C Keleshis

Rotational micro‐CT using a clinical C‐arm angiography gantry

Rotational angiography (RA) gantries are used routinely to acquire sequences of projection images of patients from which 3D renderings of vascular structures are generated using Feldkamp cone-beam reconstruction algorithms. However, these systems have limited resolution (<4 lp/mm). Micro-computed tomography (micro-CT) systems have better resolution (>10 lp/mm) but to date have relied either on rotating object imaging or small bore geometry for small animal imaging, and thus are not used for clinical imaging. The authors report here the development and use of a 3D rotational micro-angiography (RMA) system created by mounting a micro-angiographic fluoroscope (MAF) [35 microm pixel, resolution >10 microp/mm, field of view (FOV)=3.6 cm] on a standard clinical FPD-based RA gantry (Infinix, Model RTP12303J-G9E, Toshiba Medical Systems Corp., Tustin, CA). RA image sequences are obtained using the MAF and reconstructed. To eliminate artifacts due to image truncation, lower-dose (compared to MAF acquisition) full-FOV (FFOV) FPD RA sequences (194 microm pixel, FOV=20 cm) were also obtained to complete the missing data. The RA gantry was calibrated using a helical bead phantom. To ensure high-quality high-resolution reconstruction, the high-resolution images from the MAF were aligned spatially with the lower-dose FPD images, and the pixel values in the FPD image data were scaled to match those of the MAF. Images of a rabbit with a coronary stent placed in an artery in the Circle of Willis were obtained and reconstructed. The MAF images appear well aligned with the FPD images (average correlation coefficient before and after alignment: 0.65 and 0.97, respectively) Greater details without any visible truncation artifacts are seen in 3D RMA (MAF-FPD) images than in those of the FPD alone. The FWHM of line profiles of stent struts (100 microm diameter) are approximately 192+/-21 and 313+/-38 microm for the 3D RMA and FPD data, respectively. In addition, for the dual-acquisition 3D RMA, FFOV FPD data need not be of the highest quality, and thus may be acquired at lower dose compared to a standard FPD acquisition. These results indicate that this system could provide the basis for high resolution images of regions of interest in patients with a reduction in the integral dose compared to the standard FPD approach.

Implementation of a high-sensitivity Micro-Angiographic Fluoroscope (HS-MAF) for in-vivo endovascular image guided interventions (EIGI) and region-of-interest computed tomography (ROI-CT)

New advances in catheter technology and remote actuation for minimally invasive procedures are continuously increasing the demand for better x-ray imaging technology. The new x-ray high-sensitivity Micro-Angiographic Fluoroscope (HS-MAF) detector offers high resolution and real-time image-guided capabilities which are unique when compared with commercially available detectors. This detector consists of a 300 μm CsI input phosphor coupled to a dual stage GEN2 micro-channel plate light image intensifier (LII), followed by minifying fiber-optic taper coupled to a CCD chip. The HS-MAF detector image array is 1024X1024 pixels, with a 12 bit depth capable of imaging at 30 frames per second. The detector has a round field of view with 4 cm diameter and 35 microns pixels. The LII has a large variable gain which allows usage of the detector at very low exposures characteristic of fluoroscopic ranges while maintaining very good image quality. The custom acquisition program allows real-time image display and data storage. We designed a set of in-vivo experimental interventions in which placement of specially designed endovascular stents were evaluated with the new detector and with a standard x-ray image intensifier (XII). Capabilities such fluoroscopy, angiography and ROI-CT reconstruction using rotational angiography data were implemented and verified. The images obtained during interventions under radiographic control with the HS-MAF detector were superior to those with the XII. In general, the device feature markers, the device structures, and the vessel geometry were better identified with the new detector. High-resolution detectors such as HS-MAF can vastly improve the accuracy of localization and tracking of devices such stents or catheters.

Angiographic analysis of animal model aneurysms treated with novel polyurethane asymmetric vascular stent (P-AVS): feasibility study

Image-guided endovascular intervention (EIGI), using new flow modifying endovascular devices for intracranial aneurysm treatment is an active area of stroke research. The new polyurethane-asymmetric vascular stent (P-AVS), a vascular stent partially covered with a polyurethane-based patch, is used to cover the aneurysm neck, thus occluding flow into the aneurysm. This study involves angiographic imaging of partially covered aneurysm orifices. This particular situation could occur when the vascular geometry does not allow full aneurysm coverage. Four standard in-vivo rabbit-model aneurysms were investigated; two had stent patches placed over the distal region of the aneurysm orifice while the other two had stent patches placed over the proximal region of the aneurysm orifice. Angiographic analysis was used to evaluate aneurysm blood flow before and immediately after stenting and at four-week follow-up. The treatment results were also evaluated using histology on the aneurysm dome and electron microscopy on the aneurysm neck. Post-stenting angiographic flow analysis revealed aneurysmal flow reduction in all cases with faster flow in the distally-covered case and very slow flow and prolonged pooling for proximal-coverage. At follow-up, proximally-covered aneurysms showed full dome occlusion. The electron microscopy showed a remnant neck in both distally-placed stent cases but complete coverage in the proximally-placed stent cases. Thus, direct flow (impingement jet) removal from the aneurysm dome, as indicated by angiograms in the proximally-covered case, was sufficient to cause full aneurysm healing in four weeks; however, aneurysm healing was not complete for the distally-covered case. These results support further investigations into the treatment of aneurysms by flow-modification using partial aneurysm-orifice coverage.

