Purpose: Clinical performance studies of an extremity cone-beam CT (CBCT) system indicate excellent bone visualization, but point to the need for improvement of soft-tissue image quality. To this end, a rapid Monte Carlo (MC) scatter correction is proposed, and Penalized Likelihood (PL) reconstruction is evaluated for noise management. Methods: The accelerated MC scatter correction involved fast MC simulation with low number of photons implemented on a GPU (107 photons/sec), followed by Gaussian kernel smoothing in the detector plane and across projection angles. PL reconstructions were investigated for reduction of imaging dose for projections acquired at ~2 mGy. Results: The rapid scatter estimation yielded root-mean-squared-errors of scatter projections of ~15% of peak scatter intensity for 5⋅106 photons/projection (runtime ~0.5 sec/projection) and 25% improvement in fat-muscle contrast in reconstructions of a cadaveric knee. PL reconstruction largely restored soft-tissue visualization at 2 mGy dose to that of 10 mGy FBP image. Conclusion: The combination of rapid (5-10 minutes/scan) MC-based, patient-specific scatter correction and PL reconstruction offers an important means to overcome the current limitations of extremity CBCT in soft-tissue imaging.
Purpose: We describe the initial assessment of the peripheral quantitative CT (pQCT) imaging capabilities of a conebeam CT (CBCT) scanner dedicated to musculoskeletal extremity imaging. The aim is to accurately measure and quantify bone and joint morphology using information automatically acquired with each CBCT scan, thereby reducing the need for a separate pQCT exam. Methods: A prototype CBCT scanner providing isotropic, sub-millimeter spatial resolution and soft-tissue contrast resolution comparable or superior to standard multi-detector CT (MDCT) has been developed for extremity imaging, including the capability for weight-bearing exams and multi-mode (radiography, fluoroscopy, and volumetric) imaging. Assessment of pQCT performance included measurement of bone mineral density (BMD), morphometric parameters of subchondral bone architecture, and joint space analysis. Measurements employed phantoms, cadavers, and patients from an ongoing pilot study imaged with the CBCT prototype (at various acquisition, calibration, and reconstruction techniques) in comparison to MDCT (using pQCT protocols for analysis of BMD) and micro-CT (for analysis of subchondral morphometry). Results: The CBCT extremity scanner yielded BMD measurement within ±2-3% error in both phantom studies and cadaver extremity specimens. Subchondral bone architecture (bone volume fraction, trabecular thickness, degree of anisotropy, and structure model index) exhibited good correlation with gold standard micro-CT (error ~5%), surpassing the conventional limitations of spatial resolution in clinical MDCT scanners. Joint space analysis demonstrated the potential for sensitive 3D joint space mapping beyond that of qualitative radiographic scores in application to non-weight-bearing versus weight-bearing lower extremities and assessment of phalangeal joint space integrity in the upper extremities. Conclusion: The CBCT extremity scanner demonstrated promising initial results in accurate pQCT analysis from images acquired with each CBCT scan. Future studies will include improved x-ray scatter correction and image reconstruction techniques to further improve accuracy and to correlate pQCT metrics with known pathology.
In Cone Beam CT Imaging, metallic and other dense objects, such as implantable orthopedic appliances, surgical clips
and staples, and dental fillings, are often acquired as part of the image dataset. These high-density, high atomic mass
objects attenuate X-rays in the diagnostic energy range much more strongly than soft tissue or bony structures, resulting
in photon starvation at the detector. In addition, signal behind the metal objects suffer from increased quantum noise,
scattered radiation, and beam hardening. All of these effects combine to create nonlinearities which are further amplified
by the reconstruction algorithm, such as conventional filtered back-projection (FBP), producing strong artifacts in the
form of streaking. They reduce image quality by masking soft tissue structures, not only in the immediate vicinity of the
dense object, but also throughout the entire image volume. A novel, physical-model-based, metal-artifact reduction
scheme (MAR) is proposed to mitigate the metal-induced artifacts. The metal objects are segmented in the projection
domain, and a physical model based method is adopted to fill in the segmented area. The FDK1 reconstruction algorithm
is then used for the final reconstruction.
