NIST research could improve the performance of CT scanners

Scientists at the National Institute of Standards and Technology are proposing a new measurement approach to calibrate computed tomography scanners.

The approach suggests how the X-ray beams generated by CT can be measured in a manner that enables scans from various devices to be compared with one another. It also offers a way to create the first CT measurement connected to the International Standards of Units, paving the way to create a more precise definition of the units used in CT, which the field has lacked.

“If the technical community could agree on a definition, then vendors could create measurements that are interchangeable,” says Zachary Levine, a physicist at NIST and one of the authors of a paper on the proposed approach. “Right now, calibration is not as thorough as it could be.”

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One of the problems is that the CT scanner’s tube creates a beam that is the X-ray version of white light, full of photons with different wavelengths that correspond to their energy. Because a photo’s penetrating power depends on its energy, the beam’s overall effect on the phantom, which is an object that tunes the performance of an imaging device, has to be averaged out, making it challenging to define the calibration.

Other complications abound, such as the way the tube’s X-ray light has to change depending on the type of scan and denser body parts may need more penetrating X-rays, so the tube has a “color switch” that enables an operator to adjust tube voltage to range between something like a cool white and a warm white light bulb, according to researchers. Consequently, this makes it tougher to ensure that the calibration is correct for all voltages.

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Another complication is the differences that exist among various CT machine manufacturers, causing trouble for anyone who wants to link the calibration of any given scanner to a universal standard, researchers say. But if it could be done, there would be far-reaching benefits to both industry and medicine.

“You want interchangeable answers regardless of what CT machine you use and when,” Levine notes. “For one thing, you want doctors to be able to communicate between hospitals. Let’s say a patient needs a follow-up but is far from home or the same scanner got a software upgrade that changes the number of Hounsfield Units (HUs), a scale for describing radiodensity. If you can’t measure accurately, you can’t improve your technology.”

“Better comparisons among scanners might allow us to establish cut off points for disease, such as emphysema getting a particular Hounsfield score higher or lower,” he continues. “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.”

More information is available in a research paper published in the journal PLOS One.

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