[Obstetrical ultrasound: can the fetus hear the wave and feel the heat?]. / Ultraschall in der Geburtshilfe: Kann der Fötus die Ultraschallwelle hören und die Hitze spüren?

"Fetuses can hear ultrasound and the sound is as loud as a subway train entering a station." This statement originates in a single report in a non-peer reviewed journal, despite its name 1, of a presentation at a scientific meeting by researchers who reported measuring the sound intensity in the uterus of pregnant women and being able to demonstrate the above. This was later published in a peer-review journal 2 probably not very widely read by clinicians or the general public. From time to time, the popular press or various pregnancy-related websites repeat the assertion or a worried pregnant patient inquires about the truthfulness of this statement. A second, oft-quoted concern is that ultrasound leads to heating of the amniotic fluid. These two assertions may be very concerning to expectant parents and merit scientific scrutiny. In this editorial, we shall examine the known facts about the physical properties of ultrasound as they relate to these two issues. Diagnostic ultrasound employs a pulsed sound wave with positive and negative pressures and the Mayo team, quoted in the New Scientist, predicted that the pulsing would translate into a "tapping" effect 1. According to their report, they placed a tiny hydrophone inside a woman's uterus while she was undergoing an ultrasound examination. They stated that they picked up a hum at around the frequency of the pulsing generated when the ultrasound is switched on and off. The sound was similar to the highest notes on a piano. They also indicated that when the ultrasound probe was pointed right at the hydrophone, it registered a level of 100 decibels, as loud as a subway train coming into a station. Sound levels in decibels are defined for audible frequencies with the reference level being the threshold for hearing at a given frequency. Although the operating frequencies used in sonography are inaudible, it is possible for the pulsing rate (pulse repetition frequency, PRF) to be heard, thus falling in the audible range. A previous report had hinted at similar phenomena 3.Ultrasound is a pressure wave with a frequency beyond (ultra) that detectable in the human auditory system. The human ear can discern sound at roughly 20 - 20 000 cycles (hertz) per second. The frequencies of diagnostic ultrasound are roughly 1 - 10 megahertz (MHz) or 1 000 000 to 10 000 000 cycles per second. It is a form of energy and, as such, may have effects in tissues it traverses. Any consequences occurring in living tissues secondary to an external influence are called biological effects or bioeffects. This term does not imply damage or harm. The two major mechanisms for bioeffects are thermal and non-thermal. Thermal effects are secondary to ultrasound energy being converted into heat in the tissue (indirect effect of ultrasound) and non-thermal effects are secondary to the alternating positive and negative pressures generated by the wave (direct effect). The definition of moderately loud sound is 60 - 70 dB (2 × 10-3-2 × 10-2 Pa), defined as high urban ambient sound, normal conversation at 1 m, or living room music 4. In comparison, quiet conversation is 40 dB, a railway diesel engine passing at 45 mph at 100 feet is 80 - 85 dB and a rock band is 110 dB 4. There have been a few publications describing harm to fetuses exposed to elevated levels of ambient noise, particularly industrial noise 567, specifically in the aircraft and textile industries, but while there have been reports of impaired hearing in infants who were exposed to ultrasound in the womb, several rigorous studies have disproved that notion 891011. Furthermore, a study of fetuses exposed in utero to vibroacoustic stimulation 12 and a recent study of fetuses exposed to noise generated during an MR exam of the pregnant women 13 showed no ill effect on the auditory system. There have been some reports of being able to hear a "hum" during transcranial ultrasound. This may be the pulse-repetition frequency (PRF), but, if so, it would be described as a higher pitch, and probably not a "hum". To our knowledge, this phenomenon has not been investigated. Although the report mentioned above suggested that diagnostic ultrasound is detectable at measurable levels in the uterus, there is no independently confirmed, peer-reviewed, published evidence that the fetus actually hears the PRF, responds to it or is harmed by it."The fetus cannot regulate its own body temperature, so amniotic fluid can reach very high temperatures over long periods" 14. Does this statement reflect a real risk? What does it mean if this statement is scientifically true? The fear is, of course, that this will raise the temperature of the fetus. Thermally induced teratogenesis has been demonstrated in many animal studies, as well as several controlled human studies 1516. A temperature increase of 1.5 °C above the normal value has been suggested as a universal threshold 17. It is important to note that diagnostic ultrasound was not the source of the temperature elevation in any of these studies. Some believe that there are temperature thresholds for hyperthermia-induced birth defects (hence the ALARA [as low as reasonably achievable] principle), but there is some evidence that any positive temperature differential for any period of time has some effect, in other words there may be no thermal threshold for hyperthermia-induced birth defects 18. In experimental animals the most common defects are microcephaly with associated functional and behavioral problems 17, microphthalmia and cataracts. There are reports on the effects of hyperthermia and measurements of in vivo temperature induced by pulsed ultrasound but not in humans 192021. Temperature increases of 1 °C are easily reached in routine scanning 22. Elevation of up to 1.5 °C can be obtained in the first trimester and up to 4 °C in the second and third trimesters, particularly with the use of pulsed Doppler 23. When the ultrasound wave travels through tissue, its intensity diminishes with distance (attenuation). In completely homogeneous materials, the signal amplitude is reduced only by beam divergence and absorption (conversion of sound to heat). However, biologic tissues are non-homogeneous and further weakening occurs due to scattering. The issue of temperature increase in the amniotic fluid is based on the fact that the energy of the ultrasound waves is partially converted to heat in the tissue traversed by the waves. Tissues with a high absorption coefficient (such as bone) will produce a high conversion rate while the conversion will be lower in tissues with low absorption. Fluids have very low absorption characteristics and, therefore, the risk of temperature elevation in the amniotic fluid is minimal. The only available study on the topic did not demonstrate any increase in temperature in the amniotic fluid when performing diagnostic ultrasound, both in grayscale anatomic imaging (sonography) and Doppler ultrasound 24. ConclusionWhile ultrasound is a sound wave which can produce mechanical effects and temperature elevation in tissues that it traverses, the risk to human fetuses when using diagnostic ultrasound appears to be minimal if certain rules are followed, such as performing a scan when medically indicated, and observing the ALARA principle (using the lowest output power consistent with acquiring the necessary diagnostic information and keeping the exposure time as low as possible for accurate diagnosis).

