Michael T. Tomkowiak

Targeted transendocardial therapeutic delivery guided by MRI-x-ray image fusion.

Objectives: To validate a multi‐modality image fusion approach to guide catheter‐based, targeted transendocardial therapeutic delivery in a swine myocardial infarction (MI) model. Background: Biologic agents such as stem cells may curb post MI adverse ventricular remodeling if delivered by a transendocardial catheter directly into the infarct border. 3D visualization of the infarct and other cardiac surfaces is required to perform this task. We propose registering and overlaying magnetic resonance imaging (MRI) roadmaps onto live x‐ray fluoroscopy (XRF) to guide targeted transendocardial delivery. Methods: Custom software was used to register and overlay MRI models of the endocardium and infarct on live XRF by aligning common endocardial border features. In a swine MI model, transendocardial injections of co‐localizing imaging labels were performed, targeting a 20 mm perimeter around the infarct. Directed targeting error (DTE) was defined as the difference between the predicted injection site‐to‐infarct distance calculated by the image fusion system, to the actual distance determined by postprocedure in vivo MRI. The mobile image fusion system was designed to be vendor‐independent for imaging systems and transendocardial catheters. Results: Transendocardial injections were performed in all animals without complications. Mean DTE was 0.9 ± 5.0 mm (n = 8 swine). Time to register the images and establish a high quality roadmap was less than 12 min in all animals. Custom imaging tools to display injection sites and distribution were useful adjuncts during targeted injection procedures. Conclusions: Multi‐modality image fusion is a feasible and accurate platform technology to guide transendocardial injections precisely to the discrete infarct border.

Three-dimensional tracking of cardiac catheters using an inverse geometry x-ray fluoroscopy system

PURPOSE Scanning beam digital x-ray (SBDX) is an inverse geometry fluoroscopic system with high dose efficiency and the ability to perform continuous real-time tomosynthesis at multiple planes. This study describes a tomosynthesis-based method for 3D tracking of high-contrast objects and present the first experimental investigation of cardiac catheter tracking using a prototype SBDX system. METHODS The 3D tracking algorithm utilizes the stack of regularly spaced tomosynthetic planes that are generated by SBDX after each frame period (15 frames/s). Gradient-filtered versions of the image planes are generated, the filtered images are segmented into object regions, and then a 3D coordinate is calculated for each object region. Two phantom studies of tracking performance were conducted. In the first study, an ablation catheter in a chest phantom was imaged as it was pulled along a 3D trajectory defined by a catheter sheath (10, 25, and 50 mm/s pullback speeds). SBDX tip tracking coordinates were compared to the 3D trajectory of the sheath as determined from a CT scan of the phantom after the registration of the SBDX and CT coordinate systems. In the second study, frame-to-frame tracking precision was measured for six different catheter configurations as a function of image noise level (662-7625 photons/mm2 mean detected x-ray fluence at isocenter). RESULTS During catheter pullbacks, the 3D distance between the tracked catheter tip and the sheath centerline was 1.0 +/- 0.8 mm (mean +/- one standard deviation). The electrode to centerline distances were comparable to the diameter of the catheter tip (2.3 mm), the confining sheath (4 mm outside diameter), and the estimated SBDX-to-CT registration error (+/- 0.7 mm). The tip position was localized for all 332 image frames analyzed and 83% of tracked positions were inside the 3D sheath volume derived from CT. The pullback speeds derived from the catheter trajectories were within 5% of the programed pullback speeds. The tracking precision of ablation and diagnostic catheter tips ranged from +/- 0.2 mm at the highest image fluence to +/- 0.9 mm at the lowest fluence. Tracking precision depended on image fluence, the size of the tracked catheter electrode, and the contrast of the electrode. CONCLUSIONS High speed multiplanar tomosynthesis with an inverse geometry x-ray fluoroscopy system enables 3D tracking of multiple high-contrast objects at the rate of fluoroscopic imaging. The SBDX system is capable of tracking electrodes in standard cardiac catheters with approximately 1 mm accuracy and precision.

