Ernest L. Madsen

Tissue mimicking materials for ultrasound phantoms.

Up until now, no material has been found whose attenuation and speed of sound properties not only mimic those of human soft tissue, but are controllable in magnitude. We have discovered such a material in the form of water-based pharmaceutical gels containing uniform distributions of graphite powder and known concentrations of alcohol. The magnitude of the attenuation coefficient can be controlled easily between 0.2 and 1.5 dB/cm at 1 MHz, by varying the concentration of graphite. These attenuation coefficients are nearly proportional to the frequency. The speed of sound varies between 1520 and 1650 m/s at room temperature, depending primarily upon the concentration of alcohol. Bacterial invasion has been prevented by sterilization procedures and the introduction of appropriate preservatives. The ultrasonic properties exhibit temporal stability and change little over the range of room temperatures.

Tissue-mimicking phantom materials for narrowband and ultrawideband microwave applications

We propose and characterize oil-in-gelatin dispersions that approximate the dispersive dielectric properties of a variety of human soft tissues over the microwave frequency range from 500 MHz to 20 GHz. Different tissues are mimicked by selection of an appropriate concentration of oil. The materials possess long-term stability and can be employed in heterogeneous configurations without change in geometry or dielectric properties due to osmotic effects. Thus, these materials can be used to construct heterogeneous phantoms, including anthropomorphic types, for narrowband and ultrawideband microwave technologies, such as breast cancer detection and imaging systems.

Interlaboratory comparison of ultrasonic backscatter, attenuation, and speed measurements.

In a study involving 10 different sites, independent results of measurements of ultrasonic properties on equivalent tissue‐mimicking samples are reported and compared. The properties measured were propagation speed, attenuation coefficients, and backscatter coefficients. Reasonably good agreement exists for attenuation coefficients, but less satisfactory results were found for propagation speeds. As anticipated, agreement was not impressive in the case of backscatter coefficients. Results for four sites agreed rather well in both absolute values and frequency dependence, and results from other sites were lower by as much as an order of magnitude. The study is valuable for laboratories doing quantitative studies.

Method of data reduction for accurate determination of acoustic backscatter coefficients

In previous methods of data reduction used to determine ultrasonic backscatter coefficients, various approximations were made. One frequently used is that there is an abrupt cutoff in the lateral extent of the scattering volume interrogated. Another approximation in all previous methods is that the effect of time gating the received echo signals can be written as a function of the distance along the axis of the interrogating beam. In the present paper we show that the backscatter coefficient can be derived from experimental data without making such approximations. The cases of narrow-band and broadband pulses are treated, and the method is applicable whatever the distance between the interrogated volume of scatterers and the transducer face. It is shown that, for a given pulse form, the gate duration must be sufficiently long in order to attain a specified accuracy for the measured backscatter coefficient. A test of the method was done using a phantom with well-defined scattering properties. Very good agreement was found between measured values of backscatter coefficients and those calculated using a first-principles theory.

Oil-in-gelatin dispersions for use as ultrasonically tissue-mimicking materials

A form of tissue-mimicking material is reported in which oil droplets are dispersed in a water-based gelatin. Broad ranges of ultrasonic parameters, including speed of sound, attenuation coefficient, density and backscatter level, exist for this material. Very important, the attenuation coefficients are nearly proportional to the frequency as in the case of mammalian tissue and the available attenuation coefficient slopes span the range of mammalian tissues. The available range of slopes is 0.1 dB/cm/MHz through at least 2.0 dB/cm/MHz. The available speeds of sound range from a minimum below that of mammalian fat (approximately 1460 m/s) to a maximum above the accepted average for human tissue (154o m/s). Densities available range from below that of fat (approximately 0.92 gm/cm3) through about 1.00 gm/cm3. Backscatter levels are easily made negligible compared to clinical levels and compared to those exhibited in previously reported tissue-mimicking materials in which the suspended particles are solid (Madsen et al. 1978; Burlew et al., 1980). Addition of solid or hollow glass scatterers allows backscatter levels to be made comparable to those clinically observed.

