Ultrasound Physics & Doppler Endocrine University 2018 Mark Lupo, MD, FACE, ECNU
Objectives Review the essential components of ultrasound physics in neck sonography Demonstrate the importance of ultrasound artifacts in clinical imaging interpretation Highlight the utility of Doppler flow as a complement to B-mode ultrasound
Natural Ability to see with sound
Imaging with Sound Create a visual image based on sound waves instead of light Mechanical versus Electromagnetic Energy Passive Sonar - Just listening Active Sonar - Acoustic Echo: Ping a target Reflection of sound waves Creation of a 2-D or 3-D image from reflected sound waves Requires technology and understanding of physics
Sound Waves Physics of sound Role of the propagating medium Acoustic Impedance Density, Stiffness, and Speed of Sound Reflection Occurs at Boundaries of Impedance Attenuation
Principles of Sound Waves Sound is the Propagation of mechanical energy through matter. No sound in a vacuum Qualities of the transmitting medium influence sound transmission Density Stiffness Acoustic Impedance Speed of Sound (SOS)
Sound Waves Sound waves propagate by compression and rarefaction of molecules in space. Molecules of the medium vibrate around their resting position and transfer their energy to neighboring molecules. Waves carry energy, not matter.
Characteristics of waves Frequency: # cycles of compression & rarefaction in a sound wave per second (Hz) Wavelength: Distance travelled in one second Amplitude, power, intensity db
Pulse of Sound Waves Spatial Pulse Length: Cycle length * # cycles per pulse time spent emitting echoes Determinant of axial resolution (1/2 SPL) Higher Frequency: shorter SPL Better Resolution Lower Frequency: Better Depth Penetration, Lower Resolution Duty Factor (%): Time spent generating pulse (usually 1% - while 99% of the time is spent listening for signal return)
Properties of Transmitting Material Speed of Sound = Propagation Speed Constant for a given tissue Not affected by frequency or wavelength Increases with stiffness Decreases with density
Propagation Velocity of Common Tissues
Properties of Transmitting Material Acoustic Impedance (AI) Reflection (echo) of sound occurs at interfaces of Acoustic Impedance (density * propagation speed) Most tissues have heterogeneous impedance If two tissues have same AI no echo If similar weak echo If very different AIs strong echo Homogeneous medium does not reflect sound Cyst (Anechoic NO echo)
Reflection Reflection is the redirection of a portion of a sound beam back at the interface of tissues of unequal acoustic impedance. The greater the difference in impedance, the greater the reflection. Categories of Reflection Specular Mirror like (e.g., diaphragm) Reflection dependent on angle of incidence Diffuse (Soft Tissue e.g., neck ultrasound) Adjacent structures smaller than wavelength
1960s - A-mode of Thyroid A = Amplitude (strength of echo) Solid Cystic Complex Blum, 1971
From One Dimensional A-Mode to Two Dimensional B-Mode
Mid 1960s - B Mode
B Mode Scanner Fujimoto, 1967
B-Mode - Image Formation Meritt, 1998
B-Mode Thijs, 1971
Compound Arm B Mode Scanner
Gray Scale High Resolution 1979 and 2010 st e s h sh s t sh A e SM st V A e Scheible 1979 Legend: st = sternothyroid muscle, sh = sternohyoid muscle, arrows = posterior border of thyroid, A = carotid artery, V = jugular vein, sm = sternocleidomastoid muscle, e = esophagus
Key elements of an Ultrasound Scanner Sound emitter-sends sound toward the object of interest Sound detector-receives the returning reflected echoes Computer - analyze the detected signal Display shows an image of the object
Transducer Transducers both emit sound and receive the reflected echoes necessary for imaging 1% time emitting sound, 99% listening Linear transducer Curvilinear transducer
Piezoelectric Crystal Transducers have a naturally occurring or manmade crystal which can convert electrical energy to sound energy and sound energy to electric energy crystal Alternating electric current Sound wave
US Makes Assumptions Echo originates w/in main US beam Echo returns after a single reflection Depth defined by round trip time of echo SOS constant in human tissue (1540m/s) Sound beam and echo travel in straight path Acoustic energy is uniformly attenuated
Illustrating Physics by Understanding Artifacts Speed of Sound Propagation Artifacts Reflection Artifacts Shadowing and Enhancement Edge Artifact Reverberation Artifact Comet Tail Artifact
Speed Displacement (Propagation) Artifact 2009 by Radiological Society of North America Feldman M K et al. Radiographics 2009;29:1179-1189
Speed of Sound - Propagation Artifact Meritt, 1998
Attenuation Attenuation of acoustic energy results from Reflection Scatter Absorption Attenuation is frequency dependent Higher frequencies have greater attenuation Less depth of imaging with higher frequencies Artifacts due to attenuation Shadowing Enhancement Time Gain Compensation (TGC) Adjusts for loss of amplitude from echoes returning from the far-field (i.e., compensates for attenuation)
Attenuation of sound correction Time Gain Compensation T C G Returning sound
Normal Settings
Far-Field Not Seen Due to Lack of TGC for Attenuation
Increased through transmission Feldman M K et al. Radiographics 2009;29:1179-1189 BEAM ATTENUATED LESS THAN EXPECTED leads to POSTERIOR ENHANCEMENT 2009 by Radiological Society of North America
Posterior acoustic enhancement Occurs when the sound beam is attenuated less than expected; i.e. if it travels through fluid (or other medium) rather than tissue The TGC amplifies the returning echo using time (depth) as the only parameter, assuming homogeneous soft tissue in the path
