Journal 1pages FINAL 34- - National Board Of Examination

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Imaging of ThyroidVenkatesh K, Vikram Gulati, Dayananda L & Srikanth MoorthyDepartment of Radiology, Amrita Institute of Medical Sciences & Research Centre, Elamakkara, Kochi10ReviewArticleIntroductionThe thyroid gland is located in thelower anterior neck in theinfrahyoid compartment. Thegland is made up of two lobes locatedlateral to the trachea joined at the midlineby a thin bridge of tissue called isthmus.The pyramidal lobe is an accessory lobeoriginates from the isthmus or medialaspect of either lobe and extendssuperiorly along the course of distalthyroglossal duct. Imaging of thyroidgland can be done by USG, CT and MRIwhich provides anatomical informationwhile nuclear scintrigraphy providesfunctional information.Imaging modalitiesUltrasoundReal time ultrasound of the thyroid glandis usually performed with high resolutionlinear array transducers ranging from 7.5-10 MHz 1 . These allow excellentvisualization of both superficial portionsof gland and the deep structuresposteriorly to the level of spine.Ultrasound is an accurate method to usein calculating thyroid volume. Tissueharmonic imaging techniques are usedas an adjunct to conventionalsonography to improve lesion detectionand characterization .The thyroid glandis the most richly vascularised organ ofall superficially located normal structuresof the body. As a result colour and powerdoppler provides useful diagnosticinformation in thyroid disease.Ultrasound also provides guidance forperforming FNA biopsy. Evaluation ofretrotracheal and mediastinal regions isdifficult by ultrasound because ofacoustic shadowing from overlying airor bone 2 . Another limitation is that USGis inferior to CT and MRI in identifyinglymphadenopathy 3 .The normal thyroid parenchyma ishomogenous in appearance with greaterechogenicity than the adjacent strapmuscles. It is limited by a thin highlyreflective capsule. The dimensions ofnormal thyroid vary with the size ofsubject. The anteroposterior thickness isconsidered the most reliable index ofthyroid size. When it is larger than 2cmenlargement can be confidentlydiagnosed. Arterial and venous brancheswithin the thyroid parenchyma are clearlyvisible with current high sensitivity colourdoppler machines 4 . Normal peak systolicvelocities reach 20-40cm/s in the majorthyroid arteries and up to 15-30cm/s inthe intra parenchymal arteries.Computed tomographyThe thyroid gland is well seen on CTdue to its higher attenuation than softtissue caused by the physiologically highiodine content of the gland. CT isoccasionally used to demonstrate theextent of local invasion or local recurrenceof thyroid malignancy and to detect thepresence of retrosternal and retrotrachealextension of thyroid enlargement.Iodinated contrast may provide additionalinformation about lesion within thethyroid but the contrast will alterradioactive iodine uptake for about sixweeks following the study. Nuclearimaging should be performed prior toCT. On non contrast CT , normal thyroidgland has a density of approximately 80-100 HU because of its iodine content.Intravenous injection of iodinatedcontrast material results in diffuseincrease in density of the gland.Magnetic resonance imagingMRI is best performed with a dedicatedsurface coil centered over thyroid gland.MR imaging provides excellent depictionof internal parenchyma of the thyroidgland as well as relationship withadjacent structures in the neck andthoracic inlet. Like CT, MRI is useful instaging of a known thyroid malignanttumor, to identify local invasion , regionallymph node metastases and to detectthe recurrence following thyroidectomy.Using dynamic contrast enhancementcharacteristics thyroid lesions can beclassified as benign, indeterminate ormalignant. Rapid enhancement is seen in90% of malignant lesions while steadyor gradual enhancement is seen in 70%of benign cases. Normal thyroid glandis homogenously hyperintense to neckmusculature on both T1 and T2 weightedimages. The gland enhances diffusely andhomogeneously following contrastadministration.Plain filmPlain X-ray has a limited role in the thyroiddisorders. It may be useful in theassessment of retrosternal extension andtracheal displacement.Thyroid disordersThyroid disorders are further classifiedinto congenital and acquired disorders.CongenitalCongenital disease of thyroid includesagenesis, hemi-agenesis and varyingdegrees of hypoplasia and ectopias.Ultrasonography can easily demonstrateall of them except ectopias which arebest diagnosed by isotope scan.<strong>Journal</strong> of Postgraduate Medical Education, Training & ResearchVol. I, No. I & II39
Nodular disease of ThyroidOne of the most common clinicalproblem is patients presenting with asolitary palpable thyroid nodule. Themajority of thyroid nodules are benigndue to cyst, thyroiditis, adenomas orcolloid nodules. Though thyroidcarcinoma is uncommon it also veryoften presents as solitary thyroid nodule 5 .The challenge is to distinguish the fewclinically significant malignant nodulesfrom the many benign ones and toidentify the patients who are candidatesfor surgical excision.Role of Ultrasound in Thyroid nodulesGoitrousnonNodulesAdenomaThyroid nodule is defined as any discretelesion that is sonographicallydistinguishable from adjacentparenchyma.High resolution sonography has fourmajor applications in thyroid disease:1. Detection of thyroid and other neckmasses.2. Differentiation of benign frommalignant masses based on theirsonographic appearance3. Guidance for FNA/biopsy. FNAbiopsy is the most effective method fordiagnosing malignancy in thyroid nodule.4. Guidance for percutaneous treatmentof non functioning and hyperfunctioningbenign thyroid nodules and of lymphnode metastasis from thyroid carcinoma.For the differentiation of benign versusmalignant thyroid nodules sonographyhas sensitivity rates ranging from 63%-94%,specificity from 61%-95% andover all accuracy of about 80%-94% 6 .Currently no single sonographiccriterion distinguishes benign thyroidnodules from malignant thyroid noduleswith complete reliability.Non GoitrousCarcinomaFollicular Non Follicular PapillaryFollicularAnaplasticFeatures almost unique for benigngoitrous nodules are• Completely cystic appearance• Moving comet tail artifacts• Widespread cystic changes inisoechoic or highly echogenic nodules.• Hyperechoic nodules.• Thin uniform thickness perilesionalhypoechoic halo• Well defined and regular margins• Perilesional egg shell like or coarsecalcification• Doppler signal of flow around thelesion.If most of these signs are present in athyroid nodule the diagnosis of benigndisease is highly reliable and no cytologicalassessment is needed. Conversely theultrasound signs for malignancy are:• Hypoechoic nodule• Irregular margins• Thick irregular halo• Doppler signal of flow within thelesion• Microcalcifiactions• Hypervascularity• Invasion of vessels and adjacentstructures• Vessel encasementThe feature with the highestsensitivity is solid composition;however, this feature has a fairly lowpositive predictive value. The featurewith the highest positive predictivevalue is the presence ofmicrocalcifications. However thisfeature has low sensitivity. Acombination of these featuresimproves the positive predictive valueof US to some extent.The detection of additional occultnodules in patients with clinicallysolitary lesion though pathognomonicfor benign goitre, it does not totallyrule out malignancy 7 . In as many as33%-64% of patients with thyroid cancerat least one associated benign nodulewas found either surgically or atultrasound. When multinodularity isdetected each nodule should be carefullystudied. If all of them show the samecharacteristic pattern for goitrous nodules, cytological assessment is not strictlyneeded and the patient can be followedwith periodic scans. Nodules withsuspicious ultrasound criteria shouldunder go further assessment with FNA/biopsy.40<strong>Journal</strong> of Postgraduate Medical Education, Training & ResearchVol. I, No. I & II
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Journal 1pages FINAL 34- - National Board Of Examination

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