Ultrasound uses sound waves to generate images of tissues inside the body. Different tissues interact with ultrasound differently, reflecting sound waves back to the transducer in varying amounts. This allows ultrasound to distinguish between tissues and generate images of structures like the thyroid gland. The thyroid gland normally sits in the lower neck anterior to the trachea. It receives blood supply from the superior and inferior thyroid arteries and drains into neck veins. Variations can include an additional pyramidal lobe or ectopic thyroid tissue in rare cases. A variety of pathological lesions may affect the thyroid, including benign nodules, cysts, inflammation, and malignant tumors arising from follicular or parafollicular cells like papillary carcinoma

1. PHYSICS OF ULTRASOUND
PRINCIPLE
Ultrasound pulses of a particular frequency are transmitted into the tissues, and the
returning echoes are received and processed to generate an image of the underlying
tissue. Different tissues interact with ultrasound in different ways causing different
levels of acoustic energy to be reflected back to the transducer.
Ultrasound energy is subject to two basic processes as it travels through tissues:
attenuation (absorption) and scattering (reflection).These two processes, plus the
variations of sound velocities in different media, are responsible for the specific
ultrasound characteristics of tissues. Different tissues interact with ultrasound in
different ways causing different levels of acoustic energy to be reflected back to
the transducer.
ACOUSTIC IMPEDANCE
It is defined as the relationship of the acoustic pressure ("driving force") to the
molecular motion that is induced within the medium. Even a low acoustic pressure
will cause the molecules in a medium of high density and low compressibility to
vibrate. The molecules in highly compressible media such as gases must be
deflected to a greater degree before they will vibrate.
The interface between two adjacent media of different compressibility produces an
impedance mismatch, i.e., an acoustic boundary that can reflect sound waves.
Because the acoustic impedances of different human tissues are very similar, only
a very small part of the ultrasound energy is reflected at the interface between
acoustically dissimilar tissues, while most of the energy (more than 99% on
average) is transmitted through. Boundaries between soft tissue and bone reflect a
large portion of the ultrasound energy (about 50%). Most of the transmitted energy
is then lost to attenuation, leaving very little energy that can return to the
transducer as echoes from structures located within or past the bone.
B - MODE
It is the most commonly used mode in breast examination. It derives its name
from "brightness modulation," meaning that echoes are displayed on the monitor as
illuminated spots whose brightness is proportional to the amplitude of the echoes
received.
The ultrasound beam is transmitted by electronically switched crystal arrays, or by
moving crystals. The array emits numerous ultrasound beams that enter the tissue
in a linear or fan-shaped pattern, and it receives the reflected ultrasound energy in
the same position. Based on an assumed sound velocity of 1540m/s in tissue, the

2. returning echoes are processed according to their arrival time (i.e., their distance
from the transducer), and are processed and displayed line by line, generating a
two-dimensional cross-sectional image of the scanned tissue plane.
TIME GAIN COMPENSATION
Because ultrasound waves lose energy as they travel through tissues, the echoes
returned from deeper structures are weaker than those from more superficial
structures. Thus, the echoes from greater depths must be amplified more than
echoes from the near field, in order to ensure that all echoes are displayed at their
true intensity.
TGC accomplishes this by amplifying signals in proportion to the attenuation
caused by the depth of the reflector. At the same time to allow individual and inter
individual variations in the absorption and reflection characteristics of the
examined tissues, ultrasound units are equipped with slide switches for manually
adjusting the TGC slope.
TRANSDUCER

