S Rudin

Evaluation of the Micro-Angiographic Fluoroscope as a High-Resolution, Single-Photon Counting and Energy-Integrating Imager for Transmission and Emission Imaging

X-ray and radionuclide imaging are widely popular medical imaging modalities. The requirements for each imaging modality are different, and different detectors are normally used. For this paper, we demonstrate our new detector capable of both fluoroscopy and angiography, to be used as an imager for both single-photon and integral-energy imaging and applied to the dual modalities of radionuclide imaging and X-ray imaging. This newly developed micro-angiographic fluoroscope (MAF) has 1024 × 1024 pixels of 35 μm effective size and is capable of real-time imaging at 30 fps. The large variable gain of its light image intensifier (LII) provides quantum-limited operation with essentially no additive instrumentation noise. We demonstrate that the MAF can be operated in single-photon counting (SPC) mode for X-ray imaging with substantially better resolution than in energy integration (EI) mode. We may use high LII gain with very low exposure (less than 1 X-ray photon/pixel) per frame for SPC mode (with X-ray and radionuclide) and higher exposure with lower gain for EI mode (transmission imaging with X-rays). For a demonstration of the operation in both EI and SPC mode, a heavily K-edge filtered X-ray beam (average energy of 31 keV) was used to provide a nearly monochromatic spectrum. The MTF measured using a standard slit method showed a dramatic improvement for the SPC mode over the EI mode at all spatial frequencies. Images of a line-pair phantom also showed improved spatial resolution in SPC mode compared to EI mode. In SPC mode, images of human distal and middle phalanges showed the trabecular structures of the bone with far better contrast and detail. We also show MAF operation in SPC mode for radionuclide imaging using a custom-built phantom filled with I-125. A 1-mm-diameter parallel hole, medium-energy gamma camera collimator was placed between the phantom, and the MAF and was moved multiple times at equal intervals in random directions to eliminate the pattern corresponding to the collimator septa. Data was acquired at 20 fps, and multiple signal-thresholded frames were summed in SPC mode to provide an integrated frame. The sharpness of the emission image is limited by the collimator resolution and could be improved by optimized collimator design. We demonstrate that the same MAF is capable of operating in both SPC and EI modes and can be used in both X-ray transmission imaging and radionuclide emission imaging.

SU‐F‐BRA‐09: Comparison of Skin‐Dose Distributions Calculated by a Real‐Time Dose‐Tracking System with That Measured by Gafchromic Film for a Fluoroscopic C‐Arm Unit

Purpose: To assess the ability of a real‐time dose‐tracking system (DTS) to accurately represent the skin dose distribution for fluoroscopic interventional procedures by comparison to that measured using Gafchromic film (XR‐RV3, ISP, Wayne, NJ).Methods: We have developed a dose‐tracking system that calculates the radiation dose to the patients skin in real‐time using the exposure parameters and imaging‐system‐geometry obtained from the digital bus on a Toshiba Infinix C‐arm unit. The DTS presents the cumulative dose values in a color mapping on a 3D graphic of the patient for immediate feedback to the interventionalist. Gafchromic film was used to verify the spatial correspondence of the mapped distribution and the accuracy of the dose accumulation on the graphic. The film was calibrated against the readings of a 6 cc ionization chamber (PTW‐Freiburg GmbH, Freiburg, Germany) over a range of exposure values from 0 to 1500 R using cine‐radiographic exposures on the same C‐arm system. A simulated cardiac‐catheterization procedure was performed with the film wrapped around an Alderson torso phantom; the density distribution and converted dose values on the film were compared to that of the DTS graphic.Results: The DTS and film distributions were compared and excellent agreement was obtained within the cm‐sized surface elements used for the patient model demonstrating proper geometric scaling of the graphic. The dose values for individual points on the phantom surface agreed within 10% between the film and DTS even with inexact contouring of the film with the phantom in the measurement Conclusions: As shown by the agreement with Gafchromic film, the DTS provides skin‐dose distribution mapping with sufficient accuracy for use in monitoring interventional fluoroscopic procedures. NIH Grants R01‐EB002873, R01‐EB002873, R43‐FD0158401, R44‐FD0158402, and Toshiba Medical Systems Corporation.

