In situ guidance for MRI interventions using projected feedback.

PURPOSE: To develop and evaluate an augmented reality instrument guidance system for MRI-guided needle placement procedures such as musculoskeletal biopsy and arthrography. Our system guides the physician to insert a needle toward a target while looking at the insertion site without requiring special headgear. METHODS: The system is comprised of a pair of stereo cameras, a projector, and a computational unit with a touch screen. All components are designed to be used within the MRI suite (Zone 4). Multi-modality fiducial markers called VisiMARKERs, detectable in both MRI and camera images, facilitate automatic registration after the initial scan. The navigation feedback is projected directly onto the intervention site allowing the interventionalist to keep their focus on the insertion site instead of a secondary monitor which is often not in front of them. RESULTS: We evaluated the feasibility and accuracy of this system on custom-built shoulder phantoms. Two radiologists used the system to select targets and entry points on initial MRIs of these phantoms over three sessions. They performed 80 needle insertions following the projected guidance. The system targeting error was 1.09 mm, and the overall error was 2.29 mm. CONCLUSION: We demonstrated both feasibility and accuracy of this MRI navigation system. The system operated without any problems inside the MRI suite close to the MRI bore. The two radiologists were able to easily follow the guidance and place the needle close to the target without any intermediate imaging.

Wearable Mechatronic Ultrasound-integrated AR Navigation System for Lumbar Puncture Guidance.

As one of the most commonly performed spinal interventions in routine clinical practice, lumbar punctures are usually done with only hand palpation and trial-and-error. Failures can prolong procedure time and introduce complications such as cerebrospinal fluid leaks and headaches. Therefore, an effective needle insertion guidance method is desired. In this work, we present a complete lumbar puncture guidance system with the integration of (1) a wearable mechatronic ultrasound imaging device, (2) volume-reconstruction and bone surface estimation algorithms and (3) two alternative augmented reality user interfaces for needle guidance, including a HoloLens-based and a tablet-based solution. We conducted a quantitative evaluation of the end-to-end navigation accuracy, which shows that our system can achieve an overall needle navigation accuracy of 2.83 mm and 2.76 mm for the Tablet-based and the HoloLens-based solutions, respectively. In addition, we conducted a preliminary user study to qualitatively evaluate the effectiveness and ergonomics of our system on lumbar phantoms. The results show that users were able to successfully reach the target in an average of 1.12 and 1.14 needle insertion attempts for Tablet-based and HoloLens-based systems, respectively, exhibiting the potential to reduce the failure rates of lumbar puncture procedures with the proposed lumbar-puncture guidance.

Computer-assisted needle navigation for pediatric internal jugular central venous cannulation: A feasibility study.

INTRODUCTION: Vascular access for central venous catheter placement is technically challenging in children. Ultrasound guidance is recommended for pediatric central venous catheter placement, yet many practitioners rely on imprecise anatomic landmark techniques risking procedure failure due to difficulty mastering ultrasound guidance. A novel navigation system provides a visual overlay on real-time ultrasound images to depict needle trajectory and tip location during cannulation. We report the first pediatric study assessing feasibility and preliminary safety of using a computer-assisted needle navigation system to aid in central venous access. METHODS: A prospective, institutional review board-approved feasibility study was performed. All participants provided written informed consent. Ten patients (mean age: 11.4 years, five males) underwent central venous catheter placement with ultrasound and navigation system guidance. All procedures were performed by interventional radiologists expert in vascular access. Feasibility was measured through binary (yes/no) responses from participating users assessing device usability and feasibility. The number of needle passes and procedure time measures were also recorded. RESULTS: Internal jugular veins (seven right sided, three left sided) were cannulated in all patients with no complications. Users confirmed navigation system feasibility in all 10 participants. Mean vein diameter and depth was 13.3 × 9.8 ± 3.4 × 2.1 and 7.0 ± 1.7 mm, respectively. Successful cannulation occurred in all patients and required only a single needle pass in 9 of 10 patients. Mean device set-up and vascular access times were 5:31 ± 2:28 and 1:48 ± 2:35 min, respectively. CONCLUSION: This pilot study suggests that it is feasible to use a novel computer-assisted needle navigation system to safely obtain central venous access under ultrasound guidance in pediatric patients.

