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Thyroid Ultrasound

last modified on: Mon, 05/14/2018 - 10:23

 

return to: Salivary Ultrasound

  1. Background

 

  1. Terminology

Echogenicity of tissue: capacity to reflect or transmit US waves in the context of surrounding tissue. Where there is an interface of structures with different echogenicities, a visible difference in contrast appears.

Hyperechoic: white    //   Hypoechoic: grey  //  anechoic:  (black)

Examples: Bone has a bright hyperechoic rim and black or anechoic center because ultrasound casts and acoustic shadow beyond it.

Lymph nodes are hypo echoic or anechoic; muscles are hypo echoic with striate structure; fat is usually anechoic

Echogenecity = even (fine echotexture is normal for parotid/smg) or coarse (ueven echotexture seen after I131 therapy in ~ 50% of parotid glands)

Heterogeneity of Echo Signal = Heterogeneity defined for Sjogrens syndrome by: "hypoechoic areas, lines or spots or hyoechoic areas surrounded by hyperechoic lines and/or spots resembling a reticular or honeycomb image" (Saied 2013)

color Doppler: flow toward the probe = red; flow away from the probe = blue (mnemonic: BART = Blue Away, Red Toward)

Scanning planes:

Transverse = Perpendicular to the long axis of the patient (termed axial for CT imaging)

Longitudinal = Parallel to the long axis of the patient (termed sagittal for CT imaging)

Other: Oblique, Coronal

Angle of incidence: the angle at which US waves encounter the surface of a structure - best images with angle of incidence = 90 degrees (perpendicular)

Anisotropy: a tissue property responsible for US reflection even with mild changes in angel of incidence. 'now you see it, now you don't' phenomenon - 

different tissues have differing degrees of anisotropy

requires adjustments to optimize visualization via US probe manuevers of pressure, tilt, and rotation to optimize the angle of incidence to get the best reflection.

High frequency probes (10-15 MHz) are best for US imgagin of superficial structures (2-4 cm)

Mid frequency probes (5-10 MHz) preferred for slightly deeper structures (5-6 cm)

Low frequency probes (2-5 MHz) preferred for deeper still (i.e. 10 cm)

Small footprint probe: narrow probe -  advantage for in-plane needle placement (allows operator to place needle entry closer to target) - disadvantage relative to 

Larger footprint probe provides a wider picture and better lateral resolution

Gain: most US machines have an 'auto-gain knob - some have gain modes designed for structures ('nerve'/'angio'/'general') but usually not needed if appropriate probe and depth selected

Depth: reasonable to set a greater depth reading initially to get the 'big picture' and the decrease the depth when the targeted structure is found - usually 1 cm deeper that the target to be injected

Probe Manipulation: mnemonic PART (Pressure, Alignment, Rotation and Tilt)

Pressure: affects echogenicity of the tissue (also shortens distance to structure of interest)

Alignment: 'sliding' - find the structure of interest and position it optimally on the screen for needle advancement

in the middle of screen for 'out-of-plane' approach

out-of-plane with long axis of needle directed across the scanning plane (out-of-plane needling commoner catheter placement)

out-of-plane needle advancement often warrants dynamic tilting or sliding of the transducer when advancing the needle to help track the tip of the needle - visualizing the tip of the needle is challenging but essential

'hydrolocation' - small volume of saline or destrose can be injected to serve as needle tip locator

somewhat on the opposite side of the screen for 'in-plane' approach

in-plane approach: needle can be seen on the US monitor in the long-axis view (in-plane needling common for single injections)

larger needles designed for enhanced echogenicity enhances visualization - also if more favorable angle of incidence is employed

Rotation: achieve several goals -

maintain true axial view of a structure as you are sliding to keep its long axis parallel to the probe

can change view back and forth from long axis to short axis

Tilt: confusion can arise from the fact that when the probe is tilted in one direction, the US plane is sweeping to the opposite direction

Acoustic enhancement: artifact of increased echogenecity in region distal to a fluid collection

