Purpose: To develop coronary phantoms that mimic patient geometry and coronary blood flow conditions for CT imaging optimization and software validation. Materials and Methods: Five patients with varying degrees of coronary artery disease underwent 320-detector row coronary CT angiography (Aquilion ONE, Canon Medical Systems). The aorta and coronary arteries were segmented using a Vitrea Workstation (Vital Images). Patient anatomy was manipulated in Autodesk Meshmixer and 3D printed in Tango+, a flexible polymer, using an Eden260V printer (Stratasys). Phantoms were connected to a pump that simulates physiologic pulsatile flow waveforms, correlated with a simulated ECG signal. Distal resistance was optimized for all three coronary vessels until physiologically accurate flow rates and pressure were observed. Phantoms underwent coronary CT Angiography (CTA) using a standard acquisition protocol and contrast mixed in the flow loop. Image data from the phantoms were input to a CT-FFR research software and compared to those derived from the clinical data. Results: All five patient-specific phantoms were successfully imaged with CTA and the images were analyzed by the CTFFR software. The phantom CT-FFR results had a mean difference of -5.4% compared to the patient CT-FFR results. Patient and phantom CT-FFR agreed for all three coronary vessels, with Pearson correlations r = 0.83, 0.68, 0.62 (LAD, LCX, RCA). Conclusions: 3D printed patient-specific phantoms can be manipulated through material properties, flow regulations, and a pulsatile waveform to create accurate flow conditions for CT based experimentation.
Experimental prototype of a photon counting scanning slit X-ray imaging system is being evaluated for potential application in digital mammography. This system is based on a recently developed and tested “edge-on” illuminated Microchannel Plate (MCP) detector. The MCP detectors are well known for providing a combination of capabilities such as direct conversion, physical charge amplification, pulse counting, high spatial and temporal resolution, and very low noise. However, their application for medical imaging was hampered by their low detection efficiency. This limitation was addressed using an “edge-on” illumination mode for MCP. The current experimental prototype was developed to investigate the imaging performance of this detector concept for digital mammography. The current prototype provides a 60 mm field of view, 200 kHz count rate with 20% non-paralysable dead time and >7 lp/mm limiting resolution. A 0.3 mm focal spot W target X-ray tube was used for image acquisition. The detector noise is 0.3 count/pixel for 50x50 micron pixels. The count rate of the current prototype is limited by the delay line readout electronics, which causes long scanning times (minutes) and high tube loading. This problem will be addressed using multichannel ASIC electronics for clinical implementation. However, the current readout architecture is adequate for evaluation of the performance parameters of the new detector concept. It is very simple and provides a maximum intrinsic resolution of 28 micron FWHM. The prototype was evaluated using resolution, contrast detail and breast Phantoms. The MTF and DQE of the system are being evaluated at different tube voltages. The design parameters of a scanning multiple slit mammography system are being evaluated. It is concluded that a photon counting, quantum limited and virtually scatter free digital mammography system can be developed based on the proposed detector.
Three dimensional reconstruction and quantitative analysis of angiograms require vessel centerline determination and tracking. In a vessel profile, it is more straightforward to locate the valley point (local minimum) than the center. Therefore, vessel valley courses offer advantages over centerlines in terms of natural features and easy-to-locate. We propose a 'star' scan technique to generate a valley map, which is then traced to determine the valley courses. The angiogram is scanned along the horizontal, vertical, diagonal and anti-diagonal directions. The scan pattern resembles a 'star'; therefore, it is referred to as a 'star' scan. The scanning along each direction provides an image consisting of scan profiles, which may be multi-modal functions. We then detect and record the local minimum locations, thereby generating a valley map. By searching over the valley map, we can generate valley courses, which can be used for vessel quantitative analysis and 3-D reconstruction. Using the valley course, it is a straightforward process to generate centerlines. This is a robust and easily implementable algorithm for quantitative analysis of angiograms. Experimental validation of the algorithm will be reported using coronary angiograms and phantom images.
