Basic Modes Of Ultrasound Flashcards
What are the parts of a generic transducer?
Metal housing which contains:
- Acoustic insulator material
- Acoustic backing block
- Transducer piezoelectric (PZ) crystals or elements which generate the ultrasound
- Impedance matching layer
- Connecting cables – twin signal lead
Why does an ultrasound probe need a matching layer?
The matching layer or layers provide the required acoustic impedance gradient to minimise the acoustic mismatch between the transducer and the skin.
It allows for the acoustic energy from the transducer to smoothly penetrate the body tissue and for the reflected acoustic waves (the returning echo) to smoothly return to the transducer for detection.
What are the different types of ultrasound probes?
- Phased array transducers (cardiac including TOE and 3D but can also be used for lung, abdominal, obstetric)
- Linear transducers (nerves, vascular, MSK, ocular, superficial lung)
- Curvilinear transducers (abdominal, obstetric, gynaecology)
- Capacitive micromachined ultrasound transducers (CMUTs)
- Mechanical sector probes (rarely used and will not be discussed)
What are the different arrangements of transducer crystals and how do they affect the image produced?
- Linear - Crystals are arranged linearly and sequentially fired to produce parallel beams creating a rectangular field.
- Curvilinear - Crystals are arranged linearly but transducer has a curved surface producing a wide field of view at the cost of reduced lateral resolution in the far field.
- Phased - small footprint but beam is steered electronically to produce a wide field of view. The beam diverges from the same point at the transducer.
*
How is a phased transducer beam steered/shaped?
By adjusting the firing sequence of the PZ crystals and/or introducing variable delays the beam can be shaped or steered in one plane.
What are matrix transducers?
A series of phased arrays stacked upon each other. This allows steering and beam focus in two planes. This is used for Transoesophageal (TOE).
What are linear transducers and what are they commonly used for?
The PZ crystals are arranged in a row and the beams are generated in a straight direction producing a rectangular image.
They are generally high frequency. They are commonly used for vascular, MSK, ocular, nerve blocks, and solid organ imaging.
What are curvilinear transducers and what are they commonly used for?
Crystals are arranged linearly but the transducer has a curved surface producing a wide field of view at the cost of reduced lateral resolution in the far field.
They are commonly used for abdominal scanning or deep solid organ imaging.
What are CMUT transducers?
Capacitive Micromachined Ultrasound Transducers (CMUTs) are an alternative to PZ crystal transducers.
They have a wide frequency bandwith and can produce linear/curvilinear/phased images all from the same probe.
What is M mode?
It is the amplitude of the reflected ultrasound (along a single line) against depth and is displayed on the screen as a sweep (time) of either 50 or 100 mm/s.
The image displays 1D ultrasound information (vertical) against time (horizontal).
M mode shows information on fast moving structures, allows the display of multiple beats/cycles on a single trace, and has excellent tissue interface definition which can enhance measurement accuracy (HR calcs etc.).
What gives M mode such good resolution?
Motion Mode (M Mode) has very high sampling rates of 1-2000 frames/second when compared to 2D at 30-60 frames/second. This gives M Mode excellent spatial/temporal resolution.
What is B mode?
Brightness mode (B mode) is 2D imaging. It is a series of single frames merged to show a moving object. The frames are made from a series of scan lines covering the transducer sector.
What is the relationship between frame rate and sector width and depth?
A wider or deeper sector requires more scan lines, thus taking longer to generate. This decreases the frame rate.
Conversely a narrower or more shallow sector requires less scan lines, increasing frame rate and thus improving image quality.
What is the relationship in B mode between framerate and image quality?
The higher the framerate the better the image quality.
How does 3D imaging differ from 2D imaging?
3D transducers have much higher numbers of elements. Rather than capturing a 2D slice of tissue, the 3D acquisition produces a pyramid shaped block of tissue.
This block of tissue data requires cropping and image manipulation to remove the unwanted sections of the block so that the area of interest can be seen.
3D imaging has a much slower framerate than 2D imaging, as well as lower temporal resolution.
What is the most important principle for accurate doppler velocity measurement and why?
The doppler scan line must be as close to parallel to the blood flow or tissue as possible. The maximum angle acceptable is 60 degrees.
What is Continuous Wave (CW) Doppler?
CW doppler continuously transmits and receives ultrasound signals, measuring all velocities along the scan line.
It is used to measure high speed velocities that typically exceed 100cm/s, such as from valve gradients.
What is Pulsed Wave (PW) Doppler?
PW Doppler sends short pulses of ultrasound and analyses the reflected waves between sending pulses.
It is used to measure velocities at a specific point along the scan line by moving the sample volume area down the doppler line to the required depth. Only signals returning from that depth are analysed.
It can only be used for measuring velocities less than 150cm/s.
The velocity profile is hollow with a bright edge. This edge is the velocity measured at the Sample Volume area. The velocities from other depths are not displayed creating the hollow appearance.
What is Doppler Aliasing?
Aliasing is an apparent reversal of flow that occurs when the PRF is too low in comparison to the blood flow velocity.
If Doppler shifts increase echo pitch so that they return at a frequency exceeding the maximum Pulse Interval (1/PRF), detected phase shifts will be calculated based on incorrect assumptions.
What is Colour Flow Doppler?
Colour Flow Doppler (CFD) is a 2D version of PW Doppler. A 2D box is placed over a section of the B mode image and multiple doppler scans sweep the boxed area to produce a moving colour image that indicates blood flow towards (red) or away from (blue) the probe.
How does aliasing appear in CFD?
CFD aliasing appears as a bright speckled or mosaic pattern. This indicates high velocity flow.
Aliasing can indicate stenosis or regurgitation due to their production of high velocity flow.