Acoustic shadow: anechoic area behind bony structures

Attenuation: Progressive reduction of amplitude and intensity of a transmitted signal as it moves through a medium

Higher frequency waves attenuate closer to the source address by adjusting Time-gain compensation for attenuation

(Boosts signal to compensate)

Artifacts: do not correspond to real structure

Artifact lines: Comet-tail artifacts: vertical hyperchoic lines (reverberation occurring in a small space) streak of white running away (deep) often seen in colloidal cysts. Not calcium generally considered benign different from calcification which won't have this artifact

acoustic shadow

Tangential artifact. Echo-free zone

Acoustic enhancement (enhancement artifact) area behind cyst brighter because waves pass thru cyst readily)

reverberation artifact:  repetitive cycle losing energy each time. I.e. Trachea

Mirror artifact - internal reflection

Doppler: technique for imagine moving particles by the detection of a change in the frequency of the reflected ultrasound energy

Modes:  A-mode not used = amplitude mode = rejected energy is shown as peaks of different size

B-mode is used = brightness mode = reflected energy is displayed as areas of different brightness

M-mode = reflected energy is shown as areas of brightness traced from left to right on screen with time on x-axis (adjust to B-mode)

Axial resolution; distinguish structures in line with one another (depth)

lateral resolution: distinguish adjacent structures

  1. Physics

 

  1. Ultrasound = non audible sound energy
    1. Audible sound energy = 2-20kHz
    2. Ultrasound for medical application = 1,000x greater than audible energy (2 to 20MHz)
  2. Reflection of sound waves creates image based on changes of the sound waves as they traverse tissue including: attenuation, deflection, scattering, refraction, and reflection.
    1. Reflection depends on tissue impedance = measure of resistance to propagation of sound waves from one medium to another
    2. At the boundary between tissue structures with difference in impedance (impedance mismatch) an ultrasound pulse is partially reflected
    3. The impedance difference between air and tissue is greatest - and need be removed by using a coupling gel to remove air between the transducer and the skin
    4. Signal processing of the reflected acoustic signal is translated into a gray-scale (B-mode) image on the the screen
  3. The ultrasound pulse is generated by piezoelectric crystals in the transducer probe
    1. An alternative current applied to the crystals causes them to vibrate to convert electrical energy into mechanical energy to emit a pulse
    2. After the pulse is generated, the system awaits the return of the reflected signals - picked up by the same piezoelectric crystals in the transducer.
    3. This returning energy provides information based on analysis of the amplitude and timing of the signal that is translated into an image.

 

  1. Clinical Use

U/S guided needle placement hints:

initially take the operators attention off the screen and focus on attaining correct needle alignment with the US probe

only after some advancement of the needle through the skin has occurred should the probe operator shift attention back to screen

then, if the needle is visualized on the screen, further attention to the probe position and needle can be done based on feedback from the US screen

if the tip of the needle is lost from view on the screen, then look back at the bore and find the best possible way to realign the needle with the US plane

remember the US beam is thin (1 mm = the width of a credit card) so subtle movements will bring the needle in and out of view.    

to prepare the transducer to avoid exposure to body fluids and alcholol (used as asepsis for the skin) it is reasonable to use cling wrap as a protective barrier for the probe

a generous amount of US gel should be placed on the end of the transducer before placement of the protective wrap (Smith 2010)

Size Measurements

 

Lymph node evaluation

Normal lymph nodes: "No single ultrasonographic feature defines a normal froman abnormal cervical lymph node (Chan 2007)

Cancer more likely with the following (Chan 2007): "combination of characteristics such as increased size, round shape, absence of an echogenic hilus, intranodal necrosis and peripheral or displaced vascularity make malignancy more likely"

Abnormal lymph nodes (use of size criteria questionable - Van Den Brekel (1998)  - level I nodes: minimal axial diameter or short axis 4-6 mm; level II nodes 6-8 mm; level III-V nodes 4-7 mm;

As per Chan (2007) "best approach is not defining absolute size criteria"

Peripheral vascularity

Shape 

  1. Short axis-long axis ratio (S:L) - S/L ≥0.5 abnormal
  2. Nodal shape transformation from oval to round is 'most likely because of the malignant infiltration changing the architecture of the lymph node' (Chan 2007)
  3. Submandibular and parotid nodes can aslo be round and normal.

