Generally, Cone Beam CT imaging systems utilize polychromatic x-ray beams since they are
easier to generate and their output power is larger than those from monochromatic beams.
However, polychromatic beams have main drawbacks in medical imaging. For example, the
relative high content of low energy photons induced two main phenomena that degrade the
image quality. First, most of the low energy photons are absorbed within the soft tissue hardening
the beam. This beam hardening deteriorates the image quality by decreasing the uniformity.
Second, due to the polychromatic nature of the beam and beam hardening, the image is degraded
by high contrast objects. In order to solve these problems, a filter was developed and
implemented in a Flat Panel Detector-based cone Beam CT. First, the energy spectrum was
acquired and computer simulations were done to evaluate and to develop the best suitable filter
to harden the x-ray beam. Second, experiments were performed to evaluate the improvements on
image quality based on the reduction of artifacts due to high contrast objects on two objects,
namely a bone with titanium screws and a hand phantom. Lastly, the spatial resolution was
evaluated to investigate the effects of this filter. The results demonstrate that the new developed
filter improves the image quality in this area. The high contrast artifacts were also reduced as the
images from the bone phantom with the metal implant illustrates. Evidently, the use of this filter to harden to beam has increased the image quality when metal implants are present.
Several factors during the scanning process, image reconstruction and geometry of an imaging system, influence the spatial resolution of a computed tomography imaging system. In this work, the spatial resolution of a state of the art flat panel detector-based cone beam computed tomography breast imaging system is evaluated. First, scattering, exposure level, voltage, voxel size, pixel size, back-projection filter, reconstruction algorithm, and number of projections are varied to evaluate their effect on spatial resolution. Second, its uniformity throughout the whole field of view is evaluated as a function of radius along the x-y plane and as a function of z at the center of rotation. The results of the study suggest that the modulation transfer function is mainly influenced by the pixel, back-projection filter, and number of projections used. The evaluation of spatial resolution throughout the field of view also suggests that this imaging system does have a 3-D quasi-isotropic spatial resolution in a cylindrical region of radius equal to 40 mm centered at the axis of rotation. Overall, this study provides a useful tool to determine the optimal parameters for the best possible use of this cone beam computed tomography breast imaging system.
Flat-panel detector-based cone beam CT usually employs FDK algorithm as the reconstruction method. Traditionally, the
row-wise ramp linear filtering was regularized by noise-suppression windows, such as Shepp-Logan, Hamming windows
etc before the backprojection to get the final acceptable (in terms of SNR) reconstructed 3-D volume data. Though noise
was reduced, this linear filtering regularized by noise suppression window had the potential to affect the signal spatial
resolution and thus to reduce the sharpness of the structure boundaries within the breast image especially impeding the
detection of the small calcifications and very small abnormalities that may indicate early breast cancer. Furthermore, the
reconstructed images were still characterized by smudges. In order to combat the aforementioned shortcomings, a Wavelet regularization method was conducted on projection data followed by row-wise ramp linear filtering inherited
within FDK.
Duct patterns are formed by desmoplastic reactions as most breast carcinomas are. Hence, it has been suggested that
the denser a breast is, the higher the likelihood to develop breast cancer. Consequently, breast density has been one of the
suggested parameters to estimate the risk to develop breast cancer. Currently, the main technique to evaluate breast
densities is through mammograms. However, mammograms have the disadvantage of displaying overlapping structures
within the breast. Although there are efficient techniques to obtain breast densities from mammograms, mammography
can only provide a rough estimate because of the overlapping breast tissue. In this study, cone beam CT images were
utilized to evaluate the breast density of sixteen breast images. First, a breast phantom with known volumes representing
fatty, glandular and calcified tissues was designed to calibrate the system. Since cone beam CT provides 3D-isotropic
resolution images throughout the field of view, the issue of overlapping structures disappears, allowing greater accuracy
in evaluating the volumes of each different part of the phantom. Then, using cone beam CT breast images, the breast
density of eight patients was evaluated using a semi-automatic segmentation algorithm that differentiates between fatty,
glandular and calcified tissues. The results demonstrated that cone beam CT images provide a better tool to evaluate the
breast density of the whole breast more accurately. The results also demonstrated that using this semi-automatic
segmentation algorithm improves the efficiency of classifying the breast into the four classifications as recommended by
the American College of Radiology.
Tumor angiogenesis is the process by which new blood vessels are formed from the existing vessels in a tumor to
promote tumor growth. Tumor angiogenesis has important implications in the diagnosis and treatment of various solid
tumors. Flat panel detector based cone beam CT opens up a new way for detection of tumors, and tumor angiogenesis
associated with functional CBCT has the potential to provide more information than traditional functional CT due to
more overall coverage during the same scanning period and the reconstruction being isotropic resulting in a more
accurate 3D volume intensity measurement. A functional study was conducted by using CBCT to determine the degree
of the enhancement within the tumor after injecting the contrast agent intravenously. For typical doses of contrast
material, the amount of enhancement is proportional to the concentration of this material within the region of interest. A
series of images obtained at one location over time allows generation of time-attenuation data from which a number of
semi-quantitative parameters, such as enhancement rate, can be determined. An in vivo mice study with and without
mammo tumor was conducted on our prototype CBCT system, and half scan scheme is used to determine the time-intensity
curve within the VOI of the mouse. The CBCT has an x-ray tube, a gantry with slip ring technology, and a
40×30 cm Varian Paxscan 4030CB real time FPD.
