Matthew W. Gilbertson

Ergonomic control strategies for a handheld force-controlled ultrasound probe

An ergonomic, handheld, force-controlled ultrasound probe has been developed for medical imaging applications. The device, which consists of an ultrasound probe mounted to a backlash-free ball screw actuator and driven by a compact servo motor, maintains a prescribed contact force between the ultrasound probe and patients body. A control system which combines both a position and a force controller enables ergonomic operation by keeping the actuator centered within its range of motion and permits the repeated making and breaking of probe-patient contact. By controlling ultrasound probe contact force and consequently the amount of tissue deformation, the system enhances the repeatability, usability, and diagnostic capabilities of ultrasound imaging.

Probe Localization for Freehand 3D Ultrasound by Tracking Skin Features

Ultrasound probe localization with respect to the patients body is essential for freehand three-dimensional ultrasound and image-guided intervention. However, current methods for probe localization generally involve bulky and expensive equipment. In this paper, a highly cost-effective and miniature-mobile system is described for 6-DoF probe localization that is robust to rigid patient motion. In this system, skin features in the scan region are recorded at each ultrasound scan acquisition by a lightweight camera rigidly mounted to the probe. A skin map is built based on the skin features and optimal probe poses are estimated in a Bayesian probabilistic framework that incorporates a prior motion model, camera frames, and ultrasound scans. Through freehand scanning on three different body parts, it is shown that on average, for every probe travel distance of 10 mm, the translational and rotational errors are 0.91 ± 0.49 mm and 0.55 degrees ± 0.17 degrees, respectively. The 3D reconstructions were also validated by comparison with real ultrasound scans.

6-DOF probe tracking via skin mapping for freehand 3D ultrasound

Three-dimensional ultrasound (3D US) is widely useful in clinical diagnosis and therapy monitoring. However, the existing methods for 3D US are generally expensive and physically constraining. This paper describes a low-cost and unobtrusive method for 3D US, which spatially registers 2D US scans in six degrees of freedom (6 DoF). In this method, artificial skin features are created in the scan region of the body for robust feature tracking. A lightweight camera is mounted on the US probe to track the features for probe motion recovery and skin surface mapping. This algorithm does not rely on any assumption on the scene, so this system is suitable for scan regions of any size and any surface shape. In this paper, the system design and the preparation of artificial skin features are described. Performance of this method in 3D volume reconstruction is examined quantitatively through in-vitro experiments and qualitatively through in-vivo experiments.

An ergonomic, instrumented ultrasound probe for 6-axis force/torque measurement

An ergonomic, instrumented ultrasound probe has been developed for medical imaging applications. The device, which fits compactly in the hand of sonographers and permits rapid attachment & removal of the ultrasound probe, measures ultrasound probe-to-patient contact forces and torques in all six axes. The device was used to measure contact forces and torques applied by ten professional sonographers on five patients during thirty-six abdominal exams. Of the three contact forces, those applied along the probe axis were found to be largest, averaging 7.0N. Measurement noise was quantified for each axis, and found to be small compared with the axial force. Understanding the range of forces applied during ultrasound imaging enables the design of more accurate robotic imaging systems and could also improve understanding of the correlation between contact force and sonographer fatigue and injury.

Force and Position Control System for Freehand Ultrasound

A hand-held force-controlled ultrasound probe has been developed for medical imaging applications. The probe-patient contact force can be held constant to improve image stability, swept through a range, or cycled. The mechanical portion of the device consists of a ball screw linear actuator driven by a servo motor, along with a load cell, accelerometer, and limit switches. The performance of the system was assessed in terms of the frequency response to simulated sonographer hand motion and in hand-held image feature tracking during simulated patient motion. The system was found to attenuate contact force variation by 97% at 0.1 Hz, 83% at 1 Hz, and 33% 10 Hz, a range that spans the typical human hand tremor frequency spectrum. In studies with 15 human operators, the device applied the target contact force with ten times less variation than in conventional ultrasound imaging. An ergonomic, human-in-the-loop, imaging-workflow enhancing control scheme, which combines both force- and position-control, permits smooth making and breaking of probe-patient contact, and helps the operator keep the probe centered within its range of motion. By controlling ultrasound probe contact force and consequently the amount of tissue deformation, the system enhances the repeatability, usability, and diagnostic capabilities of ultrasound imaging.