Needle guidance using handheld stereo vision and projection for ultrasound-based interventions.

With real-time instrument tracking and in-situ guidance projection directly integrated in a handheld ultrasound imaging probe, needle-based interventions such as biopsies become much simpler to perform than with conventionally-navigated systems. Stereo imaging with needle detection can be made sufficiently robust and accurate to serve as primary navigation input. We describe the low-cost, easy-to-use approach used in the Clear Guide ONE generic navigation accessory for ultrasound machines, outline different available guidance methods, and provide accuracy results from phantom trials.

A freehand ultrasound elastography system with tracking for in vivo applications.

Ultrasound transducers are commonly tracked in modern ultrasound navigation/guidance systems. In this article, we demonstrate the advantages of incorporating tracking information into ultrasound elastography for clinical applications. First, we address a common limitation of freehand palpation: speckle decorrelation due to out-of-plane probe motion. We show that by automatically selecting pairs of radio frequency frames with minimal lateral and out-of-plane motions, combined with a fast and robust displacement estimation technique, greatly improves in vivo elastography results. We also use tracking information and image-quality measures to fuse multiple images with similar strains that are taken from roughly the same location so as to obtain a high-quality elastography image. Finally, we show that tracking information can be used to give the user partial control over the rate of compression. Our methods are tested on a tissue-mimicking phantom, and experiments have been conducted on intraoperative data acquired during animal and human experiments involving liver ablation. Our results suggest that in challenging clinical conditions, our proposed method produces reliable strain images and eliminates the need for a manual search through the ultrasound data in order to find radio frequency pairs suitable for elastography.

Experimental validation of an intrasubject elastic registration algorithm for dynamic-3D ultrasound images.

PURPOSE: In image-guided therapy, real-time visualization of the anatomy and adjustments in the therapy plan due to anatomical motions during the procedure is of outmost importance. 3D ultrasound has the potential to enable this real-time monitoring; however, nonrigid registration of a sequence of 3D ultrasound volumes remains to be a challenging problem. The authors present our recent results on the development of a computationally inexpensive feature-based registration algorithm for elastic alignment of dynamic-3D ultrasound images. METHODS: Our algorithm uses attribute vectors, based on the image intensity and gradient information, to perform feature-based matching in a sequence of 3D ultrasound images. Prior information from both the fixed and previous moving images is utilized to track features throughout the 3D image series. The algorithm has been compared to various publicly available registration techniques, i.e., the B-splines deformable registration, the symmetric forces Demons, and the fast free-form deformable registration method. RESULTS: Using a series of validation experiments on datasets collected from carotid artery, liver, and kidney of 20 subjects, the authors demonstrate that the feature-based, B-splines, Demons, and fast free-form deformable registration techniques can all recover volume deformations in a 3D ultrasound image series with reasonable accuracy; however, the proposed feature-based registration technique has substantial computational advantage over the other approaches. CONCLUSIONS: The proposed feature-based registration technique has the potential for real-time implementation on a computationally inexpensive platform and has the capability of recovering nonrigid deformations in tissue with reasonable accuracy.

Intra-operative ultrasound elasticity imaging for monitoring of hepatic tumour thermal ablation.

