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Evaluation of a Small Cadmium Zinc Telluride Detector for Scintimammography

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Evaluation of a Small Cadmium Zinc Telluride Detector for Scintimammography Evaluation of a Small Cadmium Zinc Telluride Detector for Scintimammography Bryon Mueller, PhD1; Michael K. O’Connor, PhD1; Ira Blevis, PhD2; Deborah J. Rhodes, MD3; Robin Smith, MD3; Douglas A. Collins, MD1; and Stephen W. Phillips, MD1 1Department of Radiology, Mayo Clinic, Rochester, Minnesota; 2Functional Imaging, General Electric Medical Systems, Haifa, Israel; and 3Department of Internal Medicine, Mayo Clinic, Rochester, Minnesota quently, the breast must be imaged in the prone lateral The purpose of this study was to evaluate a small semiconduc- position with poor visualization of lesions located in the tor-based gamma camera that may have applications in scinti- medial wall or close to the chest wall. Optimal scintimam- mammography. Methods: A small cadmium zinc telluride (CZT) mographic imaging requires a system that cannot only be ϫ detector was evaluated. The detector had a field of view of 20 positioned close to the breast but also allows the breast to be 20 cm with detector elements of 2.5 ϫ 2.5 mm in size. Both short-bore (35 mm) and long-bore (50 mm) collimators, imaging from any side with minimal interference from matched to the geometry of the detector elements, were eval- adjacent activity in the body. Conventional Anger-based uated. The imaging performance of the CZT detector was com- gamma cameras cannot be positioned in this manner be- pared with that of a conventional gamma camera equipped with cause of the 8- to 10-cm dead space between the active area all-purpose and ultra-high-resolution collimators. The perfor- and the physical edge of the detector. mance of both systems with respect to breast imaging was Several studies have reported on the development of evaluated using a water tank containing small glass spheres, small compact gamma-camera systems that overcome the 1.8–9.8 mm in diameter. The effects of variations in breast thickness, tumor depth, and tumor-to-background ratio were all problem of a large dead space. Several systems have been simulated in this phantom model. Total counts per image were developed based on position-sensitive photomultipliers adjusted to approximate the count density observed in clinical (PSPMTs) coupled to an array of NaI or CsI crystals (1–3). scintimammographic studies. Results: Sensitivity of the CZT These systems generally have excellent spatial resolution detector was 76% that of the equivalent NaI system. The sys- close to the collimator face, which is important for breast tem demonstrated excellent integral uniformity. The energy res- imaging. However, most of these systems have significantly olution of the CZT system was 6.5% for 99mTc. Spatial resolution with the long-bore collimator was superior to that of a conven- poorer energy resolution than Anger-based systems (1,3). tional large field-of -view gamma camera equipped with an An alternative to PSPMTs for light detection is a silicon ultra-high- resolution collimator, over the range 0–6 cm from photodiode usually coupled to CsI rather than NaI (4,5). the collimator face. A blinded review of breast phantom images These systems have achieved energy resolutions of 8%– showed that small spheres (Յ7 mm in diameter) were better 10%, which are comparable to those obtained with Anger- seen and had a better tumor-to-background ratio with the CZT based systems. Some PSPMT-based systems have been system than with the conventional gamma camera. Conclu- developed using a single NaI crystal (2,6). These systems sion: A small CZT detector offers superior performance to a conventional gamma camera and should permit reliable detec- have better energy resolution than the multicrystal systems tion of breast tumors Ͻ1 cm in size. but a very small field of view (6 ϫ 6 cm), making them Key Words: cadmium zinc telluride detector; scintimammogra- impractical for patient studies. phy; breast As an alternative to the above systems, over the last 5 y J Nucl Med 2003; 44:602–609 considerable progress has been made in the development of room temperature semiconductor– based gamma cameras (7–9). These systems have Ͻ1-cm dead space at the edge of the field of view and their modular nature permits detectors The quality of scintimammographic images obtained of any size to be constructed. From a performance perspec- with a conventional Anger-based gamma camera is limited tive, these systems have similar spatial resolution to PSPMT by several factors. The primary limitation is the inability to and silicon photodiode– based systems, but significantly position the gamma camera close to the breast. Conse- better energy resolution. Butler et al. (8) reported an energy resolution of Ͻ4% for 99mTc. With close proximity to the breast coupled with improved energy and spatial resolution, Received Jul. 22, 2002; revision accepted Nov. 27, 2002. cadmium zinc telluride (CZT)–based gamma cameras For correspondence or reprints contact: Michael K. O’Connor, PhD, Sec- tion of Nuclear Medicine, Charlton 1-225, Mayo Clinic, Rochester, MN 55905. should be able to detect smaller breast tumors than can be E-mail: [email protected] detected with conventional Anger-based systems. 