Mirror-image artifact can affect transcranial Doppler interpretation.

Transcranial Doppler ultrasonography (TCD) allows evaluation of blood-flow velocity in intracranial arteries detection and monitoring of vasospasm in patients with subarachnoid hemorrhage. Spectral Doppler artifacts can affect TCD data. A 1-month series of TCD findings showed marked fluctuation in blood-flow velocity values in both the middle and anterior cerebral arteries of a patient with subarachnoid hemorrhage. A mirror-image artifact of the Doppler fast Fourier transform velocity spectrum resulted in erroneous interpretation of higher flow velocity in certain vessels. This artifact may cause misinterpretation of TCD flow-velocity data and lead to improper diagnosis of the condition and treatment of patients.

Peak velocity overestimation and linear-array spectral Doppler.

Ultrasound instruments are used to evaluate blood flow velocities in the human body. Most clinical instruments perform velocity calculations based on the Doppler principle and measure the frequency shift of a reflected ultrasound beam. Doppler-only instruments use single-frequency, single-crystal transducers. Linear- and annular-array multiple-crystal transducers are used for duplex scanning (simultaneous B-mode image and Doppler). Clinical interpretation relies primarily on determination of peak velocities or frequency shifts as identified by the Doppler spectrum. Understanding of the validity of these measurements is important for instruments in clinical use. The present study examined the accuracy with which several ultrasound instruments could estimate velocities based on the identification of the peak of the Doppler spectrum, across a range of different angles of insonation, on a Doppler string phantom. The string was running in a water tank at constant speeds of 50, 100, and 150 cm/sec and also in a sine wave pattern at 100- or 150-cm/sec amplitude. Angles of insonation were 30, 45, 60, and 70 degrees. The single-frequency, single-crystal transducers (PC Dop 842, 2-MHz pulsed-wave, 4-MHz continuous-wave) provided acceptably accurate velocity estimates at all tested velocities independent of the angle of insonation. All duplex Doppler instruments with linear-array transducers (Philips P700, 5.0-MHz; Hewlett-Packard Sonos 1000, 7.5-MHz; ATL Ultramark 9 HDI, 7.5-MHz) exhibited a consistent overestimation of the true flow velocity due to increasing intrinsic spectral broadening with increasing angle of insonation.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple-element transducers.

