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U.S. Department of Health and Human Services

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FY 2000 Minimally Invasive Technologies


The rapid development of medical devices employing minimally invasive technologies has revolutionized modern health care. Diseases that once required invasive surgery for treatment (and even diagnosis) are now routinely addressed on an outpatient basis. The result 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. In FY 2000, OST investigated a number of high-priority, minimally invasive technologies in order to assist Center reviewers in the timely assessment of manufacturers' submissions of these pioneering products. Included in these technologies are 1) diffuse reflectance spectroscopy for optical diagnosis; 2) optical fibers, waveguides, and endoscopes for treatment; 3) thermal ablation using radio-frequency energy; and 4) ultrasound for imaging-based diagnosis and transdermal treatment. OST’s investigations centered on clarifying the mechanisms of interaction of the technology with the body and on developing meaningful measurement and test methods. OST’s active involvement in these areas of high future regulatory activity have kept it in the lead during a series of discussion with the Society of Vascular Interventional Radiologists over new-technology issues. Additionally, OST’s expertise in diagnostic ultrasound power measurement is such that virtually all diagnostic submissions are reviewed for dosimetric acceptability by OST scientists. This program also contributed to the development of guidance on minimally invasive optical diagnostic devices, provided consultation on the issue of cyanide production from laser ablation of uric acid stones, and contributed to reviews of device applications including fluorescence diagnostic devices and a blood irradiator. Substantial input was also provided for the development of a FDA website on LASIK. Finally, OST has established an interagency agreement with the NIH to study the application of radio-frequency thermal ablation techniques for the treatment of cancerous liver tumors. Through this arrangement, OST answered some of the critical issues in pre-market applications for ablation devices used for the treatment of soft-tissue tumors and in endometrial ablation procedures.

Determination of Tissue Properties Using Minimally Invasive Optical Techniques
Key words: tissue spectroscopy, optical disease diagnosis

This project is designed to evaluate instrumentation and techniques to deduce the optical properties of tissue over a broad spectral range. The equipment and methodology under study are expected to provide data on tissue properties to distinguish changing metabolic conditions or to identify disease. The ability to map the spectral and temporal changes in tissue optical properties is anticipated to have significant advantages over current diagnostic means. This methodology will also aid the characterization of tissue response to optical exposure, which can be used to assist in determining appropriate exposure standards for internal organs. During FY 2000, optical probes for delivering light to, and collecting light from, the target tissue were designed and analyzed. Methods have also been devised to prepare and evaluate phantoms to simulate tissue. OST is currently working to validate the optical properties of the phantoms and then assess the correlation of experimental spatial distribution measurements with Monte Carlo modeling computations.

Determination of Bone Density Using Minimally Invasive Optical Techniques
Key words: bone density, optical spectroscopy

Work was initiated to characterize bone density using near-infrared optical radiation. Initial studies will include development of bone tissue phantoms that have optical properties resembling those of bone. The figure below shows results from a section of swine shinbone cut on the long axis to a thickness of 0.06 cm and measured in a Shimadzu spectrophotometer fitted with an integrating sphere. Reflectance and transmittance values thus obtained were analyzed using the Inverse Adding Doubling program developed at the University of Oregon. The values shown in the figure below for wavelengths below 650 nm compare favorably to literature values.

Figure 5 is a graph of female bovine tibia. There are lines on the graph representing the scattering coefficient in cm-1 and the absorption coefficient in cm-1. The horizontal axis has the wavelength in nm
Figure 5

Evaluation of Endoscopes Used in Minimally Invasive Diagnostic and Therapeutic Procedures
Key words: endoscopes, optical radiation, minimally invasive diagnostics

This project is designed to characterize the optical radiation emissions from endoscopes used in various minimally invasive diagnostic and therapeutic procedures. The data obtained to date indicate that most endoscopes emit relatively little ultraviolet radiation below a wavelength of 370 nm and have greatly attenuated emissions at infrared wavelengths greater than 700 nm. In order to obtain a more comprehensive survey, more devices will be tested in FY 2001. When a variety of endoscopes have been measured, the results will provide CDRH with independent data to be used in developing reviewer guidance documents and will be used as a benchmark for evaluating the optical radiation safety of new endoscopic devices.

