Chapter 1 fact_sheet_-_principles
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Transcript of Chapter 1 fact_sheet_-_principles
Physics of Ultrasound
Ultrasound WaveUltrasound WaveNOTE: The higher the frequency the better the resolution!However you lose you loose penetration!
Compression
NOTE: The higher the frequency the better the resolution!However you lose you loose penetration!
Decompression
NOTE: The higher the frequency the better the resolution!However you lose you loose penetration!
Velocity of ultrasound is 1540 m/s in tissue and 1570 m/s in blood
NOTE: The higher the frequency the better the resolution!However you lose you loose penetration!
Medical UltrasoundMedical Ultrasound
Frequencies between 2- 10 MHz are used!
Ultrasound Wave is generated by Piezoelectric crystals
Receive signal (echo) is used to generate images!
Safety of UltrasoundSafety of UltrasoundSafety of Ultrasound
No known adverse effects of diagnostic ultrasoundNo known adverse effects of diagnostic ultrasound
Thermal effects (depdendant on intensity)
Cavitations
Ultrasound Pulse
The higher the US frequency the higher the pulse repetition frequency
NOTE: The higher the pulse repetition frequency, the higher the frame rate and image resolution!
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2D Image
2D Image
NOTE: Ultrasound is a cutplane technique. Multiple elements are used to generate a 2D image!
Types of ProbesTypes of Probes
NOTE: In echocardiography curvilinear probes are used! The advantage is its small footprint, thus it can also be used to image from intercostal spaces! Scan line density is an important factor for image quality!
Image QualityImage QualityImage Quality
Spacial resolution — lateral Contrast resolution
Spacial resolution — axial Temporal resolution
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Determinants of Spacial ResolutionDeterminants of Spacial Resolution
Lateral resolution Axial resolution
Beam width/ line density Frequency
Frequency Pulse frequency
Gain
Harmonic ImagingHarmonic Imaging
NOTE: Harmonic imaging uses the resonance characteristics of tissue! The send and receive frequency of the transducers differ. Advantage: Less artefacts, better image quality
Frame Rate — InfluenceFrame Rate — InfluenceFrame Rate — Influence
Frame Rate = Frames / sec Frame Rate = Frames / sec
Sector width Frequency
Scan lines Depth
NOTE: High frame rates are useful in combination with the image review function! (Detection of rapid motion)NOTE: High frame rates are useful in combination with the image review function! (Detection of rapid motion)NOTE: High frame rates are useful in combination with the image review function! (Detection of rapid motion)
Limitations of 2D ImagingLimitations of 2D Imaging
Attenuation Influence of tissue
Artefacts Penetration
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AttenuationAttenuationAttenuation
Definition: Decrease in amplitude and intensity as an ultrasound wave travels through a mediumDefinition: Decrease in amplitude and intensity as an ultrasound wave travels through a medium
Absorption (proportional to frequency)
Reflection
Refraction Shadowing
Transfer of energy from the beam to the tissue
Pseudoenhancement
Enemies of UltrasoundEnemies of UltrasoundEnemies of Ultrasound
Tissue Bone Air
Penetration Absorption Reflection
NOTE: Imaging is difficult in patient with small intercostal spaces (bone) and in patients with COPD (air)!NOTE: Imaging is difficult in patient with small intercostal spaces (bone) and in patients with COPD (air)!NOTE: Imaging is difficult in patient with small intercostal spaces (bone) and in patients with COPD (air)!
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Artefacts
Types of ArtefactsTypes of ArtefactsTypes of Artefacts
Near field clutter Side lobe artefact
Reverberation Beam width artefacts
Acoustic shadowing Attenuation artefact
Mirror Imaging/ double images (caused by refraction)Mirror Imaging/ double images (caused by refraction)
Specific FormsSpecific Forms
Side Lobes Reverberations
Beam Width Artefact Incorrect Gain
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When do Artefacts Occur?When do Artefacts Occur?
Good image quality (mirror artefacts)
Poor image quality
Calcification Prothetic material
Tips to Avoid ArtefactsTips to Avoid ArtefactsTips to Avoid Artefacts
Know the pitfallsBeware of strong reflections
Know the anatomy Use multiple views
NOTE: Artefacts are inconsistant!!!NOTE: Artefacts are inconsistant!!!NOTE: Artefacts are inconsistant!!!
Optimizing the 2D Image
Optimization OptionsOptimization OptionsOptimization Options
Gain Depth
TGC Imaging Frequency
Sector width Focus
Post Processing
NOTE: Know your echo machine!NOTE: Know your echo machine!NOTE: Know your echo machine!
