Physics of Colour and Doppler Lynette Hassall DMU AMS MLI Clinical Applications Specialist Sonosite Australasia Pty Ltd

Transcription

1 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. Reading these notes does not imply any qualification or competence in Ultrasound physics. Attendance at a Practical Ultrasound Training Course is highly recommended Table of contents Concepts... 2 Colour... 6 Colour Power Doppeler... 7 Spectral Doppler Artifacts Getting started - Techniques References and Bibliography 22 1

2 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). 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. The colour box may be angled at +15, 0, or -15 depending on the angle of the vessel to the transducer B-Mode 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 2

3 assigned 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). 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. 3

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. 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 4

5 It 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. 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. f D ( khz ) = f t ( khz ) 2 u c ( m ( cm / s ) / s ) And may be written as Transmit. Frequency 2 velocity. of. moving. reflector cosine. Doppler. Angle Doppler. Shift = speed. of. sound. in. soft. tissue ƒ (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. c is a constant the assumed speed of sound in soft tissue 1540 m/s ƒ (t) Transmitted frequency - this is the frequency emitted by the transducer Cos θ- 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]) U = f ( t ) f ( d ) c 2 cos θ cos The effect of this is to produce a signal which is AUDIBLE. θ Velocity. of. moving. reflector = Transmit Doppler. frequency. shift 1540 m / s 2 Co sin e. Doppler. Angle Make sure the Volume control for your system is ON so that you are able to hear the signal. 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). 5

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. You can expect the frame rate to be slower due to this double transmit However, the faster the frame rate the more real-time the colour representation of the haemodynamics will be. In order to keep the frame rate as high as possible you should keep the colour box narrowed to as small an area as practical. Any movement is best seen at 0 - movement which is either directly towards or directly away from the transducer. As this is virtually impossible to achieve in real patients an angle equal to or less than 60 is used. If the angle of flow is 90 (perpendicular) to the beam there is no movement relative to the transducer so no colour (or very poor colour representation) will be seen. This is one reason why colour and Doppler should be avoided in Transverse if we are at 90 to the direction of blood flow, we do not see colour even if flow is present. We use the colour bar at the top of the image to indicate direction of flow the colour represented at the top of the bar indicates movement towards the probe, on the bottom is away from the probe. Be aware that the machine does not know whether you are examining an artery or a vein so do not be fooled into thinking that everything that has been encoded red on your image is an artery, or blue is a vein. You control the colour encoding by pressing the Invert button. Check the display for reference. To achieve good colour resolution keep the colour box as small as possible in both height and width. *When in colour mode the gain control will increase the brightness of the colour pixel representation on the screen. 6

7 Colour Power Doppler (CPD) or Amplitude Mapping is also known as Power Doppler, Power Doppler Imaging, or Power for short this is used when we simply want to see whether there is flow in a vessel or structure, or not. CPD is displayed as one colour only so does not give any directional information to the flow. Power is useful because it is not as angle dependent as colour, is more sensitive to low flow states, and has better edge resolution, This means we will detect smaller vessels (such as those in thyroid or testes), more easily, it will also display the trickle flow in stenotic arteries. Power may be used in the transverse view to show flow in the vessel. *To be absolutely sure, you will still need to check in longitudinal to confirm whether you are interrogating a vein or artery In these image we see the radial artery in transverse and longitudinal 7

8 Spectral Doppler also known as Pulsed Wave Doppler - is a graphical representation of flow velocity over time. The brightness of the pixels represents the amplitude (strength) of the returned echo, the gain control will alter this brightness to make the graph easier to see when in Doppler mode. Doppler angle Arteries and veins have characteristic patterns of flow and these are displayed on the graph. Sample Volume The Doppler line of sight is a representation of the beam from Doppler Line of Sight the transducer crystal. The sample volume is the box which represents the area of movement we are sampling, the sample volume is three dimensional, so it is really sampling a cube of tissue. This sample volume needs to be kept as small as possible so extraneous movement is not included in the signal. Wall motion or movement from other vessels in close proximity to the vessel being sampled can be represented on the display and cause confusion. The Doppler angle is the angle between the direction of flow and the line of sight, and must be at 60 or below. Any angle higher than this and the calculations of velocity of flow become increasingly inaccurate. The movement of the blood cells has two components, relative to transducer and relative to the body [towards or away from the heart]. This combination of movement gives us a vector and the angle measurement must be calculated using the cosine of the angle. Due to the cosine part of the Doppler equation the error margin changes with changing angle of insonation, so to maintain comparable measurements the angles must be kept the same eg for ICA/CCA ratio measurements. Any series of follow-up measurements must be also taken using the same Doppler angle for the results to be comparable (so the error margin will remain the same). The 60 angle is recommended, but this angle must also be lined up along the direction of flow within the vessel. This is usually achieved by aligning the Doppler angle with the vessel walls. *However, if there is a stenotic jet, the flow may be eccentrically placed within the vessel and the walls will not be a guide to the correct placement of the angle. This image shows a typical low resistance arterial waveform. The positioning of the trace above the base line indicates continuous positive direction of the flow (relative to the transducer) throughout the cycle. 8