A novel cone-beam CT (CBCT) system has been developed with promising capabilities for musculoskeletal imaging
(e.g., weight-bearing extremities and combined radiographic / volumetric imaging). The prototype system demonstrates
diagnostic-quality imaging performance, while the compact geometry and short scan orbit raise new considerations for
scatter management and dose characterization that challenge conventional methods. The compact geometry leads to
elevated, heterogeneous x-ray scatter distributions - even for small anatomical sites (e.g., knee or wrist), and the short
scan orbit results in a non-uniform dose distribution. These complex dose and scatter distributions were investigated via
experimental measurements and GPU-accelerated Monte Carlo (MC) simulation. The combination provided a powerful
basis for characterizing dose distributions in patient-specific anatomy, investigating the benefits of an antiscatter grid,
and examining distinct contributions of coherent and incoherent scatter in artifact correction. Measurements with a 16
cm CTDI phantom show that the dose from the short-scan orbit (0.09 mGy/mAs at isocenter) varies from 0.16 to 0.05
mGy/mAs at various locations on the periphery (all obtained at 80 kVp). MC estimation agreed with dose measurements
within 10-15%. Dose distribution in patient-specific anatomy was computed with MC, confirming such heterogeneity
and highlighting the elevated energy deposition in bone (factor of ~5-10) compared to soft-tissue. Scatter-to-primary
ratio (SPR) up to ~1.5-2 was evident in some regions of the knee. A 10:1 antiscatter grid was found earlier to result in
significant improvement in soft-tissue imaging performance without increase in dose. The results of MC simulations
elucidated the mechanism behind scatter reduction in the presence of a grid. A ~3-fold reduction in average SPR was
found in the MC simulations; however, a linear grid was found to impart additional heterogeneity in the scatter
distribution, mainly due to the increase in the contribution of coherent scatter with increased spatial variation. Scatter
correction using MC-generated scatter distributions demonstrated significant improvement in cupping and streaks.
Physical experimentation combined with GPU-accelerated MC simulation provided a sophisticated, yet practical
approach in identifying low-dose acquisition techniques, optimizing scatter correction methods, and evaluating patientspecific
A flat-panel, detector-based cone beam CT system can provide advantages over a fan beam CT system in terms of 3D
isotropic spatial resolution. However, as a result of increased X-ray coverage along the rotation axis, there is also an
increase in scatter. This can lead to a decrease in low-contrast resolution as well as the appearance of non-uniform
artifacts across the reconstructed image. These effects can be minimized with the use of an anti-scatter grid; however,
further software corrections are often desirable. Software scatter correction is generally achieved through the subtraction
of an estimate of the scatter distribution from the corresponding original projection data in the linear space. While the
non-uniform artifacts effect is generally improved, a side effect of this subtractive process can be an undesirable
amplification of the apparent noise, which makes the image quality, in terms of contrast-to-noise ratio (CNR), much
worse than the images produced by fan beam CT systems. In this work, a novel modified imaging chain has been
proposed to apply separate, non-linear noise-reduction algorithms on bone and soft tissues to improve the CNR for soft
tissue as well as to maintain a high spatial resolution for the display of boney structures.
The design, initial imaging performance, and model-based optimization of a dedicated cone-beam CT (CBCT) scanner
for musculoskeletal extremities is presented. The system offers a compact scanner that complements conventional CT
and MR by providing sub-mm isotropic spatial resolution, the ability to image weight-bearing extremities, and the
capability for integrated real-time fluoroscopy and digital radiography. The scanner employs a flat-panel detector and a
fixed anode x-ray source and has a field of view of ~ (20x20x20) cm3. The gantry allows a "standing" configuration for
imaging of weight-bearing lower extremities and a "sitting" configuration for imaging of upper extremities and unloaded
lower extremities. Cascaded systems analysis guided the selection of x-ray technique (e.g., kVp, filtration, and dose) and
system design (e.g., magnification factor), yielding input-quantum-limited performance at detector signal of 100 times
the electronic noise, while maintaining patient dose below 5 mGy (a factor of ~2-3 less than conventional CT). A
magnification of 1.3 optimized tradeoffs between source and detector blur for a 0.5 mm focal spot. A custom antiscatter
grid demonstrated significant reduction of artifacts without loss of contrast-to-noise ratio or increase in dose. Image
quality in cadaveric specimens was assessed on a CBCT bench, demonstrating exquisite bone detail, visualization of
intra-articular morphology, and soft-tissue visibility approaching that of diagnostic CT. The capability to image loaded
extremities and conduct multi-modality CBCT/fluoroscopy with improved workflow compared to whole-body CT could
be of value in a broad spectrum of applications, including orthopaedics, rheumatology, surgical planning, and treatment
assessment. A clinical prototype has been constructed for deployment in pilot study trials.
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
Cone Beam Breast CT (CBBCT) acquires 3D breast images without compression to the breast. More detailed and
accurate information of breast lesions is revealed in CBBCT images. In our research, based on the observation that tumor
masses are more concentrated than the surrounding tissues, we designed a weighted average filter and a threedimensional
Iris filter to operate on the three-dimensional images. The basic process is: After weighted average filtering
and iris filtering, a thresholding is applied to extract suspicious regions. Next, after morphological processing, suspicious
regions are sorted based on their average Iris filter responses and the top 10 candidates are selected as detection results.
