Douglas P. Perrin

Force tracking with feed-forward motion estimation for beating heart surgery

The manipulation of fast-moving, delicate tissues in beating heart procedures presents a considerable challenge to the surgeon. A robotic force tracking system can assist the surgeon by applying precise contact forces to the beating heart during surgical manipulation. Standard force control approaches cannot safely attain the required bandwidth for this application due to vibratory modes within the robot structure. These vibrations are a limitation even for single degree-of-freedom systems that drive long surgical instruments. These bandwidth limitations can be overcome by the incorporation of feed-forward motion terms in the control law. For intracardiac procedures, the required motion estimates can be derived from 3-D ultrasound imaging. Dynamic analysis shows that a force controller with feed-forward motion terms can provide safe and accurate force tracking for contact with structures within the beating heart. In vivo validation confirms that this approach confers a 50% reduction in force fluctuations when compared with a standard force controller and a 75% reduction in fluctuations when compared with manual attempts to maintain the same force.

Rethinking classical internal forces for active contour models

The classical active contour model has two basic internal forces: tension and curvature. These forces are included to provide cohe sion, equal control point spacing, and locally smooth shape. These classical internal forces have undesirable attributes that are in conflict with these original desired characteristics. Tension evenly spaces the control points, but also causes the models to collapse in weak image gradients. Curvature produces locally smooth curvature, but it does so by forcing the model toward a straight line. The paper returns to the original active contour model motivations to reformulate these internal forces. The desired properties are achieved without the introduction of unwanted model behavior A new spacing force and a new constant change in curvature force are introduced and their performance characteristics are discussed. The paper includes experimental results that demonstrate the efficacy and performance of the proposed reformulations.

Real-time image-based rigid registration of three-dimensional ultrasound

Registration of three-dimensional ultrasound (3DUS) volumes is necessary in several applications, such as when stitching volumes to expand the field of view or when stabilizing a temporal sequence of volumes to cancel out motion of the probe or anatomy. Current systems that register 3DUS volumes either use external tracking systems (electromagnetic or optical), which add expense and impose limitations on acquisitions, or are image-based methods that operate offline and are incapable of providing immediate feedback to clinicians. This paper presents a real-time image-based algorithm for rigid registration of 3DUS volumes designed for acquisitions in which small probe displacements occur between frames. Described is a method for feature detection and descriptor formation that takes into account the characteristics of 3DUS imaging. Volumes are registered by determining a correspondence between these features. A global set of features is maintained and integrated into the registration, which limits the accumulation of registration error. The system operates in real-time (i.e. volumes are registered as fast or faster than they are acquired) by using an accelerated framework on a graphics processing unit. The algorithms parameter selection and performance is analyzed and validated in studies which use both water tank and clinical images. The resulting registration accuracy is comparable to similar feature-based registration methods, but in contrast to these methods, can register 3DUS volumes in real-time.

Patient-specific mitral leaflet segmentation from 4D ultrasound

Segmenting the mitral valve during closure and throughout a cardiac cycle from four dimensional ultrasound (4DUS) is important for creation and validation of mechanical models and for improved visualization and understanding of mitral valve behavior. Current methods of segmenting the valve from 4DUS either require extensive user interaction and initialization, do not maintain the valve geometry across a cardiac cycle, or are incapable of producing a detailed coaptation line and surface. We present a method of segmenting the mitral valve annulus and leaflets from 4DUS such that a detailed, patient-specific annulus and leaflets are tracked throughout mitral valve closure, resulting in a detailed coaptation region. The method requires only the selection of two frames from a sequence indicating the start and end of valve closure and a single point near a closed valve. The annulus and leaflets are first found through direct segmentation in the appropriate frames and then by tracking the known geometry to the remaining frames. We compared the automatically segmented meshes to expert manual tracings for both a normal and diseased mitral valve, and found an average difference of 0.59 +/- 0.49 mm, which is on the order of the spatial resolution of the ultrasound volumes (0.5-1.0 mm/voxel).

Image Guided Surgical Interventions

urgeons have traditionally performed procedures to treat diseases by aining direct access to the internal structures involved, and using direct isual inspection to diagnose and treat the defects. Much effort has gone nto identifying the most appropriate incisions and approaches to enable ull access inside body cavities, specific organs, or musculoskeletal tructures. Imaging has traditionally been used primarily for preoperative iagnosis and at times for surgical planning. Intraoperative imaging, hen used, was meant to provide further diagnostic information or to ssess adequacy of repair. In most cases radiograph static images or uoroscopy have been used in the operating room. As the application of ess invasive procedures has progressed, other imaging technology has een applied in an effort to address the limitations of simple radiographs r fluoroscopy. Computed tomography (CT), magnetic resonance imagng (MRI), ultrasound, nuclear radiographic imaging, and modified ptical imaging have been introduced to provide the information required o plan and perform complex interventions inside the body without the eed for direct open visual inspection. In parallel with the developments in imaging modalities, endoscopic urgery has advanced with the introduction of rigid and flexible scopes quipped with video cameras to magnify and display the image obtained. dvances in optics and digital electronics have combined to provide nparalleled image quality even with small diameter scopes, resulting in n explosion of endoscopic procedures involving virtually every structure n the body. The only real limitation to imaging has been the inability o see or “image” through opaque structures, since the irradiating or illuminating” energy provided through the scope has been almost xclusively visible light. This limitation has confined endoscopic surgery o areas where a natural body cavity or physical space can be accessed ith a scope and instruments, and filled with nonopaque medium such as gas or clear fluid. Despite these limitations, optical endoscopy has evolutionized the way many surgical procedures are performed, and has pawned a whole industry of instrument manufacturers that, in conjunc-

