Vascular pathologies such as stenosis cause changes in blood flow patterns and rates in patient vasculature. These changes are difficult to observe using conventional x-ray imaging techniques that typically operate at no more than 30 fps. However, 1000 fps High-Speed Angiography (HSA) provides sufficient temporal resolution to observe flow patterns in patient vasculature. The Actaeon detector from XCounter provides 1000 fps HSA with high spatial resolution (100 μm pixel pitch) enabling observation of detailed blood flow in 3D printed phantoms. To observe the effects of stenosis on flow, three carotid artery phantoms beginning 7.6 cm proximal to the bifurcation of the common carotid and extending 6.9 cm into the internal and external carotids with different degrees of stenosis (0%, 33%, and 66% stenosis) in the internal carotid were printed. Iodine contrast was injected into the phantom proximal to the bifurcation and images were recorded at 1000 fps. Time-density curves were generated for ROIs distal and proximal to the location of the stenosis for both branches of the carotid artery. The curves for the control phantom (no stenosis) showed little difference between the branches of the carotid while the stenotic phantoms showed an increased velocity in the flow of contrast and a lower maximum signal intensity for the stenosed internal carotid. 1000 fps HSA could be used during treatment of vascular pathologies to observe changes in flow patterns following an endovascular treatment.
While angiography may be considered the gold standard for evaluating diseases of the human vasculature, vascular flow details are unavailable due to the low temporal resolution of flat panel detectors (FPDs) which operate at a maximum of 15 – 30 fps. Higher frame rates are necessary to extract any meaningful flow detail, which may act as additional information that can be used to characterize flow-dependent disease states. These higher rates have become available with recent advances in photon-counting detector (PCD) technology. The XCounter Actaeon was used to perform high frame rate imaging at 1000 fps. The Actaeon also provides superior spatial resolution due to its 100 um pixel size and electronic charge sharing correction, making it a good candidate for small ROI imaging. With this detector, “High Speed Angiography” (HSA) was performed on a variety of 3D printed patient-specific vasculature and interventional devices, using a simulated flow loop and iodinated contrast media. The images, which illustrate pathology-dependent flow detail, were recorded in a sequence of 1 ms frames. In addition, the energy discrimination capabilities of the Actaeon were used such that with a lower energy threshold, instrumentation noise was virtually negligible. The per frame noise quality and overall patient dose were acceptable as compared to standard angiography dose rates using FPDs. The previously unseen flow detail may give new insight into the diagnosis, progression, and treatment of neuro and cardiovascular pathologies.
KEYWORDS: Attenuators, Image processing, Convolutional neural networks, Image quality, Head, Digital filtering, X-rays, Signal to noise ratio, Fluoroscopy, 3D acquisition
Patient x-ray dose during fluoroscopically-guided neuro interventions can be reduced by using a differential Region-of-Interest (ROI) x-ray attenuator. The dose in the periphery region outside the region-of-interest (ROI) treatment area is reduced while maintaining regular dose within the ROI. In this work we present a convolutional neural network to aid in restoration of image quality in dose-reduced regions. A 0.7 mm Cu attenuator with a 10 mm circular hole in the middle was used to reduce entrance dose in the periphery. A 29 layer deep CNN was developed to derive the ROI attenuator mask image from the dose-reduced images. To train the CNN, simulated ROI dose-reduced images of various backgrounds such as anthropomorphic head and chest phantoms were generated using acquired mask images of the ROI attenuator at different positions and radiological magnifications. The image quality in the dose-reduced region of the images was restored by first dividing the CNN derived mask from the dose-reduced image and then noise in the periphery region was reduced by using a combination of Gaussian and recursive temporal filtering. A total dose-area-product reduction of 70% per frame was achieved. After image processing using the CNN derived image mask of the ROI attenuator, the SNR in the dose reduced periphery regions was improved by a factor of 3. The CNN is capable of deriving the mask without any prior knowledge of ROI attenuator position or radiological magnification. Using the CNN generated mask, the image quality in the dose reduced images was restored with minimal or no boundary artifacts.
