Diagnostic ultrasound ( PDFDrive )

CHAPTER 44 Cervical Ultrasound and Preterm Birth 1497
reproducibility of TAS measurement of the cervix. Research has
demonstrated conlicting results regarding the usefulness of TAS
screens in that the cutof in TAS to identify a cervix length of
25 mm or less is variable. his suggests that higher TAS cutofs
are required to achieve high sensitivity. 14,16 he challenge is that
in a low-risk group, the prevalence of short cervix due to cervical
incompetence is low and a large sample size is required to achieve
meaningful statistics.
Transperineal/Translabial Approach
Transperineal sonography is reserved for patients for whom the
cervix cannot be adequately visualized by TAS, and in whom
TVS is unacceptable for personal or discomfort-related concerns.
Scanning is performed with an empty urinary bladder. An
abdominal transducer with a frequency of 3 MHz or higher can
be used. 17-19 To minimize the risk of transmission of infection,
the transducer is covered with plastic wrap. With the patient
supine and hips abducted, the transducer is placed between the
labia minora at the vaginal introitus. he ultrasound beam is
B
FIG. 44.3 Transperineal Scan of Normal Cervix. The cervix (calipers)
is oriented horizontally, approximately perpendicular to the ultrasound
beam. The vagina (V) is oriented in a nearly vertical plane. B, Bladder;
R, rectum.
V
R
oriented in a sagittal plane along the direction of the vagina.
he vagina is seen in a vertical plane between the bladder and
the rectum (Fig. 44.3). 15,20,21 he cervix is oriented horizontally,
at a right angle with the vagina. he full length of the cervical
canal can be visualized in 86% to 96% of patients with this
technique 17,18 ; however, in some cases the region of the external
os can be obscured by rectal gas or the symphysis pubis. Although
some reports have suggested that the reproducibility of the
measurements is poor, 21-24 others have demonstrated improved
reproducibility and accuracy in the hands of an experienced
examiner. 15,22,25 his enhanced requirement for experience limits
the utility of this study. his examination has a greater learning
curve in order to perform reliably and reproducibly than TVS
studies. Cicero et al. 25 showed that 50% of transperineal studies
were inadequate because of shadowing in the learning phase of
the irst 200 patients. Subsequently in the second phase reliable
images were obtained in 78% of cases. Because transperineal
scanning of the cervix is more dependent on gestational age and
the experience of the investigator, it should be reserved as an
alternative study in patients for whom a TVS is deemed unacceptable
for psychological, cultural, or medical reasons.
Transvaginal Sonography
TVS is the reference-standard technique for accurate determination
of the dimensions and characteristics of the cervix. 26 he
examination is performed with an empty urinary bladder. A
gynecologic table itted with stirrups is preferred, although the
examination can be carried out with the patient’s hips elevated
on a thick cushion or wedge. With the patient in a dorsal lithotomy
position, supine and hips abducted, an endovaginal transducer
(5-MHz or higher frequency) covered with plastic wrap is placed
in the vagina and oriented in a longitudinal plane. he probe is
inserted under real-time visualization until the cervix comes
into view. Usually, the transducer is inserted only 3 to 4 cm into
the vagina so that the images of the cervix are within the efective
focal range of the transducer (Fig. 44.4). Depending on the
position of the cervix in the vagina, the probe may need to be
moved anteriorly, posteriorly, and/or laterally. 27 here is a
standardized method for obtaining reproducible cervical measurements
(see box).
A
B
FIG. 44.4 TVS of Normal Cervix. (A) Suggested placement of cursors for measuring cervical length. (B) Normal cervical glandular area. The
cervical canal is seen as an echogenic line (arrow) surrounded by a hypoechoic zone resulting from endocervical glands.