As this study recruited patients already suspected of breast cancer based on other imaging studies and a physical examination, the average tumor size was large 26 mm with a range of 4— mm. Hence, the performance advantage of the dedicated PET scanners spatial resolution and sensitivity may have been negated with respect to the clinical system.
Based on the work done on the development and subsequent evaluation studies performed with the above outlined PEM systems, there are several ongoing projects aiming to further increase the performance and also evaluate new system designs for dedicated breast imaging. A group at Stanford University is actively developing a very high-resolution system using 2 flat detector panels to image compressed breast.
The effective crystal thickness for keV photons will be 1. The system is designed to provide 1-mm spatial resolution with a 3-mm DOI resolution in addition to very high system sensitivity for compressed breast imaging. System simulations indicate an ability to distinguish 2. At the University of Pennsylvania our group is developing a new generation of the BPET scanner using time-of-flight information to assist in image reconstruction of limited-angle data sets.
The new system will be composed of 2 detectors rotating around the breast and will use 1. The final system will use 2.
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Finally, a group at Washington University is using the concept of virtual pinhole PET 42 to develop a high-resolution PET insert that can be placed closed to the breast while the patient is imaged in clinical whole-body PET scanner. By collecting data simultaneously between this insert and the traditional detector ring of the whole-body scanner, it has been shown that one can achieve high spatial resolution and sensitivity needed for breast imaging while utilizing standard clinical PET systems.
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An added advantage of such a system will be the ability to image the axilla as well as the whole patient that allows diagnosis of the full extent of disease. Although the role of PEM in breast imaging has concentrated not only on screening or detection of breast tumors but also on disease staging, BSGI has focused primarily on detection of tumors, especially in women with dense breasts.
Mammography images are defined by the attenuation of x-rays within the breast tissue that varies as a function of breast density. Gamma imaging techniques such as planar scintimammography with 99m Tc can potentially be useful in these situations as the emission image is relatively independent of the breast density. Imaging is performed with slanted parallel-hole collimators and the system achieves a 6-mm spatial resolution at a distance of 3 cm from the detector head, which is similar to the midplane of a mildly compressed, average-size breast.
This study enrolled 94 female patients with a high calculated 5-year risk for development of breast cancer. All patients had a normal mammogram with a BI-RADS category of 1 or 2, and underwent a physical exam also normal within 6 months of scintmammography. Both the tests were normal. Patients were injected with 25—30 mCi — MBq of 99m Tc sestamibi and scanned 10 minutes later in the craniocaudal CC and mediolateral oblique MLO views with a 10 minutes per view per breast.
All mammograms and scintimammograms were reviewed and classified by 2 experienced radiologists. The scintimammograms were classified with a score ranging from 1 normal with no focal or diffuse uptake to 5 marked focal uptake , and were interpreted without knowledge of patient characteristics and mammography reports. In Figure 3 we show an image of a patient with increased focal uptake as seen in the scintimammogram. Out of 94 patients, 78 had a normal scintimammogram score of 1—3 who were subsequently classified as normal in a 1-year follow-up clinical examination, mammogram, and scintimammogram.
The remaining 16 patients with an abnormal scintimammogram scores of 4 and 5 underwent a directed US, out of which 11 underwent biopsy owing to focal hypoechoic finding in the US. Out of these 11 patients, 2 were found to have an invasive carcinoma.
The 5 patients who did not show any focal hypoechoic finding in the US had normal scintimammograms in the 6-months and 1-year follow-ups. The 2 patients with true-positive findings had a history of breast carcinoma, mammographic BI-RADS breast density category of 2 and 3, and both cancers measured 9 mm in the greatest diameter at pathology examinations. However, as the authors emphasize, the results of this study could be limited owing to the presence of only 2 true-positive findings. Abnormal, focused radiotracer uptake seen in a scintimammogram for a patient acquired on a Dilon system.
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Pathology demonstrated 9 mm infiltrating ductal carcinoma. Reprinted with permission from Brem et al. Recent years have seen advances in the development of semiconductor detectors such as cadmium zinc telluride CZT for direct detection of gamma rays. Owing to its cost and relatively limited size, CZT currently seems to have best use in small, application-specific cameras such as dedicated breast or cardiac scanners. The individual CZT pixels are 1. The breast is imaged with mild compression between the 2 detector heads with a 5—10 minutes scan per view. With the supplied parallel-hole collimators, the reconstructed spatial resolution is 4.
