FY 2001 Minimally Invasive Technologies
- Heat Transfer Issues in Catheter Ablation Devices
- Temperature Rise in the Eye Due to Diagnostic Ultrasound
- Low-Frequency Calibration of Medical-Use Hydrophones
- Noninvasive Optical Diagnostic Techniques
- Minimally Invasive Fiber-Optic Biosensors
- Low-Level Laser Therapy
- Lasers for Tissue Ablation
- Tracheal Tubes
The rapid development of medical devices employing minimally invasive technologies has revolutionized modern health care. Diseases that once required invasive surgery for diagnosis and treatment are now routinely addressed on an outpatient basis. The goal has been a reduction in health care costs and an increase in patient safety. In addition, many diseases can now be diagnosed much earlier, resulting in more effective treatment.
OST's Division of Physical Sciences (DPS) is investigating a number of high-priority, minimally invasive technologies in order to assist Center reviewers in the timely assessment of manufacturers' submissions. Included in these technologies are 1) diffuse reflectance spectroscopy for optical diagnosis; 2) optical fibers, waveguides, and lasers for treatment; 3) thermal ablation using radio-frequency energy; and 4) ultrasound for imaging-based diagnosis and transdermal treatment. OST's investigations center on clarifying the mechanisms of interaction of the technology with the body and on developing meaningful performance assessment procedures. Such efforts are designed to identify the critical scientific questions early in the Total Product Life Cycle for new technologies, and to provide reliable ways to answer those questions.
In the area of optical diagnosis, DPS is developing analytic techniques to correlate optical tissue properties with diffuse reflectance data, evaluating fiber optic probes used in optical diagnosis, and developing mathematical models to assist in quantifying the distribution of energy within tissues. DPS is also studying laser therapy devices in order to elucidate the mechanisms of interaction of laser energy with tissue. This program contributes to the development of guidance on minimally invasive optical diagnostic devices and contributes to reviews of device applications for fluorescence diagnostic devices. Substantial contributions have also been provided to the FDA website on LASIK.
With respect to catheter-based thermal ablation, DPS is analyzing the temperature distribution in tissue during radio-frequency ablation for the treatment of cancerous liver tumors. The goals of this research are to 1) develop quantitative methods for establishing the safety of new ablation devices; 2) generate the biophysical data needed to improve the accuracy of models used to predict the size and shape of ablated regions; and 3) determine the relationship between the results of computational models and actual ablation devices. Already, these studies have allowed DPS to identify and resolve some of the critical issues in pre-market applications for ablation devices used for the treatment of soft-tissue tumors as well as those used for endometrial ablation.
Finally, advances in ultrasound technology make it increasingly attractive for both diagnosis and treatment, either through image guidance or through direct delivery of ultrasound energy. Often these new uses require outputs different than previously employed. Virtually all ultrasound submissions are reviewed for dosimetric acceptability by DPS scientists.
Key words: ablation, heat transfer, cardiac, endometrial, oncology, radio frequency
This project is devoted to study how heat is generated and dissipated in medical devices that use radio-frequency (RF) energy to heat and destroy (ablate) diseased tissues. OST examined the physics of ablation in vitro using experimental systems and with computer simulations. The project objectives are to develop new tools for evaluating ablation devices, to determine the biophysics of ablation, and to develop appropriate safety and efficacy guidelines for the treatment of cardiac arrhythmias, liver/kidney tumors, and excessive endometrial bleeding. During FY 2001, OST scientists developed new test systems that use solid and liquid materials with electrical properties that simulate the properties of real tissues. The systems use laser instrumentation to measure temperature distributions in the tissue-simulating materials during radio-frequency heating with blood flow. This system generated independent data that was critical in deciding key issues at the FDA Clinical Panel for OB/GYN devices.
Key words: diagnostic ultrasound, eye safety, temperature rise
Previous work at CDRH showed that diagnostic ultrasound examination of the eye might produce high and possibly harmful temperature rises. Furthermore, it was shown that the TIS, the on-display estimator of temperature used by the FDA Guidance for Diagnostic Ultrasound Systems, can significantly underestimate the actual temperature during ophthalmic examinations. Therefore, the FDA implemented an output intensity limit for all eye exposures (50 mW/cm2), which is lower than that allowed for general soft tissue exposure (720 mW/cm2). This ensures that the temperature rise in the eye will always be below 1° C. This overall very conservative and limiting restriction, however, does not allow for specific cases of transducer size, frequency, and focal length. It also does not allow for cases where, although the actual rise is greater than 1° C, the TIS is greater still and is hence a conservative estimator of eye temperature.
