Background: Endovascular treatment of intracranial aneurysms (IAs) by flow diverter (FD) stents depends on flow modification. Patient-specific modeling of FD deployment and computational fluid dynamics (CFD) could enable a priori endovascular strategy optimization. We developed a fast, simplistic, expansion-free balls-weeping algorithm to model FDs in patientspecific aneurysm geometry. However, since such strong simplification could result in less accurate simulations, we also developed a fast virtual stenting workflow (VSW) that explicitly models stent expansion using pseudo-physical forces. Methods: To test which of these two fast algorithms more accurately simulates real FDs, we applied them to virtually treat three representative patient-specific IAs. We deployed Pipeline Embolization Device into 3 patient-specific silicone aneurysm phantoms and simulated the treatments using both balls-weeping and VSW algorithms in computational aneurysm models. We then compared the virtually deployed FD stents against experimental results in terms of geometry and post-treatment flow fields. For stent geometry, we evaluated gross configurations and porosity. For post-treatment aneurysmal flow, we compared CFD results against experimental measurements by particle image velocimetry. Results: We found that VSW created more realistic FD deployments than balls-weeping in terms of stent geometry, porosity and pore density. In particular, balls-weeping produced unrealistic FD bulging at the aneurysm neck, and this artifact drastically increased with neck size. Both FD deployment methods resulted in similar flow patterns, but the VSW had less error in flow velocity and inflow rate. Conclusion: In conclusion, modeling stent expansion is critical for preventing unrealistic bulging effects and thus should be considered in virtual FD deployment algorithms. Also endowed with its high computational efficiency and superior accuracy, the VSW algorithm is a better candidate for implementation into a bedside clinical tool for FD deployment simulation.
Purpose: To determine the reduction of integral dose to the patient when using the micro‐angiographic fluoroscope (MAF) compared to when using the standard flat‐panel detector (FPD) for the techniques used during neurointerventional procedures. Methods: The MAF is a small field‐of‐view, high resolution x‐ray detector which captures 1024 × 1024 pixels with an effective pixel size of 35μm and is capable of real‐time imaging up to 30 frames per second. The MAF was used in neuro‐interventions during those parts of the procedure when high resolution was needed and the FPD was used otherwise. The technique parameters were recorded when each detector was used and the kerma‐area‐product (KAP) per image frame was determined. KAP values were calculated for seven neuro interventions using premeasured calibration files of output as a function of kVp and beam filtration and included the attenuation of the patient table for the frontal projections to be more representative of integral patient dose. The air kerma at the patient entrance was multiplied by the beam area at that point to obtain the KAP values. The ranges of KAP values per frame were determined for the range of technique parameters used during the clinical procedures. To appreciate the benefit of the higher MAF resolution in the region of interventional activity, DA technique parameters were generally used with the MAF. Results: The lowest and highest values of KAP per frame for the MAF in DA mode were 4 and 50 times lower, respectively, compared to those of the FPD in pulsed fluoroscopy mode. Conclusion: The MAF was used in those parts of the clinical procedures when high resolution and image quality was essential. The integral patient dose as represented by the KAP value was substantially lower when using the MAF than when using the FPD due to the much smaller volume of tissue irradiated. This research was supported in part by Toshiba Medical Systems Corporation and NIH Grant R01EB002873.
Purpose: To evaluate the task specific imaging performance of a new 25µm pixel pitch, 1000µm thick amorphous selenium direct detection system with CMOS readout for typical angiographic exposure parameters using the relative object detectability (ROD) metric. Methods: The ROD metric uses a simulated object function weighted at each spatial frequency by the detectors’ detective quantum efficiency (DQE), which is an intrinsic performance metric. For this study, the simulated objects were aluminum spheres of varying diameter (0.05–0.6mm). The weighted object function is then integrated over the full range of detectable frequencies inherent to each detector, and a ratio is taken of the resulting value for two detectors. The DQE for the 25µm detector was obtained from a simulation of a proposed a‐Se detector using an exposure of 200µR for a 50keV x‐ray beam. This a‐Se detector was compared to two microangiographic fluoroscope (MAF) detectors [the MAF‐CCD with pixel size of 35µm and Nyquist frequency of 14.2 cycles/mm and the MAF‐CMOS with pixel size of 75µm and Nyquist frequency of 6.6 cycles/mm] and a standard flat‐panel detector (FPD with pixel size of 194µm and Nyquist frequency of 2.5cycles/mm). Results: ROD calculations indicated vastly superior performance by the a‐Se detector in imaging small aluminum spheres. For the 50µm diameter sphere, the ROD values for the a‐Se detector compared to the MAF‐CCD, the MAF‐CMOS, and the FPD were 7.3, 9.3 and 58, respectively. Detector performance in the low frequency regime was dictated by each detector's DQE(0) value. Conclusion: The a‐Se with CMOS readout is unique and appears to have distinctive advantages of incomparable high resolution, low noise, no readout lag, and expandable design. The a‐Se direct detection system will be a powerful imaging tool in angiography, with potential break‐through applications in diagnosis and treatment of neuro‐vascular disease. Supported by NIH Grant: 2R01EB002873 and an equipment grant from Toshiba Medical Systems Corporation
Purpose: ROI fluoroscopy involves the use of an x‐ray beam attenuator with higher attenuation in the periphery than the center thus allowing for dose reduction to the patient. This study presents the design considerations for placing an x‐ray ROI attenuator made of copper inside the collimator assembly of an angiographic c‐arm. Methods: The two important considerations for the design of the attenuator are the size of the ROI and the attenuation (and hence thickness of the material) needed outside the ROI. An attenuation of 80% outside the ROI, and none inside the ROI was assumed. To calculate the thickness, exposures were measured for different thicknesses of copper at various kVp's and different inherent filtration of the system. Attenuation percentage was calculated from these readings and the thickness of copper was determined. The field‐of‐view (FOV) requirement depends on the type of procedure: smaller for a neurovascular intervention and larger for a cardiac procedure. An average FOV of 33% of 21cm × 21cm at 100cm SID with a circular ROI was assumed to calculate the diameter of the ROI in the attenuator. Results: For kVp's ranging from 80 to 90, with an added filtration of 0.2mm copper, to get an average attenuation of 80%, 0.7mm of copper was needed for the thickness of the attenuator. The attenuator was placed 13cm from the focal spot and the diameter of the ROI at this distance was calculated to be 10mm. Conclusion: The ROI attenuator can be mounted inside the beam limiting mechanism of the c‐arm. This allows for the flexibility in the usage of this technique during fluoroscopic interventions, thus achieving patient‐dose reduction. Since the attenuation for copper varies with varying kVp, different masks for different kVp's are to be used for brightness equalization.
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