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Ultrasound
Ultrasound
T.R. Nelson, Ph.D.University of California, San Diego
Radiology Physics Lectures: Ultrasound
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Ultrasound
Ultrasound
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Ultrasound – Historical Roots
Professor Ian Donald1910 - 1987
Regius Chair of MidwiferyUniversity of Glasgow
medical application
Professor Pierre Curie1859 - 1906
Noble Prize in Physics 1903University of Paris
piezoelectric effect
Professor Paul Langevin1872 - 1946Sorbonne
transducer & SONAR
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Ultrasound
Medical ultrasound basic properties: • frequency 1 - 15 MHz • pulse length 3-5 cycles • speed of sound in tissue ~ 1540 m/s• attenuation ~ 1 dB / (cm-MHz)• wavelength & resolution ~ 0.5 mm• image rates up to 150 fps (30 typical)
2.5 MHz 3.5 MHz 4.0 MHz 5.0 MHz 6.0 MHz
Liver / Kidney Scan
Ultrasound Physics - Overview
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Ultrasound Physics - Doppler
Pulses reflecting off moving interfaces, (e.g. blood cells, heart valves or contrast agents) exhibit a phase shift that can be used to measure the velocity of motion along the path of the sound beam.
Typical Doppler shift range is 10 to 1000 Hz
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Ultrasound Physics - Velocity Doppler
Velocity Doppler computes the velocity in each pixel and displays a color whose hue depends on the direction and whose saturation depends on the velocity component measured
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Ultrasound Physics - Power Doppler
Power Doppler imaging computes the integral of the entire velocity distribution and displays the magnitude as a color and brightness value. Power Doppler has less angular dependence and better sensitivity to slow flow.
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Ultrasound Physics - Contrast Agents
Improved visualization of vessels and tissues is possible using gas-filled micro-bubbles to produce a large impedance discontinuity.
Contrast AgentsContrast agents offer promise in enhancing masses, visualizing blood flow, measuring perfusion, and delivering drugs and genetic agents to specific sites.
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Ultrasound Physics - Harmonics
Harmonics of the excitation frequency can improve signal-to-noise properties of images and are useful for imaging contrast agents and some tissues.
Grey-scaleVelocity Doppler
Harmonic Imaging
Power Doppler
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Ultrasound Physics - Measurements
A variety of length, area and volume measurements are possible Measurement of Length and Area
Measurement ResultsAC
(HC / BPD) (FL)
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Characteristics of Sound
• Sound is mechanical energy• Propagates longitudinally through elastic media
– Alternating zones of compression and rarefaction
• Ultrasound imaging typically uses short pulses• Energy is reflected at interfaces
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Speed of Sound, Wavelength and Frequency
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Wavelength
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Wavelength
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Pressure, Intensity and dB Scale
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Interactions of Ultrasound with Matter
• Reflection– Tissue boundaries with different acoustic impedance
• Refraction– Change in propagation direction with incidence ≠90°
• Scattering– Reflection or refraction by small particles (size << λ)
• Attenuation– Loss of intensity due to absorption and scattering
• Absorption– Conversion of mechanical energy to heat energy
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Reflection, Diffraction and Scattering
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Acoustic Impedance
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Acoustic Impedance
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Reflection
Pressure
Intensity
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Refraction - Snell’s Law
Change in propagation direction with incidence ≠90°
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Critical Angle When c2 > c1 and angle of incidence exceeds an angle theta (θc) where:
Then there will be no reflection
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Attenuation
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Absorption
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Transducers
• Device for producing and detecting ultrasound• Most transducers perform both functions• May comprise single or multiple elements• Most currently use piezoelectric material
– Changes dimension in response to electric charge– Changes electric charge in response to dimension
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Beam Properties
• Ultrasound propagates as longitudinal wave• Two primary zones • Near field (Fresnel)
– Complex field pattern
• Far field (Fraunhofer)– More coherent field pattern
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Spatial Resolution
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Lateral Resolution
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Elevational Resolution
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Ultrasound Image Acquisition
• Important Factors:– Ultrasound
• production• propagation• Interaction
– Equipment• Beam former• Pulser• Receiver• Amplifier• Scan converter• Display system
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Ultrasound Scanners
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Scan lines sweeping through different directions are used to produce a two-dimensional image
Ultrasound Imaging
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Display Modes
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Ultrasound Image Display
• Images comprised of multiple scan lines• Scan line position data arises from:
– Position sensors– Scan converters– Beam former
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Measurements
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Harmonics
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Harmonics
Conventional
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Pulse Inversion
f
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2nd Harmonic
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Contrast Agents
Micrograph of Albunex (mean size 3-5 µm)
Bubble Resonance Absorption Spectra
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Contrast Agents
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Transducer Assemblies
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Three-dimensional Ultrasound
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Image Quality and Artifacts
• Incorrect display of anatomy caused by:– Speed of sound– Refraction– Shadowing and enhancement– Reverberation– Side lobes and grating lobes– Multipath– PRF position aliasing– Slice thickness– . . .