Polyphase decomposition is a down sampling operation that produces a set of low-resolution representations of an image. Such representations themselves are different by a phase in frequency domain, hence called polyphase components. An inter-component processing operation extracts meaningful features by performing simple logical operations over selected components. This strategy is applied to angiographic analysis to develop a fast feature-oriented vessel identification technique, which consists of polyphase decomposition on a binary image, followed by inter-component processing. The inter-component processing among selected components produces a feature map in which a non-zero pixel indicates an occurrence of a vessel geometrical feature or pattern in the original image. Using feature templates, a sequence of vessel-featured maps is generated. Fast vessel identification is performed by fusing the feature maps and displaying them according to the emergence orders of vessel geometric features, such as position, diameter, length and direction. Collective display provides a method to visualize vessel features across multiple resolutions. High-speed performance is attributed to low-resolution representation of polyphase components and simple data manipulation of inter-component processing. The tradeoff of such vessel identification technique is associated with an uncertainty for accurate measurement, arising from the inherent translations in polyphase decomposition. Therefore, accurate vessel measurements will need refinement in the original image.
Region-of-interest (ROI) fluoroscopy has previously been investigated as a method to reduce x-ray exposure to the patient and the operator. This ROI fluoroscopy technique allows the operator to arbitrarily determine the shape, size, and location of the ROI. A device was used to generate patient specific x-ray beam filters. The device is comprised of 18 step-motors that control a 16 X 16 matrix of pistons to form the filter from a deformable attenuating material. Patient exposure reductions were measured to be 84 percent for a 65 kVp beam. Operator exposure reduction was measured to be 69 percent. Due to the reduced x-ray scatter, image contrast was improved by 23 percent inside the ROI. The reduced gray level in the periphery was corrected using an experimentally determined compensation ratio. A running average interpolation technique was used to eliminate the artifacts from the ROI edge. As expected, the final corrected images show increased noise in the periphery. However, the anatomical structures in the periphery could still be visualized. This arbitrary shaped region of interest fluoroscopic technique was shown to be effective in terms of its ability to reduce patient and operator exposure without significant reduction in image quality. The ability to define an arbitrary shaped ROI should make the technique more clinically feasible.
Scanning X-ray imaging systems provide significant reduction in the detected scatter radiation, cover large areas, and potentially offer high spatial resolution. Applications of one dimensional (1D) gaseous detectors and 'edge-on' illuminated silicon strip detectors for scanning slit imaging systems are currently under intensive investigation. In this work we investigate an 'edge-on' illuminated Porous Plate (PP) detector concept for applications in diagnostic X-ray imaging. As opposed to the existing X-ray imaging detectors, 'edge-on' PP detectors can provide a combination of high stopping power, high physical charge amplification, superior spatial resolution and flexible pixel shape. One common type of PP is Microchannel Plate (MCP). It has previously been investigated as a detector in surface-on illumination mode for medical X-ray imaging. However, its detection efficiency was determined to be too low for medical imaging applications. Using 'edge-on' illumination mode for MCPs with optimized structural parameters, it is possible to reach high detection efficiency. The characteristics of 'edge-on' MCP detectors are compared with the currently available X-ray imaging detectors. Possible use of other PP materials such as Porous Dielectrics, Microspheric Plates, a-Si based MCPs, Micro columnar and Micro granular Dielectrics are discussed. An 'edge-on' illuminated MCP detector for scanning X-ray imaging system is being developed in our laboratory. The details of this system and the read out electronics will be described.
Convolution-filtering methods and direct beam stop techniques have previously been investigated for estimation of scatter-glare distribution in images acquired using digital fluoroscopic systems. The purpose of this study is to compare the error in scatter-glare estimation for a direct beam stop technique and a convolution technique. Also, the change in scatter-glare intensity due to contrast material injection for ventriculography and coronary arteriography was quantified. Three different methods were used to estimate scatter-glare intensity in humanoid chest phantom and animal model images. A lead strip was used to scan the x-ray field for direct measurement of scatter-glare intensity in every pixel in the image. An array of lead beam-stops were used in order to sample scatter-glare intensity. An interpolation technique was used to estimate scatter-glare intensity for the remaining pixels in the image. Scatter-glare intensity was also estimated using a convolution filtering technique. This technique utilizes exposure parameters and image gray levels to assign equivalent Lucite thicknesses for every pixel in the image. The thickness information is then used to estimate scatter-glare intensity on a pixel-by-pixel basis. Finally, an array of lead beam-stops were used to measure the change in scatter-glare intensity due to contrast material injection. This was done by leaving the array of lead beam-stops in the x-ray beam for ventriculography and coronary arteriography procedures. The results indicate that the array of lead beam-stops produces significant errors in estimating scatter-glare intensity. This is primarily due to the fact that contrast material injection significantly changes scatter-glare intensity. The scatter- glare intensity was changed by up to 19% and 88% during coronary arteriography and ventriculography, respectively. In conclusion, contrast material injection significantly changes the scatter-glare intensity during coronary arteriography and ventriculography procedures. Therefore, convolution-filtering techniques are more suited for cardiac imaging, where scatter- glare intensity significantly changes during image acquisition.
