PHYSICS OF ULTRASOUNDPRINCIPLEUltrasound pulses of a particular frequency are transmitted into the tissues, and thereturning echoes are received and processed to generate an image of the underlyingtissue. Different tissues interact with ultrasound in different ways causing differentlevels 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 thevariations of sound velocities in different media, are responsible for the specificultrasound characteristics of tissues. Different tissues interact with ultrasound indifferent ways causing different levels of acoustic energy to be reflected back tothe transducer.ACOUSTIC IMPEDANCEIt is defined as the relationship of the acoustic pressure ("driving force") to themolecular motion that is induced within the medium. Even a low acoustic pressurewill cause the molecules in a medium of high density and low compressibility tovibrate. The molecules in highly compressible media such as gases must bedeflected to a greater degree before they will vibrate.The interface between two adjacent media of different compressibility produces animpedance mismatch, i.e., an acoustic boundary that can reflect sound waves.Because the acoustic impedances of different human tissues are very similar, onlya very small part of the ultrasound energy is reflected at the interface betweenacoustically dissimilar tissues, while most of the energy (more than 99% onaverage) is transmitted through. Boundaries between soft tissue and bone reflect alarge portion of the ultrasound energy (about 50%). Most of the transmitted energyis then lost to attenuation, leaving very little energy that can return to thetransducer 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 namefrom "brightness modulation," meaning that echoes are displayed on the monitor asilluminated spots whose brightness is proportional to the amplitude of the echoesreceived.The ultrasound beam is transmitted by electronically switched crystal arrays, or bymoving crystals. The array emits numerous ultrasound beams that enter the tissuein a linear or fan-shaped pattern, and it receives the reflected ultrasound energy inthe same position. Based on an assumed sound velocity of 1540m/s in tissue, the
returning echoes are processed according to their arrival time (i.e., their distancefrom the transducer), and are processed and displayed line by line, generating atwo-dimensional cross-sectional image of the scanned tissue plane.TIME GAIN COMPENSATIONBecause ultrasound waves lose energy as they travel through tissues, the echoesreturned from deeper structures are weaker than those from more superficialstructures. Thus, the echoes from greater depths must be amplified more thanechoes from the near field, in order to ensure that all echoes are displayed at theirtrue intensity.TGC accomplishes this by amplifying signals in proportion to the attenuationcaused by the depth of the reflector. At the same time to allow individual and interindividual variations in the absorption and reflection characteristics of theexamined tissues, ultrasound units are equipped with slide switches for manuallyadjusting the TGC slope.TRANSDUCER
PIEZOELECTRIC EFFECTIt was first described in 1880.It is defined as the application of an electric field tocertain materials causing a change in their physical dimensions, & vice versa. Thereverse 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 ingroups that function alternately as transmitters and receivers. Ultrasound is emittedfrom the transducer in parallel scan lines. The linear array provides uniformresolution over the full depth of the image filed.The focal depth can be freely selected and multiple focal points can be selected toincrease lateral resolution over the full depth of the image. The near focal regionoften starts at a depth of only 0.5cm, providing a clear, detailed image of breasttissues just beneath the skin.RESOLUTION: It is defined as the ability to discriminate two closely adjacentobjects as being separate structures. It is of two types:Lateral resolutionIt characterizes the ability to discriminate adjacent objects in a line perpendicularto the axis of the beam. Lateral resolution is inversely proportional to the width ofthe beam, and it also depends on the number and density of the adjacenttransmitted and received sound beams. High-frequency transducers emit anarrower 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, dependingon the location of the lesion in the breast and on the surrounding tissues .Axial resolutionIt is the ability to distinguish two objects that are on a line parallel to the beamaxis.Axial resolution can be improved by using a broad-bandwidth transducer whichcontains special non-piezoelectric damping materials that generate waves of lowfrequency in addition to the higher frequency waves from the piezoelectriccrystals.This “broad-band technology” produces focal regions at various levels in the scanplane, and provides almost uniform resolution over the full depth of the image.Broad-band transducers automatically decrease their transmission frequency withincreasing depth.A very high operating frequency (13MHz) provides excellent lateral resolution inthe near field, but gives relatively poor axial resolution at greater depths. An extra
5-MHz probe for patients with very large breasts can be used where the 7.5-MHzprobe may provide in sufficient depth range even at the highest power setting.ULTRASOUND ARTIFACTSPosterior enhancementDistal or posterior enhancement occurs when the ultrasound beam passes through afluid-filled cavity. Because the ultrasound traverses the fluid medium withoutabsorption or reflection, the echo signals proximal and distal to the fluid have thesame intensity. As a result, tissues lying behind the fluid appear more echogenicthan 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 projectdistally from the lateral edges of a rounded structure.Posterior acoustic shadow A high reflectivity at the tangential sites combined with high attenuation anddiffraction in those areas reduce the amplitude of echoes from reflectors behind theedges of the mass in comparison with adjacent areas.Because bone and gas-filled cavities are strongly attenuating and create a largeimpedance mismatch, they pose a barrier to ultrasound penetration. Little or noultrasound energy is available behind these structures to produce echoes that canbe imaged. The region “shaped” by these barriers appears devoid of echoes and iscalled an acoustic shadow.HARMONIC IMAGINGThis has recently been incorporated within high frequency linear transducers.Principle: It reduces noise by canceling primary sound waves & allowing passageof first or second echo broad band - two pulses with inverted phases are emitted &echoes from each are added. The linear components are cancelled and nonlinearcomponents amplified improving both contrast & spatial resolution.
