We have developed a dose-tracking system (DTS) that calculates the radiation dose to the patient’s skin in real-time by acquiring exposure parameters and imaging-system-geometry from the digital bus on a Toshiba Infinix C-arm unit. The cumulative dose values are then displayed as a color map on an OpenGL-based 3D graphic of the patient for immediate feedback to the interventionalist. Determination of those elements on the surface of the patient 3D-graphic that intersect the beam and calculation of the dose for these elements in real time demands fast computation. Reducing the size of the elements results in more computation load on the computer processor and therefore a tradeoff occurs between the resolution of the patient graphic and the real-time performance of the DTS. The speed of the DTS for calculating dose to the skin is limited by the central processing unit (CPU) and can be improved by using the parallel processing power of a graphics processing unit (GPU). Here, we compare the performance speed of GPU-based DTS software to that of the current CPU-based software as a function of the resolution of the patient graphics. Results show a tremendous improvement in speed using the GPU. While an increase in the spatial resolution of the patient graphics resulted in slowing down the computational speed of the DTS on the CPU, the speed of the GPU-based DTS was hardly affected. This GPU-based DTS can be a powerful tool for providing accurate, real-time feedback about patient skin-dose to physicians while performing interventional procedures.
skin dose; dosimetry; fluoroscopic dose; dose tracking; real-time dosimetry; fluoroscopic interventional procedures; GPU
Intracranial aneurysm treatment with flow diverters (FD) is a new minimally invasive approach, recently approved for use in human patients. Attempts to correlate the flow reduction observed in angiograms with a parameter related to the FD structure have not been totally successful. To find the proper parameter, we investigated four porous-media flow models. The models describing the relation between the pressure drop and flow velocity that are investigated include the capillary theory linear model (CTLM), the drag force linear model (DFLM), the simple quadratic model (SQM) and the modified quadratic model (MQM). Proportionality parameters are referred to as permeability for the linear models and resistance for the quadratic ones. A two stage experiment was performed. First, we verified flow model validity by placing six different stainless-steel meshes, resembling FD structures, in known flow conditions. The best flow model was used for the second stage, where six different FD’s were inserted in aneurysm phantoms and flow modification was estimated using angiographically derived time density curves (TDC). Finally, TDC peak variation was compared with the FD parameter. Model validity experiments indicated errors of: 70% for the linear models, 26% for the SQM and 7% for the MQM. The resistance calculated according to the MQM model correlated well with the contrast flow reduction. Results indicate that resistance calculated according to MQM is appropriate to characterize the FD and could explain the flow modification observed in angiograms.
flow diverter; digital subtraction angiography; aneurysms; flow resistance; permeability
Phantom equivalents of different human anatomical parts are routinely used for imaging system evaluation or dose calculations. The various recommendations on the generic phantom structure given by organizations such as the AAPM, are not always accurate when evaluating a very specific task. When we compared the AAPM head phantom containing 3 mm of aluminum to actual neuro-endovascular image guided interventions (neuro-EIGI) occurring in the Circle of Willis, we found that the system automatic exposure rate control (AERC) significantly underestimated the x-ray parameter selection. To build a more accurate phantom for neuro-EIGI, we reevaluated the amount of aluminum which must be included in the phantom. Human skulls were imaged at different angles, using various angiographic exposures, at kV’s relevant to neuro-angiography. An aluminum step wedge was also imaged under identical conditions, and a correlation between the gray values of the imaged skulls and those of the aluminum step thicknesses was established. The average equivalent aluminum thickness for the skull samples for frontal projections in the Circle of Willis region was found to be about 13 mm. The results showed no significant changes in the average equivalent aluminum thickness with kV or mAs variation. When a uniform phantom using 13 mm aluminum and 15 cm acrylic was compared with an anthropomorphic head phantom the x-ray parameters selected by the AERC system were practically identical. These new findings indicate that for this specific task, the amount of aluminum included in the head equivalent must be increased substantially from 3 mm to a value of 13 mm.
Head equivalent phantom; uniform phantom; Circle of Willis; fluoroscopy; digital subtracted angiography
Neurointervention is an ever-evolving specialty with tools including microcatheters, microwires, and coils that allow treatment of pathological conditions in increasingly smaller intracranial arteries, requiring increasing accuracy. As endovascular tools evolve, so too should the imaging.
