Caryl N. Brzymialkiewicz

Evaluation of fully 3-D emission mammotomography with a compact cadmium zinc telluride detector

A compact, dedicated cadmium zinc telluride (CZT) gamma camera coupled with a fully three-dimensional (3-D) acquisition system may serve as a secondary diagnostic tool for volumetric molecular imaging of breast cancers, particularly in cases when mammographic findings are inconclusive. The developed emission mammotomography system comprises a medium field-of-view, quantized CZT detector and 3-D positioning gantry. The intrinsic energy resolution, sensitivity and spatial resolution of the detector are evaluated with Tc-99m (140 keV) filled flood sources, capillary line sources, and a 3-D frequency-resolution phantom. To mimic realistic human pendant, uncompressed breast imaging, two different phantom shapes of an average sized breast, and three different lesion diameters are imaged to evaluate the system for 3-D mammotomography. Acquisition orbits not possible with conventional emission, or transmission, systems are designed to optimize the viewable breast volume while improving sampling of the breast and anterior chest wall. Complications in camera positioning about the patient necessitate a compromise in these two orbit design criteria. Image quality is evaluated with signal-to-noise ratios and contrasts of the lesions, both with and without additional torso phantom background. Reconstructed results indicate that 3-D mammotomography, incorporating a compact CZT detector, is a promising, dedicated breast imaging technique for visualization of tumors <1 cm in diameter. Additionally, there are no outstanding trajectories that consistently yield optimized quantitative lesion imaging parameters. Qualitatively, imaging breasts with realistic torso backgrounds (out-of-field activity) substantially alters image characteristics and breast morphology unless orbits which improve sampling are utilized. In practice, the sampling requirement may be less strict than initially anticipated.

Performance of dedicated emission mammotomography for various breast shapes and sizes

We evaluate the effect of breast shape and size and lesion location on a dedicated emission mammotomography system developed in our lab. The hemispherical positioning gantry allows ample flexibility in sampling a pendant, uncompressed breast. Realistic anthropomorphic torso (which includes the upper portion of the arm) and breast phantoms draw attention to the necessity of using unique camera trajectories (orbits) rather than simple circular camera trajectories. We have implemented several novel three-dimensional (3D) orbits with fully contoured radius-of-rotation capability for compensating for the positioning demands that emerge from different breast shapes and sizes. While a general orbit design may remain the same between two different breasts, the absolute polar tilt range and radius-of-rotation range may vary. We have demonstrated that using 3D orbits with increased polar camera tilt, lesions near the chest wall can be visualized for both large and small sized breasts (325 ml to 1,060 ml), for a range of intrinsic contrasts (three to ten times higher activity concentration in the lesion than breast background). Overall, nearly complete 3D acquisition schemes yield image data with relatively high lesion SNRs and contrasts and with minimal distortion of the uncompressed breast shape.

Analysis of a novel offset cone-beam computed mammotomography system geometry for accomodating various breast sizes.

We evaluate a newly developed dedicated cone-beam transmission computed mammotomography (CmT) system configuration using an optimized quasi-monochromatic cone beam technique for attenuation correction of SPECT in a planned dual-modality emission and transmission system for pendant, uncompressed breasts. In this study, we perform initial CmT acquisitions using various sized breast phantoms to evaluate an offset cone-beam geometry. This offset geometry provides conjugate projections through a full 360 degree gantry rotation, and thus yields a greatly increased effective field of view, allowing a much wider range of breast sizes to be imaged without truncation in reconstructed images. Using a tungsten X-ray tube and digital flat-panel X-ray detector in a compact geometry, we obtained initial CmT scans without shift and with the offset geometry, using geometrical frequency/resolution phantoms and two different sizes of breast phantoms. Acquired data were reconstructed using an ordered subsets transmission iterative algorithm. Projection images indicate that the larger, 20 cm wide, breast requires use of a half-cone-beam offset scan to eliminate truncation artifacts. Reconstructed image results illustrate elimination of truncation artifacts, and that the novel quasi-monochromatic beam yields reduced beam hardening. The offset geometry CmT system can indeed potentially be used for structural imaging and accurate attenuation correction for the functional dedicated breast SPECT system.

3D data acquisition sampling strategies for dedicated emission mammotomography for various breast sizes

The dedicated emission mammotomography system developed in our lab is in preparation for initial patient studies. As a preliminary step, we evaluate the effect of breast size and lesion location on this paradigm. The hemispherical positioning gantry allows ample flexibility in sampling a pendant, uncompressed breast. Recently acquired, realistic anthropomorphic torso (which includes the upper portion of the arm) and breast phantoms emphasize the necessity of employing unique camera trajectories (orbits) rather than simple VAOR camera trajectories. Several novel 3D orbits have been implemented with fully contoured radius-of-rotation capability to compensate for the positioning demands that are required for different breast sizes. While a general orbit design may remain the same between two different breasts, the absolute polar tilt range and ROR range may vary. We have demonstrated that with increased polar camera tilt, employing 3D data acquisition camera trajectories, lesions near the chest wall can he visualized for both large and small sized breasts.

