VIDEO: The many benefits of spectral CT for cardiovascular imaging

Spectral CT, also called dual-energy CT and more recently multi-energy CT, enables views of the anatomy at different kV energy level, which can help highlight different features. Like light through a prism, spectral CT breaks the photons down in a similar fashion, allowing various elements to be isolated, such as iodine and calcium, so they can be enhanced or subtracted from images. This allows the ability to see through heavily calcified vessels, reduce or eliminate artifacts from calcium, and show enhanced iodine mapping if soft tissues to 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; rapid kV switching back and forth between different energies; detector-based spectral CT where the different energies are sorted out by filters built into the layers of the detectors; and photon-counting CT, which bins photons by different energies when counting them. Photon-counting and detector based systems have spectral imaging baked into every scan.

"So if you scan a patient with one of these systems and you desire to get the spectral imaging information after the fact, you have the possibility to reconstruct it after the scan," Abbara said. "With the source-based spectral CT, you need use premeditated protocols, where you say 'I think I am going to need spectral information for my myocardial flow quantification,' so you have to choose that protocol. If you didn't, you have to get the patient back for another scan." 

He said CT systems that always have spectral imaging capabilities on in the background have an advantage, because it allows the radiologist or cardiologist to pull more information from the images if there is a clinical question that is left unanswered. 

"I personally like this very much, because you don't know what you are going to see at the other end when you see the images. Maybe you would desire more often than not to see the spectral information afterwards. So for systems that already have spectral information, you can get this information without any additional radiation cost to the patient," Abbara said. 

He also noted that the spectral-derived images that can be created offer additional information that is not seen on standard CT scans. This includes pulling out information specific to certain periodic table elements, such as calcium and iodine.  

"Quantification of the specific amount of iodine in the myocardium is a very hot topic," Abbaran said.   

Some spectral CT software allows you click on a specific voxel in an image to get material decomposition data to identify what the material is in the body. This includes silicone, gall stones, gout, and identifying the composition of a kidney stone. When stones are suspected but are difficult to see on a CT, the special data can help show them and determine their composition, which might influence treatment. 

Spectral CT also allows the creation of a set of virtual mono-energentic images, which may help identify things in 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 certain artifacts and get extra information that normally would be over shadowed by certain artifacts. If you go to low mono-energetic imaging, you get a better contrast-to-noise ratio. This can help salvage a suboptimal study. If you have a pulmonary embolism (PE) study and the Hounsfield units are so-so and you can't really read the exam, that radiologist may say the exam in unevaluable. But, if you had one of these scanners, then in retrospect you can ask your technologist to give you a low mono-energetic scan that gives you Hounsfield values that are much higher than the 120 kV equivalent. Early studies have shown you can go from unevaluable images to evaluable," Abbarra said. 

Adjusting kV levels virtually also allows lower dose iodine contrast exams.With the contrast shortage earlier in 2022, this might be of value to centers in case of future shortages, Abbarra said.
 

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