Assessment of patient doses from CT scanning

Abstract

Computed Tomography (CT) is an x-ray imaging technique introduced into clinical practice in the early 70's and is now considered an essential element in radiological imaging.

Notwithstanding the undoubted benefits of CT in health care, growth of the techniques has taken place without full appreciation of the relatively high patient doses involved. A national survey carried out in the UK in 1989 indicated that while CT represented about 2% of the total of all x-ray examinations, it accounted disproportionately for 20% of the resultant collective dose (Shrimpton et al. 1991). Recent studies (Shrimpton et al. 1993) suggest that CT may now represent about a third of the collective dose from diagnostic x-rays in the UK. There has been a similar proliferation of scanners throughout the world, such that the average frequency of CT examination in developed countries has been reported to be about 10 per 1000 population (Shrimpton and Wall 1992). Therefore CT practice is causing significant concern and based on the ALARA principle and other advisory bodies' recommendation (NRPB 1990), the doses to patient from this x-ray modality should periodically be monitored and kept within guideline doses.

The most common CT dosimetry approaches are standard method, Monte Carlo (MC) method and direct method. In the standard method CTDI is assessed inside a head and a body perspex phantom giving an index of the CT dose efficiency. This is used mainly for QA purposes. In the MC method the patient doses are estimated by applying this mathematical technique to simulate the interaction of CT generated radiation with matter inside a mathematical model of phantom. In the direct method, measurement of organ doses is carried out directly inside an anthropomorphic physical phantom.

We have developed the direct method by assessing CT axial and longitudinal dose distribution in an anthropomorphic phantom. Measurement of these dose distribution for any scanner allows us to quickly measure organ doses and effective dose, and hence patient doses for any examination protocol with a limited number of IV TLDs. This work describes the underlying principles and assumption of the direct CT dosimetry method we have developed and implemented. In addition the review of CT dosimetry provides a comprehensive and critical analysis of the methods developed to date. All dosimetry related concepts for the developed direct technique are considered.

Comprehensive dose measurements were carried out to estimate the level of error involved in the developed approach. The MC approach was also used to compare the results of our proposed direct method with this commonly used method in CT dosimetry for routine examinations of some scanners.

The proposed direct CT dosimetry method has now been used for several scanner models. The developed dosimetry method has been successfully implemented to assess the radiation doses resulted from various examination protocols using the recent modern CT modalities, that could not easily be investigated by other dosimetry approaches. We believe that the developed direct CT dosimetry approach overcomes many limitations imposed by other common approaches. It also provides a reliable and practical method for the assessment of patient doses from CT practices for a wide range of scan protocols. Another major advantage of the developed direct dosimetry method is that, it could easily and independently be adapted and implemented for any upcoming new model of CT scanner to be introduced into clinical practice.