Breast cancer imaging has improved over the last decade with higher and more uniform quality standards for mammography as well as through the increasing use of sonography and magnetic resonance imaging as the adjunct tools. Mammography is still the only screening tool to detect breast cancer for asymptomatic women. Due to the limitations associated with the aforementioned techniques, such as imaging of the overlapping structure with mammography, technician dependent lack of ability to detect calcifications with ultrasound, and low specificity and/or poor detection of the tiny calcium deposits with MRI, there remains an endeavor to explore new ways to better detect breast cancer.
Recently one of the most exciting ways is cone beam breast CT (CBBCT) technology [1
]. It is based on a flat panel detector, and with only one circular rotation or some other scanning path, it can provide the three-dimensional density distribution of the breast, thus greatly eliminating the imaging problem of the structure overlapping seen in mammography to enhance the contrast resolution. It has been shown that the average glandular doses of CBBCT is equivalent to mammography [5
]; so this technology might have the potential to replace mammography for breast cancer screening and diagnosis.
Among all CBBCT technologies, FDK [7
] algorithm-based circular scan scheme possesses the following advantages: a stable and simple mechanical configuration, motion artifacts reduction, computation efficiency, and so forth. However, since a single circular source trajectory does not satisfy the data sufficient condition [9
], the FDK algorithm will unavoidably induce some artifacts such as an intensity drop along the rotation axis and object geometric distortion in the area further away from the circular scanning plane when cone angle becomes large. In order to overcome these cone beam artifacts, we propose the circle plus partial helical (CH) scan scheme based on the idea that by partially filling the object support in the Radon domain (i.e., the well-known torus in 3D Radon domain) where the circular scan does not touch through the additional scanning trajectory (such as a partial helical orbit), we can acquire more information than from just a single circular scan. The idea behind the partial helical scan is to improve the image quality by correcting the aforementioned artifacts to a certain degree while not exposing the patient with too much radiation exposure. In order to maintain computation efficiency, a filtered backprojection (FBP) method is employed for the reconstruction part associated with partial helical scan.
Recently, Katsevich and Kapralov [10
] proposed a circle plus general curve scan algorithm for exact reconstruction, which is also of FBP type; moreover, it is an exact shift-invariant algorithm and very computationally efficient. The requirements for this additional scan are that, first, this additional general curve has to be a piecewise smooth curve (i.e., a straight line or helix); second, during this additional scan the circle trajectory must find its projection on the detector as it is seen from the X-ray source. General CT scanner and C-arm can easily meet this requirements and exact ROI reconstruction can be achieved by employing this algorithm. In case of CBBCT prototype, however, it is better to keep the X-ray collimation fixed (i.e., half cone illumination) during additional noncircular scan to reduce the system complexity since the scanner possesses a half cone geometry covering the whole detector. So the second requirement with respect to the aforementioned Katsevich algorithm is hard to meet. Based on this special geometric requirement of CBBCT, the proposed partial helical scan part will be reconstructed using a shift-variant filtered-backprojection [11
]. When variable size collimation is available, Katsevich type reconstruction can be conducted along a straight line scan in numerical simulation. The hybrid reconstruction method is adopted for both cases. For the proposed CH scheme, the reconstruction is composed of three parts: FDK term for circle [7
], Hui's term for circle [12
], and a shift-variant FBP term for partial helical scan, whereas for circle plus straight line (CL) scheme, the reconstruction is composed of two terms, circle and straight line reconstructions [13
]. Instead of using Hilbert reconstruction for circle part presented by original algorithm, FDK was used due to the better computational efficiency and spatial resolution [14
]. Results from both cases are compared and discussed. Overall, computer simulations based on the numerical breast phantom verified that the proposed CH scheme outperforms the FDK-based single circular scan scheme.