ABSTRACT. This study evaluated radiation dose and dose reduction in CT imaging for acute stroke. Radiation doses in three types of CT imaging (i.e. non-contrast-enhanced CT, CT perfusion (CTP) and CT angiography (CTA)) were measured with an in-phantom dosimetry system for 4-, 16-and 64-detector CT scanners in 5 hospitals. To examine the relationship between image quality and radiation dose in CTA, image contrast-to-noise ratio was evaluated. Doses to the brain, lens, salivary glands and local skin obtained with scan protocols in routine use were: 42-71 mGy, 30-88 mGy, 3.9-7.3 mGy and 40-97 mGy in non-contrast-enhanced CT; 41-75 mGy, 9.9-10 mGy, 1.5-2.1 mGy and 107-143 mGy in CTP; and 8.2-55 mGy, 26-69 mGy, 2.0-73 mGy and 32-72 mGy in CTA. For the combination of these CT examinations, on average a patient would receive 236 mGy for the maximum local skin dose and 4.2 mSv for the effective dose evaluated by the International Commission on Radiological Protection (ICRP) 103. Effective doses in CTP in this study were less than those obtained with representative protocols of Western countries. Average effective doses in each CT examination were not more than 1.5 mSv. The use of reduced kV and a narrow scan range would be effective in dose reduction of CTA and CTP, and intermittent scanning would be essential in CTP. Although lens and maximum local skin doses were far less than the thresholds for deterministic effects, since radiation risks would be increased in repeated CT examinations, efforts should be devoted to dose reduction in stroke CT examinations. Most patients with suspected stroke undergo either CT or MRI as the initial diagnostic examination [1,2]. MRI has some advantages over CT in depicting stroke lesions; however, the provision of MR scanners in general hospitals is still low, an examination takes more time and patients with pacemakers cannot undergo MRI examinations. While CT techniques carry the risk of radiation exposure, CT scanners are nearly always available [3] and scans can be done quickly. Schramm et al [4] have reported that CT images obtained by the combination of various CT examinations allow diagnostic assessment of acute stroke with a quality comparable with that of stroke MRI. So CT examination has some advantages for the diagnosis of acute stroke patients who have no time to lose.The diagnostic CT examination for acute stroke generally requires three imaging techniques: non-contrast-enhanced CT of the head; CT perfusion (CTP); and CT angiography (CTA) of intracranial vessels. In these CT examinations the local skin dose in CTP might be considerably high. In a previous study we reported that the maximum skin dose was as high as 712 mGy, 13 times higher than that in non-contrast-enhanced CT of the head [5]. Although the latest multidetector-row CT (MDCT) scanners have a dose reduction function of intermittent scanning or shuttle scanning, detailed patient doses using the function have not been reported.The CTA technique has been developing with great potential owing to high scan speed and ...