Basic physics and imaging characteristics of ultrasound.

The imaging characteristics of diagnostic ultrasound (US) are determined by the ultrasonic properties of tissue. The velocity of propagation of US and the attenuation are the 2 most important parameters. These determine the frequency with which the tissues may be imaged, which in turn sets a fundamental limit on the axial and the lateral resolution. Ultrasonic imaging equipment is designed on the premise that the ultrasonic energy propagates through tissue in a straight line and that the ultrasonic beam is very narrow. In fact, the ultrasonic energy propagates through tissue as a beam of finite dimensions set by the physical dimensions of the transducer, the way it is constructed, and the way it is energized. Also, the velocity of propagation in different tissues varies and this can lead to deviation of the ultrasonic beam from the assumed direction of propagation. This breakdown in assumptions leads to the creation of artifacts that must be appreciated in the interpretation of ultrasonic images. For this reason skilled interpreters of ultrasonic images follow 3 golden rules: never make an interpretation on a single image; just because a feature is displayed do not consider that it is necessarily real; and just because a feature is not displayed do not consider that it is necessarily not there.

Volume imaging: three-dimensional appreciation of the fetal head and face.

Quasi-three-dimensional volume imaging provides an inexpensive means of evaluating the usefulness of three-dimensional imaging. The technique works most efficiently with water-skin interfaces and therefore we investigated its application in obstetrical ultrasonography. Three-dimensional perspectives of the normal and abnormal fetal head and face were spectacular and at times provided more information than the two-dimensional images. The ability of an inexperienced observer to interpret the three-dimensional image more easily may have a role in training sonographers and counseling parents whose fetuses have structural defects. Volume imaging has certain limitations and can only be used as a complementary technique.

Transducer rotation: a useful scanning maneuver in three-dimensional ultrasonic volume imaging.

In ultrasonic volume imaging, the effect of rotating the transducer about the central axis of the imaging plane is equivalent to rotation of the insonified volume. The authors present schematics to depict the action and sonograms in two cases (a hand immersed in water and a 36-week fetus) to demonstrate this phenomenon. This maneuver can be used to "rotate" the imaged structures, which enhances their three-dimensional representation.

Correction of distortion in US images caused by subcutaneous tissues: results in tissue phantoms and human subjects.

PURPOSE: To correct distortion in ultrasonic images caused by refraction at fat-muscle interfaces in the subcutaneous tissues. MATERIALS AND METHODS: A forward-propagation technique was developed that used a priori information of the acoustic properties of the layers. The thickness and shape of these tissues were measured, and a correction was applied for the different velocities in the tissues. A synthetic-aperture scanning technique was used to allow correction to be applied with data obtained in a single scan. The technique was tested in three tissue phantoms with overlying aberrating layers (six images) and in eight volunteer subjects (37 images). The superior mesenteric artery and the aorta were used as test sites within the body, because it is relatively easy to obtain double images of these vessels owing to refraction of the scanning beam by the rectus muscles. RESULTS: Distortions such as double-image artifacts and texture disruption were corrected with use of this technique. Six of the six phantom images and 22 of the 37 images in humans were corrected with use of this technique. CONCLUSION: The forward-propagation technique compensates for refraction at subcutaneous fat-muscle interfaces.

Real-time quasi-three-dimensional viewing in sonography, with conventional, gray-scale volume imaging.

Real-time three-dimensional views of the fetus can be obtained with minor modification to conventional scanners. The modification consists of placing a divergent lens in the out-of-scan plane of the transducer to generate a beam fan-beam in the insonated volume. All of the echoes from which insonates a volume rather than a slice of tissue. Scanning is performed using oblique angles of incidence to provide total reflection conditions. Echoes are then received only from anterior surfaces, automatically generating the three-dimensional views. Intriguing portrayal of fetal detail can be obtained with this form of imaging which is likely to expand the clinical utility of sonographic examinations.

Is diagnostic ultrasound safe? Current international consensus on the thermal mechanism.

OBJECTIVE: To describe the potential risk of heating during ultrasound examinations and to report the international consensus on the safety of ultrasound in medicine. DATA SOURCES AND DATA EXTRACTION: Literature on the biological effects of hyperthermia and ultrasound. CONCLUSION: The use of B-mode grey-scale imaging is not contra-indicated on thermal grounds. Some pulsed Doppler equipment has the potential to produce biologically significant temperature increases, specifically at interfaces between bone and soft tissue. Exposures resulting in temperatures less than 38.5 degrees C can be used without reservation.

Thickness of tissues intervening between the transducer and fetus and models for fetal exposure calculations in transvaginal sonography.

In transvaginal scanning the tissues intervening between the fetus and transducer are relatively thin. The average thickness in the first trimester is 25 mm and reduces to 15 mm by the third trimester; the minimum thicknesses are 14 and 8 mm, respectively. Two models are proposed for calculating exposure in transvaginal scanning. The models are considered in terms of the overlying tissues and the target tissues. A fixed path, constant attenuation of 0.3 dB/MHz describes the properties of the overlying tissues throughout pregnancy. In the model used in first-trimester scanning, the target tissues of the embryo/fetus are considered to have properties similar to those of soft tissues, and they attenuate the energy at the same rate as the overlying tissues. In the model used in second- and third-trimester scanning, the bony structures of the fetus are the target tissues. These reflect 30% of the incident energy and attenuate all of the transmitted energy at their surface.

