Edward H. Chiang

Quantitative assessment of cartilage surface roughness in osteoarthritis using high frequency ultrasound

Osteoarthritis (OA) is a common disease which affects nearly 50% of people over age 60. Histologic evaluation suggests that fibrillations approximately 20-150 microns are among the earliest changes in the articular cartilage. We propose a technique to quantify these surface fibrillatory changes in osteoarthritic articular cartilage by considering the angular distribution of the envelope-detected backscattered pressure field from an incident 30-MHz focused transducer. The angular distribution of the scattered acoustic field from an inosonifying source will directly relate to the distribution of surface fibrillatory changes. Data are presented for three different grades (400, 500 and 600 grit) of commercially available emory paper and three samples of osteoarthritic femoral head articular cartilage, which were visually assessed as having smooth, intermediate and rough surfaces, respectively. Our preliminary results indicate a probable monotonic relationship between articular cartilage roughening and the degree of broadening in the angle-dependent pressure amplitude. When applied to the emory paper, the technique indicates sensitivity to differences as small as approximately 5-10 microns in mean roughness. This procedure may provide an extremely sensitive and reproducible means of quantifying and following the cartilage changes observed in early osteoarthritis.

Ultrasonic characterization of in vitro osteoarthritic articular cartilage with validation by confocal microscopy.

The majority of adults over the age of 65 y develop osteoarthritis (OA), a joint disease characterized by degeneration of articular cartilage and subchondral sclerosis. Early in the disease, the articular cartilage surface begins to change histologically from a smooth to a rough or fibrillated appearance. A prerequisite for any chondroprotective pharmacological intervention is detection of OA in its preclinical phase. Current diagnostic imaging modalities, such as radiographs or (nuclear) magnetic resonance imaging, either cannot directly image the cartilage surface or lack sufficient resolution to detect surface fibrillations. We have developed an ultrasonic technique that can be used to characterize these surface fibrillations directly. We present our in vitro results with validation by laser-based confocal microscopic imaging.

Quantitative assessment of surface roughness using backscattered ultrasound: The effects of finite surface curvature

We have previously described a technique to quantify surface fibrillatory changes in osteoarthritic articular cartilage. In that study, the angular distribution of the scattered acoustic field from an insonifying source directly related to the distribution of surface fibrillatory changes. In the current study, we demonstrate a more sensitive method to quantify surface roughness, the effect of global surface curvature in estimating surface roughness and the utility of using focused transducers in circumventing this potential problem for in vivo work. Phantoms composed of acrylic rods with and without sandpaper grit (about 15 to 72 microns, mean particle size) applied to the surface were scanned. A more robust angular scattering technique to measure the angle dependent data was employed, in which the integrated squared pressure amplitude over a finite time window (mean power) was measured as a function of incident acoustic angle for varying surface roughnesses and radii of curvature. We show that the potential dynamic range for making roughness discriminations diminishes with decreasing radius of curvature of the acrylic rod phantoms using an unfocused transducer. This effect is minimized with use of a focused transducer. Roughness effects are most evident at sufficiently large angles where incoherent scattering dominates. We conclude that the roughness of cylindrically curved surfaces can be quantitatively assessed using a focused ultrasound beam at sufficiently large incident angles, given that the focal spot size is sufficiently smaller than the radius of curvature of the surface.

FETAL DEPTH AND ULTRASOUND PATH LENGTHS THROUGH OVERLYING TISSUES

Measurements of minimum thicknesses, in a four-layer, overlying tissue model, on 22 pregnancies between 15 and 20 weeks gestation, yielded a global minimum and a mean of the minimum total thickness per patient of 1.7 and 2.9 cm, respectively, and a minimum and mean subcutaneous fat thickness of 0.7 and 1.4 cm. Conservative calculations of the minimum attenuation per patient, at 3.5 MHz indicated that less than 2.5% of 15 to 20 week pregnancies should fall below the lower 95% prediction line of: Attenuation (dB) = 0.10 X Maternal Weight (kg) - 3.0. The smallest calculated attenuation for any of the 21 subjects was 0.8 dB MHz-1 indicating just under a factor of two protection at 3.5 MHz of proximal fetal tissues compared with ultrasound intensities measured in water. This value is lower than those generally used in the past. The knowledge of distributions of transducer-to-fetal distances and thicknesses of overlying tissues is also important for improvement of image quality. Measurement of minimum depth of the anterior fetal thorax in 57 examinations of 25 to 40 week fetuses yielded minimum and mean values of 2.5 and 4.1 cm, respectively.

