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US physics

resident presentation Sept 2013
by

kevin armstrong

on 15 September 2015

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Transcript of US physics

A scan vs B scan
Doppler
point spread function
focus aperture
beam forming
Axial vs lateral resolution
pulse generation
reflection
reflected/refracted pulse
Reflection is of importance because it is the reflected pulse wave that results in an image
The reflected pulse wave will physically deform the crystal
The change in the crystal is interpreted as an image
Reflection
Pulse generation and reflection
the pulse generator will cause the crystal to vibrate
the voltage is applied to the crystal a number of times/s
this creates x number of pulses per second
typically 1000 pulses/sec are generated
Pulse Frequency
Frequency of
Ultrasound
greater than 20 kHz (20 000 Hz)

Frequency of medical ultrasound
2MHz- 18 MHz or
2 000- 18 000 kHz or
2 000 000 -18 000 000 Hz

Pulse frequency: the number of pulses per second
usually 1000 Hz
Some Frequency Definitions
Understanding the physics of US has the potential to improve the use of US through better
visualization of the area of interest
improvements in quality of image
improvement in understanding of image
educational opportunities
safety, quality, dosing, pt. comfort, efficiency
The physics of Ultrasound
Reverberation
Doppler
Increasing the gain
amplification of reflected signal
increases noise and signal
Time gain compensation
amplification of signal with increasing time from transmission
Gain
Echogenic: reflect echoes
anechoic: no or limited reflection compared to surrounding tissues
hyperechoic: higher reflection compared to surrounding tissues
hypoechoic: lower reflection compared to surrounding tissues
isoechoic: same reflection compared to surrounding tissues
Homogeneity- geometric texture, uniform thru tissue ie liver
Heterogeneity- contrary to homogeneous, contain dissimilar elements ie kidney
Terminology
Focusing the wave
and "Beam forming"
Lateral resolution
probably more to do with the number of crystals and focus
Frequency, pulse length and axial resolution
pulse return
Image Creation
Image Generation
Absorption
absorption
refraction
Scatter
Scatter
Attenuation
move through the tissue causing compression or rarefraction in the tissue
there will be loss of energy (attenuation) or amplitude as the pulse progresses
Attenuation (lost energy) is due to
scatter
refraction
absorption (heat)
reflection
What happens to the pulse
The movement of the pulse thru tissue
US pulse
Pulse frequency is usually 1000Hz
Comparing frequencies
Speed of an US wave
THE ASSUMED VELOCITY OF US IN THE BODY IS ASSUMED TO BE 1540 M/S (AND CONSISTENT)

The body is made of different tissue types

THIS ASSUMPTION IS USED TO DETERMINE DEPTH OF A STRUCTURE

THIS is BASED ON THE TIME A REFLECTED PULSE TAKES TO RETURN TO THE PROBE ONCE EMITTED
(since time x speed=distance)
These crystals can both be used to send and receive pulses.

a voltage applied to the crystal will cause a physical change in the crystal (vibration)

a physical change (deformation) in the crystal will produce a voltage change
The Piezoelectric Crystal
What is an US wave?

How is it created?

What is frequency?

How does frequency affect image?

What frequency is optimal?

What is a pulse?

What is the difference between frequency and pulse frequency?
Some questions about Ultrasound
Many have some sense of the principles of the physics involved

Further understanding may improve our ability to analyse and improve the image and improve efficiency
Basics

