Diagnostic ultrasound ( PDFDrive )

714 PART III Small Parts, Carotid Artery, and Peripheral Vessel Sonography
According to multiple studies of the various sonographic
features seen in thyroid nodules, microcalciications show the
highest accuracy (76%), speciicity (93%), and positive predictive
value (70%) for malignancy as a single sign. However, sensitivity
is low (36%) and insuicient to be reliable for detection of
malignancy. 29,65,72,73
Doppler Flow Pattern. It is well known from histologic
studies that most hyperplastic nodules are hypovascular lesions
and are less vascular than normal thyroid parenchyma. On the
contrary, most well-diferentiated thyroid carcinomas are
generally hypervascular, with irregular tortuous vessels and AV
shunting (see Fig. 19.16). Poorly diferentiated and anaplastic
carcinomas are oten hypovascular because of the extensive
necrosis associated with their rapid growth (see Fig. 19.26).
Quantitative analysis of low velocities is not accurate in
diferentiating benign from malignant nodules, so the only
Doppler feature that may be useful is the distribution of vessels.
With current technology, no thyroid nodule appears totally
avascular or extremely hypovascular on color and power Doppler
imaging. he two main categories of vessel distribution are nodules
with peripheral vascularity and nodules with internal vascularity
(with or without a peripheral component). 19,20,74 Studies have
demonstrated that 80% to 95% of hyperplastic, goitrous, and
adenomatous nodules display peripheral vascularity, whereas
70% to 90% of thyroid malignancies display internal vascularity,
with or without a peripheral component. 3,5,74-76 In addition, the
RI of intranodular vessels was signiicantly higher in malignant
nodules. Consequently, the vascular pattern and RI have been
reported to provide high sensitivity (92.3%) and speciicity (88%)
for the diferentiation between benign and malignant tumors. 76
According to other reports, however, color Doppler imaging was
not a reliable aid in the sonographic diagnosis of thyroid
nodules. 77-79 With the current generation of Doppler instruments,
which have extremely high sensitivity to blood low, the overlapping
of the two populations of nodules signiicantly increased,
signiicantly reducing the diagnostic reliability of Doppler
indings. 80
Findings on gray-scale and color Doppler ultrasound become
highly predictive for malignancy only when multiple signs are
simultaneously present in a nodule. 59,60,65 In a series the combination
of absent halo sign plus microcalciications plus intranodular
low pattern achieved a 97.2% speciicity for the diagnosis
of thyroid malignancy. 59 In a large study the presence of at least
one malignant sonographic inding (taller-than-wide shape,
spiculated margin, marked hypoechogenicity, microcalciication
and macrocalciication) had sensitivity of 83.3%, speciicity of
74.0%, and diagnostic accuracy of 78.0%. 65 he presence of other
indings (e.g., rim calciication) showed no statistical signiicance
in the diferentiation of a malignant nodule from a benign nodule.
In a recent pictorial review based on the ultrasound classiication
of the British hyroid Association, benign and malignant features
of thyroid nodules have been highlighted and discussed. 81
Thyroid Imaging Reporting and Data System
To facilitate interpretation and standardization among specialists,
recently a standardized system for analyzing and reporting thyroid
ultrasound and for risk stratiication (hyroid Imaging Reporting
and Data System [TIRADS]) was proposed, 82,83 following the
BIRADS system widely used for mammography.
According to this system, thyroid nodules can be categorized
in six diferent risk groups according to their ultrasonographic
characteristics:
• TIRADS 1: normal thyroid gland.
• TIRADS 2: benign conditions (0% malignancy), including
simple cyst, spongiform nodules, and isolated
macrocalciications.
• TIRADS 3: probably benign nodules (<5% malignancy).
• TIRADS 4: suspicious nodules (5%-80% malignancy rate).
Nodules in this group can be further categorized as 4a
(malignancy 5%-10%) and 4b (malignancy 10%-80%).
• TIRADS 5: probably malignant nodules (malignancy > 80%).
• TIRADS 6: biopsy-proven malignant nodules.
Contrast-Enhanced Ultrasound and Elastography
Ultrasound elastography and contrast-enhanced ultrasound
(CEUS) are advanced techniques for thyroid nodule characterization,
with a substantial amount of research in the last decade.
CEUS may depict the macrovascularization and microvascularization
of thyroid nodules, providing both qualitative and quantitative
evaluation. Nodules show heterogeneous enhancement depending
on histology. Malignant nodules most frequently have isohypovascular
contrast enhancement and slower wash-out than thyroid
parenchyma, whereas benign nodules show hypervascular
enhancement and a rimlike pattern. 84 A recent meta-analysis
based on seven studies regarding CEUS evaluation of thyroid
nodules concluded that CEUS, with pooled sensitivity, speciicity,
and positive and negative likelihood ratio (LR) of 0.853, 0.876,
5.822, and 0.195, respectively, is accurate to diferentiate benign
from malignant nodules, 85 but studies with larger sample sizes
are needed to conirm these preliminary data.
Elastography has also been applied to the study of thyroid
nodules. Elastography provides information on tissue elasticity,
based on the premise that pathologic processes such as cancer
alter the physical characteristics of the involved tissue. he purpose
is to acquire two sonographic images (before and ater tissue
compression) and track tissue displacement by assessing the
propagation of the beam, providing accurate measurement of
tissue distortion. 86,87
Elastography applied to the thyroid gland uses principally
two diferent approaches according to the type of compression
force (excitation) and elasticity evaluation: (1) freehand ultrasound
strain elastography, with its qualitative (based on colorimetric
maps and divided into four or ive classes) 88 and semiquantitative
variants (strain ratio value); and (2) quantitative approach with
transducer-induced high acoustic pulse and measures of the
speed of the shear wave generated (shear wave elastography
[SWE]). SWE can be performed using acoustic radiation force
impulse (ARFI) technology either in a small region of interest
(ROI) (point–shear wave elastography, p-SWE) or over a larger
ield of view using color-coding to visually display the stifness
values (two-dimensional shear wave elastography [2D-SWE]).
An additional semiquantitative variant of strain elastography
uses the internal (physiologic) pulsation of the carotid artery
(in vivo quasi-static elastography). In this approach, no external