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Standardization of Parameters in Radiography
for radiation protection in digital radiology
Marco LO, PhysicistM. HTA&M, CEng MIET
Joint Annual Scientific Meeting, Annual General Meeting Joint Annual Scientific Meeting, Annual General Meeting & Annual Dinner 2011& Annual Dinner 2011
Quality, Standard & Safety in RadiographyQuality, Standard & Safety in Radiography
16 April, 2011
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Justification - should only be used where it brings more good than harm
Optimization - doses should be kept as low as reasonable achievable (ALARA)
Dose limits to the individual
Three different principles are used for radiation protection in ICRP
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In radiation protection of the patient in x-ray diagnosis, the three principles introduced by the ICRP for occupational radiation protection should be applied also; it should be recognized, however, that in applying these principles a higher flexibility, compared to occupational radiation protection, is needed in order not to adversely affect the care for the patient in special situations.
THE ROLE AND DETERMINATION OF PATIENT DOSE IN X-RAY DIAGNOSIS
Flexibility in choice of exposure techniques
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EU Council Directive 97/43 EURATOMon health protection of individuals against the dangers of ionizing radiation in relation to medical exposure
“The optimization process shall include theselection of equipment, the consistent production of adequate diagnostic information as well as the practical aspects, quality assurance including quality control and the assessment and evaluation of patient doses.”
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The imaging decision
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Clinical problem Image quality Comment
Primary bone tumour
High Image may characterise the lesion
Chronic back pain with no pointers to infection or neoplasm
Medium Degenerative changes are common and non-specific. Mainly used for younger patients (e.g. less than 20 years of age, spondylolisthesis etc.) or older patients (e.g. more than 55 years of age)
Pneumonia adults: follow-up
Low To confirm clearing, etc. Not useful to reexamine patient at less than 10-day intervals as clearing can be slow (especially in the elderly)
What image quality (or diagnostic information) is neededfor a medical imaging task?
H P Busch and K Faulkner
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High Medium Low
Flat-panel (400) Flat-panel (800) Flat-panel (1600)
Storage-phosphor (200/400)
Storage-phosphor (400)
Storage-phosphor (800)
Film-screen (200) Film-screen (400) Film-screen (800)
Levels of image quality in term of speed class
simple variable speed (tailor exposure to exam) …. but more difficult to correctly use since the energy sensitivity of DR and CR is quite different than that of FS
H P Busch and K Faulkner
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•kVp compensation curves or set-up methods recommended for automatic exposure control
•recommended receptor dose for optimised images
http://www.mhra.gov.uk/home/idcplg?IdcService=SS_GET_PAGE&nodeId=263
MHRA keynote notice, “Radiation Dose Issues with Digital Radiography Systems” is more specific and states that a supplier should provide the following information with a digital radiography system:
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TG116 recommends avoiding the concept of “speed class” when referring to DR system performance. KTGT (Target Equivalent Air Kerma) values should be used to describe how one system may vary from another with respect to radiographs of a particular body part and view.
Recommended Exposure Indicator for Digital Radiography
Report of AAPM Task Group #116
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Speed and dose related metrics
Sv
mGy
Speed (Receptor dose uGy)
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appropriately independent of many details of use, such as the body part being examined and choice of collimation
proven useful for classifying and comparing the detector choices available to the radiologist
straight forward for the physicist in estimating the effect of a proposed detector change in dose to the patient population
The receptor dose needed to produce a specified display response (film density) as a measure of system speed are
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The speed class concept is widely used in CR and DR literatures
The speed concept is the starting point in transition from film/screen to digital radiology
Digital detectors are variable speed systems
Speed class can be conceptually used as the sensitivity of CR image receptor
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Why digital radiography Why digital radiography standardization?standardization?
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0.1 1 10 100 1000 0.1 1 10 100 1000
Receptor Dose (uGy)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
O.D
.
Receptor Dose (uGy)
250
500
0
750
1000
Pix
el V
alu
e
Quanta III/IOS 400Quanta Fast Detail/IOS 200Quanta Detail/IOS 60
Gamma
Latitude
Dynamic range - contrast relationship
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0.1 1 10 100 1000 0.1 1 10 100 1000
Receptor Dose (uGy)
SN
R
Receptor Dose (uGy)
SN
R
Dose - noise relationship
Quanta III/IOS 400Quanta Fast Detail/IOS 200Quanta Detail/IOS 60 non-quantum
limited region
Screens with phosphors that have the same conversion gain will have similar total noise levels, irrespective of their actual thickness.
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Maximization of image contrast can be independent of exposure dynamic range
More direct and efficient control of the trade-off between radiation dose and noise
The choice of pixel size for each application can be tailored to the tradeoff between noise and contrast resolution
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DR
Speed
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Signal contrast ratio, S
Pho
ton
dete
cted
in r
esol
utio
n ar
ea
Detectability and dose creep in digital X-rayMotz J W and Danos M. Image information content and patient exposureMed. Phys. 5 8-22, 1978
Visibility
Detectability
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The direct relationship between dose and film density, which is familiar from film/screen exposures, no longer exists in digital radiography
No consistent feedback to technologists concerning the use of optimal acquisition techniques
The reasons behind dose creep
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The wide dynamic range of a digital systems allow a high tolerance for variations in exposure techniques
Digital radiography could be seen as offering far greater opportunity for patient dose increase than decrease overall
Optimal / standard exposure techniques are needed to ensure the appropriate image quality at the lowest possible patient exposure
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amorphous silicon flat-panel detector
CR imaging platefilm screen
Frequency distribution of measured mAs forPA chest acquired on three imaging systems
Bacher K, Smeets P, Bonnarens K, De Hauwere A, Verstraete K, et al. Dose reduction in patients undergoing chest imaging: digital amorphous silicon flat-panel detector radiography versus conventional film-screen radiography and phosphor-basedcomputed radiography. AJR Am J Roentgenol 2003;181:923–9
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Optimize for human vision•signal contrast
•latitude•dynamic range (acquired vs. displayed)
•sharpness
•noise
Optimize for consistency•cassette erasure difficulties
•CR reader problems
•processing algorithm issues
•display monitor deviations
Optimize for distribution•image compression
•memory utilization
•network efficiency
Optimize for machine vision•CAD
•subtraction
•segmentation
Medical Image Processing – Many Goals
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1.0
0.8
0.6
0.4
0.2
0.00.0 0.2 0.4 0.6 0.8 1.0
FP
TP
Unsharply edged lung nodules
Hoeschen, C., Reissberg, S. and Dohring, W. The importance of optimizing the image processing for different digital x-ray detectors to get as much information as possible from the radiographs. Proc. SPIE 4682, 828–838 (2002)
ROC results of the optimization of post processing
Pathological
AZ, area under the ROC curves in the studies
Optimized spatial frequency filtering
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The detectable information in radiographs produced with digital systems is strongly dependent on the speed class and image processing used.
Inappropriate speed class would violate the ALARA principle while suboptimum image processing may lead to suppression of diagnostic information.
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Equipment design considerations and technical methods of reducing patient dose
Operational approaches to reduce unnecessary patient doses by the appropriate selection of radiological examination and technical parameters
Various aspects to the optimization of radiation protection in digital radiology ICRP 93
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Specify the medical imaging task
Determine the quality criteria
Standardize techniques and optimize processing in terms of the exposure required to produce the
specified response in the displayed image
Propose parameters in display-ready image to met quality criteria of the imaging task
Evaluate displayed
image
inconsistencies subquality
Standardization in radiography