http://www.sprawls.org/ppmi2/USPRO/
http://www.analog.com/library/analogDialogue/archives/36-03/ultrasound/index.html
http://www.usra.ca/
http://www.nysora.com/
http://www.neuraxiom.com/
http://www.howequipmentworks.com/physics/medical_imaging/ultrasound_imaging/ultrasound.html
http://www.sambahq.org/
http://rsfs.royalsocietypublishing.org/content/1/4/477.full.pdf+html
etc
Reference material
September 2015
Kevin Armstrong MD
Department of Anesthesia and Perioperative Medicine
Schulich School of Medicine and Dentistry
Western University
Basic Ultrasound Physics
Anisotropy
resonance/reverberation
Acoustic accentuation
Acoustic enhancement
shadowing
contact
Ring down/comet tails
Artifact
Doppler
Reference http://echocardiographer.org/Echo%20Physics/spectral%20doppler.html
Doppler
lateral resolution and Focus
Gain
Intensity
Focus
resolution (frequency)
Depth
Doppler
Beam Steering/Beam forming
image improvement/Alteration
Refraction
www.howequipmentworks.com
Frequency
multiple crystals?
mass is not transferred
multiple sources of pulses
Other demonstration of waves
A 3 cycle pulse
Piezoelectric Crystal
Vibration to Image
Objectives
provide insight to the physics of US
build on pre-existing knowledge
Use this understanding to more effectively utilize
US for perioperative care
Related Physics
the “loudness” of the pulse
the amount of energy applied to the crystal
the amount of movement of a particle from its neutral position
Amplitude
Reference
longitudinal vs Transverse waves

particle displaced in the same direction of travel

the density of the material changes and their is limited movement of molecules of tissue
longitudinal waves
www.howequipmentworks.com
Text
piezoelectric Crystal
Absorption is dependent on a number of factors in particular
frequency and
tissue density

High density results in greater attenuation.
The missing information
Attenuation secondary to absorption
1st step
Spark
(cc) image by nuonsolarteam on Flickr
Pulse frequency, sending and listening
If the frequency is 1000 pulses per second or 1 pulse every 1ms

Speed of US 1540 m/s
Time a pulse takes to return from a structure at 10 cm from the transducer (20 cm or 0.2 m) is 0.13ms

After a pulse is created the probe will listen (able to recieve) for 870 micsec before the next pulse

More time spent listening than sending
compound imaging
pulse wave (PW) doppler
continuous wave (CW)

PW - a pulse is sent out to a certain depth and the probe listens for the return.
Calculations are applied to the doppler shift to determine velocity at the point of interest

CW at least 2 crystals are required, one continuously sends and one listens.
Can determine peak velocities as seen in AS (4m/s), but can not determine location along the pth
summing of images to improve image
Thermal Index
(TI) The definition of the thermal index is the ratio of
the total acoustic power to that required raising
maximum temperature increase of 1°C under defined assumptions.
A thermal index of 1 indicates the acoustic power achieving
a temperature increase of 1°C.
A thermal index of 2 has the doubled power

Artifacts
Artifacts are errors in images created.
They result because of assumptions

Both have limited movement of material or particles
Visual representation of
a transverse waves
in a substance
7 compared to 4 pulses
visual representation
Therefore less energy to return to the probe (blue arrow)
in the first panel there is less energy returned to the probe than sent b/c some energy has continued onward
as the pulse continues there is also refraction
in the 2nd panel some reflection can be described as scatter
notice that the pulse continues with lower amplitude
notice there is both reflection, scatter, and refraction
Focus
some assumptions about US we frequently make

Artifacts are errors in images. They are normally caused by physical processes that affect the ultrasound beam
Artifacts can at times be helpful.

Examples?
Reverberation
Ring Down

Ring-down artifacts are produced when small crystals such as cholesterol or air bubbles resonate at the ultrasound frequency and emit sound.

Because the sound is emitted after the transducer receives the initial reflection, the system thinks the emitted sound is coming from structures deeper in the body.
Mirror Images

Sound can bounce off a strong, smooth reflector
such as the diaphragm.
The surface acts as mirror and reflects the pulse to another tissue interface. The ultrasound system crates a mirror image beyond the first surface
Enhancement

Enhancement is seen as high brightness.
This occurs when sound travels through a medium with an attenuation rate lower than surrounding tissue.

