NIH Grants R01EB002873 Research Articles

TU-FG-209-05: Demonstration of the Line Focus Principle Using the Generalized Measured-Relative Object Detectability (GM-ROD) Metric

Purpose: Demonstrate and quantify the augmented resolution due to focalspot size decrease in images acquired on the anode side of the field, for both small and medium (0.3 and 0.6mm) focal-spot sizes using the experimental task-based GM-ROD metric. Theoretical calculations have shown that a medium focal-spot can achieve the resolution of a small focal-spot if acquired with a tilted anode, effectively providing a higher-output small focal-spot. Methods: The MAF-CMOS (micro-angiographic fluoroscopic complementary-metal-oxide semiconductor) detector (75µm pixel pitch) imaged two copper wire segments of different diameter and a pipeline stent at the central axis and on the anode side of the beam, achieved by tilting the x-ray C-arm (Toshiba Infinix) to 6° and realigning the detector with the perpendicular ray to correct for x-ray obliquity. The relative gain in resolution was determined using the GM-ROD metric, which compares images on the basis of the Fourier transform of the image and the measured NNPS. To emphasize the geometric unsharpness, images were acquired at a magnification of two. Results: Images acquired on the anode side were compared to those acquired on the central axis with the same target-area focal-spot to consider the effect of an angled tube, and for all three objects the advantage of the smaller effective focal-spot was clear, showing a maximum improvement of 36% in GM-ROD. The images obtained with the small focal-spot at the central axis were compared to those of the medium focal-spot at the anode side and, for all objects, the relative performance was comparable. Conclusion: For three objects, the GM-ROD demonstrated the advantage of the anode side focal-spot. The comparable performance of the medium focal-spot on the anode side will allow for a high-output small focal-spot; a necessity in endovascular image-guided interventions. Partial support from an NIH grant R01EB002873 and an equipment grant from Toshiba Medical Systems Corp.

MO-FG-CAMPUS-IeP1-04: Kerma Area Product Calculation for Non-Uniform X-Ray Fields Using a Skin Dose Tracking System

Purpose: The functionality of the Dose-Tracking System (DTS) has been expanded to include the calculation of the Kerma-Area Product (KAP) for non-uniform x-ray fields such as result from the use of compensation filters during fluoroscopic procedures Methods: The DTS calculates skin dose during fluoroscopic interventions and provides a color-coded dose map on a patient-graphic model. The KAP is the integral of air kerma over the x-ray field and is usually measured with a transmission-ionization chamber that intercepts the entire x-ray beam. The DTS has been modified to determine KAP when there are beam non-uniformities that can be modeled. For example, the DTS includes models of the three compensation filters with tapered edges located in the collimator assembly of the Toshiba Infinix fluoroscopic C-Arm and can track their movement. To determine the air kerma after the filters, DTS includes transmission factors for the compensation filters as a function of kVp and beam filtration. A virtual KAP dosimeter is simulated in the DTS by an array of graphic vertices; the air kerma at each vertex is corrected by the field non-uniformity, which in this case is the attenuation factor for those rays which pass through the filter. The products of individual vertex air-kerma values for all vertices within the beam times the effective-area-per-vertex are summed for each x-ray pulse to yield the KAP per pulse and the cumulative KAP for the procedure is then calculated. Results: The KAP values estimated by DTS with the compensation filter inserted into the x-ray field agree within ± 6% with the values displayed on the fluoroscopy unit monitor, which are measured with a transmission chamber. Conclusion: The DTS can account for field non-uniformities such as result from the use of compensation filters in calculating KAP and can obviate the need for a KAP transmission ionization chamber. Partial support from NIH Grant R01-EB002873 and Toshiba Medical Systems Corp

SU‐C‐209‐01: Validation of the Simulated Detectability Metric (G‐ROD) Using the Experimental Generalized Measured Relative Object Detectability Metric (GM‐ROD)

