Robert Senzig

Coronary Artery Angiography Using Multislice Computed Tomography Images

Multislice CT scanners are the newest class of CT scanners and they have not one but many detectors. These scanners can acquire up to 4 slices of data from the body in the same time it takes a single-slice CT scanner to acquire one. Multislice CT allows for rapid cardiac imaging during a single breath-hold. A multislice scanner operated in helical mode provides information that can be used to reconstruct 3D cardiac images in arbitrary phases of the cardiac cycle. A 71-year-old man with hypertension, hypercholesterolemia, and known aortic and peripheral vascular disease was imaged with a LightSpeed 4-slice, multislice CT scanner (GE Medical Systems). Ten minutes before the cardiac scan, the patient received intravenous contrast material (150 mL of 300 mgI/mL) for a CT study of his abdomen. The cardiac …

A multi-source inverse-geometry CT system: initial results with an 8 spot x-ray source array

We present initial experimental results of a rotating-gantry multi-source inverse-geometry CT (MS-IGCT) system. The MS-IGCT system was built with a single module of 2 × 4 x-ray sources and a 2D detector array. It produced a 75 mm in-plane field-of-view (FOV) with 160 mm axial coverage in a single gantry rotation. To evaluate system performance, a 2.5 inch diameter uniform PMMA cylinder phantom, a 200 µm diameter tungsten wire, and a euthanized rat were scanned. Each scan acquired 125 views per source and the gantry rotation time was 1 s per revolution. Geometric calibration was performed using a bead phantom. The scanning parameters were 80 kVp, 125 mA, and 5.4 µs pulse per source location per view. A data normalization technique was applied to the acquired projection data, and beam hardening and spectral nonlinearities of each detector channel were corrected. For image reconstruction, the projection data of each source row were rebinned into a full cone beam data set, and the FDK algorithm was used. The reconstructed volumes from upper and lower source rows shared an overlap volume which was combined in image space. The images of the uniform PMMA cylinder phantom showed good uniformity and no apparent artifacts. The measured in-plane MTF showed 13 lp cm(-1) at 10% cutoff, in good agreement with expectations. 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 achieve high image resolution and large axial coverage in a single gantry rotation.

Multisource inverse-geometry CT — Prototype system integration

Todays 3rd generation CT scanners have one or two X-ray tubes, with one focal spot or “source” per vacuum chamber or “tube”. Our first multi-source inverse geometry CT prototype has eight X-ray sources. We have demonstrated multisource imaging with an 8-spot X-ray tube on a stationary gantry and a rotating phantom. We present an update on the development of the gantry-based multi-source CT scanner: we combine the multi-source X-ray tube and gantry rotation producing the first multi-source gantry-based CT scanner prototype. Currently the system is in the process of being upgraded to 32 X-ray sources to provide a larger field-of-view and to demonstrate the concept of virtual bowtie.

Effects of fixed-rate CT projection data compression on perceived and measured CT image quality

Compression of computed tomography (CT) projection data reduces CT scanner bandwidth and storage costs. Since fixed-rate compression guarantees predictable bandwidth, fixed-rate compression is preferable to lossless compression, but fixed-rate compression can introduce image artifacts. This research demonstrates clinically acceptable image quality at 3:1 compression as judged by a radiologist and as estimated by an image quality metric called local structural similarity (SSIM). We examine other common, quantitative image quality metrics from image processing, including peak signal-to-noise (PSNR), contrast-to-noise ratio (CNR), and difference image statistics to quantify the magnitude and location of image artifacts caused by fixed-rate compression of CT projection data. Masking effects caused by local contrast, air and bone pixels, and image reconstruction effects at the images periphery and iso-center explain why artifacts introduced by compression are not noticed by radiologists. SSIM metrics in this study nearly always exceeds 0.98 (even at 4:1 compression ratios), which is considered visually indistinguishable. The excellent correlation of local SSIM and subjective image quality assessment confirms that fixed-rate 3:1 projection data compression on CT images does not affect clinical diagnosis and is rarely noticed. Local SSIM metrics can be used to significantly reduce the number of viewed images in medical image quality studies.

Image quality evaluation of a LightSpeed CT750 HD computed tomography system

With the advancement of Computed Tomography technology, improving image quality while reducing patient dose has been a big technical challenge. The recent CT750 HD system from GE Healthcare provides significantly improved spatial resolution and the capability to reduce dose during routine clinical imaging. This paper evaluates the image quality of this system. Spatial resolution, dose reduction, noise, and low contrast detectability have been quantitatively characterized. Results show a quantifiable and visually discernable higher spatial resolution for both body and cardiac scanning modes without compromise of image noise. Further, equivalent image quality performance with up to 50% lower dose has been achieved.

Simulation and analysis of image quality impact from single-source ultra-wide coverage CT scanner

Future generations of CT systems would need a mean to cover an entire organ in a single rotation. A way to accomplish this is to physically increase detector size to provide, e.g., 120~160mm z (head-foot) coverage at iso. The x-ray cone angle of such a system is usually 3~4 times of that of a 64-slice (40mm) system, which leads to more severe cone beam artifacts in cardiac scans. In addition, the extreme x-ray take-off angles for such a system cause severe heel effect, which would require an increase in anode target angle to compensate for it. One shortcoming of larger target angle is that tube output likely decreases because of shorter thermal length. This would result in an increase of image noise. Our goal is to understand from a physics and math point of view, what is the clinical acceptable level of artifacts, resolution, and noise impact. The image artifacts are assessed through computer simulation of a helical body phantom and visual comparison of reconstructed images between a 140mm system and a 64-slice system. The IQ impact from target angle increase is studied analytically and experimentally by first finding the proper range of target angles that give the acceptable heel effect, then estimating the impact on peak power (flux) and z resolution using an empirical model of heel effect for given target angle and analytical models of z resolution and tube current loading factor for given target thermal length. The results show that, for a 140mm system, 24.5% of imaging volume exhibits more severe cone beam artifacts than a 64-slice system, which also brings up a patient dose concern. In addition, this system may suffer from a 36% peak power (flux) loss, which is equivalent to about 20% image noise increase. Therefore, a wide coverage CT system using a single x-ray source is likely to face some severe challenges in IQ and clinical accuracy.

Optimization of system design for 64-slice cone beam computed tomography

The technology for x-ray computed tomography (CT) has experienced tremendous growth in recent years. Since the introduction of 4-slice helical scanners in 1998, rapid improvement has been made on CT scanners in terms of the volume coverage, spatial resolution, scan speed, and the number of slices. These advancements not only significantly impact clinical applications, but also bring huge challenges to the CT system design. Because of the complexity of the volumetric CT (VCT) system, various strategies have to be utilized in the design process. These methodologies include theoretical analysis, computer simulation for system performance prediction, bench-top experiments for analysis confirmation, automated image analysis tools for automatically evaluating image performance, and double-blind tests with human observers for parameter optimization. In this paper, we present some of the system design considerations and optimization processes for a 64-slice scanner. These design processes ensure the optimal performance of the cone beam CT scanner. Initial clinical feedback has demonstrated the effectiveness of our approach.