Diagnostic ultrasound ( PDFDrive )

CHAPTER 29 Bioeffects and Safety of Ultrasound in Obstetrics 1037
A
B
C
FIG. 29.4 Effects of Altering Output Power and Receiver
Gain. (A) Output power is 100%, MI is 1.2, and TI is 0.1 (yellow
box). (B) Output power (blue box) has been reduced (76%) and
TI is low (0.1) but the image is nondiagnostic. (C) Only the receiver
gain has been increased, resulting in an image virtually identical
to A but maintaining the low output power as relected by the
unchanged MI and TI.
maternal temperature, whether from illness or exposure to heat,
can produce teratogenic efects. 20,37,38,41-56 A rise less than 2°C is
thought to be safe, 57 although any temperature increase for any
amount of time may have some efect, 46,58 and a rise of 2.5°C
may be considered signiicant. 59 A major question is whether
diagnostic ultrasound can induce a rise in temperature in the
fetus that could reach dangerous levels. 13,57,60 Temperature elevation
in the human fetus cannot be exactly measured but can be
estimated fairly accurately. 61,62 For prolonged ultrasound exposures,
temperature elevations of up to 5°C have been obtained. 57
hus any temperature increment for any period of time has some
efect; the higher the temperature diferential or the longer the
temperature increment, the greater is the likelihood of producing
an efect. Although these assumptions cannot be demonstrated
in diagnostic ultrasound and no human data exist, clinicians
should keep these facts in mind when performing obstetric
ultrasound. his also forms part of the argument against nonmedical
or nonindicated ultrasound examinations.
As with any external inluence on the pregnancy, gestational
age is a vital factor. Milder (in time or intensity) exposures
during the preimplantation period (very early gestational age)
could have similar or worse consequences than more severe
exposures during embryonic and fetal development and could
result in fetal demise and abortion or structural and functional
defects. Such a dose analysis is not available. As for many other
teratogens, the central nervous system (CNS) is most at risk
because of a lack of compensatory growth by undamaged
neuroblasts. In experimental animals the most common defects
associated with temperature increase are of the neural tube
(anencephaly, microencephaly) and the eyes (microphthalmia,
cataract). Associated with CNS defects are functional and
behavioral problems. 45 Other organ defects secondary to hyperthermia
include defects of craniofacial development (e.g., clets)
and anomalies of the axial and appendicular skeleton, 63 the body
wall, teeth, and heart.
Gestational age is critical when considering heat dispersion.
In midterm, there was no signiicant diference when guinea pig
fetal brains were exposed, alive (perfused) or postmortem
(nonperfused), in the focal region of the ultrasound beam.
However, a signiicant cooling efect of vascular perfusion was
observed when the fetuses reached the stage of late gestation
near term, when the cerebral vessels were well developed. 64 In
early human pregnancy, less than 6 weeks, the minimal fetal
perfusion may reduce heat dispersion. 65 he increased sensitivity
of Doppler devices suggests evidence of blood low within
embryonic vesicles ater heart formation, with the simultaneous
development of a uterine circulatory pathway in the developing
placenta. he low is oten referred to as “nonpulsatile” or