Latest in CT – Dual layer-based spectral detector CT

A CT scanner is the major workhorse in any radiology department. In conventional CT, single X-ray spectrum-based tissue attenuation (Hounsfield units (HU)) is used to differentiate between different tissues within the body. However, many different tissues, with different chemical composition or effective atomic number and different densities, may have overlapping HU values at a given peak tube voltage, making it difficult to differentiate.

To overcome this challenge, spectral CT scanners are introduced to measure the difference in attenuation of X-rays at two energy levels, high and low. Data collected simultaneously from these two energy levels is used to determine the Compton scatter and photoelectric components of X-ray attenuation. These components provide additional information about tissue density and atomic number that can be used to separate tissues with similar attenuation in a conventional CT scanner image.

There are two primary modes of generating spectral images – source-based and detector-based. Source-based methods include dual-source, kV-switching, twin beam and dual spin. All source-based techniques require the clinician to preselect patients for dual-energy scanning.

On the other hand, detector-based methods are dual-layer detector CT (e.g., Philips Spectral CT-7500) and photon counting CT (e.g., Siemens Naeotom Alpha). Unlike source-based spectral options, dual-layer detector method simultaneously absorbs and differentiates high and low energy available in a single poly energetic X-ray beam at the detector level. Spectral results are acquired within a single scan without the need for special modes. There are several advantages to detector-based spectral imaging. This method does not make any changes in the conventional workflow, and this method retains the same dose setting, using dose management tools as well as the same rotation speed and pitch setting. The patient is scanned as usual and a true conventional image is generated, which is identical to conventional CT scanners. Full spectral information can be generated in addition to the true conventional images. A retrospective reconstruction of the spectral information is also possible in case spectral data were not requested in the original reconstruction.

It has been reported that first-time-right diagnosis with dual-layer spectral detector CT can reduce up to 34 percent time to diagnosis along with ~25 percent reduction in repeat scans and up to 30 percent reduction in follow-up scans – these stats are really encouraging for any high-end radiology department to explore the diagnostic benefit of this new CT technology for better patient outcome.

Mono-Energetic images produce a CT image as if a single monochromatic energy was used to create it. Selecting a lower energy result enhances soft tissue contract while a higher energy can mitigate metal and beam hardening artifacts.

Spectral material identification can be used to either suppress iodine-based imaging contrast to produce a virtual non-contrast image from a single scan or to amplify the iodine signal to visualize & quantify subtle perfusion.

The Effective Atomic Number result can characterize structures based on material content.

The Electron Density result produces an image that quantifies the electron density of each voxel.

Calcium suppression images helps to assess intervertebral disc herniation and visualization of bone marrow involvement when bone fractures are present.