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Dose reduction and quality criteria in computed tomography

Obiettivo


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.

Optimization studies have been carried out on paediatric computed tomography (CT) examinations of the brain. Patients were scanned according to standard procedure followed by 2 additional scans with lower radiation doses (50-75% reduction) and images analyzed by at least 2 radiologists. Clinical information was kept in more than 95% of the images produced with lower doses. Images with lost information are being analyzed. The examination dose required was found to increase with age.

Effective doses from CT examinations were estimated for the adult Portuguese population from a survey carried out in 1990-1991. The effective doses due to examinations of head, chest, abdomen and pelvis were respectively: 1.7 5.6 7.3 5.1 mSv, with a mean effective dose per CT examination of 4.5 mSv. The collective effective dose for the Portuguese population (9.8 million) was 1009 man Sv or 0.10 mSv per caput.

Quality control was also carried out on 4 CT installations equipped with different scanners over a 10 month period.
Quality criteria for computed tomography (CT) will be defined based on the assessment of the influence of the practical implementation of quality assurance programmes on patient doses during the various CT examinations. There is indication that for some CT examinations the doses are considerable higher than can be justified by the demand for an acceptable image quality, and which may be caused by the extended use of nonoptimized standard procedures. The relevance of quality assurance for optimization in paediatric CT examinations will be demonstrated.

CT based on TLD and ionization chamber dosimetry will be developed. Dosimetric intercalibration for actual CT geometries and radiation beams will be performed between the contractors.

Both contractors have performed nationwide surveys of the clinical use of CT examinations in their respective countries (Portugal and Denmark) and performed dose measurements free-in-air in the isocentre of the CT scanners for scanning parameters used in routine examinations. These databases will be used for calculation of organ doses, taking into account the updated conversion factors available from other contractors and for calculation of collective effective dose equivalent from CT. Results from such calculations will be compared with existing results in order to make an assessment of the influence on doses from prepatient filtration and other parameters.

Part 1
Work will be undertaken towards an agreement upon objective measurements of image quality parameters (dose, noise, spatial and contrast resolutions etc.) and the assessment of the limits for such measurements in order to establish acceptable quality criteria for CT images.
A further evaluation of the methods for dosimetric measurements of CT scanners will be performed.

Part 2
The work will be developed in order to optimize some CT paediatric examinations and to investigate the role of quality assurance on patient doses and images quality.
Research will be carried out in radiology and neuroradiology departments of Lisbon hospitals with the aim of defining the minimum acceptable level of image. Relationships between image visibility of anatomical structures and details, technical factors and doses will be studied as well as their contribution to the improvement of those examinations.

Argomento(i)

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Invito a presentare proposte

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Meccanismo di finanziamento

CSC - Cost-sharing contracts

Coordinatore

AARHUS KOMMUNEHOSPITAL - AARHUS UNIVERSITY HOSPITAL
Contributo UE
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Indirizzo
44,Noerrebrogade 44
8000 AARHUS C
Danimarca

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Costo totale
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Partecipanti (1)