To detail the use of microangiography performed with a novel fluoroscope during coiling of intracranial aneurysms in 2 separate patients and discuss the benefits and potential limitations of the technology.
The microangiographic fluoroscope (MAF) is an ultra high-resolution x-ray detector with superior resolution over a small field of view. The MAF can be incorporated into a standard angiographic C-arm system for use during endovascular procedures.
The MAF was useful for improved visualization during endovascular coiling of 2 unruptured intracranial aneurysms, without adding significant time to the procedure. No significant residual aneurysm filling was identified post-coiling, and no complications occurred.
The MAF is a high-resolution detector developed for use in neurointerventional cases in which superior image quality over a small field of view is required. It has been used with success for coiling of 2 unruptured aneurysms at our institution. It shows promise as an important tool in improving the accuracy with which neurointerventionists can perform certain intracranial procedures.
Intracranial aneurysms; Microangiographic fluoroscope
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.
Use of an extensible array of Electron Multiplying CCDs (EMCCDs) in medical x-ray imager applications was demonstrated for the first time. The large variable electronic-gain (up to 2000) and small pixel size of EMCCDs provide effective suppression of readout noise compared to signal, as well as high resolution, enabling the development of an x-ray detector with far superior performance compared to conventional x-ray image intensifiers and flat panel detectors. We are developing arrays of EMCCDs to overcome their limited field of view (FOV). In this work we report on an array of two EMCCD sensors running simultaneously at a high frame rate and optically focused on a mammogram film showing calcified ducts. The work was conducted on an optical table with a pulsed LED bar used to provide a uniform diffuse light onto the film to simulate x-ray projection images. The system can be selected to run at up to 17.5 frames per second or even higher frame rate with binning. Integration time for the sensors can be adjusted from 1 ms to 1000 ms. Twelve-bit correlated double sampling AD converters were used to digitize the images, which were acquired by a National Instruments dual-channel Camera Link PC board in real time. A user-friendly interface was programmed using LabVIEW to save and display 2K × 1K pixel matrix digital images. The demonstration tiles a 2 × 1 array to acquire increased-FOV stationary images taken at different gains and fluoroscopic-like videos recorded by scanning the mammogram simultaneously with both sensors. The results show high resolution and high dynamic range images stitched together with minimal adjustments needed. The EMCCD array design allows for expansion to an M×N array for arbitrarily larger FOV, yet with high resolution and large dynamic range maintained.
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.
NEQ; Focal spot; GNEQ; ROI; MAF; EIGI; MTF; NNPS; X-ray tube; CNR
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.
Intracranial aneurysms; microangiographic fluoroscope; MAF; coil embolization; cross-correlation sensitivity
An EMCCD-based dual modular x-ray imager was recently designed and developed from the component level, providing a high dynamic range of 53 dB and an effective pixel size of 26 μm for angiography and fluoroscopy. The unique 2×1 array design efficiently increased the clinical field of view, and also can be readily expanded to an M×N array implementation. Due to the alignment mismatches between the EMCCD sensors and the fiber optic tapers in each module, the output images or video sequences result in a misaligned 2048×1024 digital display if uncorrected. In this paper, we present a method for correcting display registration using a custom-designed two layer printed circuit board. This board was designed with grid lines to serve as the calibration pattern, and provides an accurate reference and sufficient contrast to enable proper display registration. Results show an accurate and fine stitching of the two outputs from the two modules.
Fluoroscopic systems have excellent temporal resolution, but are relatively noisy. In this paper we present a recursive temporal filter with different weights (lag) for different user selected regions of interest (ROI) to assist the neurointerventionalist during an image guided catheter procedure. The filter has been implemented on a Graphics Processor (GPU), enabling its usage for fast frame rates such as during fluoroscopy.
We first demonstrate the use of this GPU-implemented rapid temporal filtering technique during an endovascular image guided intervention with normal fluoroscopy. Next we demonstrate its use in combination with ROI fluoroscopy where the exposure is substantially reduced in the peripheral region outside the ROI, which is then software-matched in brightness and filtered using the differential temporal filter. This enables patient dose savings along with improved image quality.
