Proper sizing of interventional devices to match coronary vessel dimensions improves procedural efficiency and therapeutic outcomes. We have developed a method that uses an inverse geometry x-ray fluoroscopy system [scanning beam digital x-ray (SBDX)] to automatically determine vessel dimensions from angiograms without the need for magnification calibration or optimal views. For each frame period (1/15th of a second), SBDX acquires a sequence of narrow beam projections and performs digital tomosynthesis at multiple plane positions. A three-dimensional model of the vessel is reconstructed by localizing the depth of the vessel edges from the tomosynthesis images, and the model is used to calculate the length and diameter in units of millimeters. The in vivo algorithm performance was evaluated in a healthy porcine model by comparing end-diastolic length and diameter measurements from SBDX to coronary computed tomography angiography (CCTA) and intravascular ultrasound (IVUS), respectively. The length error was −0.49±1.76 mm (SBDX – CCTA, mean±1 SD). The diameter error was 0.07±0.27 mm (SBDX − minimum IVUS diameter, mean±1 SD). The in vivo agreement between SBDX-based vessel sizing and gold standard techniques supports the feasibility of calibration-free coronary vessel sizing using inverse geometry x-ray fluoroscopy.
Proper sizing of interventional devices to match coronary vessel dimensions improves procedural efficiency and
therapeutic outcomes. We have developed a novel method using inverse geometry x-ray fluoroscopy to automatically
determine vessel dimensions without the need for magnification calibration or optimal views. To validate this method in
vivo, we compared results to intravascular ultrasound (IVUS) and coronary computed tomography angiography (CCTA)
in a healthy porcine model. Coronary angiography was performed using Scanning-Beam Digital X-ray (SBDX), an
inverse geometry fluoroscopy system that performs multiplane digital x-ray tomosynthesis in real time. From a single
frame, 3D reconstruction of the arteries was performed by localizing the depth of vessel lumen edges. The 3D model was
used to directly calculate length and to determine the best imaging plane to use for diameter measurements, where outof-
plane blur was minimized and the known pixel spacing was used to obtain absolute vessel diameter. End-diastolic
length and diameter measurements were compared to measurements from CCTA and IVUS, respectively. For vessel
segment lengths measuring 6 mm to 73 mm by CCTA, the SBDX length error was -0.49 ± 1.76 mm (SBDX - CCTA,
mean ± 1 SD). For vessel diameters measuring 2.1 mm to 3.6 mm by IVUS, the SBDX diameter error was 0.07 ± 0.27 mm (SBDX - minimum IVUS diameter, mean ± 1 SD). The in vivo agreement between SBDX-based vessel sizing and gold standard techniques supports the feasibility of calibration-free coronary vessel sizing using inverse geometry x-ray
fluoroscopy.
Low tube current scanning in combination with HYPR (HighlY constrained backPRojection) noise reduction is a proposed method for low-dose time-resolved CT myocardial perfusion imaging. We report animal studies and simulations investigating the coronary angiographic information available in these scans. Four pigs were scanned at 100 rnA and 500 rnA. A HYPR coronary angiographic image was formed from each 100 rnA scan by producing a time-averaged composite image from cardiac cycles centered on the time of peak left ventricular intensity and then multiplying the composite by a weighting image to restore image intensities. Image noise, coronary artery cross sectional area, and coronary artery intensity were measured as a function of the number of beats in the composite, weighting image filtration, and coronary artery size. HYPR images maintained coronary artery area and intensity to within 6-8% of of filtered back projection (FBP) image values, on average, for vessels with cross
sectional area greater than 2 mm2. Vessel visibility in 100-mA HYPR images was improved relative to 100-mA
FBP, in cross sectional and multiplanar reformatted images.
Scanning Beam Digital X-ray (SBDX) is a low-dose inverse geometry fluoroscopic system for cardiac interventional
procedures. The system performs x-ray tomosynthesis at multiple planes in each frame period and combines the
tomosynthetic images into a projection-like composite image for fluoroscopic display. We present a novel method of
stereoscopic imaging using SBDX, in which two slightly offset projection-like images are reconstructed from the same
scan data by utilizing raw data from two different detector regions. To confirm the accuracy of the 3D information
contained in the stereoscopic projections, a phantom of known geometry containing high contrast steel spheres was
imaged, and the spheres were localized in 3D using a previously described stereoscopic localization method. After
registering the localized spheres to the phantom geometry, the 3D residual RMS errors were between 0.81 and 1.93 mm,
depending on the stereoscopic geometry. To demonstrate visualization capabilities, a cardiac RF ablation catheter was
imaged with the tip oriented towards the detector. When viewed as a stereoscopic red/cyan anaglyph, the true orientation
(towards vs. away) could be resolved, whereas the device orientation was ambiguous in conventional 2D projection
images. This stereoscopic imaging method could be implemented in real time to provide live 3D visualization and device
guidance for cardiovascular interventions using a single gantry and data acquired through normal, low-dose SBDX
imaging.
