Steven L. Dawson

Tissue ablation with radiofrequency: Effect of probe size, gauge, duration, and temperature on lesion volume

RATIONALE AND OBJECTIVES We evaluated the parameters affecting the size and distribution of thermal tissue damage produced by radiofrequency electrodes. METHODS Thermal lesions were produced by electrodes connected to a radiofrequency generator in specimens of liver (n = 143) and muscle (n = 20). Various combinations of probe tip exposure (0.5-8 cm), gauge (12-24 gauge), duration of treatment (0.5-12 min), and temperature (80-90 degrees C) were studied. The resulting volumes of tissue coagulation were measured and compared. RESULTS Lesions equal to or less than 1.6 cm in diameter were symmetrically distributed around the electrode. Lesion diameter (but not length) increased with probe gauge and duration of treatment to a maximum of 6 min. However, lesions with mean diameters larger than 1.6 cm could not be produced using a single probe with any technique. Lesion length correlated with probe tip exposure from 1 to 8 cm (r2 = .996). Over the limited range investigated, increased temperature had minimal effects, except for tip exposures greater than 5 cm, in which larger and more uniform lesions resulted. Lesions varied equal to or less than 3 mm in diameter and equal to or less than 5 mm in length for each combination of variables. CONCLUSION Radiofrequency ablation can accurately and reproducibly cause coagulative tissue necrosis. Necrosed tissue volume increases with length of exposed probe tip, larger probes, and sessions lasting at least 6 min.

Truth cube: Establishing physical standards for soft tissue simulation

Accurate real-time models of soft tissue behavior are key elements in medical simulation systems. The need for fast computation in these simulations, however, often requires simplifications that limit deformation accuracy. Validation of these simplified models remains a challenge. Currently, real-time modeling is at best validated against finite element models that have their own intrinsic limitations. This study develops a physical standard to validate real-time soft tissue deformation models. We took CT images of a cube of silicone rubber with a pattern of embedded Teflon spheres that underwent uniaxial compression and spherical indentation tests. The known material properties, geometry and controlled boundary conditions resulted in a complete set of volumetric displacement data. The results were compared to a finite element model analysis of identical situations. This work has served as a proof of concept for a robust physical standard for use in validating soft tissue models. A web site has been created to provide access to our database: http://biorobotics.harvard.edu/truthcube/ (soon to be http://www.truthcube.org).

Designing a computer‐based simulator for interventional cardiology training

Interventional cardiology training traditionally involves one‐on‐one experience following a master‐apprentice model, much as other procedural disciplines. Development of a realistic computer‐based training system that includes hand‐eye coordination, catheter and guide wire choices, three‐dimensional anatomic representations, and an integrated learning system is desirable, in order to permit learning to occur safely, without putting patients at risk. Here we present the first report of a PC‐based simulator that incorporates synthetic fluoroscopy, real‐time three‐dimensional interactive anatomic display, and selective right‐ and left‐sided coronary catheterization and angiography using actual catheters. Significant learning components also are integrated into the simulator. Cathet. Cardiovasc. Intervent. 51:522–527, 2000.

Metrics for Laparoscopic Skills Trainers: The Weakest Link!

Metrics are widely employed in virtual environments and provide a yardstick for performance measurement. The current method of defining metrics for medical simulation remains more an art than a science. Herein, we report a practical scientific approach to defining metrics, specifically aimed at computer-assisted laparoscopic skills training. We also propose a standardized global scoring system usable across different laparoscopic trainers and tasks. The metrics were defined in an explicit way based on the relevant skills that a laparoscopic surgeon should master. We used a five degree of freedom device and a software platform capable of 1) tracking the motion of two laparoscopic instruments 2) real time information processing and feedback provision. A validation study was performed. The results show that our metrics and scoring system represent a technically sound approach that can be easily incorporated in a computerized trainer for any task, enabling a standardized performance assessment method.

The imperative for medical simulation

The practice of medicine has, for millennia, relied upon a master-apprentice system of learning, with patients providing the necessary anatomy from which one learns how to perform surgery and other procedures. The advent of high-power computing and real-time graphics representations allows medicine to advance beyond this traditional methods of teaching and to begin to educate physicians without putting patients at risk. With innovative haptics interface devices, computer-based training will enable novice physicians to learn procedures that have been developed since their training was completed. Specialty boards and credentialing organizations will, for the first, time, have metrics upon which to base the decisions regarding who is qualified to practice medicine, and both sides of the learning curve, the acquisition of skills and their deterioration, will be discovered. The paper presents the concepts, challenges, and visions of the authors, both of whom have been actively developing simulation for the specialty of interventional radiology. It includes expectations for the future of simulation in other procedural specialties.

The Effects of Testing Environment on the Viscoelastic Properties of Soft Tissues

Mechanical properties of biological tissues are needed for accurate surgical simulation and diagnostic purposes. These properties change postmortem due to alterations in both the environmental and physical conditions of the tissue. Despite these known changes, the majority of existing data have been acquired ex vivo due to ease of testing. This study seeks to quantify the effects of testing conditions on the measurements obtained when testing the same tissue in the same locations with two different instruments over time. We will discuss measurements made with indentation probes on whole porcine livers in vivo, ex vivo with a perfusion system that maintains temperature, hydration, and physiologic pressure, ex vivo unperfused, and untreated excised lobes. The data show >50% differences in steady state stiffness between tissues in vivo and unperfused, but only 17% differences between in vivo and perfused tests. Variations also exist in the time-domain and frequency domain responses between all test conditions.

