Characteristics of tissue equivalent gels intended for passive test procedures of ultrasonic scanner performance P.E. SCHUWERT The ultrasonic qualities of hydrophobic gels with various cross-linked internal structures were investigated. It was found that thermally and mechanically stable gels with ultrasonic properties resembling many mammalian tissues can be produced and may be useful as a reference among hospital clinics in quality assurance tests and training of personnel on diagnostic ultrasound scanners. This paper summarizes some of the results of these studies. KEYWORDS:
The gels were polymerized and cross-linked to different degrees and filled with different scattering obstacles, such as graphite, polyvinylacetate (PVA) or polyethylene particles, to simulate acoustical properties of soft tissues. Longitudinal phase velocity in the materials can be varied between 1500 and 1600 m s-r at 20°C and increases with polymerization (gel) concentration, where concentration is defined as percentage solid gel/monomer dry weight present in 100 cmm3 of distilled water (Fig. 1). The acoustical impedance of such gels may be altered by the inclusion of scattering obstacles which slightly lower or raise the bulk density and hence affect sound velocity and attenuation. For example, the inclusion of graphite or PVA powders leads to increased bulk density as well as an increase in longitudinal phase velocity, whereas the inclusion of polystyrene beads lowers the bulk density and sound velocity despite the higher specific velocity of homogeneous polyethylene (2200 m s-l). The dependence of attenuation on frequency may also be altered using various degrees of cross-linkage.
Modern static and real-time ultrasonic scanners offer, through their grey scale representation, the chance to image organ outlines and internal structures in abdominal diagnosis. Interest in obtaining phantom materials with ultrasonic qualities close to those found in soft tissues has been reported.lB4 Such materials can provide acoustical properties used for reference in clinical practice for routine performance tests or training purposes. Speckle patterns, frequently referred to as the texture observed on the monitor screen, are partly generated by small reflections of the ultrasonic wave along the path of propagation, but are also due to a strong dependence on transmitter/receiver characteristics. These may be the bandwidth and sensitivity of the transducer element as well as the subsequent signal processing. This is of particular interest for recent generations of ultrasonic scanners, where the highest possible frequency, amplification and focusing techniques are applied for the best axial and lateral resolution.
In this study, average particle sizes of 20 pm (graphite), 40-l 60 Mm @‘VA-powder) and 200-400 /..trnpolystyrene beads were used.
To develop a convenient, passive and system independent test procedure for the calibration of system sensitivity, depth of penetration and grey scale dynamics, as well as geometrical alignment and resolution, a few different tissue equivalent gels have been produced. Tissue equivalent
Three different hydrophobic gels were studied: a chemically cross-linked gelatine matrix, using glutardi-aldehyde (GDA) as oxidizing agent; a polyacrylamide monomer (PAM) crosslinked either through irradiation by 6oCo y rays or through the use of oxidizers; finally, a cross-linked plastisol used in eye contact lens materials. The author is in the Department of Radiation Physics, Karolinska Institutet, Box 60204,104 01 Stockholm, Sweden. Paper received 29 March 1982.
A set of 3.5,5.0,7.5 and 20 MHz broad-band transducers were used as transmitters in a water-immersion pulse transmission technique with a 0.5 mm nominal aperture hydrophone as receiver in the far field. To some extent the attenuation measurements were compared using a time-delay spectrometry technique’ including a Hewlett Packard spectrum analyser 3585 A. Discrepancies between the two methods in determining frequency dependence of attenuation are typically less than 0.4 dB relative to a common water reference at 2O’C: o/f” = 2.2 x 10B3 dB cm-’ s2 (fin MHz). Butterworth
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cross-linked and filled with polystyrene beads which leads to an attenuation power dependence expressed by (Y= 4f0*’ dB cm-’ . Lower attenuation coefficients and the contact lens materials. PAM gels shows a much higher frequency, typically expressed
were found for PAM gels In particular, attenuation in power dependence on by 0.1 f’*’ dB cm-’ (Fig. 3).
