COMPARISON OF IMAGE QUALITY AND DIFFERENT RADIATION DOSES IN COMPUTERIZED TOMOGRAPHIC DIAGNOSTICS

Abstract

Computerized tomography (CT) is one of the most important diagnostic modalities, whose use is growing from decade to decade. In the total number of radiological examinations CT is represented by 5-10%, but its contribution to the total received dose for the population is higher than 50%. Basic image quality indicators and radiation doses as well as the associated radiation risk depend on the applied radiographic technique or CT scan. In terms of good radiological practice and in accordance with the basic principles of radiation protection, it is desirable to apply the lowest possible dose to the patient while maintaining image quality and diagnostic information. The aim of the research is to define the optimal multislice CT scan protocols for the diagnosis of individual body regions, as well as to determine dose and radiation risk for patients before and after protocol optimization. The analysis included a total of 312 patients, divided into groups according to the regions of the body that were recorded: I - CT head, II - CT chest, III - CT chest and upper abdomen, IV - CT abdomen. The study has been conducted in 2 phases: in the first phase standard protocol for the scanned body region has been applied, and in the second phase CT examinations have been carried out according to the modified protocol (changing the values mAs and kV) with minimum requirements regarding the image quality. On the basis of dosimetric indicators the efficient dosage and irradiation risk for the patients in both phases have been assessed. In the study the guidelines from the Guide EUR 16262 EN (1) have been observed where parameters for the assessment of image quality have been defined in order to analyze different anatomic cross sections of certain body regions. Image quality for each patient was assessed by three-level visualization scale for each parameter of anatomic region: 0 – details are visible, 1 – details are presented, 2 – details are clearly presented. A subjective method was applied where two experienced radiologists performed the image interpretation. Final assessment of image quality of every examination corresponds to the sum of all parameters according to three-level visualization scale. Further, for the need of calculation of the size of FOM (figure of merit) the value of the index of assessment of the image quality (sum of all assessments of parameters/number of parameters) has been calculated. The value of FOM has been calculated as a quotient of the image quality assessment index and effective dosage per patient. The average value of FOM for every group of patients has offered us a relative indicator for comparison of non-optimum and optimum group of patients for the same type of examinations. By comparison of values of effective dosage in the first and second phase of the research a decrease in irradiation load for patients after protocol optimization was quantified. The results have shown that by optimum protocol selection in the sense of exposition parameters (by increasing and decreasing the value of mAs and kV) it is possible to reduce significantly the irradiation dosage at head CT examination for 5%, at chest CT examination for 2%, at chest and upper abdomen CT examination for 6% and at abdomen CT examination for 8%. By applying standard protocols, a high quality image is achieved that is sufficient for adequate radiological interpretation, and therefore a higher radiation dose than is needed. Optimal selection of protocols in terms of exposure parameters may significantly reduce the radiation dose, with better quality of diagnostic images necessary for further radiological interpretation.