Post ProcessingPost Processing
Grayscale Contrast
Compression Maps
NOTE: Use predefined settings for specific situations (i.e. patients difficult to image)NOTE: Use predefined settings for specific situations (i.e. patients difficult to image)NOTE: Use predefined settings for specific situations (i.e. patients difficult to image)
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MMode
MModeMModeMMode
AdvantageAdvantage
High Time resolutionAllows Timing of intervals
Where is it used? (see also Chapter 2)Where is it used? (see also Chapter 2)
Aorta/ left atrium (measurements, opening of aortic valve)
Left/right ventricle (measurements, LV function)
Mitral/Prosthetic valve (type of valve)
Endocarditis (motion of suspected vegetation)
Tricuspid annular plane systolic excursion (TAPSE) for RV function
Mitral valve (Mitral stenosis)
NOTE: MMode has lost a lot of its importance but is still valuable in specific situations!NOTE: MMode has lost a lot of its importance but is still valuable in specific situations!NOTE: MMode has lost a lot of its importance but is still valuable in specific situations!
Other Forms of MModeOther Forms of MModeOther Forms of MMode
Anatomical MMode Freedom of axis
Color Doppler MModeTiming of flow (i.e. flow propagation)
Tissue Doppler MModeMyocardial function, timing of events
Curved MModeDisplay of information (i.e. Strain)
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Spectral Doppler
Doppler Formula
The doppler formula allows us to calculate velocities (i.e. blood) based on the doppler shift between the send and receive signal!
NOTE: The measured velocity greatly depends on the angle between blood flow and the ultrasound beam! Always try to be as parallel as possible!
DopplerDopplerDopplerDoppler
PW - Doppler Low vel. (< 1,5 m/s) site specific
CW - Doppler High vel (> 1,5m/s) site unspecific
Tissue Doppler Lower velocity, higher amplitdueLower velocity, higher amplitdue
AliasingAliasing
Depth Velocity
Width of sample volume Doppler frequency
NOTE: Aliasing will occur if the Nyquist limit is exceeded. The Nyquist limit is qual to one-half of the pulse repetition frequency! Use the baseline shift to “stretch“ the Nyquist limit!
NOTE: Aliasing will occur if the Nyquist limit is exceeded. The Nyquist limit is qual to one-half of the pulse repetition frequency! Use the baseline shift to “stretch“ the Nyquist limit!
NOTE: Aliasing will occur if the Nyquist limit is exceeded. The Nyquist limit is qual to one-half of the pulse repetition frequency! Use the baseline shift to “stretch“ the Nyquist limit!
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Tissue Doppler ImagingTissue Doppler Imaging
Velocity Displacement
Strain Strain rate
NOTE: Tissue Doppler is also angle dependent!NOTE: Tissue Doppler is also angle dependent!NOTE: Tissue Doppler is also angle dependent!
Flow Dynamics
Laminar Flow Turbulent Flow
Nonturbulent motion of a fluid in which parallel layers have different velocities relative to each other
Flow of a fluid in which its velocity at any point varies rapidly in an irregular manner
Max. velocity about 1.5 m/s High velocity
Vortex
NOTE: Turbulent flow is usually seen in high velocity flow (i.e. in the setting of stenosis)
NOTE: Turbulent flow is usually seen in high velocity flow (i.e. in the setting of stenosis)
A spiral motion of fluid within a limited area
NOTE: Turbulent flow is usually seen in high velocity flow (i.e. in the setting of stenosis)
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Bernoulli Equation
The simplified Bernoulli Equation allows an easy estimation of pressure gradients from velocities!
Where can you apply the Bernoulli equation in the Heart?Where can you apply the Bernoulli equation in the Heart?Where can you apply the Bernoulli equation in the Heart?
Direct applications Indirect applications
Valvular stenosis AR Quantification
Defects (i.e. VSD, Coarctation, PDA)
Diastolic function
TR signal (sPAP) dp/dt (contractility)
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Color Doppler
Color EncodingColor Encoding
Flow towards the transducer is coded in red, away from the transducer in blue!Flow towards the transducer is coded in red, away from the transducer in blue!
NOTE: The way flow/ flow velocities are displayed also depends on the color map! Check your machine settings!NOTE: The way flow/ flow velocities are displayed also depends on the color map! Check your machine settings!
Color Doppler and Aliasing
If the Nyquist limit is reached, the color changes abruptly (red to blue, or blue to red). In the color Doppler display, you will see a mosaic pattern!
NOTE: The phenomenon of aliasing provides a good delineation of jets (i.e. PISA)
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Color Doppler Frame RateColor Doppler Frame Rate
Scan line density Emphasis (2D vs. Color)
Sector width (2D) Sector width (Color)
Pulse repetition frequency Depth
NOTE: Always aim for a high color Doppler frame rate!NOTE: Always aim for a high color Doppler frame rate!NOTE: Always aim for a high color Doppler frame rate!
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