Paul L. Carson

Biophysical Bases of Elasticity Imaging

Elasticity imaging is based on two processes. The first is the evaluation of the mechanical response of a stressed tissue using imaging modalities, e.g. ultrasound, magnetic resonance imaging (MRI), computed tomography (CT) scans and Doppler ultrasound. The second step is depiction of the elastic properties of internal tissue structures by mathematical solution of the inverse mechanical problem. The evaluation of elastic properties of tissues has the potential for being an important diagnostic tool in the detection of cancer as well as other injuries and diseases. The success of breast self-examination in conjunction with mammography for detection and continuous monitoring of lesions has resulted in early diagnosis and institution of therapy. Self-examination is based on the manually palpable texture difference of the lesion relative to adjacent tissue and, as such, is limited to lesions located relatively near the skin surface and increased lesion hardness with respect to the surrounding tissue. Imaging of tissue “hardness” should allow more sensitive detection of abnormal structures deeper within tissue. Tissue hardness can actually be quantified in terms of the tissue elastic moduli and may provide good contrast between normal and abnormal tissues based on the large relative variation in shear (or Young’s) elastic modulus.

ULTRASONIC POWER AND INTENSITIES PRODUCED BY DIAGNOSTIC ULTRASOUND EQUIPMENT

Abstract The ultrasonic radiation force on an absorber was measured with a feedback microbalance to determine the ultrasonic power emitted into water by 26 diagnostic ultrasound instruments. Included in the measurements were pulse echo scanners, echocardiographic units, ophthalmic units, Doppler fetal heart detectors, fetal monitors, and continuous and pulsed Doppler units for peripheral vascular studies. The temporal and spatial average intensity at the transducer was calculated for those instruments by dividing the total power by the nominal area of the radiating surface. Estimates of the spatial maximum intensity in water, both temporal average and temporal peak, were made by measurements with miniature hydrophones normalized to the power measurements. Hydrophone measurements only were performed on linearly-scanned real time arrays. For transducers with single element transmitters, the maximum obtainable power ranged from 60μW to 37 mW, and maximum obtainable intensity at the transducer face ranged from 1 to 1700 W/m2. A much smaller range of power and intensity was encountered among systems designed for a given set of applications and providing similar performance in those applications. The temporal average and temporal peak intensities at the focal point of focused transducers employed frequently in the last few years for general purpose scanning were somewhat higher than previously quoted values because of the increased focusing.

THE 3D AND 2D COLOR FLOW DISPLAY OF BREAST MASSES

A prospective study was performed in 24 women with breast masses on mammography going on to surgical biopsy. 2D and 3D power mode and frequency shift color flow Doppler scanning and display were compared. Vessels were displayed as rotatable color volumes in 3D, superimposed on gray-scale slices. The latter were stepped sequentially through the imaged volume. Radiologists rated the masses in each display (3D, 2D and videotapes) on a scale of 1 to 5 (5 = most suspicious) for each of six conventional gray-scale and six new vascular criteria. Thirteen masses proved to be benign and 11 were malignant. 3D provided a stronger subjective appreciation of vascular morphology and allowed somewhat better ultrasound discrimination of malignant masses than did the 2D images or videotapes (specificities of 85%, 79% and 71%, respectively, at a sensitivity of 90%). Only in 3D did the vascularity measures display a trend towards significance in this small study.

Imaging soft tissue through bone with ultrasound transmission tomography by reconstruction.

A phantom consisting of a 2.5-cm-diameter aluminum rod, a balloon filled with castor oil, and a ballon filled with a phenylated silicone fluid was imaged in water with ultrasound transmission tomography by reconstruction (UTTR). This phantom was chosen for investigation of the feasibility of detecting small changes in the ultrasound velocity and ultrasound attenuation coefficient of soft tissues in planes containing bones. Attenuation images of a transverse section of a leg of lamb were obtained as well. Opposed transmitting and receiving transducers were scanned on either side of the imaged object, just as an x-ray tube and detector are scanned in the pencil beam geometry employed with the first x-ray computerized axial tomographic system. More than adequate signal was transmitted through the aluminum rod and the bone to obtain reconstructions; but, in these crude images, there were numerous artifacts which maintain the uncertainty regarding the future utility of this promising technique. Although velocity imaging should not be as sensitive to reflection and refraction as attenuation imaging, a very narrow ultrasound beam must be employed to obtain reasonable resolution in velocity imaging.

What a Hospital Physicist Needs in a Transducer Characterization Standard: Are Tissue-Equivalent Test Objects Necessary?

