Douglas A. Stanton

Compressed-sensing motion compensation (CosMo): A joint prospective-retrospective respiratory navigator for coronary MRI

Prospective right hemidiaphragm navigator (NAV) is commonly used in free‐breathing coronary MRI. The NAV results in an increase in acquisition time to allow for resampling of the motion‐corrupted k‐space data. In this study, we are presenting a joint prospective–retrospective NAV motion compensation algorithm called compressed‐sensing motion compensation (CosMo). The inner k‐space region is acquired using a prospective NAV; for the outer k‐space, a NAV is only used to reject the motion‐corrupted data without reacquiring them. Subsequently, those unfilled k‐space lines are retrospectively estimated using compressed sensing reconstruction. We imaged right coronary artery in nine healthy adult subjects. An undersampling probability map and sidelobe‐to‐peak ratio were calculated to study the pattern of undersampling, generated by NAV. Right coronary artery images were then retrospectively reconstructed using compressed‐sensing motion compensation for gating windows between 3 and 10 mm and compared with the ones fully acquired within the gating windows. Qualitative imaging score and quantitative vessel sharpness were calculated for each reconstruction. The probability map and sidelobe‐to‐peak ratio show that the NAV generates a random undersampling k‐space pattern. There were no statistically significant differences between the vessel sharpness and subjective score of the two reconstructions. Compressed‐sensing motion compensation could be an alternative motion compensation technique for free‐breathing coronary MRI that can be used to reduce scan time. Magn Reson Med, 2011.

3D TEE Registration with X-Ray Fluoroscopy for Interventional Cardiac Applications

Live 3D trans-esophageal echocardiography (TEE) and X-ray fluoroscopy provide complementary imaging information for guiding minimally invasive cardiac interventions. X-ray fluoroscopy is most commonly used for these procedures due to its excellent device visualization. However, its challenges include the 2D projection nature of the images and poor soft tissue contrast, both of which are addressed by the use of live 3D TEE imaging. We propose to integrate 3D TEE imaging with X-ray fluoroscopy, providing the capability to co-visualize both the interventional devices and cardiac anatomy, by accurately registering the images using an electro-magnetic tracking system. Phantom trials validating the proposed registration scheme indicate an average accuracy of 2.04 mm with a standard deviation of 0.59 mm. In the future, this system may benefit the guidance and navigation of interventional cardiac procedures such as mitral valve repair or patent foramen ovale closure.

Quantification of AC electromagnetic tracking system accuracy in a CT scanner environment

The purpose of this study was to quantify the effects of a computed tomography (CT) scanner environment on the positional accuracy of an AC electromagnetic tracking system, the second generation NDI Aurora. A three-axis positioning robot was used to move an electromagnetically tracked needle above the CT table throughout a 30cm by 30cm axial plane sampled in 2.5cm steps. The corresponding position data was captured from the Aurora and was registered to the positioning system data using a rigid body transformation minimizing the least squares L2-norm. Data was sampled at varying distances from the CT gantry (three feet, two feet, and one foot) and with the CT table in a nominal position and lowered by 10cm. A coordinate system was defined with the x axis normal to the CT table and the origin at the center of the CT table, and the z axis spanning the table in the lateral direction with the origin at the center of the CT table. In this coordinate system, the positional relationships of each sampled point, the CT table, and the Aurora field generator are clearly defined. This allows error maps to be displayed in accurate spatial relationship to the CT scanner as well as to a representative patient anatomy. By quantifying the distortions in relation to the position of CT scanner components and the Aurora field generator, the optimal working field of view and recommended guidelines for operation can be determined such that targeting inside human anatomy can be done with reasonable expectations of desired performance.

Tracking and characterization of fragments in a beating heart using 3d ultrasound for interventional guidance

Fragments generated by explosions and similar incidents can become trapped in a patients heart chambers, potentially causing disruption of cardiac function. The conventional approach to removing such foreign bodies is through open heart surgery, which comes with high perioperative risk and long recovery times. We thus advocate a minimally invasive surgical approach through the use of 3D transesophageal echocardiography (TEE) and a flexible robotic end effector. In a phantom study, we use 3D TEE to track a foreign body in a beating heart, and propose a modified normalized cross-correlation method for improved accuracy and robustness of the tracking, with mean RMS errors of 2.3 mm. Motion analysis of the foreign body trajectory indicates very high speeds and accelerations, which render unfeasible a robotic retrieval method based on following the tracked trajectory. Instead, a probability map of the locus of the foreign body shows that the fragment tends to occupy only a small sub-volume of the ventricle, suggesting a retrieval strategy based on moving the robot end effector to the position with the highest spatial probability in order to maximize the possibility of capture.