BACKGROUND: Thermal ablation is an accepted therapy for selected hepatic malignancies. However, the reliability of thermal ablation is limited by the inability to accurately monitor and confirm completeness of tumour destruction in real time. We investigated the ability of ultrasound elasticity imaging (USEI) to monitor thermal ablation. OBJECTIVES: Capitalizing on the known increased stiffness that occurs with protein denaturation and dehydration during thermal therapy, we sought to investigate the feasibility and accuracy of USEI for monitoring of liver tumour ablation. METHODS: A model for hepatic tumours was developed and elasticity images of liver ablation were acquired in in vivo animal studies, comparing the elasticity images to gross specimens. A clinical pilot study was conducted using USEI in nine patients undergoing open radiofrequency ablation for hepatic malignancies. The size and shape of thermal lesions on USEI were compared to B-mode ultrasound and post-ablation computed tomography (CT). RESULTS: In both in vivo animal studies and in the clinical trial, the boundary of thermal lesions was significantly more conspicuous on USEI when compared with B-mode imaging. Animal studies demonstrated good correlation between the diameter of ablated lesions on USEI and the gross specimen (r = 0.81). Moreover, high-quality strain images were generated in real time during therapy. In patients undergoing tumour ablation, a good size correlation was observed between USEI and post-operative CT (r = 0.80). CONCLUSION: USEI can be a valuable tool for the accurate monitoring and real-time verification of successful thermal ablation of liver tumours.

Tracked ultrasound elastography (TrUE).

This paper presents a robust framework for freehand ultrasound elastography to cope with uncertainties of freehand palpation using the information from an external tracker. In order to improve the quality of the elasticity images, the proposed method selects a few image pairs such that in each pair the lateral and out-of-plane motions are minimized. It controls the strain rate by choosing the axial motion to be close to a given optimum value. The tracking data also enables fusing multiple strain images that are taken roughly from the same location. This method can be adopted for various trackers and strain estimation algorithms. In this work, we show the results for two tracking systems of electromagnetic (EM) and optical tracker. Using phantom and ex-vivo animal experiments, we show that the proposed techniques significantly improve the elasticity images and reduce the dependency to the hand motion of user.

A real-time intrasubject elastic registration algorithm for dynamic 2-D ultrasound images.

A new dynamic registration technique is designed to elastically align a sequence of 2-D ultrasound (US) images. The proposed algorithm tracks features over an image sequence in real-time, as opposed to our previous technique which registers images without utilizing prior information. The registration results were evaluated using a customized validation framework for elastic US registration algorithms. Experiments were performed on 600 simulated images as well as 20 image sequences obtained from 10 volunteer subjects, each sequence containing 50 images. Strategies for qualitative and quantitative evaluation consisted of visual assessment, feature overlap, similarity measures, and simulation experiments. The registration method has comparable performance to our previous registration technique; however, has the advantage of lower computational requirements and hence, is potentially more suitable for clinical applications. Rigorous performance evaluations attest to the fast speed of registration at an average of 5.5 frames per second on a conventional computing platform.

Tracked regularized ultrasound elastography for targeting breast radiotherapy.

Tracked ultrasound elastography can be used for guidance in partial breast radiotherapy by visualizing the hard scar tissue around the lumpectomy cavity. For clinical success, the elastography method needs to be robust to the sources of decorrelation between ultrasound images, specifically fluid motions inside the cavity, change of the appearance of speckles caused by compression or physiologic motions, and out-of-plane motion of the probe. In this paper, we present a novel elastography technique that is based on analytic minimization of a regularized cost function. The cost function incorporates similarity of RF data intensity and displacement continuity, making the method robust to small decorrelations present throughout the image. We also exploit techniques from robust statistics to make the method resistant to large decorrelations caused by sources such as fluid motion. The analytic displacement estimation works in real-time. Moreover, the tracked data, used for targeting the radiotherapy, is exploited for discarding frames with excessive out-of-plane motion. Simulation, phantom and patient results are presented.

Localization of pelvic anatomical coordinate system using US/atlas registration for total hip replacement.

