A framework for continuous target tracking during MR-guided high intensity focused ultrasound thermal ablations in the abdomen

Scatterplot showing percentage changes in stroke volume index (ΔSVI, %) and functional hemodynamic markers, Stroke Volume Variation (SVV, %) Pulse Pressure Variation (PPV, %), with the three tested tidal volumes (V T ), 6, 12 and 18 ml/kg during intra-abdominal hypertension. Solid line shows regression line between variables. (PDF 56 kb

A framework for continuous target tracking during MR-guided high intensity focused ultrasound thermal ablations in the abdomen

International audienceBackground: During lengthy magnetic resonance-guided high intensity focused ultrasound (MRg-HIFU) thermal ablations in abdominal organs, the therapeutic work-flow is frequently hampered by various types of physiological motion occurring at different time-scales. If left un-addressed this can lead to an incomplete therapy and/or to tissue damage of organs-at-risk. While previous studies focus on correction schemes for displacements occurring at a particular time-scale within the work-flow of an MRg-HIFU therapy, in the current work we propose a motion correction strategy encompassing the entire work-flow.Methods: The proposed motion compensation framework consists of several linked components, each being adapted to motion occurring at a particular time-scale. While respiration was addressed through a fast correction scheme, long term organ drifts were compensated using a strategy operating on time-scales of several minutes. The framework relies on a periodic examination of the treated area via MR scans which are then registered to a reference scan acquired at the beginning of the therapy. The resulting displacements were used for both on-the-fly re-optimization of the interventional plan and to ensure the spatial fidelity between the different steps of the therapeutic work-flow. The approach was validated in three complementary studies: an experiment conducted on a phantom undergoing a known motion pattern, a study performed on the abdomen of 10 healthy volunteers and during 3 in-vivo MRg-HIFU ablations on porcine liver.Results: Results have shown that, during lengthy MRg-HIFU thermal therapies, the human liver and kidney can manifest displacements that exceed acceptable therapeutic margins. Also, it was demonstrated that the proposed framework is capable of providing motion estimates with sub-voxel precision and accuracy. Finally, the 3 successful animal studies demonstrate the compatibility of the proposed approach with the work-flow of an MRg-HIFU intervention under clinical conditions.Conclusions: In the current study we proposed an image-based motion compensation framework dedicated to MRg-HIFU thermal ablations in the abdomen, providing the possibility to re-optimize the therapy plan on-the-fly with the patient on the interventional table. Moreover, we have demonstrated that even under clinical conditions, the proposed approach is fully capable of continuously ensuring the spatial fidelity between the different phases of the therapeutic work-flow

Fast 4D Ultrasound Registration for Image Guided Liver Interventions

Liver problems are a serious health issue. The common liver problems are hepatitis, fatty liver, liver cancer and liver damage caused by alcohol abuse. Continuous, long term disease may cause a condition of the liver known as the Liver Cirrhosis. Liver cirrhosis makes the liver scarred and hardened up causing portal hypertension. In such a situation the collateral vessels try to bypass the liver as blood cannot freely flow through the liver; causing internal bleeding. One of the treatments of portal hypertension is Transjugular intrahepatic portosystemic shunt (TIPS). In a TIPS procedure a tract in the liver is created that shortcuts two veins in the liver, reducing the portal hypertension. Radiofrequency ablation (RFA) is use for the treatment of liver cancer. In RFA, a needle electrode is placed through the skin into the liver tumor. High-frequency electrical currents are passed through the electrode, creating heat that destroys the cancer cells, without damaging the surrounding liver tissues. TIPS and RFA are minimally invasive procedures, where small incisions are made to perform the surgery and are alternative to open surgery. A minimally invasive alternative has large potential in reducing complication rates, minimizing surgical trauma and reducing hospital stay. However, in these procedures, due to lack of direct eyesight, three-dimensional imaging information about the anatomy and instruments during the intervention is required. The most difficult part of these procedures is the interpretation and selection of oblique views for needle/instrument insertion and target visualization. In our work we develop and evaluate techniques that enable the effective use of 3D ultrasound for image guided interventions. Ultrasound is low cost, mobile and unlike CT and X-rays does not use any harmful radiation in the imaging process. During these procedures, breathing shifts the region of interest and makes it difficult to constantly focus on a region of interest. We provide an approach to correct for the motion due to breathing. Additionally, we propose a method for image fusion of interventional ultrasound and preoperative imaging modalities such as CT for cases where the lesions are visible in CT but not visible in ultrasound. Incorporating CT data during intervention additionally adds greater definition and precision to the ultrasound based navigation system. Concluding, in this thesis, we presented methods and evaluated their accuracies that demonstrate the use of real-time 3D US and its fusion with CT in potentially improving image guidance in minimally invasive US guided liver interventions

REAL-TIME ELASTOGRAPHY SYSTEMS

Ultrasound elastography is a technique that is often used to detect cancerous tumors and monitor ablation therapy by detecting changes in the stiffness of the underlying tissue. This technique is a computationally expensive due to the extensive searching between two raw ultrasound images, that are called radio frequency images. This thesis explores various methods to accelerate the computation required for the elastography technique to allow use during surgery. This thesis is divided into three parts. We begin by exploring acceleration techniques, including multithreading techniques, asynchronous computing, and acceleration of the graphics processing unit (GPU). Elastography algorithms are often affected by out-of-plane motion due to several external factors, such as hand tremors and incorrect palpation motion, amongst others. In this thesis, we implemented an end-to-end system that integrates an external tracker system to detect the in-plane motion of two radio frequency (RF) data slices. This in-plane detection helps to reduce de-correlated RF slices and produces a consistent elastography output. We also explore the integration of a da Vinci Surgical Robot to provide stable palpation motion during the surgery. The external tracker system suffers from interference due to ferromagnetic materials present in the operation theater in the case of an electromagnetic tracker, while optical and camera-based tracking systems are restricted due to human, object and patient interference in the path of sight and complete or partial occlusion of the tracking sensors. Additionally, these systems must be calibrated to give the position of the tracked objects with respect to the trackers. Although calibration and trackers are helpful for inter-modality registration, we focus on a tracker-less method to determine the in-plane motion of two RF slices. Our technique divides the two input RF images into regions of interest and performs elastography on RF lines that encapsulate those regions of interest. Finally, we implemented the world’s first known five-dimensional ultrasound system. We built the five-dimensional ultrasound system by combining a 3D B-mode volume and a 3D elastography volume visualized over time. A user controlled multi-dimensional transfer function is used to differentiate between the 3D B-mode and the 3D elastography volume