Diagnostic ultrasound ( PDFDrive )

48 PART I Physics
EPIDEMIOLOGY
With all the potential causes for bioefects, we must now examine
the epidemiologic evidence that has been used in part to justify
the apparent safety of ultrasound. In 1988 Ziskin and Petitti 92
reviewed the epidemiologic studies conducted until then and
concluded, “Epidemiologic studies and surveys in widespread
clinical usage over 25 years have yielded no evidence of any
adverse efect from diagnostic ultrasound.” However, a 2008
review 93 of the epidemiology literature conducted by an AIUM
subcommittee resulted in a revised AIUM statement regarding
the epidemiology of diagnostic ultrasound safety in obstetrics. 94
he earlier statements indicated that no conirmed efects associated
with ultrasound exposure existed at that time. However,
the distinction being made in the current AIUM statement is
that although some efects may have been detected now, one
cannot justify a conclusion of a causal relationship based on this
evidence.
Epidemiologic studies are diicult to conduct, and data analysis
and interpretation of results can be even more diicult. Several
epidemiologic studies of fetal exposure to ultrasound have claimed
to detect certain bioefects and have also been criticized. Only
one indication of an unspeciied efect was reported in a general
survey involving an estimated 1.2 million examinations in
Canada. 95 However, this is an extremely low incidence, with no
follow-up to determine the nature of the efect. In addition, an
earlier study that included 121,000 fetal examinations reported
no efect. 96 Moore and colleagues 97 reported an increased incidence
of low birth weight, whereas Stark and colleagues 98 examined
the same data using a diferent statistical treatment and found
no signiicant increase. Scheidt and colleagues 99 noted abnormal
grasp and tonic neck relexes. hese results are diicult to
interpret, however, given the statistical treatment of the data.
Stark and colleagues 98 detected an increased incidence of dyslexia,
but the same children exhibited below-average birth weights.
General problems also plague the epidemiologic studies, including
the lack of clearly stated exposure conditions and gestational
age, problems in statistical sampling (with both positive and
negative results), and use of older scanning systems, particularly
with fetal Doppler ultrasound. Dyslexia was examined as part
of two randomized trials, including speciic long-term follow-up
that showed no statistical diference between ultrasound-exposed
and control subjects. 100,101
Ziskin and Petitti 92 also summarized the factors in evaluating
epidemiologic evidence. It is important to recognize that epidemiologic
evidence can be used to identify an association between
exposures and biologic efects, but that this does not prove the
exposure caused the bioefect. he strength of the association is
established by the statistical signiicance of the relationship. Hill 102
and Abramowicz and colleagues 93 have developed the following
seven criteria for judging causality:
1. Strength of the association
2. Consistency in reproducibility and with previous, related
research
3. Speciicity to a particular bioefect or exposure site
4. Classic time relationship of cause followed by efect
5. Existence of a dose response
6. Plausibility of the efect
7. Supporting evidence from laboratory studies
When considering these factors, no clear causal relationship
seems to exist between an adverse biologic response and ultrasound
exposure of a diagnostic nature.
Newnham and colleagues 103 reported higher intrauterine
growth restriction during a study designed to determine the
eicacy of ultrasound in reducing the number of neonatal days
and prematurity rate. herefore the study was not designed to
detect an adverse bioefect, but a statistically signiicant efect
was observed as a result of subsequent data analysis. Several
other deiciencies in methodology are evident in the selection
and exposure of the experimental groups, but in general, some
association might be inferred from the results of this wellconducted,
randomized clinical trial. In a case-control study,
Campbell and colleagues 104 reported a statistically signiicant
higher rate of delayed speech in children who were insoniied
in utero. Evidence of association is not as strong in case-control
studies as in prospective studies, and measures of delayed speech
are diicult. Subsequently, Salvesen and colleagues 105 found no
signiicant diferences in delayed speech, limited vocabulary, or
stuttering in a study of 1107 children exposed in utero and 1033
controls.
CONTROLLING ULTRASOUND
OUTPUT
he most important issue regarding potential bioefects involves
actions the physician or sonographer can take to minimize these
efects. It is essential that operators understand the risks involved
in the process, but without some ability to control the output of
the ultrasound system, this knowledge has limited use. Some
speciic methods can be used to limit ultrasound exposure while
maintaining diagnostically relevant images.
Controls for the ultrasound system can be divided into direct
controls and indirect controls (Table 2.2). he direct controls
are the application types and output intensity. Application types
are those broad system controls that allow convenient selection
of a particular examination type. hese oten come in the form
of icons that are selected by the user. hese default settings help
minimize the time required to optimize the imaging parameters
for the myriad applications for diagnostic ultrasound. hese
settings should be used only as indicated (e.g., do not use the
cardiac settings for a fetal examination). Output intensity (also
called “power,” “output,” or “transmit”) controls the overall
ultrasonic power emitted by the transducer. his control will
generally afect the intensity at all points in the image to varying
degrees, depending on the focusing. he lowest output intensity
that produces a good image should be used, to minimize the
exposure intensity. Focusing of the system is controlled by the
operator and can be used to improve image quality while limiting
required acoustic intensity. Focusing at the correct depth can
improve the image without requiring increased intensity.
he many indirect controls greatly afect the ultrasound
exposure by dictating how the ultrasonic energy is distributed
temporally and spatially. By choosing the mode of ultrasound
used (e.g., B-mode, pulsed Doppler, color Doppler), the operator