Using flow diverting Stents for intracranial aneurysm repair has been an area of recent active research. While current commercial flow diverting stents rely on a dense mesh of braided coils for flow diversion, our group has been developing a method to selectively occlude the aneurysm neck, without endangering nearby perforator vessels. In this paper, we present a new method of fabricating the low porosity patch, a key element of such asymmetric vascular stents (AVS).
We have built new asymmetric stents for minimally invasive endovascular treatment of cerebral aneurysms. Each asymmetric stent consists of a commercial stent with a micro-welded circular mesh patch. The blood flow modification in aneurysm-vessel phantoms due to these stents was evaluated using x-ray angiographic analysis. However, the density difference between the radiographic contrast and the blood gives rise to a gravity effect, which was evaluated using an initial optical dye-dilution experiment. For the radiographic evaluations, curved-vessel phantoms instead of simple straight side-wall aneurysm phantoms were used in the characterization of meshes/stents. Six phantoms (one untreated, one treated with a commercial stent, and four treated with different asymmetric stents) with similar morphologies were used for comparison. We calculated time-density curves of the aneurysm region and then calculated the peak value (Pk) and washout rate (1/τ) after analytical curve fitting. Flow patterns in the angiograms showed reduction of vortex flow and slow washout in the dense mesh patch treated aneurysms. The meshes reduced Pk down to 21% and 1/τ down to 12% of the values for the untreated case. In summary, new asymmetric stents were constructed and their evaluation demonstrates that they may be useful in the endovascular treatment of aneurysms.
aneurysm; cerebral aneurysm; angiography; time-density; blood flow; flow evaluation; flow modification; stent; asymmetric stent; interventional neuroradiology
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.
Neuro-imaging; Neuro-endovascular image guided interventions; Stent; EV3; Enterprise Stent; Pipeline Stent
In this study, we evaluated the imaging characteristics of the high-resolution, high-sensitivity micro-angiographic fluoroscope (MAF) with 35-micron pixel-pitch when used with different commercially-available 300 micron thick phosphors: the high resolution (HR) and high light (HL) from Hamamatsu. The purpose of this evaluation was to see if the HL phosphor with its higher screen efficiency could be replaced with the HR phosphor to achieve improved resolution without an increase in noise resulting from the HR's decreased light-photon yield. We designated the detectors MAF-HR and MAF-HL and compared them with a standard flat panel detector (FPD) (194 micron pixel pitch and 600 micron thick CsI(Tl)). For this comparison, we used the generalized linear-system metrics of GMTF, GNNPS and GDQE which are more realistic measures of total system performance since they include the effect of scattered radiation, focal spot distribution, and geometric un-sharpness. Magnifications (1.05-1.15) and scatter fractions (0.28 and 0.33) characteristic of a standard head phantom were used. The MAF-HR performed significantly better than the MAF-HL at high spatial frequencies. The ratio of GMTF and GDQE of the MAF-HR compared to the MAF-HL at 3(6) cycles/mm was 1.45(2.42) and 1.23(2.89), respectively. Despite significant degradation by inclusion of scatter and object magnification, both MAF-HR and MAF-HL provide superior performance over the FPD at higher spatial frequencies with similar performance up to the FPD's Nyquist frequency of 2.5 cycles/mm. Both substantially higher resolution and improved GDQE can be achieved with the MAF using the HR phosphor instead of the HL phosphor.
