Etsuko Kumamoto

Optimization of self‐reference thermometry using complex field estimation

Referenceless, or self‐reference, thermometry is a technique for mapping temperature differences in the region of interest (ROI) using the baseline phase estimated by extrapolating the field in the surrounding region for estimation (RFE) and subtracting the estimated baseline from the measured field. In the present work a self‐reference technique based on complex field estimation using 2D polynomials comprising complex‐valued coefficients was proposed and optimized. Numerical simulations with a Gaussian‐profiled phase distribution demonstrated that the ROI radius had to be 2.3–2.5 times the standard deviation (SD) of the Gaussian function in order to keep the error below 8% of the peak phase change. The area ratio between the ROI and the RFE had to be larger than 2.0 to maintain the error level. Based on the simulations, and phantom and volunteer experiments, the complex‐based method with independently optimized polynomial orders for the two spatial dimensions was compared with the phase‐based method using the similar‐order optimization strategy. The complex‐based method appeared to be useful when phase unwrapping was not removed. Otherwise, the phase‐based method yielded equivalent results with less polynomial orders. Magn Reson Med, 2006.

Method for Target Tracking in Focused Ultrasound Surgery of Liver using Magnetic Resonance Filtered Venography

The purpose of this work is to develop a magnetic resonance (MR) technique for guiding a focal point created in focused ultrasound surgery (FUS) onto a specific target position in an abdominal organ, such as the liver, which moves and deforms with respiratory motion. The translational distance, rotational angles, and amount of expansion and contraction of the organ tissue were measured by obtaining the gravity points of the veins filtered from the sagittal, cine MR images of healthy livers during free breathing. Using the locations of the vessels at each time point, the target position at which the ultrasound focus was to be placed was estimated. In the volunteer experiments (N = 2), the lower limit of the spatial matrix dimension for delineating the veins was 128 x 128. The average displacement of the liver was 19.6 + 3.6 mm in superior-inferior (SI) direction and 3.1 + 1.4 mm in anterior-posterior (AP) direction. The deformations were 3.7 + 1.1 mm in SI direction and 3.0 + 1.2 mm in AP direction. The error between the actual and the estimated target point was 0.7 plusmn 0.5 mm in SI direction, 0.6 plusmn 0.4 mm in AP direction and 1.0 plusmn 0.5 mm in distance, and less than 2.1 mm in all the trials. These results suggested that the proposed technique is sufficient for targeting the focus on a specific tissue location and for tracking the slice slab for thermometry to cover the region of focus.

Evaluation of a vessel‐tracking‐based technique for dynamic targeting in human liver

The purpose of this study was to evaluate a novel vessel‐tracking‐based technique for tracking of human liver. The novelty of the proposed technique is that it measures the translation and deformation of a local tissue region based on the displacements of a set of vessels of interest instead of the entire organ. The position of the target point was estimated from the relative positions of the center‐of‐masses of the vessels, assuming that the topological relationship between the target point and center‐of‐masses is unchanged during breathing. To reduce inaccuracy due to the delay between vessel image acquisition and sonication, the near‐future target position was predicted based on the vessel displacements in the images extracted from an image library acquired before the tracking stage. Experiments on healthy volunteers demonstrated that regardless of the respiratory condition, appropriate combinations of three center‐of‐masses from the vessels situated around the target‐tissue position yielded an estimation error of less than 2 mm, which was significantly smaller than that obtained when using a single center‐of‐mass trio. The effect of the tracking delay was successfully compensated, with a prediction error of less than 3 mm, by using over four images selected from the image library. Magn Reson Med, 2011.