Purpose:The Generalized Relative Object Detectability (G‐ROD) family of ideal observer metrics are a well‐characterized set of task‐based metrics used for quantitatively comparing imaging systems that include the detector, focal spot geometry and scatter characteristics on the basis of system abilities in imaging a given simulated object. The G‐ROD metric takes the integral over spatial frequencies of the Fourier transform of a simulated object function weighted with the detector generalized DQE (GDQE), divided by the comparable integral for another detector. When a measured image of an actual object is used, the generalized measured‐ROD (GM‐ROD) metric can compare detector systems without explicitly determining detector MTF. In this work, the results of simulated and measured ROD calculations for the same object are compared for the first time.Methods:G‐ROD calculations were performed to compare a high resolution microangiographic fluoroscopic (MAF) detector to a flat panel detector (FPD) using 5mm segments of simulated copper wires of varying diameter (d=0.05–0.6mm) as the object function for three different focal spot sizes at two magnifications. A GM‐ROD experiment with similar parameters was performed to compare the same detector system using three actual 5mm copper wires (d=0.113mm,0.238mm,0.558mm) imaged under identical but simulated conditions used in G‐ROD calculations to determine the agreement between simulated and experimentally obtained results.Results:The G‐ROD and GM‐ROD calculations examined relative detector system imaging performance. The GM‐ROD results for all three different sized wires imaged with low magnification and a small focal spot size were 2.1, 1.38, and 1.1, respectively, and the G‐ROD results for the same calculation was 1.81, 1.3, and 1.07, corresponding to percent deviations of 13%, 5.8%, and 2.7%.Conclusion:These results indicate experimental validation of the simulated metric results.Partial support from NIH grant R01EB002873 and an equipment grant from Toshiba Medical Systems Corp.

TU-FG-209-10: Phantom Simulation Method to Evaluate the Clinical Utility of the High-Resolution Micro-Angiographic Fluoroscope Complementary Metal Oxide Semiconductor (MAF-CMOS) Detector

Purpose: The purpose of this study is to develop a method to evaluate the clinical utility of a high resolution 75µ pixel MAF-CMOS image receptor compared to a 200µ flat-panel detector (FPD). Methods: A simulated procedure evaluation method was developed using an aneurysm phantom since every clinical device-deployment procedure is different and to faithfully compare the performance of two detectors in a clinical setting would require the device deployment to be stopped at different stages so that images could be acquired with both detectors. Not only is this extremely challenging to implement for a large case cohort, it also requires additional exposure and risk to the patient. To simulate a treatment procedure, a 3D printed aneurysm phantom based on patient morphology was used. An anthropomorphic head phantom was placed underneath the aneurysm phantom to simulate patient attenuation and anatomical features. An endovascular coil was deployed into the aneurysm under the guidance of both the MAF-CMOS and the FPD. To qualitatively evaluate the performance of the detectors based on images, a questionnaire containing visualization criteria was developed and a grading scale from 1 (lowest) to 10 (highest) was assigned to each question. The images from these procedures along with the questionnaire will be presented to observers to qualitatively evaluate and compare the performance of the two detectors. Results: Realistic clinical interventions were simulated using a patient-specific aneurysm phantom. Using a common set of criteria in a questionnaire to individually evaluate these images and then comparing the results for a large cohort can be an effective method to determine the significance of using the MAF-CMOS detector during clinical procedures. Conclusion: The above method can be used to effectively evaluate the clinical utility of the MAF-CMOS detector without additional risk to the patient. partial support from NIH Grant R01-EB002873 and equipment grant from Toshiba Medical Systems Corp.