602 THE JOURNAL OF NUCLEAR MEDICINE • Vol. 44 • No. 4 • April 2003 The purpose of this study was to evaluate a small CZT sured using a 99mTc line source placed at distances of 0.5–10 cm gamma camera both in terms of its performance character- from the collimator face. The line source was orientated at a istics and its application to scintimammography. precisely known oblique angle to the collimator holes to permit oversampling of the pixels. Count profiles of the line source images were generated and the full width at half maxima were MATERIALS AND METHODS measured. Linear regression analysis was used to determine the CZT Detector relationship between distance and extrinsic resolution. All images The CZT gamma camera evaluated in this study (Fig. 1) was a were acquired in an 80 ϫ 80 matrix size with a pixel size of 2.5 ϫ laboratory prototype (General Electric Medical Systems, Haifa, 2.5 mm. For convenience, this matrix was padded out to a 128 ϫ Israel). The detector contained 25 CZT modules. Each module was 128 matrix with blank pixels. 4 ϫ 4 cm with 256 (16 ϫ 16) CZT elements per module, giving an 80 ϫ 80 array of CZT elements. Each element had dimensions Gamma Camera of 2.5 ϫ 2.5 mm with a thickness of 5 mm. The 80 ϫ 80 array had To compare the CZT detector with a conventional gamma a total detector area of 20 ϫ 20 cm2. The system had a useful camera, procedures comparable to those described above were energy range of 50–200 keV and was equipped with either a performed on 1 head of a standard dual-head gamma camera general-purpose (GP) or a long-bore (LB) collimator. The colli- (Helix system; Elscint, Haifa, Israel). This system was equipped mators had square holes with dimensions of 2.3 ϫ 2.3 ϫ 35 mm with either a GP (HPC-35) or ultra-high resolution (HPC-46) and 2.3 ϫ 2.3 ϫ 50 mm for the GP and LB collimators, respec- collimator. The collimators had hexagonal holes of diameter 2.0 tively. The collimator holes were matched to the geometry of the and 2.7 mm and length 35 and 38 mm for the HPC-46 and HPC-35 detector elements. System sensitivity was measured for both col- collimators, respectively. System sensitivity was measured for limators using a point source of 99mTc placed at 10 cm from the both collimators using a standard 20% energy window centered on collimator face and a Ϫ8%/ϩ10% energy window centered on the the 140-keV photopeak of 99mTc. In addition, system sensitivity 140-keV photopeak of 99mTc. was measured for the GP CZT collimator on the Helix system by System uniformity was measured using a refillable 99mTc sheet removing the Helix collimator and placing the CZT collimator on source. Integral and differential uniformity values were determined the crystal. The parts of the crystal that were left exposed were according to the National Electrical Manufacturers Association shielded with lead. All images were acquired into 128 ϫ 128 standard (10). To determine system stability, measurements were matrix size with a zoom of 2, giving a pixel size of 2.2 mm. performed over a period of 40 d without intervening recalibration Extrinsic spatial resolution was measured using a 99mTc line source of the system. Energy resolution was measured intrinsically from placed at distances of 5–20 cm from the collimator face and the full width at half maximum of the photopeak using point aligned along the x-ory-axis of the gamma camera. Count profiles sources of 99mTc and 57Co. Extrinsic spatial resolution was mea- of the line source images were generated and the full width at half maxima were measured. Linear regression analysis was used to determine the relationship between distance and extrinsic resolu- tion. Breast Phantom A breast phantom (Fig. 2) was constructed to evaluate the expected performance of this system in scintimammography. This comprised a 20 ϫ 20 ϫ 20 cm plastic box containing a small plastic jig on which 3 glass rods containing 4 spheres each were mounted to simulate breast tumors. On each rod, the spheres were constructed to have approximate diameters of 10, 7.5, 5, and 2.5 mm. Actual measured internal diameters of the 12 spheres were 9.8, 9.3, 9.2, 7.2, 7.2, 6.7, 4.9, 4.6, 4.4, 3.2, 2.2, and 1.8 mm. Breast thickness, tumor depth, and the ratio of tumor to soft-tissue activity were all adjustable in this phantom.
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