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Can Dual-energy CT Evaluate Contrast Enhancement of Ground-glass Attenuation?

Posted May 31, 2011 Updated Apr 10, 2023
By Tatsuya Kawai MD , Yuta Shibamoto MD DMSc , Masaki Hara MD PhD , Toshinao Arakawa MD , Keiichi Nagai MD , Kazuya Ohashi BS
8 min read

Rationale and Objectives

Evaluation of contrast enhancement of pulmonary lesions with ground-glass attenuation (GGA) is difficult with conventional techniques but might be possible using contrast-mapping images (CMIs) obtained by dual-energy computed tomography. To address this issue, a phantom study was conducted, and this technique was then applied to clinical cases.

Materials and Methods

Phantoms made of agarose gel and those made of hollow resin clay, containing various concentrations of iodine or calcium, were used to simulate soft tissue and GGA, respectively. They were scanned using a dual-energy computed tomographic scanner, and the relationship between iodine concentration and calculated iodine value on CMIs was examined. The influence of calcium was also evaluated. In addition, contrast enhancement of 24 GGA lesions was evaluated on CMIs.

Results

There was a good correlation between iodine value and iodine concentration in the soft-tissue models ( r 2 = 0.996). In the GGA models, the former tended to exceed the latter when default parameters for calculating CMIs were used, but this could be corrected by modifying the parameters ( r 2 = 0.998). The iodine value increased with calcium concentration in both models. On CMIs, contrast enhancement was visible in 22 adenocarcinomas but not in a pulmonary hemorrhage and an inflammatory change.

Conclusions

Dual-energy computed tomography can evaluate contrast enhancement of GGA lesions.

Dual-energy computed tomographic (DECT) imaging was first introduced in 2005 by Siemens Medical Solutions (Forchheim, Germany) with the brand name Somatom Definition. This scanner has two x-ray tubes providing 80-kVp and 140-kVp x-rays. Using two different x-ray energies (dual-energy technique), different materials can be distinguished according to their inherent x-ray attenuation coefficients . New imaging techniques, such as lung perfusion imaging , bone-removed computed tomographic (CT) angiography , qualitative diagnosis of urinary stones , and tendon imaging in musculoskeletal system , have been reported using DECT imaging.

Liver virtual noncontrast (VNC) is a new application of the Somatom Definition and is available on the workstation. Using this application, components of contrast material can be extracted from the data sets of postcontrast images obtained using the dual-energy technique, creating VNC and “iodine-mapping” images. Therefore, contrast enhancement can be evaluated without unenhanced images. Our previous study showed that contrast enhancement of the aorta, liver, and pectoris major muscle was correlated with the iodine value obtained with this technique .

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Materials and methods

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Preparation of Phantoms

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Figure 1, Phantoms made of agarose gel (a) and hollow resin clay (b) simulating soft tissue and ground-glass attenuation (GGA) lesions, respectively. (c) Microscopic view of a GGA model. Each resin particle contains an air bubble of approximately 20 to 80 μm in diameter, simulating a bronchiole or an alveolus. (d) Iodine-adsorbed starch powder was used to achieve homogeneity of the iodinated GGA samples.

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Patients

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CT Scanning

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Material Decomposition on Contrast-mapping Images

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Figure 2, Plot of computed tomographic (CT) numbers of materials at 140 and 80 kVp. According to the material decomposition theory, CT numbers of tissues behave in a linear manner on the regressed line (solid line) determined by two or more standard attenuations of the typical tissues (in this case, fat and soft tissue). If a tissue contains high-density materials (eg, iodine, calcium), its attenuation becomes larger at low kilovoltages than that at high kilovoltages (solid circle); therefore, the concentration of such material could be estimated (broken arrow). HU, Hounsfield units.