Multiple-element transducers, commonly called arrays, contain groups of transducer elements. The complete name of an array, such as the linear sequenced array, describes both how the array is constructed (linear) and how it is operated (sequenced); however, the names most often used are incomplete descriptions, such as the commonly used linear array. The arrays are arranged as a straight or a curved line of rectangular elements (linear or convex array) or concentric rings (annular array). Except for the annular array, which focuses the beam two-dimensionally but cannot steer it, the arrays electronically scan the ultrasound beam without mechanical movement. The image formats produced are a rectangle, parallelogram, and sector. To improve image quality, arrays can electronically focus the transmitted beam at a desired depth or at multiple depths to, in effect, achieve a long focal region. Focusing of received echoes is also accomplished electronically. Dynamic aperture and apodization also improve image quality with arrays.

Doppler color imaging. Principles and instrumentation.

DCI acquires Doppler-shifted echoes from a cross-section of tissue scanned by an ultrasound beam. These echoes are then presented in color and superimposed on the gray-scale anatomic image of non-Doppler-shifted echoes received during the scan. The flow echoes are assigned colors according to the color map chosen. Usually red, yellow, or white indicates positive Doppler shifts (approaching flow) and blue, cyan, or white indicates negative shifts (receding flow). Green is added to indicate variance (disturbed or turbulent flow). Several pulses (the number is called the ensemble length) are needed to generate a color scan line. Linear, convex, phased, and annular arrays are used to acquire the gray-scale and color-flow information. Doppler color-flow instruments are pulsed-Doppler instruments and are subject to the same limitations, such as Doppler angle dependence and aliasing, as other Doppler instruments. Color controls include gain, TGC, map selection, variance on/off, persistence, ensemble length, color/gray priority. Nyquist limit (PRF), baseline shift, wall filter, and color window angle, location, and size. Doppler color-flow instruments generally have output intensities intermediate between those of gray-scale imaging and pulsed-Doppler duplex instruments. Although there is no known risk with the use of color-flow instruments, prudent practice dictates that they be used for medical indications and with the minimum exposure time and instrument output required to obtain the needed diagnostic information.

Introduction to transvaginal imaging.

Transvaginal sonography provides improved resolution and avoidance of intervening anatomic structures that can degrade the image. In this article, transvaginal transducers are described and compared with transabdominal transducers. The sagittal and coronal views used in transvaginal sonography are described. Advantages and disadvantages of transvaginal sonography are considered. Safety considerations of the decreased attenuation path to the fetus are dealt with.

Color velocity imaging: introduction to a new ultrasound technology.

Noninvasive ultrasound is the preferred methodology for the initial evaluation of carotid atherosclerosis. Since the early use of continuous-wave Doppler to assess carotid artery flow velocity blindly, neurosonology has progressed through crude B-mode imaging, spectral analysis of the Doppler signal, and gray-scale duplex Doppler/B-mode imaging, to color-flow Doppler duplex imaging. The latter allows color coding of Doppler data based on the velocity of blood flow. The combination of color-flow Doppler with gray-scale B-mode imaging allows simultaneous visual display of anatomical and hemodynamic information. Physical limitations of color-flow duplex Doppler imaging may affect the clinical utility of these techniques. Problems with pulse repetition frequency, aliasing, resolution capability of the color data, and interpolation of data make some applications difficult. Color velocity imaging uses the data contained in the gray-scale B-mode image scan lines to determine velocity of blood flow, and it offers potential advantages over conventional color-flow duplex Doppler for the assessment of carotid atherosclerosis and hemodynamics. Initial comparison of spectral Doppler and color velocity imaging data suggests that the latter is an accurate method to assess blood flow velocity. Understanding of the validity, utility, and prognostic advantages offered by color velocity imaging awaits careful prospective clinical trials.

Biomolecular absorption of ultrasound. III. Solvent interactions.

