UltrasoundPrinciples and PracticePrinciples and Practice

Thyroid/ParathyroidThyroid/Parathyroid

Why Ultrasound?• PRO

– Quick– Easy to use– Able to use at bedside or in office– Less expensive than other types of imaging studies– Complementary to other radiographic modalities.

• CON– Operator dependent– More difficult in obese patients, those having undergone previous

operations.– Many things appear similar.– Not a functional study. Anatomic only (as opposed to Sestamibi).

Types of US in General Surgery• FAST – Focused Assessment with Sonography for Trauma• Breast US

– Diagnostic, Core biopsy, Mammatome• Head/Neck

– Vasculature– Lymph Nodes, Submandibular and Parotid glands– Thyroid– Parathyroid

• Vascular• Thoracic/Cardiac

– Pericardial tamponade– Pleural effusions– Pneumothorax/hemothorax

• Abdominal – Appendix– Gallbladder– Pancreas– Spleen– Liver

• Endorectal US• Skin – identify abscess (loculations), lipomas, foreign bodies.

Imaging with (Ultra)Sound

• Create a visual image based on sound waves instead of light waves.

• Passive SONAR – Passive listening• Active sonar – Acoustic Echo

– “Ping him” - Submarines, etc.• Creation of a 2-D or 3-D image from reflected

sonic waves– Requires technology and understanding of physics.

Physics of sound• Sound waves

– Role of the propagating medium

• Acoustic impedance– Density, stiffness, and speed of sound

• Reflection– Occurs at boundaries of impedance

• Refraction– Scattering of sound waves in different directions

• Attenuation– Decrease in returning sound waves based on characteristics of

tissue reflecting sound waves back to transducer. Not all of thesound wave returns to the transducer – absorbed or scattered as it moves through different types of tissues.

Characteristics of waves

• Frequency and period– Hertz (Hz) = cycles/second

• Propagation speed• Wavelength• Amplitude, power, intensity

– dB

Image Formation

• Electrical signals produce “dots”/pixels on the screen– Brightness of the dots is proportional to the

speed/strength/and amount of the origninalimpulse (returning echo) which returns to the transducer.

– The location on the screen of the dots is determined by travel time and usually corresponds to depth of the imaged tissue elements.

Ultrasound Transducers

• Transducer – converts one form of energy to another.

• Piezolelectric effect – Property of some crystals to emit electricity when compressed.

• Types of transducers– Linear array– Curved array– Phase array

Linear Array Transducer• Multiple piezoelectric crystals in a linear array allow real

time scanning with a stationary transducer.– Is flat across the top. – Typically used for high-frequency studies.– Looks straight down, not out to the sides.– Vascular, Breast, Thyroid/Parathyroid

• Convex array– Curved top and curved image on screen at top.– Able to look at wider areas.– Typically used for lower frequency studies, deeper into the body.– Abdominal

Ultrasound probes• Higher frequency probe (7-15 mHz)

– Greater resolution (better picture), but less penetration.

– Best for superficial structures (thyroid, parathyroid, breast, superficial vessels).

• Lower frequency probe (3-7 mHz)– Less resolution, but greater penetration.– Best for intra-abdominal, retroperitoneal, and

intra-thoracic structures including aorta.

Surface Depth of penetration

Image Characteristics• Solid/Cystic• Anechoic – no echo signal returned, appears black.• Hypo/Hyperechoic – darker/lighter to compared structure• Macro/Microcalcifications – large, small calcifications• Homo/Heterogeneous – similar/dissimilar throughout

• Cyst – anechoic, smooth round borders, posterior enhancement (less attenuation through the structure)

• Solid – Contain internal echoes, little or no posterior enhancement.