ANATOMY OF THYROIDDEVELOPMENT OF THYROIDThe thyroid gland develops from an endodermal thickening of cells that originate from the 3rdbranchial pouch. In craniofacial development, the cells move to the base of the tongue to a pointknown as the foramen caecum. This is where the main anlage of the thyroid develops. (anaggregation of cells in the embryo indicating the first trace of an organ). From the base of thetongue, the anlage descends as the thyroid diverticulum, leaving the thyroglossal duct, which isconnected to the foramen caecum – passing anteriorly to the hyoid bone and thyroid cartilage. Itsettles as a bilobed organ just inferior to the thyroid cartilage, one lobe on each side of thetrachea – anterolaterally. The gland lies partly on the cricoid cartilage. The two lobes are joinedby an isthmus which unites the lobes over the trachea, anterior to the second and third trachealrings.
NORMAL THYROID ANATOMYThe thyroid gland resides in the midline of the lower neck.The gland is composed of right andleft lobes, typically interconnected by an isthmus in the midline, lying anterolateral to the larynxand 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 andbecomes 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 andsternothyroid). The thyroid gland is attached to the larynx and trachea within the visceral spaceand, therefore, moves with the larynx during swallowing.When the thyroid gland becomesenlarged, it may extend inferiorly into the superior mediastinum, commonly described as aretrosternal thyroid.VASCULAR SUPPLY
ARTERIAL SUPPLYThe superior and inferior thyroid arteries provide blood supply to the thyroid gland. Thesevessels have many anastomoses providing a rich vascular supply to this gland. The inferiorthyroid artery is a branch of the thyro-cervical trunk that arises from the subclavian artery. Itcourses anterior to the vertebral artery and longus colli muscles. The superior thyroid artery isthe first branch of the external carotid artery, arising just below the hyoid bone.VENOUS DRAINAGEVenous drainage is in the form of a plexus that drains into the internal jugular andbrachiocephalic veins. The middle and inferior cervical ganglia of the sympathetic chain providesympathetic innervation to the thyroid gland, whereas the vagus nerve provides parasympatheticregulationVARIATIONS IN PYRAMIDAL LOBEAs the thyroglossal duct involutes, a small amount of residual tissue may persist distally to formthe pyramidal lobe. The incidence of this lobe was found to be 55% in a postmortem study.It isimportant to identify this anatomic variant to avoid leaving residual tissue when totalthyroidectomy is performed. The pyramidal lobe may branch from the right or left lobe, theisthmus, or from only one lobe with absence of the isthmus . On imaging studies, the pyramidallobe demonstrates imaging characteristics similar to the normal thyroid gland. The shape of thelobe is variable, ranging from short and thick to long and thin. The pyramidal lobe is mostcommonly identified in Graves’ disease.
ECTOPIC THYROID TISSUEThe 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 avariety of additional locations, including the sublingual space, TGD, mediastinum, heart, andesophagus.The incidence of lingual thyroid tissue in clinical studies is estimated to range between 1 in 3000and 1 in 100,000 cases; however, postmortem studies have shown an incidence of approximately10%.Lingual thyroid tissue is associated with a topic thyroid gland in only 30% of cases. In theremaining 70% of cases, the lingual thyroid is then only thyroid tissue present without anassociated topic gland.
PATHOLOGICAL LESIONS OF THE THYROIDBENIGN LESIONS MALIGNANT LESIONSMultinodular goiter Papillary carcinomaHashimoto’s thyroiditis Follicular carcinomaSimple or hemorrhagic cysts Hürthle cell carcinomaFollicular adenomas Medullary carcinomaSubacute thyroiditis Anaplastic carcinoma Primary thyroid lymphoma Metastatic malignant lesionTumors can be characterized by cell of origin:Follicular cells Papillary, Follicular, Hurthle, and AnaplasticParafollicular“C” cells MedullaryTumors can also be classified according to clinical behavior and prognosis :Differentiated Papillary, Follicular, HurthlePoorly differentiated Anaplastic