The feasibility of an inverse geometry CT system with stationary source arrays MEDICAL PHYSICS
Hsieh, S. S., Heanue, J. A., Funk, T., Hinshaw, W. S., Wilfley, B. P., Solomon, E. G., Pelc, N. J.
2013; 40 (3)
Abstract
Inverse geometry computed tomography (IGCT) has been proposed as a new system architecture that combines a small detector with a large, distributed source. This geometry can suppress cone-beam artifacts, reduce scatter, and increase dose efficiency. However, the temporal resolution of IGCT is still limited by the gantry rotation time. Large reductions in rotation time are in turn difficult due to the large source array and associated power electronics. We examine the feasibility of using stationary source arrays for IGCT in order to achieve better temporal resolution. We anticipate that multiple source arrays are necessary, with each source array physically separated from adjacent ones. Key feasibility issues include spatial resolution, artifacts, flux, noise, collimation, and system timing clashes. The separation between the different source arrays leads to missing views, complicating reconstruction. For the special case of three source arrays, a two-stage reconstruction algorithm is used to estimate the missing views. Collimation is achieved using a rotating collimator with a small number of holes. A set of equally spaced source spots are designated on the source arrays, and a source spot is energized when a collimator hole is aligned with it. System timing clashes occur when multiple source spots are scheduled to be energized simultaneously. We examine flux considerations to evaluate whether sufficient flux is available for clinical applications. The two-stage reconstruction algorithm suppresses cone-beam artifacts while maintaining resolution and noise characteristics comparable to standard third generation systems. The residual artifacts are much smaller in magnitude than the cone-beam artifacts eliminated. A mathematical condition is given relating collimator hole locations and the number of virtual source spots for which system timing clashes are avoided. With optimization, sufficient flux may be achieved for many clinical applications. IGCT with stationary source arrays could be an imaging platform potentially capable of imaging a complete 16-cm thick volume within a tenth of a second.
Dynamic bowtie for fan-beam CT JOURNAL OF X-RAY SCIENCE AND TECHNOLOGY
Liu, F., Wang, G., Cong, W., Hsieh, S. S., Pelc, N. J.
2013; 21 (4): 579-590
Abstract
A bowtie is a filter used to shape an x-ray beam and equalize its flux reaching different detector channels. For development of spectral CT with energy discriminating photon-counting (EDPC) detectors, here we propose and evaluate a dynamic bowtie for performance optimization based on a patient model or a scout scan. With a mechanical rotation of a dynamic bowtie and an adaptive adjustment of an x-ray source flux, an x-ray beam intensity profile can be modulated. First, a mathematical model for dynamic bowtie filtering is established for an elliptical section in fan-beam geometry, and the contour of the optimal bowtie is derived. Then, numerical simulation is performed to compare the performance of the dynamic bowtie in the cases of an ideal phantom and a realistic cross-section relative to the counterparts without any bowtie and with a fixed bowtie respectively. Our dynamic bowtie can equalize the expected numbers of photons in the case of an ideal phantom. In practical cases, our dynamic bowtie can effectively reduce the dynamic range of detected signals inside the field of view. Although our design is optimized for an elliptical phantom, the resultant dynamic bowtie can be applied to a real fan-beam scan if the underlying cross-section can be approximated as an ellipse. Furthermore, our design methodology can be applied to specify an optimized dynamic bowtie for any cross-section of a patient, preferably using rapid prototyping technology.
Optimized control of a dynamic, prepatient attenuator MEDICAL IMAGING 2013: PHYSICS OF MEDICAL IMAGING
Hsieh, S. S., Pelc, N. J.
