Transcript Quality, Standard & Safety in Radiography (ASM 2011)

Joint Annual Scientific Meeting, Annual General Meeting
& Annual Dinner 2011
Quality, Standard & Safety in Radiography
16 April, 2011
Standardization of Parameters in
Radiography
for radiation protection in digital
radiology
Marco LO, Physicist
M. HTA&M, CEng MIET
1
Three different principles are used for radiation
protection in ICRP
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
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
EU Council Directive 97/43 EURATOM
on health protection of individuals against the dangers of ionizing
radiation in relation to medical exposure
“The optimization process shall include the
selection 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.”
The imaging decision
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)
H P Busch and K Faulkner
What image quality (or diagnostic information) is needed
for a medical imaging task?
Levels of image quality in term of speed class
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)
H P Busch and K Faulkner
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
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:
• 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
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
mGy
mSv
Speed (Receptor dose uGy)
Speed and dose related metrics
The receptor dose needed to produce a
specified display response (film density) as a
measure of system speed are
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 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
Why digital radiography
standardization?
Quanta III/IOS 400
Quanta Fast Detail/IOS 200
Quanta Detail/IOS 60
Latitude
3.0
2.5
O.D.
2.0
Gamma
1.5
1.0
Pixel Value
1000
500
250
0.5
0.0
0.1
750
1
10
100
Receptor Dose (uGy)
1000
0
0.1
1
10
100
Receptor Dose (uGy)
Dynamic range - contrast relationship
1000
Screens with phosphors that have the same conversion
gain will have similar total noise levels, irrespective of
their actual thickness.
Quanta III/IOS 400
Quanta Fast Detail/IOS 200
Quanta Detail/IOS 60
SNR
SNR
non-quantum
limited region
0.1
1
10
100
Receptor Dose (uGy)
1000
0.1
1
10
100
Receptor Dose (uGy)
Dose - noise relationship
1000
Maximization of image contrast can be
independent of exposure dynamic range
More direct and efficient control of the tradeoff between radiation dose and noise
The choice of pixel size for each application
can be tailored to the tradeoff between noise
and contrast resolution
DR
Speed
Photon detected in resolution area
Visibility
Detectability
Signal contrast ratio, S
Detectability and dose creep in digital X-ray
Motz J W and Danos M. Image information content and patient exposure
Med. Phys. 5 8-22, 1978
The reasons behind dose creep
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 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
film screen
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 filmscreen radiography and phosphor-based
computed radiography. AJR Am J Roentgenol
2003;181:923–9
CR imaging plate
amorphous silicon
flat-panel detector
Frequency distribution of measured mAs for
PA chest acquired on three imaging systems
 Optimize for human vision
• signal contrast
•latitude
•dynamic range
(acquired vs. displayed)
• sharpness
 Optimize for consistency
• cassette erasure difficulties
• CR reader problems
• processing algorithm issues
• display monitor deviations
• noise
 Optimize for machine vision  Optimize for distribution
• CAD
• image compression
• subtraction
• memory utilization
• segmentation
• network efficiency
Medical Image Processing – Many Goals
Optimized spatial frequency filtering
TP
Pathological
1.0
0.8
0.6
Unsharply edged lung nodules
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
AZ, area under the ROC curves in the studies
FP
ROC results of the optimization of post processing
Hoeschen, C., Reissberg, S. and Dohring, W. The importance of optimizing the image processing for different digital xray detectors to get as much information as possible from the radiographs. Proc. SPIE 4682, 828–838 (2002)
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.
Various aspects to the optimization of radiation
protection in digital radiology ICRP 93
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
Specify the medical imaging task
Determine the quality criteria
Propose parameters in display-ready image
to met quality criteria of the imaging task
Standardize techniques and optimize processing
in terms of the exposure required to produce the
specified response in the displayed image
inconsistencies
Evaluate
displayed
image
subquality
Standardization in radiography