The concept of radiographic equalization has previously been investigated. However, a suitable technique for digital fluoroscopic applications has not been developed. The previously reported scanning equalization techniques cannot be applied to fluoroscopic applications due to their exposure time limitations. On the other hand, area beam equalization techniques are more suited for digital fluoroscopic applications. The purpose of this study is to develop an x- ray beam equalization technique for digital fluoroscopic applications that will produce an equalized radiograph with minimal image artifacts and tube loading. Preliminary unequalized images of a humanoid chest phantom were acquired using a digital fluoroscopic system. Using this preliminary image as a guide, an 8 by 8 array of square pistons were used to generate masks in a mold with CeO<SUP>2</SUP>. The CeO<SUP>2</SUP> attenuator thicknesses were calculated using the gray level information from the unequalized image. The generated mask was positioned close to the focal spot (magnification of 8.0) in order to minimize edge artifacts from the mask. The masks were generated manually in order to investigate the piston and matrix size requirements. The development of an automated version of mask generation and positioning is in progress. The results of manual mask generation and positioning show that it is possible to generate equalized radiographs with minimal perceptible artifacts. The equalization of x-ray transmission across the field exiting from the object significantly improved the image quality by preserving local contrast throughout the image. Furthermore, the reduction in dynamic range significantly reduced the effect of x-ray scatter and veiling glare from high transmission to low transmission areas. Also, the x-ray tube loading due to the mask assembly itself was negligible. In conclusion it is possible to produce area beam compensation that will be compatible with digital fluoroscopy with minimal compensation artifacts. The compensation process produces an image with equalized signal to noise ratio in all parts of the image.
In order to quantitate anatomical and physiological parameters such as vessel dimensions and volumetric blood flow, it is necessary to make corrections for scatter and veiling glare (SVG), which are the major sources of nonlinearities in videodensitometric digital subtraction angiography (DSA). A convolution filtering technique has been investigated to estimate SVG distribution in DSA images without the need to sample the SVG for each patient. This technique utilizes exposure parameters and image gray levels to estimate SVG intensity by predicting the total thickness for every pixel in the image. At this point, corrections were also made for variation of SVG fraction with beam energy and field size. To test its ability to estimate SVG intensity, the correction technique was applied to images of a Lucite step phantom, anthropomorphic chest phantom, head phantom, and animal models at different thicknesses, projections, and beam energies. The root-mean-square (rms) percentage error of these estimates were obtained by comparison with direct SVG measurements made behind a lead strip. The average rms percentage errors in the SVG estimate for the 25 phantom studies and for the 17 animal studies were 6.22% and 7.96%, respectively. These results indicate that the SVG intensity can be estimated for a wide range of thicknesses, projections, and beam energies.
An angiographic method using first pass distribution analysis (FPA) has been investigated for determining instantaneous absolute volumetric blood flow in an angiographic perfusion phantom and in a rabbit animal model following intra-arterial injection of contrast material. The method is based on the concept of conservation of contrast material in successive angiographic images, utilizing the videodensitometric information in the arterial bed. The volume of contrast material entering the perfusion bed between two successive images was determined using videodensitometric and entrance vessel calibration techniques. In phantom studies, measured (M) and known (K) mean flow rates were related with videodensitometric and entrance vessel calibration techniques, respectively. In vivo, measured and known flow rates in the left common carotid artery of two rabbits were related with the videodensitometric and the entrance vessel calibration techniques, respectively. The results of this study demonstrated the potential utility of the FPA algorithm in conjunction with digital substraction angiography for measuring phasic blood flow.
Using ultrafast computed tomography (CT) for calcium quantification offers a potentially non- invasive way to evaluate the presence and severity of coronary artery disease. The currently applied index of ultrafast CT coronary calcium amount is the coronary calcium score of Agatston et al., but this score has not been thoroughly evaluated as to its accuracy and stability. In the present research we estimate the grey scale non-uniformity within the vascular blood pools in the anterior-posterior axis, and in the cephalad-caudad axis. We then estimate the effects of these non-uniformities on coronary calcium scores.