Diagnostic ultrasound ( PDFDrive )

692 PART III Small Parts, Carotid Artery, and Peripheral Vessel Sonography
Right
Left
Submandibular
gland
Clavicle
Internal
jugular vein
Carotid artery
A
Sternocleidomastoid
muscle
Strap muscles
Trachea
Thyroid gland
Internal jugular
vein
FIG. 19.1 Cervical “Map.” Such diagrams help communicate relationships
of pathology to clinicians and serve as a reference for follow-up
examinations.
high-frequency gray-scale and color Doppler images. he thyroid
gland is one of the most vascular organs of the body. As a result,
Doppler examination may provide useful diagnostic information
in some thyroid diseases.
Two newer techniques used for the sonographic study of the
thyroid gland are contrast-enhanced sonography and elastography.
Contrast-enhanced sonography using second-generation contrast
agents and very low mechanical index can provide useful information
for the diagnosis of select cases of nodular disease and for
ultrasound-guided therapeutic procedures. Elastography is based
on the principle that when body tissues are compressed, the
soter parts deform more easily than the harder parts. he amount
of displacement at various depths is determined by the ultrasound
signals relected by tissues before and ater they are compressed,
and the corresponding strains are calculated from these displacements
and displayed visually.
he patient is typically examined in the supine position, with
the neck extended. A small pad may be placed under the shoulders
to provide better exposure of the neck, particularly in patients
with a short, stocky habitus. he thyroid gland must be examined
thoroughly in both transverse and longitudinal planes. Imaging
of the lower poles can be enhanced by asking the patient to
swallow, which momentarily raises the thyroid gland in the neck.
he entire gland, including the isthmus, must be examined. he
examination must also be extended laterally to include the region
of the carotid artery and jugular vein to identify enlarged jugular
chain lymph nodes, superiorly to visualize submandibular
adenopathy, and inferiorly to deine any pathologic supraclavicular
lymph nodes.
In addition to the images recorded during the examination,
some operators include in the permanent record a diagrammatic
representation of the neck showing the location(s) of any abnormal
indings (Fig. 19.1). his cervical “map” helps to communicate
the anatomic relationships of the pathology more clearly to the
referring clinician and the patient. It also serves as a useful reference
for the radiologist and sonographer for follow-up
examinations.
VII
Cervical
vertebrae
Common carotid artery Esophagus Longus colli muscle
B
FIG. 19.2 Normal Thyroid Gland. (A) Transverse sonogram made
with 7.5-MHz linear array transducer. (B) Corresponding anatomic drawing.
C, Common carotid artery; J, jugular vein; Tr, tracheal air shadow.
ANATOMY
he thyroid gland is located in the anteroinferior part of the
neck (infrahyoid compartment) in a space outlined by muscle,
trachea, esophagus, carotid arteries, and jugular veins (Fig. 19.2).
he thyroid gland is made up of two lobes located along either
side of the trachea and connected across the midline by the
isthmus, a thin structure draping over the anterior tracheal wall
at the level of the junction of the middle and lower thirds of the
thyroid gland. From 10% to 40% of normal patients have a small
thyroid (pyramidal) lobe arising superiorly from the isthmus
and lying in front of the thyroid cartilage. 1 It can be regularly
visualized in younger patients, but it undergoes progressive
atrophy in adulthood and becomes invisible. he size and shape
of the thyroid lobes vary widely in normal patients. In tall
individuals the lateral lobes have a longitudinally elongated shape
on the sagittal scans, whereas in shorter individuals the gland
is more oval. In the newborn the thyroid gland is 18 to 20 mm
long, with an anteroposterior (AP) diameter of 8 to 9 mm. By
1 year of age, the mean length is 25 mm and AP diameter is 12
to 15 mm. 2 In adults the mean length is approximately 40 to
60 mm, with mean AP diameter of 13 to 18 mm. he mean
thickness of the isthmus is 4 to 6 mm. 3
Sonography is an accurate method for calculating thyroid
volume. In about one-third of cases, the sonographic measurement
of volume difers from the estimated physical size on
examination. 4 hyroid volume measurements may be useful for
goiter size determination to assess the need for surgery, permit
calculation of the dose of iodine-131 ( 131 I) needed for treating
thyrotoxicosis, and evaluate response to suppression treatments. 5
hyroid volume can be calculated with linear parameters or more
precisely with mathematical formulas. Among the linear