• Calcific – hyperechoic (bright reflectors, posterior shadowing or comet tail)

Properties of Transmitting Material

• Speed of Sound (SOS)– Constant for a given tissue– Not affected by frequency or wavelength– Increases with stiffness– Decreases with density

Properties of Transmitting Material

• Acoustic Impedance– Reflection of sound occurs at interfaces of acoustic

impedance• A single type of medium may not reflect sound

– Cysts (some) – if filled with fluid only are anechoic– Air– Bile – without stones or sludge– Blood

• Most tissues have mixed impedance– Cells, lymphatics, vessels, etc. – everything that makes up that

tissue

– Acoustic impedance depends on the speed of sound, density and stiffness of a material.

Brightest

Darkest

Acoustic Impedance

Acoustic Impedance

• Small changes in impedance occur at soft-tissue interfaces (muscle, glands,vessels, etc.)– US beam is transmitted thru tissue and then reflected

back to the transducer.– Reflected portions of beam forms image.– Slight changes in acoustic impedance allow different

structures to be seen as distinct and separate from other structures.

– Transmission of some portions of beam through tissues allows imaging of deeper structures.

Impedance cont’d

• Large changes in impedance occur at air and mineral interfaces.– The ultrasound “beam” is reflected and

absorbed to varying degrees.– The reflected portion creates an image of the

outer surface of the structure.– The remainder of the beam is absorbed.

• Structures deep to the surface of the air or mineral (bone, cartilage, tracheal rings, macrocalcifications) are not visualized.

Enhancement• Occurs deep to areas of low attenuation (fluid

filled structures).

– Beam undergoes less attenuation through fluid than other surrounding solid tissues.

– Area deep to fluid is seen as brighter since more of the original “beam” returns to the transducer.

– Allows structures to be identified as fluid filled.

Reverberation artifact• Parallel, evenly-spaced lines appear deep

to a structure.

– Multiple reflections between a more superficial structure and a deeper structure.

– The sound wave bounces back and forth between these two structures before being able to reflect all the way back up to the transducer.

• External – between transducer and an interface• Internal – between two tissue interfaces

Refraction artifact• Reduced intensity of the beam occurs as it spreads

following refraction from a curved surface.

• Occurs at rounded margins of structures which are of different acoustic density than surrounding tissue.

• Beams scatter off in different directions/angles and may not return to the transducer resulting in darker areas/blurred areas at margins of those structures.– Vessels, trachea, nodules,gallbladder, bladder, kidney.

Illustrating Physics by Understanding Artifacts

Reflection Artifacts– Shadowing and Enhancement– Edge Artifact– Reverberation Artifact

Refraction Artifacts– Phantom Image– Comet Tail artifact – macrocalcifications (yes)

vs. microcalcifications (no)

Standard Scan Planes

• Transverse

• Sagittal (Longitudinal)

• Coronal

Probe movements

• Slide -

• Spin -

• Rock and Roll – Combination of above.

Elements of Image Quality• Uniform images throughout field of view

• Adequate penetration

• Like structures look alike

• Cystic/fluid structures free of internal echoes

• All structures anatomically correct and in proper orientation

• Structures appear correct size and shape

Doppler Ultrasound• Used to evaluate blood flow and whether or not

it is directed correctly.

• Spectral Doppler – produces a picture of a blood vessel and surrounding organs. Computer converts the Doppler sounds into a graph that provides information about the velocity and direction of blood flow through the vessel being evaluated.

• Duplex doppler - combines real-time B-mode (brightness mode) imaging with Doppler flow detection.

Doppler• Color Doppler

– Produces a 2D image of vessel. Computer converts the Doppler sounds into colors that are overlaid on the image of the blood vessel, representing the velocity and direction of flow through the vessel.

– Depicts stationary objects in gray scale and objects moving towards the probe in red hues and objects moving away in blue hues.

– Lighter shades indicate faster movement.

PowerPower Doppler• More sensitive than Color Doppler

• Moving amplitudes, not frequency shift, are illustrated.

• Stationary objects are still shown in gray, but moving targets are usually shown in different hues of one color. Lighter shades represent stronger amplitudes and not faster movement.

