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Types of ultrasound

Created: 23/11/2004

- A-mode
- B-mode
- M-mode
- 2D-real time
- Pulsed-wave Doppler
- Continuous-wave Doppler
- Colour Doppler
- Power Doppler
- Duplex

Imaging modes    
A-mode Now obsolete in medical imaging.Wave spikes are represented when a single beam passes through objects of different consistency and hardness. The distance between these spikes (for example A and B) can be measured accurately by dividing the speed of sound in tissue (1540 m/sec) by half the sound travel time. A Mode
B-mode ("Brightness") Same as A-mode, but one-dimensional graphical display, with brightness corresponding to amplitude of reflected sound  
M-mode A single beam in an ultrasound scan can be used to produce an M-mode picture, where movement of a structure such as a heart valve can be depicted in a wave-like manner. Because of its high sampling frequency (up to 1000 pulses per second), this is useful in assessing rates and motion and is still used extensively in cardiac and fetal cardiac imaging. M Mode
2D-real time Most modern ultrasound devices are 2D-real time imaging systems. Multiple crystals (linear, curved or phased-array) or moving crystal.
Sequential B-mode pulses sweeping across a plane to display the image in either a linear or ‘sector’ format.
Displayed as real time imaging with up to 100 images per second.
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Doppler modes    
Pulsed-wave Doppler (PW)

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

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.

Prone to aliasing with high velocities.

Flow Toward

Trace Toward

Mitral Regurgitation

Continuous-wave Doppler
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
CW Doppler
Colour Doppler

Utilises pulsed-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.

Prone to aliasing with high velocities.

Power Doppler (CPD)

CPD = Colour Power Doppler
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, so no aliasing
CPA provides better sensitivity to slow flow states
Less angle dependent than traditional colour
But more sensitive to motion artifacts

Power Scale
Lymph Node
Directional Power Doppler

DCPD = Directional Colour Power Doppler

Combines power (amplitude) of Doppler signal with directional (phase) information

Duplex Systems or scans which combine imaging and Doppler with image guidance are often referred to as duplex systems or duplex scans  
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