SU‐FF‐I‐127: Patient Specific Angiography Phantoms for Investigating New Endovascular Image‐Guided Interventional (EIGI) Devices

Purpose: To provide patient‐specific phantoms for investigating new endovascular image‐guided interventional (EIGI) devices such as stents. Methods and Materials: A human cerebral/saccular aneurysm phantom was constructed from a patients segmented CT‐scan data. The CT‐derived vascular lumen was converted to a standard stereolithography (STL) computer file which was used, through a rapid‐prototyping process, to make a plastic resin (Accura 25) mold of the vessel with the aneurysm. In this process, an ultraviolet laser is used to selectively cure a liquid plastic layer by layer resulting in a highly accurate and very detailed positive model. RTV silicone is placed around this plastic aneurysm, allowed to cure, and then cut in half to create a mold with a cavity in the shape of the aneurysm. This mold is used to make a wax replica of the human aneurysm. The wax replica is then immersed in a silicone elastomer solution, the bubbles removed via vacuum, and the elastomer allowed to cure. The wax is then dissolved using boiling water. Results: The finished product is a clear elastomer model with a patient‐specific aneurysm cavity that may be used in both x‐ray and optical flowexperiments. Multiple identical elastomer phantoms can be generated from one mold so that new EIGI devices such as an asymmetric stent with a low porosity patch region to occlude aneurysmal flow can be variously deployed under fluoroscopic guidance and evaluated angiographically. Additionally, the effects of contrast media can be studied by comparisons with optical flow studies. Comparisons of angiographic and optical time‐density curves for an anterior cerebral artery with multi‐lobular aneurysm will be presented. Conclusions: We have developed a unique system for the creation of realistic patient‐specific phantoms which enable the evaluation of detailed vascular flow and experimental EIGI treatment techniques for clinically representative pathology. (Support: NIH R01‐NS43924, R01‐EB002873, Toshiba Corp.).

SU‐E‐I‐191: Effective‐Dose Rate Comparison between the Micro‐Angiographic Fluoroscope (MAF) and the X‐Ray Image Intensifier (XII) Used during Neuro‐Endovascular Device Deployment Procedures

Purpose: To compare the effective‐dose rate delivered to the patient using the small field‐of‐view (FOV) MAF to that of the standard larger FOV XII during clinical neuro‐interventional procedures. Methods: The MAF camera has been installed for clinical evaluation during neuro‐interventional procedures on a Toshiba Infinix fluoroscopic c‐arm system so it can be interchangeably used with the standard imaging system. This camera has a small FOV (3.6×3.6 cm) but provides high‐resolution (35—micron pixels) and high‐sensitivity. The standard system is a 9‐inch XII, with the 5‐inch mode (124‐micron pixels) being used most frequently. We used the program PCXMC 2.0 (STUK, Helsinki, Finland) to calculate effective‐dose rates for the MAF and 5‐inch mode of the XII using the technical parameters employed in patient procedures. Parameters such as kVp, mAs, beam filtration, and frame rate were logged during the procedure so the dose could be retrospectively calculated. The effective‐dose rates for the PA projection were calculated for the fluoroscopy and DSA components of 15 neuro‐interventional procedures. Results: The effective dose for fixed mAs was found to increase with increasing x‐ray tube voltage for PA imaging of the neurovasculature for both FOVs. The effective‐dose rate in the clinical procedures was substantially lower for the MAF compared to the 5‐inch mode of the XII, ranging from a factor of about 4 to 12 times lower for fluoroscopy and 12 to 46 times lower for DSA. Conclusions: Effective dose is very much dependent on the FOV. Substantial reduction in effective‐dose rate is realized using the MAF as compared to the standard imaging system; this reduction could allow the dose with the MAF to be increased by over an order of magnitude to provide increased contrast resolution without increasing the stochastic risk to the patient compared to full‐FOV imaging. (Support: Toshiba Medical Systems, NIH R01EB002873 and NIH R01EB008425)

SU-FF-I-06: A Portable Test Platform for Image Acquisition and Calibration for Cone Beam Computed Tomography (CBCT) and Region of Interest CBCT (ROI-CBCT) On a Commercial X-Ray C-Arm System

