Abstract
Recently, there has been an explosion in the use of ultrasound outside the confines of the conventional imaging department. Many specialties are waking up to the huge potential for focused ultrasound and ultrasound guidance for interventions. Unfortunately, to date, very few formal ultrasound courses are available or accessible to specialist groups. The result is, in some cases, the purchase and use of ultrasound equipment by individuals with little or no training. A good understanding of the basic physics of ultrasound is helpful, but this is of little real value unless the operator can apply this in practice. When faced with a technically challenging examination, users need to be able to fit together their knowledge of underlying principles, an awareness of what each equipment control does and an appreciation of why the scan may be difficult. The aim of this paper is to provide clinicians who are new to ultrasound with an overview of key equipment controls and to indicate how these can be optimized.
Getting started
Using an ultrasound machine is a little like driving a car or using a home personal computer. In a car, there are a number of basic controls that are common to all models, some of which are in a standardized position within the vehicle. There are numerous additional controls that are either used rarely or may be hard to locate.
Unfortunately, with ultrasound equipment, there is no clear convention for the labelling and positioning of even the most commonly used controls and even highly experienced users can be confused by the layout of a new machine. To reduce the number of keys/tabs/dials, etc. on the control panel, some manufacturers use symbols or letters rather than text to label controls. To the new or infrequent user these ‘hieroglyphics’ can be confusing. Individual tabs/keys often have multiple functions depending on the mode of operation selected and settings are often accessed via hidden on-screen menus.
Despite this apparent complexity, there are a small number of key equipment controls that are common to virtually all ultrasound systems. Only once the function of these is understood and they are located on the control panel, is it possible for the inexperienced user to start to produce good quality, diagnostic images.
This section will attempt to explain the function of each of these basic controls, where it is likely to be located and how it can influence the image.
Before using or evaluating any ultrasound equipment for the first time, the most useful starting point is to find a ‘co-pilot’ who is familiar with the specific system.
This may be an experienced colleague from the same specialty, a radiologist or a sonographer. At the time of purchase, companies provide technical support from their teams of highly experienced application specialists. If it is not practical for them to spend time with all users of the equipment, planned ‘cascading’ of information between colleagues is helpful. In practice, this does not always happen and new users may be left to fend for themselves.
Basic equipment settings
Switching on/off
An ultrasound machine is essentially a computer and, just as a home PC undergoes a start-up process, it may take a few minutes to power up. Similarly, the user should switch off at the control panel, not at the plug, on powering down.
New patient (ID/patient/data)
Before scanning, patient details, including name and hospital number, should be entered. The relevant key is normally located on the control panel, often in either the top right or left corner (Figure 1).
The new patient button is often located in the top right or left of the control panel. On some systems, this function may be accessed via an on-screen menu
Transducer selection
Basic hand-held systems will have one transducer attached and there may be no choice. On a higher specification machine, once patient details are entered, an on-screen menu may appear giving options of transducer and application. Up to four transducers may be connected at any one time and the operator will need to select the correct transducer from the menu to match the area to be examined.
Once the transducer is selected, a further menu may offer a choice of application pre-sets matched to specific body areas.
Application pre-sets
When the relevant body area is selected, the machine will default to a range of settings that are optimized by the manufacturer for that target area. This will include B-mode settings affecting penetration, resolution, frame rate, etc. and Doppler settings.
It is important to select the correct transducer/pre-set combination. It is not uncommon to observe inexperienced operators struggling to image deep into a large patient with these initial settings optimized for superficial structures. Inevitably, image quality will be poor and may be non-diagnostic.
Although pre-sets provide a very useful starting point, there is no such thing as a standard patient. The operator will still need to adjust specific controls to compensate for patient size and condition and throughout the course of an examination.
The control panel
The layout of the control panel will vary considerably between machines and manufacturers. Individual controls will also vary in appearance as well as location. Simple controls such as focus or depth may be a soft key, a dial, a paddle or on-screen. Individual tabs/keys may have multiple functions depending on the mode of operation selected.
It is for this reason that a ‘co-pilot’ who is familiar with the specific system is such a useful ally when using a machine for the first time.
Manufacturers tend to group controls depending on the function and to minimize operator movement and reach. For example, Doppler controls will normally be grouped together, as are callipers/measurement controls (Figure 2).
Controls such as measurement functions are often grouped together on the control panel to minimize operator movement and reach
Freeze
The freeze control allows the operator to produce a static image for archiving and from which measurements can be taken. This is normally a soft key. Typically, it is situated towards the right-hand side of the control panel to be reached easily with the left hand while scanning with the right. The key is normally labelled ‘freeze’. (Some of the more imaginative manufacturers use a snowflake.)
Tracker ball
This is probably the only control on an ultrasound keyboard that is common to virtually all manufacturers. This is the equivalent of computer mouse and is used to move the cursor around the screen when entering patient details, selecting transducer or typing on-screen text. The laptop equivalent tracking pad has replaced the tracker ball on hand-held systems.