Radiation dose reduction remains at the forefront of research in computed tomography. X-ray tube parameters such as
tube current can be lowered to reduce dose; however, images become prohibitively noisy when the tube current is too
low. Wavelet denoising is one of many noise reduction techniques. However, traditional wavelet techniques have the
tendency to create an artificial noise texture, due to the nonuniform denoising across the image, which is undesirable
from a diagnostic perspective. This work presents a new implementation of wavelet denoising that is able to achieve
noise reduction, while still preserving spatial resolution. Further, the proposed method has the potential to improve those
unnatural noise textures. The technique was tested on both phantom and animal datasets (Catphan phantom and timeresolved
swine heart scan) acquired on a GE Discovery VCT scanner. A number of tube currents were used to
investigate the potential for dose reduction.
KEYWORDS: Sensors, Prototyping, X-rays, Detection and tracking algorithms, Optical spheres, Computer simulations, 3D image processing, Tomography, X-ray detectors, Signal to noise ratio
The Scanning-Beam Digital X-ray (SBDX) system performs rapid scanning of a narrow x-ray beam using an
electronically scanned focal spot and inverse beam geometry. SBDX's ability to perform real-time multi-plane
tomosynthesis with high dose efficiency is well-suited to interventional procedures such as left atrial ablation, where
precise knowledge of catheter positioning is desired and imaging times are long. We describe and evaluate techniques
for frame-by-frame 3D localization of multiple catheter electrodes from the stacks of tomosynthetic images generated by
SBDX. The localization algorithms operate on gradient-filtered versions of the tomosynthetic planes. Small high
contrast objects are identified by thresholding the stack of images and applying connected component analysis. The 3D
coordinate of each object is the center-of-mass of each connected component. Simulated scans of phantoms containing
1-mm platinum spheres were used to evaluate localization performance with the SBDX prototype (5.5 × 5.5 cm detector,
3° tomographic angle) and a with new SBDX detector under design (10-cm wide × 6 cm, 6° × 3°). Z-coordinate error
with the SBDX prototype was -0.6 +/- 0.7 mm (mean+/-standard deviation) with 28 cm acrylic, 24.3 kWp source
operation, and 12-mm plane spacing. Localization improved to -0.3 +/- 0.3 mm using the wider SBDX detector and a 3-mm plane spacing. The effects of tomographic angle, plane-to-plane spacing, and object velocity are evaluated, and a
simulation demonstrating ablation catheter localization within a real anatomic background is presented. Results indicate
that SBDX is capable of precise real-time 3D tracking of high contrast objects.
C. Mistretta, O. Wieben, J. Velikina, Y. Wu, K. Johnson, F. Korosec, O. Unal, G. Chen, S. Fain, B. Christian, O. Nalcioglu, R. Kruger, W. Block, A. Samsonov, M. Speidel, M. Van Lysel, H. Rowley, M. Supanich, P. Turski, Yan Wu, J. Holmes, S. Kecskemeti, C. Moran, R. O'Halloran, L. Keith, A. Alexander, E. Brodsky, J. Lee, T. Hall, J. Zagzebski
KEYWORDS: Signal to noise ratio, Composites, Image processing, X-rays, Magnetic resonance imaging, Medical imaging, Lawrencium, Convolution, Angiography, Positron emission tomography
During the last eight years our group has developed radial acquisitions with angular undersampling
factors of several hundred that accelerate MRI in selected applications. As with all previous
acceleration techniques, SNR typically falls as least as fast as the inverse square root of the
undersampling factor. This limits the SNR available to support the small voxels that these methods
can image over short time intervals in applications like time-resolved contrast-enhanced MR
angiography (CE-MRA). Instead of processing each time interval independently, we have developed
constrained reconstruction methods that exploit the significant correlation between temporal
sampling points. A broad class of methods, termed HighlY Constrained Back PRojection (HYPR),
generalizes this concept to other modalities and sampling dimensions.