Nodal Border

  1. Sharp nodal border can be seen with normal lymph nodes - but unsharp borders are commonly thought to be a finding suggestive of normal lymph nodes
  2. Increased sharpness of nodal border thought to be due to increased acoustic impedance difference resulting rom tumor infiltration and also from loss of intranodal fat
  3. Malignant nodes with unsharp borders can represent extracapsular spread of tumor

Echogenic Hilus

  1. Represents collecting sinuses of a normal lymph node
  2. Hilar vascularity should be evaluated with color doppler
  3. Older patiens have more intranodal fat and therefore have more prominent hilus

Echogenicity (overall)

  1. Metastatic papillary thyroid nodes then to be hyperechoic (correlate with psammoma bodies)
  2. Most malignant nodes, tuberculous nodes, and normal nodes tend to be hypoechoic compared to adjacent muscle
     

Vascular Pattern

  1. Normal and reactive lymph nodes are generally either avascular or have vaculature confined to the  hilum
  2. Metastatic lymph nodes have characteristically more peripheral vascularity

Absence of hilus

Presence of nodal microcalcifications

Cystic changes

Lymph node classification per Lo et al (2016) "The Effect of Radiotherapy on Ultrasound-Guided Fine Needle Aspiration Biopsy and the Ultrasound Characteristics of Neck Lymph Nodes in Oral Cancer Pts After Treatment

Classifications

Classifications Features found to correlate with recurrent nodal disease following XRT (bold)

Classifications
Echo structure Predominantly solid Mixed cyst type  
Short-axis diameter Long-axis diameter S/L ratio Increased short and long axis diameter  
Nodal margin Regular Irregular  
Echogenicity (compared to adjacent muscle) Hypoechic Isoechoic  Hyperechoic
Echogenic hilum Present Absent  
Calcification Present Absent  
Internal echo Homogeneous Heterogeneous   
Vascular patter (Doppler- set for high sensitivity with low wall filter detection of vessels with low flow) Avascular or Hilar type Mixed/ Spotted Peripheral type
  1. Guidelines for performing and reporting thyroid and parathryoid glands ultrasonography

 

  1. Indications: (American College of Radiology, 2013) (Haugen, 2015) (Chaudhary, 2013)  
    1. Thyroid nodule, mass, or goiter on physical examination.
    2. Presentation with aerodigestive compressive symptoms (e.g. dyspnea, dysphagia, dysphonia). 
    3. Radiographic abnormality detected on other imaging studies (e.g. MRI, CT).
    4. Follow-up of previously detected thyroid abnormalities and after surgical or therapeutic (e.g radioactive iodine) interventions.
    5. Evaluation of laboratory abnormalities.
    6. To guide diagnostic (FNA cytology/biopsy) and therapeutic interventional procedures.
    7. Clinical presentation of hyperparathyroidism. 
  2. Basics of examination (American College of Radiology, 2013)
    1. Neck in hyperextension. 
    2. The right and left lobes of the thyroid gland should be imaged in the longitudinal and transverse planes. 
    3. The size of each thyroid lobe should be recorded in three dimensions, anteroposterior (AP), transverse, and longitudinal. The thickness (AP measurement) of the isthmus on the transverse view should be recorded.
    4. Thyroid abnormalities should be imaged in a way that allows for reporting and documentation of the following: 
      1. The location, size, number, and character of significant abnormalities, including measurements of nodules and focal abnormalities in three dimensions. 
      2. The localized or diffuse nature of any thyroid abnormality, including assessment of overall gland vascularity
      3. The sonographic features of any thyroid abnormality with respect to echogenicity, composition (degree of cystic change), margins (smooth or irregular), presence and type of calcification (if present), and other relevant sonographic patterns. 
      4. The presence and size of any abnormal lymph node in the lateral compartment of the neck.
  3. Selected AIUM Recommendations regarding documentation (accessed 4-21-2016 http://www.aium.org/resources/guidelines/documentation.pdf)
    1. There should be a permanent record of the U/S exam and its interpretation
    2. Variations from normal size should be accompanied by measurements
    3. Label images with patient identification, facility identification, exam date, side (R v L)
    4. "Retention of the ultrasound examination should be consistent both with the clinical needs and with relevant legal and local health care facility requirements"
    5. Limitations that compromise the quality of the exam should be noted (eg,high body mass index)
    6. Comparison with prior relevant imaging if available
    7. Final reports should be available within 24 hours or, for nonemergency cases, by the next business day