TU-FG-209-02: Effective Elimination of Aliased Signal Using An Apodized Aperture Pixel Design

Purpose: Signal and noise aliasing are major issues with high resolution direct and indirect detectors for which the Apodized Aperture Pixel (AAP) design provides an alternative solution. Methods: High resolution detectors for neurovascular interventions have MTFs that remain significant even at the Nyquist frequency. Under-sampling in such detectors, leads to aliasing of object frequencies and noise. Frequencies above the Nyquist wrap into the lower frequency range leading to aliasing. The conventional approach of convolving images with low-pass filters such as pixel binning helps in noise reduction but still allows aliasing. The AAP design proposed by Cunningham et.al, uses super sampled images with subpixels from a high resolution detector, convolved with a sinc function to get an image of the desired pixel size while apodizing the MTF beyond the Nyquist. The performance of the AAP filter in eliminating signal aliasing with respect to a standard filter such as simple 2x pixel binning are compared. Results: Three detectors were considered for this study including two indirect detectors with varying thickness of CsI (350µm, HR) and 500µm, HL) and a direct detector (aSe - 1000µm thickness – MTF after Rivetti et.al,). The presampled MTFs of respective detectors were convolved with a 2x pixel binning kernel and the AAP filter. The Fourier transform of an AAP filter, being the box function, the MTF remains high, up to Nyquist and then drops off drastically to zero. Detailed comparisons of resulting MTFs along with the original presampled MTF are shown. The AAP filter resulted in the least degradation of spatial resolution while eliminating aliasing thus outperforming 2x pixel binning Conclusion: In neurovascular imaging using high resolution detectors, the AAP design provides a viable option to effectively remove signal aliasing without compromising high spatial resolution. Partial support from NIH Grant R01-EB002873 and Toshiba Medical Systems Corp.

MO‐F‐CAMPUS‐I‐02: Stability Investigation of a Gamma Fitting Algorithm for Angiographic Parametric Imaging at Low X‐Ray Exposures Using a Patient Specific Neurovascular Phantom

Purpose:To analyze the stability of a Gamma fitting algorithm based on digital subtraction angiography sequences, at different exposure, hence noise levels.Methods:Starting with a patient CT volume, we built a 3D printed neurovascular phantom which contained a complete Circle of Willis with major arteries and five aneurysms. We placed the phantom in a 15 cm water bath and connected it to a flow loop containing a peristaltic pump which simulated physiological relevant flow conditions. We injected 10 ml contrast boluses using an automatic contrast injector at a rate of 10 ml/sec. Digital Subtraction Angiography images were acquired at 30 frames/sec and processed with a gamma fitting based algorithm, to yield parametric maps of: Mean transit time (MTT), Time‐to‐Peak (TTP), Bolus Arrival Time (BAT). Starting with the optimal exposure parameters selected by the x‐ray system automatic exposure control, while keeping the same kV, we lowered the mA and exposure per frame in four steps until we reached the minimum value allowed by the system. We analyzed the variation of the MTT, TTP and BAT for various artery signal to noise ratios using four ROIs.Results:The peak (maximum opacification) SNRs for full dose and minimum dose were: 35 and 13 for 4 mm arteries and 18 and 7 for 2 mm arteries. The parameters standard deviation expressed as a percent fraction of the value measured at the full dose value, were: MTT=2.4%, BAT=0.5% and TTP=1.9% for 4 mm vessels and MTT=3.7%, BAT=5.4% and TTP=6.3% for 2 mm vessels.Conclusion:Despite a significant decrease of the peak SNR the algorithm performed very well displaying variations less than 6.3% of the ideal conditions. Partial Support: NIH grant R01EB002873 and Toshiba Medical Systems Corp.Partial Support: NIH grant R01EB002873 and Toshiba Medical Systems Corp.

