Calibration of Alcor Post-Cryopreservation CT Scans

In 2011, Alcor began performing x-ray CT scans on neuropatients after cooling to liquid nitrogen temperature has been completed. This is done with patients remaining safely immersed under liquid nitrogen during the scan. Scans were originally performed with the intention of checking placement of acoustic fracturing sensors. However the scans were found to be even more useful for showing differences in both the extent of cryoprotection between patients (which varies depending upon circumstances of cryopreservation), and within the brains of individual patients.

The x-ray attenuation or “CT density” of tissue varies with the extent of cryoprotection by three mechanisms. First, ice formation increases space between atoms because ice is less dense than liquid water. Unfrozen tissue therefore has a higher CT density than frozen tissue. Second, solutions of cryoprotectant chemicals have a higher physical density (more electrons per unit volume to scatter x-rays) than water. CT density therefore increases as cryoprotectant concentration increases. Finally, the cryoprotectant solutions used by Alcor contain dimethyl sulfoxide (DMSO), which contains the element sulfur. The higher atomic number of sulfur compared to oxygen in water makes it better at photoelectric absorption of x-rays.

Alcor originally used CT scans for qualitative interpretation and comparison. Color scales of CT images in case reports were inconsistent as different scales were experimented with. In October, 2015, a quantitative calibration of all Alcor CT data was made in collaboration with a medical physicist. Images were obtained of reference samples of cryoprotectant solutions of known concentration. (These reference sample images are now included at the right of all Alcor CT scan pictures and movies.) The mean CT numbers within these reference samples were converted to a correspondence between Hounsfield Units (HU) at 120 kV x-ray energy and the concentration of the cryoprotectant solution, and a consistent color scale was adopted. Details of this work were presented by Steve Graber is his talk, “Alcor’s Neuropatient CT Scanning Project” at the Alcor 2015 Conference in Scottsdale, Arizona.

The justification for believing that the CT density of tissues containing cryoprotectant is similar to the CT density of liquid solutions of the same concentration is a 2006 University of Minnesota paper that studied this question in liver tissue. This paper showed that this correspondence holds to a good approximation, especially at higher concentrations that approach the concentrations needed for vitrification. In Alcor’s case, the sulfur content of the cryoprotectant solutions pushes the CT density of cryoprotected tissue far above that of un-cryoprotected tissue. For comparison, un-cryoprotected white matter of the brain has a CT density 2% greater than pure water, grey matter 4% greater, while the target concentration of M22’ cryoprotectant solution has a CT density 26% greater than pure water (257 Hounsfield Units).