Depending on the scan mode that is operating, the tracker ball is used to move on-screen graphics such as calipers, position of the Doppler sample gate, box location for high-resolution zoom or colour Doppler. The function changes as each application is selected or the select/set/priority tool is used (see below).
As one of the most commonly used controls, the layout of the control panel is often centred around the tracker ball. Other frequently used settings such as calipers, select, gain, etc. can often be located by their close proximity (Figure 3).
The tracker ball is the equivalent of a computer mouse. Its function varies depending on the scan mode selected, but may be used to move on-screen cursors including text or measurement callipers. A lap-top equivalent tracking pad has replaced the tracker ball on handheld systems
Cine-loop
On most equipment, the tracker ball also controls the cine-loop facility. This is a feature that allows the operator to scroll back through several seconds worth of captured frames once the image has been frozen. This is particularly useful when imaging non-cooperative patients and helps to minimize scan time.
Select (set/priority)
This is a generic key that is used to activate functions such as calliper placement. On some machines, this changes the function of the tracker ball (Figure 4).
The select button is a generic key, use to activate other functions such as calliper placement
Overall gain
This controls the amount of amplification given to all returning echoes regardless of the depth from which they have returned. The overall gain control is normally a large dial.
Altering the overall gain will make everything in the image appear brighter or darker (Images 1 and 2). Overall gain should be used in conjunction with time gain compensation (TGC) control and should be set to display soft tissue structures within the mid grey range and fluid filled structures as black.
Gain too high
Gain reduced
Beware, if the gain is set too high, noise is amplified and image resolution is lost. The resultant increase in artefacts within the image may produce misleading appearances, such as slice thickness artefact. This can for example mimic thrombus within the lumen of a vessel. However, if gain is set too low, weak echoes will no longer be displayed and useful information may be lost. (e.g. low-level echoes generated by true thrombus within a vessel).
Most inexperienced users tend to set the gain too high. There is almost a tendency to ‘turn up the lights’ so that everything looks brighter. It is, however, more difficult to appreciate anatomical boundaries and subtle differences in reflectivity if the gain is high. This is particularly important when searching for free fluid. With the gain set too high, amplified noise and artefactual echoes may obscure small pockets of fluid.
It is also worth noting that overall gain cannot always compensate for poor sound penetration in a large patient. It may be necessary to change to a lower frequency setting or increase the power output.
In the point-of-care setting, there is often the further complication of high ambient lighting. This can make optimization of the gain settings tricky. There may be little the operator can do to remedy this other than shading the monitor.
Time gain compensation
This usually consists of a number of sliders/paddles that correspond to specific depths within the patient (Figure 5). TGC is used to compensate for increasing attenuation with depth. On the smaller, hand-held systems, TGC is often replaced by independent near and far gain controls. The operator should aim for an image where similar structures appear at the same brightness level regardless of depth.
Time gain compensation (TGC) can be adjusted via a number of sliders/paddles that correspond to specific depths within the patient. TGC is used to compensate for increasing attenuation with depth. On the smaller, hand-held systems, TGC is often replaced by independent near and far gain controls
Depth/zoom
The depth control should be set to demonstrate the whole region of interest during an initial survey of the area (Images 3 and 4). During interventions that may be guided by ultrasound, such as central line placement, it is often more important that the needle tip avoids arteries than that the target structure is reached on first pass. Initial depth settings should therefore be set to demonstrate all of the relevant anatomy, not just the most superficial structures. This will give a better appreciation of the relative location of anatomical structures. Magnification can then be increased as the examination is focused to a specific depth. If high-resolution zoom (see below) is used on a realtime image, it is important to recognize that the top edge of the image no longer represents the skin surface. Clearly, this is critical for intervention guidance.
Standard view – no zoom. Top of image represents skin/transducer interface
Zoomed image. Top of image no longer represents skin/transducer interface
High-resolution zoom may be used to provide an expanded view of small structures. A region is selected by placing an on-screen box over the structure of interest. The location of the box is, normally, controlled by the tracker ball. Zoom is then activated to provide a high-resolution image of this small, defined area.
Focus
As the depth of interest changes from one structure to another, the depth to which the ultrasound beam is focused needs to be altered. Focusing produces a narrower beam width at the selected depth. This will improve lateral resolution making it easier to delineate small structures positioned side by side. This is particularly important when imaging structures such as nerves, which may be similar in brightness level to surrounding tissues.
The homogeneous speckle pattern characteristic of many solid organs is, in part, a product of the effective beam width. As beam width is reduced, contrast resolution will improve and structures, such as nerves, will be better visualized. Focus may need to be adjusted throughout the examination (Images 5 and 6).
