Radiation Protection in Digital Radiology

IAEAInternational Atomic Energy Agency

Radiation Protection in Digital Radiology

Fundamentals of Digital Radiography L01

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Educational Objectives

• Explain how ordinary radiographic images can be captured in digital form

• Discuss the advantages and limitations of digital images

• Explain how the dissociation of acquisition and display in DR can contribute to unnecessary radiation exposure to patients

Radiation Protection in Digital Radiology L01 Fundamentals of Digital Radiography

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What is a digital image?

• An approximation of an analog image, with regard to• spatial information• contrast information

• A computer file composed of discrete picture elements, or pixels• location in file (array or matrix)

represents image position• numeric value represents signal

intensity

Etruscan Roman Mosaic circa 50BC


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Why would we want digital images?

• Availability • a digital image can be transmitted electronically

to distant locations and can exist simultaneously at multiple locations

• Flexibility• the appearance of a digital image can be

modified• Convenience

• a digital image can be stored electronically without occupying physical space


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Conventional screen-film radiography

• Radiation strikes intensification screen(s) producing fluorescence

• Fluorescent light exposes photographic film producing latent image

• Latent image is chemically developed to produce density in film

• Film density is viewed by trans-illumination

1895


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Developed film is effectively analogue

• Density is the result of many developed silver grains

• Grains in intensification screen are quite small

[1,0,0,2,3,4][1,0,0,2,3,4][1,0,0,2,3,4][0,0,0,0,0,0]

1 0 3 42


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Three approaches to digital radiography

I. Translate developed film into digital form.II. Capture the radiographic projection by

non-photographic method and digitize during development.

III. Capture the radiographic projection or its fluorescence directly in digital form.


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Method I: Film Digitization

• Video of transilluminated radiographs• “Camera-on-a-stick”• Low cost, low quality

• LASER film digitisers• Best quality• Expensive and involves periodic maintenance

• CCD film digitisers• Less cost than LASER, less maintenance, better

quality than camera-on-a-stick• Old problems of drift, noise, non-uniform

illumination, and veiling glare – mostly rectified


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Process of film digitization

• Light is directed onto a film

• Light passing through the film is measured

• Amount of light attenuated is converted into a digital code value

Bushberg, Seibert, Leidholdt, BooneThe Essential Physics of

Medical Imaging 2nd EdRadiation Protection in Digital Radiology L01 Fundamentals of Digital Radiography

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Fundamental limitations of film digitization

• Prone to artefacts• Labour intensive – an extra step

• Best quality achievable is limited by original screen-film image

How many of us maintain capability to digitize film?Radiation Protection in Digital Radiology L01 Fundamentals of Digital Radiography

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Method II: Non-photographic capture with digital development

• Xeroradiography • Charged Selenium plate• Electrostatic latent image• Charge distribution transferred to paper using

toner• Selenium drum detector

• Selenium deposited on cylindrical Al drum• Selenium uniformly charged before exposure• X-rays partly neutralize the charge• Charge distribution measured by electrometer

array


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Computed Radiography (CR) or Photostimulable Phosphor (PSP)

Radiography

• Latent image of trapped electrons is formed when x-rays hit the imaging plate

• Latent image is read out physically instead of chemical process

• As the latent image is read out…• Stimulated light emitted with the help of LASER is directed

to a Photomultiplier Tube (PMT)• The PMT signal is digitized using Analog-to-Digital

Converter (ADC)

• The digital image consists of an array of ADC Code Values• ADC Code values represent exposure information• Array locations represent spatial information


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Imaging plateImaging plate

Latent Image slow scanslow scan

fast scanfast scan

Light guideLight guide

Photomultiplier tubePhotomultiplier tube

Rotating polygon mirrorRotating polygon mirror

LaserLaser AmplifierAmplifier

Analog-to-Digital ConverterAnalog-to-Digital Converter

Photostimulable phosphor reader

??


