Mitali J. More

Optimization of exposure parameters in full field digital mammography

Optimization of exposure parameters (target, filter, and kVp) in digital mammography necessitates maximization of the image signal-to-noise ratio (SNR), while simultaneously minimizing patient dose. The goal of this study is to compare, for each of the major commercially available full field digital mammography (FFDM) systems, the impact of the selection of technique factors on image SNR and radiation dose for a range of breast thickness and tissue types. This phantom study is an update of a previous investigation and includes measurements on recent versions of two of the FFDM systems discussed in that article, as well as on three FFDM systems not available at that time. The five commercial FFDM systems tested, the Senographe 2000D from GE Healthcare, the Mammomat Novation DR from Siemens, the Selenia from Hologic, the Fischer Senoscan, and Fujis 5000MA used with a Lorad M-IV mammography unit, are located at five different university test sites. Performance was assessed using all available x-ray target and filter combinations and nine different phantom types (three compressed thicknesses and three tissue composition types). Each phantom type was also imaged using the automatic exposure control (AEC) of each system to identify the exposure parameters used under automated image acquisition. The figure of merit (FOM) used to compare technique factors is the ratio of the square of the image SNR to the mean glandular dose. The results show that, for a given target/filter combination, in general FOM is a slowly changing function of kVp, with stronger dependence on the choice of target/filter combination. In all cases the FOM was a decreasing function of kVp at the top of the available range of kVp settings, indicating that higher tube voltages would produce no further performance improvement. For a given phantom type, the exposure parameter set resulting in the highest FOM value was system specific, depending on both the set of available target/filter combinations, and on the receptor type. In most cases, the AECs of the FFDM systems successfully identified exposure parameters resulting in FOM values near the maximum ones, however, there were several examples where AEC performance could be improved.

Integrated CT-SPECT system for small animal imaging

We are developing a scanner for simultaneous acquisition of x-ray computed tomography (CT) and single photon emission tomography (SPECT) images of small animals such as mice and rats. The scanner uses a cone beam geometry for both the x- ray transmission and gamma emission projections by using an area x-ray detector and pinhole collimator, respectively. The CT and SPECT data set are overlaid to form a coregistered structural-functional 3D image. The CT system includes a single CCD-based x-ray detector and a microfocus x-ray source. The SPECT scanner utilizes tungsten pinhole collimators and arrays of CsI(Tl) scintillation detectors. We describe considerations and the early performance of a prototype scanner.

Evaluation of gamma cameras for use in dedicated breast imaging

Due to the limitations of conventional gamma cameras for breast imaging, many researchers are developing dedicated imagers for use in scintimammography that can be positioned closer to the breast, thereby improving spatial resolution. The purpose of this study was to compare the performance characteristics of several dedicated gamma cameras with two different types of position sensitive photomultiplier tubes (PSPMTs), and four different crystal arrays. Quantities evaluated include intrinsic spatial resolution, spatial resolution vs. source-to-collimator distance, energy resolution, intrinsic non-uniformity, and system sensitivity. In order to assess the impact of changing crystal size on lesion detectability, a contrast-detail study was also performed. Our studies demonstrate that the camera with the newer PSPMTs shows superior performance in terms of uniformity and energy resolution. The contrast-detail performance of the camera with the highest spatial resolution (1.4 mm crystal pitch with a high resolution collimator) was poor, even with relatively high input fluence. However, the use of a high efficiency collimator significantly improves object detectability. The camera with the largest (3.2 mm) crystal pitch performed well with both high resolution and high efficiency collimators.

