Spencer J. Cutler

Design and development of a fully 3D dedicated x-ray computed mammotomography system

Our effort to implement a volumetric x-ray computed mammotomography (CmT) system dedicated to imaging breast disease comprises: demonstrated development of a quasi-monochromatic x-ray beam providing minimal dose and other optimal imaging figures of merit; new development of a compact, variable field-of-view, fully-3D acquisition gantry with a digital flat-panel detector facilitating more nearly complete sampling of frequency space and the physical breast volume; incorporation of iterative ordered-subsets transmission (OSTR) image reconstruction allowing modeling of the system matrix. Here, we describe the prototype 3D gantry and demonstrate initial system performance. Data collected on the prototype gantry demonstrate the feasibility of using OSTR with realistic reconstruction times. The gantry consists of a rotating W-anode x-ray tube using ultra-thick K-edge filtration, and an ~20x25cm2 digital flat-panel detector located at <60cm SID. This source/detector combination can be shifted laterally changing the location of the central ray relative to the system center-of-rotation, hence changing the effective imaging field-of-view, and is mounted on a goniometric cradle allowing <50° polar tilt, then on a 360° azimuthal rotation stage. Combined, these stages provide for positioning flexibility in a banded region about a sphere, facilitating simple circle-plus-arc-like trajectories, as well as considerably more complex 3D trajectories. Complex orbits are necessary to avoid physical hindrances from the patient while acquiring the largest imaging volume of the breast. The system capabilities are demonstrated with fully-3D reconstructed images of geometric sampling and resolution phantoms, a fabricated breast phantom containing internal features of interest, and a cadaveric breast specimen. This compact prototype provides flexibility in dedicated, fully-3D CmT imaging of healthy and diseased breasts.

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.

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.

Characterizing the contribution of cardiac and hepatic uptake in dedicated breast SPECT using tilted trajectories

A small field of view, high resolution gamma camera has been integrated into a dedicated breast, single photon emission computed tomography (SPECT) device. The detector can be flexibly positioned relative to the breast and image beyond the chest wall, allowing the system to capture direct views of the heart and liver. The incomplete sampling of these organs creates artifacts in reconstructed images, complicating lesion detection. To understand the limits imposed on a 3D acquisition trajectory, sequential tilted trajectories at increasing polar tilt are utilized to collect data of anthropomorphic phantoms filled with aqueous (99m)Tc in a clinically realistic concentration ratio. The counts collected per projection between different scans and the SNR, contrast and resolution (FWHM) of two hot lesions were compared. As expected, the counts per projection increased when the camera had direct views of the heart and liver, but remained relatively constant at other angles. The SNR, contrast and FWHM were more affected by the insufficient sampling of the data by the large polar angles than by the cardiac and hepatic activity. An upper bound on polar tilt for each azimuthal position reduces the artifacts in the reconstructed images. Such trajectories were implemented to show artifact-free reconstructed images.

Dynamic laser-guided contouring for dedicated emission mammotomography

The dedicated breast CZT-based SPECT imaging system in our lab implements novel 3D camera trajectories that can minimize breast-detector separation, thus improving resolution and image quality. Current trajectories are manually customized for each patient by measuring breast-detector separations at several positions and interpolating. This study seeks to transition from this manual method to an automated contouring solution for routine patient SPECT imaging, given the vast array of uncompressed breast shapes in women.

Comparison of reduced angle and fully 3D acquisition sequencing and trajectories for dual-modality mammotomography

A dual-modality SPECT-CT system for dedicated 3D breast cancer imaging is under development. Independent dedicated SPECT and CT imaging systems have been integrated onto a single gantry for uncompressed breast imaging. This study examines challenges and tradeoffs involved in integrating the acquisition procedures of two independent imaging systems into a single imaging protocol. The physical limitation of the rotating CT tube beneath the custom patient bed currently provides only a 294 degree scan with the bed low enough for the breast to be in the cone-beam CT field-of-view. The directly coupled SPECT system is therefore also limited if the scans are to be taken simultaneously or in an interleaved fashion. Thus, geometric phantoms are imaged to characterize image degradations due to reduced projection angles for both modalities. Two different acquisitions were performed: one with the central ray of the CT cone-beam aligned with the systems center of rotation and one offset from the center of rotation by 5 cm. Various sized activity- filled lesions in an anthropomorphic breast phantom were imaged, first with uniform aqueous background activity and then with added acrylic pieces to simulate a non-uniform background. Interleaving the SPECT and CT acquisitions into a single scan was also investigated. Iterative reconstruction algorithms are used to reconstruct the data, and the SPECT and CT images are co-registered. Both the cold rod and breast data indicate that removing 75deg of SPECT azimuth al data does not significantly reduce image quality. CT images were also minimally affected if the cone-beam is centrally aligned with the center of rotation, but degraded with the laterally offset cone-beam setup. In the course of these experiments, the patient bed was reconfigured with a larger central hole covered with flexible neoprene, gaining the ability to rotate completely around the breast and dramatically improving CT projection views through the chest wall.

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

Towards Quantification of Functional Breast Images Using Dedicated SPECT With Non-Traditional Acquisition Trajectories

Quantification of radiotracer uptake in breast lesions can provide valuable information to physicians in deciding patient care or determining treatment efficacy. Physical processes (e.g., scatter, attenuation), detector/collimator characteristics, sampling and acquisition trajectories, and reconstruction artifacts contribute to an incorrect measurement of absolute tracer activity and distribution. For these experiments, a cylinder with three syringes of varying radioactivity concentration, and a fillable 800 mL breast with two lesion phantoms containing aqueous 99mTc pertechnetate were imaged using the SPECT sub-system of the dual-modality SPECT-CT dedicated breast scanner. SPECT images were collected using a compact CZT camera with various 3D acquisitions including vertical axis of rotation, 30° tilted, and complex sinusoidal trajectories. Different energy windows around the photopeak were quantitatively compared, along with appropriate scatter energy windows, to determine the best quantification accuracy after attenuation and dual-window scatter correction. Measured activity concentrations in the reconstructed images for syringes with greater than 10 μCi/mL corresponded to within 10% of the actual dose calibrator measured activity concentration for ±4% and ±8% photopeak energy windows. The same energy windows yielded lesion quantification results within 10% in the breast phantom as well. Results for the more complete complex sinsusoidal trajectory are similar to the simple vertical axis acquisition, and additionally allows both anterior chest wall sampling, no image distortion, and reasonably accurate quantification.

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.