2013; 8668
Abstract
Dynamic attenuators are beam shaping filters that can customize the x-ray illumination field to the clinical task and for each view. These dynamic attenuators replace traditional attenuators (or “bowtie filters”) and decrease radiation dose, dynamic range, and scatter when compared to their static counterparts. We propose a one-dimensional dynamic attenuator that comprises multiple wedges with axially-dependent triangular cross-sections, and which are translated in the axial direction. These wedges together produce a time-varying, piecewise-linear attenuation function. We investigate different control methods for this attenuator and estimate the ability of the dynamic attenuator to reduce dose while maintaining the peak variance of the scan. With knowledge of the patient anatomy, the dynamic attenuator can be controlled by solving a convex optimization problem. This knowledge could be determined from a low dose pre-scan. Absent this information, various heuristics can be used. We simulate the dynamic attenuator on datasets of the thorax, abdomen, and a targeted scan of an abdominal aortic aneurysm. The dose and scatter-to-primary ratio (SPR) are estimated using Monte Carlo simulations, and the noise is calculated analytically. Compared to a system using the standard bowtie with typical mA modulation, dose reductions of 50% are observed. Compared to an optimized, patientspecific mA modulation, the typical dose reduction is 30%. If the dynamic attenuator is controlled with a heuristic, typical dose reductions are also 30%. The gains are larger in the targeted scan. The SPR is also reduced by 20% in the abdomen. We conclude that the dynamic attenuator has significant potential to reduce dose without increasing the peak variance of the scan.
Liver Imaging: Image Quality Evaluation and Comparison between Single and Dual Energy Protocols MEDICAL IMAGING 2013: PHYSICS OF MEDICAL IMAGING
Yao, Y., Megibow, A. J., Pelc, N. J.
2013; 8668
Abstract
Purpose: Some qualitative studies report a preference for blended dual-energy (DE) CT images over single energy (SE) images for liver CT imaging at the same dose. This is counter to theoretical expectations for simple tasks. We hypothesized that perhaps the broad spectrum of DE might be beneficial for a combination of tasks. We compare the CNR of SE and blended DE images for single and composite tasks, in part to see if they explain the preference. Methods: We simulated pre- and post-contrast SE abdominal CT imaging at various kVp but at constant average dose. Next, 80kVp and 140kVp scans with different dose allocations, dose matched to the SE images, were simulated. DE images were blended linearly with optimized blending ratios. The CNRs of liver against other soft tissues were used as a composite image quality metric for evaluation and comparison between the SE and DE protocols. In addition, the combination of the CNR of many tissue pairs pre- and post-contrast. Results: The CNR of pre-contrast single kVp imaging mostly increases with increasing tube voltage while 90kVp or lower energy yields higher CNR for post-contrast images, depending on the differential iodine concentration of each tissue. Similar trends are seen in the DE blended CNR curves. Results from the composite multi-CNR metric demonstrate that the SE protocol has better performance. Conclusions: Our study showed that an optimized SE protocol produces higher CNR, even for a range of tasks. This suggests that the reason for the radiologist preference must be something other than a fundamental advantage of DE.
Truncation artifact correction by support recovery MEDICAL IMAGING 2013: PHYSICS OF MEDICAL IMAGING
Hsieh, S. S., Cao, G., Nett, B. E., Pelc, N. J.
2013; 8668
Abstract
Truncation artifacts arise when the object being imaged extends past the scanned field of view (SFOV). The line integrals which lie beyond the SFOV are unmeasured, and reconstruction with traditional filtered backprojection (FBP) produces bright signal artifacts at the edge of the SFOV and little useful information outside the SFOV. A variety of techniques have been proposed to correct for truncation artifacts by estimating the unmeasured rays. We explore an alternative, iterative correction technique that reduces the artifacts and recovers the support (or outline) of the object that is consistent with the measured rays. We assume that the support is filled uniformly with tissue of a given CT number (for example, water-equivalent soft tissue) and segment the region outside the SFOV in a dichotomous fashion into tissue and air. In general, any choice for the object support will not be consistent with the measured rays in that a forward projection of the image containing the proposed support will not match the measured rays. The proposed algorithm reduces this inconsistency by deforming the object support to better match the measured rays. We initialize the reconstruction using the water cylinder extrapolation algorithm, an existing truncation artifact correction technique, but other starting algorithms can be used. The estimate of the object support is then iteratively deformed to reduce the inconsistency with the measured rays. After several iterations, forward projection is used to estimate the missing rays. Preliminary results indicate that this iterative, support recovery technique is able to produce superior reconstructions in the case of significant truncation compared to water cylinder extrapolation.
Image-based Synthetic CT: simulating arbitrary low dose single and dual energy protocols from dual energy images MEDICAL IMAGING 2012: PHYSICS OF MEDICAL IMAGING
Wang, A. S., Feng, C., Pelc, N. J.
