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Key words: ultrasound, cataracts, phacoemulsification
Metal particulate is often left in the eye after cataract surgery. The apparent source of the particulate is the surgical tool used to destroy and aspirate the lens. In one million such procedures each year, the tool commonly used is an ultrasound surgical tool known as a phacoemulsifier. Phacoemulsification allows removal of the cataractous lens through a smaller opening in the eye than other methods allow.
The destruction of the lens, however, may also result in damage to the device. The device sends strong ultrasonic waves to a hollow needle which vibrates near or within the lens. As the lens is broken into small pieces by the vibrations, the lens components are aspirated through the needle. The needle vibrations are strong enough to generate both transient and stable cavitation (bubbles which vibrate and collapse) which probably do most of the damage to the lens and which may also cause removal of material from the needle itself. The needles are often sterilized and reused a number of times, presumably leading to a greater susceptibility for damage.
After OSB-led Center Roundtables on the problem, an ad hoc committee was formed to develop further information concerning the problem and possible solutions. Manufacturers were enlisted to provide equipment to OST for investigating device performance. Testing to date has included evaluation of the acoustic output of one device, measurements of cavitation intensities for a range of device output powers, flow visualization of the irrigation and aspiration process, and metallurgical evaluation of a needle tip which showed visible damage. Figure 37 shows a video image of the bubbles formed near the irrigation sleeve along the needle. Current plans are to collect irrigation fluids under a variety of test conditions to evaluate the propensity of two manufacturers' devices to shed particulate. Both the particulate and the needles will be examined and the results compared with the acoustic and cavitation data to establish mechanisms and potential solutions to the problem of particulate left in the eyes of patients. These results will go back to the ad hoc Committee and be communicated to the manufacturers with the hope that design changes can be developed which will limit particulate formation while not limiting device performance. Also, the test methods developed here will have impact on the IEC standard for ultrasonic surgical devices. [PostMS]
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Key words: ultrasound, thermal effects, eye
Quantifying possible thermal effects during ultrasound exams is an important part of any diagnostic ultrasound submission to FDA for marketing clearance. FDA's guidance document for diagnostic ultrasound submissions uses the concepts within the American Institute of Ultrasound in Medicine/National Electrical Manufacturers Association (AIUM/NEMA) Output Display Standard (ODS). The ODS requires that diagnostic ultrasound devices display thermal indices when these indices are above a certain level. These indices mark the maximum steady state tissue temperature which could be reached during an examination. To calculate these index levels, models have been developed for ultrasound impinging on uniform soft tissue, fetal bone, and adult skull bone. The soft tissue thermal index, however, may not be relevant with respect to eye exposure, where the small ultrasound transducers and high frequencies used could produce very different temperatures than predicted.
Therefore, a theoretical study of temperature rise within the eye upon insonation was undertaken. At each plane in the direction of propagation, the focused ultrasound beam was modeled as a disk of uniform intensity. Each disk becomes a heat source, with the strength of the source depending on the beam radius and absorption characteristics of the eye structure at each particular plane. Integration over all disks provides the total temperature rise at any axial position. Calculations were done both assuming the ultrasound beam intersects the lens of the eye and assuming the beam does not intersect the lens. Both situations can be encountered during an ultrasound examination.
Results were determined for the clinically relevant frequencies of 10 MHz and 20 MHz, transducer radii of 0.25 cm and 0.5 cm, and focal lengths ranging from 0.5 cm to 3.0 cm. Perfusion was assumed to be negligible. Geometrical, thermal and acoustic parameters of the eye were taken from the literature. For every case, the ratio of maximum temperature rise to the soft tissue thermal index was calculated. For insonation through the lens, the ratio varied from 5.6 to 0.4. For insonation avoiding the lens, the ratio varied from 2.5 to 0.2.
These results indicate that in some cases the soft tissue thermal index will not be adequate to represent the temperature rise occurring within the eye upon insonation. This additional model, specific to ocular structures, will be developed further, discussed with industry and the standards-setting organizations, and incorporated into the FDA guidance. [PreME, PostMS, Stds]
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Key words: ultrasound, metrology, hydrophone
OST's ultrasound laboratory is continuing to help develop the test methods that manufacturers need to assure the safety of their diagnostic equipment. Previously, OST engineers determined that hydrophones, the instruments used to measure the amplitude of an ultrasound wave, could underestimate the Output Display Standard Mechanical Index by as much as 30%. This index, which is displayed on the equipment, gives an indication of the potential for mechanical damage to exposed tissues. The cause for this error is inadequate low frequency response in the hydrophone or associated amplifier, particularly below 1 MHz. However, at present, no commercial hydrophones provide sensitivity information below this frequency.