Endoluminal MR imaging of porcine gastric structure in vivo

Background and aimsRecently, several new endoscopic instruments have been developed. However, even with the full use of current modalities, the safety of endoscopic surgery is not guaranteed. Information regarding factors such as fibrosis and the blood vessels under the mucosa is very important for avoiding procedure-related complications. The aim of this study was to define the detailed anatomy of the gastric wall structure in vivo using original endoluminal radiofrequency coils for safer endoscopic therapy.MethodsSwine were used as the subjects and controlled with general anesthesia. Anatomical images were obtained with T1-weighted fast spin echo (T1FSE) and T2-weighted fast spin echo (T2FSE). Dynamic magnetic resonance (MR) angiography was also obtained with three-dimensional T1-weighted fast spoiled gradient recalled acquisition in the steady state (3D-DMRA) following the injection of hyaluronic acid sodium into the submucosal layer.ResultsPorcine gastric wall structure was visualized, and four layers were discriminated in the T1FSE and T2FSE images. The vascular structure was clearly recognized in the submucosa on 3D-DMRA.ConclusionEndoluminal MR imaging was able to visualize the porcine stomach with similar quality to endoscopic ultrasonography imaging. Additionally, it was possible to visualize the vascular structures in the submucosal layer. This is the first report to show that blood vessels under the gastric mucosa can be depicted in vivo.

High-resolution MR imaging of gastrointestinal tissue by intracavitary RF coil with remote tuning and matching technique for integrated MR-endoscope system

The goal of this study is to establish novel medical technologies by combining magnetic resonance imaging (MRI) with endoscopy to improve diagnostic precision and the safety of endoscopic surgeries. One of the key components of the integrated magnetic resonance (MR) endoscope system is a radio-frequency (RF) coil; this detects the MR signal from tissue and should be placed inside the body. Resonance characteristics such as the resonant frequency and the impedance of the RF coil, which affect the quality of MR images, change depending on the electric properties of the surrounding tissue and the coil deformation. Therefore, the technique of remote tuning and matching of the RF coil was developed, and its feasibility was investigated using a developed intracavitary RF coil, 1.5 tesla MR scanner, and models of phantom and resected porcine stomach. As a result, the frequency tuning and impedance matching was remotely adjusted in both models. In addition, the signal-to-noise ratio (SNR) of MR images was improved up to 134%. The developed remote tuning and matching technique was able to adjust the resonant characteristics of RF coil and can contribute the improvement of MR image quality, which would facilitate safe and precise endoscopy and endoscopic surgeries.

Navigation technique for MR-endoscope system using a wireless accelerometer-based remote control device

The MR-endoscope system can perform magnetic resonance (MR) imaging during endoscopy and show the images obtained by using endoscope and MR. The MR-endoscope system can acquire a high-spatial resolution MR image with an intraluminal radiofrequency (RF) coil, and the navigation system shows the scopes location and orientation inside the human body and indicates MR images with a scope view. In order to conveniently perform an endoscopy and MR procedure, the design of the user interface is very important because it provides useful information. In this study, we propose a navigation system using a wireless accelerometer-based controller with Bluetooth technology and a navigation technique to set the intraluminal RF coil using the navigation system. The feasibility of using this wireless controller in the MR shield room was validated via phantom examinations of the influence on MR procedures and navigation accuracy. In vitro examinations using an isolated porcine stomach demonstrated the effectiveness of the navigation technique using a wireless remote-control device.

Interactive Determination of Irradiate Condition for External Beam Radiotherapy Treatment by Satisficing Trade-off Method

In the external beam radiotherapy, it is required to optimize the number of beams, the beam angles, and the strength of irradiation, which deliver the lethal dose to the tumor while limiting the dose absorbed in the normal tissue with fewer beams. In this study, we formulate this problem as a multiobjective linear programming, and propose a planning method to balance the dose distribution and the number of beams by using satisficing trade-off method. An aspiration level to each concerning point specifies the planners direction for each objective. Therefore, we applied cluster analysis to classify the objective functions, enabling the planner to set up aspiration levels for clusters rather than each objective, which significantly ease the planning task. An illustrated plan of a treatment for cerebral tumor is given to demonstrate the effectiveness of the proposed method.