NIST proposes new universal standard for CT calibration

Scientists at the National Institute of Standards and Technology have proposed a new universal standard for calibrating computed tomography devices that they claim may improve comparisons between scans from various devices.

The newly released calibration approach was outlined in a research paper recently published in the academic journal PLOS One, the NIST said in a press release.

“If the technical community could agree on a definition, then the vendors could create measurements that are interchangeable. Right now, calibration is not as thorough as it could be. Better comparisons among scanners might allow us to establish cutoff points for disease—such as emphysema getting a particular Hounsfield score or lower,” NIST physicist and paper author Zachary Levine said in a press release.

To calibrate CT machines, radiologists use an object called a phantom, which has a known radiodensity. The Phantom is used to verify that the machine is giving the appropriate measurement in Hounsfield Units, or HUs.

But CT scanner tubes generate beams of photons with different wavelengths, according to the report, and the beams’ overall effect on the phantom must be averaged, making it “challenging to define the calibration.”

In addition, the beams can be adjusted for scanning more or less dense objects, requiring a variable spectrum of light that makes it more difficult to calibrate across voltages, NIST researchers report.

NIST researchers filled several phantoms with different concentrations of powdered chemicals common in the human body to compare HU readings across, which they said could help link HUs to the number of moles per cubic meter, which are both SI units.

“Executing this idea was tricky, because the volume of a mole depends on the size of a given chemical molecule. A mole of salt takes up more space than a mole of carbon, for example. And the air in the powders represented a further complication. Basically, we’ve shown that you can create a CT scanner performance target that any design engineer can hit. Manufacturers have been getting different answers in their machines for decades because no one told their engineers how to handle the X-ray spectrum. Only a small change to existing practice is required to unify their measurements,” Levine said in a prepared statement.

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