Priti Madhav

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.

Evaluation of tilted cone-beam CT orbits in the development of a dedicated hybrid mammotomograph

A compact dedicated 3D breast SPECT-CT (mammotomography) system is currently under development. In its initial prototype, the cone-beam CT sub-system is restricted to a fixed-tilt circular rotation around the patients pendant breast. This study evaluated stationary-tilt angles for the CT sub-system that will enable maximal volumetric sampling and viewing of the breast and chest wall. Images of geometric/anthropomorphic phantoms were acquired using various fixed-tilt circular and 3D sinusoidal trajectories. The iteratively reconstructed images showed more distortion and attenuation coefficient inaccuracy from tilted cone-beam orbits than from the complex trajectory. Additionally, line profiles illustrated cupping artifacts in planes distal to the central plane of the tilted cone-beam, otherwise not apparent for images acquired with complex trajectories. This indicates that undersampled cone-beam data may be an additional cause of cupping artifacts. High-frequency objects could be distinguished for all trajectories, but their shapes and locations were corrupted by out-of-plane frequency information. Although more acrylic balls were visualized with a fixed-tilt and nearly flat cone-beam at the posterior of the breast, 3D complex trajectories have less distortion and more complete sampling throughout the reconstruction volume. While complex trajectories would ideally be preferred, negatively fixed-tilt source-detector configuration demonstrates minimally distorted patient images.

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.

Investigation of cone-beam acquisitions implemented using a novel dedicated mammotomography system with unique arbitrary orbit capability (Honorable Mention Poster Award)

We investigate cone-beam acquisitions implemented on a novel dedicated cone-beam transmission computed mammotomography (CmT) system with unique arbitrary orbit capability for pendant, uncompressed breasts. We use a previously reported optimized quasi-monochromatic beam technique together with orbits made possible with a novel CmT gantry system, to evaluate Vertical-Axis-Of-Rotation (VAOR), Circle-Plus-Two-Arcs (CP2A), and Saddle trajectories. Aquisition parameters include: W target, 60 kVp tube potential, 100th VL Nd filtration, 1.25 mAs, 55 cm SID, CsI(Tl) digital flat panel x-ray detector, and 7.7cm diameter uniform disc (Defrise) and resolution phantoms. Complex orbits were also performed for a realistic breast phantom. Reconstructions used an iterative ordered subsets transmission (OSTR) algorithm with 4x4 binned projections, 8 subsets, and 10 iterations, with 0.125 mm3 voxels. We evaluate the results for image artifacts, distortion, and resolution. Reconstructed images of the disc coronal and sagittal slices show significant distortion of the discs and phantom interfaces away from the central plane of the cone-beam for VAOR, less distortion for CP2A, and minimal distortion for the complex 3D Saddle orbit. Resolution phantoms indicate no loss of resolution with the Saddle orbit, with the smallest 1.1mm diameter rods clearly resolved. Other image artifacts such as streaking were also significantly reduced in the Saddle orbit case. Results indicate that arbitrary orbits of pendant uncompressed breasts using cone-beam acquisitions and OSTR iterative reconstructions can be successfully implemented for dedicated CmT to improve angular sampling with significant reduction in distortion and other image artifacts. This capability has the potential to improve the performance of dedicated CmT by adequately sampling the breast and anterior chest volumes of prone patients with pendant, uncompressed breasts.

Initial Development of a Dual-Modality SPECT-CT System for Dedicated Mammotomography

Dual-modality systems offer great promise in improving detection and evaluation of cancer through enhancement of the visual quality and quantitative accuracy of radionuclide imaging. A compact SPECT-CT system for dedicated 3D breast imaging is in development. The SPECT components include a 16times20cm2 CZT-based compact gamma camera with 2.5mm square pixels, attached to a goniometer with polar positioning capability. The CT component includes a heavily filtered W-target X-ray source producing a quasi-monochromatic cone beam and CsI(Tl) digital detector. Both systems are coupled to a common rotation stage and have a common field of view. The CT system has a stationary polar orientation, and is laterally offset from the center of rotation for imaging pendant uncompressed breasts that are larger than the detectors field of view. The constraints and performance of this initial configuration are being investigated. Previous measurements with combined system components show that optimal placement is limited by physical constraints rather than signal cross-contamination. Results on a combined system show that emission projection images are contaminated by X-ray scatter photons resulting in the reconstructed emission images having a higher signal level. Emission contamination also increases noise in the transmission image resulting in reducing the SNR in reconstructed CT images. Having both modalities on a single gantry is expected to simplify data acquisition, SPECT-CT image registration, and necessary image corrections.