Therefore, a more detailed theoretical analysis of eye temperature rise was performed to determine if a less restrictive set of output limitations could be developed and still ensure safety. It was found that for all types of eye exams, transducer diameters and focal lengths currently used an upper intensity limit of 131 mW/cm2, 137 mW/cm2, 118 mWcm2 and 168 mW/cm2 (for frequencies of 7 MHz, 10 MHz , 15 MHz and 20 MHz, respectively) which provide for safe use of the device. No change in the overall limit (720 mW/cm2) is needed for frequencies of 40 MHz and above. If specific focal lengths are taken into account, intensities may rise to as much as 584 mW/cm2, 634 mW/cm2, 619 mW/cm2, for 7 MHz, 10 MHz, and 15 MHz, respectively. No change would be allowed for the 20-MHz case.
Key words: ultrasound, exposimetry, hydrophone
OST’s ultrasound laboratory is continuing to help develop the test methods and evaluate commercial measurement devices that manufacturers use to assure the safety of their biomedical equipment. Previously, OST engineers have shown that the low-frequency response of medical-use hydrophones is important for accurate pulse measurements, particularly of the peak rarefactional pressure, an important quantity for assessing the likelihood of cavitation onset. The Mechanical Index, a related quantity displayed on diagnostic ultrasound equipment, gives an indication of the potential for mechanical damage to exposed tissues. However, frequency response data below 1-2 MHz typically are not provided for commercial hydrophones designed for measurements in medical ultrasound fields.
To address this problem, OST developed a calibration technique in which broadband pressure pulses are used to measure the low-frequency response (0.2-2 MHz) of miniature hydrophones used in biomedical ultrasonics. A chief advantage of the technique is its ability to obtain broadband calibration data in a single measurement, as opposed to more time consuming single-frequency methods. However, expensive commercially available electronics are required for generating the high-voltage pulses needed, a disadvantage that makes the technique unsuitable for adoption by voluntary standards organizations. Therefore, OST has developed a new and economical voltage pulser that meets or exceeds the performance of the commercial device in this application. An initial design analysis has been published, and the final design is being completed for distribution to relevant standards setting organizations.
Key words: spectroscopy, mathematical modeling
Accurate in vivo optical property data are essential for understanding light propagation in tissue and for developing optical diagnostic techniques such as fluorescence spectroscopy. However, such information is scarce for many internal organs. Several studies for determining tissue optical properties have demonstrated the efficacy of diffuse reflectance measurements with illumination-collection fiber separations of up to tens of millimeters. But, these large separation distances are difficult to achieve for illumination probes used with gastrointestinal endoscopes. In FY 2001, OST scientists performed a preliminary study analyzing an optical property measurement system for the 300-750 nm wavelength range that uses small fiber separation distances. The approach involved the use of a neural network trained on radial profiles of diffuse reflectance generated by Monte Carlo simulations (incorporating accurate fiber apertures) to determine absorption and reduced scattering coefficients from experimental reflectance data. Results from 45 Monte Carlo simulations covering absorption coefficients from 0.1 to 30 cm-1 and reduced scatting coefficients from 5 to 25 cm-1 were used to train the neural network. Measurements were performed at a wavelength of 633 nm using solutions of Intralipid and ink as well as human skin tissue in vivo. Initial results indicated moderate agreement with optical property data in the literature. Simulation results indicated that increases in scattering and absorption accentuate the decay of radial reflectance distributions. Thus, the optimal fiber separation distances for determination of optical properties in the 300-750 nm wavelength regions are likely smaller than those which are optimal for near-infrared measurements performed in many previous studies. Simulations also indicate that errors of 10% or more in predicted reflectance can occur by not accounting for fiber aperture. Systems may also benefit from measuring light returning through the illumination fiber, a technique that has not been previously tested.
Key words: optical fibers, laser delivery, biosensors
OST scientists are continuing to investigate techniques for the delivery of laser radiation to tissues for optical diagnostic procedures. Optical fibers are becoming more useful in modern biomedical systems such as minimally invasive techniques for laser diagnostics, therapy, and optical imaging. A fiber optic-based biosensor system includes two principal optical components: an effective laser delivery system and a sensitive sensor probe. In FY 2001, OST scientists worked to improve fundamental features of both optical-fiber biosensor components.
