Luis Felipe Gutierrez

X-Ray Fused With Magnetic Resonance Imaging (XFM) to Target Endomyocardial Injections: Validation in a Swine Model of Myocardial Infarction

Background— Magnetic resonance imaging (MRI) permits 3-dimensional (3D) cardiac imaging with high soft tissue contrast. X-ray fluoroscopy provides high-resolution, 2-dimensional (2D) projection imaging. We have developed real-time x-ray fused with MRI (XFM) to guide invasive procedures that combines the best features of both imaging modalities. We tested the accuracy of XFM using external fiducial markers to guide endomyocardial cell injections in infarcted swine hearts. Methods and Results— Endomyocardial injections of iron-labeled mesenchymal stromal cells admixed with tissue dye were performed in previously infarcted hearts of 12 Yucatan miniswine (weight, 33 to 67 kg). Features from cardiac MRI were displayed combined with x-ray in real time to guide injections. During 130 injections, operators were provided with 3D surfaces of endocardium, epicardium, myocardial wall thickness (range, 2.6 to 17.7 mm), and infarct registered with live x-ray images to facilitate device navigation and choice of injection location. XFM-guided injections were compared with postinjection MRI and with necropsy specimens obtained 24 hours later. Visual inspection of the pattern of dye staining on 2,3,5-triphenyltetrazolium chloride–stained heart slices agreed (&kgr;=0.69) with XFM-derived injection locations mapped onto delayed hyperenhancement MRI and the susceptibility artifacts seen on the postinjection T2*-weighted gradient echo MRI. The distance between the predicted and actual injection locations in vivo was 3.2±2.6 mm (n=64), and 75% of injections were within 4.1 mm of the predicted location. Conclusions— Three-dimensional to two-dimensional registration of x-ray and MR images with the use of external fiducial markers accurately targets endomyocardial injection in a swine model of myocardial infarction.

Technology Preview: X-Ray Fused With Magnetic Resonance During Invasive Cardiovascular Procedures

We have developed and validated a system for real‐time X‐ray fused with magnetic resonance imaging, MRI (XFM), to guide catheter procedures with high spatial precision. Our implementation overlays roadmaps—MRI‐derived soft‐tissue features of interest—onto conventional X‐ray fluoroscopy. We report our initial clinical experience applying XFM, using external fiducial markers, electrocardiogram (ECG)‐ gating, and automated real‐time correction for gantry and table movement.

Antegrade Percutaneous Closure of Membranous Ventricular Septal Defect Using X-Ray Fused With Magnetic Resonance Imaging

OBJECTIVES We hypothesized that X-ray fused with magnetic resonance imaging (XFM) roadmaps might permit direct antegrade crossing and delivery of a ventricular septal defect (VSD) closure device and thereby reduce procedure time and radiation exposure. BACKGROUND Percutaneous device closure of membranous VSD is cumbersome and time-consuming. The procedure requires crossing the defect retrograde, snaring and exteriorizing a guidewire to form an arteriovenous loop, then delivering antegrade a sheath and closure device. METHODS Magnetic resonance imaging roadmaps of cardiac structures were obtained from miniature swine with spontaneous VSD and registered with live X-ray using external fiducial markers. We compared antegrade XFM-guided VSD crossing with conventional retrograde X-ray-guided crossing for repair. RESULTS Antegrade XFM crossing was successful in all animals. Compared with retrograde X-ray, antegrade XFM was associated with shorter time to crossing (167 +/- 103 s vs. 284 +/- 61 s; p = 0.025), shorter time to sheath delivery (71 +/- 32 s vs. 366 +/- 145 s; p = 0.001), shorter fluoroscopy time (158 +/- 95 s vs. 390 +/- 137 s; p = 0.003), and reduced radiation dose-area product (2,394 +/- 1,522 mG.m(2) vs. 4,865 +/- 1,759 mG.m(2); p = 0.016). CONCLUSIONS XFM facilitates antegrade access to membranous VSD from the right ventricle in swine. The simplified procedure is faster and reduces radiation exposure compared with the conventional retrograde approach.

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.

A practical global distortion correction method for an image intensifier based x-ray fluoroscopy system.

X-ray images acquired on systems with image intensifiers (II) exhibit characteristic distortion which is due to both external and internal factors. The distortion is dependent on the orientation of the II, a fact particularly relevant to IIs mounted on C arms which have several degrees of freedom of motion. Previous descriptions of distortion correction strategies have relied on a dense sampling of the C-arm orientation space, and as such have been limited mostly to a single arc of the primary angle, alpha. We present a new method which smooths the trajectories of the segmented vertices of the grid phantom as a function of a prior to solving the two-dimensional warping problem. It also shows that the same residual errors of distortion correction could be achieved without fitting the trajectories of the grid vertices, but instead applying the previously described global method of distortion correction, followed by directly smoothing the values of the polynomial coefficients as functions of the C-arm orientation parameters. When this technique was applied to a series of test images at arbitrary alpha, the root-mean-square (RMS) residual error was 0.22 pixels. The new method was extended to three degrees of freedom of the C-arm motion: the primary angle, alpha; the secondary angle, beta; and the source-to-intensifier distance, lambda. Only 75 images were used to characterize the distortion for the following ranges: alpha, +/- 45 degrees (Deltaalpha = 22.5 degrees); beta, +/- 36 degrees (Deltabeta = 18 degrees); lambda, 98-118 cm (Deltalambda = 10 cm). When evaluated on a series of test images acquired at arbitrary (alpha, beta, lambda), the RMS residual error was 0.33 pixels. This method is targeted at applications such as guidance of catheter-based interventions and treatment planning for brachytherapy, which require distortion-corrected images over a large range of C-arm orientations.

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.

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.

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.

Single-view 2D/3D registation for X-ray guided bronchoscopy

X-ray guided bronchoscopy is commonly used for targeting peripheral lesions in the lungs which cannot be visualized directly by the bronchoscope. The airways and lesions are normally not visible in X-ray images, and as a result, transbronchial biopsy of peripheral lesions is often carried out blindly, lowering the diagnostic yield of bronchoscopy. In response to this problem, we propose to superimpose the lesions and airways segmented from preoperative 3D CT images onto 2D fluoroscopic images. A feature-based 2D/3D registration method is used for image fusion between the two datasets. The algorithm extracts features of the bony structures from both CT and X-ray images to compute the registration. Phantom and clinical studies were carried out to validate the algorithms performance, showing an accuracy of 3.48±1.38mm. The convergence range and speed of the algorithm were also evaluated to investigate the feasibility of using the algorithm clinically. The results are presented.