MTF; DQE; GMTF; GDQE; NPS; GNNPS; MAF; FPD; MAF-HR; MAF-HL
Aneurysm treatment using flow diversion could become the treatment of choice in the near future. While such side-wall aneurysm treatments have been studied in many publications and even implemented in selected clinical cases, bifurcation aneurysm treatment using flow diversion has not been addressed in detail. Using angiographic imaging, we evaluated treatment of such cases with several stent designs using patient-specific aneurysm phantoms. The aim is to find a way under fluoroscopic image guidance to place a low-porosity material across the aneurysm orifice while keeping the vessel blockage minimal. Three pre-shaped self-expanding stent designs were developed: the first design uses a middle-flap wing stent, the second uses a two-tapered-wing-ended stent, and the third is a slight modification of the first design in which the middle-flap is anchored tightly against the aneurysm using a standard stent. Treatment effects on flow were evaluated using high-speed angiography (30 fps) and compared with the untreated aneurysm. Contrast inflow was reduced in all the cases: 25% for Type 1, 63% for type 2 and 88% for Type 3. The first and the second stent design allowed some but substantially-reduced flow inside the aneurysm neck as indicated by the time-density curves. The third stent design eliminated almost all flow directed at the aneurysm dome, and only partial filling was observed. In the same time Type 1 and 3 delayed the inflow in the branches up to 100% compared to the untreated phantom. The results are quite promising and warrant future study.
Flow Diverter; Asymmetric Vascular Stent; Time Density Curves; Intracranial Bifurcation Aneurysm; Patient Specific Phantoms; Branch Jailing
We evaluate a new method for measuring the presampled modulation transfer function (MTF) using the noise power spectrum (NPS) obtained from a few flat-field images acquired at one exposure level. The NPS is the sum of structure, quantum, and additive instrumentation noise, which are proportional to exposure squared, exposure, and a constant, respectively, with the spatial-frequency dependence of the quantum noise depending partly on the detector MTF. Cascaded linear-systems theory was used to derive an exact and generic relationship that was used to isolate noise terms and enable determination of the MTF directly from the noise response, thereby circumventing the need for precision test objects (slit, edge, etc.) as required by standard techniques. Isolation of the quantum NPS by fitting the total NPS versus exposure obtained using 30 flat-field images each at six or more different exposure levels with a linear regression provides highly accurate MTFs. A subset of these images from indirect digital detectors was used to investigate the accuracy of measuring the MTF from 30 or fewer flat-field images obtained at a single exposure level. Analyzing as few as two images acquired at a single exposure resulted in no observable systematic error. Increasing the number of images analyzed resulted in an increase in accuracy. Fifteen images provided comparable accuracy with the most rigorous slope approach, with less than 5% variability, suggesting additional image acquisitions may be unnecessary. Reducing the number of images acquired for the noise response method further simplifies and facilitates routine MTF measurements.
MTF; two-dimensional MTF; NPS; detector; performance; flat panel detector; SSXII; image quality; quality assurance
Performance of indirect digital x-ray imagers is typically limited by the front-end components. Present x-ray-to-light converting phosphors significantly reduce detector resolution due to stochastic blurring and k-fluorescent x-ray reabsorption. Thinner phosphors improve resolution at the cost of lowering quantum detection efficiency (QDE) and increasing Swank noise. Magnifying fiber optic tapers (FOTs) are commonly used to increase the field-of-view of small sensor imagers, such as CMOS, CCD, or electron-multiplying CCD (EMCCD) based detectors, which results in a reduction in detector sensitivity and further reduces the MTF. We investigate performance trade-offs for different front-end configurations coupled to an EMCCD sensor with 8 μm pixels. Six different columnar structured CsI(Tl) scintillators with thicknesses of 100, 200, 350, 500, and 1000 μm type high-light (HL) and a 350 μm type high-resolution (HR) (Hamamatsu) and four different FOTs with magnification ratios (M) of 1, 2.5, 3.3, and 4 were studied using the RQA5 x-ray spectrum. The relative signal of the different scintillators largely followed the relative QDE, indicating their light output per absorbed x-ray was similar, with the type HR CsI emitting 57% of the type HL. The efficiency of the FOTs was inversely proportional to M2 with the M = 1 FOT transmitting 87% of the incident light. At 5 (10) cycles/mm, the CsI MTF was 0.38 (0.22), 0.33 (0.17), 0.37 (0.19), 0.23 (0.09), 0.19 (0.08), and 0.09 (0.03) for the 100, 200, 350HR, 350, 500, and 1000 μm CsI, respectively and the FOT MTF was 0.89 (0.84), 0.80 (0.72), 0.70 (0.60), and 0.69 (0.37) for M = 1, 2.5, 3.3, and 4, respectively. The 1000, 500, and 350HR μm CsI had the highest DQE for low, medium, and high spatial frequency ranges of 0 to 1.6, 1.6 to 4.5, and 4.5 to 10 cycles/mm, respectively. Larger FOT M resulted in a reduction in DQE. Quantifying performance of different front-end configurations will enable optimal selection of components for task-specific designs.
