Cone-beam CT (CBCT) technique has been used by orthopedists to monitor bone graft growth after
orthopedic surgery. In order to correct severe metal artifacts in reconstructed images caused by metal
implants used in bone grafting, a three-dimensional metal artifact correction method has been previously
proposed. The implants' mathematic boundaries were generated to help to segment metal from
reconstructed images. The segmented metal implants were
forward-projected onto the detector to create
metal-only projections to compensate for beam-hardening effect. This method was proved effective with
the metal implants of regular shape which can be simulated by simple 3D primitives, such as cuboid,
cylinder and cone. But for metal implants of arbitrary shape, their boundaries are difficult to define
mathematically. To solve this problem, this paper proposed a method by setting up an implant image
library and using the implants' a priori shape information from the library during the artifact correction.
The implants were acquired and scanned before the surgery and their a priori information were stored in a
library. During the artifact correction, the library was called to provide the shape information of the
implants to help to do the implant segmentation. The segmented implants were forward-projected onto the
detector to generate implant-only projections by a cone-beam forward-projection technique. Beam-hardening
effect in the original projections was then compensated by high polynomial orders of implant
projections. Finally, the corrected projections were back-projected to produce artifacts-reduced images.
Both phantom studies and patient studies were conducted to test this correction method. Results from both
studies show the artifacts have been greatly reduced and the accuracy of bone volume measurement has
been increased.
Cone-beam CT (CBCT) technique is needed by orthopaedists in their new studies to monitor bone volume growth
and blood vessel growth of structural bone grafts used in reconstruction surgery. However, titanium plate and screws,
which are commonly used to connect bone grafts to host bones, can cause severe streaking artifacts and shading
artifact in the reconstructed images due to their high attenuation of x-rays. These metal artifacts will distort the
information of the bone and cause difficulties when measuring bone volume growth and the inside blood vessel
growth. To solve this problem and help orthopaedists quantitatively record the growth of bone grafts, we present a
three-dimensional metal artifact correction technique to correct the streaking artifacts generated by titanium implants.
In this project not only the artifacts need to be corrected but also the correct information of the bone is required in the
image for the quantitative measurements. Both phantom studies and animal studies were conducted to test this
correction method. Images without metal correction and images with metal correction were compared together, as well
as the reference bone images acquired without metal. It's shown the streaking and shading artifacts were greatly
reduced after metal correction. The accuracy of bone volume measurements was also greatly increased by 79% for
phantom studies and 53% for animal studies.
The purpose of the study is to characterize the imaging performance of the recently built novel cone beam breast CT (CBBCT) scanner. This CBBCT scanner system has one x-ray source and one flat panel detector (Varian's PaxScan 4030CB) mounted on a rotating assembly. A patient table is mounted above the rotating tube/detector assembly. The table has a hole through it that allows a woman's breast to hang pendant in the imaging volume at the rotation axis. The tube/detector assembly rotates around the rotation axis and acquires multiple 2D projection images of the uncompressed breast located at the rotation axis in 10 seconds. Slip ring technology allows continuous rotation of the x-ray tube/detector assembly concentric to the opening in the table to achieve multiple circle scans. Also, it has a controlled vertical motion during the rotation to perform a spiral scan over 20 cm of travel. The continuous 360° rotation is designed to have speeds up to 1 rev/sec. This system was validated through a series of breast-imaging phantom studies and and patient studies. The results show that the image quality of the CBBCT scanner is excellent and all phantom masses (tissue-equivalent carcinomas) and calcifications as well as human subjects' masses, calcifications and abnormalities can be detected faithfully using the CBBCT technique with a glandular dose level less than or equal to that of a single two-view mammography exam. The results indicate that the CBBCT imaging system has much better detectability of small breast tumors compared to the conventional mammography system.
A new breast-shaped compensation filter has been designed and fabricated for breast imaging using our newly built
breast imaging (CBCTBI) system, which is able to scan an uncompressed breast with pendant geometry. The shape of
this compensation filter is designed based on an average-sized breast phantom. Unlike conventional bow-tie
compensation filters, its cross-sectional profile varies along the chest wall-to-nipple direction for better compensation for
the shape of a breast. Breast phantoms of three different sizes are used to evaluate the performance of this compensation
filter. The reconstruction image quality was studied and compared to that obtained without the compensation filter in
place. The uniformity of linear attenuation coefficient and the uniformity of noise distribution are significantly improved,
and the contrast-to-noise ratios (CNR) of small lesions near the chest wall are increased as well. Multi-normal image
method is used in the reconstruction process to correct compensation flood field and to reduce ring artifacts.
The purpose of this study is to demonstrate the feasibility of performing cone beam CT angiogenesis imaging (CBCTAI) using a cone beam CT scanner prototype. We have developed a cone beam breast CT imaging technique that is able to detect a ~2 mm tumor in live mice and have expanded the imaging to include visualization of tumor vessels. The CBCT prototype consisted of a modified clinical CT scanner and a flat panel detector. This CBCT scanner prototype was used for a series of preliminary contrast studies with live mice: contrast-to-scan time delay, contrast for tumor vessel enhancement (comparison with histology) and early tumor vessel development imaging. The results of the live mice studies demonstrate that good image quality can be achieved with this prototype demonstrating the feasibility of CBCTAI. Achieving the CBCTAI technique on the cone beam breast CT imaging modality will significantly advance breast cancer detection, diagnosis and treatment.
