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)

Assessment of Vascular Geometry for Bilateral Carotid Artery Ligation to Induce Early Basilar Terminus Aneurysmal Remodeling in Rats

Bilateral common carotid artery (CCA) ligation in rabbits is a model for basilar terminus (BT) aneurysm formation. We asked if this model could be replicated in rats. Fourteen female Sprague Dawley rats underwent bilateral CCA ligation (n=8) or sham surgery (n=6). After 7 days, 5 ligated and 3 sham rats were euthanized for histological evaluation of BT aneurysm formation, while the remaining rats were imaged with magnetic resonance angiography, euthanized, and subjected to corrosion casting of the Circle of Willis (CoW). 3D micro computed tomography images of CoW casts were used for flow simulations at the rat BT, and electron micrographs of the casts were analyzed for aneurysmal and morphological changes. Results from these analyses were compared to rabbit model data (n=10 ligated and n=6 sham). Bilateral CCA ligation did not produce aneurysmal damage at the rat BT. While the surgical manipulation increased rat basilar artery flow, fluid dynamics simulations showed that the initial hemodynamic stress at the rat BT was significantly less than in rabbits. Rats also exhibited fewer morphological and pathological changes (minor changes only occurred in the posterior CoW) than rabbits, which had drastic changes throughout the CoW. A comparison of CoW anatomies demonstrated a greater number of branching arteries at the BT, larger CoW arteries in relation to basilar artery, and a steeper BT bifurcation angle in the rat. These differences could account for the lower hemodynamic stress at the BT and in the cerebrovasculature of the rat. In conclusion, bilateral CCA ligation in rats does not recapitulate the rabbit model of early flow-induced BT aneurysm. We suspect that the different CoW morphology of the rat lessens hemodynamic insults, thereby diminishing flow-induced aneurysmal remodeling.

Evaluation of intracranial aneurysm coil embolization in phantoms and patients using a high-resolution Microangiographic Fluoroscope (MAF)

Intracranial aneurysm (IA) embolization using Gugliemi Detachable Coils (GDC) under x-ray fluoroscopic guidance is one of the most important neuro-vascular interventions. Coil deposition accuracy is key and could benefit substantially from higher resolution imagers such as the micro-angiographic fluoroscope (MAF). The effect of MAF guidance improvement over the use of standard Flat Panels (FP) is challenging to assess for such a complex procedure. We propose and investigate a new metric, inter-frame cross-correlation sensitivity (CCS), to compare detector performance for such procedures. Pixel (P) and histogram (H) CCS's were calculated as one minus the cross-correlation coefficients between pixel values and histograms for the region of interest at successive procedure steps. IA treatment using GDC's was simulated using an anthropomorphic head phantom which includes an aneurysm. GDC's were deposited in steps of 3 cm and the procedure was imaged with a FP and the MAF. To measure sensitivity to detect progress of the procedure by change in images of successive steps, an ROI was selected over the aneurysm location and pixel-value and histogram changes were calculated after each step. For the FP, after 4 steps, the H and P CCSs between successive steps were practically zero, indicating that there were no significant changes in the observed images. For the MAF, H and P CCSs were greater than zero even after 10 steps (30 cm GDC), indicating observable changes. Further, the proposed quantification method was applied for evaluation of seven patients imaged using the MAF, yielding similar results (H and P CCSs greater than zero after the last GDC deposition). The proposed metric indicates that the MAF can offer better guidance during such procedures.

Overcoming x-ray tube small focal spot output limitations for high-resolution region of interest imaging

We investigate methods to increase x-ray tube output to enable improved quantum image quality with a higher generalized-NEQ (GNEQ) while maintaining a small focal-spot size for the new high-resolution Micro-angiographic Fluoroscope (MAF) Region of Interest (ROI) imaging system. Rather than using a larger focal spot to increase tube-loading capacity with degraded resolution, we evaluated separately or in combination three methods to increase tube output: 1) reducing the anode angle and lengthening the filament to maintain a constant effective small focal-spot size, 2) using the standard medium focal spot viewed from a direction on the anode side of the field and 3) increasing the frame rate (frames/second) in combination with temporal filter. The GNEQ was compared for the MAF for the small focal-spot at the central axis, and for the medium focal-spot with a higher output on the anode side as well as for the small focal spot with different temporal recursive filtering weights. A net output increase of about 4.0 times could be achieved with a 2-degree anode angle (without the added filtration) and a 4 times longer filament compared to that of the standard 8-degree target. The GNEQ was also increased for the medium focal-spot due to its higher output capacity and for the temporally filtered higher frame rate. Thus higher tube output, while maintaining a small effective focal-spot, should be achievable using one or more of the three methods described with only small modifications of standard x-ray tube geometry.