TU-F-CAMPUS-I-05: Investigation of An EMCCD Detector with Variable Gain in a Micro-CT System

Purpose: To investigate the performance of a newly built Electron Multiplying Charged Coupled Device (EMCCD) based Micro-CT system, with variable detector gain, using a phantom containing contrast agent of different concentrations. Methods: We built a micro- CT system with an EMCCD having 8 microns pixels and on-chip variable gain. We tested the system using a phantom containing five tubes filled with different iodine contrast solutions (30% to 70%). First, we scanned the phantom using various x-ray exposures values at 40 kVp and constant detector gain. Next, for the same tube currents, the detector gain was increased to maintain the air value of the projection image constant. A standard FDK algorithm was used to reconstruct the data. Performance was analyzed by comparing the signal-to-noise ratio (SNR) measurements for increased gain with those for the low constant gain at each exposure. Results: The high detector gain reconstructed data SNR was always greater than the low gain data SNR for all x-ray settings and for all iodine features. The largest increases were observed for low contrast features, 30% iodine concentration, where the SNR improvement approached 2. Conclusion: One of the first implementations of an EMCCD based micro- CT system was presented and used to image a phantom with various iodine solution concentrations. The analysis of the reconstructed volumes showed a significant improvement of the SNR especially for low contrast features. The unique on-chip gain feature is a substantial benefit allowing the use of the system at very low x-ray exposures per frame.Partial support: NIH grant R01EB002873 and Toshiba Medical Systems Corp. Partial support: NIH grant R01EB002873 and Toshiba Medical Systems Corp.

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

SU-E-I-55: The Contribution to Skin Dose Due to Scatter From the Patient Table and the Head Holder During Fluoroscopy

Purpose: To determine contributions to skin dose due to scatter from the table and head holder used during fluoroscopy, and also to explore alternative design material to reduce the scatter dose. Methods: Measurements were made of the primary and scatter components of the xray beam exiting the patient table and a cylindrical head holder used on a Toshiba Infinix c-arm unit as a function of kVp for the various beam filters on the machine and for various field sizes. The primary component of the beam was measured in air with the object placed close to the x-ray tube with an air gap between it and a 6 cc parallel-plate ionization chamber and with the beam collimated to a size just larger than the chamber. The primary plus scatter radiation components were measured with the object moved to a position in the beam next to the chamber for larger field sizes. Both sets of measurements were preformed while keeping the source-to-chamber distance fixed. The scatter fraction was estimated by taking the ratio of the difference between the two measurements and the reading that included both primary and scatter. Similar measurements were also made for a 2.3 cm thick Styrofoam block which could substitute for the patient support. Results: The measured scatter fractions indicate that the patient table as well as the head holder contributes an additional 10–16% to the patient entrance dose depending on field size. Forward scatter was reduced with the Styrofoam block so that the scatter fraction was about 4–5%. Conclusion: The results of this investigation demonstrated that scatter from the table and head holder used in clinical fluoroscopy contribute substantially to the skin dose. The lower contribution of scatter from Styrofoam suggests that there is an opportunity to redesign patient support accessories to reduce the skin dose. Partial support from NIH grant R01EB002873 and Toshiba Medical Systems Corporation Equipment Grant

MO‐F‐CAMPUS‐I‐01: A System for Automatically Calculating Organ and Effective Dose for Fluoroscopically‐Guided Procedures

Purpose:A system was developed that automatically calculates the organ and effective dose for individual fluoroscopically‐guided procedures using a log of the clinical exposure parameters.Methods:We have previously developed a dose tracking system (DTS) to provide a real‐time color‐coded 3D‐ mapping of skin dose. This software produces a log file of all geometry and exposure parameters for every x‐ray pulse during a procedure. The data in the log files is input into PCXMC, a Monte Carlo program that calculates organ and effective dose for projections and exposure parameters set by the user. We developed a MATLAB program to read data from the log files produced by the DTS and to automatically generate the definition files in the format used by PCXMC. The processing is done at the end of a procedure after all exposures are completed. Since there are thousands of exposure pulses with various parameters for fluoroscopy, DA and DSA and at various projections, the data for exposures with similar parameters is grouped prior to entry into PCXMC to reduce the number of Monte Carlo calculations that need to be performed.Results:The software developed automatically transfers data from the DTS log file to PCXMC and runs the program for each grouping of exposure pulses. When the dose from all exposure events are calculated, the doses for each organ and all effective doses are summed to obtain procedure totals. For a complicated interventional procedure, the calculations can be completed on a PC without manual intervention in less than 30 minutes depending on the level of data grouping.Conclusion:This system allows organ dose to be calculated for individual procedures for every patient without tedious calculations or data entry so that estimates of stochastic risk can be obtained in addition to the deterministic risk estimate provided by the DTS.Partial support from NIH grant R01EB002873 and Toshiba Medical Systems Corp.

