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AN OVERVIEW OF OPTIMISED DOSE MEASUREMENTS IN CONVENTIONAL

DIAGNOSTIC RADIOLOGY

Refuoe Pepenene, Sibusiso Jozela

Test & Measurements conference 2017 31 July 2017

• Introduction

• Effects of ionizing radiation

• The role of dosimetry in diagnostic radiology

• Why dose measuring instruments do not always agree?

• Background (Quantities used to estimation and quantify organdose, specification of x-ray beam qualities, Establishment ofradiation beam qualities)

• Experimental, results and conclusion

Overview

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• Across South Africa, millions of medical radiographic proceduresare performed on patients.

• Patient are expose to a wide range of radiation doses fromvarious imaging modalities.

• Although these procedures are beneficial, the potential forradiation-induced injuries to the patient exists.

• International studies have shown that patient dose duringdiagnostic examinations from one health facility to another mayvary by a factor of 100.

• Hence, for ethical and efficient use of clinical imaging, thecontrol of doses and the optimization of X-ray imaging systemsis fundamental.

Introduction

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• Two categories (Non-stochastic and stochastic effects)

• Deterministic (Non-stochastic) effects - occur only when theradiation dose exceeds a certain threshold e.g. Skin Erythema,Cataract, sterility.

• Stochastic effects –may be induced at any dose (probabilities)e.g. Cancer and heredity defects.

• Although, doses from radiological procedures are generallylower than the level required to produce deterministic effects,these procedures may result to the stochastic effects

Effects of ionizing radiation

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• Asses dose received by the patient

• Asses equipment performance

• Establish Diagnostic Reference Levels (DRLs)

• Setting and checking standards of good practice

• Used to determine the risks so that diagnostic techniquescan be properly justified

• Assist in cases where accidental overexposure may berequired to be investigated

Role of dosimetry in diagnostic radiology

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• Energy response

• Rate dependence (Detector non-linearity & Electronic non-linearity)

• Thresholds (do measurements starts and end on the same basis)

Why dose measuring instruments do not always agree?

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• The Effective dose is considered a good indicator of radiologicalrisk

• It is a risk-weighted measure of radiation to organs in the bodyassociated with radiological examination.

• The estimation of organ doses from conventional radiographyprocedures requires free-in-air entrance surface dose (ESD)measured.

• ESD can be measured directly using thermoluminescentdosimeters or computed from measurements made withionization chamber

• To quantify the patient dose during radiological examination –Energy imparted is used (Transmission ionization chamber).

• Measurements need to be traceable through an unbroken chainof comparison to national and international standard.

Background

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• NCRP guidelines - diagnostic radiography units operated at tube potential above 70 kVp are required by regulation to contain a total filtration e.g. tube wall and aluminium equivalent added filtration.

• This filtration absorbs the low-energy X-rays in the beam which would otherwise increase the dose received by the patient.

• The most complete specification of the x-rays beam quality is given by their spectral distribution.

• Alternatively, radiation beam and second half-value layers. qualities maybe specified in terms of the tube voltage and the first

Background

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• Generally, the radiation qualities used for the calibration of diagnostic detectors are categorized into 5 qualities, namely RQR, RQR-M, RQA, RQA_M and RQT.

• These qualities are established according to the International Code of Practice, Technical Reports Series No. 457, IAEA, Vienna (2007) which is based on IEC 61267 (IEC, 2004)

• The first step in realizing the X-ray beam qualities is to establish the RQR series, which represents the X-ray beam incident on the patient in general radiography, fluoroscopy and dental applications

Radiation Beam Qualities

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Cont….

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Radiation quality Radiation Origin Phantom Material Application

RQR Unfiltered beam No phantomGeneral radiography, Fluoroscopy, Dental

RQR-M Unfiltered beam No Phantom Mammography

RQA Added filter AluminiumBehind patient (on Image intensifier

RQA-M Added filter AluminiumBehind patient (on Image intensifier

RQT Added filter Copper Computer tomography

• The instruments were calibrated against air kerma standard, free in air using X-ray beam produced from a constant potential generator

Experimental

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• The UUT tends to measure either slightly higher or lower than the standard

Results

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• Detector no 1, no2 and no 5, uncertainty of measurement (k=2)was 3%.

• Detector no.3 and no.5 uncertainty was 5% and 2%.

Results

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• The calibration factor for some detectors tends to deviatemore from unity, a behaviour attributed to energydependency of the detectors.

• This may induce errors in cases where the detector is onlycalibrated at the specific beam qualities and the interpolationis utilised to determine the factors for intermediate qualities.

• Calibration factors confirm that some of the detectors mayresult in maximum error of 6% in dose measurements.

Results cont…

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• Through traceable calibration, the optimization of dosemeasurements for national healthcare facilities may beachieved.

• Standardized radiation dose will reduce the inconsistences indiagnostic radiation dose measurements by national healthfacilities

• This will have a positive effect in establishing the diagnosticreference levels and optimization of patient protection.

Conclusion

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• R.A Parry et. Al. The AAPM/RSNA physics tutorial for residents, Imaging & Therapeutic Technology, 1999

• J. Vassileva, M Rehani, Diagnostic Reference Levels, AJR, January 2015

• Ernest K. Osei, Johnson Darko, A Survey of Organ Equivalent and Effective Doses from Diagnostic Radiology Procedures, ISRN Radiology, Volume 2013, 2012

• Medical x-ray, electron beam, and gamma ray protection for energies up to 50 MeV (equipment design, performance and use). NCRP report no 102. Bethesda, Md: National Council on Radiation Protection and measurements, 1989.

• Technical Reports Series no. 457, Dosimetry in Diagnostic Radiology: An international Code of Practice

References

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