Region-of-Interest Micro-Angiographic Fluoroscope Detector Used in Aneurysm and Artery Stenosis Diagnoses and Treatment

Due to the need for high-resolution angiographic and interventional vascular imaging, a Micro-Angiographic Fluoroscope (MAF) detector with a Control, Acquisition, Processing, and Image Display System (CAPIDS) was installed on a detector changer, which was attached to the C-arm of a clinical angiographic unit at a local hospital. The MAF detector provides high-resolution, high-sensitivity, and real-time imaging capabilities and consists of a 300 µm-thick CsI phosphor, a dual stage micro-channel plate light image intensifier (LII) coupled to a fiber optic taper (FOT), and a scientific grade frame-transfer CCD camera, providing an image matrix of 1024×1024 35 µm effective square pixels with 12 bit depth. The changer allows the MAF region-of-interest (ROI) detector to be inserted in front of the Image Intensifier (II) when higher resolution is needed during angiographic or interventional vascular imaging procedures, e.g. endovascular stent deployment. The CAPIDS was developed and implemented using Laboratory Virtual Instrumentation Engineering Workbench (LabVIEW) software and provides a user-friendly interface that enables control of several clinical radiographic imaging modes of the MAF including: fluoroscopy, roadmapping, radiography, and digital-subtraction-angiography (DSA). The total system has been used for image guidance during endovascular image-guided interventions (EIGI) for diagnosing and treating artery stenoses and aneurysms using self-expanding endovascular stents and coils in fifteen patient cases, which have demonstrated benefits of using the ROI detector. The visualization of the fine detail of the endovascular devices and the vessels generally gave the clinicians confidence on performing neurovascular interventions and in some instances contributed to improved interventions.

A theoretical and experimental evaluation of the microangiographic fluoroscope: A high-resolution region-of-interest x-ray imager

PURPOSE

The increasing need for better image quality and high spatial resolution for successful endovascular image-guided interventions (EIGIs) and the inherent limitations of the state-of-the-art detectors provide motivation to develop a detector system tailored to the specific, demanding requirements of neurointerventional applications.

METHOD

A microangiographic fluoroscope (MAF) was developed to serve as a high-resolution, region-of-interest (ROI) x-ray imaging detector in conjunction with large lower-resolution full field-of-view (FOV) state-of-the-art x-ray detectors. The newly developed MAF is an indirect x-ray imaging detector capable of providing real-time images (30 frames per second) with high-resolution, high sensitivity, no lag and low instrumentation noise. It consists of a CCD camera coupled to a Gen 2 dual-stage microchannel plate light image intensifier (LII) through a fiber-optic taper. A 300 microm thick CsI(T1) phosphor serving as the front end is coupled to the LII. The LII is the key component of the MAF and the large variable gain provided by it enables the MAF to operate as a quantum-noise-limited detector for both fluoroscopy and angiography.

RESULTS

The linear cascade model was used to predict the theoretical performance of the MAF, and the theoretical prediction showed close agreement with experimental findings. Linear system metrics such as MTF and DQE were used to gauge the detector performance up to 10 cycles/mm. The measured zero frequency DQE(0) was 0.55 for an RQA5 spectrum. A total of 21 stages were identified for the whole imaging chain and each stage was characterized individually.

CONCLUSIONS

The linear cascade model analysis provides insight into the imaging chain and may be useful for further development of the MAF detector. The preclinical testing of the prototype detector in animal procedures is showing encouraging results and points to the potential for significant impact on EIGIs when used in conjunction with a state-of-art flat panel detector (FPD).

Evaluation of a second-generation self-expanding variable-porosity flow diverter in a rabbit elastase aneurysm model

BACKGROUND AND PURPOSE

The self-expanding V-POD is a second-generation flow-diverting device with a low-porosity PTFE patch on a self-expanding microstent. The authors evaluated this device for the treatment of elastase-induced aneurysms in rabbits.

