FRCR1 US1 14 - DCPB€¦ · FRCR/MSc_US1 AJW - 2014 Diagnostic Ultrasound Amanda Watson Ultrasound Physics Department of Clinical Physics F-Block Basement Western Infirmary Glasgow

FRCR/MSc_US1 AJW - 2014

Diagnostic Ultrasound

Amanda Watson

Ultrasound PhysicsDepartment of Clinical Physics

F-Block BasementWestern InfirmaryGlasgow G11 6NT

0141 211 2129

Vascular Laboratory, L9, WIG0141 211 2551

[email protected]


Week 1 – Basics to B-Mode, SafetyRITI 8a_120,121,122,123

Week 2 - Doppler Ultrasound, ArtefactsRITI 8a_129, 130, RITI 8a_134

Week 3 (March) – FRCR only - Multiple choice


Ultrasound in context

http://www.england.nhs.uk/statistics/wp-content/upl oads/sites/2/2013/04/KH12-release-2012-13.pdf


Same phenomenon as audible sound (non-ionising!)

Mechanical vibrations of particles of the medium of propagation

Propagates through tissue as a longitudinal wave� Particles do not move through the medium - they oscillate about

their mean positions in the direction of sound propagation� Energy is transferred as kinetic energy of particles

Beyond human perception i.e. > 20 kHz

Most clinical applications use ultrasound in the range 2 to 15 MHz

Fundamental Principles – What is Ultrasound?


Sound travels in the form of a wave.Sound waves transmit vibrational energy

Tissue transmits longitudinal waves

Wave motion leads to regions of compression and rarefraction - “peaks”and “troughs”

The transducer is essentially a vibrating piston in contact with tissue.

Mechanical Pressure Waves

λλλλ = c/f

Wavelength/Frequency is an important parameter in ultrasound as it is related to the spatial resolution of ultrasound.

Penetration of ultrasound is also frequency dependent.


The Speed of Sound, c, is a property of the material that the sound is propagating through.

c = √√√√(K/ρρρρ)Where K is the bulk modulus (stiffness) of the material and ρρρρ is the density (weight per unit volume)

Speed of Sound

Material Speed of Sound (m/s)

Air 330 Water 1480

Fat 1460 Liver 1555 Blood 1560 Kidney 1565 Bone 3190 - 3406

Average Soft Tissue

1540

Ultrasound machines are calibrated with an “average soft tissue” speed of sound of 1540 m/s .

This allows the position of echoes to be determined using the Range Equation


Ultrasound: The Pulse Echo Principle


The Pulse-Echo Technique

Pulses are needed to give positional information

Use the Range Equation!


2

cTD =

The Pulse-Echo Technique

The Range Equation - the round-trip time for an echo at a d istance D from a pulse transmitter is T = 2D/c where c is the spee d of sound.


Time or Depth Gain Compensation - weak echoes at depth may need to be boosted and strong echoes near the surface may need to be suppressed.


Swept Gain CompensationSwept gain, compensates for the loss of signal at depth due to

attenuation. The scanner will perform this automatically.

TGC or DGC slider controls allow the user to apply local amplification of suppression of signals. It can be useful for minimizing bright signals close to the surface


A plot of the amplitude of the signal along one beam is known as A-Mode . This technique is sometimes used in ophthalmology.

Alternatively the different signal amplitudes and positions can be represented by “dots” of grey with the shade of grey dependant on the signal amplitude


B- Mode

In B-mode each peak of intensity is represented by a dot. The intensity or brightness of the dot depends on the amplitude. An image may be built up by sweeping a beam through a plane of tissue to build up a single “frame” of an image. If the sweeping is done fast enough, then the image will be able to respond to changes occurring in the scan plane in real time.


The rate of sweeping is the frame rate

The number of beam positions making up the image is the beam density or number of scan lines



If a single B-Mode scan line is fixed in space, then the echoes can be mapped in time. This is known as M-Mode and is used for assessing moving structures - Cardiology


Ultrasound Generation, Detection, Focusing and Resolution


Transducer Technology

PiezoelectricityTransducer ConstructionArrays and beam-forming Resolution/Penetration limitations


Piezoelectricity

Ultrasound transducers contain a piezoelectric ceramicor crystal - usually lead zirconate titanate, PZT.The piezoelectric effect is the conversion of electrical energyinto mechanical energy (movement).

Receive Transmit


Transducer Construction

The transducer will have a resonant frequency or frequency bandwidth at which it is most efficient. Depends on the thickness of the element. λ = 2 x thickness

Backing material produces dampening to shorten pulse lengths.

The impedance of the transducer differs greatly from that of tissue so the front face must be “matched ” to improve transmission - quarter wavelength matching is used.

Coupling gel is important to eliminate reflections from air between the transducer face and the skin


Frequency, Bandwidth and Resolution

The bandwidth is the spread of frequencies contained within a pulse.

