A new measurement approach proposed by scientists at the National Institute of Standards and Technology (NIST) could lead to a better way to calibrate computed tomography (CT) scanners, potentially streamlining patient treatment by improving communication among doctors.
The approach suggests how the x-ray beams generated by CT can be measured in a way that allows scans from different devices to be usefully compared to one another. It also offers a pathway to create the first CT measurement standards connected to the International System of Units (SI) by creating a more precise definition of the units used in CT — something the field has lacked.
An object’s ability to block x-rays — its “radiodensity” — is measured in Hounsfield Units (HUs), named for the Nobel Prize winning co-inventor of CT. Calibration of a CT machine, something every radiology facility has to perform regularly, involves scanning an object of known radiodensity called a phantom and checking whether these measurements give the right number of HUs.
A problem is that a CT scanner’s tube — essentially its x-ray generating “light bulb” — 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 photon’s penetrating power depends on its energy, the beam’s overall effect on the phantom has to be averaged out, making it challenging to define the calibration.
The NIST team had to overcome the uncertainties created by the tube’s broad x-ray spectrum and tube voltage setting. Their idea was to fill several phantoms with different concentrations of powdered chemicals that are common in the body, and compare the phantoms’ radiodensity using CT. The comparison would help link HUs to the number of moles per cubic meter, which are both SI units.