This brief introduction considers the physics needed to get basic ultrasound images from a typical point of care machine.

Transcription

1 AAGBI seminar on Focused echocardiography for anaesthesia Craig Morris Physics This brief introduction considers the physics needed to get basic ultrasound images from a typical point of care machine. The echo probe emits a low frequency pulse (2-3MHz vs higher frequency 7-10MHz in a vascular access or nerve block probe) of ultrasound, which travels through the tissues, meets a reflective surface (eg pericardium) and returns to the probe for interpretation. The intensity of the reflection and the distance (ie time needed to return) are integrated to generate the image you see in 2D. A higher frequency probe gives a sharper (better resolution) image but suffers with deeper structures as it gets absorbed- to see the heart relatively low frequencies are needed compromising resolution. The pulse of ultrasound is actually an onion shaped packet of ultrasound, a few mm dimension travelling at approx 1540ms -1 through tissue, 330ms -1 through air. Ultrasound is far happier travelling through tissue than air. This ultrasound packet, and it s reflection of a given interface between materials, is the basis for all ultrasound imaging and is termed the pulse-echo principle. While the ultrasound image in 2-D is a full screen it is important to realise this is a 1mm or so slice that has been taken through the body from the probe footplate. While the echo probe footplate looks round, particularly an echo probe, it does not shine out a beam like a torch but rather like a wedge or wallpaper scraper of ultrasound. The practitioner can therefore make up a 3-D image in their head by using tilting, rotating or movement of the probe across the body. Air is the enemy of ultrasound closely followed by bone which both effectively stop it s passage dead. Naturally the heart is surrounded by bone and air and conditions eg COPD which increase the air in the chest make ultrasound difficult! Quite aside from breaking the ice a good covering of gel allows ultrasound to enter the body without an air interface ie couplant. Ultrasound puts energy into patients and the effects of this are potentially harmful. In practice, in adults, echocardiography is very safe and whilst we should never image patients for longer than is needed the risks are so low as to be difficult to quantify. Piezo electric effect The probe contains a piezo electric material which emits ultrasound when electrically stimulated, and vice versa ie electric when stimulated by ultrasound. This allows it therefore to be a transmitter and receiver, by firing the ultrasound into the tissues then waiting for the returning ultrasound which generates an electric signal which can be detected and interpreted as a tissue interface. The pulse of ultrasound is actually an onion shaped packet of ultrasound, a few mm dimension travelling at approx 1540ms -1 through tissue, 330ms -1 through air. Ultrasound is far happier travelling through tissue than air. This ultrasound packet, and it s reflection of a given interface between materials, is the basis for all ultrasound imaging and is termed the pulse-echo principle. 1

2 The probe foot plate contains elements which can be stimulated in sequence to generate an image eg a linear array vascular probe. In fact the phased array probe for echocardiography uses precise electronic stimulation of the piezo electric elements to generate a fan of ultrasound from a very narrow base, ideally suited to scanning between ribs as in (Transthoracic) echocardiography. While phased array echo probes have a square looking footplate, it does not emit a torchbeam. Rather a very thin slice of ultrasound in the shape of a fan comes out in the plane of the black line here/ Any pathology above or below this black line ie outside a very narrow 1mm is simply not seen. Waves, velocity and distance For clinicians getting used to ultrasound it often feels like returning to school physics classes! Ultrasound is a wave and as such has a velocity (c) and frequency (f) and wavelength (λ) described by C= f λ In most tissues c is relatively fixed at 1540 ms -1 as compared to air 330. The velocity in fat is slightly slower typically nearer The ultrasound machine therefore measures distance through time- by emitting the ultrasound pulse, allowing it s outward passage, reflection then detection and halving this time, assuming c= 1540, to determine distance. It is important to realise that ultrasound may well travel through a medium eg air, but it is at the interface between different media eg tissue and air, where a difference in acoustic impedance causes reflection and becomes apparent as a structure. This is somewhat akin to the radiology silhouette sign where differences in radiological density allow structure to be inferred. Most probes have an operating frequency which is relatively fixed, mainly due to the physical dimensions it has been built to: the thickness of the piezo electric crystals in the foot plate. Therefore an echo probe will never give good quality superficial images and similarly a vascular 2

3 probe will rarely penetrate deep enough to see much beyond pericardium, and there is little that can be done to alter this apart from purchasing a variety of appropriate probes. Resolution Is the term used for being able to distinguish closely related points as separate points rather than merged. This is essential to allow fine detail to be seen rather than generating a big blur. One way of thinking about this is taking a photo of someone a long way away where you can only just about see them. This tiny image can be easily enlarged many time so that person now fills the screen but you would never be able to make out the details of their face ie the detail has not been resolved. An ultrasound probe emits many scan lines across it s foot plate, the exact manner and sequence of which depends upon the probe itself eg linear or curvilinear (ie a vascular or abdominal probe) vs phased array (ie an echo probe) see Resolution occurs in 2 planes axial ie along the scan line or lateral which is distinguishing points across the scan plane 3