Using 2 detector heads provides higher system sensitivity for the dual-detector head designs compared with single-detector head systems. Image provided by GE Healthcare. A clinical study based on patients was recently performed to evaluate the benefit of the dual-detector CZT system as opposed to a single-head CZT system.
BSGI was performed before the biopsy. The patients had a mean age of 59 years and the BI-RADS assessment of lesions was either category 4 or 5 in all patients with an almost equal distribution between the 2 categories. Upon completion of each patient study, the images were reviewed together with images from other imaging modalities. If the BSGI image showed additional lesions that were not seen elsewhere, additional diagnostic examinations were performed to evaluate these lesions.
After all patients had been imaged and histopathology reports for all lesions obtained, the BSGI images were read by 3 radiologists independent of other patient information. These readings were performed blindly, first using images from only 1 detector to mimic a single-head detector followed by images from both detectors.
Each BSGI image was classified with a score ranging from 1 no focal uptake to 5 intense focal uptake. If abnormal uptake was observed, then the radiologist also marked the location and intensity of the abnormal uptake in the CC and MLO views. Images with lesions scoring 2 or higher were considered positive for this study. A total of lesions were confirmed as cancer in 88 patients. Figure 5 shows an image of a patient with a 4-mm cancerous lesion that was not visible with the single-detector view but was clear with the dual-detector view.
This study also demonstrated the benefits of a high-resolution, dual-head BSGI system, but does not compare it directly with standard diagnostic mammography or US. An year-old patient with 4-mm invasive lobular carcinoma as marked by the arrows.
Molecular breast imaging findings were negative when only images from detector 1 were available. However, together with detector 2, cancer was identified with a high focal uptake. Reprinted with permission from Hruska et al. Another prospective clinical study 49 using a much larger patient population was recently completed with an aim to compare the performance of a prototype version of the dual-head GE NM Discovery b and mammography in screening women with mammographically dense breasts.
Enrolled patients were mostly 25 years or older and were undergoing routine screening mammography, or else less than 50 years old but had not undergone mammography at the time of enrollment. Mammography was either screen based or digital with 2 views per breast.
Mammography images were independently read by trained breast radiologists in a standard manner without any knowledge of the BSGI results. Images from the BSGI system were reviewed by 2 dedicated breast radiologists blinded to mammography results and assigned an abnormal tracer uptake score ranging from 1 no abnormal uptake to 5 uptake highly suspicious for malignancy.
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Uptake scores of 3, 4, or 5 were considered to indicate a positive result. Positive cancer status in patients was defined on the basis of a positive histopathologic results from a biopsy performed within days on the initial study mammography. Negative cancer status was defined as a result of: 1 negative or benign results from follow-up imaging performed within — days after the initial study mammography, 2 benign histopathologic results, or 3 medical record review or patient interview confirming no cancer diagnosis.
Out of patients with verified cancer status, 11 were diagnosed with positive cancer status. The cancers detected in BSGI images only not visible in mammography were invasive with size range of 0. A group at the Mayo Clinic has been a primary driving force in the development of CZT-based BSGI scanners and built a single-head prototype system 50 , 51 before the commercial introduction of the dual-head systems from Gamma Medica and GE. The detector head used by the Mayo group is the same as the one now used in the GE NM Discovery b system, with the breast being imaged with mild compression and using parallel-hole collimators.
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The group at the University of Virginia has developed a system that uses a single-detector head for gamma-ray imaging incorporated in a vertical, mammography-like gantry together with an x-ray source and detector. Similarly, the gamma-ray detectors acquire data with limited rotation around the breast, and an iterative limited-angle reconstruction algorithm provides a reconstructed single photon image. Limited clinical studies have been performed with this system, as part of its design evaluation.
Customized detector trajectories are used to collect all the projection data needed to achieve a tomographic image. Dedicated PEM and BSGI devices have been developed to provide higher spatial resolution and sensitivity relative to multipurpose clinical systems. BSGI systems have seen enough interest that commercially there are 3 devices currently available from different manufacturers.
The improved spatial resolution of these commercial, as well as research BSGI, systems has shown some benefit of single photon imaging, primarily in the area of detection of small lesions in high-density breasts. Based on the studies completed, PEM has shown higher sensitivity or specificity or both in the detection of small lesions 1—2 cm in size relative to mammography. The higher spatial resolution of the existing PEM systems 1. Most of the existing dedicated PEM systems, including the only commercially available system from Naviscan, have been developed to image a compressed breast in an analogous and often in conjunction with mammography systems.