LabVIEW Graphical User Interface for a New High Sensitivity, High Resolution Micro-Angio-Fluoroscopic and ROI-CBCT System

A graphical user interface based on LabVIEW software was developed to enable clinical evaluation of a new High-Sensitivity Micro-Angio-Fluoroscopic (HSMAF) system for real-time acquisition, display and rapid frame transfer of high-resolution region-of-interest images. The HSMAF detector consists of a CsI(Tl) phosphor, a light image intensifier (LII), and a fiber-optic taper coupled to a progressive scan, frame-transfer, charged-coupled device (CCD) camera which provides real-time 12 bit, 1k × 1k images capable of greater than 10 lp/mm resolution. Images can be captured in continuous or triggered mode, and the camera can be programmed by a computer using Camera Link 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. The program, written in LabVIEW, has the following capabilities: camera initialization, synchronized image acquisition with the x-ray pulses, roadmap and digital subtraction angiography acquisition (DSA), flat field correction, brightness and contrast control, last frame hold in fluoroscopy, looped play-back of the acquired images in angiography, recursive temporal filtering and LII gain control. Frame rates can be up to 30 fps in full-resolution mode. The user friendly implementation of the interface along with the high frame-rate acquisition and display for this unique high-resolution detector should provide angiographers and interventionalists with a new capability for visualizing details of small vessels and endovascular devices such as stents and hence enable more accurate diagnoses and image guided interventions.

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).

Real time implementation of distortion corrections for a tiled EMCCD-based solid state x-ray image intensifier (SSXII)

The new Solid State X-ray Image Intensifier (SSXII) is being designed based on a modular imaging array of Electron Multiplying Charge Couple Devices (EMCCD). Each of the detector modules consists of a CsI(Tl) phosphor coupled to a fiber-optic plate, a fiber-optic taper (FOT), and an EMCCD sensor with its electronics. During the optical coupling and alignment of the modules into an array form, small orientation misalignments, such as rotation and translation of the EMCCD sensors, are expected. In addition, barrel distortion will result from the FOTs. Correction algorithms have been developed by our group for all the above artifacts. However, it is critical for the systems performance to correct these artifacts in real-time (30 fps). To achieve this, we will use two-dimensional Look-Up-Tables (LUT) (each for x and y coordinates), which map the corrected pixel locations to the acquired-image pixel locations. To evaluate the feasibility of this approach, this process is simulated making use of parallel coding techniques to allow real-time distortion corrections for up to sixteen modules when a standard quad processor is used. The results of this simulation confirm that tiled field-of-views (FOV) comparable with those of flat panel detectors can be generated in ~17 ms (>30 fps). The increased FOV enabled through correction of tiled images, combined with the EMCCD characteristics of low noise, negligible lag and high sensitivity, should make possible the practical use of the SSXII with substantial advantages over conventional clinical systems. (Support: NIH Grants R01EB008425, R01NS43924, R01EB002873)

TH‐C‐210A‐04: New High Resolution Dynamic Detectors and Flow Modifying Stents for Neuro‐Endovascular Image Guided Interventions

As the field of minimally invasive endovascular image‐guided interventions (EIGI) advances, there has been progress in the development of new endovascular devices such as the asymmetric vascular stent for treatment of aneurysms as well as progress in the image guidance systems needed to improve both the diagnoses and interventions and the methods used to evaluate the improved imaging performance. High resolution imagingsystems with MTFs extending past 8 Lp/mm and with ability to visualize not only radioopaque markers but the detailed structure of devices such as stents have motivated the development of new asymmetric devices such as the blood flow modifying asymmetric vascular stent (AVS). The AVS has now come through a number of generations from balloon expandable stainless steel strutted structures with laser micro‐welded mesh flow diverters to new super‐elastic nitinol closedcell self‐expanding stents with organic material flow diverters. The methods of laser machining, surface finishing, and deployment will be described with progress in animal models reported. In parallel with advances in EIGI devices has come new high resolution detector development. The detectors have a unique combination of features such as far superior spatial resolution compared with conventional dynamic flat panel detectors, large dynamic range of sensitivity with negligible lag and ghosting to enable both fluoroscopy and angiography, and low noise to enable quantum limited performance over the full useful range including during lowdose fluoroscopy. The Micro‐Angiographic Fluoroscope (MAF) consists of an x‐ray converter phosphor sensed by a micro‐channel plate light image intensifier which is in turn coupled to a high performance CCD camera using a fiber optic taper. The MAF is a region of interest (ROI) imager with 4 cm field of view centered at the interventional site and may be moved in front of a larger conventional detector when improved resolution is needed. The Solid State X‐ray Image Intensifier (SSXII) while having much of the benefits of the MAF in superior imaging capability achieves its great sensitivity using only electron multiplying CCDsensors. An array design is being developed so that the imaging FOV may be expanded by adding modules each with its own EMCCD‐based detector. To more fully characterize detectors, new evaluation methods are being explored. For example, the accurate determination of MTF from measurements of noise only, without the need for a slit or edge will be reported. Also from a careful analysis of noise, the exposure range for detector quantum limited performance can be well demarcated by the instrumentation noise equivalent exposure (INEE). Finally, more realistic linear system parameters that include focal‐spot size, geometry, and scatter provide generalized MTFs and DQEs or GMTFs and GDQEs. All told, there is much happening in EIGI. Learning Objectives 1. Appreciate the progress being made in improved EIGI devices and in particular flow modifiers such as the asymmetric vascular 2. stent (AVS) for aneurysm treatment. 3. Understand the operation of new high‐resolution micro‐angiographic systems including the MAF and SSXII. 4. Understand new objective image detector evaluations including INEE, GMTF, GDQE, and determination of MTF from noise 5. measurements alone. (Supported in part by NIH Grants R01EB002873, R01 NS43924, R01EB008425, and the Toshiba Medical Systems Corp.)