Quantitative evaluation for accumulative calibration error and video-CT registration errors in electromagnetic-tracked endoscopy

PurposeElectromagnetic (EM)-guided endoscopy has demonstrated its value in minimally invasive interventions. Accuracy evaluation of the system is of paramount importance to clinical applications. Previously, a number of researchers have reported the results of calibrating the EM-guided endoscope; however, the accumulated errors of an integrated system, which ultimately reflect intra-operative performance, have not been characterized. To fill this vacancy, we propose a novel system to perform this evaluation and use a 3D metric to reflect the intra-operative procedural accuracy.MethodsThis paper first presents a portable design and a method for calibration of an electromagnetic (EM)-tracked endoscopy system. An evaluation scheme is then described that uses the calibration results and EM-CT registration to enable real-time data fusion between CT and endoscopic video images. We present quantitative evaluation results for estimating the accuracy of this system using eight internal fiducials as the targets on an anatomical phantom: the error is obtained by comparing the positions of these targets in the CT space, EM space and endoscopy image space. To obtain 3D error estimation, the 3D locations of the targets in the endoscopy image space are reconstructed from stereo views of the EM-tracked monocular endoscope. Thus, the accumulated errors are evaluated in a controlled environment, where the ground truth information is present and systematic performance (including the calibration error) can be assessed.ResultsWe obtain the mean in-plane error to be on the order of 2 pixels. To evaluate the data integration performance for virtual navigation, target video-CT registration error (TRE) is measured as the 3D Euclidean distance between the 3D-reconstructed targets of endoscopy video images and the targets identified in CT. The 3D error (TRE) encapsulates EM-CT registration error, EM-tracking error, fiducial localization error, and optical-EM calibration error.ConclusionWe present in this paper our calibration method and a virtual navigation evaluation system for quantifying the overall errors of the intra-operative data integration. We believe this phantom not only offers us good insights to understand the systematic errors encountered in all phases of an EM-tracked endoscopy procedure but also can provide quality control of laboratory experiments for endoscopic procedures before the experiments are transferred from the laboratory to human subjects.

Image registration based 3D TEE-EM calibration

Real-time 3D trans-esophageal echo (TEE) is being integrated into routine clinical practice for minimally invasive cardiac therapy. Electromagnetically (EM) tracking of the TEE probe inside the body may facilitate intraprocedural heart surgery and therapy on a beating heart. This requires an accurate calibration between the 3D TEE and the EM tracking system. The objective of calibration is to find the transformation that converts the coordinates of voxels in ultrasound volumes into the coordinate system of a position sensor attached to the TEE probe. An image registration based ultrasound calibration method is presented and validated using phantoms. The experiments indicate a post-calibration image reconstruction precision of 1.56 mm and accuracy of 0.85 mm (relative accuracy of 1.86%), and TRE of 2.37 mm. The potential impact of this work is that it can provide an easy to use calibration for clinical use.

Fast and accurate calibration of an X-ray imager to an electromagnetic tracking system for interventional cardiac procedures

Cardiovascular disease affects millions of Americans each year. Interventional guidance systems are being developed as treatment options for some of the more delicate procedures, including targeted stem cell therapy. As advanced systems for such types of interventional guidance are being developed, electromagnetic (EM) tracking is coming in demand to perform navigation. To use this EM tracking technology, a calibration is necessary to register the tracker to the imaging system. In this paper we investigate the calibration of an X-ray imaging system to EM tracking. Two specially designed calibration phantoms have been designed for this purpose, each having a rigidly attached EM sensor. From a clinical usability point-of-view, we propose to divide this calibration problem into two steps: i) in initial calibration of the EM sensor to the phantom design using an EM tracked needle to trace out grooves in the phantom surface and ii) segmentation from X-ray images and 3D reconstruction of beads embedded in the phantom in a known geometric pattern. Combining these two steps yields and X-ray-to-EM calibration accuracy of less than 1 mm when overlaying an EM tracked needle on X-ray images.

