Martin O. Culjat

A Review of Tissue Substitutes for Ultrasound Imaging

The characterization and calibration of ultrasound imaging systems requires tissue-mimicking phantoms with known acoustic properties, dimensions and internal features. Tissue phantoms are available commercially for a range of medical applications. However, commercial phantoms may not be suitable in ultrasound system design or for evaluation of novel imaging techniques. It is often desirable to have the ability to tailor acoustic properties and phantom configurations for specific applications. A multitude of tissue-mimicking materials and phantoms are described in the literature that have been created using a variety of materials and preparation techniques and that have modeled a range of biological systems. This paper reviews ultrasound tissue-mimicking materials and phantom fabrication techniques that have been developed over the past four decades, and describes the benefits and disadvantages of the processes. Both soft tissue and hard tissue substitutes are explored.

THz Medical Imaging: in vivo Hydration Sensing

The application of THz to medical imaging is experiencing a surge in both interest and federal funding. A brief overview of the field is provided along with promising and emerging applications and ongoing research. THz imaging phenomenology is discussed and tradeoffs are identified. A THz medical imaging system, operating at ~525 GHz center frequency with ~125 GHz of response normalized bandwidth is introduced and details regarding principles of operation are provided. Two promising medical applications of THz imaging are presented: skin burns and cornea. For burns, images of second degree, partial thickness burns were obtained in rat models in vivo over an 8 hour period. These images clearly show the formation and progression of edema in and around the burn wound area. For cornea, experimental data measuring the hydration of ex vivo porcine cornea under drying is presented demonstrating utility in ophthalmologic applications.

Tactile Feedback Induces Reduced Grasping Force in Robot-Assisted Surgery

Robot-assisted minimally invasive surgery has gained widespread use over the past decade, but the technique is currently operated in the absence of haptic feedback during tissue manipulation. We have developed a complete tactile feedback system, consisting of a piezoresistive force sensor, control system, and pneumatic balloon tactile display, and mounted directly onto a da Vinci surgical robotic system. To evaluate the effect of tactile feedback on robotic manipulation, a group of novices (n = 16) and experts ( n = 4) were asked to perform three blocks of peg transfer tasks with the tactile feedback system in place. Force generated at the end-effectors was measured in all three blocks, but tactile feedback was active only during the middle block. All subjects used higher force when the feedback system was inactive. When active, subjects immediately used substantially less force and still maintained appropriate grip during the task. After the system was again turned off, grip force increased significantly to prefeedback levels. These results demonstrate that robotic manipulations without tactile feedback are done with more force than needed to grasp objects. Therefore, the addition of tactile feedback allows the surgeon to grasp with less force, and may improve control of the robotic system and handling of tissues and other objects.

A Haptic Feedback System for Lower-Limb Prostheses

A haptic feedback system has been developed to provide sensory information to patients with lower-limb prostheses or peripheral neuropathy. Piezoresistive force sensors were mounted against four critical contact points of the foot to collect and relay force information to a system controller, which in turn drives four corresponding pneumatically controlled balloon actuators. The silicone-based balloon actuators were mounted on a cuff worn on the middle thigh, with skin contacts on the posterior, anterior, medial, and lateral surfaces of the thigh. Actuator characterization and human perceptual testing were performed to determine the effectiveness of the system in providing tactile stimuli. The actuators were determined to have a monotonic input pressure-vertical deflection response. Six normal subjects wearing the actuator cuff were able to differentiate inflation patterns, directional stimuli and discriminate between three force levels with 99.0%, 94.8%, and 94.4% accuracy, respectively. With force sensors attached to a shoe insole worn by an operator, subjects were able to correctly indicate the movements of the operator with 95.8% accuracy. These results suggest that the pneumatic haptic feedback system design is a viable method to provide sensory feedback for the lower limbs.

Terahertz sensing in corneal tissues.

This work introduces the potential application of terahertz (THz) sensing to the field of ophthalmology, where it is uniquely suited due to its nonionizing photon energy and high sensitivity to water content. Reflective THz imaging and spectrometry data are reported on ex-vivo porcine corneas prepared with uniform water concentrations using polyethylene glycol (PEG) solutions. At 79% water concentration by mass, the measured reflectivity of the cornea was 20.4%, 14.7%, 11.7%, 9.6%, and 7.4% at 0.2, 0.4, 0.6, 0.8, and 1 THz, respectively. Comparison of nine corneas hydrated from 79.1% to 91.5% concentration by mass demonstrated an approximately linear relationship between THz reflectivity and water concentration, with a monotonically decreasing slope as the frequency increases. The THz-corneal tissue interaction is simulated with a Bruggeman model with excellent agreement. THz applications to corneal dystrophy, graft rejection, and refractive surgery are examined from the context of these measurements.