Force Feedback in a Three-Dimensional Ultrasound-Guided Surgical Task

Three-dimensional ultrasound (3D US) is a novel imaging modality that allows real-time visualization of internal body structures such as the heart, even through visually opaque blood and tissue. The real-time nature of 3D US allows minimally invasive manipulations to be carried out without an endoscopic camera. The quality of the images is not ideal, however, other senses might be used to augment a surgeon’s performance in a 3D US-guided procedure. We investigated the combination of haptics under 3D US in a force control task. Results suggest that stiffness of the tissue plays a significant role as to the relative importance of vision versus haptics in this type of task.

Temporal Enhancement of 3D Echocardiography by Frame Reordering

We describe a method to increase the frame rate for 3-dimensional ultrasound sequences of periodically moving cardiac structures by reordering the acquired volume series. The frame rate is especially important in studying intracardiac structures such as valve leaflet motion in which valve closing times are on the order of milliseconds. Current commercially available systems for volumetric ultrasound imaging are limited to approximately 10 to 20 volumes per second. Although this frame rate is sufficient for real-time observation of basic cardiac morphology, understanding cardiac dynamics requires faster frame rates. The presented work achieves higher frame rates by sampling over several beats and using a simultaneous electrocardiography signal to accurately place the frame within the cardiac cycle. The proposed method relies on periodicity of the heart motion and that within the temporal regions of highest velocity, the structural motions of interest have the lowest beat-to-beat variability.

A novel actuated tether design for rescue robots using hydraulic transients

In the world of search and rescue robotics, particularly for search, smaller is better. Small robots can get into tighter places and are more maneuverable. With diminishing size, however, providing adequate power and communications becomes a problem. Communication is problematic if the search site is a collapsed building were transmitted signals have to travel through layers of concrete and steel. Tethers are good for providing power and communication, but tethers get caught and small robots have difficulty with the added drag of the tether. This work proposes a self-actuating tether capable of moving its own weight and remaining free while traversing around corners. Tether motion is due to induced high pressure water transients formed by rapidly arresting flow through the tether. A number of tests performed on a constructed tether prototype are presented. A simplified model of the water transients to better understand design parameters is outlined and simulated, and force measurements are collected to validate the simulation results.

Mitral annulus segmentation from four-dimensional ultrasound using a valve state predictor and constrained optical flow.

Measurement of the shape and motion of the mitral valve annulus has proven useful in a number of applications, including pathology diagnosis and mitral valve modeling. Current methods to delineate the annulus from four-dimensional (4D) ultrasound, however, either require extensive overhead or user-interaction, become inaccurate as they accumulate tracking error, or they do not account for annular shape or motion. This paper presents a new 4D annulus segmentation method to account for these deficiencies. The method builds on a previously published three-dimensional (3D) annulus segmentation algorithm that accurately and robustly segments the mitral annulus in a frame with a closed valve. In the 4D method, a valve state predictor determines when the valve is closed. Subsequently, the 3D annulus segmentation algorithm finds the annulus in those frames. For frames with an open valve, a constrained optical flow algorithm is used to the track the annulus. The only inputs to the algorithm are the selection of one frame with a closed valve and one user-specified point near the valve, neither of which needs to be precise. The accuracy of the tracking method is shown by comparing the tracking results to manual segmentations made by a group of experts, where an average RMS difference of 1.67±0.63mm was found across 30 tracked frames.

Robotic Force Stabilization for Beating Heart Intracardiac Surgery

The manipulation of fast moving, delicate tissues in beating heart procedures presents a considerable challenge to surgeons. We present a new robotic force stabilization system that assists surgeons by maintaining a constant contact force with the beating heart. The system incorporates a novel, miniature uniaxial force sensor that is mounted to surgical instrumentation to measure contact forces during surgical manipulation. Using this sensor in conjunction with real-time tissue motion information derived from 3D ultrasound, we show that a force controller with feed-forward motion terms can provide safe and accurate force stabilization in an in vivo contact task against the beating mitral valve annulus. This confers a 50% reduction in force fluctuations when compared to a standard force controller and a 75% reduction in fluctuations when compared to manual attempts to maintain the same force.