MATERIALS AND METHODS

Three V-POD types (A, circumferential patch closed-cell stent [n = 9]; B, asymmetric patch closed-cell stent [n = 7]; and C, asymmetric patch open-cell stent [n = 4]) were evaluated by using angiography, conebeam micro-CT, histology, and SEM. Aneurysm flow modifications were expressed in terms of immediate poststent/prestent ratios of maximum CA volume entering the aneurysm dome tracked on procedural angiograms. Flow modifications were correlated with 4 weeks' follow-up angiographic, micro-CT, histologic, and SEM results.

RESULTS

Mechanical stent-deployment difficulties in 4 aneurysms (1 type A; 3 type B) led to suboptimal results and exclusion from analysis. Of the remaining 16 aneurysms, 4-week post-treatment angiograms showed no aneurysm filling in 10 (63%), 3 (∼19%) had no filling with a small remnant neck, and 3 (∼19%) had <0.25 filling. Successfully treated aneurysms (n = 16) demonstrated an immediate poststent/prestent CA maximum volume ratio of 0.13 ± 0.18% (0.0%-0.5%). Favorable contrast-flow modification on immediate angiography after deployment correlated significantly with aneurysm occlusion on follow-up angiography, micro-CT, and histology. The occlusion percentage derived from micro-CT was 96 ± 6.8%. Histology indicated advanced healing (grade ≥3) in the aneurysm dome in 13 of 16 cases. SEM revealed 15 of 16 stents in an advanced state of endothelialization.

CONCLUSIONS

This study showed the feasibility and effectiveness of V-POD for aneurysm healing in a rabbit elastase model.

Study of Stent Deployment Mechanics Using a High-Resolution X-ray Imaging Detector

To treat or prevent some of the 795,000 annual strokes in the U.S., self-expanding endo-vascular stents deployed under fluoroscopic image guidance are often used. Neuro-interventionalists need to know the deployment behavior of each stent in order to place them in the correct position. Using the Micro-Angiographic Fluoroscope (MAF) which has about 3 times higher resolution than commercially available flat panel detectors (FPD) we studied the deployment mechanics of two of the most important commercially available nitinol stents: the Pipeline embolization device (EV3), and the Enterprise stent (Codman). The Pipeline stent's length extends to about 3 times that of its deployed length when it is contained inside a catheter. From the high-resolution images with the MAF we found that upon the sudden release of the distal end of the Pipeline from a helical wire cap, the stent expands radially but retracts to about 30% (larger than for patient deployments) of its length. When released from the catheter proximally, it retracts additionally about 50% contributing to large uncertainty in the final deployed location. In contrast, the MAF images clearly show that the Enterprise stent self expands with minimal length retraction during deployment from its catheter and can be retrieved and repositioned until the proximal markers are released from clasping structures on its guide-wire thus enabling more accurate placement at the center of an aneurysm or stenosis. The high-resolution imaging demonstrated in this study should help neurointerventionalists understand and control endovascular stent deployment mechanisms and hence perform more precise treatments.

Component Level Modular Design of a Solid State X-ray Image Intensifier for an M×N Array

The Solid-State X-ray Image Intensifier (SSXII) is a novel dynamic x-ray imager, based on an array of electron-multiplying CCDs (EMCCDs), that can significantly improve performance compared to conventional x-ray image intensifiers (XIIs) and flat panel detectors (FPDs). To expand the field-of-view (FOV) of the SSXII detectors while maintaining high resolution, a scalable component level modular design is presented. Each module can be fit together with minimum dead-space and optically coupled to one contiguous x-ray converter plate. The electronics of each of the modules consists of a detachable head-board, on which is mounted the EMCCD, and a driver board. The size of the head-boards is minimized to ensure that the modules fit together properly. The driver boards connect with the head-boards via flat cables and are designed to be plugged into the main mother-board that contains an FPGA chip that generates the driving clock signals for the EMCCDs and analog-to-digital converter (ADC). At the front-end, a high speed ADC on each of the driver boards samples and digitizes the EMCCD analog output signal and an extensible modular digital multiplexer back-end is used to acquire and combine image data from multiple modules. The combined digital data is then transmitted to a PC via a standard Camera Link interface. Eventually, this modular design will be extended to a 3×3 or larger array to accomplish full clinical FOVs and enable the SSXII to replace conventional lower-resolution XIIs or FPDs.