• No specific hemodynamic information is given

• Only minimal angle of incidence needed to pick up motion vs. that with color doppler (need less than a 60 degree angle to the vessel.– Better for smaller vessels – Assess vascularity within thyroid nodules– See feeding (polar) vessel to parathyroid gland or lymph nodes that

ordinarily wouldn’t be able to be seen with Color Doppler

How to do a Thyroid ultrasound examination

• Obtain transverse view over trachea, slide off to one side and obtain views from superior to inferior, keeping lobe in middle of screen. Then image isthmus and opposite lobe.

• Then obtain longitudinal views – obtain full longitudinal view of carotid a., then slide medially in same orientation to view thyroid longitudinally.

• US bilateral jugular chains and central neck compartment to look for enlarged lymph nodes.

• Image, label, and archive abnormalities for future comparison.

• All abnormalities should be documented in both transverse and longitudinal views.

How to do a Parathyroid Ultrasound examination

• Perform normal thyroid ultrasound – identify nodules and possible intrathyroidal parathyroid glands.

• Parathyroid glands usually appear hypoechoic, ovoid and abut the thyroid. They can appear heterogeneous or isoechoic or even cystic. Document in two views.

• For superior parathyroid glands, look all along the paratracheal/paraesophageal space posterior to the thyroid lobe. Superior glands sit in a plane posterior to the thyroid gland.

• For inferior glands, look in the same plane as the inferior tip of the thyroid lobe. Inferior glands sit in a more anterior plane. Also look within the thymic tract anteriorlyall the way down to the innominate vein.

• Look in the carotid sheath for ectopic glands.

US of lymph nodes• Normally are somewhat ellipsoid, flattened and long, <1cm

with echogenic “hilar line” in center and surrounded by more hypoechoic outer rim.

• LN’s > 1cm are concerning but not always abnormal.

• If hilar line is replaced or missing, is a sign of malignant infiltration.

• Papillary thyroid cancer can have cystic degeneration seen commonly within the lymph nodes.

• Look for matting or coalescence of the lymph nodes.

Normal Right thyroid lobe, transverse view

Rt. Carotid a.

isthmus

Trachea w/ bright cartilage

Refraction artifact from rounded carotid artery

Posterior enhancement

Acoustic impedance from trachea

Ovoid, hypoechoic right superior parathyroid

Normal longitudinal view, left thyroid lobe.

Sternothyroid m.

inferior

SCM Strap mm.

R Jugular v.

R Carotid a.

R thyroid lobe

trachea

Transverse view R lobe

Complex mass with septations

Longitudinal view, same patient

Actually two separate nodules upon exploration…

Transverse view R lobe

Smooth borders, heterogeneous appearance

Macrocalcificationwith comet tail

Same complex nodule from previous slide.

Longitudinal view R lobe

Left lobe transverse view

Complex nodule, solid and cystic portions -heterogeneous, abundant colloid on FNA.

Right lobe, transverse view

Hyperechoic solid nodule, slightly irregular border.

Longitudinal view, left thyroid lobe

Complex fluid filled nodule with large “papillary projection”.

Path: Papillary cancer

Posterior enhancement

Cystic nodule, anechoic

Solid, isoechoicnodule with “halo” sign.

Power doppler with vascular flow around outside of nodule.

A “halo” sign suggests a benign process.

Increased vascular flow w/in nodule suggests potential malignancy.

Stable cyst with septation.

Esophagus

(Do not biopsy!)

trachea

L carotid a.

Complex nodule, posterior enhancement

Complex nodule

Simple Cyst, anechoic

Posterior enhancement

Coarse calcification, posterior shadowing

Lobulated irregular nodule, microcalcifications (note no comet tails), depth > width, invasive into overlying strap muscles.

Sternohyoid m.

Sternothyroid m.

Invasion of muscles

SCM

Carotid a.