Intraoperative Imaging Guidance for Sentinel Node Biopsy in Melanoma Using a Mobile Gamma Camera

Objective:To evaluate the sensitivity and clinical utility of intraoperative mobile gamma camera (MGC) imaging in sentinel lymph node biopsy (SLNB) in melanoma. Background:The false-negative rate for SLNB for melanoma is approximately 17%, for which failure to identify the sentinel lymph node (SLN) is a major cause. Intraoperative imaging may aid in detection of SLN near the primary site, in ambiguous locations, and after excision of each SLN. The present pilot study reports outcomes with a prototype MGC designed for rapid intraoperative image acquisition. We hypothesized that intraoperative use of the MGC would be feasible and that sensitivity would be at least 90%. Methods:From April to September 2008, 20 patients underwent Tc99 sulfur colloid lymphoscintigraphy, and SLNB was performed with use of a conventional fixed gamma camera (FGC), and gamma probe followed by intraoperative MGC imaging. Sensitivity was calculated for each detection method. Intraoperative logistical challenges were scored. Cases in which MGC provided clinical benefit were recorded. Results:Sensitivity for detecting SLN basins was 97% for the FGC and 90% for the MGC. A total of 46 SLN were identified: 32 (70%) were identified as distinct hot spots by preoperative FGC imaging, 31 (67%) by preoperative MGC imaging, and 43 (93%) by MGC imaging pre- or intraoperatively. The gamma probe identified 44 (96%) independent of MGC imaging. The MGC provided defined clinical benefit as an addition to standard practice in 5 (25%) of 20 patients. Mean score for MGC logistic feasibility was 2 on a scale of 1–9 (1 = best). Conclusions:Intraoperative MGC imaging provides additional information when standard techniques fail or are ambiguous. Sensitivity is 90% and can be increased. This pilot study has identified ways to improve the usefulness of an MGC for intraoperative imaging, which holds promise for reducing false negatives of SLNB for melanoma.

Optimization of breast imaging procedure with dedicated compact gamma cameras

Results are presented on studies conducted with various prototypes of a dedicated small field-of-view (SFOV) gamma camera for use in radiopharmaceutical studies of the breast. Since the experience in the clinical use of such instruments is limited, these experiments were conducted to test various clinical imaging implementations. Both planar and tomographic techniques were utilized to image various compressed and noncompressed breast phantoms. Lesion contrast was used to quantify the lesion visibility of each case. The results of this study indicate that lesion contrast is optimized with planar imaging of the compressed breast and that contrast is also dependent on lesion-to-detector distance. Based on these observations, planar imaging conducted with a system comprised of two opposed detectors providing compression to the breast would be optimal. The opposed views would ensure the minimization of lesion-to-detector distance, especially for lesions whose location is not known a-priori.

Evaluation of Gamma Cameras for Use in Dedicated Breast Imaging

Due to the limitations of conventional gamma cameras for breast imaging, many researchers are developing dedicated imagers for use in scintimammography that can be positioned closer to the breast, thereby improving spatial resolution. The purpose of this paper was to compare the performance characteristics of several dedicated gamma cameras with two different types of position sensitive photomultiplier tubes (PSPMTs), and four different NaI crystal arrays. Quantities evaluated include intrinsic spatial resolution, spatial resolution versus source-to-crystal distance, energy resolution, intrinsic nonuniformity, and system sensitivity. In order to assess the impact of changing crystal size on lesion detectability, a contrast-detail study was also performed. Our studies demonstrate that the camera with the newer PSPMTs (Hamamatsu model H8500) shows superior performance in terms of uniformity and energy resolution. The contrast-detail performance of the camera with the highest spatial resolution (1.4-mm crystal pitch with a high resolution collimator) was poor, even with relatively high input fluence. However, the use of a high-efficiency collimator significantly improves object detectability. The camera with the largest (3.2 mm) crystal pitch performed better than the others tested, with both high resolution and high efficiency collimators. However, its performance for small lesion size was better with the high-resolution collimator. Hence, based on the results of the contrast-detail study, of the cameras evaluated here, the camera with the 3.2-mm crystal pitch and the high-resolution collimator is best suited for imaging breast tumors

X-ray stereotactic lesion localization in conjunction with dedicated scintimammography

We are developing a dual modality system that combines digital X-ray mammography with gamma emission scintigraphy, on an upright mammography gantry in which the breast is held under mild compression by a support structure that is independent of the detectors. The X-ray source and detectors can be rotated around a fixed rotation axis permitting multiple views of the breast with fixed compression. Two such views can be combined as a stereotactic pair to obtain the three-dimensional location of breast lesions. Information about the location of the lesion within the breast permits corrections for attenuation and detector spatial resolution, resulting in more accurate estimation of the true lesion-to-background concentration ratio, based on the image lesion-to-background counts ratio. In this paper, we describe the model used to make these corrections, and present the results of phantom experiments designed to test the accuracy of our calculations.