9 By convention, write arterial flow above the baseline and venous flow below the baseline to provide a quick reference point for observers. Differentiation between arteries and veins using ultrasound: To begin with we observe the vessel in B-Mode *Remembering that we always assess the vessel in both transverse and longitudinal planes. Ultrasound is a dynamic examination; so follow the course of the vessel for a minimum of several cm before making any decision. An artery will display Pulsatility in response to systole and diastole Limited response to extrinsic compression, walls will not co-apt easily No change in caliber will be noted with breathing The three layers of the artery walls will be seen the intima and adventitia are seen as hyperechoic lines, separated by a hypoechoic layer the muscular medial layer A vein will display Phasicity in response to breathing, or as response to the Valsalva manoeuvre the vein will increase and decrease in caliber as the patient breathes, if it is close to the trunk. Small peripheral veins will not display phasicity but will show increased caliber if a tourniquet is applied to the limb. The vein walls will co-apt if compression is applied to the vessel. In a normal vein the vessel walls will collapse completely and touch each other (co-apt) if they do not then you are pressing on an artery OR the vein has either acute or chronic thrombus within it. *Compression should only be performed in the transverse plane, if you apply compression with the transducer in the longitudinal plane, it is easy to slip off the plane of the vessel and mistakenly think it is co-apting. No pulsatility will be demonstrated, however the closer you get to the heart, the more likely you will see transmitted pulsations, superimposed over the normal phasicity. Only a single layer to the vessel wall will be seen, there is no muscular layer visible 9

10 Veins have valves, and a valve sinus (widening) at the level of the valve, arteries do not. Colour variation between arteries and veins The flow towards the transducer is demonstrated as either red or blue in colour mode. Unfortunately the system does not automatically detect arterial flow and render the arteries red or venous flow and render the veins blue you need to determine whether the vessel is a vein or an artery and then choose which colour to display the vessel appropriately. By convention and for convenience - we tend to display the arteries red and the veins blue so as not to confuse observers. (If you look at any anatomy text book, arteries are always coloured red, and veins blue). On the colour display we observe the hue of the colour slow flow is displayed in darker hues, while faster flow is displayed as lighter brighter hues. In a typical artery we may demonstrate a darker colour close to the walls, shading to a brighter colour in the centre indicating parabolic flow fastest in the middle of the artery. Arteries will display a pulsatile flow in response to the cardiac cycle. In systole there will be an increased flow with a change in colour from a darker to a lighter red. In diastole you may see a flash of the opposite colour as we get a reverse component to the flow in diastole. This rhythmic change in colour in conjunction with the cardiac cycle is typical of the appearance of the arteries in colour mode. Veins will display a monophasic colour pattern, there will be no rhythmic change in flow pattern with cardiac cycle, instead the flow may cease completely if the limb is dependent, if a tourniquet is applied, or if the patient performs the valsalva manoeuvre. Another way to check whether you are observing a vein or an artery is to see if the flow augments with distal compression. If you are observing an arm vein and squeeze the hand then the flow in the vein will increase and a lighter hue colour will result. This augmentation does not occur with arteries. Flow in the vein and artery will be in different directions you should have one blue and one red vessel In this image we have the vertebral artery and vein running side by side, demonstrating different flow direction as different colours. 10

11 Spectral variation between arteries and veins When you are interrogating the arteries and veins using spectral Doppler you will see characteristic flow patterns in arteries and veins (more on this subject later) If you have the sound turned on you will hear a difference in the sound generated by the vessels. Veins have a characteristic, low pitched whooshing sound, arteries have a higher pitched, rhythmic variation which increases in pitch in systole, decreases in diastole and corresponds with the cardiac cycle. Turbulent or stenotic areas of flow will demonstrate loud sounds, almost like the sound the static on your TV screen makes if you are off channel. (this is difficult to describe on paper, but unmistakable when you have heard it) A point to note is that you do not NEED to view the vessel in Colour mode, prior to positioning the Doppler sample volume. You can simply position the sample volume within the vessel without activating the colour control. However, the Colour image is used to give us a quick look at the vessel to provide us with a quantitative impression of the flow characteristics. The colour image also allows us to position our sample volume quickly and accurately, within the fastest flow. The Doppler scale setting should be adjusted so the waveforms fill the window, without aliasing this allows us to measure the velocities more accurately, and obtain a better appreciation of the haemodynamics. 11

