The effect on apparent size of simulated pulmonary nodules of using three standard CT window settings

This study was undertaken to determine whether three commonly used combinations of CT display window settings permit accurate measurement of pulmonary nodule size during thoracic CT examination. Forty-five rounded nodules of synthetic material of approximate soft tissue density (+70 Hounsfield units; HU), varying in size from 5.8 mm to 21.4 mm, were suspended in surgical gauze (-900 HU to -950 HU) to simulate pulmonary nodules. CT images were obtained for each nodule using 10 mm and 2 mm CT collimation and the size of each nodule was measured at three window settings: (a) 'soft tissue setting': window width (WW) 400 HU, window level (WL) +20 HU; (b) 'lung setting'. WW 850 HU, WL -750 HU; (c) 'broad lung setting': WW 1350 HU, WL -550 HU. Measurements obtained using either lung setting were highly accurate irrespective of collimation while measurements obtained using soft tissue settings were inaccurate. The choice of display window settings is an important consideration when CT is used to assess the size of pulmonary nodules.