A tracking system has been developed to provide real-time feedback of skin dose and dose rate during interventional fluoroscopic procedures. The dose tracking system (DTS) calculates the radiation dose rate to the patient’s skin using the exposure technique parameters and exposure geometry obtained from the x-ray imaging system digital network (Toshiba Infinix) and presents the cumulative results in a color mapping on a 3D graphic of the patient. We performed a number of tests to verify the accuracy of the dose representation of this system. These tests included comparison of system–calculated dose-rate values with ionization-chamber (6 cc PTW) measured values with change in kVp, beam filter, field size, source-to-skin distance and beam angulation. To simulate a cardiac catheterization procedure, the ionization chamber was also placed at various positions on an Alderson Rando torso phantom and the dose agreement compared for a range of projection angles with the heart at isocenter. To assess the accuracy of the dose distribution representation, Gafchromic film (XR-RV3, ISP) was exposed with the beam at different locations. The DTS and film distributions were compared and excellent visual agreement was obtained within the cm-sized surface elements used for the patient graphic. The dose (rate) values agreed within about 10% for the range of variables tested. Correction factors could be applied to obtain even closer agreement since the variable values are known in real-time. The DTS provides skin-dose values and dose mapping with sufficient accuracy for use in monitoring diagnostic and interventional x-ray procedures.
skin dose; dosimetry; radiation safety; cardiac fluoroscopic procedures; fluoroscopic dose; dose tracking; real-time dosimetry; fluoroscopic interventional procedures
The low electronic noise, high resolution, and good temporal performance of electron-multiplying CCDs (EMCCDs) are ideally suited for applications traditionally served by x-ray image intensifiers. In order to improve an expandable clinical detector’s field-of-view and have full control of the system performance, we have successfully built a solid-state x-ray detector. The photon transfer technique was used to quantify the EMCCD quantum performance in terms of sensitivity (or camera gain constant, K), read noise (RN), full-well capacity (FW), and dynamic range (DR). Measured results show the system maintains a K of 11.3 ± 0.9 e−/DN at unit gain, with a read noise of 71.5±6.0 e−rms at gain 1, which decreases proportionally with higher gains. The full well capacity was measured to be 31.3±2.7 ke−, providing a dynamic range of 52.8±0.7 dB using the chip manufacturer specified clocking scheme. Similar performance was measured with other commercial camera systems. The manufacturer data sheet indicates a dynamic range of 66 dB is plausible with improved read noise and full well capacity. Different clocking schemes are under investigation to assess their impact on improving performance towards idealized values. EMCCD driver clock voltage levels were adjusted individually to check the influence on quantum performance. The clocks work to transfer charge from the image area to readout amplifier through the storage area, horizontal and multiplication registers. Results indicate that the clock that contributes to lateral overflow drain bias is essential to the system performance in terms of dynamic range and full well capacity. The serial register clocks used for transporting charge stored in the pixels of the memory lines to the output amplifier had the largest effect on RN, while others had less of an impact. Initial adjustment of these clocks resulted in a variability of 16% in the performance of dynamic range, 38% in read noise and 56% in full well capacity. Quantifying the quantum performance provides valuable insight into overall performance and enables optimal adjustment of the clocking scheme. Further improvements are expected.