Heating of guinea-pig fetal brain during exposure to pulsed ultrasound.

Ultrasound-induced temperature elevations in fresh and formalin-fixed fetal guinea-pig brains were measured during in vitro insonation, with a stationary beam in a tank containing water at 38 degrees C. The pulsing regimen used 6.25 microseconds pulses, repeated at a frequency of 4 kHz emitted from a focussed transducer operating with a centre frequency of 3.2 MHz. The greatest temperature rise in brain tissue occurred close to bone and correlated with both gestational age and progression in bone development. After a 2 min insonation with a spatial peak temporal average intensity (ISPTA) of 2.9 W/cm2, a mean temperature elevation of 5.2 degrees C was recorded in fetuses of 60 days gestation (dg). The same exposure produced an increase of 2.6 degrees C in the centre of whole brains of 60 dg fetuses when the bony cranium was removed. As most of the heating occurs within 40 s, these findings have implications for the safety of pulsed Doppler examinations where dwell-time may be an important factor.

Is the quality of transvaginal images superior to transabdominal ones under matched conditions?

Transvaginal images, under matched conditions, are in general superior in quality to transabdominal sonograms. The difference is, however, not dramatic and instances frequently occur when the same quality and even inferior images are obtained on transvaginal examinations. The dominant factor is the amount of overlying tissues. For the same thickness, subcutaneous tissues have greater degrading characteristics. Subcutaneous tissues also mar the quality of images by giving rise to strong multiple reflection artifacts. The major factors for image degradation are associated with a moderate increase in the width of the ultrasonic beam and a large increase in the side lobes. The dramatic superiority of transvaginal imaging described in previous comparison studies is due principally to the use of higher frequencies, more strongly focused beams and closer positioning of tissues to the transducer.

New class of pulsed Doppler US ambiguity at short ranges.

A form of range ambiguity in Doppler ultrasound has been identified. It occurs when a high pulse repetition frequency (prf) is used with a superficial range gate. "Phantom" range gates are then generated at depths beyond the desired range gate, and these may produce artifactual signals. Extremely high prf values should not normally be used, and duplex machines should be programmed to display phantom range gates wherever they fall within the displayed field of view.

A sonographic technique to reduce beam distortion by curved interfaces.

Ultrasonic beams are distorted during propagation between two media with different velocities if the traversed interface is curved within the ultrasonic beamwidth. The distorting effects of various interfaces on a beam generated by a multi-element transducer are analyzed and a technique to reduce these by controlling the time delays to the individual elements is presented.

Transvaginal pulsed Doppler ultrasound assessment of blood flow to the corpus luteum in IVF patients following embryo transfer.

The waveforms of vessels supplying the ovaries of women on an in-vitro fertilization (IVF) programme were studied using transvaginal B-mode and Doppler ultrasound. There were 125 scans recorded in 65 women at weekly intervals from 3 days after embryo transfer or 5 days after gamete intrafallopian transfer (GIFT) until confirmation of pregnancy or onset of menses. At each examination the signals obtained from vessels supplying the ovaries were recorded and quantified using a resistance index (RI). Fifteen patients became pregnant of whom one has subsequently miscarried. There was a highly significant difference in the RI values between patients who became pregnant and those who did not; no patient who became pregnant had a RI greater than 0.5. Oestrogen to progesterone ratios were calculated in the subgroup of non-pregnant patients and there was no correlation between these values and the RI values. This new technique enables prediction of IVF treatment failure earlier than has been reported previously and may reflect the inadequacy of the corpus luteum.

Causes of shadowing in breast sonography.

As ultrasonic beams propagate through tissues, there is a loss of energy by absorption, reflection and scattering. This loss is displayed in the image as shadowing and is an important sonographic sign for the detection and diagnosis of breast disease. Shadowing may result because of reflection of most of the energy by a large impedance discontinuity. The degree of shadowing by interfaces is dependent on their size relative to the ultrasonic beam. Interfaces smaller than the beam may allow sufficient energy to bypass the reflector to permit good visualization of posterior detail. Shadowing may also result from beam spreading due to refraction. Attenuation is the acoustic parameter which specifies total loss of energy and is due predominantly to absorption. Echogenicity is determined by the reflecting and scattering characteristics and therefore attenuation and echogenicity should be regarded as independent entities in the interpretation of breast images.

Sister chromatid exchange frequency in human lymphocytes after long duration exposure to pulsed ultrasound.

The aim of this study is to determine whether sister chromatid exchange (SCE) frequency increases in human lymphocytes that are exposed to pulsed ultrasound conditions within the range typically employed in ultrasound Doppler measurements of fetal blood flow. Whole blood diluted with culture medium was exposed in vitro to 3.1 MHz pulsed ultrasound at spatial peak pulse average (SPPA) intensities from 15 to 135 W/cm2. Insonation for 5 min did not significantly alter the base level frequency of SCEs compared to that observed in sham-insonated controls. To avoid missing an effect, due to differing sensitivities of the division stages within the mitotic cycle, insonation was also applied for 24 hr to include one complete cell division cycle. No change in SCE was observed after this extended insonation at SPPA intensity of 33 W/cm2.