Fibers and Waveguides Used for Minimally Invasive Surgery
Key words: optical fibers, fiber transmission

Using funding obtained via an interagency agreement with the Armed Forces Office of Scientific Research, work continued on the testing of infrared fiber optics, ultraviolet optics and x-ray optics designed for medical use. OST continued to evaluate tapered hollow glass tubing for radiation transmission in the ultraviolet and x-ray spectral regions. Tapers have proved valuable for their ability to homogenize the laser beam profile, allowing delivery of uniform energy profiles to phantom materials and tissues. The use of optical fibers with evanescent fiber tips allow laser energy to be accurately delivered to a tissue surface while avoiding the exposure of surrounding tissue. OST has also evaluated selected fiber optics components for use in a confocal imaging system. These instruments could be designed to provide more finely detailed images of tissue through fiber imaging systems used with conventional endoscopes. Data obtained from these studies will develop expertise that will be useful in evaluating optical fiber delivery systems used with medical devices.

Therapeutic Laser Devices Used for Tissue Ablation
Key words: angina, laser therapy, heart disease

Two recently developed procedures for minimally invasive treatment of angina are the subject of intense clinical investigation: transmyocardial laser revascularization (TMR) and percutaneous laser myocardial revascularization (PMR). These procedures use lasers to cut channels in a beating heart to relieve the symptoms of angina. Two recent PMR clinical studies have shown that the mechanism(s) responsible for a therapeutic effect remain in question. Other studies indicate that two mechanisms, angiogenesis and denervation, play a major role in the relief of angina. Both of these latter two mechanisms depend upon the amount of tissue damage that occurs during laser channel formation in the myocardium. OST has initiated a laboratory study to provide data to support developing review guidelines for TMR and PMR. The effort is focusing on laser-tissue ablation performance, as well as acoustic-mechanical performance. These studies will be carried out in a phantom material, polyacrylamide gel. Polyacrylamide gels have been used to study the performance of one of the common lasers, the Ho:YAG laser, that has been used in the clinical studies.

Heat Transfer Issues in Catheter Ablation Devices
Key words: ablation, heat transfer radio frequency

This project is devoted to studying how heat is generated and dissipated in medical devices that use radio-frequency energy sources to destroy (ablate) diseased tissues. OST is one of the few groups examining the physics of ablation in in vitro experimental systems and with computer simulation models. The main objectives of this project are to develop new tools for evaluating ablation devices, to research the physics of the ablation process, and to develop appropriate safety and efficacy guidelines for cardiac arrhythmia and liver tumor ablation devices. OST scientists have developed a new test system that uses both solid and liquid versions of blood and soft tissue-simulating materials with the same electrical properties as real tissues. The system uses laser instrumentation to measure temperature distributions in the tissue-simulating materials during radiofrequency heating with blood flow. This system has been used to manipulate the various parameters affecting the ablation process.

Measurement of Laser-Induced Acoustic Stresses
Key words: Laser-tissue interaction, optical fiber, laser-induced stress, Er:YAG laser

Er:YAG lasers are being studied as candidates for surgical procedures in liquid environments, such as in ophthalmology. For example, the small tissue penetration depth (a few micrometers) along with minimal thermal damage makes the Er:YAG laser an appealing candidate for the removal of epiretinal membranes in vitrectomy. However, the strong absorption that makes the Er:YAG laser such a precise cutting tool is also responsible for thermoelastic and collapse-induced stress transients that must be quantified and evaluated for potential harm. Therefore, OST scientists in collaboration with the Department of Biomedical Engineering at Tel Aviv University have begun a study of Er:YAG laser-induced stress waves. Using a high-fidelity acoustic pressure sensor constructed by OST engineers, measurements were acquired beneath a biological membrane submerged in a saline bath. Results yielded pressures peaks of 300-600 mbar beneath the uncut membrane, which could be harmful for the optic nerve if located directly below the treatment area. Acoustic waves representative of direct laser-liquid interactions were observed immediately following membrane rupture and yielded much larger pressures. These morphological changes in the acoustic wave could be used as a feedback signal to indicate when the membrane has been cut. OST will continue to study potential acoustic damage to the delicate structures of the eye because of the increasing use of lasers for many types of eye surgery. For example, acoustic damage to the cornea’s endothelial cells could lead to long-term problems as the eye ages.