Reflectors at depths greater than the weak attenuation are abnormally bright in comparison with neighboring tissues.
Attenuation

Tissues deeper than strongly attenuating objects, such as calcification, appear darker because the intensity of the transmitted beam is lower.
Useful Artifacts
shadowing
enhancement
reverberation
ring down
Non useful artifacts

Acoustic attenuation
Contact artifact
Comet Tails
Ultrasound lung comets (ULCs)
seen in interstitial lung disease
explanation is not clear

Experimental models permitted us to discover that ring-down artifacts are produced only by single and double layers of bubbles in specific structural settings.
Reverberation between bubbles with a critical radius seems to be at the origin of ring-down artifacts
B lines
Sonographic Interstitial Syndrome
The Sound of Lung Water ournal of Ultrasound in Medicine JUM February 1, 2009 vol. 28 no. 2 163-174
Sonographic Interstitial Syndrome
The Sound of Lung Water ournal of Ultrasound in Medicine JUM February 1, 2009 vol. 28 no. 2 163-174
Ultrasound lung comets (ULCs) are an echographic sign of uncertain biophysical characterisation mostly attributed to water-thickened subpleural interlobular septa, but invariably associated with increased extravascular lung water.
Role and importance of ultrasound lung comets in acute cardiac care European Heart Journal: Acute Cardiovascular Care April 2015 vol. 4 no. 2 103-112
Sonography is useful in the diagnosis of lung diseases in which the alveolar air content is impaired and interstitial and alveolar fluids are increased and also when air or fluids are collected in the pleural space.
Lung Sonography JUM January 1, 2013 vol. 32 no. 1 165-171
Doppler
Color flow Doppler ultrasound
Continuous wave Doppler ultrasound
Duplex Doppler ultrasound
Doppler ultrasound flowmeter

Pulse wave Doppler ultrasound


Color flow Doppler ultrasound

a form of pulse wave Doppler in which the energy of the returning echoes is displayed as an assigned color;
by convention flow towards the transducer are seen as shades of red, those flow away from the transducer are seen as shades of blue.
The color display is usually superimposed on the B-mode image, thus allowing simultaneous visualization of anatomy and flow dynamics.

Older technology
dedicated cystal for sending and receiving
adv: higher velocity assessment
Some phased array probes have a sharing (2D & CW) arrangement

All other returning ultrasound information is essentially "ignored".
Pulse Wave vs CW doppler
It can usually be said that when an operator wants to know where a specific area of abnormal flow is located that pulsed wave Doppler is indicated. When accurate measurement of elevated flow velocity is required, then CW Doppler should be used.
Doppler Ghosting.

Doppler systems convert frequency shifts into a spectrum of colors Frequency shifts arise from movement. An ultrasound machine measures the movement of Red Blood Cells. Since the blood flows through a
vessel that has walls that pulsate, this pulsation causes it’s own Doppler
shift so the machine creates a color map that “bleeds” into the surrounding tissue (ghosting).

Mechanical Cardiac probe function
samples the returning Doppler shift data from a given region in addition to 2D imaging
pulse will impact on axial resolution
1540 m/s
3 vibration pulse
Potential to increase pulse frequency
this will require more processing power
waves generally diverge as the propagate
Focusing is created by beam forming
Examples in periop US- B lines
Examples we see in periop US are

needles (multiple images)

A lines in Lung US

The circumstances around reverberation is a highly reflective interface
Shadowing can allow identification of tissue type

Enhancement can also assist with tissue identification

reverb-A line

Ring down- B lines
user sets from which depth, pulses with a doppler shift frequencies will be analysed and displayed
Lateral resolution is degraded in the near-field and far-field. On the other hand, delaying and summing echo samples from receive channels dynamically allows us to focus receive echo signals at every depth; the method is called
dynamic receive focusing
Range ambiguity


Is created when an echo from a deep reflector arrives after another pulse is already created.
When the Pulse Repetition Frequency (PRF) is too high
the machine assumes that the echo came from the second pulse and displays the echo to shallow on the image.
Full transcript