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Physics of Colour and Doppler Rev 1 - Cut Surgery

Physics of Colour and Doppler Lynette Hassall DMU AMS MLI Clinical Applications Specialist Sonosite Australasia Pty Ltd 1 Table of contents 2 Colour ..6 Colour Power Doppeler .. 7 Spectral Doppler .. 12 15 Getting started - Techniques .. 21 References and Bibliography 22 These notes are intended to be an introduction to the subject only. I have tried to make the Doppler Physics as simplistic as possible and convey a limited, basic overview of a very complex subject. I am attempting to provide you with a general understanding of how and why Doppler Ultrasound produces an image, and how to optimize those images to achieve the best from your machine, for the benefit of your patients. Please refer to Physics text books and articles for more complete explanations.

Physics of Colour and Doppler Lynette Hassall DMU AMS MLI Clinical Applications Specialist Sonosite Australasia Pty Ltd 2 B-Mode 0° +15° -15° Concepts All Colour and Doppler Applications in Ultrasound work from the ‘Doppler Principle’.

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Transcription of Physics of Colour and Doppler Rev 1 - Cut Surgery

1 Physics of Colour and Doppler Lynette Hassall DMU AMS MLI Clinical Applications Specialist Sonosite Australasia Pty Ltd 1 Table of contents 2 Colour ..6 Colour Power Doppeler .. 7 Spectral Doppler .. 12 15 Getting started - Techniques .. 21 References and Bibliography 22 These notes are intended to be an introduction to the subject only. I have tried to make the Doppler Physics as simplistic as possible and convey a limited, basic overview of a very complex subject. I am attempting to provide you with a general understanding of how and why Doppler Ultrasound produces an image, and how to optimize those images to achieve the best from your machine, for the benefit of your patients. Please refer to Physics text books and articles for more complete explanations.

2 Reading these notes does not imply any qualification or competence in Ultrasound Physics . Attendance at a Practical Ultrasound Training Course is highly recommended Physics of Colour and Doppler Lynette Hassall DMU AMS MLI Clinical Applications Specialist Sonosite Australasia Pty Ltd 2B-Mode 0 +15 -15 Concepts All Colour and Doppler Applications in Ultrasound work from the Doppler Principle . The Doppler equation calculates the difference in received frequency from the transmitted frequency sent from the probe, measured in Hertz. From this, the Ultrasound machine calculates the velocity of the blood flow (using a complex mathematical formula called the Fast Fourier Transform [FFT]) and displays the result as a Colour overlay on the B-mode display, an audible sound, or as a graph (Spectral Display).

3 Red Blood Cells (RBC) act as Rayleigh scatterers this is a special type of ultrasound reflection pattern, the scatter is equal in all directions ie because the reflectors are so small the reflection is not angle dependent the main point is that, for Doppler purposes - the red blood cells act as point sources of ultrasound. We use this reflection pattern when we are obtaining a Colour image or Doppler trace. Colour : The Colour box is displayed as an overlay onto the B-Mode image. The crystals first send out a pulse of sound which produces the B-mode image, then a selected group of crystals send out a second pulse to form the Colour image you see on the screen. Each Colour line of sight has numerous sampling sites along it so you need to keep the Colour box as narrow as possible to keep the frame rate high, which in turn provides a much faster update process and a more real-time display of the haemodynamics.

4 The Colour box may be angled at +15 , 0 , or -15 depending on the angle of the vessel to the transducer plus or or When set up correctly, the Colour display provides you with a qualitative impression of the flow characteristics of the vessel you are observing. No numerical value can be Physics of Colour and Doppler Lynette Hassall DMU AMS MLI Clinical Applications Specialist Sonosite Australasia Pty Ltd 3assigned to the image, but the Colour image can assist you in quickly and easily placing the spectral sample volume in the area of fastest flow. When positioning the Colour box you are trying to achieve an angle with the direction of the blood flow in the vessel (this is the reason for the ability to change the angle of the Colour box in some systems).

