In current dedicated breast computed tomography (mammotomography) systems, comfortable patient positioning on a
stationary bed restricts the practicable range of source-detector trajectories, thus compromising the system's ability to
adequately image the patient's anterior chest wall. This study examines the effect on detecting small, low-contrast
lesion-like-spheres using limited angle x-ray source-detector trajectories and trajectories that intentionally raise the
tomographic imaging system mid-acquisition. These modified acquisition paths may increase chest wall visualization,
simplify the design of the imaging system and increase patient comfort by allowing the design of an improved patient
bed. Thin walled balloons of various volumes filled with iodine act as surrogate high contrast lesions to initially
investigate the effect of these novel trajectories. Then, stacks of 5mm acrylic spheres regularly spaced in concentric
circles are placed in water to simulate a low contrast environment in a uniform scatter medium. 360° azimuthal scans are
acquired at various bed heights with contiguous projections subsequently removed to create limited angle acquisitions
from 240-360°. Projections from the different bed heights are interwoven to form trajectories that mimic discontinuously
raising the imaging system mid-acquisition. The resulting iteratively reconstructed volumes are evaluated with an
observer study. Initial images suggest that using limited angles and raising the system is possible while increasing the
observer's ability to visualize objects near the chest wall. Based on the results of this study, an improved patient bed to
facilitate chest wall imaging will be designed, and the feasibility of vertical system motion to increase imaged breast
A hybrid, dual modality single photon emission computed tomography (SPECT) and x-ray computed mammotomography (CmT) scanner for dedicated breast and axillary imaging is under development. CmT imaging provides high resolution anatomical images, whereas SPECT provides functional images albeit with coarser resolution. As is being seen clinically in whole body imaging, integration of the images is expected to enhance (visually) and improve (with attenuation correction of SPECT) information provided by either modality for the detection, characterization and potentially staging of breast cancer. The registration of these images considers variations in object positions between the different modalities and imaging parameters (pixel size, conditions of acquisition, scan limitations). Automatic methods can be used which find the geometric transformations of the different imaging modalities involved. Here we demonstrate the initial stages of iterative 2-dimensional registration and fusion of SPECT with parallel beam geometry and CmT with offset cone-beam acquisition geometry for mammotomography with images acquired and reconstructed independently on each system. Two registration algorithms are considered: the first is an intrinsic correlation, Mutual Information (MI) method based on intrinsic image content; the second is a rigid body transform method, Iterative Closest Point (ICP) method based on identification of fiducial markers visible to both emission (SPECT) and transmission (CmT) imaging modalities. Experiments include use of a geometric resolution/frequency phantom imaged under different conditions, and two different anthropomorphic breast phantom sizes (325 and 935mL). Initial results with the geometric phantom demonstrate that MI can be misled by highly symmetric features, and ICP using control points is more accurate to within fractions of a voxel. Initial breast phantom studies indicate that object size and SPECT resolution limitations may contribute to registration errors.
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 ~20x25cm<sup>2</sup> 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.
With the development of several classes of dedicated emission and transmission imaging technologies utilizing ionizing radiation for improved breast cancer detection and in vivo characterization, it is extremely useful to have available anthropomorphic breast phantoms in a variety of shapes, sizes and malleability prior to clinical imaging.
These anthropomorphic phantoms can be used to evaluate the implemented imaging approaches given a known quantity, the phantom, and to evaluate the variability of the measurement due to the imaging system chain. Thus, we have developed a set of fillable and incompressible breast phantoms ranging in volume from 240 to 1730mL with nipple-to-chest distances from 3.8 to 12cm. These phantoms are mountable and exchangeable on either a uniform chest plate or anthropomorphic torso phantom containing tissue equivalent bones and surface tissue. Another fillable ~700mL breast phantom with solid anterior chest plate is intentionally compressible, and can be used for direct comparisons between standard planar imaging approaches using mild-to-severe compression, partially compressed tomosynthesis, and uncompressed computed mammotomography applications. These phantoms can be filled with various fluids (water and oil based liquids) to vary the fatty tissue background composition. Shaped cellulose sponges with two cell densities are fabricated and can be added to the breasts to simulate connective tissue. Additionally,
microcalcifications can be simulated by peppering slits in the sponges with oyster shell fragments. These phantoms have a utility in helping to evaluate clinical imaging paradigms with known input object parameters using basic imaging characterization, in an effort to further evaluate contemporary and next generation imaging tools. They may additionally provide a means to collect known data samples for task based optimization studies.