12 Spectral Doppler The sample volume needs to be kept as small as possible to accurately and unambiguously represent the maximum flow velocity in the vessel you are interrogating. If the sample volume is too wide then you are obtaining signals from all structures moving within the volume, including the vessel walls, and possibly any other vessel or moving structure adjacent to it. In this example the sample volume is much too large for the vessel. The wall movement, the slower blood flow at the periphery, and the very much faster flow in the centre of the vessel are going to be displayed. This will produce a very noisy spectral trace with a very wide range of velocities displayed. This wide range of velocities has been termed spectral broadening. True spectral broadening occurs at an area of stenosis, due to the turbulence of the flow, but if your sample volume is too large this can be produced artifactually. In this second example the sample volume has been decreased in size and now only the fastest flow in the centre of the vessel will be displayed, producing a much cleaner signal, with no artifactual spectral broadening. When you have the colour set up correctly you should take the time to systematically set up the Doppler line of sight, sample volume and Doppler angle to achieve the best Doppler signal possible for that vessel Align the Doppler line of sight so that it is parallel to the sides of the colour box When this is done set the Doppler sample volume so that it is as small as possible, and completely within the centre of the vessel we do not want to inadvertently sample the walls or surrounding tissue 12

13 Finally align the Doppler angle to 60 to the Doppler line of sight, BUT ALSO ALONG the direction of the fastest flow within the vessel. A normal vessel will usually have parabolic flow, which means that if you line up the Doppler angle with the vessel walls, the angle to the direction of flow should be approximately correct Remember this flow may not necessarily be aligned with the vessel walls if there is a stenosis and an eccentric jet, or if there is a bifurcatiorn or tortuosity of the vessel walls. Do not align the Doppler angle across the line of flow as you will only display a noisy spectral trace, with spectral broadening. Doppler angle across direction of flow Doppler angle aligned with direction of flow What do the signals mean? The graphical display gives us a picture of velocity along the y -axis and time across the x -axis of the graph. The brightness of the pixels on the graph displays the strength or amplitude of the returned signal this may be changed using the Gain control, when in Pulsed Wave mode. The Baseline represents zero flow direction with reference to the transducer; it can be referred to as the base line or the zero crossing line. Velocity Y axis Time Baseline X axis 13

14 The arterial wave forms are traditionally written above the base line, and venous waveforms are written below the baseline. High resistance arterial Waveform Low resistance arterial Waveform Venous Waveform Arterial waveforms usually show a rhythmic pattern of flow which corresponds with the cardiac cycle, a high resistance waveform displays a sharp upstroke in systole, with reverse flow (or no flow) in the diastolic phase. The third component of elastic recoil of the vessel walls, displays as a second smaller peak, before the cycle starts again. The low resistance waveform displays as a continuous forward flow throughout the whole cycle, with the velocity never dropping below the baseline. Venous flow displays as a monophasic continuous flow towards the heart. There is variation in velocity with respiration, or augmentation (flow increase) on distal compression. The closer you are to the heart, the more likely there will be transmitted pulsations superimposed on the phasic pattern. A display demonstrating spectral broadening shows infill of the spectral window (the clear area under the spectral trace) there are no clearly defined velocities so this appearance demonstrates either Turbulence Disturbed pattern of blood flow at a bifurcation Post stenotic area Sample volume is too large Sample volume too close to wall of vessel Doppler angle is across the lines of blood flow Spectral Broadening A pattern such as this one with spectral broadening and a high amplitude spike would indicate that the sample volume has been placed too close to the pulsing wall of an artery Wall Thump / Spectral Broadening 14