Purpose: We have developed a unique portable test platform (PTP) which enables CBCT for specimens and phantoms on standard commercial clinical x‐ray systems. This PTP can be used to acquire ROI‐CBCT projection images, where a lower resolution, lower dose image peripheral to a high resolution ROI is acquired. This is achieved either by acquiring an image using an Image Intensifier (II) with an ROI filter in the x‐ray beam or by combining images acquired separately with low and high resolution x‐ray detectors.Method and Materials: The CBCTimages are acquired as the object rotates on the computer‐controlled rotary table of the PTP. For ROI‐CBCT, a micro‐angiography (MA) detector or an ROI filter is mounted on the PTP. The PTP also provides for relative X, Y, Z adjustments. After coarse alignment adjustments of the PTP, fine translational and angular adjustments are made based on fluoroscopic imaging of a cylindrical calibration phantom. Results: The PTP allows quick assembly of the parts required for CBCT or ROI‐CBCT reconstruction, reduces initial setup time to < 45 min, and provides for setup reproducibility. The system can be aligned to within one pixel (43 micron for the MA detector), with angular alignments of pitch and roll of the object better than 0.7° and 0.1° respectively. Conclusion:, The PTP allows fast and reliable set‐up and alignment of CBCT specimens, for standard and for ROI‐CBCT applications. The PTP may enable wider use of CBCT and ROI‐CBCT for specimens and phantoms without a costly dedicated system. (Partial support from NIH Grants R01‐NS43924, R01‐EB02873, R01‐HL52567, R01‐EB02916, and Toshiba Medical Systems Corporation).

Factors affecting the accurate determination of cerebrovascular blood flow using high speed droplet imaging

Detailed cerebrovascular blood flow can be more accurately determined radiographically from the new droplet tracking method previously introduced by the authors than from standard soluble contrast techniques. For example, arteriovenous malformation (AVM) transit times which are crucial for proper glue embolization treatments, were shown to be about half when using droplets compared to those measured using soluble contrast techniques. In this work, factors such as x-ray pulse duration, frame rate, system spatial resolution (focal spot size), droplet size, droplet and system contrast parameters, and system noise are considered in relation to their affect on the accurate determination of droplet location and velocity.

SU-D-134-03: Design Considerations for a Dose-Reducing Region of Interest (ROI) Attenuator Built in the Collimator Assembly of a Fluoroscopic Interventional C-Arm

PURPOSE ROI fluoroscopy involves the use of an x-ray beam attenuator with higher attenuation in the periphery than the center thus allowing for dose reduction to the patient. This study presents the design considerations for placing an x-ray ROI attenuator made of copper inside the collimator assembly of an angiographic c-arm. METHODS The two important considerations for the design of the attenuator are the size of the ROI and the attenuation (and hence thickness of the material) needed outside the ROI. An attenuation of 80% outside the ROI, and none inside the ROI was assumed. To calculate the thickness, exposures were measured for different thicknesses of copper at various kVps and different inherent filtration of the system. Attenuation percentage was calculated from these readings and the thickness of copper was determined. The field-of-view (FOV) requirement depends on the type of procedure: smaller for a neurovascular intervention and larger for a cardiac procedure. An average FOV of 33% of 21cm × 21cm at 100cm SID with a circular ROI was assumed to calculate the diameter of the ROI in the attenuator. RESULTS For kVps ranging from 80 to 90, with an added filtration of 0.2mm copper, to get an average attenuation of 80%, 0.7mm of copper was needed for the thickness of the attenuator. The attenuator was placed 13cm from the focal spot and the diameter of the ROI at this distance was calculated to be 10mm. CONCLUSION The ROI attenuator can be mounted inside the beam limiting mechanism of the c-arm. This allows for the flexibility in the usage of this technique during fluoroscopic interventions, thus achieving patient-dose reduction. Since the attenuation for copper varies with varying kVp, different masks for different kVps are to be used for brightness equalization.

The micro-angiographic fluoroscope (MAF) in High Definition (HD) mode for improved contrast-to-noise ratio and resolution in fluoroscopy and roadmapping

During image guided interventional procedures, superior resolution and image quality is critically important. Operating the MAF in the new High Definition (HD) fluoroscopy mode provides high resolution and increased contrast-to-noise ratio. The MAF has a CCD camera and a 300 micron cesium iodide x-ray convertor phosphor coupled to a light image intensifier (LII) through a fiber-optic taper. The MAF captures 1024 × 1024 pixels with an effective pixel size of 35 microns, and is capable of real-time imaging at 30 fps. The HD mode uses the advantages of higher exposure along with a small focal spot effectively improving the contrast-to-noise ratio (CNR) and the spatial resolution. The Control Acquisition Processing and Image Display System (CAPIDS) software for the MAF controls the LII gain. The interventionalist can select either fluoroscopic or angiographic modes using the two standard foot pedals. When improved image quality is needed and the angiography foot-pedal is used for HD mode, the x-ray machine will operate at a preset higher exposure rate using a small focal spot, while the CAPIDS will automatically adjust the LII gain to achieve proper image brightness. HD mode fluoroscopy and roadmapping are thus achieved conveniently during the interventional procedure. For CNR and resolution evaluation we used a bar phantom with images taken in HD mode with both the MAF and a Flat Panel Detector (FPD). It was seen that the FPD could not resolve more than 2.8 lp/mm whereas the MAF could resolve more than 5 lp/mm. The CNR of the MAF was better than that of the FPD by 60% at lower frequencies and by 600% at the Nyquist frequency of the FPD. The HD mode has become the preferred mode during animal model interventions because it enables detailed features of endovascular devices such as stent struts to be visualized clearly for the first time. Clinical testing of the MAF in HD mode is imminent.

