A feasibility study is presented for a small, low-cost, dedicated device for positron emission mammography. Two detector arrays above and below the breast would be placed in a conventional mammography unit. These detectors are sensitive to positron annihilation radiation, and are connected to a coincidence circuit and a multiplane image memory. Images of the distribution of positron-emitting isotope are obtained in real time by incrementing the memory location at the intersection of each line of response. Monte Carlo simulations of a breast phantom are compared with actual scans of this phantom in a conventional PET scanner. The simulations and experimental data are used to predict the performance of the proposed system. Spatial resolution experiments using very narrow bismuth germanate BGO crystals suggest that spatial resolutions of about 2 mm should be possible. The efficiency of the proposed device is about ten times that of a conventional brain scanner. The scatter fraction is greater, but the scattered radiation has a very flat distribution. By designing the device to fit in an existing mammography unit, conventional mammograms can be taken after the injection of the radio-pharmaceutical allowing exact registration of the emission and conventional mammographic images.
We are developing a high specificity technique for detecting the increased metabolic rate of breast tiiinours. The glitcose analog FDG is known to concentrate in breast tiinlours rendering them easily detectable in conventional PET scans. Since PET is a relatively expensive imging technique it has not been used routinely in the detection of breast cancer. Positron emission mtnrnography (PEM) will provide a highly eficient, high sputial resolution, and low cost positron irtulging system whose rnetabolic irnages are co-registered with conventional mnmtnography. Coincidences betweeti hoo BGO blocks CUI into 2 x 2 tnrn squares coitpled to two 7.5 crn square inuiging PMTs are detected and back-projected to form reultirne multiple plane irnages. The design is about 20 tinles more sensitive than a conventional multi-slice PET body scanner, so much less radio-pharmaceiitical can be wed, reducing the patient dose and cost per scan. Protoepe detectors hose been t n d e and extensive rneasurernents done. The device is expected to have an in-plane spatial resolution about 2 intn FWHM. Besides the application as N secondup screening tool the device may be beneficial in rrzeasiiring a tiirrwur's response to ruriio-therapy or cherno-therapy as well as uiding the surgeon in optimizing the removal of inalignant tissue.
Performance characteristics of a positron emission mammographic (PEM) instrument were studied. This dedicated metabolic breast imaging system has spatial resolution of 2.8-mm full width at half maximum (FWHM), coincidence resolving time of 12-nsec FWHM, and absolute efficiency of 3%. Hot spots with diameter of 16 mm in a phantom with signal-to-background activity ratio of 6:1 were distinguishable with a scanning time of 5 minutes.
Each of two detectors used in our Positron Emission Mammography (PEM) system consists of four 36 mm x 36 mm x 20 mm bismuth germanate (BGO) crystal detector blocks coupled to a crossed-wire anode position-sensitive photomultiplier tube (PS-PMT). To facilitate high spatial-resolution imaging, the crystal blocks have been finely pixelated using a diamond saw. In each detector, 36 x 36 1.9 mm x 1.9 mm crystal elements are coupled directly to the PMT window and, on the opposite face of the blocks, 35 x 35 elements are offset by 1.0 mm along both the .Y-and y-axis of the PS-PMT. As part of a system calibration routine, a novel method for crystal element identification has been developed. This algorithm successfully identifies 59 x 49 crystal elements on each detector face. These results are used to generate a Look-Up- Table &UT) that is accessed during image formation for the effective correction of spatial distortion inherent in the detectors. Crystal identification also facilitates the capability for accurate energy discrimination, since the detector gain is considered on an element-by-element basis by accessing an energy LUT. Employing a third LUT, which contains the relative efficiencies of individual crystal elements results in improvement in image uniformity from 50% to 13%.
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