One of the problems associated with assuring an adequate low frequency response for hydrophones is the lack of available calibration techniques below 1 MHz. Therefore, OST engineers have begun developing a technique for efficient calibration below 1 MHz using broadband, plane wave pulses generated by electrical shock excitation of thick piezoceramic transducers. One important aspect of this technique is the lower frequency limit for which it is useful. Therefore, the frequency response was analyzed theoretically for a thickness-expansion drive transducer. In this analysis two factors were found that limit the useful range of the technique at low frequencies. The first is related to the negative capacitance that appears in the equivalent circuit of a thickness expander. This element can be ignored in some transducer applications, but its presence must be accounted for in the low frequency case. The second factor is due to the finite measurement time available, which for small transmitter/receiver separation distances is equal to the acoustic transit time through the piezoelectric disk. OST engineers determined that for the practical case of an air-backed, 2.5 cm-thick lead zirconate titanate transducer (Navy Type I) radiating into water, the lower limit considering both factors is approximately 200 kHz. This limit is acceptable for determining the frequency response of hydrophones used in diagnostic ultrasound field measurements, but it is insufficient for characterizing hydrophones used to measure extracorporeal shockwave lithotripsy pressure pulses.
After publishing these results, the ultrasound laboratory will concentrate on measuring the low-frequency performance of commercial hydrophones now in use for characterizing the acoustic output of diagnostic ultrasound devices. This will be done with the goal of developing the rationale for revising both FDA guidance and U.S. standards with respect to acceptable hydrophone low frequency response limits. [PreME, PostMS, Stds, ProA]
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Key words: Doppler, phantom, signals
ODE's guidance to ultrasound manufacturers requests data establishing that Doppler ultrasound can accurately measure blood velocity. Doppler accuracy is the only effectiveness data required in ultrasound imaging submissions. OST is developing a phantom to assist industry in making such accuracy determinations. Accuracy testing of pulsed Doppler in a preliminary model of a torus phantom showed spurious signals. Design modifications eliminated those signals. Preliminary accuracy tests of color and pulsed Doppler were performed at velocities up to 170 cm/s (figure 38). These studies showed possible effects of the ultrasound instrument's wall filter frequency on the color Doppler accuracy. The filter is used to remove low velocities to prevent small movements of solid tissues such as the heart muscle from being confused with flowing blood. The velocity measured by color Doppler was quite accurate when a 100 Hz wall filter was used (figure 39). However, when a 600-Hz wall filter was used, the Doppler measured velocity shifted upward by about 13 cm/s. This may be because random fluctuations are biased to a higher velocity when the higher wall filter is used. Results validating the phantom for Doppler accuracy testing will be made available to industry through publication in the scientific literature. [ProA, PreME]
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Model Development
Key words: ultrasound, bioeffects, stress proteins, biomarker, cardiac function, avian embryo
In obstetrics, the application of ultrasound covers a broad spectrum. During pregnancy, ultrasound is used alone to visualize and monitor fetal development, and it is used as an adjunct with other procedures, such as amniocentesis, to determine the well being of the developing fetus. The rapid development and expansion in the use of diagnostic ultrasound has been largely due to the perceived medical benefits of this technology. In parallel, the expanded clinical utility raises new questions into the potential adverse effects associated with ultrasound exposure. Ultrasound has been shown to produce alterations in cardiac function in the developing chicken heart. Studies are being designed, in collaboration with Children's National Medical Center, to determine if alterations in genetic expression are associated with these ultrasound-induced changes in cardiac function and if changes in genetic expression can be used as a sensitive indicator of cardiac damage. A major question to be discerned is whether ultrasound bioeffects, if any, result from heat generation or factors independent of heat.
Initial studies have been conducted using the chick embryo as a model to evaluate potential ultrasound bioeffects. Heat was used as a positive control in order to determine the "thermal dose" required for the alteration of gene expression in developing embryos. Studies are being conducted to compare the effects of direct heating to the effects of ultrasound by examining the expression of stress proteins and cardiac function in 72-hour chick embryo. Heat-shocked embryos were placed in a water bath set at 37, 40, 41, 42, 43, 44, or 45oC for 5, 20, 40, or 60 minutes. Ultrasound treated chicks were exposed for 1 hour to 0 (sham), 50, 100, 250 or 500 mW/cm2 (ISATA) continuous wave ultrasound in the near field. Embryos were filmed before and after all exposures for functional analysis and were then labeled with 35S-methionine and the proteins separated by SDS-PAGE. A comparison of control and heat-treated embryos showed the enhanced de novo synthesis of three proteins with the molecular weights of 21kD, 74kD, and 94kD. All three of the induced proteins demonstrated a dose-dependent response to temperature and exposure duration. Western blots probed with monoclonal antibodies generated against heat shock proteins (hsp) and glucose regulated proteins (grp) were used to identify the bands as hsp25, hsp70, and grp94 respectively. Ultrasound treatment at 500mW/cm2 produced an increase in embryonic temperature to 43.5oC. Lower intensities were not found to cause heating of the embryo by more than 2oC. However, intensities of both 250 and 500 mW/cm2 did enhance the synthesis of the same three proteins as were induced by heat alone. Preliminary cinephoto-analysis demonstrated that ventricular shortening fraction was decreased by heat at a temperature of 45oC, but was unaffected at the lower temperatures. In contrast, shortening fraction was decreased by ultrasound at intensities of 100, 250, and 500 mW/cm2. These data indicate that ultrasound can produce changes in protein synthesis and heart function that are not induced by direct heating to comparable temperatures. [ProA]