A new measurement approach 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). 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. (If the human eye could see X-rays, you could run the tube’s beam through a prism and see it break into a spectrum of colors.) 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.
Further complicating the situation is the way the tube’s X-ray light has to change depending on the type of scan. Denser body parts need more penetrating X-rays, so the tube has a sort of color switch, allowing its operator to adjust the tube voltage to match the job. Adjusting the tube’s voltage alters the spectrum of the beam, 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.
Adding these complications to the differences that exist among various CT machine manufacturers is troublesome when linking the calibration of any given scanner to a universal standard.
Better calibration could make diagnosis more efficient and less costly. Better comparisons among scanners might allow the establishment of cutoff points for disease, such as emphysema getting a particular Hounsfield score or lower. 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. This second procedure could be eliminated.