Detector-based spectral CT with a novel dual-layer technology: principles and applications

Abstract

Detector-based spectral computed tomography is a novel dual-energy CT technology that employs two layers of detectors to simultaneously collect low- and high-energy data in all patients using standard CT protocols. In addition to the conventional polyenergetic images created for each patient, projection-space decomposition is used to generate spectral basis images (photoelectric and Compton scatter) for creating multiple spectral images, including material decomposition (iodine-only, virtual non-contrast, effective atomic number) and virtual monoenergetic images, on-demand according to clinical need. These images are useful in multiple clinical applications, including- improving vascular contrast, improving lesion conspicuity, decreasing artefacts, material characterisation and reducing radiation dose. In this article, we discuss the principles of this novel technology and also illustrate the common clinical applications. Teaching points • The top and bottom layers of dual-layer CT absorb low- and high-energy photons, respectively.• Multiple spectral images are generated by projection-space decomposition.• Spectral images can be generated in all patients scanned in this scanner.

Keywords: CT; Cardiac; Contrast; Dual-energy; Spectral.

Fig. 1

Spectral technology. (a) Illustration depicting the configuration of the dual-layer detector-based spectral CT. The top layer is made up of an yttrium-based garnet scintillator, which absorbs the low-energy photons and the bottom layer is made up of gadolinium-oxysulphide, which absorbs the high-energy photons. Attached to each layer, there is a thin front-illuminated photodiode (FIP), which converts the light photons to an electrical signal and an application-specific integrated circuit (ASIC) for analogue to digital conversion (not shown here). (b) Diagram showing the technique of image generation from the SDCT scanner. Data from the two layers is utilised to generate photoelectric and Compton scatter basis pairs. Linear combination gives virtual monoenergetic images, while material decomposition gives iodine-density, virtual non-contrast and effective atomic number-based images

Fig. 2

SDCT images. (a) Iodine-only image, highlighting tissues containing iodine. (b) Virtual non-contrast image, obtained after removal of pixels containing iodine. (c) Effective atomic number-based image, where the colour depends on the effective atomic number. (d) Virtual monoenergetic images at 40 keV, 60 keV, 80 keV and 100 keV. The contrast signal is highest in the 40-keV image, with the signal progressively decreasing at higher energy levels

Fig. 3

Salvage of a suboptimal vascular study. (a) Coronal 120-kVp routine diagnostic CT image in a patient being evaluated for radiofrequency ablation of pulmonary veins, shows poor contrast opacification of the vascular structures, particularly the left atrium (arrow), due to contrast extravasation during the scanning. (b) 45-keV virtual monoenergetic image (VMI) at the same level shows significant improvement in the contrast attenuation, especially in the left atrium (arrow). This obviates the need for an additional scan with another contrast bolus, saving radiation and contrast dose

Fig. 4

Low-contrast dose study. (a) Coronal 120-kVp routine diagnostic CTA image in a patient who received only 20 ml of intravenous contrast, shows poor contrast opacification of the abdominal aorta as well as parenchymal organs. (b) 40 keV VMI at the same level shows significant improvement of vascular contrast in the abdominal aorta. Thus, the low-energy VMI allows the use of low contrast dose, which is valuable in patients with severe renal dysfunction

Fig. 5

Improved lesion conspicuousness. (a) Axial 120-kVp routine diagnostic CT image through the upper abdomen shows a vague ill-defined hypoattenuating lesion in the pancreatic head (arrow). (b) 50-keV VMI at the same level shows excellent contrast opacification of the pancreas, with much improved conspicuity of the hypoattenuating pancreatic head lesion (arrow)

Fig. 6

Metallic artefact reduction. (a) Axial 120-kVp routine diagnostic CT image through the head shows dense metallic opacity in the right temporal region (arrow). (b) 120-keV VMI at the same level shows significant improvement in the metallic artefact (arrow)

Fig. 7

Beam hardening artefact reduction. (a) Axial 120-kVp routine diagnostic image at the level of the right axilla shows beam hardening artefact (arrow) originating from dense contrast within the right subclavian vein, which limits evaluation of adjacent structures. (b) 100-keV VMI at the same level shows near-total elimination of the beam hardening artefact (arrow)

Fig. 8

Urinary calculus composition. (a) Coronal 120-kVp routine diagnostic image in a patient with acute abdominal pain shows a 7-mm calculus in the inferior pole of the right kidney (arrow). (b) Effective atomic number-based reconstruction at the same level shows that the calculus has high atomic number (arrow) consistent with a calcium calculus. (c) Uric acid image shows no focal abnormality in the area corresponding to calculus (arrow), indicating that the calculus does not have uric acid

Fig. 9

Perfusion imaging with iodine maps. Sagittal iodine overlay image in a patient with acute chest pain shows a wedge-shaped perfusion defect in the right lower lobe (arrow), which is consistent with an acute pulmonary embolism. CTA in this patient (not shown here) did not show the clot clearly

Fig. 10

Lesion characterisation. (a) 120-kVp routine-diagnostic (conventional) CT image through the upper abdomen shows a well-defined hypoattenuating lesion in the left kidney (arrow), which has a mean attenuation of 49.5 HU, which is too high for a simple cyst. This can either be a complicated cyst or enhancing solid lesion. (b) VNC reconstruction at the same level shows that the attenuation of the lesion (arrow) is 45.2 HU, indicating that the high attenuation was inherent in the scan and not due to contrast administration (c) Iodine map at the same level confirms the above, with an insignificant (0.2 mg/ml) iodine uptake in the lesion (arrow). This constellation of findings helps in diagnosing this as a complicated renal cyst than an enhancing solid mass

Fig. 11

Virtual non-contrast. (a) Axial 120-kVp routine diagnostic image at the level of kidneys shows a contrast-filled stent graft lumen within the proximal abdominal aorta. (b) Virtual non-contrast reconstruction at the same level has removed the pixels which contained iodine. (c) Compare this with the true non-contrast image obtained at the same level, which shows similar appearance and attenuation values as the virtual non-contrast. In multi-phasic studies, especially in vascular indications, the true non-contrast phase can be excluded, resulting in significant radiation dose savings