TH-AB-201-01: A Real-Time Skin-Dose Mapping System for Region-Of-Interest (ROI) Fluoroscopy

Purpose: The real-time dose-tracking system (DTS) which we developed for fluoroscopically–guided procedures has been upgraded so that it can track the patient skin dose when using a region-of-interest (ROI) beam attenuator. Methods: ROI fluoroscopy is a method for greatly reducing patient dose by inserting an attenuator with an aperture into the x-ray beam. The aperture defines an ROI with normal dose and image quality, while under the attenuator the dose and quality are reduced. A moveable copper ROI attenuator has been installed in the collimator assembly of a Toshiba Infinix system. The DTS provides a 3D-dose mapping on a patient graphic and, to track dose for ROI fluoroscopy, the software now models the attenuator shape and position. A calibration file provides values for the transmission of the attenuator as a function of kVp and beam filtration and dose corrections are performed if a ray to a given point on the patient graphic is determined to intersect the attenuator. Backscatter is included in the skin-dose determination and is dependent on the ROI beam size projected onto the patient. In the current version, a spatially invariant factor corrects for backscatter within the ROI and from the ROI region into the attenuated region. The agreement of the dose distribution calculated by the DTS with the actual distribution was verified by measurement with calibrated radiochromic film and a 20-cm PMMA phantom. Results: The DTS is able to accurately model the size and position of the ROI attenuator as projected onto the patient graphic. Measurements with radiochromic film show the skin-dose estimation agreement to be within +/−12%, with most error arising from the spatial variation of backscatter. Conclusion: The dose distribution on the patient can now be tracked in real-time with acceptable accuracy when the new dose-saving feature of ROI fluoroscopy is used. This research was supported in part by Toshiba Medical Systems Corporation and 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-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

SU-E-I-53: Comparison of Kerma-Area-Product Between the Micro-Angiographic Fluoroscope (MAF) and a Flat Panel Detector (FPD) as Used in Neuro-Endovascular Procedures

Purpose: To determine the reduction of integral dose to the patient when using the micro-angiographic fluoroscope (MAF) compared to when using the standard flat-panel detector (FPD) for the techniques used during neurointerventional procedures. Methods: The MAF is a small field-of-view, high resolution x-ray detector which captures 1024 × 1024 pixels with an effective pixel size of 35μm and is capable of real-time imaging up to 30 frames per second. The MAF was used in neuro-interventions during those parts of the procedure when high resolution was needed and the FPD was used otherwise. The technique parameters were recorded when each detector was used and the kerma-area-product (KAP) per image frame was determined. KAP values were calculated for seven neuro interventions using premeasured calibration files of output as a function of kVp and beam filtration and included the attenuation of the patient table for the frontal projections to be more representative of integral patient dose. The air kerma at the patient entrance was multiplied by the beam area at that point to obtain the KAP values. The ranges of KAP values per frame were determined for the range of technique parameters used during the clinical procedures. To appreciate the benefit of the higher MAF resolution in the region of interventional activity, DA technique parameters were generally used with the MAF. Results: The lowest and highest values of KAP per frame for the MAF in DA mode were 4 and 50 times lower, respectively, compared to those of the FPD in pulsed fluoroscopy mode. Conclusion: The MAF was used in those parts of the clinical procedures when high resolution and image quality was essential. The integral patient dose as represented by the KAP value was substantially lower when using the MAF than when using the FPD due to the much smaller volume of tissue irradiated. This research was supported in part by Toshiba Medical Systems Corporation and NIH Grant R01EB002873.