3. PIEZOELECTRIC EFFECT
It was first described in 1880.It is defined as the application of an electric field to
certain materials causing a change in their physical dimensions, & vice versa. The
reverse of the piezoelectric effect converts the energy back to its original form.
A linear-array transducer consists of a large number of small crystals arranged in
groups that function alternately as transmitters and receivers. Ultrasound is emitted
from the transducer in parallel scan lines. The linear array provides uniform
resolution over the full depth of the image filed.
The focal depth can be freely selected and multiple focal points can be selected to
increase lateral resolution over the full depth of the image. The near focal region
often starts at a depth of only 0.5cm, providing a clear, detailed image of breast
tissues just beneath the skin.
RESOLUTION: It is defined as the ability to discriminate two closely adjacent
objects as being separate structures. It is of two types:
Lateral resolution
It characterizes the ability to discriminate adjacent objects in a line perpendicular
to the axis of the beam. Lateral resolution is inversely proportional to the width of
the beam, and it also depends on the number and density of the adjacent
transmitted and received sound beams. High-frequency transducers emit a
narrower beam than low-frequency transducers.
Frequencies of 7.5MHz or higher have a resolution of approximately 0.2mm,
meaning that they can define structures as small as 0.2mm in diameter, depending
on the location of the lesion in the breast and on the surrounding tissues .
Axial resolution
It is the ability to distinguish two objects that are on a line parallel to the beam
axis.
Axial resolution can be improved by using a broad-bandwidth transducer which
contains special non-piezoelectric damping materials that generate waves of low
frequency in addition to the higher frequency waves from the piezoelectric
crystals.
This “broad-band technology” produces focal regions at various levels in the scan
plane, and provides almost uniform resolution over the full depth of the image.
Broad-band transducers automatically decrease their transmission frequency with
increasing depth.
A very high operating frequency (13MHz) provides excellent lateral resolution in
the near field, but gives relatively poor axial resolution at greater depths. An extra

4. 5-MHz probe for patients with very large breasts can be used where the 7.5-MHz
probe may provide in sufficient depth range even at the highest power setting.
ULTRASOUND ARTIFACTS
Posterior enhancement
Distal or posterior enhancement occurs when the ultrasound beam passes through a
fluid-filled cavity. Because the ultrasound traverses the fluid medium without
absorption or reflection, the echo signals proximal and distal to the fluid have the
same intensity. As a result, tissues lying behind the fluid appear more echogenic
than tissues of equal quality and depth lying adjacent to the fluid-filled cavity.
This phenomenon is useful for recognizing fluid –filled breast masses at ultrasound
(cysts, Hemangiomas, etc.). Edge shadows are band-like shadows that project
distally from the lateral edges of a rounded structure.
Posterior acoustic shadow
A high reflectivity at the tangential sites combined with high attenuation and
diffraction in those areas reduce the amplitude of echoes from reflectors behind the
edges of the mass in comparison with adjacent areas.
Because bone and gas-filled cavities are strongly attenuating and create a large
impedance mismatch, they pose a barrier to ultrasound penetration. Little or no
ultrasound energy is available behind these structures to produce echoes that can
be imaged. The region “shaped” by these barriers appears devoid of echoes and is
called an acoustic shadow.
HARMONIC IMAGING
This has recently been incorporated within high frequency linear transducers.
Principle: It reduces noise by canceling primary sound waves & allowing passage
of first or second echo broad band - two pulses with inverted phases are emitted &
echoes from each are added. The linear components are cancelled and nonlinear
components amplified improving both contrast & spatial resolution.

5. ANATOMY OF THYROID
DEVELOPMENT OF THYROID
The thyroid gland develops from an endodermal thickening of cells that originate from the 3rd
branchial pouch. In craniofacial development, the cells move to the base of the tongue to a point
known as the foramen caecum. This is where the main anlage of the thyroid develops. (an
aggregation of cells in the embryo indicating the first trace of an organ). From the base of the
tongue, the anlage descends as the thyroid diverticulum, leaving the thyroglossal duct, which is
connected to the foramen caecum – passing anteriorly to the hyoid bone and thyroid cartilage. It
settles as a bilobed organ just inferior to the thyroid cartilage, one lobe on each side of the
trachea – anterolaterally. The gland lies partly on the cricoid cartilage. The two lobes are joined
by an isthmus which unites the lobes over the trachea, anterior to the second and third tracheal
rings.