Longer pulses give a “purer tone” i.e. just one frequency … but the pulse length will affect the axial resolution


Axial Resolution

Axial resolution depends on the pulse duration (PD)

PD = N / f

where N is the number of cycles in the pulse and f is the frequency

Good dampening and higher frequencies give shorter pulse lengths


Beam Characteristics and Lateral Resolution

A beam of ultrasound is well collimated near the transducer (near field ) then starts to diverge (far field )

The length of the near field zone and the amount of divergence depend on the wavelength of ultrasound and the diameter of the transducer

Near Field Length = d2 / 4λλλλ or r2 / λλλλ


Beam Shape and Lateral Resolution

The beam shape will therefore depend on the wavelength (or frequency) and size of the transducer.

Lateral resolution is determined by the beam shape

The beam width can be made more narrow by focusing and electronic beamforming


Electronic scanning using arrays

Electronic scanning produces well defined narrow beams of ultrasound.

A: A beam is formed from a group of elementsB: One element is dropped from the right and one picked up on the left.


Dynamic Apertures

Varying the number of elements used to form the beam helps control the beam width at depth.


Phased Arrays have asmall “footprint” butuse electronic beam steering to give a widefield of view.


Conventional Focusing

Focusing narrows the beam and locally improves lateral resolution


Electronic Focusing

The use of electronic arrays makes it easy to focus at different depths to improve the image quality.

Transmit focusing

Receive focusing


To have three focal zones on at the one time would mean threesweeps for each complete frame this will slow down the frame rate.

OK for relatively still tissues but not so good for movingstructures - e.g. heart, foetus.

Multiple Focal Zones


Beam Dimensions - Summary

Lateral resolution is determined by the beam shape which depends on frequency, and beamforming(focus, aperture size)

Axial resolution depends on pulse length which depends on frequency and number of cycles

Slice thickness depends on element height i.e. transducer construction


Temporal Resolution & Frame Rates

As ultrasound imaging is a real time technique, the frame rate is important. But there often is a trade off between image quality and frame rates e.g. multiple focal zones.

Many image enhancement features require more time for processing >> they can compromise the real time nature of ultrasound.

T = 2D/c = 13µs x D


Frame rate

A deeper image requires more time to process each scan line - so slower frame rates

More scan lines will give a better image but slower frame rates

A wider field of view with the same image quality will give a lower frame rate.


Frame Rates


Frame Rates


Reflection and Scatter


Ultrasound in TissueAs an ultrasound wave propagates through tissue, it is attenuated.

Attenuation is caused by

•Absorption

•Reflection•Refraction•Scattering

Ultrasound images rely on the ultrasound being reflectedfrom the underlying tissue.

Reflection may be total or partialPartial reflection may be specular or diffuseScattering gives characteristic “texture”

In soft tissue, absorption account for up to 90% of the total attenuation of the ultrasound beam. Absorption falls off exponentially with distance, the same fraction of the incoming energy is lost in each unit distance travelled.


12

12

ZZ

ZZr

+−=

The amount of reflection depends on the different valuesof impedance, Z, of the tissues.

The pressure amplitude reflection coefficient, r

For intensity use R=r 2

cZ ×= ρ

density Speed of sound

Interface r MUSCLE/BLOOD 0.07%

FAT/MUSCLE 1.08%

SOFT TISSUE/WATER 0.23%

SOFT TISSUE/AIR 99.9%

SOFT TISSUE/BONE 41.2%

Low values of r mean that the ultrasound can penetrateto the tissues below.

High values mean most is reflected

Non-perpendicular reflective interfaces cause refraction

KZ ⋅= ρ

c = √√√√(K/ρρρρ)


Non-perpendicular reflection/transmission causes reflections and transmission at different angles.

sin θθθθ2/sin θθθθ1 = C2/C1

θ2

θ1

Non-perpendicular reflection/transmission also causes refraction at the interface i.e. a change of direction


Scattering gives tissues their characteristic “texture” inthe ultrasound image.

•Hyperechoic - higher scattering levels than the surrounding

•Hypoechoic - lower scattering levels than the surrounding

•Rayleigh scatterers -much smaller than the wavelength


is the process by which some of the mechanical energy of the ultrasound is converted into heat in the tissues.

In soft tissue, absorption account for up to 90% of the total attenuation of the ultrasound beam. Absorption falls off exponentially with distance, the same fraction of the incoming energy is lost in each unit distance travelled.

The absorption coefficient µa depends on the characteristics of the medium and is also approximately proportional to ultrasound frequency.

The intensity, I, at a distance x can be expressed by:

I=I0exp(- µa x) where I0 is the initial intensity at x = 0,

Absorption


This parameter takes account of Absorption and Scattering

The attenuation coefficient is proportional to frequency (some cases closer to f2). As with the absorption coefficient it is expressed in decibels.

