1

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

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

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 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.”

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)

What image quality (or diagnostic information) is neededfor a medical imaging task?

H P Busch and K Faulkner

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

•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:

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

Speed and dose related metrics

Sv

mGy

Speed (Receptor dose uGy)

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

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 Why digital radiography standardization?standardization?

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

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.

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

DR

Speed

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

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

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

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

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

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

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.

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

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