SU‐E‐I‐22: Dependence On Calibration Phantom and Field Area of the Conversion Factor Used to Calculate Skin Dose During Neuro‐Interventional Fluoroscopic Procedures

Purpose:To determine the appropriate calibration factor to use when calculating skin dose with our real‐time dose‐tracking system (DTS) during neuro‐interventional fluoroscopic procedures by evaluating the difference in backscatter from different phantoms and as a function of entrance‐skin field area.Methods:We developed a dose‐tracking system to calculate and graphically display the cumulative skin‐dose distribution in real time. To calibrate the DTS for neuro‐interventional procedures, a phantom is needed that closely approximates the scattering properties of the head. We compared the x‐ray backscatter from eight phantoms: 20‐cm‐thick solid water, 16‐cm diameter water‐filled container, 16‐cm CTDI phantom, modified‐ANSI head phantom, 20‐cm‐thick PMMA, Kyoto‐Kagaku PBU‐ 50 head, Phantom‐Labs SK‐150 head, and RSD RS‐240T head. The phantoms were placed on the patient table with the entrance surface at 15 cm tube‐side from the isocenter of a Toshiba Infinix C‐arm, and the entrance‐skin exposure was measured with a calibrated 6‐cc PTW ionization chamber. The measurement included primary radiation, backscatter from the phantom and forward scatter from the table and pad. The variation in entrance‐skin exposure was also measured as a function of the skin‐entrance area for a 30×30 cm by 20‐cm‐thick PMMA phantom and the SK‐150 head phantom using four different added beam filters.Results:The entranceskin exposure values measured for eight different phantoms differed by up to 12%, while the ratio of entrance exposure of all phantoms relative to solid water showed less than 3% variation with kVp. The change in entrance‐skin exposure with entrance‐skin area was found to differ for the SK‐150 head compared to the 20‐cm PMMA phantom and the variation with field area was dependent on the added beam filtration.Conclusion:To accurately calculate skin dose for neuro‐interventional procedures with the DTS, the phantom for calibration should be carefully chosen since different phantoms can contribute different backscatter for identical exposure parameters.Research supported in part by Toshiba Medical Systems and NIH Grants R43FD0158401, R44FD0158402 and R01EB002873

SU‐E‐CAMPUS‐I‐04: Automatic Skin‐Dose Mapping for An Angiographic System with a Region‐Of‐Interest, High‐Resolution Detector

Purpose:Our real‐time skin dose tracking system (DTS) has been upgraded to monitor dose for the micro‐angiographic fluoroscope (MAF), a high‐resolution, small field‐of‐view x‐ray detector.Methods:The MAF has been mounted on a changer on a clinical C‐Arm gantry so it can be used interchangeably with the standard flat‐panel detector (FPD) during neuro‐interventional procedures when high resolution is needed in a region‐of‐interest. To monitor patient skin dose when using the MAF, our DTS has been modified to automatically account for the change in scatter for the very small MAF FOV and to provide separated dose distributions for each detector. The DTS is able to provide a color‐coded mapping of the cumulative skin dose on a 3D graphic model of the patient. To determine the correct entrance skin exposure to be applied by the DTS, a correction factor was determined by measuring the exposure at the entrance surface of a skull phantom with an ionization chamber as a function of entrance beam size for various beam filters and kVps. Entrance exposure measurements included primary radiation, patient backscatter and table forward scatter. To allow separation of the dose from each detector, a parameter log is kept that allows a replay of the procedure exposure events and recalculation of the dose components. The graphic display can then be constructed showing the dose distribution from the MAF and FPD separately or together.Results:The DTS is able to provide separate displays of dose for the MAF and FPD with field‐size specific scatter corrections. These measured corrections change from about 49% down to 10% when changing from the FPD to the MAF.Conclusion:The upgraded DTS allows identification of the patient skin dose delivered when using each detector in order to achieve improved dose management as well as to facilitate peak skin‐dose reduction through dose spreading.Research supported in part by Toshiba Medical Systems Corporation and NIH Grants R43FD0158401, R44FD0158402 and R01EB002873

