DOPPLER AND CONTRAST ULTRASONOGRAPHY
S. Prathaban
Professor and Head, Department of Clinics Madras Veterinary College, Chennai –600 007
Doppler dramatically increased the diagnostic capabilities of cardiac ultrasound. This modality allows detection and analysis of moving blood cells and tells us about the direction, velocity, character and timing of blood flow. The haemodynamic information provided by Doppler echocardiography allows definitive diagnosis in most cardiac examination.
Three types of Doppler may be used during an echocardiographic examination
- Pulsed wave (PW)
- Continuous wave (CW)
- Color flow (CF)
Pulsed wave Doppler:
Pulsed wave Doppler is site specific in which it can be directed and set to sample flow at specific places within the heart. However PW Doppler is limited in its capacity to detect higher frequency (velocity) shifts. Continuous wave Doppler (CW) can detect high frequency shifts and therefore high flow velocities with virtually no limits. Because sound is transmitted and received continuously it is not possible and interrogate at specific depths within the heart. Although this may sound disadvantageous, the information provided is valuable. Colour flow Doppler (CF) is a form of pulsed wave Doppler, frequency shifts are encoded varying lines and intensities of colour. Flow information is graphic and detection of abnormal flow is easier with CF Doppler however quantitative information is limited.
Doppler shift:
Christian Johann Doppler an Austrian physicists and mathematician was first to describe the Doppler effect. He found all types of waves (light, sound etc) change in wave length when position between the source of the wave and the receiver of the wave changes. For example if we move towards sound source the pitch are frequency of that sound increases and if we move away from that sound source the frequency decreases. The change in the frequency between sound that is transmitted and the sound that is received is the Doppler shift when the source and reflecting surfaces both are stationary the transmitted and reflected wave lengths are equal. When the reflecting structure is moving toward the sources sound waves encounted more often increasing the number of waves (high frequency) reflected back towards the source. When the reflecting structure is moving away from the source sound waves travels ahead of the transmitted wave front and are encounted less frequently, decreasing the number of sound waves (Low frequency) reflected back to the source. The Doppler shifts that we use are in ultrasound in is the difference in frequency transmitted by the transducer and received frequency reflected from blood cells.
Doppler tracing:
Doppler derived frequency shift (fd) is equal to the reflected frequency minus transmitted frequency therefore objects moving towards the source result in the positive frequency shifts where as the object moving away from the source result in negative frequency shifts. The site (gate) for the Doppler flow interrogation is selected by the examiner and is represented on the Doppler display as a line (base line). Positive frequency shift (flow moving towards the transducer) produce wave forms up from the base line, whereas negative frequency shifts (flow moving away from the transducer) producing downward deflection on the Doppler tracings. These images are called spectral tracings.
Pulsed Wave (PW) Doppler:
Pulsing the sound waves allows a transducer to act as a receiver for the signal only during the time interval specified by a sample depth. With PW Doppler the transducer will receive frequency shifts only during the time interval dictated by the depth of the sample site ignoring all the returning the echoes. New sound waves will not be transmitted until the transducer has received echoes from the previous burst. Range resolution is the ability to measure velocity within a normal cell at a specified depth along the ultrasound beam. The gate is the site at which sampling is set to occur. The examiner usually sets gate while watching a two dimensional image.
Continous Wave (CW) Doppler :
CW Doppler continuously sends out and continuously receives sound. It is not possible to range gate CW Doppler because the transducer as no way of detecting the depth of the reflected signal. CW Doppler detects frequency shifts all along the ultrasound beam with no range resolution and is steered in one of two wave imaging CW systems use a cursor as colour representing the Doppler sound beam. The cursor is placed over the two dimensional image and frequency shifts are calculated all along beam in imaging CW system non imaging CW system use a dedicated CW pulse without a two dimensional image. These systems require recognition of characteristic flow profiles.
Velocity alone the beam varies and a full spectrum of frequency shifts are detected with a CW Doppler. When CW Doppler is used properly the highest velocity along line of interrogation are recorded. The highest flow velocity are what is of interest and diagnostically important. Lower velocity flows are hidden within the higher flow profiles. Flow patterns for the various valves and vessels in the heart are characteristic and usually are identified easily with both PW and CW Doppler.
The Doppler equation:
Doppler ultrasound can determine blood cell velocities within the heart or peripheral vessels based on the Doppler shifts. Accurate measurements of velocity are affected by transducer frequency and intercept angle.
