Medical City · Device Lab 17 of 69
Cryotherapy System
One probe, placed freehand through the skin under imaging, and one gas channel behind it. The ice grows as a single ball around the active length, and everything about the treatment is decided by where that one probe went. There is nothing to overlap and no spacing to plan, so the whole of the geometry is a single choice made once with a needle in a hand.
- 10 parts
- 2 configurations in the Device Lab
- Also called: Cryotherapy, Cryosurgery, Cryoablation
Configurations
The versions in the Device Lab.
Single-probe percutaneous cryoablation under imaging guidance
One probe, placed freehand through the skin under imaging, and one gas channel behind it. The ice grows as a single ball around the active length, and everything about the treatment is decided by where that one probe went. There is nothing to overlap and no spacing to plan, so the whole of the geometry is a single choice made once with a needle in a hand.
Multi-probe array with overlapping isotherms
Several probes driven from a manifold, each on its own valve, placed to a template so the spacing is a designed number rather than an estimate. The ice from the probes becomes confluent and so, if the spacing is right, do the lethal isotherms, and a warming catheter protects the one structure nearby that must not freeze.
How it works
Explore it in the Device Lab.
The gap between what you can see and what you did
Set a probe, a tissue and a freeze time, and read four numbers together: the ice radius, the radius of the twenty-below boundary, the radius of the forty-below boundary, and the fraction of the ice volume inside each.
Two gases, one hole, opposite signs
Compute the Joule-Thomson coefficient for argon, nitrogen and helium at room temperature from their van der Waals constants, and compute each gas’s inversion temperature from the same two numbers.
The cycle changes the criterion, not the field
Run one cycle, then two, with the probe, the tissue, the pressure and the freeze time identical, and compare the iceball with the destroyed volume.
Perfusion as a length
Step the tissue from resting muscle to liver to kidney and read two things: the perfusion length, and the fraction of the iceball that reached the destructive threshold.
Spacing, and the waist nobody planned for
Two probes, one spacing, and two questions that look like the same question and are not: at what separation do the iceballs merge, and at what separation is the destroyed zone between the probes as wide as one probe’s own?
The parts
Part by part.
Argon and helium cylinders and their yokes
Two steel cylinders standing in a yoke beside the console, chained at two heights, each with a handwheel on top and an outlet keyed so that it will only mate with its own regulator. One is marked argon and one helium.
Two-stage regulator and high-pressure supply line
A stainless body clamped to the cylinder outlet, with two round gauges on it and a handwheel underneath. The upper gauge reads what is left in the cylinder; the lower one reads what is being delivered to the console.
High-pressure circuit, filter and burst disc
Behind the cabinet wall: a tall filter canister with a drain at the bottom, a machined block carrying three solenoid coils, a stainless pipe run rather than hose, and a small brass housing on the side with a thin metal disc clamped inside it. The disc is the least impressive object in the assembly and it is the only part of the circuit guaranteed to fail before anything else does.
Counter-current heat exchanger
Inside the probe shaft, in the last five centimetres before the tip: a fine helix of tubing wound round a central line, sitting inside a return annulus. It is the part of this device that looks least like a refrigerator and it is the reason the device is one.
Joule-Thomson orifice and expansion chamber
A small plate across the gas path, near the base of the active length, with a hole in it sixty micrometres across. Downstream of it, a chamber the length of the tip.
Vacuum-jacketed probe shaft
A rigid steel needle, a couple of millimetres across, with ten-millimetre marks etched along it and a visible collar some way from the point. Everything on the hub side of that collar is a vacuum flask.
Freezing tip and active length
The bare metal at the end of the probe: a couple of centimetres of polished steel closed by a trocar point. It is the only part of this device that is a refrigerator, and its surface area is the single number the entire thermal calculation is built on.
Low-pressure return and exhaust
A fat corrugated hose from each probe hub back to a port low on the console, a silencer inside the cabinet, and a stack leaving the top of it. The hose is much fatter than the supply hose even though it carries exactly the same gas at exactly the same rate, and the reason it is fatter is the whole of what this component teaches.
Interstitial thermocouple set
Four fine needles, each less than a millimetre across, with an amber hub and a lead back to a connector block on the console. They are the least impressive item in the tray and they are the only direct evidence anybody will ever have about the quantity that decides whether this treatment worked.
Front panel, gauges and displays
A raked face at reading height carrying two windows, a mechanical pressure gauge, a row of channel lamps and three keys. The left window shows the cycle as a timeline; the right one shows what the thermocouples are reading.
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