Investigation of micro-columnar scintillators on an optical fiber coupled compact imaging system

A compact imaging system with a novel front-end detector is under investigation and development. Unique aspects of this collimatorless system include the use of thin arrays of many thousands of microcolumnar (<10 /spl mu/m diameter) CsI front-end scintillators that are coupled through a four-times reducing fiber-optic (FO) bundle to a metal-channel multianode position sensitive photodetector. The tested arrays are 140 or 200 /spl mu/m tall on faceplates of plane glass, FO and FO with statistical extramural absorbers (EMAs). The highly discrete nature of the scintillator microcolumn arrays ensures very fine intrinsic spatial resolution, limited by the particle penetration and backscatter in the detector assembly. Their retro-reflector-tipped front ends facilitate light propagation toward the photodetector, ensuring good light collection, Monte Carlo simulations confirmed the limiting nature of beta particle penetration on measurable resolution. With this system, absolute light output was higher for the taller arrays, which indicates that these sizes are below the optimum for light output and energy absorption from the energetic beta particles; even taller scintillators, however, would suffer from increased backgrounds from annihilation radiation with positron detection. While MTF measurements with an X-ray source and microslit indicate the best response with the arrays on FO+EMA substrates, measurements with high and medium (1.7 MeV and 635 keV) energy beta line sources yield the best responses with the plane glass substrate, indicating that energy thresholding affects resolution in the classical way, even with these highly miniaturized arrays. Experiments with complex positron emission distributions along with large gamma-ray backgrounds, as may be expected during surgery, yield images with small background contamination and no distortions.

Comparison of compact gamma cameras with 1.3 mm and 2.0 mm quantized elements for dedicated emission mammotomography

In an effort to image smaller breast lesions, two compact gamma cameras with different intrinsic NaI(Tl) pixel sizes are evaluated for use in the application specific emission tomography system for mammotomographic emission imaging. Comparison measurements were made with two scintillator arrays with 1.3/spl times/1.3/spl times/6mm/sup 3/ or 2.0/spl times/2.0/spl times/6mm/sup 3/ elements on exactly the same set of PMTs, electronics and control/processing hardware. Uniformity, sensitivity and energy resolution were assessed with flood field phantoms. Spatial resolution measurements included: a /sup 99m/Tc (140 keV) activity filled capillary tube imaged in planar mode from 1 - 10 cm distance; two such tubes separated by 2 cm were also imaged with simple circular tomography from 3 - 7 cm radii-of-rotation (RORs); and a /sup 99m/Tc filled mini-cold rod phantom was imaged at 5 cm ROR with a simple circular orbit. Finally, a freely suspended and uniformly filled 950 ml breast phantom containing four fillable lesions (4 - 10 mm dia) was imaged with a lesion-to-uniform-background activity concentration ratio of 15:1, using simple and complex 3D orbits and minimal RORs. The measured sensitivity varied by the crystal fill-factor; uniformity had <4% variability; and mean energy resolutions of each camera were /spl les/12% full-width at half-maximum (FWHM). The planar spatial resolutions correspond to calculated values, with smaller pixels yielding 2 - 13% better resolution with decreasing distance, corroborating the change from collimator-limited to intrinsic resolution with decreasing separation distance; tomographic results ranged from 3.2 - 5.2 mm FWHM at 3 - 7cm, with nominally better contrast-resolution for the smaller pixel camera. Consistent with signal detection characteristics for these measurement conditions, quantitative SNRs and contrasts from lesion imaging with the uniform breast background illustrate better overall performance under nearly all conditions and for all lesions for the larger pixel camera.

Measurements of an optimized beam for x-ray computed mammotomography

Simulation results from previous studies indicate that a quasi-monochromatic x-ray beam can be produced using a newly developed beam filtration technique. This technique utilizes heavy filtration with novel high Z filter materials having k-edges just above those of CsI, producing a near monochromatic beam with mean energy optimized for detection. The value of a near monochromatic x-ray source for a fully 3D tomography application is the expected improved ability to separate tissues with very small differences in attenuation coefficients for a range of uncompressed breast sizes while maintaining dose levels at or below existing dual view mammography. In this study, we experimentally investigate a set of filter materials (Al, Cu, Ag, Ce, W, and Pb), filter thicknesses (10th, 100th, and 200th VL), and tube potentials (40-80 kVp) using a newly constructed test apparatus. Initial experimental results corroborate simulations and indicate that this approach can improve image quality (SNR) at constant dose. Al, Cu, W, and Pb provide optimal exposure efficiency results at 60 kVp and above. Decreasing relative improvements are observed above 100th VL filter thickness at 78 cm SID. Results are obtained without significant tube heating (except at 40 kVp). In addition, simulations indicate significant reductions in beam hardening. This optimized beam will be incorporated into a novel cone-beam x-ray computed mammotomography sub-system together with an emission tomograph in a dual modality CT/SPECT application specific emission and transmission tomography system for fully 3D uncompressed breast imaging.