Superficial cyst
Posterior Acoustic Enhancement Seen behind cysts, blood vessels and often lymph nodes Can occur with solid nodules, particularly if hypervascular Turn on Doppler to confirm a hypoechoic structure w/ posterior enhancement is a cyst or solid nodule
Enhancement is seen behind cysts - But can see it behind homogeneous solid objects as well Benign hyperechoic nodule seen in Hashimoto s Thyroiditis
Shadowing LARGE AI MISMATCH 2009 by Radiological Society of North America Feldman M K et al. Radiographics 2009;29:1179-1189
Reflection Artifacts Eggshell Calcification
Shadowing and Enhancement
Reflection Artifacts Edge Artifact
Reverberation Occurs when sound bounces back and forth between two highly reflective surfaces multiple times before returning to the transducer The effect is to produce a step ladder of echoes in longitudinal orientation, each a little smaller Image
Reverberation Artifact Meritt, 1998
Reverberation Artifact
Reverb artifact Adding harmonics
Tunneling R IJ Catheter Reverberation Artifact
Comet Tail (Ring Down) Artifact
Microcalcification vs Comet Tail
Focus and Resolution Focused beam width determines Lateral and Azimuthal Resolution Near field (Fresnel Zone) Large variations of intensity Focal Zone - Area of maximal narrowing (Adjustable) Far field (Fraunhofer Zone) Intensity more uniform Pulse duration determines Axial Resolution (axial resolution = 1/2 SPL) Practical Considerations As frequency increases, axial resolution improves, but depth of imaging decreases. The depth of the focal zone is adjustable and indicated on the display
Frequency and Resolution With higher frequency the resolution of the near field is improved, but the deep structures are better visualized with the lower frequency.
Lateral resolution Ability to discriminate two objects perpendicular to the sound beam The narrower the beam, the better the lateral resolution. Higher frequencies have narrower beams Image 3 MHz 10 MHz
Focusing the Beam: Lateral resolution All beams have a focal zone, the narrowest part of the beam which has the best quality sound Near field Far field Focused Focal zone
Focal zone
Image Optimization Create the sharpest image to allow tissue discrimination. Equipment factors: Quality of Transducer Quality of Electronics Image Enhancement and Compound Imaging User Adjustments: Depth, Gain, Frequency Focal zones Number and Location Compound Imaging Tissue Harmonic Imaging Dynamic range Time Gain Compensation
Optimal Gain
Image Optimization Dynamic range May increase conspicuity of subtle lesions
Adjustment of number and position of focal zones
Doppler Physics When an interface is moving with respect to the reflected sound wave, the frequency of the reflected sound is altered. Frequency shifted up by approaching target Frequency shifted down by receding target Degree of frequency shift proportional to velocity Doppler Signal Processing and Display Doppler Frequency Spectrum Color Flow Power Mode
Illustration of Doppler Shift
Uses of Doppler Vascular Imaging and flow analysis of blood vessels Vascularity of tissue Probability of Malignancy Graves v. Thyroiditis Amiodarone Thyrotoxicosis Image Clarification
Color and Power Doppler Meritt, 1998
Color and Power Doppler Meritt, 1998
Color and Power Doppler Color Doppler Provides information regarding direction and velocity. More useful in vascular studies Power Doppler No information regarding velocity Less angle dependence Less noise Increased sensitivity for detection of flow
Doppler Issues High Sensitivity vs. Conventional Power User Variability Grading Scales (for thyroid nodules) 1 (absent vascularity) 2 (mostly peripheral vascularity) 3 (peripheral greater than intranodular) 4 (predominately intranodular)
Grade 1 No flow to nodule
Grade 2 Peripheral Flow
Grade 3 Moderate Central Flow
Grade 3 - Moderate Central Flow Suspicious for cystic papillary carcinoma
Grade 4 High Velocity Penetrating Flow
Increased Intranodular Flow Predictive of Malignancy? B-Mode Power Doppler NO LONGER CONSIDERED AN INDEPENDENT RISK
Vascularity - What to do? Consider whether features are suspicious for papillary or follicular cancers. Hypoechoic or features of papillary: Vascularity may be less important Iso or hyperechoic with variable thickness halo: Consider intranodular vascularity as possible risk. AACE & 2009 ATA guidelines do consider vascularity. Korean guidelines do not consider vascularity ATA 2015 guidelines do not consider vascularity **Don t be reassured by absence of vascularity.**
Doppler in the diagnosis of Thyrotoxicosis Graves Thyroid inferno Peak systolic velocity (PSV) 8-20 cm/sec Thyroiditis Various vascular patterns absent to hypervascular Thyrotoxicosis Factitia Minimal intrathyroidal vascular flow Peak systolic velocity (PSV) 3-5 cm/sec Amiodarone Induced Type 1 (Grave s like) increased or normal flow Type 2 (Destructive) decreased flow
Graves Disease Thyroid Inferno
Doppler of Lymph Nodes Suspicious Peripheral Normal Central/Hilar Kim et al, JUM 2013
Doppler for the Clarification of interpretation Central Compartment Lymph Nodes? Blood Vessels, Not Nodes
Clarification of Interpretation - Cyst?
Doppler Before Biopsy
Doppler Parathyroid Glands
Summary Sound transmission is dependent on the conducting medium. Sound is reflected at interfaces of acoustic impedance mismatch. Resolution is dependent on frequency and beam focal width. Artifacts such as shadowing and enhancement provide useful clinical imaging information. Doppler complements B-mode sonography in the evaluation of nodules, parathyroid, lymph nodes and thyrotoxicosis etiology