Millions of CAT scans take place every year in the U.S alone, but over time the outputs from the devices have a tendency to “drift” according to NIST physicist Zachary Levine. .Scientists at NIST have developed a new Standard Reference Material (SRM) that would enable hospitals to link important tissue density measurements made by CAT scans to international standards.
Computed tomography (CT or CAT) uses computer processing to combine multiple x-ray images into three dimensional scans that resemble slices of the body. These cross sections are useful for spotting changes that are difficult to discern from ordinary x-ray images for changes in lung tissues that might indicate cancer or emphysema.
Scanners are calibrated daily, but over periods of months you still see variations”, says Levine. “Some of the variations come from the physical degradation of the machine, or even from changes made by software upgrades. Doctors get to know their own machine’s idiosyncrasies and are able to make good diagnoses but they would still like to be more certain about what they are seeing, especially when it comes to lung tissue.”
Lung tissue is among the lightest in the body and varies constantly in density depending on how much air is trapped in it, therefore, a lung tissue SRM must span a range of densities. Emphysema makes lung tissue grow less dense but when a tumor exists, the density increases.
Scientists who work on CAT scans often express this density range as a CT number where a change in the CT number by a few points can mean the difference between one diagnosis and another. The NIST team has now made it possible for SRM 2008, fashioned from polyurethane foam of different densities to link CAT scans to the International System of Units.