4 In general the ultrasound machine is far better at resolving points in the axial plane rather than lateral and a typical echo probe would be able to distinguish (resolve) points 1mm apart in the axial plane vs 2-3 mm in the lateral. Resolution is improved by using higher frequency probes, but tissue penetration is compromised; a trade off is required. This has some practical implications eg two lesions separated by < these distances will appear as one, and by applying M-mode in the axial plane resolution is typically crisp. Seeing slices It is essential to realise ultrasound is a 2D representation of a 3D structure and as such the phased array probe interrogates a slice of tissue approx 1-2mm thick and it is for the clinician to interrogate in multiple planes and in most cases re-construct a 3D image in their minds. Similarly pathology outside the scan plane will not be seen eg regional wall motion abnormalities or eccentric jets of valve regurgitation REGARDLESS OF THEIR SEVERITY. This is one of many reasons why scanning structures in multiple planes from multiple view points is essential to avoid missing pathology. There are many analogies but for me I like to think of showing someone a loaf of sliced bread who had never seen one before. I pass them one slice at a time and they can look at it, then hand it back to see another slice- then you ask what does a loaf of bread look like? It is this ability to put slices together in your mind that allows you to interpret the structure as a loaf of bread rather than many random slices. A further implication here is anisotropy which is the phenomenon of getting a poor image definition when not scanning at quite 90 degrees. This is quite common during nerve or vascular work but will occur with echo too where your probe is in the right place but very minor tilts of the probe sharpen up the image enormously ie don t move the probe, adjust it s angle slightly 4

5 Probe focusing Rather like any emitter of energy, the probe emits a controlled yet divergent pattern of ultrasound. To achieve best results this requires focusing onto the area of interest. The Sonosite system automatically focuses for the depth selected so there are no controls to adjust on the machine, but on systems which use a focus this should be placed just beyond the zone of interest. Doppler shift Familiar to most as the changing siren when an ambulance passes, essentially ultrasound gets bunched up in front of a moving object (frequency increases) and stretched behind it (frequency decreased). By measuring this frequency shift the velocity of an object can be measured eg police traffic speed traps. In echo this is used to measure blood velocities and derive valve gradients or data on diastolic function. It is possible to measure myocardial velocities (tissue Doppler) and derive very useful information on diastolic heart function but this is beyond the remit of point of care scanning. While the Doppler equation need not be remembered when scanning patients one must remember the angle of insonation is essential, and it should be parallel to the flow ie θ= 0. In the extreme where scanning perpendicular to flow no Doppler shift can be detected eg in the PLAX view Doppler of the aortic and mitral valves is little more than a screen for turbulence. 5

6 Additional modes The vast majority of point of care ultrasound is performed using 2D imaging. It is very much the case that additional modes are add on luxuries and almost all the information which will matter is available on 2D. Furthermore, the additional modes are dependent upon a good quality 2D image eg if the 2D is poor the colour flow will be equally poor. However, there are a few other things probes can do and this course attempts to introduce these as they can help refine a diagnosis M-mode is essentially 2D fired out along one scan line. As such the selected scan line can be interrogated very rapidly and allows for highly detailed axial resolution. This mode is good therefore for measuring heart cambers as it reveals the endocardial surface very clearly Colour flow mapping is a pulse wave Doppler technique which is quite impressive in it s function! A sampling window is selected and each pixel is then interrogated with pulse wave 6

7 Doppler and the average Doppler velocity displayed as a colour (BART blue away, red towards transducer). This is clearly a lot of information being processed very quickly and this is a good mode for assessing particularly jets of regurgitation in a semi-quantitative way. Spectral Doppler consists of 2 modes Continuous wave Doppler where the probe emits and receives data continuously measuring the Doppler shift of typically blood. As such it can measure high velocities eg across stenosed valves and use of a modified Bernoulli equation (gradient mmhg= 4v 2 ) a gradient determined. Continuous Doppler has no way of knowing where this Doppler shift came from along the path interrogated, but clearly stenotic areas will usually have the highest velocities Pulse wave Doppler which is a variant of colour flow above. Here the probe emits ultrasound and waits for it s return- knowing the velocity in tissue it can therefore interrogate a given point or area eg at the mitral valve in flow or aortic valve to determine stroke volume. Hence while it may know where this Doppler shift came from, because it has to wait for the signal to return it cannot measure very fast velocities much above 2ms -1. It is a curious effect that faster velocities are seen as reversing direction or aliasing, rather like the wagon wheels going round the wrong way in a cowboy film if the frame rate is not able to sample at least twice each rotation. The combination of 2D, M-mode, colour flow and spectral Doppler may bne regarded as the basis for a full departmental study but again, everything depends upon a good quality 2D image and certainly at the start most significant pathology can be detected with 2D. Machine and ultrasound safety Infection control perhaps not the first thought but US machines transfer bugs very effectively. Standard principles using approved cleaning agents for the probes and hand washing are mandatory. Ultrasound imaging puts ultrasound energy into patients which is usually dissipated as heat. Full guidance on ultrasound safety is available through the British Medical Ultrasound Society 7