MO‐FF‐A4‐03: Testing of the High‐Resolution ROI Micro‐Angio Fluoroscope (MAF) Detector Using a Modified NEMA XR‐21 Phantom

Purpose: To test the MAF in conditions and tasks specific to minimally invasive neurovascular procedures. Materials and Methods: A high‐sensitivity, high‐resolution MAF detector was built and incorporated into a standard angiographic C‐Arm system. This detector consists of a 300μm CsI input phosphor coupled to a dual stage GEN2 micro‐channel‐plate light image intensifier, followed by minifying fiber‐optic taper coupled to a CCD chip. The detector is attached to a very stable detector‐changer onto a Flat‐Panel (FP) C‐arm angiographic unit to allow facile placement of the detector into the field‐of‐view whenever high resolution is needed. A NEMA XR21‐2000 phantom was modified to evaluate neurovascular x‐ray imaging systems. The phantom was restructured to be head‐equivalent; two aluminum plates shaped to fit into the NEMA phantom geometry were added to a 15cm thick section. Digital subtraction angiography(DSA)testing was enabled by adding a removable central section with a hollow slot which allows insertion of various angiographic test blocks. DSA and DA were tested using a standard removable insert having simulated arteries with thicknesses of 4, 2 and 1 mm and 15mg/cm3 iodine contrast and with stenoses and aneurysms (AAPM Report 15). Features on the central plates of the NEMA XR21 phantom such as bar pattern and iodine‐detail‐contrast‐targets were also imaged. The results of the evaluation of the MAF with the modified phantom were compared with the images obtained with a standard flat panel. Results: The phantom imaging results presented as (MAF‐detected‐features/Flat‐Panel‐detected‐features) are: bar pattern — (5.0/3.1) lines/mm; smallest iodine‐contrast target group detectable — (10/10) mg/cm2, details of smallest simulated vessel in DSA — (1/2) mm. Conclusions: The MAF detector performs at least as well as a standard FP in detection of low‐contrast objects, and is superior in the visualization and identification of the small details. (Support: NIH grant R01‐EB002873, R01EB0008425).

SU-FF-I-45: Labview Graphical User Interface for Micro Angio-Fluoroscopic High Resolution Detector

Purpose: A graphical user interface based on LabView software was developed to control a Micro Angio‐Fluoroscopic detector (MAF) for real‐time acquisition, display and rapid frame transfer of high resolution images of a region‐of‐interest. Method and Materials: A MAF detector was built by our group using a CsI(Tl) phosphor, fiber‐optic taper and Light Amplifier optically coupled to a progressive scan charged coupled device(CCD)camera which provides real‐time 12 bit, 1k × 1k images. During image acquisition, the MAF detector is inserted in the x‐ray beam of an angiographic unit, between the x‐ray image intensifier and the patient. Images can be captured in continuous or triggered mode and the camera can be programmed by a computer using the serial communication. A graphical user interface was developed to control the camera modes such as gain and pixel binning as well as to acquire, store, display, and process the images.Results: The program, written in LabView, has the following capabilities: camera initialization, synchronized image acquisition with the x‐ray pulses, flat field correction, window and level adjustment, brightness and contrast control, and looped play‐back of the acquired images. Acquisition starts when the first triggering pulse is read by the interface. The acquired sequence of images is automatically displayed in a loop after completion of acquisition and the images can be stored or deleted at the users discretion. Frame rates can be up to 30fps in 2×2 binning mode and 25fps unbinned. Conclusion: The user friendly implementation of the interface along with the high frame rate acquisition and display for this unique high resolution detector may provide angiographers a new capability for visualizing details of small endovascular devices such as stents and hence enable more accurate image guided localization. (Support: NIH Grants R01NS43924, R01EB002873).