Details of blood flow patterns and rates can provide useful information to physicians when deciding whether to treat diseased vessels and in assessing the effectiveness of treatments. These blood flow details are difficult to see using ‘realtime’ imaging techniques of 30 fps. 1000 fps High-Speed Angiography (HSA) provides the temporal resolution needed to record details of flow within patient vasculature. The Actaeon detector from XCounter is capable of x-ray imaging at 1000 fps providing sufficient temporal and spatial resolution (100 μm pixel pitch) for the quantification of flow details. A new method for experimentally obtaining flow details in patient-specific geometries demonstrates microspheres tracking vascular flow similar to methods used in optical laser-based particle image velocimetry (PIV). The microspheres are prepared by soaking them in iodinated contrast medium to provide radio-opacity and injected into 3D-printed, patient-specific vascular phantoms during x-ray exposure. Images were acquired at 1000 fps for 2.4 seconds using the Actaeon’s High-Sensitivity mode. Changes in particle positions were tracked through consecutive frames and the position data was used to calculate velocities. The velocities were then mapped to the initial position and binned to reduce apparent variation in individual particle paths. This method provides quantitative data regarding the flow details within a vessel and also qualitative information regarding flow at different points in time during the acquisition. These methods could enable new measurements of flow properties in patient-specific vasculature.
Temporal and spatial details of vascular flow patterns and rates in neuro and cardio vascular pathology are difficult to evaluate even with so-called “real-time” 15 or 30 fps x-ray imaging using flat panel detectors (FPDs). Higher frame rates of 1000 fps with high spatial resolution of 100 um pixels can demonstrate vascular flow details previously unseen by any other means. A new ultra-high temporal and spatial resolution detector with the above capabilities, the XCounter’s Actaeon, is a direct conversion photon counting detector (PCD) with built-in electronic charge sharing correction and dual energy threshold settings. This PCD was used to image a 3D-printed realistic aneurysm flow phantom injected with both iodine and gas bubble contrast media. The flow patterns, including details of the vortex flow and velocities of individual gas bubbles, were recorded in a sequence of 1 ms images and compared with frames from standard 30 fps angiographic imaging where flow patterns were blurred and individual gas bubble movement could not be observed. Because of the low-energy threshold capability of the PCD, instrumentation noise was virtually negligible enabling quantum limited performance such that, at standard angiography dose rates for FPDs, the per frame noise quality for the 1 ms frames of the PCD sequences was acceptable. This resulted in a combination of temporal, spatial, and contrast resolution unseen previously.
Photon counting detectors (PCDs) with their low noise, high spatial and contrast resolution, and dual energy imaging capabilities are shown to be prospective candidates for Cone-Beam CT (CBCT) vascular imaging. The ultra-high frame rate capability of such detectors can enable CBCT at scan speeds far greater than that of current CBCT scanners which can only go up to about 0.14 rev/s (50 degrees/sec) compared to current multislice CT scanners that can achieve 3-4 rev/s. XCounter’s Actaeon PCD with frame rates up to 1000 fps can achieve CBCT scans at multi-slice CT scanner rates. The CdTe Actaeon PCD with pixel pitch of 100 μm and dual energy direct acquisition capability has built-in electronic anti-charge sharing correction. CBCT of a moving wire was demonstrated with the Actaeon PCD at these higher speeds and shown to be able to eliminate the effect of acquisition system image motion blur. Also a patient-specific 3D printed vascular phantom with a stent deployed was imaged at high frame rates using the Actaeon for CBCT acquisition with reconstruction using an FDK algorithm and the maximum intensity projection images clearly showed individual strut detail. This feasibility study will lead to exploration of new possibilities of high frame-rate imaging with PCDs and their potential applications.
Neuro-endovascular image-guided interventions (EIGIs) are aided by use of detectors with improved spatial resolution. A new detector is capable of switching between standard resolution, flat-panel detector (FPD) zoom and high-definition (Hi-Def) zoom modes, with 194 and 76 μm pixels respectively. The relative performance of the two zoom modes to image specific, high-resolution objects was quantitatively investigated. Detector DQEs were measured for both zoom modes and used to determine the Relative Object Detectability (ROD), which compares two imaging detectors’ relative abilities to image a simulated object by integrating the DQE of one detector with the square of the Fourier transform of the simulated object function and dividing the result by a similar integral for a second reference detector. Initial evaluations used a pre-whitened matched filter (PWMF) ideal-observer model. Comparisons were also made using the generalized-ROD (G-ROD) which uses the generalized-DQE (GDQE) that includes the effects of clinically relevant parameters such as magnification, focal-spot size, and scatter and the generalized-measured-ROD (GM-ROD) which uses the square of the Fourier transform of the actual images of the object acquired with the detectors. Each of the metrics demonstrated improved performance of the Hi-Def zoom mode over the standard FPD mode in imaging a wide array of objects such as stents, wires, and spheres. Of particular note is the greater performance of the Hi-Def when considering the high spatial frequencies necessary for visualizing fine image details of a pipeline stent. These initial investigations demonstrate the great potential of the Hi-Def zoom mode during neuro-interventions.
