Baorui Ren

Design and performance of the prototype full field breast tomosynthesis system with selenium based flat panel detector

We have developed a breast tomosynthesis system utilizing a selenium-based direct conversion flat panel detector. This prototype system is a modification of Selenia, Hologic’s full field digital mammography system, using an add-on breast holding device to allow 3D tomosynthetic imaging. During a tomosynthesis scan, the breast is held stationary while the x-ray source and detector mounted on a c-arm rotate continuously around the breast over an angular range up to 30 degrees. The x-ray tube is pulsed to acquire 11 projections at desired c-arm angles. Images are reconstructed in planes parallel to the breastplate using a filtered backprojection algorithm. Processing time is typically 1 minute for a 50 mm thick breast at 0.1 mm in-plane pixel size, 1 mm slice-to-slice separation. Clinical studies are in progress. Performance evaluations were carried out at the system and the subsystem levels including spatial resolution, signal-to-noise ratio, spectra optimization, imaging technique, and phantom and patient studies. Experimental results show that we have successfully built a tomosynthesis system with images showing less structure noise and revealing 3D information compared with the conventional mammogram. We introduce, for the first time, the definition of “Depth of Field” for tomosynthesis based on a spatial resolution study. This parameter is used together with Modulation Transfer Function (MTF) to evaluate 3D resolution of a tomosynthesis system as a function of system design, imaging technique, and reconstruction algorithm. Findings from the on-going clinical studies will help the design of the next generation tomosynthesis system offering improved performance.

A new generation FFDM/tomosynthesis fusion system with selenium detector

A new generation of digital breast tomosynthesis system has been designed and is commercially available outside the US. The system has both a 2D mode and a 3D mode to do either conventional mammography or tomosynthesis. Uniquely, it also has a fusion mode that allows both 3D and 2D images to be acquired under the same breast compression, which results in co-registered images from the two modalities. The aim of this paper is to present a technical description on the design and performance of the new system, including system details such as filter options, doses, AEC operation, 2D and 3D images co-registration and display, and the selenium detector performance. We have carried out both physical and clinical studies to evaluate the system. In this paper the focus will be mainly on technical performance results.

Radiation dosimetry in digital breast tomosynthesis: Report of AAPM Tomosynthesis Subcommittee Task Group 223

The radiation dose involved in any medical imaging modality that uses ionizing radiation needs to be well understood by the medical physics and clinical community. This is especially true of screening modalities. Digital breast tomosynthesis (DBT) has recently been introduced into the clinic and is being used for screening for breast cancer in the general population. Therefore, it is important that the medical physics community have the required information to be able to understand, estimate, and communicate the radiation dose levels involved in breast tomosynthesis imaging. For this purpose, the American Association of Physicists in Medicine Task Group 223 on Dosimetry in Tomosynthesis Imaging has prepared this report that discusses dosimetry in breast imaging in general, and describes a methodology and provides the data necessary to estimate mean breast glandular dose from a tomosynthesis acquisition. In an effort to maximize familiarity with the procedures and data provided in this Report, the methodology to perform the dose estimation in DBT is based as much as possible on that used in mammography dose estimation.

Lesion visibility in low dose tomosynthesis

Visibility of lesions in mammography are significantly reduced by the presence of anatomical, or structure, noise. Breast tomosynthesis offers the possibility of reducing this noise. We have compared the detection of low contrast and microcalcification objects with tomosynthesis imaging as a function of dose to full field digital mammography (FFDM) performed at a standard screening dose. The measurements were performed with a variety of phantoms and complex backgrounds. The complex backgrounds greatly reduced object visibility using FFDM; much less so for the tomosynthesis images. In summary, visibility of low contrast objects using tomosynthesis was superior to visibility of these objects in FFDM, even when the tomosynthesis imaging was performed at 1/4 or less of a FFDM dose. Tomosynthesis also showed superior visibility to FFDM for 160-180 micron microcalcifications at 1/2 the FFDM dose.

The dependence of tomosynthesis imaging performance on the number of scan projections

In general, the use of more projections results in fewer tomosynthesis reconstruction artifacts. However, under a fixed dose, an excess number of projections will make the detector noise more pronounced in each of the x-ray shots and thus degrade image quality. Even in the absence of detector noise the advantages of higher projection numbers eventually have diminishing returns, making more projections unnecessary. In this study, we explore the dependence of tomosynthesis imaging performance on the number of projections, while keeping other factors fixed. We take the contrast-to-noise ratio as the figure of merit to search for the range of optimal projection number. The study is carried out through both simulations and experiments, with phantoms consisting of micro-calcification and mass objects, and a cadaver breast. The goal of this paper is to describe our methodology in general, and use a prototype tomosynthesis system as an example. The knowledge learned will help the design of future generation clinical tomosynthesis systems.

