R Rana

Anti-scatter grid artifact elimination for high resolution x-ray imaging CMOS detectors

Higher resolution in dynamic radiological imaging such as angiography is increasingly being demanded by clinicians; however, when standard anti-scatter grids are used with such new high resolution detectors, grid-line artifacts become more apparent resulting in increased structured noise that may overcome the contrast signal improvement benefits of the scatter-reducing grid. Although grid-lines may in theory be eliminated by dividing the image of a patient taken with the grid by a flat-field image taken with the grid obtained prior to the clinical image, unless the remaining additive scatter contribution is subtracted in real-time from the dynamic clinical image sequence before the division by the reference image, severe grid-line artifacts may remain. To investigate grid-line elimination, a stationary Smit Rӧntgen X-ray grid (line density: 70 lines/cm, grid ratio 13:1) was used with both a 75 micron-pixel CMOS detector and a standard 194 micron-pixel flat panel detector (FPD) to image an artery block insert placed in a modified uniform frontal head phantom for a 20 x 20cm FOV (approximately). Contrast and contrast-to-noise ratio (CNR) were measured with and without scatter subtraction prior to grid-line correction. The fixed pattern noise caused by the grid was substantially higher for the CMOS detector compared to the FPD and caused a severe reduction of CNR. However, when the scatter subtraction corrective method was used, the removal of the fixed pattern noise (grid artifacts) became evident resulting in images with improved CNR.

SU-E-I-48: Noise Reduction with Over-Sampling for High Resolution Detectors Using a Spread Function Convolution Method

Purpose: A method to reduce noise resulting from the use of higher resolution x-ray detectors being developed to meet the demands of image guided vascular interventions is demonstrated. Methods: New direct detectors based on amorphous Se can have MTFs that remain high even at their Nyquist frequency. Since such detectors can be made with smaller pixels than may be required for even the high resolution requirements of many neurovascular applications, the resulting over-sampled images can be convolved with various functions to lower the noise. The effect on resolution can then be compared with a simple pixel binning. The general method proposed by Cunningham et al as the apodized-aperture pixel (AAP) design was compared with the Result for various simple 1D convolution spread functions with the effects on total noise and MTF compared to that of the binning case for a 25 μm aSe CMOS detector’s MTF published by the University of Waterloo group lead by KS Karim. Results: Assuming a white noise image, various convolution kernels resulted in similar reductions of total standard deviation. Detailed comparisons were made with the simple the 2x binning case. While reducing noise, the over-sampling convolution method using simple convolution low pass filters did not show advantage over 2x binning with regard to modifying the MTF; however, significant improvement was evidenced for the more complex sinc function used in the AAP design. Conclusion: As higher resolution detectors are being developed to meet the increasing demands for improved images to guide finer vascular interventions, use of super-sampled aSe detectors where resolution is maintained with reduced noise may well fill this requirement. Partial support from NIH Grant R01EB002873 and Toshiba Medical Systems Equipment Grant

Workflow for the use of a high-resolution image detector in endovascular interventional procedures

Endovascular image-guided intervention (EIGI) has become the primary interventional therapy for the most widespread vascular diseases. These procedures involve the insertion of a catheter into the femoral artery, which is then threaded under fluoroscopic guidance to the site of the pathology to be treated. Flat Panel Detectors (FPDs) are normally used for EIGIs; however, once the catheter is guided to the pathological site, high-resolution imaging capabilities can be used for accurately guiding a successful endovascular treatment. The Micro-Angiographic Fluoroscope (MAF) detector provides needed high-resolution, high-sensitivity, and real-time imaging capabilities. An experimental MAF enabled with a Control, Acquisition, Processing, Image Display and Storage (CAPIDS) system was installed and aligned on a detector changer attached to the C-arm of a clinical angiographic unit. The CAPIDS system was developed and implemented using LabVIEW software and provides a user-friendly interface that enables control of several clinical radiographic imaging modes of the MAF including: fluoroscopy, roadmap, radiography, and digital-subtraction-angiography (DSA). Using the automatic controls, the MAF detector can be moved to the deployed position, in front of a standard FPD, whenever higher resolution is needed during angiographic or interventional vascular imaging procedures. To minimize any possible negative impact to image guidance with the two detector systems, it is essential to have a well-designed workflow that enables smooth deployment of the MAF at critical stages of clinical procedures. For the ultimate success of this new imaging capability, a clear understanding of the workflow design is essential. This presentation provides a detailed description and demonstration of such a workflow design.