5 Imagine the acute angle of the Colour box is an arrow and point the arrow towards the most superficial portion of the vessel. This is the first step in optimizing the Colour display. If the vessel itself is at an angle to the transducer, or is very deep, a straight box may be used. Pulse Repetition Frequency (PRF) may also be called the Velocity range or Scale. The PRF is the rate at which the lines of sight are sent out and received back to the transducer. The higher the PRF is, the greater the velocity of flow we can accurately display. We need to correlate this with the speed of flow in the vessel we are interrogating too high a PRF and we will not detect slow venous flow, too low a setting for the PRF and we will see aliasing in all vessels.

6 Physics of Colour and Doppler Lynette Hassall DMU AMS MLI Clinical Applications Specialist Sonosite Australasia Pty Ltd 4 Filter also called Wall Filter, High Pass Filter, or Wall Thump filter blood moves within the vessel, the walls of the vessel also move slightly, and so does the surrounding tissue. This movement from the vessel walls and tissue are high amplitude (very strong) but low frequency (they are not moving as fast as the blood within the vessels) in order to avoid displaying these confusing signals, the filter can be set to remove echoes below a certain frequency. If the filter is set too low the wall signal will not be removed, and if it is set too high the signals from slow moving blood in a vein may also be removed.

7 Slower flow towards the wall of the artery may also be removed and we will not see the Colour fill to the vessel walls. The wall filter needs to be set appropriately for the vessel you are interrogating a good start is - High for arteries, Low for veins If the angle is incorrect there will be very poor representation of flow Physics of Colour and Doppler Lynette Hassall DMU AMS MLI Clinical Applications Specialist Sonosite Australasia Pty Ltd 5It is important that you have a basic understanding of the Doppler equation used for this calculation as it helps to understand how to manipulate the probe and machine factors to obtain the best result possible. The Colour image, the Power image, and the Spectral Trace all work on the same principle, only the mode of display has changed.

8 It will also help you to understand why sometimes, even though there is flow in the vessel, you may not be scanning at the correct angle to demonstrate it. This is a form of the Doppler equation calculating Doppler shift. )/(cos)/(2)()(smcscmukHzfkHzftD = And may be written as = (d) Doppler shift the difference between the transmitted and received frequencies -this is measured by the system and is calculated in kHz. u - is the velocity of the moving reflector (RBC) relative to the transducer this is what we are attempting to quantify in our spectral Doppler trace. 2 is used due to the pulse-echo effect from the Rayleigh scatterer or Red Blood Cells (RBC) the pulse from the transducer to a moving reflector (RBC) produces a Doppler shift at time of reception of the pulse, then a further Doppler shift occurs when the pulse from the RBC returns to the transducer.

9 C is a constant the assumed speed of sound in soft tissue 1540 m/s (t) Transmitted frequency - this is the frequency emitted by the transducer Cos - the angle of approach of the flow direction of the red blood cells to the Doppler beam must be allowed for , this is done using the cosine of the Doppler angle we are using this to calculate the vector at which the red blood cell is moving relative to the transducer. By changing the formula around to solve the velocity of the moving reflector (written as u ) the system is able to calculate the speed of the reflector in cm/second (using the Fast Fourier Transform [FFT]) The effect of this is to produce a signal which is Make sure the Volume control for your system is ON so that you are able to hear the signal.

10 By listening to the pitch of the sound you will be able to hear when you are sampling and artery ( higher pitch, rhythmic variation), or a vein (lower pitch, whooshing sound). cos2)()(tdUfcf= = Physics of Colour and Doppler Lynette Hassall DMU AMS MLI Clinical Applications Specialist Sonosite Australasia Pty Ltd 6 Colour gives directional information for blood flow. This directional information is relative to the face of the transducer ie towards the transducer or away from the transducer. A transducer crystal is able to resonate to produce the sound wave, then stop and listen for the return echo it cannot both transmit and receive simultaneously. The Colour box is superimposed over the B-mode Image;- first the B-mode image is generated then a second pulse of sound is transmitted (from a selected number of crystals) to produce the Colour display.


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