Thyroid & parathyroid

• Basic physics principles
  • Ultrasound probe both emits and receives sound waves
  • Medical ultrasound: 2 - 20 MHz
  • Lower frequencies: better penetration, lower resolution
  • Higher frequencies: lower penetration, higher resolution
• B-Mode (Brightness Mode):
  • 2D black and white image in < 1mm "slice"
  • Emitted waves are reflected back from the target material relative to the degree of the material's acoustic impedance, which is dependent on density
  • Higher density materials generally "reflect" more and look brighter
  • For example, bone is more "reflective" than soft tissue; thus bony structures appear brighter on ultrasound images in contrast to darker surrounding tissue
• Doppler mode:
  • Measures direction and speed of tissue / blood motion
• M-Mode (Motion mode):
  • Pulses are emitted in quick succession and each time, an image is taken; over time, this is analogous to recording a video in ultrasound (used for heart valves)
• References: Arch Pathol Lab Med 2010;134:1541 and Radiol Clin North Am 2011;49:417

• Sonographic features of benign thyroid nodules
  • Small size (< 1 cm)
  • Fluid filled
  • Honeycomb morphology
  • Hyperechoic (colloid nodule or focal nodular Hashimoto thyroiditis)
  • Large nodules if they are predominantly cystic (cystic change accounting for > 50% of nodule)
  
  
• Sonographic features of thyroid nodules suspicious for malignancy (Diagn Cytopathol 2008;36:390, Eur J Endocrinol 2009;161:103)
  • Solid oval nodules (anterior-posterior dimension: transverse dimension ratio is > 1)
  • Presence of discrete coarse echogenic foci and/or microcalcifications
  • Hypoechoic (medullary and papillary thyroid cancers)
  • Thin capsules or irregular borders (suggestive of extracapsular spread)
  • Intranodular vascularity
  • Nodal metastases
  
  
• Sonographic features of benign lymph nodes
  • Oval shape (short axis: long axis ratio ≤ 0.5)
  • Hypoechoic cortex and echoic hilum (due to adipose tissue)
  • Clearly demarcated margin from surrounding tissue
  • Central vascularization
  
  
• Sonographic features of lymph nodes suspicious for malignancy (Eur J Endocrinol 2009;161:103)
  • Round shape (short axis: long axis ratio > 0.5)
  • Echogenic heterogeneity of cortex and absent fatty hilum
  • Irregular margin with surrounding tissue
  • Increased or abnormally located vascularity
  • Features suggestive of metastatic thyroid cancer: cystic appearance, hyperechoic punctations / calcifications
  
  

• Strongly recommend ultrasonic examination of thyroid and cervical lymph nodes if thyroid nodules; recommend FNA for sampling if > 1 cm in greatest dimension and high suspicion sonographic pattern (estimates a 70% - 90% risk of malignancy), including:
  • Solid hypoechoic nodule or nodule that is partially solid and hypoechoic and partially cystic with one or more of the following features:
    • Irregular margins (infiltrative, microlobulated)
    • Microcalcifications
    • Oval (taller than wide) shape
    • Rim calcifications with an extrusive soft tissue component
    • Evidence of extrathyroidal extension (Thyroid 2016;26:1)

• Anechoic: black (e.g. blood, cystic fluid)
• Azimuthal plane: midsagittal plane of transducer, "beam" used to guide needle in UGFNA
• Hyperechoic: brighter than surrounding tissue (e.g. bone)
• Hypoechoic: darker than surrounding tissue (e.g. soft tissue vs. bone)
• Isoechoic: same intensity as surrounding tissue
  
  
• Ultrasound artifacts:
  • Posterior (acoustic) shadowing: strong reflectors (air) or absorbers (stones, bones) block visualization of structures beyond them in relation to the beam
  • Posterior (acoustic) enhancement: anechoic structures (cysts) show brighter signals from areas beyond them in relation to the beam
  • Eggshell calcification: nodules surrounded by a layer of calcium have bright anterior and posterior walls due to a reflection from the surface, but posteriorly there is acoustic shadowing; this phenomenon also leads to edge artifact in which parallel dark lines extend posteriorly from the sides of nodules
  • Reverberation artifact: sound waves reflect off a very reflective surface and are re-reflected from the skin, resulting in phantom images behind the target image
  • Comet tail artifact: reverberation artifact from front and back of a very strong reflector / absorber (air bubble, metal fragment) - can also happen with dense colloid
  