Real time implementation of anti-scatter grid artifact elimination method for high resolution x-ray imaging CMOS detectors using Graphics Processing Units (GPUs)

Scatter is one of the most important factors effecting image quality in radiography. One of the best scatter reduction methods in dynamic imaging is an anti-scatter grid. However, when used with high resolution imaging detectors these grids may leave grid-line artifacts with increasing severity as detector resolution improves. The presence of such artifacts can mask important details in the image and degrade image quality. We have previously demonstrated that, in order to remove these artifacts, one must first subtract the residual scatter that penetrates through the grid followed by dividing out a reference grid image; however, this correction must be done fast so that corrected images can be provided in real-time to clinicians. In this study, a standard stationary Smit-Rontgen x-ray grid (line density - 70 lines/cm, grid ratio - 13:1) was used with a high-resolution CMOS detector, the Dexela 1207 (pixel size - 75 micron) to image anthropomorphic head phantoms. For a 15 x 15 cm field-of-view (FOV), scatter profiles of the anthropomorphic head phantoms were estimated then iteratively modified to minimize the structured noise due to the varying grid-line artifacts across the FOV. Images of the head phantoms taken with the grid, before and after the corrections, were compared, demonstrating almost total elimination of the artifact over the full FOV. This correction is done fast using Graphics Processing Units (GPUs), with 7-8 iterations and total time taken to obtain the corrected image of only 87 ms, hence, demonstrating the virtually real-time implementation of the grid-artifact correction technique.

SU-C-209-03: Anti-Scatter Grid-Line Artifact Minimization for Removing the Grid Lines for Three Different Grids Used with a High Resolution CMOS Detector

PURPOSE Demonstrate the effectiveness of an anti-scatter grid artifact minimization method by removing the grid-line artifacts for three different grids when used with a high resolution CMOS detector. METHOD Three different stationary x-ray grids were used with a high resolution CMOS x-ray detector (Dexela 1207, 75 µm pixels, sensitivity area 11.5cm × 6.5cm) to image a simulated artery block phantom (Nuclear Associates, Stenosis/Aneurysm Artery Block 76-705) combined with a frontal head phantom used as the scattering source. The x-ray parameters were 98kVp, 200mA, and 16ms for all grids. With all the three grids, two images were acquired: the first for a scatter-less flat field including the grid and the second of the object with the grid which may still have some scatter transmission. Because scatter has a low spatial frequency distribution, it was represented by an estimated constant value as an initial approximation and subtracted from the image of the object with grid before dividing by an average frame of the grid flat-field with no scatter. The constant value was iteratively changed to minimize residual grid-line artifact. This artifact minimization process was used for all the three grids. RESULTS Anti-scatter grid lines artifacts were successfully eliminated in all the three final images taken with the three different grids. The image contrast and CNR were also compared before and after the correction, and also compared with those from the image of the object when no grid was used. The corrected images showed an increase in CNR of approximately 28%, 33% and 25% for the three grids, as compared to the images when no grid at all was used. CONCLUSION Anti-scatter grid-artifact minimization works effectively irrespective of the specifications of the grid when it is used with a high spatial resolution detector. Partial support from NIH Grant R01-EB002873 and Toshiba Medical Systems Corp.

WE-G-204-04: Focal Spot Deblurring For High Resolution Amorphous Selenium (aSe) Complementary Metal Oxide Semiconductor (CMOS) X-Ray Detector