The prospect of improved low noise, high speed, and dual-energy imaging that may be associated with the use of photon-counting imaging detectors (PCD) has motivated this evaluation of a newly upgraded version of a prototype PCD. The XCounter Actaeon was evaluated in its four acquisition modes each based upon varying signal processing firmware including a mode with charge sharing correction that enables neighboring pixels that share the energy from one incident x-ray photon detection to be counted only once at the proper summed energy in the pixel with the largest charge deposition. Since this PCD is a CdTe-based direct detector with 100 μm pixels, such charge sharing for typical medical x-ray energy photons may occur frequently and must be corrected to achieve more accurate counts. This charge sharing correction is achieved with an Anti-Coincidence Circuit (ACC) which prevents double pixel counting from one event as well as prevents counting from either event if they are below a preset threshold. Various physical parameters of the PCD were evaluated including linearity, sensitivity, pulse pile-up effects, dark noise, spatial resolution, noise power spectrum, and detective quantum efficiency.
Real-time visualization of fine details ranging to 100 um or less in neuro-vascular imaging guided interventions is important. A separate high-resolution detector mounted on a standard flat panel detector (FPD) was previously reported. This device had to be rotated mechanically into position over the FPD for high resolution imaging. Now, the new detector reported here has a high definition (Hi-Def) zoom capability along with the FPD built into one unified housing. The new detector enables rapid switching, by the operator between Hi-Def and FPD modes. Standard physical metrics comparing the new Hi-Def modes with those of the FPD are reported, demonstrating improved imaging resolution and noise capability at patient doses similar to those used for the FPD. Semi-quantitative subjective studies involving qualitative clinician feedback on images of interventional devices such as a Pipeline Embolization Device (PED) acquired in both Hi-Def and FPD modes are presented. The PED is deployed in a patient specific 3D printed neuro-vascular phantom embedded inside realistic bone and with tissue attenuating material. Field-of-view (FOV), exposure and magnification were kept constant for FPD and Hi-Def modes. Static image comparisons of the same view of the PED within the phantom were rated by expert interventionalists who chose from the following ratings: Similar, Better, or Superior. Generally, the Hi-Def zoomed images were much preferred over the FPD, indicating the potential to improve endovascular procedures and hence outcomes using such a Hi-Def feature.
As neuro-endovascular image-guided interventions (EIGIs) make use of higher resolution detectors, gantry rotational motion-induced blur becomes more noticeable in acquired projections as well as reconstructed images by reducing the visibility of vascular and device features whose visualization could be critical in the treatment of vascular pathology. Motion-induced blur in projections views is a function of an object’s position in the field-of-view (FOV), gantry rotational speed, and frame capture or exposure time. In this work different frame rates were used to investigate the effects of blurring from individual projections on the reconstructed image. To test the effects of these parameters on reconstructed images, a regular pattern phantom of small objects was simulated and projection views were generated at various different frame rates for a given simulated rotational velocity. The reconstruction was made using a linear interpolation of filtered backprojections. Images reconstructed from lower frame rates showed significant blurring in the azimuthal direction, increasingly worse towards the periphery of the image. However, those reconstructed from higher frame rates showed significantly less blur throughout the entire FOV. While lower frame rates could be used with slower gantry speeds this would increase the risk of voluntary or involuntary patient motion contributing to blur over the entire FOV. A high frame rate used with high gantry speeds could reliable provide images without gantry-motion blur while reducing the risk of patient-motion blur. Frame rates exceeding 2000 fps available with photon counting detectors such as the X-counter Actaeon are available.
The spectroscopic capabilities of a newly upgraded version of a prototype imaging photon counting detector (PCD) was investigated. The XCounter Actaeon has four acquisition modes in which signal processing is varied including one mode having a charge sharing correction so that neighboring pixels that share a detected event will not be erroneously counted twice, hence it is designated the Anti-Coincidence Circuit On or ACC On mode. Since this CdTe-based direct conversion PCD has 100 μm pixels, such charge sharing may frequently occur for typical medical x-ray energies. Each pixel of this PCD has two scalers and two energy discriminators that enable counting without instrumentation noise of events above each threshold level; hence, a spectrum can be obtained by sequentially moving the thresholds of both discriminators. It became evident from the spectra for the various acquisition modes that only those obtained with the charge sharing correction enabled, compared favorably with theoretically predicted spectra. After verifying the energy calibration using the mono-energetic emissions from an Am-241 source, spectra at various kVps from a standard medical x-ray generator were obtained. The spectra generated by ACC On mode for 70 kVp and 110 kVp were the closest match to the theoretical spectra generated by SpekCal. For dual energy applications, ACC On mode with charge sharing correction circuitry would be the best choice among various acquisition modes. Also investigated was the dual energy imaging capability of the Actaeon PCD with ACC On mode to separate Aluminum and Iodine while imaging an artery stenosis phantom.
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