15 Artifacts. As with B-mode, colour mode and Doppler mode also have intrinsic artifacts associated with them. Attenuation As the ultrasound beam travels through tissue it is attenuated because the signals from red blood cells are much weaker than those from surrounding tissues, you need to use a lower frequency than you would expect for a vessel of given depth. You may also need to increase the gain to produce a colour image from a deep vessel. In a deep vessel you may not be able to demonstrate colour flow due to this beam attenuation. Place your Doppler sample volume into the vessel and try to obtain a spectral trace anyway, as the Spectral Doppler has a higher intensity and you may get the information needed. Acoustic shadowing Has a different effect sometimes the structure causing the acoustic shadowing will not be visible in the image, due to the angles of the lines of sight for the colour box and Doppler differing from the B-mode this becomes especially confusing if you can see the vessel clearly in the B-Mode image. For example a Subclavian artery may have a shadow over it from the clavicle, you would expect, because you can see the artery in B- Mode, that you would receive a Doppler signal from it however If the angle is incorrect you are trying to place your Doppler signal through the bone and so will produce acoustic shadowing for the Doppler signal different to that from the B-mode. In this first illustration the B-mode shadowing is clearly defined you can see the Subclavian vessel and the shadow from the clavicle 15

16 In this second illustration, when we activate the colour box, we observe a colour acoustic shadow and will not see colour in the vessel, even though the vessel is still clearly visible in the B-mode image. Colour signal does not travel through bone no colour fill is seen in vessel This image shows an abrupt lack of colour within the colour box, even though the vessel walls and lumen can still be seen in B-mode 16

17 Aliasing Is an indication that you are sampling too slowly for the velocities you are trying to document. Aliasing is demonstrated on your colour image by a change in colour from paler red hues, through white, to paler blue hues. This may not necessarily be an indication that you have disease present, with increased flow velocities, it may be that you have the settings incorrect. Aliasing is not necessarily a bad thing if you are trying to find flow in a vein and have your settings for low velocity flow, if you slide onto an artery the flow will be too fast, and you will see the aliasing and realize that you are not in the vein. If the vessel is stenotic then aliasing may provide a clue which tells you that there is an area of increased flow velocity and you need to examine that portion of the vessel carefully. In Doppler trace the signal will wrap around the baseline. Look at the soft keys, you have PRF (which stands for Pulse Repetition Frequency) and to the left of this control there is a separate control with labels for Low, Medium and High (these are the Sensitivity settings). The Scale controls the big jumps large changes in velocity The PRF control is the fine tuning for the colour flow. In simple terms if you have very fast flow change to HIGH, if you are looking for very slow flow or small vessels change to LOW. To overcome aliasing Lower the baseline Increase the PRF change the scale from Med to High Reduce the transmitted frequency of the beam change the Res or Gen setting to Pen Reduce the depth of the sample volume (although this should already be at the most superficial depth possible, for good B-mode optimization) Change of direction The colour display always provides directional information in relation to its motion - either towards or away from the transducer If you observe the colour bar at the corner of your image you can decide what colour has been allocated to flow towards the transducer. This colour bar displays red on the top, so red in this illustration represents flow towards the transducer 17

18 Tortuosity - A tortuous vessel can be confusing as it seems to have two directions of flow within it. If you think logically, you understand that the blood flow is in the same direction with respect to the body, so any confusing signal in the vessel needs to be interpreted - In this illustration of a tortuous vessel, the overall flow is towards the marker side of the image - however the flow begins towards the transducer (displayed as red), briefly it has no motion at all relative to the transducer (so no colour is displayed -shown as a black area), then the flow is away from the transducer (displayed as blue). To alleviate this problem the colour box can made smaller laterally, and the colour box angled appropriately, so only one side of the image has colour encoding at any one time. The flow hasn t changed, only our perceptions. be Alternatively the CPD may be activated to avoid confusion. 18

19 Electrical Interference Colour and Doppler signals are subject to interference from other sources of electrical signals. These signals are usually composed of straight lines superimposed over the colour or Doppler display. Very little can be done to minimize these artifacts other than recognize them for what they are. One possible solution is to unplug the system from the mains electrical supply and run on battery power. Incorrect Filter setting Incorrect adjustment of the Filter may cause errors in registration of the flow. In vessels with low flow, a high filter setting may filter out all of the flow signal, so no flow will be seen in the vessel. In a vessel with higher flow, the high filter may limit the colour fill and artifactually cause a loss of colour close to the vessel walls, mimicking fresh thrombus formation. In doppler With a spectral trace we chop off the lower velocities of flow and do not represent these in the image we see a black area close to the baseline 19