Computer-enhanced laparoscopic training system (CELTS): bridging the gap

BackgroundThere is a large and growing gap between the need for better surgical training methodologies and the systems currently available for such training. In an effort to bridge this gap and overcome the disadvantages of the training simulators now in use, we developed the Computer-Enhanced Laparoscopic Training System (CELTS).MethodsCELTS is a computer-based system capable of tracking the motion of laparoscopic instruments and providing feedback about performance in real time. CELTS consists of a mechanical interface, a customizable set of tasks, and an Internet-based software interface. The special cognitive and psychomotor skills a laparoscopic surgeon should master were explicitly defined and transformed into quantitative metrics based on kinematics analysis theory. A single global standardized and task-independent scoring system utilizing a z-score statistic was developed. Validation exercises were performed.ResultsThe scoring system clearly revealed a gap between experts and trainees, irrespective of the task performed; none of the trainees obtained a score above the threshold that distinguishes the two groups. Moreover, CELTS provided educational feedback by identifying the key factors that contributed to the overall score. Among the defined metrics, depth perception, smoothness of motion, instrument orientation, and the outcome of the task are major indicators of performance and key parameters that distinguish experts from trainees. Time and path length alone, which are the most commonly used metrics in currently available systems, are not considered good indicators of performance.ConclusionCELTS is a novel and standardized skills trainer that combines the advantages of computer simulation with the features of the traditional and popular training boxes. CELTS can easily be used with a wide array of tasks and ensures comparability across different training conditions. This report further shows that a set of appropriate and clinically relevant performance metrics can be defined and a standardized scoring system can be designed.

Palliation of Malignant Bile Duct Obstruction with Metallic Biliary Endoprostheses: Technique, Results, and Complications☆

Expandable metallic stents were placed in 34 patients with pathologically proved malignant bile duct obstruction to determine ease of insertion, benefits of a one-stage insertion, and cost-effectiveness relative to conventional plastic stents. Thirty-eight strictures, ranging in length from 1 to 7 cm (mean, 3.2 cm), were present in the 34 patients. Strictures were located in the lower common bile duct (n = 22), middle of the common bile duct (n = 6), and hilar confluence (n = 10). In 13 patients (38%) metallic stents were placed at the time of initial biliary drainage (one-stage procedure), while the remaining patients underwent stent placement within 1-7 days of biliary drainage (two-stage procedure). Biliary obstruction was relieved in 31 of 34 patients (91%). Three patients died within 14 days of stent insertion of unrelated causes, without any change in biliary status. Mean duration of follow-up for all patients was 5.3 months (range, 0.5-14 months). Four episodes of stent occlusion occurred in three patients (12% occlusion rate); each episode was treated successfully. The average length of hospital stay for patients who underwent a one-stage procedure was 13 days (range, 3-33 days) and was 20 days (range, 9-42 days) for patients who underwent a two-stage procedure. The facility of one-step insertion, low occlusion rate, and the many strategies available for treatment of occluded stents make metallic stents an attractive alternative to conventional plastic stents in palliating patients with malignant biliary obstruction.

Thirty Years' Experience with Balloon Dilation of Benign Postoperative Biliary Strictures: Long-term Outcomes

PURPOSE To determine the effectiveness of percutaneous balloon dilation of benign postoperative biliary strictures. MATERIALS AND METHODS We received approval from our institutional review board to undertake this retrospective HIPAA-compliant study, and informed consent was waived. From April 1, 1977, to April 1, 2007, percutaneous biliary balloon dilation (PBBD) was performed in 85 patients with benign biliary strictures. In the 75 patients with follow-up (31 male, 44 female; mean age, 56 years; mean follow-up, 8 years), 205 PBBD procedures were performed during 112 treatments of 84 biliary strictures. PBBD of the stricture was performed with a noncompliant balloon (8-12-mm diameter). PBBD procedures were repeated at 2- to 14-day intervals until cholangiography demonstrated free drainage of contrast material to the bowel and no residual stenosis. An internal-external biliary drain was left in situ for a mean of 14-22 days and removed after a clinical trial of catheter clamping and a normal cholangiogram. RESULTS All procedures were technically successful, and 52, 11, 10, and two patients underwent a total of one, two, three, and four PBBD treatments, respectively. Four of 205 procedures (2%) led to major complications: two subphrenic abscesses, one hepatic arterial pseudoaneurysm, and one case of hematobilia treated with transfusion. Six patients died from unrelated causes and three from hepatitis C-related liver failure. The probability of a patient not developing clinically significant restenosis at 5, 10, 15, 20, and 25 years was 0.52, 0.49, 0.49, 0.41, and 0.41, respectively, after the first PBBD treatment and 0.43, 0.30, 0.20, 0.20, and 0.20, respectively, after the second PBBD treatment. No significant difference was found in the rate of clinically significant restenosis after the first PBBD between strictures at anastomotic and nonanastomotic sites (P = .75). During the follow-up period, 56 of 75 patients (75%) had successful management with PBBD. CONCLUSION PBBD of benign strictures demonstrates long-term effectiveness. No significant difference was found in the rate of clinically significant restenosis after PBBD of biliary strictures at anastomotic and nonanastomotic sites.

Procedural Simulation: A Primer

Procedural simulation will be a revolutionary change in how health care providers maintain their proficiency and skill. Visionary leaders are already examining how this educational technique will be integrated into traditional curricula, and interventional specialties will be at the leading edge of this revolution. The role must be defined that simulation will play, new educational models must be developed around it, and studies must be performed that will meet the demands of a skeptical profession. Only then will the first truly revolutionary change in medical learning have been achieved since the advent of animal experimentation nearly 1,000 years ago.