Absorption coefficients for several biomolecules have been measured to determine the role of solvent interactions in the absorption characteristics of biomolecules at neutral pH. The presence of phosphate buffer ions in aqueous solvent dramatically increases the absorption of small biomolecules (sugars and amino acids). These increases suggest the importance of solvent interactions in absorption. For large molecules (proteins and polysaccharides), buffer ion effects are lessened and the role of solvent interactions is not as clear. Absorptions of hemoglobin are essentially the same in each of five aqueous solvents and are greater in nonaqueous solvents. Solvation interactions may be inhibited by the tertiary structure of globular proteins. Solvation apparently does contribute to absorption in biomolecules along with other physicochemical processes.

Interlaboratory comparison of ultrasonic attenuation and speed measurements.

A set of test samples, all containing ultrasonically equivalent tissue-mimicking material, was produced and measurements of ultrasonic speed and ultrasonic attenuation coefficients were made at seven laboratories using various techniques. The ultrasonic speed values agree well with one another, having a spread of about 0.3 per cent; thus, speed values for tissue parenchyma appearing in the literature are likely to be accurate. Values of ultrasonic attenuation coefficients agree fairly well with one another, with differences between individual values and the group mean of generally less than 20 per cent of the group mean.

Artifacts in ultrasound imaging.

Ultrasound imaging artifacts of acoustic origin relating to resolution, propagation path, and attenuation are reviewed. Lateral and axial resolution limitations are artifactual in nature since a failure to resolve means a loss of detail and two adjacent structures may be visualized as one. Apparent resolution close to the transducer (speckle) is not directly related to tissue texture but is a result of interference effects from the distribution of scatterers in the tissue. Reverberation produces a set of equally spaced artifactual echoes distal to the real reflectors. The mirror image artifact is the presentation of objects that are present on one side of a strong reflector, appearing on the other side as well. Shadowing and enhancement are useful artifacts for determining the nature of masses. Enhancement results from low attenuation objects in the sound path while shadowing results from strongly reflecting or strongly attenuating objects. Additional artifacts include section thickness, refraction, multipath, side lobe, grating lobe, focal enhancement, comet tail, ring down, speed error, and range ambiguity.

A comparison of sector and linear array scanners for the measurement of the fetal femur.

This study was designed to determine whether differences between sector and linear array scanners introduce variability in fetal femur length measurements, used to determine gestational age. Twenty transducers from 15 machines were tested with the manufacturers' approval. Machines were coded to maintain anonymity. Three different sized sections of plastic straws were scanned in a water bath. All measurements were made by the same investigator. Measurements were adjusted to compensate for the difference in sound velocity in water, which introduced a 3.36 per cent error in apparent depth of the image for linear array scanners, and a 3.36 per cent error in apparent depth and length for sector scanners. A large variation in measurements was obtained, although the differences were usually within acceptable limits (approximately 4 per cent). Linear array scanners were found to give similar readings as sector scanners.

Biomolecular absorption of ultrasound: II. Molecular structure.

Measurements of absorption coefficients in several globular and linear proteins yield no correlations of absorption with alpha-helix content or with the number of polypeptide chains in the protein. Removal of all but the primary structure with denaturing agents that convert proteins to random chains causes only small changes in the absorption of globular proteins. Complete denaturing of linear muscle proteins results in large reductions in absorption. Therefore, it is concluded that absorption in globular proteins is insensitive to structural characteristics while in linear proteins it is dependent upon the amount of alpha-helix content. An alternative explanation of the results is that alpha-helix contributes to absorption in both globular and linear proteins but tertiary structure in globular proteins reduces absorption because of inhibited solvent interactions.

Biomolecular absorption of ultrasound. I: Molecular weight.

Amino acid solutions have absorptions which are generally small compared to those for proteins. Proteolytic enzyme treatment of proteins in solution reduces their absorption. These observations suggest that absorption increases with molecular weight. However, measurements of sugars, polysaccharides, amino acids, and proteins yield no correlations of absorption with molecular weight within these groups. Therefore, it is concluded that absorption increases in these molecules with increasing molecular weight only in a threshold sense, with absorption increasing significantly only in a restricted molecular weight range. This range may approximate that observed for polyethylene glycol and dextran, viz., 1 to 100 monomer units. However, there is some indication that the transition range may be narrower than a factor of 100 in molecular weight.