- Imaging/biopsy system for breast cancer detection assessed
Caren B. Les
Researchers have completed initial testing of a positron emission mammography/tomography breast cancer detection system that finds tumors half the size of the smallest ones detected by standard imaging systems. Providing axial resolution below 2 mm, the imager is designed for women with dense or fibroglandular breasts, for whom conventional x-ray mammography offers detection that is insufficient.
This schematic drawing of the positron emission mammography/tomography breast imaging and biopsy system shows the four rotating detector units and the biopsy arm. The system incorporates motion control electronics and data acquisition computers (not depicted). Images reprinted with permission of Physics in Medicine and Biology.
Scientists at the US Department of Energy’s Thomas Jefferson National Accelerator Facility in Newport News, Va., at the West Virginia University School of Medicine in Morgantown and at the University of Maryland School of Medicine in Baltimore designed and constructed the apparatus. Built to perform both imaging and biopsy, the system requires no breast compression.
In the dual-function system, PET images reveal suspicious uptake of a radiotracer and guide biopsy of the area, while limited-angle positron emission mammography verifies the biopsy needle position prior to tissue sampling. Designed to image the contours of the breast, the system produces high-resolution 3-D images.
This positron emission mammography/tomography image of a standard test phantom was reconstructed with the 3-D ordered set expectation maximization algorithm. The diameters of radioactive rods (in millimeters) are shown. A maximum acceptance angle of 20° was used to create the image.
The scanner incorporates two sets of rotating planar detector heads. Each of the four detectors is made of a 4 × 3 array of flat panel position-sensitive photomultipliers coupled to a 96 × 72 array of 2 × 2 × 15-mm lutetium yttrium orthosilicate detector elements set at a pitch of 2.1 mm. Each detector has a computer that transfers the data it acquires to a single computer, where coincidence events are identified.
The detectors are mounted on a gantry. Specifically, each is attached to a computer-controlled linear slide so that its distance from the system’s center can be varied. The slides are mounted to a computer-controlled linear stage; the gantry computer coordinates detector motion and data acquisition.
The biopsy arm can be moved 360° around the scanner’s field of view, allowing biopsies from any angle. The biopsy gun is mounted on a computer-controlled three-axis stage, allowing the radioactive-tipped biopsy needle to be positioned automatically using the images of the breast.
Image reconstruction is performed with a three-dimensional ordered set expectation maximization algorithm parallelized to run on a multiprocessor computer. The reconstructed field of view is 15 × 15 × 15 cm.
To assess the system’s capabilities, the researchers used the scanner to image a standard test phantom containing radioactive rods of various diameters that are separated by twice their diameters. The results showed spatial resolution at the center of the field of view of 2.01 ± 0.9 mm (radial), 2.04 ± 0.08 mm (tangential) and 1.84 ± 0.07 mm (axial). At a radius of 7 cm from the center of the scanner, the resolution was 2.11 ± 0.08 mm (radial), 2.16 ± 0.07 mm (tangential) and 1.87 ± 0.08 mm (axial). Maximum system detection sensitivity was 488.9 kcps/μCi/ml (6.88%).
The investigators are working on minor improvements in the detection systems and on image reconstruction software, and they are planning to add components for taking x-ray CT scans. Initial clinical trials are planned for after the completion of system testing.
Physics in Medicine and Biology, Feb. 7, 2008, pp. 637-653.
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