Quantitative Digital Subtraction Angiography Measurement of Arterial Velocity at Low Radiation Dose Rates.

PURPOSE: Quantitative digital subtraction angiography (qDSA) has been proposed to quantify blood velocity for monitoring treatment progress during blood flow altering interventions. The method requires high frame rate imaging [~ 30 frame per second (fps)] to capture temporal dynamics. This work investigates performance of qDSA in low radiation dose acquisitions to facilitate clinical translation. MATERIALS AND METHODS: Velocity quantification accuracy was evaluated at five radiation dose rates in vitro and in vivo. Angiographic technique ranged from 30 fps digital subtraction angiography ( 29.3 ± 1.7 mGy / s at the interventional reference point) down to a 30 fps protocol at 23% higher radiation dose per frame than fluoroscopy ( 1.1 ± 0.2 mGy / s ). The in vitro setup consisted of a 3D-printed model of a swine hepatic arterial tree connected to a pulsatile displacement pump. Five different flow rates (3.5-8.8 mL/s) were investigated in vitro. Angiography-based fluid velocity measurements were compared across dose rates using ANOVA and Bland-Altman analysis. The experiment was then repeated in a swine study (n = 4). RESULTS: Radiation dose rate reductions for the lowest dose protocol were 99% and 96% for the phantom and swine study, respectively. No significant difference was found between angiography-based velocity measurements at different dose rates in vitro or in vivo. Bland-Altman analysis found little bias for all lower-dose protocols (range: [- 0.1, 0.1] cm/s), with the widest limits of agreement ([- 3.3, 3.5] cm/s) occurring at the lowest dose protocol. CONCLUSIONS: This study demonstrates the feasibility of quantitative blood velocity measurements from angiographic images acquired at reduced radiation dose rates.

An X-Ray C-Arm Guided Automatic Targeting System for Histotripsy.

OBJECTIVE: Histotripsy is an emerging noninvasive, nonionizing and nonthermal focal cancer therapy that is highly precise and can create a treatment zone of virtually any size and shape. Current histotripsy systems rely on ultrasound imaging to target lesions. However, deep or isoechoic targets obstructed by bowel gas or bone can often not be treated safely using ultrasound imaging alone. This work presents an alternative x-ray C-arm based targeting approach and a fully automated robotic targeting system. METHODS: The approach uses conventional cone beam CT (CBCT) images to localize the target lesion and 2D fluoroscopy to determine the 3D position and orientation of the histotripsy transducer relative to the C-arm. The proposed pose estimation uses a digital model and deep learning-based feature segmentation to estimate the transducer focal point relative to the CBCT coordinate system. Additionally, the integrated robotic arm was calibrated to the C-arm by estimating the transducer pose for four preprogrammed transducer orientations and positions. The calibrated system can then automatically position the transducer such that the focal point aligns with any target selected in a CBCT image. RESULTS: The accuracy of the proposed targeting approach was evaluated in phantom studies, where the selected target location was compared to the center of the spherical ablation zones in post-treatment CBCTs. The mean and standard deviation of the Euclidean distance was 1.4 ±0.5 mm. The mean absolute error of the predicted treatment radius was 0.5 ±0.5 mm. CONCLUSION: CBCT-based histotripsy targeting enables accurate and fully automated treatment without ultrasound guidance. SIGNIFICANCE: The proposed approach could considerably decrease operator dependency and enable treatment of tumors not visible under ultrasound.

Three-dimensional catheter navigation of airways using continuous-sweep limited angle fluoroscopy on a C-arm.

Purpose: To develop an imaging-based 3D catheter navigation system for transbronchial procedures including biopsy and tumor ablation using a single-plane C-arm x-ray system. The proposed system provides time-resolved catheter shape and position as well as motion compensated 3D airway roadmaps. Approach: A continuous-sweep limited angle (CLA) imaging mode where the C-arm continuously rotates back and forth within a limited angular range while acquiring x-ray images was used for device tracking. The catheter reconstruction was performed using a sliding window of the most recent x-ray images, which captures information on device shape and position versus time. The catheter was reconstructed using a model-based approach and was displayed together with the 3D airway roadmap extracted from a pre-navigational cone-beam CT (CBCT). The roadmap was updated in regular intervals using deformable registration to tomosynthesis reconstructions based on the CLA images. The approach was evaluated in a porcine study (three animals) and compared to a gold standard CBCT reconstruction of the device. Results: The average 3D root mean squared distance between CLA and CBCT reconstruction of the catheter centerline was 1 ± 0.5 mm for a stationary catheter and 2.9 ± 1.1 mm for a catheter moving at ∼ 1 cm / s . The average tip localization error was 1.3 ± 0.7 mm and 2.7 ± 1.8 mm , respectively. Conclusions: The results indicate catheter navigation based on the proposed single plane C-arm imaging technique is feasible with reconstruction errors similar to the diameter of a typical ablation catheter.