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Image Evaluation

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Statistical Analysis

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Results

Homogeneity of the GGA Models

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Evaluation of Soft Tissue Models

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Figure 3, Transaxial images of the soft tissue models containing iodine (a) or calcium (b) obtained by dual-energy scanning. (Top row) weighted average image (WAI); (bottom row) contrast-mapping image (CMI). Visually, attenuations of samples with no iodine and those with low concentrations of calcium (left) on the CMI were almost the same as the background level. (c,d) Correlations between iodine values on the CMI and iodine (c) or calcium (d) concentrations are shown.

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Evaluation of GGA Models and Modification of Decomposition Parameters

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Figure 4, Relationship between computed tomographic (CT) numbers at 80 and those at 140 kVp. At low kilovoltage, actual CT numbers of the ground-glass attenuation (GGA) models were higher than predicted by default parameters (solid line) in the attenuation range of −520 to −640 Hounsfield units (HU). Linear regression revealed that by changing the default setting of fat attenuation (−110 HU) to −91 HU, the broken line for actual CT numbers of GGA samples almost perfectly matched the solid line. A magnified image around fat attenuation is shown at the bottom right of the graph.

Table 1

Parameters for Material Decomposition (Excerpt)

Tube Voltage (kVp) Attenuation (Hounsfield Units) Default Modified Fat 80 −110 −91 140 −96 −96 Tissue 80 60 60 140 54 54 Minimum −300 −700 Maximum 3071 3071

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Figure 5, Computed tomographic (CT) number versus iodine value of ground-glass attenuation model samples with different attenuation. Note that each model contains no iodine. Using the modified parameters, iodine value became almost zero and the pseudoenhancement effect was eliminated. CMI, contrast-mapping image; HU, Hounsfield units.

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Figure 6, (a,b) Weighted average image (WAI) and contrast-mapping image (CMI) of the ground-glass attenuation (GGA) models containing iodine (a) or calcium (b) calculated using the default parameters (middle row) and the modified parameters (bottom row). (c) In iodinated GGA models, the pseudoenhancement effect observed when using the default parameters could be eliminated using the modified parameters, with no change in the slope of the regression line. (d) In GGA models containing calcium, pseudoenhancement derived from the GGA sample itself could also be eliminated using modified parameters, but iodine value increased with calcium concentration to almost the same extent as in the iodinated models.

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Evaluation of Clinical Cases

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Figure 7, Two cases with ground-glass attenuation (GGA) lesion. Case A (a–c) : adenocarcinoma with mixed subtypes. Case B (d–f) : pulmonary hemorrhage with chronic inflammation. (a,d) Unenhanced lung window computed tomographic images obtained by conventional single-energy scan. (b,d) Postcontrast weighted average image (WAI) under soft tissue window setting. (c,f) Contrast-mapping image (CMI) created using modified parameters. Contrast enhancement of the GGA was difficult to identify on the WAI but could be visualized on CMI in the case of adenocarcinoma.

Table 2

Mean Visual Scores of Contrast Enhancement in Various Types of Ground-glass Attenuation Lesions

Pathological Diagnosis_n_ Score Reviewer 1 Reviewer 2 Reviewer 3 Total Adenocarcinoma ∗ 18 2.1 ± 0.7 1.9 ± 0.8 2.3 ± 0.8 2.1 ± 0.7 Bronchioloalveolar carcinoma ∗ 3 2.7 ± 0.6 2.0 ± 1.2 2.3 ± 1.2 2.3 ± 0.9 Atypical adenomatous hyperplasia 1 2 2 3 2.3 ± 0.6 Pulmonary hemorrhage ∗ 1 0 0 0 0 Inflammatory change ∗ 1 0 0 0 0

Data are expressed as mean ± standard deviation.

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Figure 8, Scatterplot of the calculated contrast enhancement versus the iodine value in 24 patients with pulmonary ground-glass attenuation lesions. Each lesion was measured three times by three radiologists separately. There was a moderate correlation ( r 2 = 0.39, P < .0001). CMI, contrast-mapping image; HU, Hounsfield units.

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Discussion

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Conclusions

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Academic Radiology, Volume 18, Issue 6
Journals General Radiology
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