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Ultrasound and physical principles

Created: 23/11/2004
 
Click the following links for sections in this article:

Ultrasound system concept
Transducer frequency and wavelength
Position of displayed echos?
Piezoelectric effect
Image resolution
Doppler in ultrasound
What is colour Doppler?
Colour vs CPA?


What is ultrasound?

Ultrasound imaging uses the transmission and reflection of high-frequency longitudinal, mechanical waves ("ultrasonic sound" waves) in tissues.

Image information is provided by the energy of these waves as they are reflected from the surfaces between different tissues. The reflections are due to differences in the mechanical properties of the tissues.

Using the wave propagation speed in tissues, time of reflection information can be converted into distance of reflection information.



Infrasound - 0-20 Hz
Audible sound - 20 Hz to 20,000 Hz
Ultrasound - >20,000 Hz (or 20 KHz)
Medical ultrasound - 2.5 MHz to 15 MHz

 

Wavelength

How is the image formed on the monitor?

The strength or amplitude (brightness) of each reflected wave is represented by a dot
The position of the dot represents the depth from which the returning echo was received
These dots are combined to form a complete image

Strong reflections = white dots
Diaphragm, gallstones, bone
Weaker reflections = grey dots
Most solid organs, thick fluid
No reflections = black dots
Fluid within a cyst, urine, blood

 

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Ultrasound system concept  
System beamformer initiates sound wave  
Scanhead produces “pulses” of ultrasound  
Transmission through medium  
Reflection from tissue interfaces  
Signal returns to system  

An image of all reflections is formed on the monitor

 

 

System Design

 

Transducers turn electrical energy into mechanical waves and vice versa. After encountering reflective structures, some of the sound waves travel deeper into the body. These will be refelected from deeper structures.

 

 

 

 
   Sound is reflected any time a wave changes mediums
 
 Energy peaks occur when the transducer receives a reflection
 
 The size of the peak depends on the strength of reflection
 
 Delay of reflection is interpreted as distance from the probe
 Sound travels at 1540 m/s in tissue at 37°C

 

Position of displayed echos?

Display screen divided into a matrix of PIXELS (picture elements)

Pixels

 

How does the system know the depth of the reflection?.........TIMING
The system calculates how long it takes for the echo to return to the scanhead using:
Distance = Velocity/Time
The velocity in tissue is assumed constant at 1540m/sec

 

Piezoelectric Effect

Crystal

 

Characteristic of transducer elements (crystals) which converts electrical to mechanical energy and vice versa
An electric field, set up by a voltage applied to two electrodes on its surface, causes a dimensional change of the crystal
Most ultrasonic transducers use artificial polycrystaline ferroelectric materials such as lead zirconate titanates (PZT)
Element thickness determines the resonant frequency

What is Doppler?

Doppler effect based on work by Austrian physicist Johann Christian Doppler.
Apparent change in received frequency due to relative motion between a sound source and sound receiver

Source moving TOWARD receiver = HIGHER frequency
Source moving AWAY from receiver = LOWER frequency

Click here for an animated audio example of the Doppler effect...

 

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Transducer frequency and wavelength

ArrowFrequency = ArrowResolution

ArrowFrequency = ArrowPenetration

A 12 MHz scanhead has very good resolution, but cannot penetrate very deep into the body
A 3 MHz scanhead can penetrate deep into the body, but the resolution is not as good as the 12 MHz scanhead

Use the highest frequency transducer that will reach the required depth

Penetration
____

 


Image resolution

The ultimate goal of any ultrasound system is to make like tissues look alike and unlike tissues look different:


Pieces


Resolving capability of the system depends on:
axial/lateral resolution
spatial resolution
contrast resolution
temporal resolution

Axial resolution
Specifies how close together two objects can be along the axis of the beam, yet still be detected as two separate objects.
Frequency (wavelength) affects axial resolution

Lateral resolution
The ability to resolve two adjacent objects that are perpendicular to the beam axis as separate objects.
Beamwidth affects lateral resolution

Spatial resolution
Also called "detail resolution".
It is the combination of AXIAL and LATERAL resolution

Contrast resolution
The ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects

 

Contrast Resolution

 

Doppler in ultrasound

Used to evaluate blood flow
Scanhead is the sound source and receiver
Flow is in motion relative to the scanhead
Doppler produces an audible signal as well as a graphical representation of flow = spectral waveform

The Doppler shift produced by moving blood flow is calculated by the ultrasound system using the following equation:

Doppler Equation Where Ft is the transmitted Doppler frequency, V is the speed of blood flow, CosØ is the Cosine of the bloodflow to beam angle and C is the speed of sound in tissue.

Types of Doppler

Pulsed-wave (PW)

 

Produces short bursts/pulses of sound
Uses the same crystals to send and receive the signal
Provides depth precision (sample volume)
Maximum velocity is limited. The signal will always alias at a given point, based on the transducer frequency.

Continuous -wave (CW)

 

 
Uses different crystals to send and receive the signal
One crystal constantly sends a sound wave of a single frequency, the other constantly receives the reflected signal
No depth precision
Does not alias


What if the velocity is too high to display?


This effect is called ALIASING (wraparound).
The pulsed Doppler sample rate is not adequate for high-frequency shifts.
The "peaks" are cut off and displayed below baseline

Aliasing

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The spectral waveform represents the audible signal and provides information about:

The direction of the flow
How fast the flow is travelling (velocity)
The quality of the flow (normal vs abnormal)

Due to the nature of the COS function, unacceptable velocity estimation errors can occur when the Doppler angle exceeds 60 degrees.


Flow Toward
Flow coming TOWARD the scanhead is represented above the baseline

Flow Perpendicular

Flow Away
Flow traveling AWAY from the scanhead is represented below the baseline


Forward Trace Doppler
 

Away Trace Doppler

What is colour Doppler?

Utilises pulse-echo Doppler flow principles to generate a colour image
This image is superimposed on the 2D image
The red and blue display provides information regarding DIRECTION and VELOCITY of flow

Regardless of colour, the top of the bar represents flow coming towards the scanhead and the bottom of the bar represents flow away from the scanhead.

Colour Flow   Colour Direction
Umbilical Flow

Colour vs CPA?

CPA = Colour Power Angiography
Flow information is generated based on the AMPLITUDE or STRENGTH of the blood cell motion
The CPA image is superimposed on the 2D grayscale image

 

  The colour maps for Power are represented by a single continuous colour
Power does not provide DIRECTIONAL information.Therefore, Power does not display aliasing
CPA provides better sensitivity to slow flow states
Less angle dependent than traditional colour
But more sensitive to motion artifacts
Power Scale

Power Doppler 


ArticleDate:20041123
SiteSection: Article
 
   
    
                                            
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