MO‐D‐332‐07: Update On the Development of a New Dual Detector (Micro‐Angiographic Fluoroscope/Flat Panel) C‐Arm Mounted System for Endovascular Image Guided Interventions (EIGI)

Purpose: To develop a dual detector C‐arm unit, capable of high‐resolution microangiography and fluoroscopy, and Region‐of‐Interest Cone‐Beam CT (ROI‐CBCT). Method and Materials: The Microangiographic Fluoroscope (MAF) (1024×1024×12bits, 35μm pixels, 4 cm field‐of‐view, FOV) was attached with a specially designed holder to a standard C‐arm Flat‐Panel (FP) system. The MAF consists of a 300 μm CsI input phosphor coupled to a dual stage GEN2 micro‐channel plate light image intensifier (LII), followed by a minifying fiber‐optic taper coupled to a 30 fps CCDcamera. The LII has a large variable gain allowing usage for very low (fluoroscopic) exposures while maintaining very good image quality. The holder allows facile placing of the new detector into the FP FOV when use is required or parking when not. The source‐to‐image distance and the orientation of this detector are selected using the same controls as for the standard C‐arm unit. A special switch attached onto the holder allows automatic collimation of the x‐ray beam to the active area of the MAF. The new system was tested in multiple experiments involving phantoms and animals for reliability and capability to perform EIGI procedures and also for dual detector ROI‐CBCT. Results: The new system is being used routinely for EIGI fluoroscopic guidance and microangiography in our research lab. The design allows a variable SID between 69 and 104 cm. During rotational angiography and dual detector ROI‐CBCT, 194 projections are acquired, one every degree. Interventional devices such as endovascular stents placed in the animals and phantoms were reconstructed with great accuracy, and virtually without artifacts. Conclusion: Implementation of such a high‐resolution imager on a clinical system could bring substantial benefits for the treatment of cerebrovascular disease and also potentially increase the motivation to develop improved and more effective endovascular devices. (Funding: NIH Grants R01 EB002873, NS43924).

SU-GG-I-12: Effect of Geometric Unsharpness On the Reconstructed Image in Region of Interest (ROI) μCT

Purpose: To demonstrate the effects of variation of focal‐spot size and magnification on the spatial resolution of reconstructed images of a micro‐computed tomography (μCT) system which is attached to a standard angiographic C‐arm gantry to enable Region‐of‐Interest cone‐beam CT (ROI‐CBCT). Method and Materials: High‐resolution ROI projection data of a vascular phantom were acquired using a new high‐sensitivity, microangiographic fluoroscope (HSMAF) detector (35 μm pixels), which was attached to the C‐arm gantry and able to be positioned in front of a standard full field‐of‐view, low‐resolution commercial flat‐panel detector(FPD) (194 μm pixels). The HSMAF consists of a CsI phosphor viewed by a 4‐cm diameter light image‐intensifier with large variable dynamic range whose output is coupled via a fiber‐optic taper to a CCDcamera. The test objects in the vascular phantom were a stent (100 micron struts) inside of a catheter in a cylindrical water bath. The phantom was placed on a portable test platform (PTP) enabling CBCT image acquisition by the HSMAF every 1°. Six μCT runs were performed using two focal‐spot sizes (0.3 and 0.6 mm) and three magnification factors (1.15, 1.29, and 1.48). Profiles were extracted from the reconstructed struts, and the full width half‐maximum (FWHM) were measured. Results: The reconstructed data show that using the optimal configuration (smallest magnification with small focal spot) compared to the worst configuration (largest magnification and the large focal spot) resulted in a 47% reduction in the FWHM in the object plane (175 μm versus 375 μm). Conclusion: Micro‐CBCT can provide more accurate visualization of fine device features; however, geometric unsharpness and/or large focal spots can substantially degrade resolution reducing the quality of the μCBCT reconstructions. (Research sponsored by: NIH Grants R01‐NS43924, R01‐EB002873, Toshiba Medical Systems Corporation)