THE EFFECT OF WINDOW WIDTH ON RADIATION-D OSE IN CT-EXAMINATIONS OF HIGH-CONTRAST STRUCTURES

Purpose: To establish the relationship between window width and dose t o be applied in low-dose high-resolution (HR) computed tomography (CT) . Material and methods: Low-dose HR CT-scans of the petrous bone displ ayed with different window values were analyzed for identification of osseous details. For two homogenous phantoms, standard deviation of CT -numbers were measured in order to calculate the fraction of pixels no t displayed within the correct grey level. Results: The broadest windo w used (4000 HU) allowed the best distinction of osseous structures. S tandard deviation of CT-numbers in the phantoms varied between 28.5 an d 43.2 (mean = 33.6) HU. Thus, only 26.6% of all pixels are displayed in the correct grey level for a 1000 HU-window, but 82.3% for a 4000 H U-window. Conclusion: When using low doses and HR-reconstruction algor ithms, large window widths allow for an optimal assessment of high-con trast structures. Under these conditions, even high standard deviation s of the CT-numbers will not compromise image evaluation. Due to the r estricted number of grey-levels that can be distinguished by the human eye, image noise is compressed into a smaller number of discernable g rey-levels. Because of the inverse square root function between dose t o be applied and window width used (derived in the article), an even m inor enlargement of window permits considerable reductions of patient exposure while the discernable image noise remains constant.