Figure 4. a–c.

Equivalency of virtual unenhanced image to true unenhanced image in patient evaluated for acute chest pain. Conventional polyenergetic axial CT image of the chest (a) obtained before the injection of contrast showing no contrast in the aorta and other vascular structures. Conventional contrast-enhanced image (b) obtained just after contrast injection showing increased attenuation in the aorta. Virtual unenhanced image (c) created from spectral data obtained just after contrast injection. Like the true unenhanced image at approximately the same level, the virtual unenhanced image shows no contrast in the aorta and other vascular structures.