An asymmetric stent with low porosity patch across the intracranial aneurysm neck and high porosity elsewhere is designed to modify the flow to result in thrombogenesis and occlusion of the aneurysm and yet to reduce the possibility of also occluding adjacent perforator vessels. The purposes of this study are to evaluate the flow field induced by an asymmetric stent using both numerical and digital subtraction angiography (DSA) methods and to quantify the flow dynamics of an asymmetric stent in an in vivo aneurysm model. We created a vein-pouch aneurysm model on the canine carotid artery. An asymmetric stent was implanted at the aneurysm, with 25% porosity across the aneurysm neck and 80% porosity elsewhere. The aneurysm geometry, before and after stent implantation, was acquired using cone beam CT and reconstructed for computational fluid dynamics (CFD) analysis. Both steady-state and pulsatile flow conditions using the measured waveforms from the aneurysm model were studied. To reduce computational costs, we modeled the asymmetric stent effect by specifying a pressure drop over the layer across the aneurysm orifice where the low porosity patch was located. From the CFD results, we found the asymmetric stent reduced the inflow into the aneurysm by 51%, and appeared to create a stasis-like environment which favors thrombus formation. The DSA sequences also showed substantial flow reduction into the aneurysm. Asymmetric stents may be a viable image guided intervention for treating intracranial aneurysms with desired flow modification features.
Stent; Aneurysm; Digital Subtraction Angiography; Computational Fluid Dynamics; Image guided interventions; CT; image-based finite element models of physiology; image-based biomechanical models; circulation; porosity; wall shear stress
A new high-resolution, high-sensitivity, low-noise x-ray detector based on EMCCDs has been developed. The EMCCD detector module consists of a 1kx1k, 8μm pixel EMCCD camera coupled to a CsI(Tl) scintillating phosphor via a fiber optic taper (FOT). Multiple modules can be used to provide the desired field-of-view (FOV). The detector is capable of acquisitions over 30fps. The EMCCD’s variable gain of up to 2000x for the pixel signal enables high sensitivity for fluoroscopic applications. With a 3:1 FOT, the detector can operate with a 144μm effective pixel size, comparable to current flat-panel detectors. Higher resolutions of 96 and 48μm pixel size can also be achieved with various binning modes. The detector MTFs and DQEs were calculated using a linear-systems analysis. The zero frequency DQE was calculated to be 59% at 74 kVp. The DQE for the 144μm pixel size was shown to exhibit quantum-noise limited behavior down to ~0.1μR using a conservative 30x gain. At this low exposure, gains above 30x showed limited improvements in DQE suggesting such increased gains may not be necessary. For operation down to 48μm pixel sizes, the detector instrumentation noise equivalent exposure (INEE), defined as the exposure where the instrumentation noise equals the quantum-noise, was <0.1μR for a 20x gain. This new technology may provide improvements over current flat-panel detectors for applications such as fluoroscopy and angiography requiring high frame rates, resolution, dynamic range and sensitivity while maintaining essentially no lag and very low INEE. Initial images from a prototype detector are also presented.
DX; DCT; METR; SIM; SYS
We study the properties of a new microangiographic system, consisting of a Region of Interest (ROI) microangiographic detector, x-ray source, and patient. The study was performed under conditions intended for clinical procedures such as neurological diagnostic angiograms as well as treatments of intracranial aneurysms, and vessel-stenoses. The study was performed in two steps; first a uniform head equivalent phantom was used as a “filter”. This allowed us to study the properties of the detector alone, under clinically relevant x-ray spectra. We report the detector MTF, NPS, NEQ, and DQE for beam energies ranging from 60–100kVp and for different detector entrance exposures. For the second step, the phantom was placed adjacent to the detector, allowing scatter to enter the detector and new measurements were obtained for the same beam energies and detector entrance exposures. Different radiation field sizes were studied, and the effects of different scatter amounts were investigated. The spatial distribution of scatter was studied using the edge-spread method and a generalized system MTF was obtained by combining the scatter MTF weighted by the scatter fraction with the detector MTF and focal spot unsharpness due to magnification. The NPS combined with the generalized MTF gave the generalized system NEQ and DQE. The generalized NEQ and the ideal object detectability were used to calculate the Dose Area Product to the patient for 75% object detection probability. This was used as a system optimization method.
