Douglas Kieper

Optimizing pinhole and parallel hole collimation for scintimammography with compact pixellated detectors

The relative resolution and sensitivity advantages of pinhole and parallel hole collimators for planar scintimammography with compact, pixellated gamma detectors were investigated using analytic models. Collimator design was studied as follows. A desired object resolution was specified for a pixellated detector with a given crystal size and intrinsic spatial resolution and for a given object-to-collimator distance. Using analytic formulas, pinhole and parallel hole collimator parameters were calculated that satisfy this object resolution with optimal geometric sensitivity. Analyses were performed for 15 cm /spl times/ 20 cm field of view detectors with crystal elements 1.0, 2.0, and 3.0 mm on a side and 140 keV incident photons. The sensitivity for a given object resolution was greater for pinhole collimation at smaller distances, as expected. The object distance at which the pinhole and parallel hole sensitivity curves cross each other is important. The crossover distances increased with larger crystal size for a constant object resolution and increased as the desired object resolution decreased for a constant crystal size. For example, for 4 mm object resolution and a pinhole collimator with focal length 13 cm, these distances were 5.5 cm, 6.5 cm, and 8 cm for the 1 mm, 2 mm, and 3 mm crystal detectors, respectively. The results suggest a strategy of parallel hole collimation for whole breast imaging and pinhole collimation for imaging focal uptake. This could be accomplished with a dual detector system with a different collimator type on each head or a single head system equipped with two collimators and a rapid switching mechanism.

Analysis of factors affecting positron emission mammography (PEM) image formation

Image reconstruction for positron emission mammography (PEM) with the breast positioned between two parallel, planar detectors is usually performed by backprojection to image planes. Three important factors affecting PEM image reconstruction by backprojection are investigated: 1) image uniformity (flood) corrections, 2) image sampling (pixel size) and 3) count allocation methods. An analytic expression for uniformity correction is developed that incorporates factors for spatial-dependent detector sensitivity and geometric effects from acceptance angle limits on coincidence events. There is good agreement between experimental floods from a PEM system with a pixellated detector and numerical simulations. The analytic uniformity corrections are successfully applied to image reconstruction of compressed breast phantoms and reduce the necessity for flood scans at different image planes. Experimental and simulated compressed breast phantom studies show that lesion contrast is improved when the image pixel size is half of, rather than equal to, the detector pixel size, though this occurs at the expense of some additional image noise. In PEM reconstruction counts usually are allocated to the pixel in the image plane intersected by the line of response (LOR) between the centers of the detection pixels. An alternate count allocation method is investigated that distributes counts to image pixels in proportion to the area of the tube of response (TOR) connecting the detection pixels that they overlay in the image plane. This TOR method eliminates some image artifacts that occur with the LOR method and increases tumor signal-to-noise ratios at the expense of a slight decrease in tumor contrast. Analysis of image uniformity, image sampling and count allocation methods in PEM image reconstruction points to ways of improving image formation. Further work is required to optimize image reconstruction parameters for particular detection or quantitation tasks.

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.

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.

A large field of view positron emission mammography imager

Dedicated positron emission mammography (PEM) systems provide a potentially high sensitivity, high-resolution, low attenuation and lower cost alternative to whole body PET. We have designed, built, and performed initial evaluation of a large field-of-view PEM system. This system uses two 15 cm /spl times/ 20 cm planar detectors. Each detector has an array of 3 mm /spl times/ 3 mm /spl times/ 10 mm LGSO (90% LSO, 10% GSO) scintillator crystals read out by an 6 /spl times/ 8 array of 25 mm /spl times/ 25 mm position-sensitive photomultipliers. The detectors are mounted on a gantry that is used in conjunction with an X-ray mammography unit such that when the film holder is rotated away the upper detector is above the compression paddle, which is used to compress the breast against the lower PEM detector.

Detecting infiltrating lobular carcinoma using scintimammographic breast specific gamma imaging.

UNLABELLED This study evaluated the effectiveness of scintimammography performed with gamma cameras optimized for breast imaging in the detection of infiltrating lobular carcinoma. This new procedure, Breast Specific Gamma Imaging (BSGI), was conducted on 105 patient presenting with 113 breast lesions. Studies were conducted at two medical centers using three prototype cameras [14, 16]. Biopsy and pathology reports were obtained for all cases and, of the 34 detected carcinomas, 6 were determined to be infiltrating lobular type without mixed component other than lobular carcinoma in situ. Of the 6 lesions, 4 were smaller than 1 cm, the smallest moasuring 3 mm at biopsy. BSGI detected all 6 of the lobular carcinomas and correctly identified the secondary lesion in the only multifocal case. The BSGI foci sizes matched the lesion size at biopsy to within +/-5.5 mm, with about an equal number of cases ovar and under estimated. CONCLUSION BSGI provides an effective tool for the detection of lobular carcinoma and in the determination of lesion size and multifocality.

Improved lesion visibility in a dedicated dual head scintimammography system - phantom results

Phantom test results are presented simulating a dedicated dual head compact gamma imaging system for scintimammography. These studies were completed with various detector prototypes constructed from pixellated scintillators coupled to various arrays of position sensitive photomultiplier tubes. A 15cm /spl times/ 20cm dual-head prototype system is currently under development that will provide sufficient imaging area for breast imaging studies. The results indicate that the use of two detector heads placed on opposing sides of breast under mild compression substantially improves the lesion contrast regardless of lesion depth in the tissue (especially for small, centrally located lesions). In addition, there is significantly improved lesion signal by using a geometric mean technique, Square Root (Image 1* Image2), to combine the images from the opposing views. In count poor cases, a smoothing kernel was applied prior to image multiplication to reduce high spatial frequency noise.

Data analysis methods for a small field-of-view combined scintimammography/digital X-ray system in breast lesion management

Abstract Recently, a multidisciplinary research collaboration began a clinical study employing a dual modality, small field-of-view breast imaging system. The system, comprised of a mini gamma camera and digital X-ray detector, was designed to examine the possible clinical advantages of dual modality breast imaging in the management of patients with suspicious findings (BIRADS 3–5) from a screening mammogram. In addition, dynamic time–resolution studies of radiotracer uptake and washout were evaluated for their value in differentiation of lesion type. This preliminary report focuses on the development and implementation of these techniques and presents patient data as evidence of their effectiveness. The results of this study indicate that applying these techniques may significantly improve the diagnostic value of scintimammography by increasing specificity to 97.7% by differentiation of true positive and false positive lesions.

Breast lesion dynamic radiotracer uptake quantification method utilizing a combined digital X-ray/gamma system

Recently, a multidisciplinary research collaboration began a clinical study employing a dual modality, small field-of-view (SFOV) breast imaging system. The system, comprised of a mini gamma camera and digital X-ray detector, was designed to provide a spatially coregistered image containing both X-ray and radiotracer data. Images were evaluated to quantify the spatial correlation of X-ray structures and radiotracer uptake distribution.