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Characteristics of PSP systems

• Generally, but not exclusively, cassette-based systems

• Moderate initial capital investment• Simple retrofit to existing radiographic

equipment• Individual scanner can serve multiple

acquisition devices• Workflow comparable to daylight

loader film processing


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Method III: X-rays are converted immediately into digital signals without

latent image • Fluorescent screen with video camera (video-

fluoroscopy, image intensifiers)• Fluorescent screen with Charged-Coupled Devices (CCD)

or Complementary Metal Oxide Semiconductor (CMOS) array• Optical lens coupling

• Secondary quantum sink• Fiber optic coupling

• Small area

• Hydrogenated Amorphous Silicon (a-Si:H) with Thin Film Transistors (TFT)• alternative = switching diode• requires x-ray converter (Gadolinium oxysulphide or Caesium

Iodide)• Amorphous Selenium (a-Se) electronically coupled to TFT

“Fla

t pan

els

”


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Characteristics of “Direct” capture systems

+ Rapid acquisition and processing+ Typically integrated with x-ray

generator+ No mechanical scan mechanism—High initial capital investment—Challenging manufacturing

processes—Limited systems for bedside

radiography? Brief history of clinical operation

? Life cycle issues unknown (durability?)

? Image rendering unknown? Exposure factor issues

Courtesy JA Seibert, UC-Davis


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How good an approximation does the digital image make?

• Spatial information depends on …•Dimensions of the pixels (matrix size)•Blur

• Contrast information depends on …•Grayscales (Code values) per pixel (i.e.

quantization)•Characteristic function (Code values vs.

exposure)•Noise


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Effects of Pixel size and Bit-depth

1024 x 1024 64 x 64 32 x 32 16 x 16

For an image of M x N matrix, k bytes/pixel, the memory needed to store the image is k x M x N bytes

8 bits/pixel3 bits/pixel2 bits/pixel1 bit/pixel

Larger pixels

More bit-depth


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Practical resolution is less than the Nyquist frequency

• Factors besides sampling compromise sharpness; depends on• X-ray focal spot dimensions• Blur in Indirect DR and CR• Optical and mechanical imprecision

in IDR and CR• Afterglow in fast-scan dimension in

CR

• Limit of resolution is where Modulation Transfer Function (MTF) has decreased to 10%


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MTF of DR depends on more than just sampling


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Noise interferes with our ability to detect contrast

σ = √N

SNR = N/σ = √N

Air KERMA

(µGy)

Photons

/100mX100µm

Noise

(%)

9.0 1333 2.7

0.9 133 8.6

0.09 13 27.4

Bushberg, Seibert, Leidholdt, Boone The Essential Physics of Medical Imaging 2nd Ed


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Combination of quantum noise and anatomic noise limits low contrast

detection

DR Image CT Image


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Detective Quantum Efficiency (DQE) of DRCombines SNR and Resolution

(200 um)

(143 um)

(200 um)

Ideal detector


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0

0.5

1

1.5

2

2.5

3

0.1 1 10 100 1000

Air kerma (μGy)

Den

sity

(O

D)

1

10

100

1000

10000

Inte

nsit

y (r

el)

Film/screenPSL

1023

0 Raw Plate Exposure0.1 µGy 1000 µGy

High kVL=2.2, S=50Over-Exposed

Low kV, L=1.8, S=750Under-Exposed

ED

R S

igna

l

DR has wide dynamic range (latitude)

Histogram re-scaling


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Rescaling of DR images is a double-edged sword

• Variations in exposure factor selection are automatically compensated by rescaling for consistent appearance

• Inappropriately high or low exposures are not immediately apparent from the appearance of the image until the range-of-adjustment is exceeded• Underexposure makes noisy images• Overexposure makes crisp, noiseless images,

that are preferred by radiologists


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There is a documented tendency to overexpose in CR and DR

• Oversight of exposure factor selection is impossible without an exposure indicator

Barry Burns, UNC

Freedman et al. SPIE 1897 (1993),472-479.Gur D et al. Proc 18th European Congress of Radiology. Vienna Sep 12-17.(1993)154.


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What would be the appropriate exposure?

• Seibert, et al Acad Radiol (1996) 4: 313-318• QA based on exposure indicator reduces doses

• Willis Ped Radiol (2002) 32: 745-750• 33% dose reduction if exposure indicator target followed

• AAPM Task Group #116 is effort to standardize indicators


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Five ways of making a radiographic projection with lower dose to the

patient

• Each method has consequences with respect to the five aspects of image quality


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Important information about DR acquisition and processing is in

metadata

• CR vs. DX object• Mandatory vs.

optional vs. private tags

• Automatic vs. manual entry of data

• PACS interpretation of metadata


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New artefacts from the discrete nature

of DR

• Interference pattern between fixed grid lines and down-sampling rate for display

• Disappeared on zoom

• Bad choices• Display default

magnification factor

• Line rate of grid


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Photon Energy (keV)