Effect of Doppler frequency:
PW Doppler measures the frequency shifts location within the heart. Just like two dimensional and M – mode imaging, the reflected single must be received before the next pulse is transmitted or the recorded singles will be ambiguous. The time interval between the pulses is also referred to as pulse repetition frequency (PRF) must be two times the sample depth. The time between pulses must increase as sample depth increases, resulting in decrease PRF. Decrease PRF decrease the Doppler frequency shift that can be measured accurately. The sampling rate must be atleast two times the frequency shift for the transducer to receive unambiguous flow of information. Maximum Doppler shift that can be recorded accurately is equal to one half the PRF
Doppler shift = ½ PRF
One half of the PRF is referred to as niquist limit. When the niquist limit is exceeded, the flow profile wraps around the image, this can be corrected moving the baseline up or down on the monitor allowing the entire profile to be recorded accurately. When the niquist limit is exceed by large degrees the aliased signal no longer displays the characteristic flow profile and direction can no longer be determined. Switching to CW Doppler allows flow velocities that exceeded the niquist limit to be recorded accurately. The maximum velocity that can be recorded without aliasing is inversely proportional to depth for any given transducer frequency the niquist limit is exceeded for sooner at deeper gates for a given interrogation frequency.
Normal blood flow is typically laminar for all blood cell within a vessel out flow tract are chamber move in the same direction with similar flow velocity. Vessel and chamber walls create friction for the blood cell moving adjacent to wall surfaces and velocities are some what slower along the periphery of the flow stream than in the centre of the flow stream. PW Doppler appears hollow within the spectral broadening when flow is laminar intercept angle close to zero and the niquist limit is not exceeded spectral broadening in the filling of typically hollow waveform. Spectral broadening a pulse wave signal may be result of improper gain settings large intercepts angle are non laminar flow (turbulent). When flow becomes abnormal it is turbulent. Turbulent flow has blood cells moving in many directions at various velocities. This kind of flow is seen with stenotic lesions, shunts and valvular regurgitation. Doppler singnals produce from turbulent flow have a lot of spectral broadening because of many velocities and flow direction present in the jet. CW Doppler shows spectral broadening even when flow is laminar because flow velocities detected all along the transmitted sound beam vary tremendously .
Colour Flow Doppler :
CF Doppler is a form of pulse wave Doppler real time images and colour flow mapping are done at the same time with alternating scan lines dedicating towards real time image generation and Doppler signals. Blood flow in colour mapping is perceived by the machine either moving towards the transducer or moving away from it via negative or positive frequency shift. Flow moving towards the sound source is plotted in hues of red and flow moving away from the transducer is mapped in shades of blue. No flow generates no frequency shift has no colour is assigned. Enhanced colour maps available in most equipment display flow velocity information as well as direction. Colour range from deep red for slow blood flow to bright yellow for rapid red flow towards the transducer. Slow blood flow away from the transducer is mapped in deep blue colours and whereas rapid blood flow away from the transducer is mapped in shades of light blue and white. CF Doppler depends on two important factors. Pulse repetition frequency and frequency of the transducer. As spectral Doppler the sound source frequency dictates the maximal velocity but can be accurately mapped at any given depth before aliasing occurs. Aliasing in CF Doppler involves a reversal of colour resulting in a mosaic of mixing of the red and blue hues. Aliasing in CF Doppler involves a reversal of colour while using high frequency transducer when in activating there is normal flow and the aliasing is only a function of transducer frequency. A aliasing would be eliminated if a lower frequency transducer was used.
Many ultrasound machines have variance maps. These machines map turbulent flow in hues other then blue or red.
Frame rate refers to the number of times a real time or colour flow image is generated per minute . A frame rate of atleast 15 times per minute is required for smooth transition and the appearance of a continuously moving image as a sector. Frame rate in CF Doppler is equal to PRF divided by scan lines per colour sector. The operator can alter the width of the sector . Decreasing the colour wedge decreases the amount of time necessary for sampling and increases the frame rate. The operator also can eliminate the real time image that extends beyond the width of the colour sector. Decreasing the colour wedge decreases the amount of time necessary for sampling and increases the frame rate. The operator also can eliminate the real time image that extents beyond the width of colour sector. This also decreases the time necessary for image generation and enhances colour flow mapping.
Many machine allow the operator to decrease the depth of the colour wedge . This has no effect on the frame rate, because total image depth is unchanged . It merely decreases the information the mind has two process.
The number of time the line of sound is sampled is referred as its packet size. Increasing packet size improve the image quality and fills in the colour display but at the expense of frame rate. Packet sizes usually can be selected by the operator. Decreasing packet size will increase the frame rate, but decrease sampling time. Information may be lost with short sampling times . However, this may be necessary width rapid heart rate. Increasing packet size will increase the time required for sampling and will decrease the frame rate. But it results in greater accuracy in measuring velocity and map colour.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1665223/
https://www.pashudhanpraharee.com/therapeutic-ultrasound-in-veterinary-practice/