In this study we develop a novel ECG-gated method of HYPR (HighlY constrained backPRojection) CT reconstruction for low-dose myocardial perfusion imaging and present its first application in a porcine model. HYPR is a method of reconstructing time-resolved images from view-undersampled projection data. Scanning and reconstruction techniques were explored using x-ray projections from a 50 sec contrast-enhanced axial scan of a 47 kg swine on a 64-slice MDCT system. Scans were generated with view undersampling factors from 2 to 10. A HYPR reconstruction algorithm was developed in which a fully-sampled composite image is generated from views collected from multiple cardiac cycles within a diastolic window. A time frame image for a heartbeat was produced by modifying the composite with projections from the cycle of interest. Heart rate variations were handled by automatically selecting cardiac window size and number of cycles per composite within defined limits. Cardiac window size averaged 35% of the R-R interval for 2x undersampling and increased to 64% R-R using 10x undersampling. The selected window size and cycles per composite was sensitive to synchrony between heart rate, gantry rate, and the view undersampling pattern. Temporal dynamics and perfusion metrics measured in conventional short-scan (FBP) images were well-reproduced in the undersampled HYPR time series. Mean transit times determined from HYPR myocardial time-density curves agreed to within 8% with the FBP results. The results indicate potential for an order of magnitude reduction in dose requirement per image in cardiac perfusion CT via undersampled scanning and ECG-gated HYPR reconstruction.
The feasibility of making regional perfusion measurements using a tomosynthetic digital subtraction angiography (TDSA) acquisition has been demonstrated. The study of tomosynthetic perfusion measurements was motivated by the clinical desire for perfusion measurements in an interventional angiography suite. These pilot studies were performed using the scanning-beam digital x-ray (SBDX) system which is an inverse-geometry imaging device which utilizes an electromagnetically-scanned x-ray source, and a small CdTe direct conversion photon counting detector. The scanning electron source was used to acquire planar-tomographic images of a 12.5 x 12.5 cm field of view at a frame rate of 15 frames/sec during dynamic contrast injection. A beagle animal model was used to evaluate the tomosynthetic perfusion measurements. A manual bolus injection of iodinated contrast solution was used in order to resolve the parameters of the contrast pass curve. The acquired planar tomosynthetic dataset was reconstructed with a simple back-projection algorithm. Digital subtraction techniques were used to visualize the change in contrast agent intensity in each reconstructed plane. Given the TDSA images, region of interest based analysis was used in the selection of the image pixels corresponding to the artery and tissue bed. The mean transit time (MTT), regional cerebral blood volume (rCBV) and regional cerebral blood flow (rCBF) were extracted from the tomosynthetic data for selected regions in each of the desired reconstructed planes. For the purpose of this study, the arterial contrast enhancement curve was fit with a combination of gamma variate terms, and the MTT was calculated using a deconvolution based on the singular value decomposition (SVD). The results of the contrast pass curves derived with TDSA were consistent with the results from perfusion measurements as implemented with CT acquisition.
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.
A prototype scanning-beam digital x-ray system for cardiac fluoroscopy has been constructed. Source-to-detector distance is 94 cm with the subject positioned near the source. The 4-kW source operates at 70-110 kVp and has an electromagnetically-scanned 25-cm-diameter transmission target. The target is at ground potential and is directly liquid cooled for continuous full-power operation. The source collimator has 22,000 holes whose axes are aligned with the center of the detector array. Beam divergence through the 0.38-mm-diameter collimator holes is matched to the 1.8-cm diameter of the detector array. The detector is a 96- element scintillator array optically coupled to a 96-channel photomultiplier tube. A narrow (0.6 degree half-angle) x-ray beam scans the 19-cm-diameter field of view at 30 frames/sec. A two-dimensional shift-and-add reconstruction algorithm produces a narrow-angle classical tomographic view of the subject in real time. The small detector area and large patient- detector distance result in negligible detected x-ray scatter. Signal-to-noise ratio is calculated to be equal to conventional fluoroscopic systems with ten times less patient skin exposure and better than four times less patient integral dose. Exposure reduction is due to the elimination of x-ray scatter and the anti-scatter grid, increased detector DQE, and geometric considerations.
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