Medical City · Device Lab 37 of 69
Transcranial Doppler
One two-megahertz element about ten millimetres across, in a moulded grip with an orientation flat on it, held against the side of a head by a person who is listening. There is no image of any kind. What the operator has is a depth, a direction, a spectrum and a sound, and finding the vessel means sweeping the probe across the window until the sound is right and then holding still.
- 10 parts
- 2 configurations in the Device Lab
Configurations
The versions in the Device Lab.
Hand-held non-imaging probe, positioned by the operator
One two-megahertz element about ten millimetres across, in a moulded grip with an orientation flat on it, held against the side of a head by a person who is listening. There is no image of any kind. What the operator has is a depth, a direction, a spectrum and a sound, and finding the vessel means sweeping the probe across the window until the sound is right and then holding still.
Head-frame mounted bilateral probes for continuous monitoring
Two probes, each about eight millimetres across, carried in ball-and-socket gimbals on a frame that clamps round the head with an occipital strap and two tension screws. The gimbals are aimed once, by hand, listening, and then locked, and after that nobody touches them. The frame exists for one reason: so that the angle in the Doppler equation, whatever it is, is the same angle tomorrow morning as it was tonight.
How it works
Explore it in the Device Lab.
The cosine nobody measured
Set the angle to zero and step it outwards, watching four numbers at once: the velocity, the pulsatility index, the per-degree sensitivity and the displayed wrap limit.
What the unknown cannot touch
Set a large angle, forty degrees, which under-reads every velocity by twenty-three per cent, and compare the pulsatility index the machine computes against the index of the waveform it was computed from.
Two products and no free direction
Raise the pulse repetition frequency and watch the wrap velocity rise and the unambiguous depth fall. Then multiply them together.
Where the budget goes
Compute the round-trip loss through the bone and through the brain separately, and compare them.
The gate, and what a longer one costs
Lengthen the gate and watch the signal rise and the axial extent of the sample volume rise with it.
The parts
Part by part.
Mains inlet, isolation and protective earth
A filtered appliance inlet, a fuse, an equipotential stud and a rating plate on the back of a box the size of a shoebox, and behind them a transformer whose entire job is to make certain that no single failure anywhere in this machine can put a voltage onto a metal-shelled probe that somebody is pressing firmly against a patient's head.
Transmit pulser and burst gate
A high-voltage switch that slams a few tens of volts across a piece of ceramic, a counter that decides how many cycles of that to allow through, and a protection switch that has to disconnect a microvolt amplifier from all of it and reconnect it a few microseconds later.
Two megahertz element and backing block
A disc of piezoelectric ceramic about a centimetre across and a millimetre thick, with a heavy lossy block cemented to the back of it, and between them they decide nearly everything about what this device can and cannot reach.
Quarter-wavelength matching layer and lens
A layer tens of micrometres thick between the ceramic and the outside world, and a moulded silicone face over the top of it, and between them they are the difference between a probe that works and a probe that reflects nearly everything back into itself.
Coupling gel and the acoustic window path
A bottle of warmed gel in a clip on the trolley and a film of it a millimetre thick between a lens and a scalp, and it is not an accessory. It is the first component of the acoustic path and the commonest reason the path stops existing.
Probe cable and connector
A metre or two of screened coaxial cable with a multi-pin connector on the end, carrying tens of volts one way and a few microvolts the other way down the same conductor, a few microseconds apart.
Preamplifier and depth-dependent gain
A low-noise first stage under a shielding can, followed by an amplifier whose gain climbs as the echo gets later, because a later echo has travelled further and lost more.
Quadrature demodulator and direction resolution
Two mixers, a phase splitter that feeds them the same reference ninety degrees apart, and the reason this device knows whether blood is coming towards the probe or going away from it. One mixer on its own could never tell.
Range gate and sample volume selector
A delay and a sample-and-hold, with a number on the screen that says millimetres. The number is a time wearing a costume: the machine waits, then looks, and multiplies the waiting by half the speed of sound.
Wall and clutter filter
A high-pass filter with a switchable corner, and the quietest way there is to make this machine produce a wrong answer that looks better than the right one.
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