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NIST Develops Better Medical Device Calibration for CT Scanners

National Institute of Standards and Technology (NIST) scientists have developed a new x-ray measurement approach designed to improve medical device calibration for CT scanners.

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By Fred Donovan

- National Institute of Standards and Technology (NIST) scientists have developed a new x-ray measurement approach designed to improve medical device calibration for computed tomography (CT) scanners.

X-ray beams generated by CT scanners can be measured in a manner that enables scans from difference devices to be compared to one another.

“You want interchangeable answers regardless of what CT machine you use and when,” said Zachary Levine, a NIST physicist and one of the authors of a research paper detailing the approach.

“For one thing, you want doctors to be able to communicate between hospitals. Let’s say a patient needs a follow up but is somewhere far from home, or the same scanner got a software upgrade that changes the number of HUs [Hounsfield units]. If you can’t measure accurately, you can’t improve your technology,” he added.

NIST said the approach offers a way to create the first CT measurement standards connected to the International System of Units by developing a more precise definition of the units used in CT scanning.

“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,” Levine said.

Better calibration could also make diagnosis more efficient and less costly.

“Better comparisons among scanners might allow us to establish cutoff points for disease—such as emphysema getting a particular Hounsfield score or lower,” Levine said. “It’s also common for CT scans to turn up suspicious growths that might be cancerous, and a doctor commonly orders an MRI as a follow up. We might eliminate the need for that second procedure.”

Every radiology facility must calibrate its CT machine on a regular basis. Calibration entails scanning a test object of known radiodensity and determining whether the measurement provides the right number of HUs.

The CT scanner’s tube creates a beam of photons with different wavelengths that correspond to their energy. Because a photon’s penetrating power depends on its energy, the beam’s overall effect on the test object must be averaged out, making it hard to define the calibration, NIST explained.

In addition, the tube’s X-ray light needs to change depending on the type of scan. Denser body parts need more penetrating X-rays, so the tube has a switch allowing the operator to adjust the tube voltage to match the job.

This adjustment alters the beam’s spectrum, so that it ranges between something like a “cool white” and a “warm white” light bulb. The variable spectrum makes it tougher to ensure that the calibration is correct for all voltages.

Also, differences among various CT machine manufacturers create additional problems for anyone who wants to link the calibration of any scanner to a universal standard. But if it could be done, there would be far-reaching benefits to both industry and medicine.

To overcome the problems, the NIST team had to overcome the uncertainties created by the tube’s broad X-ray spectrum and tube voltage setting. The scientists filled several test objects with different concentrations of powdered chemicals that are common in the body and compared their radiodensity using CT. The comparison helped link HUs to the number of moles per cubic meter. 

“Executing this idea was tricky, because the volume of a mole depends on the size of a given chemical molecule,” Levine said. “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,” Levine said. “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,” he concluded. 


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