Medium αααα (dB/cm/MHz) HVL at 3 MHz (cm) Blood 0.2 5 Liver 0.6 1.7

Muscle 1 1

Fat 0.4 – 1.4 2.5 – 0.7

Bone 22 0.04

Water 0.0022 (dB/cm/MHz2) 150 Air 1.6 (dB/cm/MHz2) 0.2

Ultrasound Attenuation Coefficient

Lower frequencies can penetrate further than higher frequ encies


But remember the relationship between frequencyand wavelength?

c = f *λλλλ

High frequencies have smaller wavelengths whichmeans they have narrower beam widths and can be used to image smaller targets. i.e. They have better resolution.

Generally: Penetration Resolution

Penetration Resolution

Resolution vs. Penetration Trade - Off


The B-Scanner


Signal Processing

Beamformer and pulserTransmit powerAmplification and gainDemodulation and rejectionDigitisation, dynamic range and CompressionScan ConverterSummary of scan controlsIntroduction to tissue harmonic imagingSpatial compounding


Signal Processing – How to make an image

For each received echo weneed to know two things:

� Where it comes from• Range equation,

received beam

� What shade of grey togive it

• Dynamic range, compression


Signal Processing

Transmitter or PulserProvides the electrical signal for the excitation of the transducer. The amplitude of the output pulses may be controlled by the user using the Output Power Control


Receiver and Processing

Amplification - Pre-amplification will have already been completed in order to make very small signals large enough for further processing and to protect the amplifier from excessively large signals.

Digitisation – in modern scanners is done at an early stage making the processing of signals into images much faster.

The degree of amplification is known as the gain.

Digitisation


GainOverall Gain

Swept Gain or CompensationAutomatically compensates for the attenuation at depth


Dynamic Range and CompressionThe dynamic range of image display devices (i.e. the number of different shades of grey that can be displayed) is considerably lower than the dynamic range that can be handled by the receiver. The signal therefore has to be compressed (large signals reduced and small signals boosted) to make it suitable for display.


60dB 40dB

High dynamic range gives more greysLower dynamic range gives more contrast


Scan Converter

The memory device in which images are formed, stored, made suitable for "real-time" display on a monitor or for output to various hard-copy devices.

The digital matrix is a straight forward rectangular array for linear transducers e.g. one column of pixels for each element or beam.

Curved or phased arrays and any beam steering require more complex vector considerations to form the image. Widely spaced scan lines will require interpolation i.e. the pixels in between lines will be assigned an average value based on the surrounding pixel values.


Summary of Image Controls

Patient DetailsApplication Preset TransducerFrequency(Monitor)DepthFocal zones (number and position)GainsOutput PowerDynamic rangeZoom (read and write)


Tissue Harmonic Imaging

Harmonic imaging is a technique that takes advantage of the fact that as the ultrasound propagates in tissue, the waveform within the pulse is distorted so that the returning echoes will contain, not just the fundamental frequency, but a small amount of signal at twice the fundamental frequency i.e. the first harmonic frequency.

direction of propagation

Pure Tone Increasing Harmonics

C+∆C

C-∆C

C~1.5 km/s


fo 2fo

C onven tion a l Im aging :reverb no ise a t fo= > ____

H arm onic Im aging :R ece iver rem oves fo

& reverb n o ise

Spectral analysis allows the first harmonic (2 x fundamental) frequency to be extracted from the receive signal.

The harmonic signal has not suffered from the aberrations and distortions of subcutaneous fat and is a “cleaner” signal.


Conventional imaging ignores the harmonic content and displays the fundamental. Reverberations from the ribs will appear below the ribs.

Harmonic imaging will only display the information from below the ribs - cutting out the reverberation clutter.


Real time Compound Imaging

Frequency compounding is a broadband technique that sums frames of the image created from different parts of the frequency spectrum.

Spatial compounding sums the information from multiple lines of sight to create a single frame to minimise shadowing and improve edge detection. E.g. SonoCT

Ultrasound image compounding requires rapid processing capabilities to maintain frame rates.


Spatial compounding



Sources of Information

Textbooks – Old ones good for the fundamentals, newer ones will bring technology up to date e.g.

� Diagnostic Ultrasound: Physics and Equipment by Peter Hoskins, Abigail Thrush, Kevin Martin, Cambridge University Press, 2010

� Ultrasound Physics and Instrumentation by Wayne R. Hedrick, David L. Hykes, and Dale E. Starchman, Mosby, 2005

� Essentials of Ultrasound Physics by James A. Zagzebski, Mosby, 1996

Images in this presentation are fromHoskins and Zagzebski.


Sources of Information

Hangiandreou NJ. AAPM/RSNA physics tutorial for residents. Topics in US: B-mode US: basic concepts and new technology. Radiographics. 2003 Jul-Aug;23(4):1019-33 http://radiographics.rsnajnls.org/cgi/content/full/23/4/1019


Any Questions?

Documents

FRCR1 US1 14 - DCPB€¦ · FRCR/MSc_US1 AJW - 2014 Diagnostic Ultrasound Amanda Watson Ultrasound Physics Department of Clinical Physics F-Block Basement Western Infirmary Glasgow