Ultrasound 2 Flashcards
What do transducers do (as a source and as a receiver)?
Transducers convert mechanical movement into an electrical signal or vice versa
Electrical - mechanical movement or the other way around
What is the piezoelectric effect?
Piezoelectric crytstal is heated up / poled, so that charges become aligned
What happens to the piezoelectric crystal when the transducer is acting as a source vs as a receiver?
As a source, a voltage is applied to cause the piezoelectric crystal to compress and to rarefract, producing a sound wave
As a receiver, the wave’s mechanical movements cause the crystal electrodes to compress or to stretch, and depending on the direction of movement, a voltage with a different polarity is produced. 2 electrodes
What contains the piezoelectric element in the ultrasound transducer construction?
2 electrodes
What is the thickness of the piezoelectric element and why is it chosen as such?
Half of the wavelength: when the sound travels through the PZT element, it will have traveled half the wavlenegth. As the matching layer has a lower impedance to the PZT element, some of the waves are reflected and some are transmitted. The reflected waves are inverted, so destructively interfere with each other, and the transmitted waves are in phase so they constructively interfere to produce waves with a greater amplitude.
What is the purpose of using a backing layer?
The backing layer allows the sound to reflect at the back of the piezoelectric element so that it can be transmitted eventually to the tissue and the detecting lens.
What are the relative impedances of the backing layer, the PZT element and the matching layer?
The impedance of the matching layer should be equal to the square root of the impedance of the PZT element and the tissue
What do the lenses at the end of the matching layer do?
The lenses are curved, and as the sound approaches the curved surface at different poins, they will be refracted at different points. The lens curvature is designed so that the waves are refracted towards the beam axis and focus the beam at a particular point
What is the point of focusing (on transmission or reception) in diagnostic imaging?
To improve spatial localization, helps concentrate energy onto a specific region (resolution)
What is the focal length?
The distance between the end of the matching layer and beam axis
What are the names of the 2 different planes in the beam axes?
- The elevation plane
- The scan plane
What is the width of the beam at -3dB
half of the maximum intensity
What is the width of the beam at -6dB
1/4 of the maximum intensity
What are the 3 different types of probe types and what are the main characteristics?
Linear: PZT elements arranged linearly straight
Curvilinear: PZT elements curve around a structure. They are placed straight parallel to each other, but the surface they adhere to is curved
Phased: PZT elements are placed at angles to each other which increase as the array is more towards the outside, and the surface to which they are adhered to is straight.
What is the name of the surface to which the element arrays are adhered to?
the field of view
What are the applications of linear transducer array?
Imaging superficial structures such as blood vessels in the neck
What are the applications of curvilinear transducer array?
Imaging abdominal organs and fetuses
What are the applications of phased transducer array?
Small footprints used where there is a limited window for imaging at the skin surface but a wider field of view is needed at depth
What is the element width approximately equal to?
The wavelength of the ultrasound wave
What is the element length approximately equal to ?
30*wavelength + lens
What are the small spaces between elements called ?
kerf
What is the centre to centre distance of elements called?
Pitch
What is the field divided into?
2 sections: the near field and the far field
What is the near field main characteristics in terms of path differences between wavelets?
There are large path differences between wavelets arriving from different points on the source, resulting in a complex pattern of pressure minima and maxima
What is the far field main characteristics in terms of path differences between wavelets?
the path differences are smaller, as all points are far from the source, and there is a more simple distribution or radial profile of pressure, the beam begins to diverge
What does the position of the last axial maximum depend on?
The source radius and wavelength of the ultrasound
What happens for small low frequency sources?
They have a large wavelength as wavelength is inversely proportional to frequency, such that the last axial maximum is close to the source, and the field diverges quickly (as the tranition between the near and far fields occurs quicker)
What happens for a large frequency source?
As the wavelength is inversely proportional to frequency, the last axial maximum will be farther and the beam will be well collimated (well directed)
What is the equation for the last axial maximum?
z_max = a^2 / wavelength
What happens to a beam with a high axial maximum in terms of the relative pressure amplitude ?
A beam with a high axial maximum will be well collimated, producing a complex pattern of pressure minima and maxima
If the source radius»_space; wavelength of sound in the material, what happens to the wavefronts of the beam?
They are quite long and narrow
What happens when the source radius «_space;wavelength of sound in the material?
the source acts like a point source, with sound radiated over a wide angle from close to the source due to diffraction effects
What is the equation used to calculate the angle of divergence of the far field?
sin(theta) = 0.61 * wavelength / source radius
What does the equation for the angle of divergence of the far field tell us?
The angle of the central, axial region, the main lobe, is surrounded by sidelobes areas.
The sidelobes areas are areas of alternate high and low pressure which look like rings around the main lobe
In practice, how is imaging done?
using arrays of small elements and the total field is the sum of the fields form the individual elements in the active group of elements
How is the distance of structures from the source determined and used to form an image?
Short pulses are used for imaging
Why are short pulses used so that the distance of structures from the source can be determined and used to form an image?
Short pulses contain a range of frequencies, and there are fewer wavefronts to interfere as:
1. the near field beam patterns are far simpler
2. the pressure amplitude on axis is more uniform
What is the central frequency of the pulse spectrum?
The frequency that produces the highest amplitude pulse
What does the width of the pulse spectrum give?
The bandwidth: the range of frequencies contained in the pulse
What happens to a shorter pulse?
It has a wider bandwidth
How can images be formed?
From a series of scan lines which make up the image
What occurs to the small elements used to form the image vs the wavelength?
They result in a divergent field, so to make a narrower beam, the elements are pulsed sequentially in groups.
What is the term for when small elements are pulsed sequentially in groups?
the transmit beam
What’s the point of doing transmit beam?
Small elements result in a divergent field
What is the result of transmit beam?
The beams result in several elements combined, so they are less divergent
How can a focus be formed at different depths?
Using electronic steering where each element is fired at a slightly different time
How are the beams making up the image scan lines formed?
By an active group of elements
In which the active group is moved along the array by 1 element each time
How is the focus depth chosen?
By the operator, or based on an imaging preset
How does the machine adjust the focus depth?
Accounts for differences in path length between each element and focus position
how does the machine account for differences in path length between each element and focus position on transmit vs on receive?
On transmit: by relative delay between signals on elements
On receive: by adding relative delays to signals before summing
What happens to the echoes after they are received by the transducer elements?
Added together
Process of ultrasound imaging:
- echo reception
- signal addition
- signal processing
3.1 compenation for attenuation
3.2 envelope detection
3.3 brightness profile formation
Echo reception
When an ultrasound probe sends out sound waves into the body, they bounce off different tissues and structures. The echoes are picked up by the transducer elements in the probe.
Signal addition
The echoes received by the trasnducer elements are combined or added together. The process enhances the signal qualty