Appendix: System Controls and Their Uses

In addition to the basic choice of transducer type and frequency for the examination in hand, there are many other factors on a Doppler ultrasound system which need to be adjusted. Despite the efforts of the manufacturers to automate and simplify things, it is still necessary to adjust continually many of the scan and Doppler parameters during the course of an examination. Is the vessel superficial or deep? Is flow fast or slow, high volume or low volume? Most systems now come with a variety of preset programs for different situations: peripheral arteries, veins, cerebrovascular, etc.; together with automatic image and spectral display optimisation. Many also have the facility to allow users to save their own program preferences, which is a convenient option to store preferred settings once a satisfactory set up has been achieved for a particular type of examination. Whilst the manufacturer’s presets allow the basic appropriate settings to be employed at the start of an examination, fine adjustments will still be required during the course of the examination to make the most of the available information. Familiarity with the ultrasound system, together with experience, enable skilled operators to set up their system appropriately for the examination being performed. Different manufacturers sometimes use different names for the same controls or functions; it is not possible to give an exhaustive list of all the possible options on the range of modern equipment now available but the following notes describe the basic controls, or parameters on most systems that can be adjusted during the course of an examination to improve and maximise the information that is obtained from the examination.

General Principles

TRANSDUCER FREQUENCY

The highest frequency which will achieve the highest resolution consistent with adequate penetration for imaging is normally chosen. The Doppler frequency used by any transducer is often 1–2 MHz below the imaging frequency, although modern equipment has a wide range of receive frequencies such as 5–14 MHz. In addition to the basic imaging requirements, it should be remembered that the deeper a vessel lies, the longer it takes a sound pulse to travel there and back, so that the Nyquist limit becomes very relevant ( Chapter 1 ), limits the Doppler frequencies that can be used and therefore the frequency shifts/velocities that can be recorded accurately.

B-MODE IMAGE

For colour Doppler examinations this should be set up with relatively low overall gain, so that the image is a little on the dark side as the software tends to allocate colour to darker areas, rather than to areas which contain echoes. See ‘ Colour write priority ’ below.

TRANSMIT POWER

The transmit power of the system should be set at the lowest level consistent with an adequate examination, especially during obstetric and gynaecological examinations. It is better to start at a medium level and increase the power only after other measures to improve system sensitivity have been tried, such as adjusting colour gate size, removing filters, adjusting the scale/pulse repetition rate. For low mechanical index (MI) contrast studies (see Chapter 17 ) the transmit power should be set as low as possible and the MI reading ideally should be less than 0.4. Modern systems often have contrast-agent-specific programs already installed, or the relevant settings can be obtained from the supplier of the contrast agent.

UPDATE/DUPLEX/TRIPLEX

In duplex ultrasound there is the ability to acquire and display both real time imaging and spectral Doppler information either simultaneously, or alternately. Simultaneous display results in degradation of both the image and the spectral display as the computer has to process data from both sources. The update facility allows the operator to set the system to handle either imaging data, or Doppler data. This results in a higher quality of display for the selected mode and it is usually used for acquisition of the best-quality spectral display for analysis. Simultaneous duplex scanning is of some value in the initial stages of an examination in order to position the sample volume in a specific area of interest. Triplex mode refers to the simultaneous acquisition, processing and display of colour Doppler, spectral display and imaging information. As with duplex scanning, this requires significant division of processing power and consequent compromise in the quality of the display. Newer systems with more powerful computers are less prone to these problems but there is still a division of processing power and experienced operators will still tend to switch between imaging and spectral Doppler for optimal results.

AUTOMATIC IMAGE OPTIMISATION

Many systems now have automatic image optimisation controls which adjust factors such as time gain compensation, receiver gain and colour Doppler gain to improve the image. In some cases this facility can also be applied to the spectral display. These can be useful in getting a reasonable initial display but should not be relied upon entirely and the operator needs to make the final adjustments to obtain the best images.

Colour Doppler Controls

COLOUR BOX

This defines the area of the image over which colour Doppler data are acquired. It can be adjusted for size, position and direction/angulation. It is better to keep the box as small and as superficial as practical because larger areas take more processing power and time. Larger sizes and greater angulations may therefore reduce frame rates. Greater angulation also reduces sensitivity, which can therefore be improved by reducing the degree of angulation of the colour box, for instance when examining deeper segments of the femoral vein in larger legs. Deeper boxes need slower pulse repetition frequencies, therefore limiting the size of shift that can be measured without aliasing.

COLOUR MAP

Choose a colour map that has good contrast for the identification of aliasing. Maps which grade to white at each end, or to very pale colours, do not give as much information about aliasing as those which have significantly different colours, such as pale green and pale orange. Variance maps were said to register the amount of spectral broadening in the colour map, although this is not easy to appreciate and may, in fact, not be true as this ‘variance’ seems to reflect the velocity, rather than spectral broadening. Variance maps are most frequently used in cardiological examinations. In addition, colour tags can be put into the scale so that velocities above or below the chosen range can be identified.

COLOUR GAIN

The colour gain is set to the optimal level. This can be identified by turning up the gain until noise/colour speckle is seen in the colour box and then backing off slightly on the overall colour gain. It should also be adjusted to remove any spill of the colour map over the boundaries of the vessel seen on the B-mode images; this is sometimes called colour bleeding. Colour gain often needs to be increased if the colour box is moved to a deeper location.

COLOUR SCALE

The colour scale should be set to levels appropriate for the range of velocities under investigation. It should be remembered that colour Doppler only gives information on the mean Doppler shift in a pixel; the mean velocity in a pixel is only calculated when an angle correction for that pixel is performed. The colour scale is closely related to the pulse repetition frequency, or sampling rate; this has to be above the Nyquist limit for the frequency shift being measured. If it is set too low then aliasing will occur; this can be rectified by increasing the colour scale range, also known as the pulse repetition frequency. If it is set too high then there will be poor colour sensitivity for slower velocities, resulting in inadequate colour fill-in across the vessel.

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