It is evident from the experimental results that the high water content in gels with weak scattering ability resembles body fluids with a frequency power dependence reaching 1.5, whereas the more structured gel matrices yield a power dependence close to unity. Physical properties The cross-linked gel matrices as described above achieve a bulk structure ranging from soft to an almost stiff state which is thermally stable up to 60°C. Weakly scattering gels can be made translucent and bulk density is typically between 1.030 and 1.100 kg rns3, with a slight decrease in density (0.1%). A follow-up on sealed matrices is currently being made.
Gel concentration @] Fig. 1 Longitudinal phase velocity as a function of gel concentration in percentage dry weight gel or polymer to 100 cmm3 water. l - cross-linked gelatine matrices, q - polyacrylamide monomers (PAM), o -eye contact lens plastisols.
Velocity measurements were made with a pulse propagation timedelay method using 0.5 to 2.0 cm thick samples. Such readings may be prone to errors6, but with sufficiently thick samples a 0.5% standard deviation was obtained. The set of samples were all moulded between plane parallel glass plates spaced by Teflon rings. Results The frequency dependence of the amplitude attenuation coefficient 01,expressed in dB cm-‘, was determined by a regression analysis assuming a frequency power dependence of the form a! = Qp as a first approximation, where the coefficients a and b are typically found to be 0.8 and 1.l respectively, for abdominal tissues such as the human liver in vitro7. Among the more highly attenuating structures found in soft tissues, frequency dependence appears to have a power dependence much closer to unity than for fluids such as cystic liquids (b = 1.5 - 1.7). This tendency is also found for the gel matrices reported by Madsen et a14, when fitted to a power relation CY=af’, as well as among the soft tissues compiled by Goss et a1.7 Close approaches to soft tissue attenuation frequency dependence and speed of sound have been obtained with a 20-25% concentration of gelatin solution cross-linked with GDA and filled with 15% dry weight of PVA powder (Fig. 2). An even higher relative attenuation, but with a lower power dependence, is found in sample 2, a 20% gelatin matrix
Frequency [MHz] Fig. 2 Attenuation against frequency and speed of sound in a few cross-linked gelatine matrices and two tissues in vitro? f is in MHz. Frequency Longitudinal power dependence velocity idI3 cm-‘s*l [m s-‘I
. A X * 0 .
Tendon in vitro 20% gelatine, 4% polystyrene beads Liver in vitro 20% gelatine, 15% PVA powders 25% gelatine, 7.5% graphite powder 25% gelatine, 6% PVA powder
4.5f a8 4.0f”.’ ;:;;::: 0.7fl.l 0.451.3
1580 1550 1550 1590 1570 1540
more structured gels and tissues, indicating that losses due to relaxational absorption dominate and that the discrete relaxational frequencies are in the low- or sub- MHz domain.
Discussion From Figs 2 and 3 it can be seen that when attenuation is raised due mainly to the increase in scattering strength, that is, proportional to an increase in the number of scattering obstacles per unit volume, the frequency dependence approaches a linear relationship (b - 1 ,O) compared to a power dependence where b - 1.5 in more fluid-like media such as cyst-liquids. This finding is in conflict with the expected Rayleigh scattering behaviour (b - 4) when weakly scattering obstacles with dimensions much less than a wavelength are present. This behaviour is not easily explained, but it has been suggested’ that a substantial contribution to attenuation is due to a series of discrete relaxation frequencies leading to the total attenuation being almost linearly dependent on frequency.
Frequency [MHZ] Fig. 3 Attenuation against frequency and speed of sound in fluidlike gels and in vitro tissues.6 f is in MHz.
Specimen A 0 . X 0 .