SU‐E‐I‐22: Performance Evaluation of a Commercial CMOS Detector for Dynamic High‐Resolution Imaging

Purpose: Present state‐of‐the art flat‐panel detectors have limitations in terms of noise and resolution for dynamic imaging. With recent developments, CMOS‐based detectors show promise to fulfill the ever increasing need for better image quality. Here, we evaluated a CMOS‐based detector by measuring its sensitivity, MTF, DQE and INEE. Methods: The detector (Hamamatsu Model # C10901D‐40) featured 12‐bit image acquisition, CsI phosphor, 100 μm detector pixel pitch and an active area of 9.7cm × 6.7cm and had a maximum frame rate of 30 fps. The sensitivity and instrumentation noise equivalent exposure (INEE) were calculated using flat‐field images taken at various exposures (4.2 μR to 334 μR). The detector resolution was evaluated by measuring the modulation transfer function (MTF) using a tilted edge. The noise power spectra (NPS) and detective quantum efficiency (DQE) were measured using the RQA5 spectrum for three different exposures (4.2 μR, 16.5 μR and 174 μR). Results: The signal vs. exposure curve showed excellent linearity of the detector and the sensitivity of the detector was found to be 5.8 DN/μR. The slope and the intercept of the variance vs. exposure curve were used to calculate the INEE of the detector, which was found to be 7.46 μR The measured MTF values were 0.61, 0.21 and 0.07 at 10%, 50% and 90% of the Nyquist frequency of 5 cycles/mm. The corresponding values of the DQE at these indicated frequencies were 0.47(0.56), 0.19(0.32) and 0.03(0.09) at 4.2 μR (16.5 μR). Conclusion: The imaging performance of Hamamatsu detector was evaluated in terms of sensitivity, linear system metrics and INEE. The INEE of this detector appears higher than desirable for fluoroscopy, but the detector shows good DQE at high exposure when the effect of the high instrumentation noise is not significant.(Support: NIH grants R01EB002873 and R01EB008425) NIH grants R01EB002873 and R01EB008425

SU-E-I-27: Effect of Focal Spot Choice on the System Performance of a High- Resolution Micro-Angiographic Fluoroscope (MAF) Detector.

The generalized metrics of GMTF and GDQE were used to evaluate the effect of focal-spot choice on system performance of the Micro- Angiographic Fluoroscope (MAF) imager with a geometric magnification common for neuro-endovascular interventions. The MAF, a newly developed high-resolution detector with very low instrumentation noise and large variable gain, was used for the study. It has 35-micron pixels and a 500-micron thick CsI HR-type phosphor. The detector MTF was measured using the slanted edge method and the focal spot MTF's were measured using a standard pin-hole assembly. For the comparison analysis, the GMTF and GDQE were determined for three different focal spots (0.3 mm, 0.5 mm and, 0.8 mm) with a magnification factor of 1.25. A stainless- steel stent was also imaged with the same magnification for all three focal spots. The MAF's performance is affected significantly by the choice of focal-spot size because of its very high resolution. The GMTF and GDQE comparison for three focal spots showed significant degradation, increasing from small to large focal spot. The GMTF values were found to be 0.09, 0.05 and 0.01 at 5 cycles/mm for the small, medium and large focal spot, respectively. The corresponding values for GDQE were 0.1, 0.03 and 0.001. These results demonstrating the effect of choice of focal spot on the image quality are supported by line profiles obtained across the stent images. The results of this study demonstrate the significance of focal-spot size on system performance when using a high-resolution detector with a realistic magnification and shows the importance of choosing the minimum focal-spot size. (Support: NIH-Grant R01EB002873) NIH Grants R01-EB008425, R01-EB002873 and an equipment grant from Toshiba Medical Systems Corp.