0 20 40 60 80 100 120 140

0.01

0.1

1.0

Qu

a ntu

m E

f fic

ien

c y

BaFBr 100 mg/cm² (CR)

Gd2O2S:Tb 120 mg/cm2 (Lanex)

CsI:Tl 45 mg/cm2 (a-Si/CsI)

Martin Yaffe/Tony Seibert

A-Selenium 25 mg/cm2

X-ray energy sensitivity differs among detectors

Would you expect to use the same kVp?Radiation Protection in Digital Radiology L01 Fundamentals of Digital Radiography

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Emerging digital technologiesThe EOS Imaging system

32

Two simultaneous digital planar radiographs (PA and LAT) in the standing position by linear scanning of a fan-shaped collimated X ray beam from 5 cm to 180 cm (whole body).

Charpak Detector: Detector amplification by photon gaz cascade

High gain signal, sensitivity maximized

X ray tubes

Detectors

EOS allows for a dose reduction up to 10 times compared to CR


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Advantages of digital radiology

• Digital imaging has practical technical advantages compared with film techniques:•wide contrast dynamic range, •post-processing functionality, •multiple image viewing options, •electronic transfer, •electronic archiving possibilities.


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How to move from film-screen to digital?

• Training should be planned with anticipation.• Selection of equipment, connectivity and

quality control requires good advice (not only from the manufacturers) and visits to other installations.

• Patient dose and image quality should be audited carefully during the transition. The risk of increasing patient doses exists.


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How to move from film-screen to digital?

• Typically the initial setting for the automatic exposure control using CR, should be the same or similar to the existing with film-screen. Optimisation (including possible kV changes) and potential dose reduction should be initiated later once radiologists become familiar with digital.

• Audit DICOM header. It contains a lot of useful information.


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Checklist of practical advice (ICRP 93)

• When introducing a new digital system into clinical practice, the system should be set up to achieve the best balance between image quality and patient dose.

• Avoid deleting non diagnostic images at the workstation and carry out a statistical rejection rate analysis periodically.


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• Be familiar with your workstation capabilities (post-processing capabilities, options in the monitor to visualise the images, etc).

• Identify correctly all the images to avoid their loss in the PACS.

• Ask for a calibration of the automatic exposure control that is appropriate for the sensitivity range of the digital system and the selected post processing.


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• Avoid manual exposures if the automatic exposure control is usable. However, ensure it is used correctly, as only some possible errors are correctable by post processing.

• Control the number of images per examination to maintain it at a number similar (or lower) than that for conventional radiology.


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• Make available a workstation for radiographers to post-process images in order to avoid retakes.

• Pay attention to the dose indication on the panel of the x-ray system or in the in-room monitors and utilise the information to manage patient doses.

• Establish easy access to the PACS to review previous images, in order to avoid retakes.


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• Pay attention to proper collimation for the desired anatomical area. Once the image is acquired, numerical methods (i.e. software) may automatically crop part of the image, and when the image is received for reading the radiologist will not be aware that a larger anatomical area than necessary was irradiated.

• Select the correct pre-programmed technique e.g. using an abdominal technique (70-80 kV) for chest imaging (120-130 kV) will result in a higher entrance surface dose.


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Summary• A digital radiographic image is a matrix of numbers with discrete

physical pixel dimensions and gray-levels.• The discrete nature of the DR image is the source of its

advantages and limitations• Digital radiographs can be produced by three methods:

• Digitization of screen-film radiographs• Non-photographic capture and digitization• Direct capture with or without conversion to light

• DR technologies have advantages of availability, flexibility, and convenience over conventional screen-film

• The utility of the DR image is enhanced by demographic, exam, and processing information in the DICOM header

• Except for digitized radiography, DR has the potential for unnecessary patient radiation exposure


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Answer True or False

• A digital image contains numeric value representing signal intensity.

• In computed radiography (CR), the electronic latent image is developed by chemical process

• Amorphous selenium is used in direct capture flat panel detectors


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Answer True or False

• True. A digital image is a computer file composed of discrete elements or pixels representing signal intensity.

• False. The electronic latent image in the CR plate is read out using LASER

• True. Amorphous selenium (aSe) is electronically coupled to the thin film transistor (TFT) in a direct capture flat panel detector.


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References

• Managing patient dose in Digital Radiology ICRP Publication 93 Ann ICRP 2004 Elsevier.


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