Which milliampere-seconds (mas) setting is most likely to produce quantum mottle?

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Introduction
Implications for practice
Introduction
Section snippets
Conflict of interest statement
Acknowledgements
Cited by (6)
Recommended articles (6)
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What is the effective solution to reduce quantum mottle?
Why is it important for the radiographer to observe the milliampere seconds mAs readout at the end of each exposure when using automatic exposure control AEC )?

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Abstract

Introduction

Radiographers have a duty to ensure that radiation doses to patients are as low as reasonably achievable. With digital technologies, exposure factors which achieve the optimum balance between image noise and patient dose must be sought. In digital radiography, Deviation Index (DI) values provide the radiographer with feedback on the appropriateness of individual exposures but can also be tracked as part of a departmental quality assurance programme.

Methods

In November 2017, exposure logs were extracted from six digital radiography (DR) x-ray systems, collated and analysed. Five examinations were identified which frequently produced DI values outside the manufacturer's recommended Optimal Range (-3 to +2). Incremental improvements were made to the default exposure settings for these examinations via a cyclical process of modification and re-evaluation. A full data collection exercise was then repeated in April 2019.

Results

At baseline, 10,658 out of 29,637 (36.0%) exposures had DI values outside the manufacturer's recommended Optimal Range, but for some individual examinations the proportion was as high as 547 out of 725 (74.5%). Following multiple optimisation cycles, the overall proportion of examinations outside the Optimal Range had fallen to 7611 out of 26,759 (28.4%). Default milliampere-seconds (mAs) values for these examinations were reduced by between 22% and 50%.

Conclusion

A marked reduction in patient doses can be achieved through a departmental programme of DI value monitoring and targeted optimisation of default exposure settings.

Implications for practice

DI values should be routinely monitored as part of routine quality assurance programmes. Radiographers have a responsibility to ensure that they possess a clear understanding of DI values and that appropriate exposure settings are selected for each individual patient.

Introduction

Radiographers have a duty to ensure that radiation doses to patients are as low as reasonably achievable (ALARA). However, in clinical practice, radiographic exposures are often greater than required to achieve adequate image quality.1 This may be due to default exposures that are set too high,2 failure by radiographers to adjust exposure factors based on individual patient size or a gradual increase in average exposures over time, frequently referred to as dose creep.1,3, 4, 5, 6, 7

The wide dynamic range of Computed Radiography (CR) and Digital Radiography (DR) permits apparently satisfactory images to be produced over a wide range of exposures. Underexposed digital images exhibit significant quantum mottle and appear noisy.6 Increasing the exposure provides images with reduced noise. As exposure rises, however, diminishing improvements are obtained from further increases in dose.8 Only when an image is grossly overexposed does the digital detector become ‘saturated’ and image quality degraded.9 Patients may therefore be exposed to doses several orders of magnitude greater than necessary before impairment of the resultant images becomes evident.10

Exposure factors which achieve the optimum balance between image quality and patient dose must be sought; an approach referred to as optimisation. To provide the radiographer with feedback on the appropriateness of their exposure CR and DR equipment usually provides an estimate of the quantity of radiation incident on the image receptor, generically referred to as the Exposure Index (EI).

Proprietary EI metrics (e.g. LgM, REX, EXP, S-value, Sensitivity Number or DEI) have now largely been replaced by a universal standardised Exposure Index.11,12 Under this system each examination type is assigned a target Exposure Index (EIT) value which is considered to represent the optimal balance between dose and image quality. The EIT may be defined by either the manufacturer or the clinical site. The Deviation Index (DI) then provides a simple numerical indication of the extent to which the EI value of an exposure deviates from the intended EIT for that examination.13 EI values equal to the EIT produce a DI value of zero.14 EI values greater than the EIT result in a positive DI value and those less than the EIT result in a negative DI value (Fig. 1). Selection of appropriate EI targets is essential for DI values to be meaningful.

Previous studies conducted in clinical departments have reported high proportions of exposures with EI or DI values outside of the target range recommended by the manufacturer.3,7,16, 17, 18, 19, 20, 21, 22 In some cases examinations were identified where more than 50% of the exposures fell outside of the recommended range.3,7,19,21 The evidence base relating to EI values in the United Kingdom (UK), however, is very limited.23

This study investigated whether DI values collected on a department-wide basis could provide valuable feedback to support dose optimisation activities.

Section snippets

Methods

Full Research Ethics Committee approval was not required for this study. Local approval was granted by the hospital Research and Development department under Governance Arrangements for Research Ethics Committees (GafREC) provisions.

The study involved six Discovery XR656 DR X-ray systems (GE Healthcare, UK) at a large acute tertiary teaching hospital in the UK which undertakes over 190,000 radiographic examinations per year. The systems are used for both adult and paediatric patients, serving

Baseline

Logs relating to 30,000 exposures were extracted as baseline data. 324 exposures that had DI values less than −10 (262) or greater than 10 (62) were excluded. The DI value was missing for 39 exposures. 29,637 sequential exposures (98.8%) were included in the subsequent analysis. Approximately one-third of the exposures (10,658 of 29,637) resulted in a DI value outside the Optimal Range; 2735 (9.2%) were underexposed and 7923 (26.7%) overexposed (Table 3).

19.5% (1618 of 8294) of Automatic

Discussion

This study has provided a snapshot of the DI values achieved within a UK imaging department and demonstrated that a monitoring programme for DI values can support the optimisation of default exposure parameters. In keeping with previous studies, the pre-optimisation data revealed a high proportion of DI values outside the Optimal Range.3,7,16, 17, 18, 19, 20, 21 Direct comparisons with the results of previous studies is problematic due to differing examination types included, definitions of

Conclusion

Radiographers have a responsibility to understand DI values and how they can be used to inform the choice of appropriate exposure settings for individual patients. In this study exposure optimisation activities were conducted for five body areas using feedback provided by DI data collected on a department-wide basis. The study demonstrated that notable dose reductions could be achieved through a departmental programme of DI value monitoring and targeted modification of default exposure

Conflict of interest statement

None

Acknowledgements

The authors would like to sincerely thank Mr Peter Ekwo for his assistance with data collection and Mr Nicholas Woznitza of the College of Radiographers (CoR) Formal Radiography Research Mentorship (FoRRM) scheme for his assistance in the preparation of this manuscript.