Tissue-mimicking agar/gelatin materials for use in heterogeneous elastography phantoms

Five 9 cm x 9 cm x 9 cm phantoms, each with a 2-cm-diameter cylindrical inclusion, were produced with various dry-weight concentrations of agar and gelatin. Elastic contrasts ranged from 1.5 to 4.6, and values of the storage modulus (real part of the complex Youngs modulus) were all in the soft tissue range. Additives assured immunity from bacterial invasion and can produce tissue-mimicking ultrasound and NMR properties. Monitoring of strain ratios over a 7 to 10 month period indicated that the mechanical properties of the phantoms were stable, allowing about 1 month for the phantom to reach chemical equilibrium. The only dependable method for determining the storage moduli of the inclusions is to make measurements on samples excised from the phantoms. If it is desired to produce and accurately characterize a phantom with small inclusions with other shapes, such as an array of small spheres, an auxiliary phantom with the geometry of the cylindrical inclusion phantoms or the equivalent should be made at the same time using the same materials. The elastic contrast can then be determined using samples excised from the auxiliary phantom. A small increase of about 10% in volume of the cylindrical inclusions occurred-a tolerable increase. Interestingly, the smallest increase (about 5%) occurred in the phantom with the largest elastic contrast.

LIQUID OR SOLID ULTRASONICALLY TISSUE-MIMICKING MATERIALS WITH VERY LOW SCATTER

A new tissue-mimicking material for ultrasound, using evaporated milk as the primary absorption component, is described. It has very low backscatter but still exhibits the 1540 m s-1 propagation speed and proportionality of attenuation coefficient and frequency over the diagnostic frequency range. The material can be produced in solid or liquid form with attenuation coefficient slopes spanning the range 0.1-0.7 dB cm-1 MHz-1. The liquid form is useful in phantoms where detailed beam patterns are to be determined, either involving translation of measurement devices in the liquid or phantoms with fibers present for causing the only detectable echoes. In the latter case, the liquid quality allows removal of liquid with one attenuation coefficient slope and replacement with another. The solid form may be more useful than the liquid for two reasons. First, many simulated lesions (including ones that produce essentially no internal echoes) can lie in the scan slice with positions extending over the entire image area without enhancement or shadowing effects being of concern. Second, the lack of significant backscatter from the material in the absence of added scatterers allows the backscatter coefficient to be varied over a considerable range. A critical result is that intrinsic material contrast between targets and surroundings can be accurately predicted in terms of the concentrations of added scatterers and, assuming all scatterers are of the same type, the contrast will be completely independent of frequency. Use of the fungicide thimerosal eliminates deterioration, and ultrasonic properties have been shown to be stable over 2.5 years.

Ultrasound transmission measurements through the os calcis

SummaryA method of measuring ultrasonic propagation in the os calcis was devised for assessing bone properties in humans, Speed-of-sound (SOS) and broadband ultrasound attenuation (BUA) were measured using broadband acoustic pulses transmitted and received by a pair of focused transducers. The transducers are mounted coaxially in a water tank with the subjects heel in between. Reproducibility of results in an adult male was 10% for the BUA and 1.2% for the SOS. Both SOS and BUA changed when the transmission path through the os calcis was varied. For a population of normal male subjects, SOS and BUA were correlated with densitometry results on the os calcis, but less well correlated to area density at remote sites.

Ultrasonic shear wave properties of soft tissues and tissuelike materials

Determinations of shear wave speeds of sound and attenuation coefficients are reported for soft tissues, a silicone rubber reference material, and a gel used in manufacturing ultrasonically tissue-mimicking materials. Fresh bovine tissues were investigated, including calfskin, liver, cardiac muscle, and striated muscle. Because of the very large shear wave attenuation coefficients, reasonably accurate determinations of shear wave properties are difficult to make. The quantity measured directly was the complex reflection coefficient for shear waves at a planar interface between the sample and fused silica. Measurements were made at frequencies spanning the range 2-14 MHz. The shear wave attenuation coefficients increase with frequency and are of the order of 10(4) times the longitudinal wave attenuation coefficients. The shear wave speeds of sound also increase with frequency but are only a few percent of the longitudinal wave speeds of sound. The results are accurate enough to allow frequency dependencies to be proposed.

Interlaboratory Comparison of Ultrasonic Backscatter Coefficient Measurements From 2 to 9 MHz

As are the attenuation coefficient and sound speed, the backscatter coefficient is a fundamental ultrasonic property that has been used to characterize many tissues. Unfortunately, there is currently far less standardization for the ultrasonic backscatter measurement than for the other two, as evidenced by a previous American Institute of Ultrasound in Medicine (AIUM)–sponsored interlaboratory comparison of ultrasonic backscatter, attenuation, and speed measurements (J Ultrasound Med 1999; 18:615–631). To explore reasons for these disparities, the AIUM Endowment for Education and Research recently supported this second interlaboratory comparison, which extends the upper limit of the frequency range from 7 to 9 MHz.