Purpose: CMOS-based aSe detectors compared to CsI-TFT-based flat panels have the advantages of higher spatial sampling due to smaller pixel size and decreased blurring characteristic of direct rather than indirect detection. For systems with such detectors, the limiting factor degrading image resolution then becomes the focal-spot geometric unsharpness. This effect can seriously limit the use of such detectors in areas such as cone beam computed tomography, clinical fluoroscopy and angiography. In this work a technique to remove the effect of focal-spot blur is presented for a simulated aSe detector. Method: To simulate images from an aSe detector affected with focal-spot blur, first a set of high-resolution images of a stent (FRED from Microvention, Inc.) were acquired using a 75µm pixel size Dexela-Perkin-Elmer detector and averaged to reduce quantum noise. Then the averaged image was blurred with a known Gaussian blur at two different magnifications to simulate an idealized focal spot. The blurred images were then deconvolved with a set of different Gaussian blurs to remove the effect of focal-spot blurring using a threshold-based, inverse-filtering method. Results: The blur was removed by deconvolving the images using a set of Gaussian functions for both magnifications. Selecting the correct function resulted in an image close to the original; however, selection of too wide a function would cause severe artifacts. Conclusion: Experimentally, focal-spot blur at different magnifications can be measured using a pin hole with a high resolution detector. This spread function can be used to deblur the input images that are acquired at corresponding magnifications to correct for the focal spot blur. For CBCT applications, the magnification of specific objects can be obtained using initial reconstructions then corrected for focal-spot blurring to improve resolution. Similarly, if object magnification can be determined such correction may be applied in fluoroscopy and angiography.

SU‐E‐I‐50: Investigation of the Effect of Anti‐Scatter Grid Septa Size and Orientation On Artifacts with High Resolution Detectors When Decreased Pixel Size Approaches Septa Width

Purpose: As x-ray detectors with greatly increased spatial resolution become available, their decreased pixel dimensions can approach the size of anti-scatter grid septa resulting in loss of primary x-ray detection and imaging artifacts. We investigate this effect and any potential advantage gained by rotating anti-scatter grids 45 degrees. Methods: We calculated the percent of pixel area unobstructed by grid septa for the maximum single pixel coverage of septa-pixel alignment for both 0 degree and 45 degree grid orientations for a broad range of ratios of pixel to septa sizes. We also calculated the percent benefit gained from a 45 degree grid orientation over the 0 degree orientation. Results: There is some benefit provided by the 45 degree orientation for pixel-to-septa ratios between 0.75 and 1.21 with a peak benefit of 8.6% at a ratio of 1.0 for exact alignment of the centers of the pixel and septa. For ratios below 0.75 total loss of data may occur regardless of grid orientation for some pixels. For ratios above 1.21, the 45 degree orientation corresponds increasingly to septa covering up a greater pixel area along the pixel diagonal. Large pixel-to-septa ratios for most current systems meet this criteria with flat panel detector pixels typically of 150 to 200 um and grid septa of 20 to 30 um implying that the 45 degree orientation is inappropriate. For new and proposed high resolution 25 to 35 um pixel detectors, septa coverage and the consequent image artifacts must be considered. Conclusion: The analysis of grid septa coverage is dependent on the pixel-to-septa ratio. Although some small benefit in using a 45 degree orientation for ratios near 1 is possible, other image processing and/or mechanical grid movement methods will have to be explored to minimize image artifacts resulting from grid usage with high resolution detectors. Support in part by NIH grant R01EB002873

WE‐G‐204‐06: Grid‐Line Artifact Minimization for High Resolution Detectors Using Iterative Residual Scatter Correction

Purpose: Anti-scatter grid-line artifacts are more prominent for high-resolution x-ray detectors since the fraction of a pixel blocked by the grid septa is large. Direct logarithmic subtraction of the artifact pattern is limited by residual scattered radiation and we investigate an iterative method for scatter correction. Methods: A stationary Smit-Rοntgen anti-scatter grid was used with a high resolution Dexela 1207 CMOS X-ray detector (75 µm pixel size) to image an artery block (Nuclear Associates, Model 76-705) placed within a uniform head equivalent phantom as the scattering source. The image of the phantom was divided by a flat-field image obtained without scatter but with the grid to eliminate grid-line artifacts. Constant scatter values were subtracted from the phantom image before dividing by the averaged flat-field-with-grid image. The standard deviation of pixel values for a fixed region of the resultant images with different subtracted scatter values provided a measure of the remaining grid-line artifacts. Results: A plot of the standard deviation of image pixel values versus the subtracted scatter value shows that the image structure noise reaches a minimum before going up again as the scatter value is increased. This minimum corresponds to a minimization of the grid-line artifacts as demonstrated in line profile plots obtained through each of the images perpendicular to the grid lines. Artifact-free images of the artery block were obtained with the optimal scatter value obtained by this iterative approach. Conclusion: Residual scatter subtraction can provide improved grid-line artifact elimination when using the flat-field with grid “subtraction” technique. The standard deviation of image pixel values can be used to determine the optimal scatter value to subtract to obtain a minimization of grid line artifacts with high resolution x-ray imaging detectors. This study was supported by NIH Grant R01EB002873 and an equipment grant from Toshiba Medical Systems Corp.

