Ultrasound

Ultrasound

T.R. Nelson, Ph.D.University of California, San Diego

Radiology Physics Lectures: Ultrasound

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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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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

f

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Pulse Inversion

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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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