One area of study focused on the evaluation of an optical waveguide concept for efficient delivery of laser and x-ray radiation. It is based on a simple lens-free method for coupling laser radiation to optical delivery fibers. OST scientists utilized an uncoated glass hollow taper as a laser-to-fiber coupler. It is funnel-shaped and, utilizing the grazing-incidence effect, provides an efficient way of direct launching of laser radiation into delivery fibers.
A second area of study involved evaluating a novel approach for switching laser radiation on and off while being delivered into a precise tissue area. The method uses tissue activated optical fiber probes with specially shaped angled tips. It provides a safe method for laser delivery that includes only two states of tissue illumination: (1) off-state (no tissue illumination), when the fiber tip is out of the tissue area – the laser emission is back reflected at the angled tip due to total-internal reflection; and (2) on-state (tissue illumination), when the fiber tip is contacting the absorbing tissue area, and the laser energy is coupled into the absorber.
Key words: laser therapy, biostimulation
The use of lasers for the therapeutic treatment of disease is growing. One type of photochemical laser application is known as photodynamic therapy. In this application a drug is first administered, and a selected wavelength of laser light is used to trigger a reaction that kills cancerous cells or stops a normally progressive disease. More people are now utilizing another photochemical application known as laser therapy (biostimulation or low-level laser therapy). This therapy does not use drugs but relies only on application of laser light to tissue. There is little knowledge of how these devices work to diminish pain or to accelerate wound healing. In trying to determine the mechanism of interaction of low levels of laser energy with tissues, OST scientists initiated a collaboration with the Uniformed Services University for the Health Sciences to use minimally invasive fiber optic sensors in an attempt to identify changes induced in tissue by laser light. This work will continue in FY 2002.
Key words: laser, ablation, cardiology
Lasers are increasingly being used to treat diseases via tissue ablation. Except for the excimer laser used in ophthalmology for photorefractive surgery, there are no guidelines for measuring the ablative performance of a laser device. In FY 2001, OST scientists began experiments to study the ablative performance of the laser devices used in percutaneous myocardium revascularization (PMR) and transmyocardial revascularization (TMR). There is a need to understand the mechanism(s) of the procedure and the science of the ablation process, as well as to develop guidelines for future PMA submissions. The therapeutic mechanism is not known, although a number of potential effects have been postulated. Depending upon the chosen mechanism, the ablative performance may directly influence the laser-tissue interaction and ultimately the patient’s outcome. It is also important not to perforate the myocardium during PMR procedures. It is therefore critically important that the ablation rate be accurately known, so that during a PMR procedure the amount of laser radiation used will not cause a perforation. There are a number of different laser devices being investigated for TMR and PMR, including Ho:YAG, CO2, and excimer lasers. Each has different laser-tissue interactions. Subsequent changes to these laser devices will require the presentation of additional laser ablation data, in order to substantiate their equivalence without additional clinical trials.
OST scientists are continuing to develop laboratory phantoms that will ablate in a manner similar to swine myocardium. These phantoms may be made from swine and bovine skin, or polyacrylamide, or swine myocardium. Literature studies have shown that the ablative performance of pulsed laser devices depends upon the mechanical properties of the target tissue. The use of liquid plastics whose hardness or strength can be easily changed and molded into different shapes is being studied in order to simulate various tissues. In FY 2002, the mechanical properties of the phantom materials will be established, and ablation performance studies will begin. From these results, ablative performance guidelines will be developed which specify the ablative phantom’s strength. It is expected that this will greatly reduce the variability in ablation performance that is currently found using heterogeneous animal tissues.
Key words: lasers, tracheal tubes, medical devices, patient protective covers, standards
In FY 2001, an OST representative was appointed as U.S. Delegation Leader for ISO TC 172/SC 9. This subcommittee continues to work on modifying the standard for determining the laser resistance of the shafts of tracheal tubes. When the standard was published in 1999, it was decided that a New Work Item [NWI] would be initiated to address comments which had been received when the document was circulated as a Draft International Standard [DIS]. The revised document is now at the stage of a DIS and will be circulated as a Final Draft International Standard [FDIS] in FY 2002. Work is also progressing on a standard for the laser resistance of surgical drapes and patient protective covers. The project group prepared a NWI that was circulated and approved as a DIS in the first half of FY 2001. The document will be circulated as an FDIS in FY 2002.