MTF; NPS; NEQ; DQE; system; generalized; detectability; observer; angiography; microrangiography
Standard objective parameters such as MTF, NPS, NEQ and DQE do not reflect complete system performance, because they do not account for geometric unsharpness due to finite focal spot size and scatter due to the patient. The inclusion of these factors led to the generalization of the objective quantities, termed GMTF, GNNPS, GNEQ and GDQE defined at the object plane. In this study, a commercial x-ray image intensifier (II) is evaluated under this generalized approach and compared with a high-resolution, ROI microangiographic system previously developed and evaluated by our group. The study was performed using clinically relevant spectra and simulated conditions for neurovascular angiography specific for each system. A head-equivalent phantom was used, and images were acquired from 60 to 100 kVp. A source to image distance of 100 cm (75 cm for the microangiographic system) and a focal spot of 0.6 mm were used. Effects of varying the irradiation field-size, the air-gaps, and the magnifications (1.1 to 1.3) were compared. A detailed comparison of all of the generalized parameters is presented for the two systems. The detector MTF for the microangiographic system is in general better than that for the II system. For the total x-ray imaging system, the GMTF and GDQE for the II are better at low spatial frequencies, whereas the microangiographic system performs substantially better at higher spatial frequencies. This generalized approach can be used to more realistically evaluate and compare total system performance leading to improved system designs tailored to the imaging task.
MTF; NPS; GMTF; GNNPS; GDQE; Image Intensifier; generalized; microangiography; performance
In order to satisfy the high resolution (3 to 10 cycles/mm) imaging requirements in neurovascular image-guided interventional (IGI) procedures, a micro-angiographic fluoroscope (MAF) is being developed to enable both rapid sequence angiography (15 fps) at high exposure levels (hundreds of μR/frame) as well as fluoroscopy at high frame rates (30 fps) and low exposure levels (5 to 20 μR/frame). The prototype MAF consists of a 350-μm-thick CsI(Tl) scintillator coupled by a 2:1 fiber-optical taper to an 18 mm diameter variable-gain light image intensifier with two-stage microchannel plate (MCP) viewed by a 12-bit, 1024x1024, 30 fps CCD camera with digital interface board. The optical set-up enables variation of effective pixel-size from 31 to 50 micron. The first frame lag of the MAF in fluoroscopic 30 fps mode (2:1 binning) was less than 0.8% at exposures of 5-23 μR/frame. MTF, NPS, and DQE in angiographic mode were measured for IEC standard spectrum RQA 5. At spatial frequencies of 4 and 10 cycles/mm the MTF was 14% and 1.5%, and the DQE was 12% and 1.2%, respectively, while the DQE(0) was 60%. Acquisition software was developed to acquire 15 fps angiography and 30 fps fluoroscopy for real-time dark field and flat field correction or real-time roadmapping. Images obtained with the MAF in small animal IGI procedures are demonstrated. The linearity versus x-ray intensity and MCP working range effects has been studied. We plan to expand the current 3.6 cm diameter field of view to 6 cm in the next model of the MAF.
x-ray detector; fluoroscopy; angiography; image-guided intervention; MTF; DQE; region-of-interest radiography; micro-angiography; MCP; linearity
A new microangiographic system (MA) integrated into a c-arm gantry has been developed allowing precise placement of a MA at the exact same angle as the standard x-ray image intensifier (II) with unchanged source and object position. The MA can also be arbitrarily moved about the object and easily moved into the field of view (FOV) in front of the lower resolution II when higher resolution angiographic sequences are needed. The benefits of this new system are illustrated in a neurovascular study, where a rabbit is injected with contrast media for varying oblique angles. Digital subtraction angiographic (DSA) images were obtained and compared using both the MA and II detectors for the same projection view. Vessels imaged with the MA appear sharper with smaller vessels visualized. Visualization of ~100 μm vessels was possible with the MA whereas not with the II. Further, the MA could better resolve vessel overlap. Contrast to noise ratios (CNR) were calculated for vessels of varying sizes for the MA versus the II and were found to be similar for large vessels, approximately double for medium vessels, and infinitely better for the smallest vessels. In addition, a 3D reconstruction of selected vessel segments was performed, using multiple (three) projections at oblique angles, for each detector. This new MA/II integrated system should lead to improved diagnosis and image guidance of neurovascular interventions by enabling initial guidance with the low resolution large FOV II combined with use of the high resolution MA during critical parts of diagnostic and interventional procedures.
x-ray detectors; x-ray tomosynthesis; detector technology; digital angiography; microangiography; image intensifier; multiview; image guided interventions; image reconstructions