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EMG and NCS EP. Basic machine checks. Section 1: Prepare equipment for investigating the peripheral nervous system. 1.1The equipment to be used is checked for safe condition and operates correctly, including all relevant peripheral devices.
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EMG and NCS EP Basic machine checks M Stevenson & R Pottinger
M Stevenson & R Pottinger Section 1: Prepare equipment for investigating the peripheral nervous system. 1.1The equipment to be used is checked for safe condition and operates correctly, including all relevant peripheral devices. 1.2 Any faults are identified, reported and rectified (if possible) in accordance with Departmental procedure. 1.3 There is adequate storage volume for the investigation 1.4 A basic machine check is performed in all modalities to be used. 1.5 A formal record of machine performance (as described in 1.4) is made 1.6 Any faults are identified, reported and rectified (if possible) in accordance with Departmental procedure. Performance evidence required 4 Assessments in CTS Equipment log (machine function and faults).
M Stevenson & R Pottinger 1.1 Check the equipment is in a safe condition to use. external inspection of the equipment, mains leads, connectors etc. Including the recording and stimulation components. check the expiry date on any electrical safety test sticker. know the electrical safety class of the piece of equipment. verify that the equipment boots up correctly and that appropriate programs can be selected. Amplifiers should be turned on. Stimulator function should be tested. The printer should be checked for adequate paper and ink. A simple screen copy will confirm printer function.
M Stevenson & R Pottinger 1.2 Identify faults and rectify and/or report them properly. Not likely to occur in most assessments, it is often necessary to test standards are met by questioning the candidate about the fault reporting procedure in their department. It is useful for the candidate to have this written up in their portfolio, including examples of previously reported problems. It is essential that the candidate recognises the importance of fuse ratings in mains leads, and how to calculate the correct fuse for any equipment.
M Stevenson & R Pottinger 1.3 Identify the amount of storage space required by the test and check there is sufficient storage space on the equipment. The available amount of storage space can be found by selecting ‘my computer’ then right clicking the appropriate disc, then select ‘properties’. The available space is indicated :
M Stevenson & R Pottinger This information is only useful if the candidate knows the typical volume of space required to store the information for a particular study…
M Stevenson & R Pottinger 1.4 Perform a basic machine check on the machine. Time and date:
M Stevenson & R Pottinger Default settings: Click on the ‘settings’ key, ensuring that HFF, LFF and sensitivity are correct for the modality to be used. Also check the stimulator parameters. On some machines the settings are displayed on the recording screen.
M Stevenson & R Pottinger Using an internal square wave calibration pulse: On the Viking series, the calibration pulse is selected by using a hard key function on the Viking keyboard. The value for amplitude and duration of the square wave is different for different programs. Remember, the amplitude and the appearance of the square wave is affected by the filters used in the program.
M Stevenson & R Pottinger The following example uses our program for median/ulnar sensory studies. The default filter settings are 20Hz-3KHz.
M Stevenson & R Pottinger I would recommend that the first step in this process is to take out the filters as much as possible Note: The filters cannot be taken off completely – the calibration pulse will therefore not produce a perfect square wave on the display.
M Stevenson & R Pottinger Selecting the calibration pulse in this program gives a square wave of 20uV amplitude and 2 ms duration. At these kinds of sensitivities, it is inevitable that the square wave will be ‘noisy’. The simplest, ‘visual check’ is performed by lining up the square wave with the graticule markers on the screen. on this screen shot, each dot is 1 ms apart in time and 5uV in amplitude (for trace 1). The square wave can be seen to be ‘about right’.
M Stevenson & R Pottinger Cursors These can be applied to the square wave (or even to the graticules) to check for accuracy in the ability of the machine to measure amplitude and latency. Note again the effects of noise on the square wave if the signal averager is not used. Also note where I have placed the latency marker; it’s half way up the ascending limb of the square wave (this is due to the effects of the high frequency filter which cannot be taken off completely) upon the morphology of the calibration pulse.
M Stevenson & R Pottinger The square wave could also be averaged to remove much of the noise – but this assumes that the processing involved in the signal averager is working correctly…
M Stevenson & R Pottinger Another method of testing the accuracy of the latency markers is to use 50Hz artefact. Note the appearance of 50Hz with the time bases used in NCS is very different to that in EEG. I would recommend that students build up a set of common artefacts in their portfolios, and thoroughly familiarise themselves with the appearance of as many artefacts as possible. This will certainly come up in the Part II exam.
M Stevenson & R Pottinger NOTE: You do not have to use ALL of these methods to check latency markers – one will do.
M Stevenson & R Pottinger High frequency filter: A visual check only is possible using the internal calibration pulse. produce a series of calibration pulses, reducing the HFF value for each trace. The candidate should recognise and point out the salient areas of the square wave which are affected, and comment whether the appearance is as expected. Trace 1 – HFF 20kHz, Trace 2 – 1.5kHz, Trace 3 500Hz. LFF 0.2Hz all traces.
M Stevenson & R Pottinger Effect of low frequency filters Remember to put the HFF back to its’ original value. Produce a set of waves with varying LFF values. Again, the candidate should recognise and point out the salient areas of the square wave which are affected, and comment whether the appearance is as expected. Trace 1 – LFF 0.2Hz, Trace 2 LFF 20Hz, Trace 3- LFF 150Hz, HFF 20kHz all traces.
M Stevenson & R Pottinger It is worth the candidate remembering that filters affect both the amplitude and the phase of the waveform. It can be useful for them to produce a piece of work on the effects of filters on sensory and motor NCS for their portfolio. The candidate should be able to identify which parts of the square wave represent high or low frequency signal.
M Stevenson & R Pottinger Signal Averager function: A signal averager acts by adding the responses together and dividing by the number of runs performed (hence the term ‘signal average’). If an identical waveform is applied to an averager (for example, a calibration pulse) then the size, duration and morphology of the wave should not change, irrespective of the number of averages performed. But.. the trace will become less noisy. In the following example, there are 3 traces, with 1, 5 and 20 averaged runs…
M Stevenson & R Pottinger 1.5 Produce a formal record of the machine performance If the student produces print-outs of the above, this should satisfy the performance criterion. NOTE: The ‘calibration’ procedure described above should be performed for each amplifier used for sensory studies. In our set-up, only 1 amplifier is used. So no additional checks need to be performed on other channels. This will be different for other tests, for example evoked potentials, where up to 4 amplifiers are typically used to produce a series of waveforms. The same procedure can be applied to performing motor NCS. As different settings are used, I would expect a student to be prepared to perform a calibration for both motor and sensory NCS.