Measurement of breast density with digital breast tomosynthesis

Breast density is known as a strong risk factor for breast cancer. Clinical assessment of breast density during screening mammography is often done by radiologists through visual evaluation or by a computer program. Automated computer methods offer the potential for non-subjective density assessments. With the rapid development and increased utilization of tomosynthesis clinically, there is a practical need for systems to provide automated breast density measurements in tomosynthesis like those available in mammography. QuantraTM is a software package using physical modeling of mammography systems, and performs volumetric assessment of breast tissue composition for conventional mammography. In this paper, we describe recent developments to extend Quantra to calculate breast density using tomosynthesis projection images. Our development took advantage of the combo imaging mode of Hologic Selenia DimensionsTM system, which allowed co-registered conventional 2D mammogram and 3D tomosynthesis images to be acquired in a single compression. We used the Quantra results of 2D mammograms as a reference to refine the new processing algorithm for tomosynthesis images. This paper describes details of the new algorithm and provides some preliminary results.

The CNR method in scan angle optimization of tomosynthesis and its limitations

The performance optimization of tomosynthesis is very challenging as it involves multiple system parameters to be optimized towards multiple figures of merit (FOM). Common approach is to take a selected few FOMs and optimize them under more confined conditions. While this kind of study helps us to gain more insights, extra precautions are needed when one tries to generalize the conclusions. Several reported works have shown that increasing the scan angle improves the contrast to noise ratio (CNR), which made the authors conclude that from the CNR perspective, large scan angle has advantages over small angle in tomosynthesis. In this study, we investigated the dependence of CNR on the scan angle while other system parameters were fixed. We found that improvement of CNR with large scan angle in those published studies was actually due to reconstruction algorithm and associated filtering effect but not due to the scan angle itself. To reveal this property, we selected six filters to cover a board range of possible shapes, and showed CNR variations with different filters. Besides, we also studied the ML-EM and SART iterative reconstruction algorithms, and obtained their equivalent Fourier filters numerically. The change of the equivalent filter shapes of iterative methods at different scan angle explained the observed CNR dependence on the scan angles. We conclude that larger scan angle does not have any intrinsic CNR advantage over small one in tomosynthesis. The observed CNR gain at large angle is an effect from the reconstruction filters. Therefore CNR based optimization study need to be carried out without the potential bias from filters.

Initial experience with dual-energy contrast-enhanced digital breast tomosynthesis in the characterization of breast cancer

An assessment is ongoing of the ability of dual energy contrast-enhanced digital breast tomosynthesis (CE-DBT) to depict the morphologic and vascular characteristics of breast cancer in comparison with breast MRI and digital mammography (DM). Eight patients with newly diagnosed breast cancer were imaged with an automated dual-energy CE-DBT system. High energy/low energy image pairs of the index breast were obtained at 1 pre- and 3 post-contrast timepoints. Post-contrast images were obtained after intravenous administration of Visipaque (1 mL/kg). Anatomic images were reconstructed using filtered backprojection, and contrast-enhanced images were generated using simple backprojection followed by temporal or dual-energy subtraction. Dual-energy CE-DBT was able to demonstrate the index malignant lesion in 7 of 8 patients (9 of 10 lesions). Morphologic characteristics including margin detail and associated microcalcifications were qualitatively concordant with DM. Vascular characteristics were identifiable qualitatively on post-processed images in some cases, and judged to be qualitative concordant with breast MRI.

Dual energy iodine contrast imaging with mammography and tomosynthesis

We have developed dual energy (DE) iodine contrast imaging functions with a commercial mammography and tomosynthesis system. Our system uses a tungsten target x-ray tube and selenium direct conversion detector. Conventional low energy (LE) images were acquired with existing Rh, Ag and Al filters at the screening doses while the high energy images (HE) were acquired with new Cu filters at half of the screening doses. In DE 2D mode, a pair of LE and HE images was taken with one second delay time between and with anti-scatter grid. In DE 3D mode, 22 views of alternating LE and HE were taken over 15 degrees angle in seven seconds without grid while tube was scanned continuously. We used log-subtraction algorithm to obtain clean DE images with the subtraction factor K derived empirically. In 3D mode, the subtraction was applied to each pair of LE and HE slices after reconstruction. The x-ray technique optimization was done with simulation and phantom study. We performed both phantom and patient studies to demonstrate the advantage of iodine contrast imaging. Among several new things in our work, a selenium detector optimized for DE imaging was tested and a large dose advantage was demonstrated; 2D and 3D DE images of a breast under same compression were acquired with a unique DE combo mode of the system, allowing direct image quality comparison between 2D and 3D modes. Our study showed that new DE system achieved good image quality. DE imaging is be a promising modality to detect breast cancer.

Investigation of practical scoring methods for breast density

Breast density is known as a strong risk factor for breast cancer Clinically, physicians often use the BI-RADS or the Boyd categories to describe the density of breast, measured by observing mammograms More accurately, breast density is measured by the percentage of glandular tissue in a breast For all these methods, there might be more easily interpretable clinical value if the breast density was reported with scoring methods which are correlated to the patient distribution In this paper two practical scoring methods will be discussed The first one is population-based, with each segment of the continuous scores matching the patient BI-RADS distribution found in large scale clinical study The second one is statistics-based, with the breast density result compared with the mean and the standard deviation from a reference population Both methods will be described in details, together with preliminary results from an evaluation study with a total of 942 patients.