 

  1. Ultrasound Findings

    1. Normal thyroid and parathyroid glands: (Blum, 2016) (Chaudhary, 2013) 
      • Normal thyroid lobe dimensions are: 18-20 mm longitudinal and 8-9 mm antero-posterior (AP) diameter in newborn; 25 mm longitudinal and 12-15 mm AP diameter at one year age; and 40-60 mm longitudinal and 13-18 mm AP diameter in adult population. The limits of normal thyroid volume (excluding isthmus, unless its thickness is >3 mm) are 10-15 ml for females and 12-18 ml for males.
      • The thyroid gland is slightly more echo-dense than the adjacent structures because of its high iodine content.
      • The thyroid gland has homogenous ground glass appearance.
      • There are frequently 1-2 mm echo-free zones on the surface and within the thyroid gland that represent blood vessels. 
      • The parathyroid glands are observed only when they are enlarged and are less dense ultrasonically than thyroid tissue because of the absence of iodine.
    2.  Thyroid nodule. See: Thyroid nodule evaluation
    3. Grave's disease: (Knipe, 2016) (Floyd, 2015)
      • Thyroid gland is normal to moderately enlarged and can be hyperechoic.
      • Heterogeneous thyroid echotexture
      • Hypervascular; may demonstrate a "thyroid inferno" pattern on colour Doppler which consists of multiple small areas of colour flow seen diffusely throughout the gland representing increased vascularity and arteriovenous shunting. 
      • Relative absence of nodularity in uncomplicated cases
    4. Hashimoto's thyroiditis: (Knipe, 2016) (Chaudhary, 2013)
      • Diffusely enlarged thyroid gland.
      • Heterogeneous thyroid echotexture.
      • Presence of hypoechoic micronodules (1-6 mm) with a surrounding echogenic septations is also considered to have a relatively high positive predictive value.
      • Colour Doppler study usually shows normal or decreased flow, but occasionally there might be hypervascularity and may demonstrate a "thyroid inferno" pattern which consists of multiple small areas of colour flow seen diffusely throughout the gland representing increased vascularity and arteriovenous shunting. 
      • Reactive cervical lymph nodes, common at level VI with normal morphologic features
    5. De Quervain's thyroiditis (subacute granulomatous thyroiditis): (Chaudhary, 2013) 
      • Characteristic focal hypoechoic areas (map like) and enlargement of one or both thyroid lobes.
      • Color Doppler sonography shows decrease or absent blood flow within abnormal map-like hypoechoic areas.
      • Level VI chain lymph nodes (pre-tracheal, the preferential site of thyroid drainage) are found to be enlarged in majority of patients.
      • Complete recovery is characteristic and occurs in weeks to months.
    6. Acute thyroiditis (suppurative/infectious thyroiditis): (Chaudhary, 2013) 
      • The involved lobe appears heterogeneous and hypoechoic.
      • Abscess and cyst formation may be seen. 
      • Rarely, retropharyngeal abscess, tracheal obstruction, jugular vein thrombosis and mediastinitis may complicate acute thyroiditis.
    7. Riedel's thyroiditis (chronic fibrous thyroiditis/invasive fibrous thyroiditis): (Chaudhary, 2013) 
      • Diffuse hypoechoic process with ill-defined margins and marked fibrosis.

 

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