Assessing duchenne muscular dystrophy with force-controlled ultrasound

In this paper, we present a technique for quantitative discrimination of Duchenne Muscular Dystrophy (DMD). Our ultrasound image data is generated with a novel force-controlled ultrasound acquisition system that allows precise ultrasound image acquisition at a predetermined force. We use the texture of ultrasound images, as calculated by the Canny edge detector, as the input image feature for our analysis algorithm. After statistically sieving through the edge detection parameters on our training set, we identify the set of parameters significant within a threshold. Decision trees are then trained on these significant parameters over a training dataset with cross-validation, and evaluated on accuracy, precision, specificity and sensitivity on a separate test dataset. We discuss the performance of our system, by muscle groups, on data collected with our device in a recent clinical study. Using depth of the image as a proxy for image regions, we evaluate the extent to which the performance of our system is robust to region-of-interest selection. Our method holds significant promise for automated assessment of Duchenne Muscular Dystrophy using force-controlled ultrasound image acquisition in a reliable and robust manner.

Computer-guided ultrasound probe realignment by optical tracking

In longitudinal studies and localized therapies, tissue changes are commonly tracked by repeated ultrasound scans at a fixed location marked on the patient body. However, the accuracy of this probe realignment approach is sometimes inadequate, especially when maintaining the insonification angle is essential. This paper describes a system that provides real-time visual guidance for accurate realignment of the ultrasound probe in six degrees of freedom (6 DoF). This system uses a small camera rigidly mounted on the probe to track artificial skin features, from which the current probe pose relative to the target pose is estimated. A virtual pyramid is created in the skin map and shown in the camera frame to intuitively indicate the probe movement required to achieve the target pose. Performance of this system was examined in vivo, and it was shown that this system significantly improves alignment of tissue structures in repeated ultrasound scans.

Evaluating the clinical relevance of force-correlated ultrasound

In this paper, we propose a new modality for automated diagnostic assessment of tissues in the context of Duchenne Muscular Dystrophy (DMD). In this force-correlated ultrasound imaging method, we first perform an automated extraction of a multitude of ultrasound images captured across a range of contact forces - a force video, or a force sweep. These images are then coupled with a mechanism to enhance the diagnostic fidelity of the image with regard to DMD. For this, we propose a variance map, which we compute as the pixel-wise standard deviation image for a multiscale stack generated using each image. Using a biomarker quantification scheme of mean gray scale level (GSL) on the enhanced fidelity force-correlated ultrasound images, a k-means clustering is then performed to discriminate the DMD subjects from the control subjects. We present our results on the use of this novel modality in the diagnostic assessment of DMD on data gathered from a clinical study with our system.

Impedance-controlled ultrasound probe

An actuated hand-held impedance-controlled ultrasound probe has been developed. The controller maintains a prescribed contact state (force and velocity) between the probe and a patients body. The device will enhance the diagnostic capability of free-hand elastography and swept-force compound imaging, and also make it easier for a technician to acquire repeatable (i.e. directly comparable) images over time. The mechanical system consists of an ultrasound probe, ball-screw-driven linear actuator, and a force/torque sensor. The feedback controller commands the motor to rotate the ball-screw to translate the ultrasound probe in order to maintain a desired contact force. It was found that users of the device, with the control system engaged, maintain a constant contact force with 15 times less variation than without the controller engaged. The system was used to determine the elastic properties of soft tissue.

Trajectory-based deformation correction in ultrasound images

Tissue deformation in ultrasound imaging poses a challenge to the development of many image registration techniques, including multimodal image fusion, multi-angle compound image and freehand three-dimensional ultrasound. Although deformation correction methods are desired to provide images of uncompressed tissue structure, they have not been well-studied. A novel trajectory-based method to correct a wide range of tissue deformation in ultrasound imaging was developed. In order to characterize tissue deformation under different contact forces, a force sensor provides contact force measurement. Template based image-flow techniques were applied to RF A-lines under different contact forces. A two-dimensional displacement trajectory field was constructed, where pixel coordinates in each scan were plotted against the corresponding contact force. Nonlinear extrapolation algorithms are applied to each trajectory to relocate the corresponding pixel to where it would have been had there been no contact, thereby correcting tissue deformation in the images. This method was validated by using a combination of FEM deformation and ultrasound simulation. It was shown that deformation of the simulated pathological tissue could be corrected. Furthermore, nonlinear polynomial regression was found to give better estimates, than linear regression, when large deformation was present. Estimation accuracy was not improved significantly for a polynomial regression larger than second order.