A cone beam CT (CBCT) imaging system has been developed based on a high-speed flat panel detector specially
designed for cone beam CT. This system has shown great potential for early breast cancer detection with high contrast
and spatial resolution, and the potential for high-speed dynamic studies. Image lag always plays an important role in
cone beam CT imaging by affecting the image quality. This paper investigated the relationship of the image lag with
various parameters of this new system, which include frame number, detector mode, frame rate, detector signal strength
and kVp value. A simulated breast phantom was designed to demonstrate how image lag causes artifacts in the image
and affects the reconstructed linear attenuation coefficient of tumor in different tissues. Experimental results illustrate
that lag on this system is less than 3%, and is independent of kVp value and detector mode. When the detector signal
strength increases from 10% to 80% saturation, lag decreases by 10%. Lag is also a function of the frame rate increasing
by 20% when the frame rate changed from 3.75 fps to 30 fps. Computer simulations reveal that lag on this new system
caused less than 6 HU reduction in the CT# for simulated tumors and soft tissue and 20 HU reduction for high contrast
objects. This work tells us that lag on this new CBCT system for breast imaging can be ignored. However, when this
system is used for dynamic study, which requires a high readout speed from the detector, image lag correction will need
to be considered to achieve good image quality.
The clinical goal of breast imaging is to detect tumor masses when they are as small as possible, preferably less
than 10 mm in diameter. Conventional screen-film mammography is the most effective tool for the early detection of
breast cancer currently available. However, conventional mammography has relatively low sensitivity for the detection
of small breast cancers (under several millimeters). Specificity and the positive predictive value of mammography
remain limited owing to an overlap in the appearance of benign and malignant lesions, and surrounding structure. We
propose to address the limitations accompanying conventional mammography by incorporating a cone beam CT
reconstruction technique with a recently developed flat panel detector (FPD). We have performed a computer
simulation study and preliminary phantom studies to prove the feasibility of developing an FPD-based cone beam CT
breast imaging technique for a small size normal breast phantom. In this study, we report the design and construction
of a novel FPD-based cone beam breast CT scanner prototype. In addition, we present the results of phantom studies
performed on our current FPD-based cone beam CT scanner prototype, which uses the same flat panel detector
proposed for the cone beam breast CT scanner prototype, to predict the image performance of the novel cone beam
breast CT scanner, while we are completing the construction of the system.
The purpose of this study is to characterize a newly built flat panel detector (FPD)-based cone beam CT
(CBCT) prototype for dynamic imaging. A CBCT prototype has been designed and constructed by completely
modifying a GE HiSpeed Advantage (HSA) CT gantry, incorporating a newly acquired large size real-time FPD (Varian
PaxScan 4030CB), a new x-ray generator and a dual focal spot angiography x-ray tube that allows the full coverage of
the detector. During data acquisition, the x-ray tube and the FPD can be rotated on the gantry over Nx360 degrees due
to integrated slip ring technology with the rotation speed of one second/revolution. With a single scan time of up to 40
seconds , multiple sets of reconstructions can be performed for dynamic studies. The upgrade of this system has been
completed. The prototype was used for a series of preliminary phantom studies: different sizes of breast phantoms, a
Humanoid chest phantom and scatter correction studies. The results of the phantom studies demonstrate that good
image quality can be achieved with this newly built prototype.
The sensitivity to detect small breast cancers and the specificity of conventional mammography (CM) remain limited owing to an overlap in the appearances of lesions and surrounding structure. We propose to address the limitations accompanying CM using flat panel detector (FPD)-based cone beam CT breast imaging (CBCTBI). The purpose of the study is to determine optimal x-ray operation ranges for different sizes of normal breasts and corresponding glandular dose levels. The current CBCT prototype consists of a modified GE HighSpeed Advantage CT gantry, an x-ray tube, a Varian PaxScan 4030CB FPD, a CT table and a PC. Two uncompressed breast phantoms, with the diameters of 10.8 and 13.8 cm, consist of three inserts: a layer of silicone jell simulating a background structure, a lucite plate on which five simulated carcinomas are mounted, and a plate on which six calcifications are attached. With a single scan, 300 projections were acquired for all phantom scans. The optimal x-ray techniques for different phantom sizes were determined. The total mean glandular doses for different size phantoms were measured using a CT pencil ionization chamber. With the optimal x-ray techniques that result in the maximal dose efficiency for the different tissue thickness, the image quality with two different phantoms was evaluated. The results demonstrate that the CBCTBI can detect a few millimeter-size simulated carcinoma and ~ 0.2 mm calcification with clinically acceptable mean glandular doses for different size breasts.
Quantum noise in cone beam CT (CBCT) imaging was studied to provide quantitative relationships among 3D cone beam image noise level and CT acquisition and reconstruction parameters, which include entrance exposure level, number of projections, and single detector size. It showed that the level of reconstructed image noise, which was caused by quantum noise in projection data, was spatially variant and related to the shape of the scan object, and that the image noise level was inversely proportional to the square root of entrance exposure level per projection, square root of number of projections, and square of detector size. Both computer simulations and real phantom studies were conducted to verify the derived quantitative relationships between image noise level and CT parameters. Shepp-logan head phantom was used in computer simulations to verify the theoretical relation between noise level and detector size, while a real cylindrical oil-uniformed phantom was studied to verify the theoretical relation between noise level and entrance exposure level. The real phantom studies were carried out on a flat panel detector (FPD)-based CBCT system available in our Lab. This work can provide a guide on how to balance various CBCT parameters to achieve satisfactory image quality with desired signal-to-noise ratio, specified spatial resolution, low contrast detectability and minimal x-ray radiation to patients.
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