6. NORMAL THYROID ANATOMY
The thyroid gland resides in the midline of the lower neck.The gland is composed of right and
left lobes, typically interconnected by an isthmus in the midline, lying anterolateral to the larynx
and trachea at approximately the level of the second and third tracheal rings.
The normal thyroid gland weighs approximately 30 g. It is slightly heavier in women and
becomes enlarged during pregnancy.
The thyroid gland resides within the visceral space, anterior to the prevertebral space,
surrounding the trachea and lying posterior to the infrahyoid strap muscles (sternohyoid and
sternothyroid). The thyroid gland is attached to the larynx and trachea within the visceral space
and, therefore, moves with the larynx during swallowing.When the thyroid gland becomes
enlarged, it may extend inferiorly into the superior mediastinum, commonly described as a
retrosternal thyroid.
VASCULAR SUPPLY

7. ARTERIAL SUPPLY
The superior and inferior thyroid arteries provide blood supply to the thyroid gland. These
vessels have many anastomoses providing a rich vascular supply to this gland. The inferior
thyroid artery is a branch of the thyro-cervical trunk that arises from the subclavian artery. It
courses anterior to the vertebral artery and longus colli muscles. The superior thyroid artery is
the first branch of the external carotid artery, arising just below the hyoid bone.
VENOUS DRAINAGE
Venous drainage is in the form of a plexus that drains into the internal jugular and
brachiocephalic veins. The middle and inferior cervical ganglia of the sympathetic chain provide
sympathetic innervation to the thyroid gland, whereas the vagus nerve provides parasympathetic
regulation
VARIATIONS IN PYRAMIDAL LOBE
As the thyroglossal duct involutes, a small amount of residual tissue may persist distally to form
the pyramidal lobe. The incidence of this lobe was found to be 55% in a postmortem study.It is
important to identify this anatomic variant to avoid leaving residual tissue when total
thyroidectomy is performed. The pyramidal lobe may branch from the right or left lobe, the
isthmus, or from only one lobe with absence of the isthmus . On imaging studies, the pyramidal
lobe demonstrates imaging characteristics similar to the normal thyroid gland. The shape of the
lobe is variable, ranging from short and thick to long and thin. The pyramidal lobe is most
commonly identified in Graves’ disease.

8. ECTOPIC THYROID TISSUE
The foramen cecum at the base of the tongue is the most common ectopic thyroid location
(Lingual thyroid) , accounting for 90% of cases. Up to 10% of ectopic thyroid tissue is found in a
variety of additional locations, including the sublingual space, TGD, mediastinum, heart, and
esophagus.
The incidence of lingual thyroid tissue in clinical studies is estimated to range between 1 in 3000
and 1 in 100,000 cases; however, postmortem studies have shown an incidence of approximately
10%.Lingual thyroid tissue is associated with a topic thyroid gland in only 30% of cases. In the
remaining 70% of cases, the lingual thyroid is then only thyroid tissue present without an
associated topic gland.

9. PATHOLOGICAL LESIONS OF THE THYROID
BENIGN LESIONS MALIGNANT LESIONS
Multinodular goiter Papillary carcinoma
Hashimoto’s thyroiditis Follicular carcinoma
Simple or hemorrhagic cysts Hürthle cell carcinoma
Follicular adenomas Medullary carcinoma
Subacute thyroiditis Anaplastic carcinoma
Primary thyroid lymphoma
Metastatic malignant lesion
Tumors can be characterized by cell of origin:
Follicular cells
Papillary,
Follicular,
Hurthle, and
Anaplastic
Parafollicular“C” cells
Medullary
Tumors can also be classified according to clinical behavior and prognosis :
Differentiated
Papillary,
Follicular,
Hurthle
Poorly differentiated
Anaplastic

10. IMAGING MODALITIES OF THYROID