A cryostat is an instrument used to sustain very low temperatures for medical, engineering, and scientific uses. It uses liquid helium or nitrogen and has a precision cutting tool for dissecting tissue. Cryostats are used in MRI machines, tissue sample storage, and frozen sections for surgical examinations.
The word cryostat is a word of compound meaning from the roots cryo, meaning cold, and stat, meaning static or stationary. A cryostat is an instrument used to sustain very low temperatures at -238 degrees Fahrenheit (-150 degrees Celsius) or lower, for many medical, engineering and scientific uses. The chemicals to maintain these temperatures are liquid helium baths for tissue samples or liquid nitrogen for cryonics patients. The inner vessel of a cryostat is similar in construction to a vacuum flask; and there is a precision cutting tool for dissecting tissue known as a microtome within the freezer portion of the cryostat.
There is a metal cold plate in contact with the liquid helium bath inside a cryostat on which tissue samples can be stored. Since a vacuum pump on helium evaporates some of the helium isotopes into vapor, heat is released in the process which keeps liquid temperatures low and super-insulation materials in the shielding layers around the liquid bath insulate the liquid and the they keep fresh. Some cryostats are supplied with more liquid helium from storage dewars, and other cryostat designs have an attached mechanical refrigerator that draws in the helium vapor, cools it down, and reintroduces it into liquid form once again, in a closed-loop operation. Multistage cryostats can achieve lower temperatures than normal cyrostats by using dry dilution or magnetic refrigeration plus cooler isotopes of helium in a special pan within the mix.
Cryostats can be found inside magnetic resonance imaging (MRI) machines used to keep the coil of superconducting magnet superconducting wire in a superconducting state so that the wire has no electrical resistance. The coil is immersed in liquid helium and the vapor of this helium is replenished in the closed loop design by an attached refrigeration unit. Cryostats are also used to slice and store tissue samples in medicine, and portable cryostats of this type can be run by generators or vehicle inverters to keep them cool for transporting tissue samples. Slicers can slice as fine as a micrometer for examination of the slide under a microscope.
The most common use of frozen sections is for examinations during surgical procedures. Reasons for this practice may be if a tumor appears to have metastasized, any suspected metastases are rushed to cryosection to confirm, and if so, the surgeon may stop the surgery. Another use is in exploratory surgeries and excision of an internal lesion can identify the cause of the patient’s symptoms. Frozen sections can sometimes reveal disease antigens that have been masked in ordinary formalin solutions. When it comes to sentinel node surgeries, a surgeon may excise tumor tissue for a benign or malignant verdict, before deciding if further lymph node removals are needed.
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