The detection results are marked out and provided to radiologists as CAD system output. In our experiment, our method
detects 12 mass locations out of 14 pathology-proven malignant clinical cases.
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.
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 timeattenuation
data from which a number of semi-quantitative parameters, such as enhancement rate, can be determined.
Computer simulations prove the superiority of half scan over full scan in terms of more accurately delineating the
time-intensity curve, and all the simulation parameter settings are based on the actual CBCT prototype. An
experiment study was conducted on our prototype CBCT system, and a full and half scan scheme is used to
determine the time-intensity curve within the ROI of the mouse. The CBCT has an x-ray tube, a gantry with slip
ring technology, and a 40x30 cm Varian Paxscan 4030CB real time FPD.
Flat panel detector based cone beam breast imaging CT can provide 3-D image of the scanned breast with 3-D isotropic spatial resolution, overcoming the disadvantage of the superimposition of structure associated with X-ray projection mammography that makes a small carcinoma (a few millimeters in size) difficult to detect when it is occult or in dense breast, which leads to a high false-positive biopsy rate. Circular scan CBBCT is the most desirable mode due to
its simple geometrical configuration and potential applications in functional imaging. Only circular scan, however, can't provide the sufficient information for nearly exact reconstruction, and thus resulting in the reconstructed image artifacts, such as density drop and geometrical deformation when the cone angle becomes large. In order to combat this drawback, a circle plus sparse helical line scan scheme is proposed. Computer simulation on mathematic breast phantom testifies the practical feasibility of the new scheme and correction of those artifacts to a certain degree.
Phase-contrast imaging uses the phase coefficient rather than the attenuation coefficient alone to image objects.
Consequently, it may resolve some structures that have similar attenuation coefficients but different phase coefficients
as their surroundings. Phase contrast imaging is also an edge-enhanced imaging technique. With this method, the
boundary of inside small structures could be easily determined. In this paper, the possibility of incorporating the phase
contrast in-line method into the current cone beam CT (CBCT) system was explored. Starting from the interference
formula of in-line holography, some mathematical assumptions were made and thus, the terms in the interference
formula could be approximately expressed as a line integral that is the requirement for all CBCT algorithms. So, the
CBCT reconstruction algorithms, such as the FDK algorithm could be applied for the in-line holographic projections,
with some mathematical imperfection. A point x-ray source and a high-resolution detector were assumed for computer
simulation. The reconstructions for cone-beam CT imaging were studied. The results showed that all the lesions in the
numerical phantom could be observed with an enhanced edge. However, due to the edge-enhancement nature of the inline
holographic projection, the reconstructed images had obvious streak artifacts and numerical errors. The image
quality could be improved by using a hamming window during the filtering process. In the presence of noise, the
reconstructions from the in-line holographic projections showed clearer edges than the normal CT reconstructions did.
Finally it was qualitatively illustrated that small cone angle and weak attenuation were preferred in this method.
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.
Proc. SPIE. 5745, Medical Imaging 2005: Physics of Medical Imaging
KEYWORDS: Radon, Fluctuations and noise, 3D image reconstruction, Detection and tracking algorithms, Sensors, Signal attenuation, Temporal resolution, Reconstruction algorithms, Algorithm development, Breast imaging
A modified Feldkamp (FDK) half-scan (MFDKHS) algorithm was developed to conduct the circular half-scan scheme on a flat panel detector (FPD)-based CBCT. X-ray source scans the object in a circular trajectory in the range of 180 degrees plus full fan angle rather than 360 degrees for a full scan. The redundant data is weighted by a weighting function in 3-D case based on the ones proposed by Parker1, FDK algorithm is then applied on this weighted data, and another FBP term developed by Dr. Hu is also added to get the final reconstruction. The MFDKHS is expected to correct the attenuation coefficient drop of the reconstructed object associated with FDK in the place where Z (rotation axis) is away from scanning plane and improve the temporal resolution as well.
A modified Feldkamp (FDK) 3-D cone beam algorithm was developed to conduct the half-scan scheme on a flat panel detector (FPD)-based prototype CBCT system which is available in our Lab. X-ray source scans the object in a circular trajectory in the range of 180 degrees plus full cone angle rather than 360 degrees for a full scan. The redundant data is weighted by a weighting function in 3-D case based on the ones proposed by Parker1, FDK algorithm is then adopted to get the reconstruction image. The breast phantom and breast specimen are used in the experiment and the objects reconstructed from half-scan CBCT are compared with those reconstructed from full scan. The applicability of the half-scan scheme is testified in terms of image noise level, contrast resolution. The half-scan scheme is expected to improve the temporal resolution and may also reduce the patients’ X-ray dose level. The result showed an encouraging potential usage of the Half-scan CBCT in the functional 4-D CT imaging.