Technical and clinical study of x-ray-based surface echo probe tracking using an attached fiducial apparatus.

PURPOSE: Several types of structural heart intervention (SHI) use information from multiple imaging modalities to complete an interventional task. For example, in transcatheter aortic valve replacement (TAVR), placement and deployment of a bioprosthetic aortic valve in the aorta is primarily guided by x-ray fluoroscopy (XRF), and echocardiography provides visualization of cardiac anatomy and blood flow. However, simultaneous interpretation of independent x-ray and echo displays remains a challenge for the interventionalist. The purpose of this work was to develop a novel echo/x-ray co-registration solution in which volumetric transthoracic echo (TTE) is transformed to the x-ray coordinate system by tracking the three-dimensional (3D) pose of a probe fiducial attachment from its appearance in two-dimensional (2D) x-ray images. METHODS: A fiducial attachment for a commercial TTE probe consisting of rings of high-contrast ball bearings was designed and fabricated. The 3D pose (position and orientation) of the fiducial attachment is estimated from a 2D x-ray image using an algorithm in which a virtual point cloud model of the attachment is iteratively rotated, translated, and forward-projected onto the image until the average sum-of-squares of grayscale values at the projected points is minimized. Fiducial registration error (FRE) and target registration error (TRE) of this approach were evaluated in phantom studies using TAVR-relevant gantry orientations and four standard acoustic windows for the TTE probe. A patient study was conducted to assess the clinical suitability of the fiducial attachment prototype during TTE imaging of patients undergoing SHI. TTE image quality for the task of guiding a transcatheter procedure was evaluated in a reviewer study. RESULTS: The 3D FRE ranged from 0.32 ± 0.03 mm (mean ± SD) to 1.31 ± 0.05 mm, depending on C-arm orientation and probe acoustic window. The 3D TRE ranged from 1.06 ± 0.03 mm to 2.42 ± 0.06 mm. Fiducial pose estimation was stable when >75% of the fiducial markers were visible in the x-ray image. A panel of reviewers graded the presentation of heart valves in TTE images from 48 SHI patients. While valve presentation did not differ significantly between acoustic windows (P > 0.05), the mitral valve did achieve a significantly higher image quality compared to the aortic and tricuspid valves (P < 0.001). Overall, reviewers perceived sufficient image quality in 76.5% of images of the mitral valve, 54.9% of images of the aortic valve, and 48.6% of images of the tricuspid valve. CONCLUSIONS: Fiducial-based tracking of a commercial TTE probe is compatible with clinical SHI workflows and yields 3D target registration error of less than 2.5 mm for a variety of x-ray gantry geometries and echo probe acoustic windows. Although TTE image quality with respect to target valve anatomy was sufficient for the majority of cases examined, prescreening of patients for sufficient TTE quality would be helpful.

A Quantitative Digital Subtraction Angiography Technique for Characterizing Reduction in Hepatic Arterial Blood Flow During Transarterial Embolization.

OBJECTIVE: There is no standardized and objective method for determining the optimal treatment endpoint (sub-stasis) during transarterial embolization. The objective of this study was to demonstrate the feasibility of using a quantitative digital subtraction angiography (qDSA) technique to characterize intra-procedural changes in hepatic arterial blood flow velocity in response to transarterial embolization in an in vivo porcine model. MATERIALS AND METHODS: Eight domestic swine underwent bland transarterial embolizations to partial- and sub-stasis angiographic endpoints with intraprocedural DSA acquisitions. Embolized lobes were assessed on histopathology for ischemic damage and tissue embolic particle density. Analysis of target vessels used qDSA and a commercially available color-coded DSA (ccDSA) tool to calculate blood flow velocities and time-to-peak, respectively. RESULTS: Blood flow velocities calculated using qDSA showed a statistically significant difference (p < 0.01) between partial- and sub-stasis endpoints, whereas time-to-peak calculated using ccDSA did not show a significant difference. During the course of embolizations, the average correlation with volume of particles delivered was larger for qDSA (- 0.86) than ccDSA (0.36). There was a statistically smaller mean squared error (p < 0.01) and larger coefficient of determination (p < 0.01) for qDSA compared to ccDSA. On pathology, the degree of embolization as calculated by qDSA had a moderate, positive correlation (p < 0.01) with the tissue embolic particle density of ischemic regions within the embolized lobe. CONCLUSIONS: qDSA was able to quantitatively discriminate angiographic embolization endpoints and, compared to a commercially available ccDSA method, improve intra-procedural characterization of blood flow changes. Additionally, the qDSA endpoints correlated with tissue-level changes.