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Refraction
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Shadowing and Enhancement
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Reverberation
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Speed of Sound
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1.48 mm/msec ATS Phantom Imaged at 1.54 mm/msec
(Courtesy of Larry Mo, Zonare Corp.)
Work in progress
Sound Speed Correction
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1.48 mm/msec ATS Phantom Imaged at 1.48 mm/msec
Image Rescaled to 1.54 mm/msec Dimensions
(Courtesy of Larry Mo, Zonare Corp.)
Sound Speed Correction
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Side Lobes
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Reverberation
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Doppler Ultrasound
• Measurement of motion – Derived from frequency shift of returning echo– Multiple measurement strategies
• Measures velocity component in beam direction– Does not measure absolute (vector) velocity
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Doppler Imaging
Grey-scale
Velocity Doppler
Harmonic Imaging
Power Doppler
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System Performance and Quality Assurance
• System Performance– Sensitivity– Dynamic range– Spatial resolution– Contrast sensitivity– Distance accuracy– PRF– Power output
• Quality assurance – Measure parameters to ensure meets specifications
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Quality Assurance Tests
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• Ultrasound has diagnostic value
• ~250 million fetal ultrasound examinations per year in USA
• No evidence that diagnostic ultrasound produces harm
• Wider range of US studies and new technologies having higher acoustic output levels in more patients
• Current acoustic output much greater than earlier equipment
• Subtle or transient effects not well understood
• Diagnostic ultrasound should be used prudently
• Ultrasound examinations should only be performed by trained, competent personnel
• Essential to maintain vigilance to ensure continued safety• BMUS Safety Statement (2000); Clinical Safety Statement for Diagnostic Ultrasound, European Committee of Medical Ultrasound Safety
(2006)
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• This Output Display Standard (ODS) consists of the thermal index (TI) and the mechanical index (MI)
• The MI indicates the potential for the ultrasound to induce inertial cavitation in tissues.
• The TI expresses the potential for a rise in temperature at the ultrasound beam’s focal point.
• While not perfect, TI and MI should be accepted as the most sensible methods of thermal and non-thermal risk estimation
• Implementation of the ODS puts much greater responsibility for patient safety on the ultrasound end user
• Adherence to the ALARA principle is recommended
• A major recommendation of the ODS document was education of end users.
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• Non-diagnostic uses of diagnostic ultrasound equipment– repeated scans for training– equipment demonstration using normal subjects– production of fetal souvenir pictures or videos
• Instrument power levels– Avoid frequent exposure of same subject – the TI should always be less than 0.5 – the MI should always be less than 0.3– Follow safe scanning guidelines and ALARA
• First trimester scans should not be carried out for:– the sole purpose of producing souvenir videos or photographs– their production should not increase exposure levels or extend the
scan times beyond those needed for clinical purposes.
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Ultrasound
T.R. Nelson, Ph.D.University of California, San Diego
Radiology Physics Lectures: Ultrasound
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