Virtual non-contrast in second-generation, dual-energy computed tomography: Reliability of attenuation values

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Abstract

Purpose

To evaluate the reliability of attenuation values in virtual non-contrast images (VNC) reconstructed from contrast-enhanced, dual-energy scans performed on a second-generation dual-energy CT scanner, compared to single-energy, non-contrast images (TNC).

Materials and methods

Sixteen phantoms containing a mixture of contrast agent and water at different attenuations (0–1400 HU) were investigated on a Definition Flash-CT scanner using a single-energy scan at 120 kV and a DE-CT protocol (100 kV/SN140 kV). For clinical assessment, 86 patients who received a dual-phase CT, containing an unenhanced single-energy scan at 120 kV and a contrast enhanced (110 ml Iomeron 400 mg/ml; 4 ml/s) DE-CT (100 kV/SN140 kV) in an arterial (n = 43) or a venous phase, were retrospectively analyzed. Mean attenuation was measured within regions of interest of the phantoms and in different tissue types of the patients within the corresponding VNC and TNC images. Paired t-tests and Pearson correlation were used for statistical analysis.

Results

For all phantoms, mean attenuation in VNC was 5.3 ± 18.4 HU, with respect to water. In 86 patients overall, 2637 regions were measured in TNC and VNC images, with a mean difference between TNC and VNC of −3.6 ± 8.3 HU. In 91.5% (n = 2412) of all cases, absolute differences between TNC and VNC were under 15 HU, and, in 75.3% (n = 1986), differences were under 10 HU.

Conclusions

Second-generation dual-energy CT based VNC images provide attenuation values close to those of TNC. To avoid possible outliers multiple measurements are recommended especially for measurements in the spleen, the mesenteric fat, and the aorta.

Introduction

Dual-energy Computed Tomography (DE-CT) is a promising imaging technique that provides better tissue characterization compared to single-energy computed tomography [1], [2], [3]. Based on two synchronous CT acquisitions at different tube voltages, this technology allows the identification and visualization of materials that have different X-ray absorptions on low and high kV [2]. The differentiation of two materials works best for materials with a high difference in atomic numbers, such as iodine-based contrast agents and normal tissue. This feature enables virtual ‘subtraction’ of the iodine content from a contrast-enhanced, dual-energy CT scan, resulting in a virtual non-contrast (VNC) image. In addition, by avoiding a true non-contrast scan, the overall radiation dose is reduced for the patient. Currently, two technical designs of CT scanners allow the use of dual energy in clinical routine, the rapid kVp switching technique and the dual-source scanners [1]. Previous studies have concluded that VNC is a possible alternative to true non-enhanced CT scans; however, most of the available data originates from studies that used the first generation of dual-energy scanners available [2], [4], [5], [6].

These first clinical dual-energy scanners were available in 2006, with the limitation of a narrow field of view. Furthermore, due to an energy spectra overlap between 100 and 140 kV, these scanners had to be operated at 80 and 140 kV, resulting in the inability to be used in the abdomen due to a low penetration depth and beam hardening artifacts [7]. Fortunately, the recently introduced second-generation of dual-energy scanners has overcome these problems by using a greater field of view and a tin filter, which reduces the X-ray spectra overlap, and facilitates the improved use of this technology in the abdomen [1], [4], [8], [9]. For the new generation of dual-source CT scanners only one study that has assessed VNC has been published (14) and to our knowledge, currently no data about the reliability of different tissue attenuation in virtual non-contrast images is available.

Therefore, the aim of our study was to evaluate the reliability of attenuation values in virtual non-contrast images (VNC) reconstructed from contrast enhanced dual-energy scans performed on a second generation dual-source CT, compared to single-energy, non-contrast images (TNC). As a secondary aim, we assessed the influence of contrast agent concentration and body mass index on the reliability of attenuation measurements in VNC images.

Section snippets

Study design

The study included a phantom component and a clinical component. The research protocol for this retrospective study was approved by the Institutional Review Board of our institution (Medical University of Vienna, Vienna, Austria; protocol #573/2010). All examinations had been exclusively performed for clinical purposes. Due to the retrospective design of the study, the need for informed consent was waived. All patient data were completely anonymized at the beginning of the study and were not

Phantom scans

The attenuation measurements at different contrast agent concentrations in the phantoms are shown in Table 1. In contrast to the attenuation of the phantoms (0–1400 HU) measured at single-energy (120 kV), the attenuation values of the reconstructed VNC images of all phantoms should be equal to the attenuation of water (i.e., 0 HU). The actual measured mean attenuation for all phantoms was 5.3 ± 18.4 HU. Stratifying the phantoms according to their iodine concentration, the phantoms that had

Discussion

In our study, we found a high reliability for attenuations values of virtual non-contrast images up to contrast agent-caused attenuations of 730 HU; beyond 730 HU, the error of VNC attenuation values increases compared to TNC. This finding was confirmed in the phantom component and the clinical component of the study; the difference between non-contrast and virtual non-contrast was lower than 15 HU in 91.5% of all measurements. The highest differences were found in the aorta, in the mesenteric fat

Conflict of interest

None.

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Presented: RSNA 2010: “Density Measurements in Virtual Non-contrast Images Using the Novel Dual-Energy CT.”.

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