Focus too deep and gain not optimized. IJV appears to be full of low-level echoes
Focus, depth and gain-adjusted IJV clearly demonstrated
On some hand-held systems, focus is optimized for the entire depth of view and is altered automatically as the depth setting is changed. On these systems, focus cannot be altered independently.
Frequency
Modern transducers utilize broad bandwidth technology and transmit over a range of frequencies. If the correct transducer/pre-set combination has been selected, it is rarely necessary to change frequency.
On most equipment, it is possible for the operator to optimize the image for deep or superficial structures (or for slender or challenging patients). By selecting the right pre-set a number of parameters are altered, including the range of frequencies from which the image is generated. This means that a single transducer can be used for a broad range of patient size and depth of interest.
Tissue harmonic imaging
This is an optional scan mode available on most equipment. Tissue harmonic imaging utilizes the second harmonic signal that is generated within the tissues due to non-linear propagation of the sound pulses. The frequency of the second harmonic is twice the transmit frequency. For example, a transducer with a central transmit frequency of 3 MHz would produce a harmonic signal of 6 MHz.
Key advantages of harmonic imaging are the reduction of artefacts and improvement in lateral resolution. Only the central portion of the main beam is of a high enough intensity to generate a harmonic signal. The result is a reduction in beam width, and hence an improvement in lateral resolution. Clutter within the image is markedly reduced. In practice, harmonic imaging may be of benefit in larger patients.
Compound imaging
In conventional B-mode imaging, each frame of information is generated by firing groups of piezoelectric elements, in a single sweep across the face of the transducer. Each pulse of sound is fired at 90° to the array.
Compound imaging techniques use electronic beam steering to fire pulses from a number of separate transmit angles. Realtime imaging is then achieved by combining the data from multiple lines of sight to generate successive frames. By scanning from a number of different angles, artefacts such as clutter, noise, posterior enhancement and shadowing are reduced, and real structures are reinforced.
The key drawback of this technique is that the system is unable to distinguish between artefacts that reduce image quality and ‘helpful’ artefacts that may alert the operator to the presence of pathology or help to characterize a lesion. Compound imaging effectively removes or reduces the appearance of all shadowing (Image 7).
(a) Compound imaging on. Shadowing from renal calculus is markedly reduced. (b) Compound imaging off. Shadowing from calculus visible (arrows)
Doppler controls
Most manufacturers group Doppler controls together in one area of the control panel. However, frequently, they are adjusted via an on-screen menu.
In whatever context Doppler is used, correct setting of a number of equipment variables is vital. Understanding of these controls and a good awareness of the limitations of the equipment will enable the operator to make an accurate interpretation of blood flow. Lack of understanding can lead to some serious confusion. As with other applications, manufacturers provide a number of examination-specific pre-sets for vascular. These generally include arterial and venous options. Although these provide an excellent starting point, a number of adjustments will need to be made to optimize demonstration of both normal haemodynamics and pathology.
As with other soft tissue structures, selection of transmit frequency, focal depth and gain are equally applicable in both B-mode and Doppler assessment of vessels.
(For an excellent introduction to Doppler equipment, see Thrush A, Hartshorn T. Peripheral Vascular Ultrasound: How, When, Why. 3rd edn. Edinburgh: Churchill Livingstone Elsevier, 2009).
Velocity scale (pulse repetition frequency)
This setting determines the range of blood flow velocities that can accurately be displayed. In simple terms, if investigating high-velocity flow, the pulse repetition frequency (PRF) needs to be high. For low-velocity flow, PRF needs to be reduced. This applies to both colour and spectral Doppler.
The scale is visualized as a velocity range on the vertical axis of the spectral trace in pulsed wave Doppler and as a colour bar in colour flow mapping.
Wall filter
This is used to remove the high amplitude, low-frequency signal from vessel walls. If set too high, this can result in the appearance of loss of low-velocity flow. Although wall filter settings generally change automatically as the PRF is altered, it may be worth checking if very slow flow is suspected.
Colour gain
This controls the amount of overall amplification given to the Colour Doppler signal. At the start of the examination, increase the Colour gain so that you can see colour artefact throughout the soft tissue structures. The gain should then be reduced until the colour artefact just disappears from the soft tissue and colour filling remains within any vessels.
Putting it all into practice
Whatever the ultrasound examination, selecting the correct transducer/pre-set combination is key. If you get this right, you stand a better chance of producing images of a reasonable diagnostic quality. However, where the examination is technically difficult, skilful adjustment of a few basic settings can make a big difference. Knowing what to adjust can be baffling initially, but soon makes sense if the operator can apply their understanding of the underlying physics and their awareness of what each control alters. Practice is key. In the same way that it takes time to master the art of driving a car, use of an ultrasound machine can feel cumbersome, and at times irritating, during the early stages of learning. However, with appropriate training and experience, point of care users will find this to be a tremendously useful tool.