20 Mirror image artifact Occurs when there is a large impedance mismatch between structures The result is a structure being displayed twice one a mirror image of the other Soft tissue / bone interface Soft tissue / Lung interface Diaphragm / Lung Interface Bladder / Rectum or Bowel Interface The most interesting thing about the mirror image artifact is that it is present in all modes B-Mode, Power, Colour AND Doppler and we can achieve a spectral trace from the artifact, as well as the from the real vessel. Note in the last image, the sample volume is placed within the artifactual vessel Colour Flash Artifact Colour is written within the colour box over EVERYTHING that is moving, it does not simply pick out the blood flow and display it for us. Because the colour is written over everything within the colour box that moves, we get a flash artifact if we move the transducer, or if the patient twitches or moves their muscles, or if peristalsis occurs, or if breathing occurs, or if the heart beats. To avoid this artifact keep your transducer face very still, ask the patient to keep still, but realize that sometimes this artifact is unavoidable, as we can t always control breathing, peristalsis or heart motion 20

21 Getting started - Techniques Physics of Colour and Doppler 1 - Optimise the B-Mode Image You will not be able to obtain a good colour image or Doppler trace if you have not understood the principles of optimization in B-mode. 2 Apply generous quantities of gel to the skin surface. 3 Adjust the depth so that the structure you are interested in is as large as possible on the screen (the object fills the screen) 4 Optimize the Gain and Time Gain Compensation (TGC Near and Far Gain) controls to achieve a balanced image 5 Survey the vessel in both longitudinal and transverse 6 Observe the B-mode characteristics of the vessel and try to determine whether it is a vein or an artery 7 Turn into longitudinal and display the full length of the vessel across the screen. Appearance on screen 8 Tip or tilt the transducer produce an angle with the vessel you may need to make a puddle of gel and rest one side of the transducer in it to obtain an angle, this provides contact with the gel, and does not compress small superficial vessels. Appearance on screen 9 Activate the colour Box 10 Point the arrow side of the colour box towards the most superficial part of the vessel, to optimize the angle between Colour Doppler lines of sight and flow 21

22 If there is no flow displayed, Lift the probe pressure are you pushing too hard and compressing the vessel? Decrease the PRF or Scale to Low Turn the wall filter to Low Increase the colour Gain Augment the vessel distally If there is still no flow displayed Change to CPD and reassess 11 When you have colour flow within the colour box, optimize the colour parameters to achieve the optimum colour image. Decide whether you are looking at a vein or an artery and make sure the colour is set up so that if the vessel is an artery it is displayed as red, and if it is a vein display it as blue. 12 Narrow the colour box, also make it as short vertically as practical, to keep the frame rate as high as possible. You will not be able to display a good Spectral trace if you start with a poor Colour Image. Colour and Spectral Doppler work on the same principle so Optimise the colour first then the Spectral Doppler. 13 Activate the pulsed wave Doppler 14 Align the Doppler line of sight so that it is parallel to the sides of the colour box 15 Then set the Doppler sample volume so that it is as small as possible, and completely within the vessel you are interrogating we do not want to inadvertently sample the walls or surrounding tissue 16 Finally align the Doppler angle to 60 to the Doppler line of sight BUT ALSO ALONG the direction of the fastest flow within the vessel. *Remember this may not necessarily be aligned with the vessel walls if there is a stenosis with an eccentric jet, or bifurcation or tortuosity within the vessel itself. There is no use lining up the Doppler angle across the line of flow as you will only display a noisy spectral trace. 17 Look at the trace on the screen o Is there spectral broadening? o Is it arterial or venous? 18 Listen to the audible signal 19 Optimise the signal you are achieving. o Decrease the size of the sample volume o Align the angle correctly to the flow o Increase the Gain 20 Freeze the image 21 Measure velocities (if necessary) 22 Document, by Saving the image. 22

23 References & Bibliography Allen, P; Dubbins, P; Pozniak, M; McDicken, W N; Clinical Doppler Ultrasound, 2003, Churchill Livingstone (Pub) Gent, R Applied Physics and Technology of Diagnostic Ultrasound (1997) Openbook (Pub) Kremkau F Diagnostic Ultrasound Principles and Instruments (2006) 7 th Ed, Saunders Elsevier (Pub) O Regan, S Haemodynamics of peripheral arterial waveforms Article soundeffects issue 4, 2008 pp Rose, M Did you put colour on that? Article soundeffects Issue 2, 2006, pp Stockdale, A Doppler at 60 - Magical or mythical Article soundeffects issue 3, 2006, pp Szabo T, Diagnostic Ultrasound Imaging Inside Out (2004) Elsevier (Pub) Zweibel, W Introduction to Vascular Ultrasonography 4 th Ed (2000) Saunders (Pub) 23