Multimodality image fusion to guide peripheral artery chronic total arterial occlusion recanalization in a swine carotid artery occlusion model: unblinding the interventionalist.

To demonstrate the feasibility of magnetic resonance imaging (MRI) to X‐ray fluoroscopy (XRF) image fusion to guide peripheral artery chronic total occlusion (CTO) recanalization.

Calibration-free device sizing using an inverse geometry x-ray system

PURPOSE Quantitative coronary angiography (QCA) can be used to support device size selection for cardiovascular interventions. The accuracy of QCA measurements using conventional x-ray fluoroscopy depends on proper calibration using a reference object and avoiding vessel foreshortening. The authors have developed a novel interventional device sizing method using the inverse geometry scanning-beam digital x-ray (SBDX) fluoroscopy system. The proposed method can measure the diameter and length of vessel segments without imaging a reference object and when vessels appear foreshortened. METHODS SBDX creates multiple tomosynthetic x-ray images corresponding to planes through the patient volume. The structures that lie in the plane are in focus and the features above and below the plane are blurred. Three-dimensional localization of the vessel edges was performed by examining the degree of blurring at each image plane. A 3D vessel centerline was created and used to determine vessel magnification and angulation relative to the image planes. Diameter measurements were performed using a model-based method and length measurements were calculated from the 3D centerline. Phantom validation was performed by measuring the diameter and length of vessel segments with nominal diameters ranging from 0.5 to 2.8 mm and nominal lengths of 42 mm. The phantoms were imaged at a range of positions between the source and the detector (+/- 16 cm relative to isocenter) and with a range of foreshortening angles (0 degrees-75 degrees). RESULTS Changes in vessel phantom position created magnifications ranging from 87% to 118% relative to isocenter magnification. Average diameter errors were less than 0.15 mm. Average length measurements were within 1% (0.3 mm) of the true length. No trends were observed between measurement accuracy and magnification. Changes in vessel phantom orientation resulted in decreased apparent length down to 28% of the original nonforeshortened length. Average diameter errors were less than 0.25 mm across all vessel angulations; errors were less than 0.1 mm for smaller diameter vessels and low to moderate vessel angles. Diameter errors increased with true diameter and vessel angle relative to the image plane. Average length measurement errors were also within 1% (0.3 mm) for each angulation. CONCLUSIONS Tomosynthetic imaging with SBDX can accurately measure dimensions of vessels in various magnifications and angulations without calibration. This method may be more accurate and convenient than conventional QCA techniques.

Calibration-free coronary artery measurements for interventional device sizing using inverse geometry x-ray fluoroscopy: in vivo validation

Abstract. Proper sizing of interventional devices to match coronary vessel dimensions improves procedural efficiency and therapeutic outcomes. We have developed a method that uses an inverse geometry x-ray fluoroscopy system [scanning beam digital x-ray (SBDX)] to automatically determine vessel dimensions from angiograms without the need for magnification calibration or optimal views. For each frame period (1/15th of a second), SBDX acquires a sequence of narrow beam projections and performs digital tomosynthesis at multiple plane positions. A three-dimensional model of the vessel is reconstructed by localizing the depth of the vessel edges from the tomosynthesis images, and the model is used to calculate the length and diameter in units of millimeters. The in vivo algorithm performance was evaluated in a healthy porcine model by comparing end-diastolic length and diameter measurements from SBDX to coronary computed tomography angiography (CCTA) and intravascular ultrasound (IVUS), respectively. The length error was −0.49±1.76  mm (SBDX – CCTA, mean±1 SD). The diameter error was 0.07±0.27  mm (SBDX − minimum IVUS diameter, mean±1 SD). The in vivo agreement between SBDX-based vessel sizing and gold standard techniques supports the feasibility of calibration-free coronary vessel sizing using inverse geometry x-ray fluoroscopy.