SU-E-I-13: Comparison of CCD and CMOS Micro-Angiographic Fluoroscope (MAF) Detector Systems Using Contrast Measurements for Specific Imaging Tasks Related to Neuro-Endovascular Image-Guided Interventional (EIGI) Procedures

PURPOSE To compare the performance of two high-resolution detector systems for use during neuro-EIGI procedures. METHODS The task-specific performance of custom high-resolution CCD and CMOS micro-angiographic fluoroscopes (MAF) was compared. To simulate the attenuation of x-rays by a patient in neuro-EIGIs, a modified ANSI head phantom was used. A bar phantom with 0.05-mm thick lead was used to simulate the contrast and size of EIGI devices used in neuro EIGIs. The bar phantom, placed at an angle of 6.5 degrees relative to the pixel rows of both detectors, was imaged for 88kVp using the small 0.3mm focal spot in order to decrease the geometric unsharpness and preserve the high-resolution capabilities of both detectors. The resultant images have a geometric object magnification of 1.25, similar to that of a clinical neuro-EIGI, and a fixed field size was used to keep the scatter the same for both detectors. For images obtained using each detector, we calculated and compared the contrast of the averaged line profiles for the 5.0, 5.6 and 6.3 lp/mm bar pattern groups, which have lead-line widths of 100, 89, and 79 microns, respectively. RESULTS For the MAF-CCD, the measured contrasts were 0.020, 0.019 and 0.013 for the 5.0, 5.6 and 6.3 line pair groups, respectively, and for the MAF-CMOS the contrasts were 0.025, 0.019 and 0.014 for the 5.0, 5.6 and 6.3 line pair groups, respectively. CONCLUSION For the very small lines of the bar pattern, the MAF-CMOS appears to provide almost the same contrast as compared to the MAF-CCD, within the measurement error; however, noise and dynamic range characteristics remain to be studied before concluding that the MAF-CMOS can replace the MAF-CCD during task-specific neuro-EIGI procedures where high spatial resolution is required. Supported by NIH Grant: 2R01EB002873 and an equipment grant from Toshiba Medical Systems Corporation.

SU-E-I-83: Parallel Programming Upgrades for the Control Acquisition, Processing and Image Display System (CAPIDS) of the Micro Angiographic Fluoroscope (MAF)

PURPOSE CAPIDS is a unique software platform designed to control and acquire images from the high-resolution MAF detector, process them and display them in a clinical environment. The images are then stored for optional playback at a later stage. CAPIDS also acquires and records the exposure parameters from the x-ray unit. We present new parallel programming modifications using the host computer systems Graphics Processing Unit (GPU) and Central Processing unit (CPU) to improve the system performance for the various MAF imaging tasks. METHODS Multicore CPUs allow for concurrent tasks to be executed at the same time in parallel. During runtime, CAPIDS has three concurrent tasks: image acquisition and processing, image saving, and exposure parameter acquisition. Parallel programming constructs from LabVIEW allow each tasks to be executed on a separate core.GPUs allow for the same task to be performed on independent data sets in parallel. During runtime, all the image processing including flat-field correction, digital image subtraction, image averaging, and temporal recursive filtering are performed on the GPU. RESULTS The new version of CAPIDS with all the parallel programming updates was successfully used for the first time to control the MAF, acquire the images, process the images and display the images during an actual clinical intervention. The images were acquired under fluoroscopy, digital subtraction angiography, and roadmap modes. CONCLUSION Distributing concurrent tasks to different cores of a multicore CPU results in an efficient utilization of resources, efficient power management and increases operation speed. Use of GPUs for image processing further enhances the speed of operation. Supported by NIH Grant: 2R01EB002873 and an equipment grant from Toshiba Medical Systems Corporation.