Iterative reconstruction methods have emerged as a promising avenue to reduce dose in CT imaging. Another,
perhaps less well-known, advance has been the development of inverse geometry CT (IGCT) imaging
systems, which can significantly reduce the radiation dose delivered to a patient during a CT scan compared
to conventional CT systems. Here we show that IGCT data can be reconstructed using iterative methods,
thereby combining two novel methods for CT dose reduction.
A prototype IGCT scanner was developed using a scanning beam digital X-ray system - an inverse geometry
fluoroscopy system with a 9,000 focal spot x-ray source and small photon counting detector. 90 fluoroscopic
projections or "superviews" spanning an angle of 360 degrees were acquired of an anthropomorphic phantom
mimicking a 1 year-old boy. The superviews were reconstructed with a custom iterative reconstruction
algorithm, based on the maximum-likelihood algorithm for transmission tomography (ML-TR). The
normalization term was calculated based on flat-field data acquired without a phantom. 15 subsets were used,
and a total of 10 complete iterations were performed.
Initial reconstructed images showed faithful reconstruction of anatomical details. Good edge resolution and
good contrast-to-noise properties were observed. Overall, ML-TR reconstruction of IGCT data collected by a
bench-top prototype was shown to be viable, which may be an important milestone in the further development
of inverse geometry CT.
We investigate a real-time digital tomosynthesis (DTS) imaging modality, based on the scanning beam digital
x-ray (SBDX) hardware, used in conjunction with an electromagnetic navigation bronchoscopy (ENB) system
to provide improved image guidance for minimally invasive transbronchial needle biopsy (TBNbx). Because the
SBDX system source uses electron beams, steered by electromagnets, to generate x-rays, and the ENB system
generates an electromagnetic field to localize and track steerable navigation catheters, the two systems will affect
each other when operated in proximity. We first investigate the compatibility of the systems by measuring the
ENB system localization error as a function of distance between the two systems. The SBDX system reconstructs
DTS images, which provide depth information, and so we investigate the improvement in lung nodule visualization
using SBDX system DTS images and compare them to fluoroscopic images currently used for biopsy verification.
Target localization error remains below 2mm (or virtually error free) if the volume-of-interest (VOI) is at least
50cm away from the SBDX system source and detector. Inside this region, tomographic angle ranges from 3° to
10° depending on the VOI location. Improved lung nodule (≤ 20mm diameter) contrast is achieved by imaging
the VOI near the SBDX system detector, where the tomographic angle is maximized. The combination of the
SBDX image guidance with an ENB system would provide real-time visualization during biopsy with improved
localization of the target and needle/biopsy instruments, thereby increasing the average and lowering the variance
of the yield for TBNbx.
An advanced Scanning-Beam Digital X-ray (SBDX) system for cardiac angiography has been constructed. The 15-kW source operates at 70 - 120 kVp and has an electron beam that is electromagnetically scanned across a 23-cm X 23-cm transmission target. The target is directly liquid cooled for continuous full-power operation and is located behind a focused source collimator. The collimator is a rectangular grid of 100 X 100 apertures whose axes are aligned with the center of the detector array. X-ray beam divergence through the collimator apertures is matched to the 5.4-cm X 5.4 cm detector, which is 150 cm from the source. The detector is a 48 X 48 element CdZnTe direct-conversion photon-counting detector. A narrow x-ray beam scans the full field of view at up to 30 frames per second. A custom digital processor simultaneously reconstructs sixteen 1,0002 pixel tomographic images in real time. The slices are spaced 1.2 cm apart and cover the entire cardiac anatomy. The small detector area and large patient-detector distance result in negligible detected x-ray scatter. Image signal-to-noise ratio is calculated to be equal to conventional fluoroscopic systems at only 12% of the patient exposure and 25% of the staff exposure. Exposure reduction is achieved by elimination of detected scatter, elimination of the anti-scatter grid, increased detector DQE, and increased patient entrance area.
A prototype scanning-beam digital x-ray (SBDX) system for cardiac fluoroscopy has been constructed. The unique geometry and absence of detected x-ray scatter in the SBDX image promises to provide image quality equivalent to a conventional image-intensifier-based fluoroscopic system at substantially reduced x-ray exposure to patient and staff. In order to measure the SBDX exposure advantage, a contrast- detail study was performed comparing SBDX and a conventional cardiac fluoroscopic system. Low-contrast deductibility as a function of the phantom entrance exposure was determined. The expected SBDX exposure advantage was 3.0 to 3.4, for low-contrast objects ranging in diameter from 2 to 10 mm. This exposure advantage is applicable to the AP projection through an average-size cardiac patient. Based on these results, calculations show that angulated views and larger patients will experience significantly greater exposure reductions. In addition, the results also indicate that SBDX system design modifications can provide a greater exposure reduction from that measured with this prototype.