8 In adult patients the use of 2D echocardiography in routine circumstances cannot expose patients to harmful levels of ultrasound energy. This is not the case with other modes and remember other tissues may be insonated on the way eg unexpected foetus in the pelvis! It is important to realise increasing gain does not affect patients as this is processed after the patient. However, increasing depth or applying additional modes (eg M-mode or spectral Doppler) can greatly increase the energy patients experience. The principle of ALARA (as low as reasonably achievable) is frequently cited Machine controls The knobology is a source of wonderment and confusion when starting out so we will keep it brief. Most point of care machines are designed to be point and go so with the appropriate presets they generally need little adjustment to get acceptable images. There is a very nice manual available for the Sonosite system 8

9 The image above is from a Philips portable system but the principles are the same acrss brands and systems. ON/OFF (OK we ll take that as read) Keyboard to enter patient and clinical details. It is important that diagnostic images are identifiable and recorded, and while in many areas of practice eg vascular access images may be rarely recorded, when a diagnosis or serial assessments are needed then details are essential for governance and operational reasonins Tracker ball or touch pad often forms the centre of controls allowing cursor points or captured image frames to be reviewed FREEZE is perhaps the most important button when starting as when you saw something good then it s gone, hit freeze and review your images- most systems keep a live store of preceeding frames for a few seconds and it will usually be there. In theiry freeze should also be used when the probe is off the patient to stop it heating up Depth will require lower frequency (poor resolution) and higher intensity (tissue damage). There will be a compromise. Try and make the structure of interest fill the screen. A good starting depth for an echo is approx 16cm Sector width. The breadth of the pizza slice being recorded can be varied from narrow (eg 1/8 slice) to very broad (close to ½ the pizza) and while a broad image gets more of the heart in, it also needs more scan lines, less resolution and reduced frame rate. Use the narrowest sector to give you perspective and do not ask the machine to process data at the periphery which isn t needed PRESETS most machines have a preset configuration eg adult cardiac or vascular which selects depth, probe frequency etc and is usually pretty close to what you want. If your scan looks wrong eg very jerky or too deep, then you may have the wrong preset. Some of us loving playing with buttons and many machines can have your own preset put in but when starting and doing an echo try cardiac! FOCUS doesn t appear on Sonosite systems (it is automatically adjusted to depth selected) but if present should be just past the area of interest for best resolution GAIN makes the image more bright and shouty but only increases the exposure of the image you have captured ie it doesn t generate a stronger image as such. It is usually better to dim the room and leave gain lower 9

10 Time gain compensation (TGC) allows the gain to be adjusted to the depth scanned and typically looks like a line of sliders or on Sonosite systems individual dials for near/ far. If a structure needs to be enhanced in the far view due to tissue attenuation then TGC may help 2D is what we usually look at or B-mode for brightness, it is the typical imaging modality we see M-Mode is close to B Mode and fires out one line of ultrasound and plots this against time. It is really good for measurement eg size of the LV Colour Doppler places an adjustable box onto the 2D image and the analyses each pixel for the average velocity. It is very good for valve regurgitation and identifying turbulence Spectral Doppler is used for measuring velocities and may be pulsed (you know where the data comes from but can t cope much >2ms due to aliasing) or continuous (copes with any velocity likely to be seen but the data may come anywhere along the scan line selected). Doppler is used for measuring valve gradients (modified Bernoulli equation 4v 2 ) eg aortic stenosis or RVSP, stroke volume and cardiac output, valve areas Penetration/ frequency are Sonosite presets. Some machines allow the operator to slide the frequency continuously Grayscale/ dynamic range is helpful by assigning shades of grey to the image. An echo isn t too interested in myocardium- far more use to get the sharp distinction between endocardium and blood so a very black and white image is good, this would not help imaging a homogenous organ eg the liver Tissue harmonic imaging (THI) is typically turned on an essentially allows more clear delineation of linear structure Most machines have a calculations package which allows ready access to standard measurements eg fractional shortening, ejection fraction or calculations eg valve gradients. It is a case of familiarising yourself with the system as to where the buttons live (When using a Sonosite for starters try 16cm depth, penetration and greyscale -3 and hit AUTOGAIN) Resources Some proprietary modes eg Sono multibeam (MB) allow improved images using closely guarded algorithms and processing of the raw ultrasound data When/dp/ /ref=sr_1_1?s=books&ie=UTF8&qid= &sr=1-1 Medicine/dp/ X/ref=sr_1_1?ie=UTF8&qid= &sr=8-1 Bowra/dp/ /ref=sr_1_1?s=books&ie=UTF8&qid= &sr=1-1 10

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