FY 2000 Computational Modeling
Continuing advances in computer power are now making computational modeling a powerful tool for evaluating new devices. Product designers are making increasing use of such modeling in the development and evaluation of new technologies and products. These techniques allow both designers and FDA scientists to manipulate a wide range of variables without having to construct a laboratory bench test mechanism for each possibility. Moreover, a well-optimized complementation of clinical trials with computer modeling holds excellent promise for both reducing costs and increasing the informational value of such trials. In FY 2000, OST investigated these modeling techniques with the goal of assisting manufacturers in the analysis their products through the use of computation methods where cost/benefit advantages exist. Sponsors are increasingly turning to computational modeling to provide answers to problems not answerable by other techniques, and this is particularly true for heart valve technology. In FY00, OST scientists reviewed a number of applications where computational modeling provided the best available estimate of the large transient stresses induced in mechanical heart valves when the valve slams shut. This program also contributed to the development of the ASTM Standard Test Method for Measurement of Radio Frequency Induced Heating near Passive Implants in the Magnetic Resonance Imaging (MRI) Environment and to the review of device applications with claims of MRI compatibility and safety.
MR Compatibility: Evaluation of Patient Heating
Key words: magnetic resonance imaging radiofrequency MRI safety, Implants, SAR, magnetic fields
This project is investigating the use of computer modeling to evaluate the undesirable heating of certain patients undergoing magnetic resonance imaging (MRI) examinations. This heating occurs because of the interaction of metallic implants with the strong radio frequency (RF) magnetic field produced by a MRI device. Commercially available software XFDTD was adapted to model and calculate the rate of RF energy absorption (Specific Absorption Rate or SAR) and the SAR distribution in a realistic model of the human body. The body model contains a metallic implant and is placed in a model of birdcage body coil of a 1.5 Tesla MRI (RF magnetic fields at 64 MHz). The result of extensive computations show that the magnitude of the increased tissue heating due to the presence of the metallic implant depends on the dimensions, the orientation, the shape of the metallic objects, and the location of the metallic implants in the patient. The increased heating of surrounding tissues primarily concentrates in a small volume near the tip of the metallic wire. Scientists obtained a calculated maximum SAR value of 41 W/kg (averaged over one gram of tissue) at this location. However, a maximum value of 310 W/kg was calculated when the absorption is averaged over 0.125 gram of tissue.
Calculation of Virus Transport through Barriers as a Function of Pore Geometry
Key words: virus transmission, transport modeling, computational fluid dynamics, barrier evaluation
When stressed during use, synthetic barriers such as surgical gloves can develop tears that are undetectable by the user. While post-operation tests can detect the presence of holes in the glove, they provide little information regarding how much virus may have been transmitted during use. OST scientists employed a mathematical model to predict levels of virus transmission through a compromised barrier as a function of pore geometry and trans-membrane pressure. It was found that during conditions modeling the manipulation of surgical instruments, up to 300 hepatitis B viruses per second are transmitted through a slit 1 micron high and 4 microns wide. The calculations help CDRH to meaningfully quantify the risk associated with barrier failure.
Computational Studies of Vascular Grafts
Key words: vascular grafts, blood flow, modeling methods
A computational study has begun to determine how vascular prostheses affect blood flow and the concentration of chemicals activated by the prosthetic material. Earlier studies have shown trapped particles at the downstream junction between a graft and artery, which correlates well with the clinical finding of increased tissue overgrowth there. Preliminary computational studies also show an enhanced concentration of dissolved species at this site. This study aims to elucidate factors leading to clinical graft failure and to provide expertise in modeling methods likely to be used in future applications of endovascular grafts, stents, and other cardiovascular devices.