Radiation dose issues in digital radiography systems

The widespread use of Computed Radiography (CR) and the more recent introduction of Direct Digital Radiography (DDR) has brought a number of advantages to radiology departments. Digital radiology systems usually have a much larger dynamic range than film screen systems and so can accept a much wider range of exposures. This has the advantage that the number of repeat examinations due to under or over exposure may be reduced. Also, such systems should allow better dose optimisation for individual examinations. However, the wide exposure dynamic range of such systems may have the disadvantage that, if the x-ray generator Automatic Exposure Control (AEC) develops a fault or the output calibration drifts, the dose increase/decrease may not be identified readily. Also, the wide exposure dynamic range means there is significant potential for the initial set-up of such systems not to be optimised.

The implementation of the following recommendations will help to minimise the above mentioned disadvantages:

1. The company supplying new digital equipment should supply information on the recommended receptor exposures, which give diagnostic images with the lowest possible patient dose for each particular examination.

2. This figure for the patient dose, should be proved with an in-beam phantom during hand-over; the manufacturer should also specify the phantom parameters. These dose values could then be adjusted at each local site such as the practice applied to Image Intensifiers. It is important for departments to set up QA systems to routinely monitor factors including clinical exposure constancy, imaging system sensitivity and (where applicable) AEC performance (see paragraph 5).

3. Digital radiography systems may have different x-ray energy responses to film screen systems. Therefore the generator's Automatic Exposure Control (AEC) compensation characteristics should be different from that used for film screen systems.

For existing systems which have been upgraded to use DDR or CR, the existing AEC compensation characteristics will need reprogramming. X-ray equipment manufacturers should be in a position to supply any necessary generator firmware, software or hardware upgrades (there may be a cost associated with this).

Mobile X-ray sets should not be affected, as most do not normally use Automatic Exposure Control; however departmental exposure protocols should be adjusted to reflect the different characteristics of the digital image receptors.

4. Each image, whether produced on film or soft copy display, should ideally have an associated number to indicate the level of exposure to the detector. Currently all CR systems have a sensitivity index which is related to detector exposure, however, DDR systems are generally not supplied with this feature.

5. Once CR and DDR are in use, the constancy of applied exposure factors can be monitored on a regular basis when checked in accordance with new IPEM guidelines.

The MHRA suggests that readers refer to reports from the British Institute of Radiology: 'Quality Assurance in the Radiology Department'; and the IPEM's Report 77: 'Recommended Standards for the Routine Performance Testing of Diagnostic X-ray Imaging Systems'. 

The MHRA would like to thank IPEM and the NHS-PaSA evaluation centre KCARE for their help in preparing this guidance.

For Technical Assistance on this subject, please contact:

Alistair Mackenzie
Principal Physicist, KCARE
Tel: 020 7346 1625
E-mail: info@kcare.co.uk

Cliff Double
Senior Medical Device Specialist, MHRA
Tel: 020 7084 3039
E-mail: cliff.double@mhra.gsi.gov.uk




Page last modified: 05 September 2006