Spectral CT Makes Detecting Early-Stage COVID-19 Easier

Even though CT scans have not been widely used to diagnose COVID-19 infection, new research shows that using spectral CT with electron density (ED) imaging can make it much easier to assess the extent of lung lesion in patients who have early-stage disease.

Using a small group of case studies, published Oct. 21 in the American Journal of Roentgenology, a team of clinicians from Antony’s Private Hospital in France showed that electron density (ED) imaging can improve ground-glass opacity (GGO) visualization compared to conventional CT studies.

“ED imaging may improve the detection of early-stage COVID-19, a stage for which conventional CT has shown limited sensitivity for detection,” said lead study author Beatrice Daoud, an associate radiologist at Antony's. “Spectral CT may also provide a better assessment of the extent of the lung lesions.”

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Although GGOs do show up on conventional CT scans, the visualization is subtle, Daoud’s team said, revealing only very small increases in attenuation compared with normal lungs. Consequently, accurate identification can be difficult, particularly during early infection. This study marks the first time investigators have looked into whether spectral CT scans can improve providers’ abilities to assess lung lesion extent.

Fig. 1—39-year-old man with coronavirus disease. A, Initial conventional axial CT image shows ground-glass opacities in right upper lobe and combination of consolidation and ground-glass opacities in right lower lobe. Note barely distinguishable lung lesions (within red box) in left upper lobe. B, Spectral electron density image clearly shows lesions (within red box) in left upper lobe. C, Follow-up conventional axial CT image obtained 2 days after images in A and B shows clearly visible lesions (within red box) in left upper lobe. Courtesy: American Journal of Roentgenology

To make this determination, the team retrospectively examined data from spectral CT scans conducted in four patients who have had confirmed COVID-19 cases with an RT-PCR test, they said. They reconstructed the spectral CT images using the same standard soft kernel (filter B) and a similar iterative method that is used to capture conventional CT images. They, then, compared initial conventional CT images with the follow-up conventional CT scans.

Based on their analysis, all 45 GGOs identified in the four patients showed up better on the ED images than the conventional CT scans, and the follow-up conventional CT images confirmed their presence. In addition, the lesion extent, which they assessed with semi-quantitative reporting scale denoting surface area involvement for each lobe, was much easier to determine on ED images, the team said.

“We reviewed conventional chest CT images obtained with a parenchyma kernel and standard lung window setting, as is usually the case in everyday radiology practices,” Daoud said.

The team compared the ED images to conventional images obtained using a soft mediastinum kernel and standard lung window setting, as well as conventional images captured with a soft mediastinum kernel and narrow lung window setting, virtual low-monoenergy images, virtual high-monoenergy images, and electron density images.

Based on these comparisons, the team discovered ED images made a difference in evaluating each of the four patients:

Patient 1: A 39-year-old man diagnosed via conventional CT with lesions in both the right upper and lower lobes was found – with ED imaging – to have lesions in the upper left lobe that were barely noticeable previously.

Patient 2: A 41-year-old woman with GGOs in both lower lobes discovered with conventional CT had additional lesions identified in the upper right lobe with ED imaging.

Patient 3: A 64-year-old man had a conventional CT that showed peripheral bilateral diffuse GGOs. ED imaging revealed the GGOs to be more extensive and conspicuous than initially believed.

Patient 4: An 80-year-old man had barely noticeable peripheral GGOs in the left upper lobe. Follow-up ED imaging showed the extent of these GGOs.

Overall, these results show that ED imaging offers greater detail for early assessment of GGOs that can be overlooked with conventional CT. The results are most promising, they said, by enhancing the contrast of GGOs compared with the normal lung.

“Our results suggest that the better ground-glass opacity visualization obtained using electron density imaging may be chiefly related to the increased visual noise in the image with soft kernel reconstruction and narrow lung window setting compared with electron density imaging, for which narrowing the window does not impair image quality,” they said.

Still, they said, a more systematic analysis of the different available image reconstruction is needed.

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