SU-E-I-25: Performance Evaluation of a Proposed CMOS-Based X-Ray Detector Using Linear Cascade Model Analysis.

The need for high-resolution, dynamic x-ray imaging capability for neurovascular applications has put an ever increasing demand on x-ray detector technology. Present state-of-the-art detectors such as flat panels have limited resolution and noise performance. A linear cascade model analysis was used to estimate the theoretical performance for a proposed CMOS-based detector. The proposed CMOS-based detector was assumed to have a 300-micron thick HL type CsI phosphor, 35-micron pixels, a variable gain light image intensifier (LU), and 400 electron readout noise. The proposed detector has a CMOS sensor coupled to an LII which views the output of the CsI phosphor. For the analysis the whole imaging chain was divided into individual stages characterized by one of the basic processes (stochastic/deterministic blurring, binomial selection, quantum gain, additive noise). Standard linear cascade modeling was used for the propagation of signal and noise through the stages and an RQA5 spectrum was assumed. The gain, blurring or transmission of different stages was either measured or taken from manufacturer's specifications. The theoretically calculated MTF and DQE for the proposed detector were compared with a high-resolution, high-sensitive Micro-Angio Fluoroscope (MAF), predecessor of the proposed detector. Signal and noise for each of the 19 stages in the complete imaging chain were calculated and showed improved performance. For example, at 5 cycles/mm the MTF and DQE were 0.08 and 0.28, respectively, for the CMOS detector compared to 0.05 and 0.07 for the MAF detector. The proposed detector will have improved MTF and DQE and slimmer physical dimension due to the elimination of the large fiber-optic taper used in the MAF. Once operational, the proposed CMOS detector will serve as a further improvement over standard flat panel detectors compared to the MAF which is already receiving a very positive reception by neuro-vascular interventionalists. (Support:NIH-Grant R01EB002873) NIH Grants R01- EB008425, R01-EB002873 and an equipment grant from Toshiba Medical Systems Corp.

SU-E-I-28: Development of Graphic Patient Models for a Real-Time Skin Dose Tracking System (DTS) for Fluoroscopic Interventional Procedures.

To develop a library of graphic human models that closely match patients undergoing interventional fluoroscopic procedures in order to obtain an accurate estimate of their skin dose. A dose tracking system (DTS) has been developed that calculates the dose to the patient's skin in real time during fluoroscopic procedures based on a graphical simulation of the x-ray system and the patient. The calculation is performed using a lookup table containing values of mGy per mAs at a reference point and inverse-square correction using the distance from the source to individual points on the skin. For proper inverse-square correction, the external shape of the graphic should closely match that of the patient. We are in the process of developing a library of 3D human graphic models categorized as a function of basic body type, sex, height and weight. Two different open- source software applications are being used to develop graphic models with varying weights and heights, to 'morph' the shapes for body type and to 'pose' them for proper positioning on the table. The DTS software is being designed such that the most appropriate body graphic can be automatically selected based on input of several basic patient dimensional metrics. A series of male and female body graphic models have been developed which vary in weight and height. Matching pairs have been constructed with arms at the side and over the head to simulate the usual placement in cardiac procedures. The error in skin dose calculation due to inverse-square correction is expected to be below 5% if the graphic can match the position of the patient's skin surface within 1 cm. A library of categorized body shapes should allow close matching of the graphic to the patient shape allowing more accurate determination of skin dose with the DTS. Support for this work was provided in part by NIH grants R43FD0158401, R44FD0158402, R01EB002873 and R01EB008425, and by Toshiba Medical Systems Corporation.