculty Heidelberg, reference number S-438/2018) and consistent with the Health Information Portability and Accountability Act of 1996 (HIPAA), with written informed consent of every participant enrolled in the study.
The FUSION-X-US-II consists of the ACUSON S2000 Automated Breast Volume Scanner (ABVS, Siemens Healthcare GmbH, Mountainview, CA, USA) and the MAMMOMAT Inspiration (Siemens Healthcare GmbH, Forchheim, Germany), both FDA approved as well as CE certified and used in routine clinical practice. The prototype combining both techniques is a research device and is not commercially available.
For this study evaluating the performance of ABUS in healthy volunteers, only the ACUSON S2000 ABVS device was used and the participants were not exposed to any radiation. The 5-14 MHz linear ultrasound transducer with an array length of 154 mm is integrated into the compression paddle, so the ABUS can be performed under the same compression and position as tomosynthesis. A specially developed compression paddle was used, which is composed of a rigid frame and specially woven gauze being radiolucent and permeable for ultrasound lotion. The gauze is able to sustain forces adequate for performing mammography/tomosynthesis and ABUS of over 200 N. In comparison to the previous FUSION-X-US, the paddle was adapted in terms of weight and size to provide better positioning and an improved tightening mechanism of the gauze enables a more conform compression. The transducer system is connected to the ACUSON S2000 ABVS device, where the acquired scans are displayed and saved. To further improve the contact between the breast surface and the ultrasound probe in FUSION-X-US-II, a special air cushion has been designed, which is inflated after lowering the compression paddle and adds homogeneous pressure from caudal, pushing the peripheral parts of the breast towards the gauze (Fig. 1). The inflation is controlled manually by the radiology technologist until optimal contact with the gauze is reached.
The breast is positioned in a standard view (craniocaudal (CC), mediolateral-oblique (MLO) or mediolateral (ML)) and the compression paddle is lowered. The air cushion is inflated and coupling lotion is applied on the gauze either manually or through an integrated automatic dispensing device. The transducer moves automatically over the breast, covering an area of maximum 30 x 15 cm2 with a maximum penetration depth of 10 cm. When the final position of the probe is reached, the gauze is lifted to release the breast.
The data is transferred to the working station and processed to generate images for interpretation by the physician.
The study was performed in September 2018 with 30 healthy women volunteering to participate. We included women aged 18 or older, non-pregnant, capable of understanding the study constraints after written informed consent. All participants received ABUS using the prototype, without however conducting tomosynthesis or mammography, so no radiation was applied.
The breast was positioned under the compression paddle by a radiology technician. The force and the breast thickness during compression were documented for each position. Before the first scan was started, the ultrasound probe was positioned manually on the breast for real time adjustment of frequency, focal depth and gain at the discretion of the investigator. Each breast was scanned in CC projection and additional scans in MLO or ML (projection) were obtained. CC orientation was defined as the standard view of ABUS. In cases of excellent patient´s tolerability, we decided to perform an additional scan in a second orientation (ML/MLO). The intention was to test if also the other orientations could be scanned using the prototype, because this is necessary if the prototype is used to obtain ABUS in combination with tomosynthesis (which will often be performed in ML/MLO). We aimed at an equal distribution of ML and MLO scans.
ABUS images were evaluated by an experienced physician with over 10 years of experience with ABUS systems using the syngo.breast ultrasound software (Software Version VA25, ©2012-2013 Siemens Medical Solutions USA, Inc., PA, USA). According to the ACR guidelines, the breast density was assessed in three categories (homogeneous fatty background texture, homogeneous fibroglandular background texture, heterogeneous background texture). Image quality was rated subjectively by the physician on a scale ranged from 1 to 5. Category 1 represents a very low quality, with no identifiable breast structures, category 2 a quality below diagnostic applicability with identifiable breast structures, category 3 a sufficient quality for diagnostic applicability (lower than HHUS), category 4 a quality close to/comparable to HHUS, and category 5 an equal or higher quality compared to HHUS.
All detected lesions were measured and classified using BI-RADS® scores .
To quantify the breast coverage by the ABUS, the level of the nipple region in the US image was used as a reference of comparison. The breast area depicted in this ABUS image (solid outline in Fig. 2) was measured using the software Fiji (ImageJ, Version 2.0.0, 2018, ã2010-2020). The estimated total breast area was determined through manual extrapolation along the skin (dotted outline in Fig. 2), assuming a continuous skin contour. The quotient of the measured breast area (covered by ABUS) and the extrapolated total breast area was used to estimate the percentage of the breast area covered in ABUS.
After the scan, the participants were asked to rate the tolerability of the breast compression from 1 (comfortable) to 5 (very uncomfortable) as well as their perception of the additional force applied by the inflated air cushion.
This is an explorative study based on descriptive statistics. Values are given as mean with standard deviation or median with quartiles dependent on the level of measurement. The differences in quality over the projections were tested with Fisher´s exact test. The resulting p-value has to be interpreted descriptively.