In Total Hip Replacement (THR) procedures, misalignment of the acetabular component can lead to dislocation and impingement. For the successful alignment of acetabular component, precise estimation of pelvic anatomical coordinate system is necessary. Conventional navigation systems use CT scan or fluoroscopy, or involve implanted bone fiducials or invasive probing of bony landmarks to locate the anatomical coordinate. In this paper, an ultrasound-based approach is proposed that exploits prior knowledge about the anatomy of the pelvis in the form of a 3D surface atlas. Tracked ultrasound images are utilized to extract sample points from the surface of the pelvis. A generic coordinate system in the specific patient is localized by registering these points to a statistical atlas of the pelvis in which a canonical anatomical coordinate system had been defined. This technique has been evaluated using simulation, dry bone, and cadaver experiments and was able to localize the anatomical coordinate system with the accuracy of about 1 degree.

Ultrasound elastography: a dynamic programming approach.

This paper introduces a 2-D strain imaging technique based on minimizing a cost function using dynamic programming (DP). The cost function incorporates similarity of echo amplitudes and displacement continuity. Since tissue deformations are smooth, the incorporation of the smoothness into the cost function results in reduced decorrelation noise. As a result, the method generates high-quality strain images of freehand palpation elastography with up to 10% compression, showing that the method is more robust to signal decorrelation (caused by scatterer motion in high axial compression and nonaxial motions of the probe) in comparison to the standard correlation techniques. The method operates in less than 1 s and is thus also potentially suitable for real time elastography.

Experimental validation of a 4D elastic registration algorithm.

This paper presents an extensive validation study of an elastic registration algorithm for dynamic 3D ultrasound images (also known as a 4D image). The registration algorithm uses attribute vectors from both a fixed and previous moving images to perform feature-based alignment of a series of images. The 4D method reduces computational requirements and increases the effective search space for the location of corresponding features, resulting in enhanced registration speed when compared to a static 3D registration technique. Experimental analysis revealed up to 32% improvement in speed when using the 4D method, which makes the algorithm attractive for real-time applications.

Intra-subject elastic registration of 3D ultrasound images.

3D registration of ultrasound images is an important and fast-growing research area with various medical applications, such as image-guided radiotherapy and surgery. However, this registration process remains extremely challenging due to the deformation of soft tissue and the existence of speckles in these images. This paper presents a technique for intra-subject, intra-modality elastic registration of 3D ultrasound images. Using the general concept of attribute vectors, we define the corresponding voxels in the fixed and moving images. Our method does not require presegmentation and does not employ any numerical optimization procedure. As the computational requirements are minimal, the method has potential use in real-time applications. The technique is implemented and tested on 3D ultrasound images of human liver, captured by a 3D ultrasound transducer. The results show that the method is sufficiently accurate and robust even in cases where artifacts such as shadows exist in the ultrasound data.

Towards real-time registration of 4D ultrasound images.

In this paper, we demonstrate a method for fast registration of sequences of 3D liver images, which could be used for the future real-time applications. In our method, every image is elastically registered to a so called fixed ultrasound image exploiting the information from previous registration. A few feature points are automatically selected, and tracked inside the images, while the deformation of other points are extrapolated with respect to the tracked points employing a fast free-form approach. The main intended application of the proposed method is real-time tracking of tumors for radiosurgery. The algorithm is evaluated on both naturally and artificially deformed images. Experimental results show that for around 85 percent accuracy, the process of tracking is completed very close to real time.

Elastic registration of 3D ultrasound images.

3D registration of ultrasound images is an important and fast-growing research area with various medical applications, such as image-guided radiotherapy and surgery. However, this registration process is extremely challenging due to the deformation of soft tissue and the existence of speckles in these images. This paper presents a novel intra-modality elastic registration technique for 3D ultrasound images. It uses the general concept of attribute vectors to find the corresponding voxels in the fixed and moving images. The method does not require any pre-segmentation and does not employ any numerical optimization procedure. Therefore, the computational requirements are very low and it has the potential to be used for real-time applications. The technique is implemented and tested for 3D ultrasound images of liver, captured by a 3D ultrasound transducer. The results show that the method is sufficiently accurate and robust and does not easily get trapped with local minima.