20-, 23-, and 25-gauge vitreous cutters: performance and characteristics evaluation.

Purpose: The flow rate of nine vitreous cutters was tested in two different viscosity environments. Physical features of each probe, such as flexibility and internal lumen diameter, were measured. Methods: For each probe, a total of 26 aspiration tests were performed in two different liquids. The stiffness of the shaft of each probe was calculated by measuring the displacement of the tip of the probe under a known constant force. All the probes were dissected and the internal lumen diameter was measured. Results: Significant differences in flow rates were observed among the three separate conditions, among the three different gauge groups (25-, 23-, and 20-gauge vitreous cutter), and also within each gauge group. Internal lumen diameters and probe stiffness characteristics are presented. Conclusion: Vacuum, internal lumen diameter, and duty cycle seem to be the critical features governing actual flow rate. Knowledge of flow rate and stiffness of the various probes may aid surgeons in making an educated choice and may improve patient care.

Feasibility study of intraocular robotic surgery with the da Vinci surgical system.

Purpose: To assess the feasibility of performing intraocular robotic surgery with the da Vinci Surgical System (Intuitive Surgical, Sunnyvale, CA). Methods: Using modified robotic instruments, 25-gauge pars plana vitrectomy, intraocular foreign body removal, and anterior capsulorhexis were performed with the da Vinci system on porcine eyes. We assessed the surgical system’s ability to provide the control, dexterity, maneuverability, and visualization necessary for intraocular surgery. Results: Control of the robotic wristlike instruments allowed for full range of movement. The dexterity of the robotic arms was excellent, with steady instrument motion. Controlling the robotic arms was not as intuitive as moving the wrist. A high stable point of rotation induced motion-related stress at the site of instrument insertion. Visualization of the external operative field during intraocular procedures required camera realignment, and absent retroillumination made anterior segment surgery hard to perform. Conclusions: The da Vinci Surgical System provided adequate dexterity for performing delicate intraocular manipulations. In the current design, the kinematics of the robotic arms was found to be insufficient for standard intraocular surgery. The system’s endoscope did not did not yield the same detail acquired by an ophthalmic microscope.

A Multielement Tactile Feedback System for Robot-Assisted Minimally Invasive Surgery

A multi-element tactile feedback (MTF) system has been developed to translate the force distribution, in magnitude and position, from 3times2 sensor arrays on surgical robotic end-effectors to the fingers via 3times2 balloon tactile displays. High detection accuracies from perceptual tests (> 96%) suggest that MTF may be an effective means to improve robotic control.

Imaging of human tooth enamel using ultrasound

This paper reports the results of a complete circumferential scan of a human tooth and its underlying dentino-enamel junction using ultrasound at frequencies in the 10-MHz range. The imagery shows clearly a two-dimensional contour of the dentino-enamel junction with a depth and lateral resolution of approximately 100 /spl mu/m and 750 /spl mu/m, respectively. The resulting sonograph is compared with an optical micrograph of the same tooth to verify the accuracy of the ultrasonic technique. The results are a significant step toward the biolocation of submillimeter size features within the tooth volume.

In vivo terahertz imaging of rat skin burns

A reflective, pulsed terahertz (THz) imaging system was used to acquire high-resolution (d(10-90)/λ~1.925) images of deep, partial thickness burns in a live rat. The rats abdomen was burned with a brass brand heated to ~220°C and pressed against the skin with contact pressure for ~10 sec. The burn injury was imaged beneath a Mylar window every 15 to 30 min for up to 7 h. Initial images display an increase in local water concentration of the burned skin as evidenced by a marked increase in THz reflectivity, and this likely correlates to the post-injury inflammatory response. After ~1 h the area of increased reflectivity consolidated to the region of skin that had direct contact with the brand. Additionally, a low reflecting ring of tissue could be observed surrounding the highly reflective burned tissue. We hypothesize that these regions of increased and decreased reflectivity correlate to the zones of coagulation and stasis that are the classic foundation of burn wound histopathology. While further investigations are necessary to confirm this hypothesis, if true, it likely represents the first in vivo THz images of these pathologic zones and may represent a significant step forward in clinical application of THz technology.