Spectral CT in abdominal disorders

Etienne Danse, MD, PhD, Director of the CT Unit, Department of Radiology, Cliniques Universitaires St-Luc, Brussels, Belgium

Begüm Demirler Șimșir, MD, visiting research fellow, Department of Radiology, Cliniques Universitaires St-Luc, Brussels, Belgium

Emmanuel Coche, MD, PhD, Head of Department of Radiology, Cliniques Universitaires St-Luc, Brussels, Belgium

Introduction

The IQon Spectral CT uses dual-energy spectral technology to provide users with enhanced imaging capabilities, helping to improve diagnostic confidence. Dual-energy CT (DECT) has existed in clinical practice for many years. There are a number of publications and clinical studies available that demonstrate the major capabilities of DECT based on its image quality improvement with monoenergetic reconstructions, virtual non-enhanced reconstructed images, and material decomposition images.1

In this chapter, we will focus on the significance and benefits of spectral CT in abdominal imaging, using several powerful examples based on our experiences to support our discussion, as well as a comparison of both phantom and patient images obtained from the IQon Spectral CT versus those from a single layer 64-row CT. After a thorough review, a statistically significant increase in global image quality in images obtained from the spectral CT, including visualization of the splanchnic vascular network, has been demonstrated.2

The tools derived from spectral CT for abdominal analysis

Virtual non-contrast reconstructed images, monoenergetic images at different energy levels from 40 to 200 keV, iodine maps, and Z effective maps are the spectral results commonly reviewed when spectral CT examinations are performed for abdominal imaging. The only requirement is to scan the patients with an X-ray tube operated at 120 kVp or 140 kVp.

Virtual non-contrast images

The iodine extraction is optimal on portal series and can be done on arterial phase when liver protocols are performed.3 Presently, software is unable to extract excretory phases containing an excess of iodine contrast within the urinary tract.4 Skipping the true noncontrast series when they were initially needed helps to reduce the radiation dose. In a study including 202 patients with acute abdominal pain, mean CT attenuation values were similar on VNC compared to true non-contrast images, with good image quality, mild noise, and good acceptability. Detection of hemorrhage was similar by both VNC and true non-contrast images, whereas the radiation dose was reduced by 33-47% when true non-contrast images were omitted.5 With spectral CT systems, the attenuation values of different tissues of the abdominal area have showed comparable and valid values between VNC and true non-contrast acquisition, apart from subcutaneous fat tissue.4 It should be noted though that some recent studies have shown contradictory results and demonstrated substantial differences between fluid, fat, and renal tissues on virtual non-contrast images and true non-contrast acquisitions.6

Monoenergetic images

Virtual monoenergetic (MonoE) images could be reconstructed from 40 to 200 keV. For the analysis, image quality from reconstructed 70 keV spectral images and conventional images acquired at 120 kVp were compared. The 70 keV spectral images were found to demonstrate better image quality compared to the conventional images.7

In routine practice, we recommend using virtual monoenergetic images reconstructed at a low energy level to help enhance iodine contrast within vessels as well as the viscerae. This assists clinicians in better detection of hypervascular lesions at the arterial phase compared to conventional images.8

Low-energy monoenergetic images

On unenhanced acquisitions, low-energy reconstruction helps increase detection of initially non-visible structures, like gallbladder and biliary stones9,10 (Figure 1). On enhanced CT acquisitions, the availability of lower energy reconstructed images has multiple advantages:11,12

• For strictly vascular applications, the amount of iodine volume can be reduced to 50%, without impacting diagnostic capabilities

• The use of higher iodine contrast medium is not required

• Detection of hypervascular lesions at the arterial phase is optimized

• Detection of bleeding sources is optimized in acute settings (Figure 2)

• Assessment of organ perfusion is optimized, particularly for the bowel and the liver

Excellent quality images are also obtained with 350 to 370 mg/ml iodine contrast concentrations, which means that higher iodine concentrations are not required for common angiographic CT examinations of the abdomen.

In patients with renal function impairment, when the clinical question is focused on a vascular problem or a hypervascular condition visible at the arterial phase, spectral CT images can be useful in enabling the use of reduced contrast medium with the combination of image reconstruction at lower energies.13

For vascular applications, this approach has great diagnostic value for demonstrating vessel patency, or for endoleak detection when endovascular prosthesis controls are performed with CT angiography.11,14

High-energy monoenergetic images

This option is useful for optimal detection of slightly calcified gallbladder stones.15 In addition, when combined with a dedicated metal artifact reduction software such as OMAR, this type of spectral result can greatly reduce streak artifact caused by the presence of metal objects.16

Iodine maps

Iodine maps can be a great help with perfusion assessments of the liver, kidney, pancreas, and bowel wall, as well as perfusion defects within the gallbladder wall.

The amount of iodine in the area selected or region of interest (ROI) is measured in mg/ml. In the abdominal area, it can help to confirm the presence or absence of iodine uptake (>= 0.5 mg/ml is the cut-off value).

Z effective values of the tissue

The Z effective color map of a selected slice can be of great clinical value in the identification or assessment of an organ perfusion, helping to enhance tissue structures and allowing a clinician to more clearly visualize a normal status versus perfusion abnormalities.

Detection of cholesterol stones in the biliary tract and the amount of cholesterol within the considered stone are possible by simply looking at the color content of the gallbladder stones which are similar to the fatty tissue of the abdomen (Figure 1).

(a) Conventional CT shows enlarged common bile duct (arrow) without identification of the cause.

(b) Monoenergetic image at 40 keV demonstrates a hypoattenuating round-shaped lesion (arrow).

(c) Virtual non-contrast image demonstrates lack of calcium content of the lesion. (d) Z effective map demonstrates the lesion with a low atomic number color coded in orange/red (arrow) related to an entrapped cholesterol stone (which was later endoscopically removed).

Patient with acute necrotizing pancreatitis and active bleeding detected with CT. (a) 3D reconstruction conventional series: bleeding is shown (frame), the left gastric artery is not visible. (b) 3D reconstruction monoenergetic (40 keV): improved visualization of the vessels, a part of the left gastric artery is visible (arrow). (c) Angiography: left gastric artery (arrows) is demonstrated as the source of the bleeding (frame).

Improved detection of right-sided renal infarct with spectral data (upper row). (a) Conventional portal phase image: demonstrates a hypodense parenchymal area (arrow). (b) Iodine no Water image: demonstrates low iodine content (0.56 mg/ml), (upper to the iodine uptake threshold value of 0.5 mg/ml) in this area (arrow) compared to normal appearing adjacent parenchyma (5.64 mg/ml). (c) Z effective map: demonstrates the infarct area color coded in yellow (arrow) and normal appearing adjacent renal parenchyma color coded in dark blue.

Improved detection of pyelonephritis (lower row). (a) Conventional portal phase image: demonstrates a hypodense paranchymal area (arrow). (b) Iodine no Water image: demonstrates the hypoperfused area with lower iodine content (arrow) compared to normal appearing renal parenchyma (iodine contents; 3.49 mg/ml and 5.42 mg/ml respectively). (c) Z effective map demonstrates this area color coded in light blue (arrow) and normal appearing renal parenchyma color coded in dark blue.

As well as improving detection of renal infarcts and pyelonephritis, spectral CT could also help in differential diagnosis by allowing iodine quantification when conventional CT appearances of renal infarct and pyelonephritis overlap.