Analysis of position-dependent Compton scatter in scintimammography with mild compression

During the past several years, we have been developing a breast scanning system that combines digital X-ray mammography with breast scintigraphy using a dedicated small field of view gamma camera. The relatively low uptake of /sup 99/mTc-sestamibi in the breast compared to other organs such as the heart results in a large fraction of the detected events being Compton scattered gamma rays. In this study, our goal was to determine whether generalized conclusions regarding scatter-to-primary ratios at various locations within the breast image are possible, and if so, to use them to make explicit scatter corrections to the breast scintigrams. Energy spectra were obtained from patient scans for contiguous regions of interest (ROIs) centered left to right within the image of the breast, and extending from the chest wall edge of the image to the anterior edge. An anthropomorphic torso phantom with fillable internal organs and a compressed-shape breast containing water only was used to obtain realistic scatter-only spectra for each ROI. The measured patient energy spectrum was fitted with a linear combination of the scatter-only spectrum from the anthropomorphic phantom and the scatter-free spectrum from a point source. This procedure was repeated for each of the ROIs. We found that although there is a very strong dependence on location within the breast of the scatter-to-primary ratio, the spectra are well modeled by a linear combination of position-dependent scatter-only spectra and a position-independent scatter-free spectrum, resulting in a set of position-dependent correction factors. These correction factors can be used along with measured emission spectra from a given breast to correct for the Compton scatter in the scintigrams. However, the large variation among patients in the magnitude of the position-dependent scatter makes the success of universal correction approaches unlikely.

Limited Angle Dual Modality Breast Imaging

We are developing a dual modality breast scanner that can obtain x-ray transmission and gamma ray emission images in succession at multiple viewing angles with the breast held under mild compression. These views are reconstructed and fused to obtain three-dimensional images that combine structural and functional information. Here, we describe the dual modality system and present results of phantom experiments designed to test the systems ability to obtain fused volumetric dual modality data sets from a limited number of projections, acquired over a limited (less than 180 degrees) angular range. We also present initial results from phantom experiments conducted to optimize the acquisition geometry for gamma imaging. The optimization parameters include the total number of views and the angular range over which these views should be spread, while keeping the total number of detected counts fixed. We have found that in general, for a fixed number of views centered around the direction perpendicular to the direction of compression, in-plane contrast and SNR are improved as the angular range of the views is decreased. The improvement in contrast and SNR with decreasing angular range is much greater for deeper lesions and for a smaller number of views. However, the z-resolution of the lesion is significantly reduced with decreasing angular range. Finally, we present results from limited angle tomography scans using a system with dual, opposing heads.

Combined structural and functional imaging of the breast.

Scintimammography, or single gamma nuclear imaging of the breast, has shown promise as a way of characterizing certain biological properties of suspicious breast masses. Conventional scintimammography, performed using large clinical gamma cameras and prone patient positioning suffers from several drawbacks including poor sensitivity for small (< 1 cm) lesions and no reliable method for correlating scintigraphic findings with those of other imaging modalities. We are developing a system designed to overcome some of these problems. The system combines x-ray mammography with scintimammography on a common gantry. The x-ray and gamma ray images are obtained in quick succession, with the breast in a common configuration under mild compression. A digital x-ray detector is used, permitting rapid assessment of lesion location prior to gamma imaging, and enabling fusion of the x-ray transmission and gamma emission information in a single digital image. In a pilot clinical diagnostic study, the system has demonstrated high pathology-proven accuracy in differentiating benign and malignant masses.