Figure 3:

Images in a 71-year-old man scanned with (A) energy-integrating detector (EID) CT and (B, C) photon-counting detector (PCD) CT with dual-source geometry to achieve 66-msec temporal resolution. Axial images are shown in left column, and oblique coronal images are shown in right column. While the EID CT examination is limited to single-energy data (A) at this temporal resolution, the multienergy capabilities of the PCD CT system allowed creation of low-energy (45, 55 keV) virtual monoenergetic images (VMIs) (B), which showed increased iodine signal (shown as mean Hounsfield unit measurements in regions of interest) compared with EID CT despite an 18% decrease (A: 110 mL vs B, C: 90 mL iohexol [Omnipaque 350, GE Healthcare]) in volume of administered contrast material (mean CT numbers for the black circular regions of interest are given in the left column of images). The use of VMIs adds to the inherently higher iodine contrast-to-noise ratio possible with PCD CT and provides clearer delineation of a branch of the left coronary artery (arrowheads). Increasing the VMI energy (65 keV or higher) decreased calcium blooming relative to EID CT (arrows in A and B). Absolute iodine concentration was measured using the iodine map images (mean concentration in mg/cm³ unit shown in region of interest) and virtual noncontrast (VNC) images used to visualize calcifications (arrow in C) with attenuation similar to that of iodinated blood. Reconstruction kernels used were as follows: Bv40 (body-vascular, sharpness level 40) (EID CT 90-kV, in A), Bv48 (PCD CT VMIs in B), Qr40 (quantitative-regular, sharpness level 40) (PCD CT iodine map and VNC images in C) Display windows and levels were as follows: 2000 HU and 200 HU for EID CT and PCD CT VMIs, 30/15 mg · mL⁻¹ for iodine map, and 1000 HU and 100 HU for VNC image.