Acoustic generation of bubbles in excised canine urinary bladders

A high-intensity, 555-kHz acoustic field was used to generate bubbles within urinary bladders excised from dogs. Following the exposure, bubbles were visualized on a diagnostic ultrasound scanner with a 5-MHz in-line mechanical sector scanhead. Scattering of the high-intensity ultrasound by the bubbles was also observed during the exposure as high-amplitude scan lines. The bladders used had been surgically removed after tying off the ureters and urethra to prevent urine loss and exposure to external contaminants. Each bladder was sealed in a plastic bag filled with a degassed saline solution. The bladder was centered in a sealed degassed water path at the common focus of a 7-cm-diam transducer and a 10-cm-diam brass reflector. The 555-kHz transducer and reflector were both focused at 10 cm and were aligned coaxially. Using various acoustic pressure amplitudes, two, 10-s low-frequency exposures, separated by approximately 30 s, occurred at approximately 2-min intervals. Experiments on a single bladder lasted as long as 45 min. The sizes of the largest bubbles, which were easily imaged, were estimated from rise velocity measurements as 50-70 microns in radius, and pressure amplitudes used to generate those large bubbles were estimated as 10-20 bars. The detection of smaller bubbles was limited by the inability to clearly distinguish bubble echoes from artifacts caused by the reverberant field within the bladder. Visual inspection of the exterior and interior bladder wall showed no significant discoloration within the high intensity beam path.(ABSTRACT TRUNCATED AT 250 WORDS)

CONSTANT SOFT TISSUE DISTANCE MODEL IN PREGNANCY

To evaluate the effects of the standoff, the same plane was imaged in several patients with a linear array and with the sector scanner, with and without a standoff. Total thicknesses measured with the standoff and the linear array agreed within + 1 mm, while direct contact sector scanning depressed the thickness of overlying tissues by an average of 3.5 mm more than the other two methods. Attenuation coefficients used to calculate attenuation in the various tissue layers were the same as those employed by the NCRP (1983) and Carson (1988) except that skin is added separately here. In dBcm-IMHz -2, they were: skin - 1.5; subcutaneous fat - 0.46; abdominal (skeletal) muscle 0.51; preperitoneal fascia - 0.51; myometrium (smooth muscle) - 0.29. Fig. 1 shows individual measurements of thickness of overlying tissue layers and total overlying tissue thicknesses as a function of maternal weight. Linear regressions are shown as the solid lines. The one case at 97 kg was not included in the linear regressions, because the data suggested that different anatomical relations may exist in this obese case.

Ultrasound attenuation coefficient in the fetal liver as a function of gestational age

An apparent increase in the ultrasound attenuation coefficient per unit frequency, alpha f, of fetal liver as a function of gestational age has been observed. Measurements were made in utero with a 25 megasample/sec RF digitizer and a real time ultrasound system with a 5 MHz scan head. A precise measurement of alpha f was employed in which the intercept was tied to 0 at a frequency of 0. In 178 examinations of normal pregnancies, the linear regression of the alpha f increased 26% between 26 and 40 weeks gestation. This statistically significant increase (p less than 0.0001) is consistent with several observations, those of Parker et al. of increased attenuation in liver when glycogen is added, the increasing glycogen storage in the liver before birth, and our own pre- and postnatal measurements reported elsewhere. A noninvasive assay for glycogen content would have important applications in medicine and biomedical science. However, an increase in measurement accuracy and precise correlation with glycogen content will be required to make meaningful predictions in individual cases, as opposed to the present statistical trends.

Pre- to postnatal reduction in ultrasound attenuation coefficient of the liver.

A recent study showed the ultrasound attenuation coefficient of fetal liver between 26 and 40 weeks of gestation to be 26% higher than after birth. To test the hypothesis that ultrasound attenuation is sensitive to fetal liver glycogen concentration, the livers of 24 fetuses were examined at 5 MHz just prior to and just after birth. The mean pre- to post-delivery reduction in attenuation coefficient was 0.08 dB cm-1 MHz-1 +/- 0.02 (SEM), or 17% of the post-delivery mean. This is consistent with the increase in attenuation measured by others in liver homogenate when glycogen was added. An increase in measurement accuracy, correlation with glycogen content, and, possibly, control for biological variability will be required to make predictions in individual cases, as opposed to these averages. A simple test of glycogen content would be of value scientifically and in prenatal and postnatal management.

Development Of An Intravascular Ultrasonic Catheter Imaging System

A prototype ultrasonic imaging system typical of the size necessary to be embedded in a catheter delivery system was developed to examine the feasibility of high resolution, intravascular ultrasonic imaging in vivo. Although the current system is too large to be useful in examination of proximal coronary arteries, the signal-to-noise ratio (SNR) and resolution obtained clearly indicate that catheter-borne, intravascular ultrasonic imaging is indeed feasible and could be very informative as an adjunct to angioplasty or vascular surgery.

Observation of acoustic cavitation in excised canine urinary bladders

A high‐intensity ultrasonic field has been used to generate bubbles within urinary bladders excised from dogs. Following the exposure, bubbles were easily visualized using an ATL UltraMark 9 diagnostic scanner with a 5‐MHz mechanical sector scanhead. Scattering of the high‐intensity ultrasound by the bubbles was also observed as uniformly high amplitude scan lines during the irradiation. The bladders were removed surgically after tying off the ureters and urethra to prevent urine loss and exposure to external contaminants. Each bladder was sealed in a plastic bag filled with a degassed aqueous solution of sodium chloride (0.9% by weight) and sodium azide (1 g/liter). A bag containing a bladder was centered in a sealed degassed water path at the common focal point of a 7‐cm‐diam, 500‐kHz transducer, and a 10‐cm‐diam brass reflector. The transducer and reflector were both focused at 10 cm and were aligned coaxially. Exposures were 10 s in duration but intensity or pressure values were not available as of th...