Augmenting CT cardiac roadmaps with segmented streaming ultrasound

Static X-ray computed tomography (CT) volumes are often used as anatomic roadmaps during catheter-based cardiac interventions performed under X-ray fluoroscopy guidance. These CT volumes provide a high-resolution depiction of soft-tissue structures, but at only a single point within the cardiac and respiratory cycles. Augmenting these static CT roadmaps with segmented myocardial borders extracted from live ultrasound (US) provides intra-operative access to real-time dynamic information about the cardiac anatomy. In this work, using a customized segmentation method based on a 3D active mesh, endocardial borders of the left ventricle were extracted from US image streams (4D data sets) at a frame rate of approximately 5 frames per second. The coordinate systems for CT and US modalities were registered using rigid body registration based on manually selected landmarks, and the segmented endocardial surfaces were overlaid onto the CT volume. The root-mean squared fiducial registration error was 3.80 mm. The accuracy of the segmentation was quantitatively evaluated in phantom and human volunteer studies via comparison with manual tracings on 9 randomly selected frames using a finite-element model (the US image resolutions of the phantom and volunteer data were 1.3 x 1.1 x 1.3 mm and 0.70 x 0.82 x 0.77 mm, respectively). This comparison yielded 3.70±2.5 mm (approximately 3 pixels) root-mean squared error (RMSE) in a phantom study and 2.58±1.58 mm (approximately 3 pixels) RMSE in a clinical study. The combination of static anatomical roadmap volumes and dynamic intra-operative anatomic information will enable better guidance and feedback for image-guided minimally invasive cardiac interventions.

Effects of sensor orientation on AC electromagnetic tracking system accuracy in a CT scanner environment

The purpose of this study was to examine the effects of different sensor orientation on the positional accuracy of an AC electromagnetic tracking system, the second generation NDI Aurora, within a CT scanner environment. A three-axis positioning robot was used to move three electromagnetically tracked needles above the CT table throughout a 30cm by 30cm by 30cm volume sampled in 2.5cm steps. All three needle tips were held within 2mm of each other, with the needle axes orthogonally located in the +x, +y, and +z directions of the Aurora coordinate system. The corresponding position data was captured from the Aurora for each needle and was registered to the positioning system data using a rigid body transformation minimizing the least squares L2-norm. For all three needle orientations the largest errors were observed farthest from the field generator and closest to the CT table. However, the 3D distortion error patterns were different for each needle, demonstrating that the sensor orientation has an effect on the positional measurement of the sensor. This suggests that the effectiveness of using arrays of reference sensors to model and correct for metal distortions may depend strongly on the orientation of the reference sensors in relation to the orientation of the tracked device. In an ideal situation, the reference sensors should be oriented in the same direction as the tracked needle.

Multimodality image guidance system integrating X-ray fluoroscopy and ultrasound image streams with electromagnetic tracking

This work presents an integrated system for multimodality image guidance of minimally invasive medical procedures. This software and hardware system offers real-time integration and registration of multiple image streams with localization data from navigation systems. All system components communicate over a local area Ethernet network, enabling rapid and flexible deployment configurations. As a representative configuration, we use X-ray fluoroscopy (XF) and ultrasound (US) imaging. The XF imaging system serves as the world coordinate system, with gantry geometry derived from the imaging system, and patient table position tracked with a custom-built measurement device using linear encoders. An electromagnetic (EM) tracking system is registered to the XF space using a custom imaging phantom that is also tracked by the EM system. The RMS fiducial registration error for the EM to X-ray registration was 2.19 mm, and the RMS target registration error measured with an EM-tracked catheter was 8.81 mm. The US image stream is subsequently registered to the XF coordinate system using EM tracking of the probe, following a calibration of the US image within the EM coordinate system. We present qualitative results of the system in operation, demonstrating the integration of live ultrasound imaging spatially registered to X-ray fluoroscopy with catheter localization using electromagnetic tracking.