Generalized two-dimensional (2D) linear system analysis metrics (GMTF, GDQE) for digital radiography systems including the effect of focal spot, magnification, scatter, and detector characteristics

The MTF, NNPS, and DQE are standard linear system metrics used to characterize intrinsic detector performance. To evaluate total system performance for actual clinical conditions, generalized linear system metrics (GMTF, GNNPS and GDQE) that include the effect of the focal spot distribution, scattered radiation, and geometric unsharpness are more meaningful and appropriate. In this study, a two-dimensional (2D) generalized linear system analysis was carried out for a standard flat panel detector (FPD) (194-micron pixel pitch and 600-micron thick CsI) and a newly-developed, high-resolution, micro-angiographic fluoroscope (MAF) (35-micron pixel pitch and 300-micron thick CsI). Realistic clinical parameters and x-ray spectra were used. The 2D detector MTFs were calculated using the new Noise Response method and slanted edge method and 2D focal spot distribution measurements were done using a pin-hole assembly. The scatter fraction, generated for a uniform head equivalent phantom, was measured and the scatter MTF was simulated with a theoretical model. Different magnifications and scatter fractions were used to estimate the 2D GMTF, GNNPS and GDQE for both detectors. Results show spatial non-isotropy for the 2D generalized metrics which provide a quantitative description of the performance of the complete imaging system for both detectors. This generalized analysis demonstrated that the MAF and FPD have similar capabilities at lower spatial frequencies, but that the MAF has superior performance over the FPD at higher frequencies even when considering focal spot blurring and scatter. This 2D generalized performance analysis is a valuable tool to evaluate total system capabilities and to enable optimized design for specific imaging tasks.

Progress in the Development of a new Angiography Suite including the High Resolution Micro-Angiographic Fluoroscope (MAF), a Control, Acquisition, Processing, and Image Display System (CAPIDS), and a New Detector Changer Integrated into a Commercial C-Arm Angiography Unit to Enable Clinical Use

Due to the high-resolution needs of angiographic and interventional vascular imaging, a Micro-Angiographic Fluoroscope (MAF) detector with a Control, Acquisition, Processing, and Image Display System (CAPIDS) was installed on a detector changer which was attached to the C-arm of a clinical angiographic unit. The MAF detector provides high-resolution, high-sensitivity, and real-time imaging capabilities and consists of a 300 μm-thick CsI phosphor, a dual stage micro-channel plate light image intensifier (LII) coupled to a fiber optic taper (FOT), and a scientific grade frame-transfer CCD camera, providing an image matrix of 1024×1024 35 μm square pixels with 12 bit depth. The Solid-State X-Ray Image Intensifier (SSXII) is an EMCCD (Electron Multiplying charge-coupled device) based detector which provides an image matrix of 1k×1k 32 μm square pixels with 12 bit depth. The changer allows the MAF or a SSXII region-of-interest (ROI) detector to be inserted in front of the standard flat-panel detector (FPD) when higher resolution is needed during angiographic or interventional vascular imaging procedures. The CAPIDS 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 or SSXII including: fluoroscopy, roadmapping, radiography, and digital-subtraction-angiography (DSA). The total system has been used for image guidance during endovascular image-guided interventions (EIGI) using prototype self-expanding asymmetric vascular stents (SAVS) in over 10 rabbit aneurysm creation and treatment experiments which have demonstrated the system's potential benefits for future clinical use.