  
• Bayonet sign:
  • Due to speed propagation artifact - machines use average speed of sound to calculate depth
  • If sound actually travels faster in the tissue (anechoic or hypoechoic structures), a reflector will appear closer to the transducer than its actual depth and vice versa
  • A needle with its tip in a cyst or nodule with differing echogenicity from surrounding tissue will appear to have its tip bent due to this artifact, looking like a bayonet
  
  
• See Arch Pathol Lab Med 2010;134:1541 and Radiol Clin North Am 2011;49:417

Risk of thyroid cancer

Ultrasound FNA techniques

ATA algorithm for
patients with
thyroid nodules

ATA association

Abbreviations: AIT, amiodarone induced thyrotoxicosis; ESR, erythrocyte sedimentation rate
See: Radiol Clin North Am 2011;49:391

• Indications for Ultrasound Guided Fine Needle Aspiration (UGFNA)
  • Nonpalpable or difficult to palpate nodules, most commonly of thyroid
  • Targeting specific areas in complex and cystic nodules, such as solid areas
  • For repeat FNA, when a prior palpation guided FNA sample was insufficient (Diagn Cytopathol 2008;36:390)
  • Followup for patients post partial or total thyroidectomy for malignancy, such as to sample thyroid bed (J Ultrasound Med 2013;32:1319)
  
  
• Benefits of UGFNA compared to conventional FNA alone
  • Higher rates of successful biopsy (J Clin Ultrasound 1994;22:535, Thyroid 1998;8:15, Thyroid 1998;8:283)
  • Potentially lowers risk of damage to surrounding structures, which can be visualized
  • Varying reports on cost effectiveness (BMC Endocr Disord 2009;9:14, Thyroid 2006;16:555)
  • Efficacy of UGFNA is improved by the presence of a cytopathologist onsite, decreasing the overall inadequacy rate from 9.3% to 6% (Br J Radiol 2014;87:20130571)
  
  
• Complications and contraindications: Same as conventional FNA
  
  
• Overview of procedure - Focused thyroid U / S and biopsy
  • Image each thyroid lobe in the transverse and longitudinal planes to determine the overall appearance and locate nodules
  • After completing ultrasonic assessment of the thyroid, relocate the position and measure size of suspicious nodules
  
  
• Approaches to UGFNA relative to the transducer beam (azimuthal plane)
  • For each approach, orient the needle with the bevel tip up to create the greatest reflection
  • Parallel approach "in beam"
    • Point the needle down along the plane of the beam towards the nodule
    • Maintain needle and transducer in the same plane, parallel to the plane of the transducer and advance the needle into the nodule
    • Advantage: entire length of needle seen
  • Perpendicular approach "out of beam"
    • Point the needle towards the midpoint of the transducer's side (long axis)
    • The perpendicular approach will result in visualization of the needle as it transversely crosses the plane of the beam at 90 degrees
    • The nodule and needle point will be centered in the midpoint of the transducer's long axis
    • Advantage: desirable due to anatomy in some locations
    • Disadvantage: entire length of needle not visualized
  
  
• Needle based sample collection techniques
  • With aspiration / suction
    • 27 or 25G needle attached to a 10 cc syringe (with or without extension tubing) withdrawn so that 1 - 2 cc of negative pressure induces aspiration
  • Without aspiration / nonsuction
    • 27 or 25G needle (with or without stylet, may attach open syringe) is introduced into nodule and capillary action causes uptake of cellular material into the needle
  
  
• Sample preparation: same as for palpation guided FNA

• Baskin, Jack, Duick, and Levine: Thyroid Ultrasound and Ultrasound-Guided FNA, 3rd Edition, 2013