Virtual Monochromatic Reconstruction of Dual-Energy Unenhanced Head CT at 65–75 keV Maximizes Image Quality Compared with Conventional Polychromatic CT

To determine the virtual monochromatic imaging (VMI) energy levels that maximize brain parenchymal image quality in dual-energy unenhanced head computed tomography (CT) and to assess the improvement with this technique compared with conventional polychromatic scanning.Institutional review board approval was obtained with no informed consent required for this HIPAA-compliant retrospective analysis. Twenty-five consecutive unenhanced head CT scans were acquired with a 64-section dual-energy scanner with fast tube voltage switching (80-140 kVp). Scans were retrospectively reconstructed at VMI energy levels from 40 to 140 keV in 5-keV increments and were analyzed by using four quality indexes: gray matter (GM) signal-to-noise ratio (SNR), white matter (WM) SNR, GM-WM contrast-to-noise ratio (CNR), and posterior fossa artifact index (PFAI). Optimal mean values for each parameter were compared with those from 50 consecutive scans obtained with the same scanner in 120-kVp single-energy mode. Repeated-measures analysis of variance and Dunnett post hoc t test were then used to determine significance.Maximal GM SNR, WM SNR, and GM-WM CNR values were observed at 65 keV, and minimal PFAI was observed at 75 keV. These values were significantly better than those of conventional polychromatic CT (P < .01); quality index improvement ratios (corrected for radiation dose) ranged from 17% to 50%.Virtual monochromatic reconstruction of dual-energy unenhanced head CT scans at 65-75 keV (optimal energy levels) maximizes image quality compared with scans obtained with conventional polychromatic CT.