Basic explanation of the various imaging modes used in ultrasound imaging
B-mode (Brightness mode)
Brightness mode is the most basic greyscale imaging mode. It consists of a two dimensional image where the brightness of an area is stronger in places where the echo returns most strongly.
M-mode (Motion mode)
M-Mode is based upon B-mode but is used to track motion, such as the beating of a heart. Short movies are taken in M-mode and can usually be exported to a computer or PACS system to be viewed.
Color Doppler (Color flow Doppler)
Color Doppler uses standard ultrasound methods to produce a picture of a blood vessel. In addition, a computer converts the Doppler sounds into colors that are overlaid on the image of the blood vessel and that represent the speed and direction of blood flow through the vessel.
Generally the color above the bar indicates the flow towards the transducer , while the color below the bar indicates the flow away from the transducer, the brighter the color, the faster the flow speed, whereas the darker the color, the slower the flow speed.
Color Doppler has poor temporal resolution/flow dynamics. (frame rate can be low when scanning deep)
Power Doppler is also referred to as energy Doppler, amplitude Doppler and Doppler angiography. Power Doppler does not display flow direction or different velocities. Power mode is used to image blood flow by displaying the density of red blood cells, as opposed to their velocity. Power increases the sensitivity range of the soundwaves allowing even small flows of blood through organs to be tracked rather than the faster, more substantial flow in arteries and veins detected by Color Doppler.
Power Doppler is:
- sensitive to low flows,
- No directional information in some modes,
- Very poor temporal resolution,
- Susceptible to noise
The transducer sends and receives sound waves that are amplified through a microphone. The sound waves bounce off solid objects, including blood cells. The movement of blood cells causes a change in pitch of the reflected sound waves (called the Doppler effect). If there is no blood flow, the pitch does not change. Information from the reflected sound waves can be processed by a computer to provide graphs or pictures that represent the flow of blood through the blood vessels.
- Examines flow at one site,
- Detailed analysis of distribution of flow,
- Good temporal resolution - can examine flow waveform,
- Allows calculations of velocity and indices
Spectral steered doppler
Produces a spectral doppler graph but the direction of the imaging can be steered to different locations rather than in only one direction. Spectral steered doppler can be used in both PW and CW Doppler.
Duplex combines Spectral and Color Doppler images side by side on the screen for easy comparison.
When using color flow imaging with pulsed wave Doppler, the color flow/B-mode image is frozen while the pulsed wave Doppler is activated. Recently, some manufacturers have produced concurrent color flow imaging and pulsed wave Doppler, sometimes referred to as triplex scanning.
PW Doppler (Pulsed wave)
Pulsed wave Doppler is used to provide analysis of the flow at specific sites in the vessel under investigation. PW produces a series of pulses used to study the motion of blood flow at a small region along a desired scan line. The x-axis of the graph represents time while the Y axis represent Doppler frequency shift. The shift can be converted into velocity and flow if an appropriate angle between the beam and blood flow is known Pulsed wave systems suffer from a fundamental limitation.
The time interval between sampling pulses must be sufficient for a pulse to make the return journey from the transducer to the reflector and back. If a second pulse is sent before the first is received, the receiver cannot discriminate between the reflected signal from both pulses and ambiguity in the range of the sample volume ensues. As the depth of investigation increases, the journey time of the pulse to and from the reflector is increased, reducing the pulse repetition frequency for unambiguous ranging. The result is that the maximum fd measurable decreases with depth.
When pulses are transmitted at a given sampling frequency (known as the pulse repetition frequency), the maximum Doppler frequency fd that can be measured unambiguously is half the pulse repetition frequency. If the blood velocity and beam/flow angle being measured combine to give a fd value greater than half of the pulse repetition frequency, ambiguity in the Doppler signal occurs. This ambiguity is known as aliasing. A similar effect is seen in films where wagon wheels can appear to be going backwards due to the low frame rate of the film causing misinterpretation of the movement of the wheel spokes.
CW Doppler (Continuous wave)
Continuous wave systems use continuous transmission and reception of ultrasound. Doppler signals are obtained from all vessels in the path of the ultrasound beam (until the ultrasound beam becomes sufficiently attenuated due to depth). Continuous wave Doppler ultrasound is unable to determine the specific location of velocities within the beam and cannot be used to produce color flow images. CW allows the operator to measure extremely high velocities. CW is not limited like PW with respect to the ability to measure high velocities as it is always sending and receiving signals, it does not have to wait for a signal to return before sending out another.