Improved contrast and spatial resolution with Single Photon Counting (SPC) for an area x-ray imager, the newly developed high-resolution Micro-Angiographic Fluoroscopic (MAF) detector

Although in radiological imaging, the prevailing mode of acquisition is the integration of the energy deposited by all x-rays absorbed in the imaging detector, much improvement in image spatial and contrast resolution could be achieved if each individual x-ray photon were detected and counted separately. In this work we compare the conventional energy integration (EI) mode with the new single photon counting (SPC) mode for a recently developed high-resolution Micro-Angiographic Fluoroscopic (MAF) detector, which is uniquely capable of both modes of operation. The MAF has 1024×1024 pixels of 35 microns 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 and enables the MAF to operate in both EI and the very sensitive low-exposure SPC modes. We used high LII gain with very low exposure (<1 x-ray photon/pixel) per frame for SPC mode and higher exposure per frame with lower gain for EI mode. Multiple signal-thresholded frames were summed in SPC mode to provide an integrated frame with the same total exposure as EI 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 frequencies. Images of a line pair phantom also showed improved spatial resolution with 12 lp/mm visible in SPC mode compared to only 8 lp/mm in 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. These improvements with the SPC mode should be advantageous for clinical applications where high resolution and/or high contrast are essential such as in mammography and extremity imaging as well as for dual modality applications, which combine nuclear medicine and x-ray imaging using a single detector.

Partially polyurethane-covered stent for cerebral aneurysm treatment

Partially polyurethane-covered stent (PPCS) is proposed for the treatment of cerebral aneurysms. The PPCSs were observed to substantially modify the flow entering the aneurysm in a patient-specific aneurysm phantom (PSAP). These stents can act as flow modulators and the polyurethane (PU) membrane can provide a smooth scaffold for restoring the structural integrity of the diseased vessel. Partial coating of the stent aids in sealing only the entrance to the aneurysm while keeping the perforators around the aneurysm open and patent. Biocompatibility of the PU membrane was monitored using contact angle measurements to show that critical surface tension (CST) values remained in the thromboresistant range of 20-30 mN/m. Stent flexibility, stiffness, and pressure-diameter relationship showed no significant change after asymmetric PU film application. No delamination of the PU membrane from the stent was observed within the working strains of the stent. The flow modulating capability of the PPCS was monitored by intentionally orienting the stent to cover either the proximal or the distal regions along the neck of the PSAP. Time density curves (TDCs) compared the relative metrics of input rate, washout rate, residence time, and influx in the aneurysm before and after the stent placement.

Modification of the NEMA XR21-2000 cardiac phantom for testing of imaging systems used in endovascular image guided interventions

X-ray equipment testing using phantoms that mimic the specific human anatomy, morphology, and structure is a very important step in the research, development, and routine quality assurance for such equipment. Although the NEMA XR21 phantom exists for cardiac applications, there is no such standard phantom for neuro-, peripheral and cardio-vascular angiographic applications. We have extended the application of the NEMA XR21-2000 phantom to evaluate neurovascular x-ray imaging systems by structuring it to be head-equivalent; two aluminum plates shaped to fit into the NEMA phantom geometry were added to a 15 cm thick section. Also, to enable digital subtraction angiography (DSA) testing, two replaceable central plates with a hollow slot were made so that various angiographic sections could be inserted into the phantom. We tested the new modified phantom using a flat panel C-arm unit dedicated for endovascular image-guided interventions. All NEMA XR21-2000 standard test sections were used in evaluations with the new "head-equivalent" phantom. DSA and DA are able to be tested using two standard removable blocks having simulated arteries of various thickness and iodine concentrations (AAPM Report 15). The new phantom modifications have the benefits of enabling use of the standard NEMA phantom for angiography in both neuro- and cardio-vascular applications, with the convenience of needing only one versatile phantom for multiple applications. Additional benefits compared to using multiple phantoms are increased portability and lower cost.

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.

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 system's 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.

Component analysis of a new Solid State X-ray Image Intensifier (SSXII) using photon transfer and Instrumentation Noise Equivalent Exposure (INEE) measurements