Radiation Dose Reduction in CT Myocardial Perfusion Imaging Using SMART-RECON.

In this paper, a newly developed statistical model-based image reconstruction [referred to as Simultaneous Multiple Artifacts Reduction in Tomographic RECONstruction (SMART-RECON)] is applied to low dose computer tomography (CT) myocardial perfusion imaging (CT-MPI). This method uses the nuclear norm of the spatial-temporal image matrix of the CT-MPI images as a regularizer, rather than a conventional spatial regularizer that incorporates image smoothness, edge preservation, or spatial sparsity into the reconstruction. In addition to providing the needed noise reduction for low-dose CT-MPI, SMART-RECON provides images with spatial resolution and noise power spectrum (NPS) properties, which are independent of contrast and dose levels. Both numerical simulations and in vivo animal studies were performed to validate the proposed method. In these studies, it was found that: 1) quantitative accuracy of perfusion maps in CT-MPI was well maintained for radiation dose level as low as 10 mAs per image frame, compared with the reference standard of 200 mAs for conventional filtered backprojection; 2) flow-occluded myocardium in the porcine heart was well delineated by SMART-RECON at 10 mAs per frame when compared with model-based image reconstruction using spatial total variation (TV) as the regularizer (referred to as TV-SIR) or spatial-temporal TV (ST-TV-SIR); the CT-MPI results were confirmed with positron-emission tomography imaging; 3) image sharpness in SMART-RECON images was nearly independent of image contrast level and radiation dose level, in stark contrast to TV-SIR and ST-TV-SIR, which displayed a strong dependence on both image contrast and radiation dose levels; and 4) the structure of the dose-normalized NPS for the SMART-RECON method did not depend on dose, while the TV-SIR and ST-TV-SIR NPS structure was dose-dependent.

Feasibility of low-dose single-view 3D fiducial tracking concurrent with external beam delivery.

PURPOSE: In external-beam radiation therapy, existing on-board x-ray imaging chains orthogonal to the delivery beam cannot recover 3D target trajectories from a single view in real-time. This limits their utility for real-time motion management concurrent with beam delivery. To address this limitation, the authors propose a novel concept for on-board imaging based on the inverse-geometry Scanning-Beam Digital X-ray (SBDX) system and evaluate its feasibility for single-view 3D intradelivery fiducial tracking. METHODS: A chest phantom comprising a posterior wall, a central lung volume, and an anterior wall was constructed. Two fiducials were placed along the mediastinal ridge between the lung cavities: a 1.5 mm diameter steel sphere superiorly and a gold cylinder (2.6 mm length × 0.9 mm diameter) inferiorly. The phantom was placed on a linear motion stage that moved sinusoidally. Fiducial motion was along the source-detector (z) axis of the SBDX system with ±10 mm amplitude and a programmed period of either 3.5 s or 5 s. The SBDX system was operated at 15 frames per second, 100 kVp, providing good apparent conspicuity of the fiducials. With the stage moving, detector data were acquired and subsequently reconstructed into 15 planes with a 12 mm plane-to-plane spacing using digital tomosynthesis. A tracking algorithm was applied to the image planes for each temporal frame to determine the position of each fiducial in (x,y,z)-space versus time. A 3D time-sinusoidal motion model was fit to the measured 3D coordinates and root mean square (RMS) deviations about the fitted trajectory were calculated. RESULTS: Tracked motion was sinusoidal and primarily along the source-detector (z) axis. The RMS deviation of the tracked z-coordinate ranged from 0.53 to 0.71 mm. The motion amplitude derived from the model fit agreed with the programmed amplitude to within 0.28 mm for the steel sphere and within -0.77 mm for the gold seed. The model fit periods agreed with the programmed periods to within 7%. CONCLUSIONS: Three dimensional fiducial tracking with approximately 1 mm or better accuracy and precision appears to be feasible with SBDX, supporting its use to guide radiotherapy.