Sternohyoid m. Sternothyroid m. SCM

Transverse view L superior parathyroid tucked posterior to thyroid lobe

Left superior thyroid lobe

L sup thyroid lobe

Longitudinal view

Left superior parathyroid gland

Hypoechoic, ovoid

Tucked posterior to thyroid lobe

Longitudinal view right inferior parathyroid

Thyroid lobe longitudinal view

Inferior para

Inferior parathyroid gland sitting in same plane as tip of thyroid lobe

Anterior means an inferior gland.

Glands in a posterior plane to the thyroid are usually superior glands.

Posterior glands inferior to tip of thyroid (low in the neck) are superior glands that have descended to an ectopic position.

Right thyroid lobe

Right superior parathyroid gland

R Carotid a.

Longitudinal view

of superior parathyroid gland that has descended in paratracheal space

Thyroid lobe

Anterior and inferior tip of thyroid lobe

scm

Carotid a.

L.N. with hilar line

Right carotid a.

Lymph node with hypoechoic area surrounding Hilar line

Enlarged (1.92cm) normal appearing lymph node.

Absence of hilar line and ellipsoid shape = malignant node

Large L.N. metastatic papillary thyroid cancer, coalescing, possible matted nodes.

Definitions• Acoustic Impedance – Product of density and velocity of sound in a particular material. The

amount of reflection of a sound beam is determined by the difference in the impedances of the two tissues.

• Acoustic Shadow (Comet tail) – Posterior loss of sound.• Amplitude (A-mode) – Strength or height of the wave, measured in decibels.• Anechoic – structure which returns no echoes (cystic or fluid filled)• Attenuation – Reduction in amplitude and intensity as sound travels due to scatter, reflection

and absorption. Dependent on frequency; higher frequencies give less penetration.• Axial resolution – Depth resolution; ability to separate two objects lying in tandem along the axis

of the beam.• B-mode – Brightness mode. This is the mode we use for clinical applications.• Complex – structure that is heterogeneous and may contain both cystic and solid components.

Splenic hemangiosarcomas, and normal kidneys are complex.• Cystic – structure that does not return echoes. Also have enhanced through transmission of

sound since they don’t attenuate sound passing through them. Causes hyperechogenicitybehind the cyst. Often display a penumbral shadow along the lateral margins of the cyst.

• Frequency – The number of times in a given interval of time that a particular action occurs. US frequency is referred to as hertz or megahertz.

• Gain – Amplification (brightness) of returning echoes to compensate for less of transmitted sound caused by absorption and reflection.

• Gray scale – Method of translating the electronic echo amplitude into an interpretable image. Varying amplitudes of echoes are displayed on the monitor in varying shades of gray

Definitions• Hertz – one cycle per second.• Heterogeneous – structures with uneven distribution of echo patterns.• Homogeneous – structures with an even distribution of echo patterns.• Hyperechoic – more echogenic compared to a nearby structure or than its expected echo

texture.• Hypoechoic – less echogenic compared to a nearby structure or than its expected echo texture.• Isoechoic – the same echotexture in comparison to another part • Pulse-echo prinicple – sending pulses of ultrasound into the body so that they react with tissue

and return reflections.• Refraction – bending of waves as they pass from one medium to another.• Resolution – the ability to discern to points in space.• Reverberation – the phenomenon of multiple reflections within a closed system which causes

the echoes to be misplaced in the display thereby representing a false image.• Time Gain compensation – Control that compensates for the loss (attenuation) of the sound

beam as it passes through tissue.• Transducer – An electromechanical device that is part of an US system. Contacts the patients

and converts electrical energy into mechanical energy and vice versa.• Velocity of propagation – speed of sound in a medium. Velocity changes across different

tissues. Velocity = Wavelength x Frequency.• Wavelength – distance a wave travels in a single cycle. As frequency becomes higher,

wavelength becomes smaller.