C HARACTERIZATION and standardization of pulse-echo ultrasonic transducers for diagnostic ultrasound or nondestructive evaluation (NDE) has always been a thorny problem. The Standards Committee of the American Institute of Ultrasound in Medicine (AIUM) and the Institute for Basic Standards of the National Bureau of Standards cosponsored a meeting on June 16,1977 to examine this problem. This meeting was well attended by members of the medical ultrasound and NDE engineering communities. Following this meeting, the AIUM standards committee began developing a standard for single-element pulse-echo transducers.

Simplified technique for the calibration and use of a miniature hydrophone in intensity measurements of pulsed ultrasound fields

A simplified technique for calibration of the response of a miniature hydrophone as a function of frequency was evaluated experimentally for reproducibility and for its uncertainty in calibration of hydrophones intended for intensity measurements in broadband medical ultrasound fields. In this hydrophone calibration technique, the known total acoustic power passing through a plane lying normal to an ultrasound beam is compared with the surface integral of the intensity over that plane. Broadband source transducers calibrated by the National Bureau of Standards were driven in a narrow‐band tone‐burst mode to calibrate the hydrophone at multiple discrete frequencies. A least squares polynomial fit to the discrete values provided a continuous estimate curve of the hydrophone intensity response factor (Kf2) as a function of frequency. Good reproducibility of this multipoint technique was demonstrated by an independent, but similar, calibration of the hydrophone by the Bureau of Radiological Health. This hydro...

A modeled study for diagnosis of small anechoic masses with ultrasound.

Hemispherical holes created in the bases of uniform natural sponges were imaged in water to determine how anechoic masses fill in with echoes at increased gains. The increase in gain, from the threshold for outlining a hole, to the fill-in of the hole with echoes, was obtained for a range of hole diameters and imaging conditions. These gain changes fit on a smooth curve when plotted against the hole diameter divided by the ultrasound beam width. The main sources of fill-in, beam width and multiple reverberations, appear to behave similarly in vivo. Understanding these effects may help reduce diagnostic errors.

ULTRASOUND TRANSAXIAL TOMOGRAPHY BY RECONSTRUCTION

The diagnostic potential of transmission imaging with ultrasound has stimulated many attempts to utilize the large differences in ultrasound attenuation between various soft tissues. Most such attempts have met with very limited success due to practical difficulties including scattering and phase distortion by bones, air spaces, and fat. Imaging a transverse plane in the body by transmission scanning of that plane from a large number of angles, and subsequent reconstruction of the transmission image by computer may provide the long desired diagnostic information in many areas of the body. Just as with computerized axial tomography now used successfully for enhanced x-ray imaging, ultrasound transaxial tomography by reconstruction (UTTR) displays those objects causing distortions of the ultrasound and allows, in effect, averaging of data from scans at many angles. Others have investigated UTTR for soft tissue imaging. The present results simulate with phantom measurements the image of soft tissues adjacent to bone-equivalent objects.

Assessment of errors in intensity measurements of pulse echo ultrasound using miniature hydrophones

Total transmitted power and intensity distributions from diagnostic ultrasound systems have been measured with a radiation force balance and with a miniature hydrophone and instrumentation available in many medical centers. In assessing the accuracy of absolute intensity and power determinations from measurements of acoustic pressure with a hydrophone, ultrasonic power was computed from hydrophone measurements. This power value was compared with the power determined by a radiation force technique. On five pulse echo ultrasound systems, the ratio of the power measured with a radiation force balance to the power determined with the hydrophone varied from 0.25 to 5.5. These differences are attributed mainly to the known large variations in hydrophone response as a function of frequency, and possible time, since several other possible sources of error were evaluated and estimated to have a reasonably small net effect of 25%. The errors evaluated were those dealing primarily with measurement procedures.

Transport phenomena in laminar flow of blood

Recently it has been shown experimentally that transport of heat and gas (specifically oxygen and helium) are augmented in the laminar flow of aqueous suspensions of polystyrene spheres 50 and 150 micrometer in diameter. In this report, data on heat and gas transport are correlated. Application of this correlation to flowing blood leads to the following conclucions. There is no significant augmentation of oxygen and heat transport in flowing blood even at shear rates much higher than physiological shear rates; an observation which is in accord with the experimental results. The augmentation of the diffusion coefficient of plasma proteins in flowing blood, though not very high, appears to be measurable. Of the total measured augmentation of about 6000--30 000% in platelet diffusivity in flowing blood, quoted from the literature, about 500% is attributable from this correlation to fluid mechanical forces, and the balance is hypothetically attributed to other forces (electrical or biochemical) present in blood.