2012; 8313
Abstract
While the imaging protocol determines radiation dose and image quality, it is difficult to find the lowest dose protocol (kVp, mAs, filtration) that provides appropriate diagnostic quality images. Recently, we developed a method for retrospectively synthesizing CT scans of arbitrary protocols using a previously acquired dual energy scan that relies on projection space data. Here, we propose a new variant of synthetic CT that only requires reconstructed dual energy images to simulate realistic images from arbitrary low dose protocols. Axial scans of a phantom were acquired on a GE CT750 HD system at 80 kVp and separately at 140 kVp, enabling material decomposition. Additional scans at 100 and 120 kVp and at different exposures were made to compare with synthesized results. Raw data for any spectrum can be estimated by forward transmission through the material decomposition images, but these have degraded spatial resolution. To avoid blurring, the synthesized image is represented as a linear combination (i.e., mixed or blended image) of the 80/140 kVp images. Noise with the correct statistics is then added so that the total noise matches the expected noise of the simulated protocol (estimated from forward transmission). For the studied object, the resulting synthesized images are indistinguishable from the actual images. Synthetic CT enables users to visualize the impact of protocol changes on the contrast and noise of CT scans, which can be used to develop lower dose protocols by demonstrating dose/noise/protocol trade-offs. Our new image domain implementation significantly increases the accessibility of synthetic CT to potential users.
A volumetric reconstruction algorithm for stationary source inverse-geometry CT MEDICAL IMAGING 2012: PHYSICS OF MEDICAL IMAGING
Hsieh, S. S., Pelc, N. J.
2012; 8313
Abstract
Stationary source inverse-geometry CT (SS-IGCT) has been proposed as a new system architecture that has several key advantages over traditional cone beam CT (CBCT). One advantage is the potential for acquiring a large volume of interest with minimal cone-beam artifacts and with very high temporal resolution. We anticipate that SS-IGCT will use large, stationary source arrays, with gaps in between separate source array modules. These gaps make reconstruction challenging because most analytic reconstruction algorithms assume a continuous source trajectory. SS-IGCT is capable of producing the same dataset as a traditional scanner taking multiple overlapping axial scans, but with segments of the views missing from each axial scan because of gaps. We propose the following, two-stage volumetric reconstruction algorithm. In the first stage, the missing rays are estimated in a spatially varying fashion using available data and geometric considerations, and reconstruction proceeds with standard algorithms. The missing data are then re-estimated by a forward projection step. These new estimates are quite good and the reconstruction can be performed again using any algorithm that supports multiple parallel axial scans. Although inspired by iterative reconstruction, our algorithm only needs one "iteration" of forward- and back-projection in practice and is efficient. Simulations of a thorax phantom were performed showing the efficacy of this technique and the ability of SS-IGCT to suppress cone-beam artifacts compared to conventional CBCT. The noise and resolution characteristics are comparable to that of CBCT.
A comparison of dual kV energy integrating and energy discriminating photon counting detectors for dual energy x-ray imaging MEDICAL IMAGING 2012: PHYSICS OF MEDICAL IMAGING
Wang, A. S., Pelc, N. J.
2012; 8313
Abstract
Dual energy imaging enables material decomposition and requires attenuation measurements with at least two different energies. Today's clinically available implementations use two separate exposures, at a low and high tube voltage (dual kV). Photon counting x-ray detectors (PCXDs) are an alternative technology that takes advantage of an x-ray source's broad spectrum by counting the number of transmitted photons at each energy from a single exposure. The richness of the information contained in these measurements can depend heavily on the detector's energy response, itself dependent on count rate. We compare the material decomposition precision of dual kV with energy integrating detectors to that of realistic PCXDs. We model the three primary effects that degrade PCXD performance: count rate limitations, energy resolution, and spectrum tailing. For example, the high flux rates required for clinical imaging pose a serious challenge for PCXDs. The Cramer-Rao Lower Bound is used to predict the best possible material decomposition variance as a function of these detector imperfections. For dual kV and photon counting, we determined the optimal kV, mAs, and filtration for a broad range of imaging tasks, subject to dose and tube power constraints. We found that a well-optimized dual kV protocol performs on par with the estimated performance of today's PCXDs. For dual kV protocols, it is helpful to increase the energy separation between the spectra by increasing the kV separation and adding filtration. For PCXDs, the detector's count rate capabilities must be increased and the spectrum tailing reduced for photon counting to become a competitive technology at the high intensities of clinical imaging.