Routine quality control assessments of medical equipment are crucial for an accurate patient medical treatment as
well as for the safety of the patient and staff involved. These regular evaluations become especially important when
dealing with radiation-emitting equipment. Therefore, a quality control (QC) program has been developed to
quantitatively evaluate imaging systems by measuring standard parameters related to image quality such as the
Modulation Transfer Function (MTF), the Noise Power Spectrum (NPS), uniformity, linearity and noise level among
others. First, the methods of evaluating the aforementioned parameters have been investigated using a cone beam CT
imaging system. Different exposure techniques, phantoms, acquisition modes of the flat panel detector (FPD) and
reconstruction algorithms relevant to a clinical environment were all included in this investigation. Second, using the
results of the first part of this study, a set of parameters for the QC program was established that yields both, an accurate
depiction of the system image quality and an integrated program for easy and practical implementation. Lastly, this QC
program will be implemented and practiced in our cone beam CT imaging system. The results using our available
phantoms demonstrate that the QC program is adequate to evaluate stability and image quality of this system since it
provides comparable parameters to other QC programs.
The Modulation Transfer Function (MTF) of any system is the frequency response to a delta signal. This response is degraded by several factors such as the inherent veiling glare of the detector and
the focal spot size among others. Consequently, the MTF has been one of the physical characteristics that
are commonly used to quantitatively measure the physical performance of a system. In this article, the
MTF of two flat panel detectors (FPD) and of two Cone Beam CT systems is evaluated. First, the MTF of
PaxScan 2520 and of PaxScan 4030CB is evaluated. One of the standard techniques to evaluate the MTF
of a FPD is by using an edge of a metal with high atomic number. For instance, it has been suggested by
IEC 62220-1 to use an opaque edge to evaluate the MTF of a FPD. Yet, it was found that different metals
yield slightly different MTF. In this study, the effects on the MTF evaluation of different metals and exposure parameters was studied and analyzed. First, the MTF was evaluated using different kVps and exposures levels. Second, the MTF was evaluated using aluminum edges of different thickness. Third, the MTF was evaluated using the following four different metals: Aluminum, Copper, Steel and Lead. Finally, the MTF obtained previously were compared to the MTF obtained by using a pinhole. In the second part of this study, the MTF of two systems using these two FPDs were also evaluated using different wires, filters and acquisition modes. The preliminary results demonstrate that the MTF is independent of kVp and exposure level. Yet, it is dependent on the material used to evaluate it.
The Modulation Transfer Function (MTF) of any system is the frequency response to a delta
signal. Ideally, this response should be a step function for a flat panel detector (FPD); it should be one for
frequencies less than the Nyquist frequency and zero for frequencies above. Yet, this response is degraded
by several factors such as the veiling glare of the detector and the focal spot size. Consequently, the MTF
has been one of the physical characteristics that are commonly used to quantitatively measure the physical
performance of a system. One of the standard techniques to evaluate the MTF is by using an edge of a
metal with a high atomic number. For instance, it has been suggested by IEC 62220-1 to use an opaque
edge to evaluate the MTF of FPDs. In a previous study, it was found that different metals yield slightly
different evaluation of the MTF. The effects of these slightly different MTFs on image quality were
investigated. The evaluation of the MTFs of a PaxScan 4030CB and PaxScan 2520 from a previous study
were used in this study. A ball, a cylindrical water phantom, a breast phantom, a living mouse and three
breasts of patients from a pilot study were analyzed for improvements in image quality after PSF deconvolution
post-processing. The results of this study suggest that the detector's MTF de-convolution
post-processing achieved a CNR's increase while it also enhanced the edges and uniformity.
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.
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 physical performance of two flat panel detectors (FPD) has been evaluated using a standard x-ray beam quality set
by IEC, namely RQA5. The FPDs evaluated in this study are based on an amorphous silicon photodiode array that is
coupled to a thallium-doped Cesium Iodide scintillator and to a thin film transistor (TFT) array. One detector is the
PaxScan 2520 that is designed for fluoro imaging, and has a small dynamic range and a large image lag. The other
detector is the PaxScan 4030CB that is designed for cone beam CT, and has a large dynamic range (>16-bit), a reduced
image lag and many imaging modes. Varian Medical Systems manufactured both detectors. The linearity of the FPDs
was investigated by using an ionization chamber and aluminum filtration in order to obtain the beam quality. Since the
FPDs are used in fluoroscopic mode, image lag of the FPD was measured in order to investigate its effect on this study,
especially its effect on DQE. The spatial resolution of the FPDs was determined by obtaining the pre-sampling
modulation transfer function for each detector. A sharp edge was used in accordance to IEC 62220-1. Next, the
Normalized Noise Power Spectrum (NNPS) was calculated for various exposures levels at RQA5 radiation quality.
Finally, the DQE of each FPD was obtained with a modified version of the international standard set by IEC 62220-1.
The results show that the physical performance in DQE and MTF of the PaxScan 4030CB is superior to that of PaxScan2520.
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