The SSXII is a novel x-ray imager designed to improve upon the performance limitations of conventional dynamic radiographic/fluoroscopic imagers related to resolution, charge-trapping, frame-rate, and instrumentation-noise. The SSXII consists of a CsI:Tl phosphor coupled via a fiber-optic taper (FOT) to an electron-multiplying CCD (EMCCD). To facilitate investigational studies, initial designs enable interchangeability of such imaging components. Measurements of various component and configuration characteristics enable an optimization analysis with respect to overall detector performance. Photon transfer was used to characterize the EMCCD performance including ADC sensitivity, read-noise, full-well capacity and quantum efficiency. X-ray sensitivity was measured using RQA x-ray spectra. Imaging components were analyzed in terms of their MTF and transmission efficiency. The EMCCD was measured to have a very low effective read-noise of less than 1 electron rms at modest EMCCD gains, which is more than two orders-of-magnitude less than flat panel (FPD) and CMOS-based detectors. The variable signal amplification from 1 to 2000 times enables selectable sensitivities ranging from 8.5 (168) to over 15k (300k) electrons per incident x-ray photon with (without) a 4:1 FOT; these sensitivities could be readily increased with further component optimization. MTF and DQE measurements indicate the SSXII performance is comparable to current state-of-the-art detectors at low spatial frequencies and far exceeds them at higher spatial frequencies. The instrumentation noise equivalent exposure (INEE) was measured to be less than 0.3 μR out to 10 cycles/mm, which is substantially better than FPDs. Component analysis suggests that these improvements can be substantially increased with further detector optimization.

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.

The Solid-State X-Ray Image Intensifier (SSXII): An EMCCD-Based X-Ray Detector

The solid-state x-ray image intensifier (SSXII) is an EMCCD-based x-ray detector designed to satisfy an increasing need for high-resolution real-time images, while offering significant improvements over current flat panel detectors (FPDs) and x-ray image intensifiers (XIIs). FPDs are replacing XIIs because they reduce/eliminate veiling glare, pincushion or s-shaped distortions and are physically flat. However, FPDs suffer from excessive lag and ghosting and their performance has been disappointing for low-exposure-per-frame procedures due to excessive instrumentation-noise. XIIs and FPDs both have limited resolution capabilities of ~3 cycles/mm. To overcome these limitations a prototype SSXII module has been developed, consisting of a 1k x 1k, 8 mum pixel EMCCD with a fiber-optic input window, which views a 350 mum thick CsI(Tl) phosphor via a 4:1 magnifying fiber-optic-taper (FOT). Arrays of such modules will provide a larger field-of-view. Detector MTF, DQE, and instrumentation-noise equivalent exposure (INEE) were measured to evaluate the SSXIIs performance using a standard x-ray spectrum (IEC RQA5), allowing for comparison with current state-of-the-art detectors. The MTF was 0.20 at 3 cycles/mm, comparable to standard detectors, and better than 0.05 up to 7 cycles/mm, well beyond current capabilities. DQE curves indicate no degradation from high-angiographic to low-fluoroscopic exposures (< 2% deviation in overall DQE from 1.3 mR to 2.7 muR), demonstrating negligible instrumentation-noise, even with low input signal intensities. An INEE of < 0.2 muR was measured for the highest-resolution mode (32 mum effective pixel size). Comparison images between detector technologies qualitatively demonstrate these improved imaging capabilities provided by the SSXII.

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 playback 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 framerate 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. (Support: NIH Grants R01NS43924, R01EB002873).

Investigation of new flow modifying endovascular image-guided interventional (EIGI) techniques in patient-specific aneurysm phantoms (PSAPs) using optical imaging

Effective minimally invasive treatment of cerebral bifurcation aneurysms is challenging due to the complex and remote vessel morphology. An evaluation of endovascular treatment in a phantom involving image-guided deployment of new asymmetric stents consisting of polyurethane patches placed to modify blood flow into the aneurysm is reported. The 3D lumen-geometry of a patient-specific basilar-artery bifurcation aneurysm was derived from a segmented computed-tomography dataset. This was used in a stereolithographic rapid-prototyping process to generate a mold which was then used to create any number of exact wax models. These models in turn were used in a lost-wax technique to create transparent elastomer patient-specific aneurysm phantoms (PSAP) for evaluating the effectiveness of asymmetric-stent deployment for flow modification. Flow was studied by recording real-time digitized video images of optical dye in the PSAP and its feeding vessel. For two asymmetric stent placements: through the basilar into the right-posterior communicating artery (RPCA) and through the basilar into the left-posterior communicating artery (LPCA), the greatest deviation of flow streamlines away from the aneurysm occurred for the RPCA stent deployment. Flow was also substantially affected by variations of inflow angle into the basilar artery, resulting in alternations in washout times as derived from time-density curves. Evaluation of flow in the PSAPs with real-time optical imaging can be used to determine new EIGI effectiveness and to validate computational-fluid-dynamic calculations for EIGI-treatment planning.