Spectral computed tomography, also called dual-energy computed tomography and more recently multi-energy computed tomography, can visualize anatomy at different kV energy levels, which can help highlight different features. Like light through a prism, spectral computed tomography breaks down photons in the same way, allowing various elements, such as iodine and calcium, to be isolated so that they can be enhanced or subtracted from images. This allows seeing through heavily calcified vessels, reducing or eliminating calcium artifacts, and showing improved iodine mapping if soft tissues show blood flow deficits such as infarcts or areas of ischemia.
To achieve this, Abbara said CT systems use four different techniques: dual X-ray sources at different kV energies; fast kV switching between different energies; spectral CT based on detectors where the different energies are sorted by filters integrated in the layers of the detectors; and photon-counting CT, which classifies photons by different energies as they are counted. Photon counting and detector-based systems have spectral imaging built into each scan.
“So if you scan a patient with one of these systems and want to get the spectral imaging information afterwards, you have the option of reconstructing it after the scan,” Abbara said. “With source-based spectral CT, you have to use premeditated protocols, where you say ‘I think I’m going to need spectral information for quantification of my myocardial flow’, so you have to choose that protocol. If you didn’t, you have to pick up the patient for another scan.”
He said CT systems that still have spectral imaging capabilities in the background have an advantage because they allow the radiologist or cardiologist to derive more information from the images if there is a clinical question that remains. without answer.
“I personally like that a lot, because you don’t know what you’re going to see on the other end when you see the images. Maybe you want to see the spectral information afterwards more often than not. So , for systems that already have spectral information, you can get that information at no additional radiation cost to the patient,” Abbara said.
He also noted that the spectrum-derived images that can be created offer additional information not visible on standard CT scans. This includes extracting information specific to certain elements of the periodic table, such as calcium and iodine.
“Quantifying the specific amount of iodine in the myocardium is a very hot topic,” Abbaran said.
Some spectral computed tomography software allows you to click on a specific voxel in an image to obtain material breakdown data to identify the material present in the body. This includes silicone, gallstones, gout, and identifying the makeup of a kidney stone. When stones are suspected but difficult to see on a CT scan, special data can help show them and determine their composition, which could influence treatment.
Spectral CT also allows the creation of a set of mono-energetic virtual images, which can help identify elements of the anatomy that are not visible at higher or lower energies, from 40 to 200 kilo-electron Volt ( keV).
“If you go high, you can reduce some artifacts and get additional information that would normally be obscured by some artifacts. If you go for low mono-energy imaging, you get a better contrast-to-noise ratio. This can help save a suboptimal study If you have a pulmonary embolism (PE) study and the Hounsfield units are poor and you can’t really read the exam, that radiologist may say the exam is not evaluable But, if you had one of these scans, then in retrospect you can ask your technologist to give you a low-energy mono-scan which gives you Hounsfield values well above the 120 kV equivalent. studies have shown that you can go from non-evaluable to evaluable images,” Abbarra said.
Adjusting the kV levels also practically allows low dose iodine contrast examinations. With the contrast shortage at the start of 2022, this could be useful for centers in the event of a future shortage, Abbarra said.
Find more SCCT content
Find more cardiac CT content
Find more RSNA content