Detector, collimator and real-time reconstructor for a new scanning-beam digital x-ray (SBDX) prototype

Scanning-beam digital x-ray (SBDX) is an inverse geometry fluoroscopy system for low dose cardiac imaging. The use of a narrow scanned x-ray beam in SBDX reduces detected x-ray scatter and improves dose efficiency, however the tight beam collimation also limits the maximum achievable x-ray fluence. To increase the fluence available for imaging, we have constructed a new SBDX prototype with a wider x-ray beam, larger-area detector, and new real-time image reconstructor. Imaging is performed with a scanning source that generates 40,328 narrow overlapping projections from 71 x 71 focal spot positions for every 1/15 s scan period. A high speed 2-mm thick CdTe photon counting detector was constructed with 320x160 elements and 10.6 cm x 5.3 cm area (full readout every 1.28 s), providing an 86% increase in area over the previous SBDX prototype. A matching multihole collimator was fabricated from layers of tungsten, brass, and lead, and a multi-GPU reconstructor was assembled to reconstruct the stream of captured detector images into full field-of-view images in real time. Thirty-two tomosynthetic planes spaced by 5 mm plus a multiplane composite image are produced for each scan frame. Noise equivalent quanta on the new SBDX prototype measured 63%-71% higher than the previous prototype. X-ray scatter fraction was 3.9-7.8% when imaging 23.3-32.6 cm acrylic phantoms, versus 2.3- 4.2% with the previous prototype. Coronary angiographic imaging at 15 frame/s was successfully performed on the new SBDX prototype, with live display of either a multiplane composite or single plane image.

Depth-resolved registration of transesophageal echo to x-ray fluoroscopy using an inverse geometry fluoroscopy system

PURPOSE Image registration between standard x-ray fluoroscopy and transesophageal echocardiography (TEE) has recently been proposed. Scanning-beam digital x-ray (SBDX) is an inverse geometry fluoroscopy system designed for cardiac procedures. This study presents a method for 3D registration of SBDX and TEE images based on the tomosynthesis and 3D tracking capabilities of SBDX. METHODS The registration algorithm utilizes the stack of tomosynthetic planes produced by the SBDX system to estimate the physical 3D coordinates of salient key-points on the TEE probe. The key-points are used to arrive at an initial estimate of the probe pose, which is then refined using a 2D/3D registration method adapted for inverse geometry fluoroscopy. A phantom study was conducted to evaluate probe pose estimation accuracy relative to the ground truth, as defined by a set of coregistered fiducial markers. This experiment was conducted with varying probe poses and levels of signal difference-to-noise ratio (SDNR). Additional phantom and in vivo studies were performed to evaluate the correspondence of catheter tip positions in TEE and x-ray images following registration of the two modalities. RESULTS Target registration error (TRE) was used to characterize both pose estimation and registration accuracy. In the study of pose estimation accuracy, successful pose estimates (3D TRE < 5.0 mm) were obtained in 97% of cases when the SDNR was 5.9 or higher in seven out of eight poses. Under these conditions, 3D TRE was 2.32 ± 1.88 mm, and 2D (projection) TRE was 1.61 ± 1.36 mm. Probe localization error along the source-detector axis was 0.87 ± 1.31 mm. For the in vivo experiments, mean 3D TRE ranged from 2.6 to 4.6 mm and mean 2D TRE ranged from 1.1 to 1.6 mm. Anatomy extracted from the echo images appeared well aligned when projected onto the SBDX images. CONCLUSIONS Full 6 DOF image registration between SBDX and TEE is feasible and accurate to within 5 mm. Future studies will focus on real-time implementation and application-specific analysis.