Contact lens gel, 5% PVA powder 20% gelatine, 10% cross-linkage Blood in vitro 40% PAM gel, 10% cross-linkage 20% PAM gel, 10% cross-linkage CyStic fluid (human breast in vitro)
Frequency power dependence [dB cm-‘s*l
Longitudinal velocity [m s-‘I
“0:; ;::,” 0.2 f1.2 0.1 f1.7 0.02f1.5 0.01 f1.7
1580 1560 1580 1510 1500 1500
Most published attenuation data show agreement on frequency dependence, but the absolute magnitude in dB cm-’ may vary by a factor of 2 or more. This may be due to the techniques used, but also to the intensity levels employed during the measurements. A pressurepulse peak value in excess of 10’ Pa may introduce non-linear effects in the attenuation value recorded.” In this study peak-to-peak pressures of lo4 Pa were typically used.
The gels are further assumed to be viscoelastic and may be described by the Voigt model behaviour, where elastic and viscous damping components are in parallel action.8 The following relations can be derived on a par with the relations for human soft tissues: -I
withM* =M’ t iiI4”. c1 denotes the velocity of longitudinal waves, M the plane wave bulk modulus, M’ the storage modulus, and M” the loss modulus due to losses arising from absorption and the relaxation frequency influence. The loss factor is defined as tan 6 = 5
and it may be seen from Fig. 4 that losses due to M” increase strongly with frequency for fluid-like tissues and gels, indicating that losses due to viscous absorption dominate. On the other hand, M” appears to decrease for
Fig. 4 Frequency dependence of the loss factor tan 6 = M”/M’. Specimens: l -tendon in vitro; A - 20% gelatine, 4% polystyrene beads; X - liver in vitro; o - 20% gelatine, 7.5% graphite powder; q - 20% PAM gel, 10% cross-linkage; b -cystic fluid (human breast).
University of Denmark, for valuable discussions and suggestions. This work was supported in part by the National Swedish Board of Technical Development and performed in a cooperation with the Industrial Acoustics Research Group, Technical University of Denmark. References
Sommer, F.C. A Phantom for Imaging Biological Fluids by Ultrasound and CT Scanning, Ultrasound Med. and Biol. 6 (2) (1980) 135 Astrahan, M. Communication&fed. Phys. 6 (3) (1979) 235 Burlew, M., Madsen, E., Zagzebski, J. A new Ultrasound Tissue Equivalent Materia1,RadioZog.y (134) (Feb 1980) 517-520 Zagzebski, J., Madsen, E. Ultrasonic Phantoms, IEEE Trans.
It has been demonstrated that cross-linked hydrophobic gels may attain very good ultrasonic properties resembling soft tissues. The attenuation dependence on frequency, that is, its power dependence, can be varied between that equivalent to body fluids and that reported for muscle or tendon tissues. It has also been possible to develop a series of volumes with different back-scatter generating obstacles within these materials, thus allowing for a passive dynamic grey scale and sensitivity check (Fig. 5).
on Nucl. Sci. (1980) 1177-1181 Lewin, P. Calibration and performance
1 Fig. 5 Texture scanned at 2.25
pattern of a series of weak scattering MHz with a linear array transducer
of a Miniature Ultrasonic Hydrophone using Time Delay Spectrometry, abstract IEEE Ultrasonic Meeting (Ott 1981) (in press) Goss, S.A., Johnston, R.L., Maynard, V., Nider, L., Frizzell, L.A., O’Brien, W.D., Dunn, F. NBS special publication on tissue characterization II, No 525 (1979) 44 Goss, S.A., Dunn, F. Ultrasonic Properties of Mamalian Tissues, Compilation No II,J. Acoust. Sot. Am. No (68) (July 1980) 93-108 Ahuja, A.S. Ultrasonic Attenuation in Soft Tissues: Reasons for Large Magnitude and Linear Frequency Dependence, Ulfrasoniclmagiing (2) (1980)
Acknowledgement I am grateful to Professor L. [email protected]
$ and to Dr. P. Lewin, of the Industrial Acoustics Research Group, The Technical
Wells, P.N.T. Medical Physics of CT and Ultrasound, American Assoc. of Physicists in Medicine (AAPM), Monograph No 6 (1980) 383-384 Carstensen, E.L., Muir, T.G. Demonstration of Nonlinear Acoustical Effects at Biological Frequencies and Intensities UPrasoundMed.