Dose reduction and quality criteria in computed tomography

Measurement of image quality parameters has been carried out and assessment made of the limits for acceptable quality criteria in computed tomography (CT). A further evaluation of the methods for dosimetric measurements of CT scanners has been performed, by comparison of free in air and phantom dose measurements for new CT systems both with thermoluminescence (TL) and with a pencil shaped ionisation chamber. Calculations of organ doses and an assessment of the collective effective dose from CT to the Danish population were performed on the basis of a 1989 survey. The highly collimated X-ray beams in CT result in steep dose gradients resulting in large variations in organ doses. For a thorax examination performed on an Alderson phantom the location of the upper slice was changed by 2 cm. Thermoluminscence measurements in the thyroid, the sensitive organ in the region, showed a factor 3 increase in dose and this caused an increase in the calculated effective dose of 17% from 6.5 mSv to 7.6 mSv for a standard thorax examination. The collective effective dose from CT examinations in Denmark was estimated to be 250 man Sv giving 0.05 mSv per head or 5-8% of the collective effective dose from conventional X-ray examinations.