Monoplane stereoscopic imaging method for inverse geometry x-ray fluoroscopy

Scanning Beam Digital X-ray (SBDX) is a low-dose inverse geometry fluoroscopic system for cardiac interventional procedures. The system performs x-ray tomosynthesis at multiple planes in each frame period and combines the tomosynthetic images into a projection-like composite image for fluoroscopic display. We present a novel method of stereoscopic imaging using SBDX, in which two slightly offset projection-like images are reconstructed from the same scan data by utilizing raw data from two different detector regions. To confirm the accuracy of the 3D information contained in the stereoscopic projections, a phantom of known geometry containing high contrast steel spheres was imaged, and the spheres were localized in 3D using a previously described stereoscopic localization method. After registering the localized spheres to the phantom geometry, the 3D residual RMS errors were between 0.81 and 1.93 mm, depending on the stereoscopic geometry. To demonstrate visualization capabilities, a cardiac RF ablation catheter was imaged with the tip oriented towards the detector. When viewed as a stereoscopic red/cyan anaglyph, the true orientation (towards vs. away) could be resolved, whereas the device orientation was ambiguous in conventional 2D projection images. This stereoscopic imaging method could be implemented in real time to provide live 3D visualization and device guidance for cardiovascular interventions using a single gantry and data acquired through normal, low-dose SBDX imaging.

Feasibility of CT-based 3D anatomic mapping with a scanning-beam digital x-ray (SBDX) system.

This study investigates the feasibility of obtaining CT-derived 3D surfaces from data provided by the scanning-beam digital x-ray (SBDX) system. Simulated SBDX short-scan acquisitions of a Shepp-Logan and a thorax phantom containing a high contrast spherical volume were generated. 3D reconstructions were performed using a penalized weighted least squares method with total variation regularization (PWLS-TV), as well as a more efficient variant employing gridding of projection data to parallel rays (gPWLS-TV). Voxel noise, edge blurring, and surface accuracy were compared to gridded filtered back projection (gFBP). PWLS reconstruction of a noise-free reduced-size Shepp-Logan phantom had 1.4% rRMSE. In noisy gPWLS-TV reconstructions of a reduced-size thorax phantom, 99% of points on the segmented sphere perimeter were within 0.33, 0.47, and 0.70 mm of the ground truth, respectively, for fluences comparable to imaging through 18.0, 27.2, and 34.6 cm acrylic. Surface accuracies of gFBP and gPWLS-TV were similar at high fluences, while gPWLS-TV offered improvement at the lowest fluence. The gPWLS-TV voxel noise was reduced by 60% relative to gFBP, on average. High-contrast linespread functions measured 1.25 mm and 0.96 mm (FWHM) for gPWLS-TV and gFBP. In a simulation of gated and truncated projection data from a full-sized thorax, gPWLS-TV reconstruction yielded segmented surface points which were within 1.41 mm of ground truth. Results support the feasibility of 3D surface segmentation with SBDX. Further investigation of artifacts caused by data truncation and patient motion is warranted.

Monte Carlo simulation of inverse geometry x-ray fluoroscopy using a modified MC-GPU framework.

Scanning-Beam Digital X-ray (SBDX) is a technology for low-dose fluoroscopy that employs inverse geometry x-ray beam scanning. To assist with rapid modeling of inverse geometry x-ray systems, we have developed a Monte Carlo (MC) simulation tool based on the MC-GPU framework. MC-GPU version 1.3 was modified to implement a 2D array of focal spot positions on a plane, with individually adjustable x-ray outputs, each producing a narrow x-ray beam directed toward a stationary photon-counting detector array. Geometric accuracy and blurring behavior in tomosynthesis reconstructions were evaluated from simulated images of a 3D arrangement of spheres. The artifact spread function from simulation agreed with experiment to within 1.6% (rRMSD). Detected x-ray scatter fraction was simulated for two SBDX detector geometries and compared to experiments. For the current SBDX prototype (10.6 cm wide by 5.3 cm tall detector), x-ray scatter fraction measured 2.8-6.4% (18.6-31.5 cm acrylic, 100 kV), versus 2.2-5.0% in MC simulation. Experimental trends in scatter versus detector size and phantom thickness were observed in simulation. For dose evaluation, an anthropomorphic phantom was imaged using regular and regional adaptive exposure (RAE) scanning. The reduction in kerma-area-product resulting from RAE scanning was 45% in radiochromic film measurements, versus 46% in simulation. The integral kerma calculated from TLD measurement points within the phantom was 57% lower when using RAE, versus 61% lower in simulation. This MC tool may be used to estimate tomographic blur, detected scatter, and dose distributions when developing inverse geometry x-ray systems.