Efficacy of Fixed Filtration for Rapid kVp-Switching Dual Energy X-ray Systems: Experimental Verification MEDICAL IMAGING 2012: PHYSICS OF MEDICAL IMAGING
Yao, Y., Wang, A. S., Pelc, N. J.
2012; 8313
Abstract
The dose efficiency of dual energy imaging can be improved if the spectra are filtered to increase spectral separation. Our preliminary simulations showed that a fixed Gd filter can be efficacious in rapid kVp-switching systems. We conducted physical experiments on a table top x-ray system to verify the performance improvement before moving forward to a real CT scanner. We chose a commercial Gd2O2S screen as our filter and used an acrylic-copper step wedge phantom. Data were collected at 70 and 125 kVp, with and without the filter. The tube current was adjusted to make the exposure rate with and without the filter to be roughly the same. The data were decomposed into basis material images and the variance of the decomposition was measured for each acrylic-copper thickness pair. Simulations were done with the same experimental settings for comparison and validation. The experiments verified that a Gd filter can reduce the variance at fixed dose. The variance reduction is monotonically stronger as the object becomes more attenuating. This study demonstrates the potential of fixed Gd filtration to improve the dose efficiency and material decomposition precision for rapid kVp-switching dual energy systems.
Data normalization method for a multi-source inverse geometry CT system MEDICAL IMAGING 2012: PHYSICS OF MEDICAL IMAGING
Baek, J., Pelc, N. J.
2012; 8313
Abstract
The multi-source inverse-geometry CT(MS-IGCT) system is composed of multiple sources and a small 2D detector array. Each source is activated sequentially and covers a small portion of the field-of-view (FOV) and a full FOV reconstruction is acquired by combining projection data from all sources. During the data acquisition, the intensity of each x-ray source could change, e.g. because of instability in the power supply, leading to artifacts in the reconstructed image. To reduce the image artifacts, we developed a data normalization algorithm for the MS-IGCT system. The projection data of each source shares an overlap region in 2D Radon space with another source. Thus, substantially same projection data can be generated from different sources at different gantry positions. Since at least one source can illuminate a reference channel and therefore its data can be easily normalized. This normalized projection data can be used to normalize the raw data of another source with which it shares an overlap region. By performing this normalization process sequentially, the intensity variations of all sources can be corrected. The proposed method was tested with Shepp-Logan phantom using 10% random source intensity fluctuations. While the reconstructed image showed image artifacts that result from uncorrected fluctuations, after applying the proposed normalization algorithm, image artifacts were removed.
Initial results with a multi-source inverse-geometry CT system MEDICAL IMAGING 2012: PHYSICS OF MEDICAL IMAGING
Baek, J., Pelc, N. J., Deman, B., Uribe, J., Harrison, D., Reynolds, J., Neculaes, B., Inzinna, L., Caiafa, A.
2012; 8313
Abstract
The multi-source inverse-geometry CT(MS-IGCT) system is composed of multiple sources and a small 2D detector array. An experimental MS-IGCT system was built and we report initial results with 2×4 x-ray sources, a 75 mm inplane field-of-view (FOV) and 160 mm z-coverage in a single gantry rotation. To evaluate the system performance, experimental data were acquired from several phantoms and a post-mortem rat. Before image reconstruction, geometric calibration, data normalization, beam hardening correction and detector spectral calibration were performed. For reconstruction, the projection data were rebinned into two full cone beam data sets, and the FDK algorithm was used. The reconstructed volumes from the upper and lower source rows shared an overlap volume which was combined in image space. The reconstructed images of the uniform cylinder phantom showed good uniformity of the reconstructed values without any artifacts. The rat data were also reconstructed reliably. The initial experimental results from this rotating-gantry MS-IGCT system demonstrated its ability to image a complex anatomical object without any significant image artifacts and to ultimately achieve large volumetric coverage in a single gantry rotation.