Investigating novel patient bed designs for use in a hybrid dual modality dedicated 3D breast imaging system

A hybrid SPECT-CT system for dedicated 3D breast cancer imaging (mammotomography) is in development. Using complex 3D imaging acquisition trajectories, the versatile integrated system will be capable of contouring and imaging an uncompressed breast suspended in a 3D volume located below a radio-opaque patient bed, providing co-registered volumetric anatomical and functional information. This study examines tradeoffs involved in the design of the patient bed to satisfy concomitant and competing technical and ergonomic requirements specific to this imaging paradigm. The complementary source-detector arrangement of the CT system is geometrically more restrictive than that of the single detector SPECT system. Additionally, the compact dimensions and size of the CT system components (primarily the x-ray tube) are key constraints on the bed design and so the focus is concentrated there. Using computer-aided design software, several design geometry options are examined to simultaneously consider and optimize the following parameters: image magnification, imaged breast volume, azimuthal imaging span, and patient comfort. Several CT system source to image distances are examined (55-80cm), as well as axial patient tilt up to 35°. An optimal patient bed design for a completely under-bed hybrid imaging system was determined. A 60cm SID, magnification factor of ~1.5, and patient bed angled at ~15° provided the optimal dimensions. Additional bed dimensions allow the CT projection beam to nearly entirely image the chest wall, however at the cost of reduced angular sampling for CT. Acquired x-ray mammotomographic image data is used to assess the feasibility of this reduced angle acquisition approach.

Patient Bed Design for an Integrated SPECT-CT Dedicated Mammotomography System

This study investigates the significant challenges and tradeoffs associated with designing a patient positioning system for a novel, compact SPECT-CT system dedicated to 3D breast and chest wall imaging. The primary challenge is to maintain patient comfort while optimally positioning the patient for maximal chest wall imaging, given the physical and geometric dimensions of the dual-modality system. The dedicated breast SPECT system comprises a 16times20cm2 CZT-based gamma camera attached to a versatile gantry, capable of varying polar tilt, radial distance and azimuth. The dedicated breast cone beam CT system, initially designed here without dynamic polar tilt, includes a tungsten target source and a 20times25cm2 CsI(Tl) detector laterally offset from the central ray, separated by a 60cm SID. The integrated imaging system with a common field-of-view is positioned below the custom-designed, 3-dimensionally flexible bed positioning system. An additional consideration of the bed is that it is radio-opaque, which minimizes both emission contamination from the patients body into the transmission CT system, while simultaneously minimizing any X-ray scatter onto the patients body. This preliminary implementation and investigation indicates the need for novel camera trajectories to satisfy both chest wall proximity imaging and patient comfort. The effects of breast volume imaged within the common FOV and table angles, which are assumed to correspond to patient comfort, are examined. A manufactured galvanized steel prototype demonstrates the practicability of the bed.

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.

A novel method to characterize the MTF in 3D for computed mammotomography

A novel phantom has been developed to measure the modulation transfer function (MTF) in 3D for x-ray computed tomography. The phantom consists of three tungsten wires, positioned nearly orthogonal to each other. Simultaneous measurements of the MTF are taken at various locations along the three orthogonal reconstructed planes. Our computed mammotomography (CmT) system uses a Varian Paxscan 2520 digital x-ray detector which can be positioned anywhere in ~2pi steradian band and can have arbitrary trajectories. With a half-cone beam geometry and with the phantom positioned near the center of rotation, projection images are acquired over 360 degrees. Various 3D orbits are evaluated including vertical axis of rotation and saddle. Reconstructions were performed using an iterative ordered-subsets transmission algorithm on rebinned projection images, using various numbers of iterations. Rotation of reconstructed slices isolated each wire into its own plane. At various locations along the length of each wire, corresponding MTFs were calculated from 1D line spread functions. Through measurement, accuracy of wire method was verified by comparison of the projection MTFs computed from a wire and a standard edge device. Results indicated minor variations in MTF among the three orthogonal planes, which imply a high degree of uniform sampling in the imaged volume. Findings indicate that the phantom can be used to assess the intrinsic image resolution in 3D as well as potential degradative effects of measurements in various media.

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.