Comparison of scintimammography and dedicated emission mammotomography

Using a 16 cmtimes20 cm medium field of view CZT camera and a compressible breast phantom containing deformable lesions of various sizes and activity concentrations, a detailed comparison is made between 2D, planar scintimammography utilizing various degrees of breast compression and fully 3D, dedicated, uncompressed breast SPECT, or emission mammotomography. A 700 mL compressible anthropomorphic breast phantom attached to a chest plate was developed in order to compare 2D and 3D emission (or transmission) imaging of a breast containing small lesions in the same phantom, while providing physical attributes that mimic realistic imaging conditions including hindrances that could limit otherwise ideal imaging of an isolated breast phantom. Thin walled, tillable, deformable lesions from 40 to 500 microL volume suspended on narrow polyethylene tubing are used so that their shape would change with different degrees of breast compression and also to provide minimal lesion wall and support thicknesses. Experiments were performed with low noise, and lesion-to-background concentration ratios range from 3:1 to 12:1. Scintimammography is performed for equivalent times for compression thicknesses from 6 cm to 12 cm (fully uncompressed) using a single medio-lateral view, and mammotomography is performed for the uncompressed breast for vertical axis of rotation, simple tilted parallel beam, and a trajectory based on a 3-lobed sinusoid projected onto a hemisphere. Image quality, based on lesion SNRs and contrasts, as well as degree of sampled breast volume are evaluated. Dedicated mammotomography appears to be nearly twice as effective as planar scintimammography under these measurement conditions

Investigating the effects of energy resolution in dedicated emission mammotomography

This study probes the recent debate over the necessity for good energy resolution for uncompressed breast, 3D lesion imaging with dedicated single photon emission mammotomography. Here, the imaging system consists of a commercial, discretized CZT gamma camera having ~6% FWHM intrinsic energy resolution (at 140 keV) and intrinsic spatial resolution corresponding to the 2.5 mm square pixilation, and is used on a fully-3D positioning gantry. Wider energy windows are used on list mode acquired data as a surrogate for having otherwise identical detection systems with poorer energy resolution characteristics. Scans using simple circular trajectories are first obtained of an aqueous Tc-99m filled mini resolution cold-rod phantom at various radii-of-rotation, and also immersed in a larger uniform water bath. Multiple 3D orbits about Tc-99m filled anthropomorphic breast and torso phantoms are acquired, with the breast containing two large lesions. The list mode data files were multiply processed to obtain images of varying energy window widths (from symmetric 6% to an asymmetric 18% (-12+6)) but with the same projection image count density. Counts were randomly subsampled from the entire list mode data set in order to maintain equivalent levels of count density for several bootstrap realizations. All data was then reconstructed using OSEM for various iterations. Profiles were obtained from the cold rod images, and regions of interest were drawn in and about the spherical lesions to determine signal-to-noise ratios and contrasts for each iteration. Results clearly illustrate both visual and quantitative differences between the various energy windows, with smaller energy windows (corresponding to better energy resolution) having better image quality

Characterizing the MTF in 3D for a Quantized SPECT Camera Having Arbitrary Trajectories

The emergence of application-specific 3D tomographic small animal and dedicated breast imaging systems has stimulated the development of simple methods to quantify the spatial resolution or modulation transfer function (MTF) of the system in three dimensions. Locally determined MTFs, obtained from line source measurements at specific locations, can characterize spatial variations in the system resolution and can help correct for such variations. In this study, a method is described to measure the MTF in 3D for a compact SPECT system that uses a 16 times 20 cm2 CZT-based compact gamma camera and 3D positioning gantry capable of moving in different trajectories. Image data are acquired for a novel phantom consisting of three radioactivity-filled capillary tubes, positioned nearly orthogonally to each other. These images provide simultaneous measurements of the local MTF along three dimensions of the reconstructed imaged volume. The usefulness of this approach is shown by characterizing the MTF at different locations in the reconstructed imaged 3D volume using various (1) energy windows; (2) iterative reconstruction parameters including number of iterations, voxel size, and number of projection views; (3) simple and complex 3D orbital trajectories including simple vertical axis of rotation, simple tilt, complex circle-plus-arc, and complex sinusoids projected onto a hemisphere; and (4) object shapes in the cameras field of view. Results indicate that the method using the novel phantom can provide information on spatial resolution effects caused by system design, sampling, energy windows, reconstruction parameters, novel 3D orbital trajectories, and object shapes. Based on these measurements that are useful for dedicated tomographic breast imaging, it was shown that there were small variations in the MTF in 3D for various energy windows and reconstruction parameters. However, complex trajectories that uniformly sample the breast volume of interest were quantitatively shown to have slightly better spatial resolution performance than more simple orbits.