Transducers and focussing Flashcards
List the components of an ultrasound probe
Transducer - piezoelectric material, generates ultrasound and receives echoes by compression/expansion
Backing layer - absorbs ultrasound energy that would otherwise travel from transducer to probe - unwanted echoes
Matching layer - Acoustic impedance of transducer is different to that of tissue. Would cause reflection.
Coupling gel - excludes air
How is the ultrasound beam formed?
Individual elements fire
Ultrasound waves from each element in all directions
Within the beam, these add constructively
Outside the beam, these cancel each other
Huygen’s principle
Each point on the transducer is a small independent transmitter of ultrasound
Superposition
When two or more ultrasound waves arrive at the same point, the resulting amplitude is determined by the relative phase of the combining waves
The amplitude varies from maximum (when the waves are in phase) to minimum (when half a cycle out)
How does beam width affect intensity
As the beam narrows, the same amount
Define the formula for calculation of the diffraction limit divergence angle
Angle = sin-1 (1.22 x wavelength) / A
A = aperture
What is the diffraction limit?
The minimum angle of divergence from the ultrasound transducer
The beam can never been narrower than the divergence angle
In the focal zone, the width of the beam is equal to the diffraction limit
Angle = sin-1 (1.22 x wavelength) / A
Define the formula for beam width at focus
(2.44 x wavelength x F) / A
F = depth of focus
A = aperture diameter
How does transmit focussing work?
To aim for point P, the outermost elements transmit first, followed by the elements next to them and so on. The timing is arranged so that the elements lie on a circle centred at P.
How does transmit beam forming work?
The user sets the depth at which the machine will be focussed
The machine then calculates necessary delays to focus at the focal point
How does receive focussing work?
This is automatic and requires no input from the user
What are the benefits of matrix arrays?
2 dimensional transducer element arrangement
They can focus the beam in the scan plane and also in the elevation plane
Improved slice thickness
Can steer the beam in a number of different directions, allowing 3D without mechanical movement of the probe
Can produce real time 3D due to the speed 3D can be produced
Describe sidelobes
Ultrasound intensity varies across the beam
The intensity is maximum at the centre of the beam and weakens towards the edges
A low-level sidelobe is seen either side of the main beam
Describe slice thickness
The slice thickness (elevation plane) is much smaller than the aperture diameter
Therefore ability to focus in this plane is reduced
This can result in slice thickness artifact
Define the formula to calculate the nearzone length
NZL = A^2 / 4 x wavelength
