Transcript IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L 8: Factors affecting image quality IAEA International.

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
L 8: Factors affecting image quality
IAEA
International Atomic Energy Agency
Introduction
A review is made of:
• Definitions of image quality
• The factors that affect image quality
• The common image quality related problems
encountered by radiologists in routine
practice
• The image criteria concept as a tool to help
to achieve good image quality with the use
of low radiation dose per radiograph
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Topics
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Image quality evaluators
Image contrast
Blur or lack of sharpness
Distortion and Artifacts
Image noise
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Overview
• To become familiar with the factors that
determine the image clarity and the way the
image quality can be improved
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 8: Image quality
Topic 1: Basic Image Quality Evaluators
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Imaging quality
• Efficient diagnosis requires
• acceptable noise
• good image contrast
• sufficient spatial resolution or image sharpness
• These factors are linked
• “Objective” measurement of quality is
difficult
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Factors affecting image quality
Contrast
Blur or
Unsharpness
Image quality
Distortion
& artifact
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Noise
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Image quality evaluators/descriptors
• Basic evaluators
• Contrast
• Resolution
• Noise
Contrast
detail
Signal-to-noise
Wiener spectra
• Linking evaluators
analysis
Ratio S/N
• Modulation transfer
Rose model
• Signal-to-noise ratio
CONTRAST
RESOLUTION
• Wiener spectra
ROC analysis
• Overall evaluators
• Contrast detail analysis
Modulation transfer
• Rose Model
Function MTF
• ROC analysis
NOISE
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 8: Image quality
Topic 2: Image contrast
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Image contrast
Low
Contrast
Medium
Contrast
High
Contrast
Image contrast refers to the difference in optical density
between two regions of an image
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Some factors influencing contrast
• Radiographic or subject • Image contrast
contrast
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Tissue thickness
Tissue density
Tissue electron density
Effective atomic number Z
X Ray energy in kV
X Ray spectrum (Al filter)
Scatter rejection
• Collimator
• Grid
• …
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• The radiographic contrast
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plus :
Film and processing
characteristics
Screen characteristics
Windowing level of CR, DR,
CT, and DSA
…
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Technique factors (1)
• Peak voltage value has an influence on the
beam hardness (beam quality)
• It has to be related to medical question
• What is the anatomical structure to investigate?
• What is the contrast level needed?
• For a thorax examination : 130 - 150 kV is suitable to
visualize the lung structure
• While only 65 kV is necessary to see bone structure
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Technique factors (2)
• The higher the energy, the greater the penetrating
power of X Rays
• At very high energy levels, the difference between
bone and soft tissue decreases and both become
equally transparent
• Image contrast can be enhanced by choosing a
lower kVp so that photoelectric interactions are
increased
• Higher kVp is required when the dynamic range is
high (chest)
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X Ray penetration in human tissues
60 kV - 50 mAs
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70 kV - 50 mAs
80 kV - 50 mAs
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X Ray penetration in human tissues
Improvement of image contrast
(lung)
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Technique factors (3)
• The mAs controls the quantity of X Rays (intensity
or number of X Rays)
• X Ray intensity is directly proportional to the mAs
• Over- or under-exposure can be controlled by
adjusting the mAs
• If the film is too “white”, increasing the mAs will
increase the exposure and optical density
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X Ray penetration in human tissues
70 kV - 25 mAs
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70 kV - 50 mAs
70 kV - 80 mAs
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Receptor contrast
• The film as receptor has a major role to play in
altering the image contrast
• There are high contrast and high sensitivity films
• The characteristic curve of the film describes the
intrinsic properties of the receptor (base + fog,
sensitivity, mean gradient, maximum optical
density)
• N.B.: Film processing has strong effect on fog and
contrast
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LCD Display
• The LCD display is commonly used in fluoroscopy
and digital imaging
• The display adds flexibility in the choice of image
contrast
• The dynamic range of the monitor is limited (limitation in
displaying wide range of exposures)
• Increased flexibility in displaying image contrast is
achieved by adjustment of the window width and
level, or gray levels, of a digital image
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Contrast agents
• Nature has provided limited contrast in the body
• Man-made contrast agents have frequently been
employed to achieve contrast when natural
contrast is lacking (iodine, barium)
• The purpose is to produce contrast differences
between vessels or potential spaces, e.g., the GI
tract, and surrounding tissue
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 8: Image quality
Topic 3: Blur or lack of sharpness
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Blur or lack of sharpness
• The boundaries of an organ or lesion may be
very sharp but the image shows a lack of
sharpness
• Different factors may be responsible for such a
degree of unsharpness or blurring
• The radiologist viewing the image might express
an opinion that the image lacks “detail” or
“resolution” (subjective reaction of the viewer to
the degree of sharpness present in the image)
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Resolution
• Smallest distance that two objects can be
separated and still appear distinct
• Example of limits
• Screen-film system: 0.01 mm
• CT: 0.5 mm
• Other definition: “Point-spread” function
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Characteristic of a “point” object
Point object expected to be point in image
Blurring due to imperfections of imaging system
Measurement: full-width-at-half-maximum (FWHM)
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Factors affecting image sharpness
Subject
Unsharpness
Geometric
Unsharpness
Image
Unsharpness
Motion
Unsharpness
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Receptor
Unsharpness
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Geometric blur
• If the focal spot is infinitesimally small, the blur is
minimized because of minimal geometric
unsharpness
• As the focal spot size increases, the blur of the image
increases
Small focal spot
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Large focal spot
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Geometric blur
• Another cause of geometric unsharpness is the
distance of the receptor from the object
• Moving the receptor away from the object results
in an increased lack of sharpness
• N.B.: The smaller the focal size and the closer the
distance between the object and the image
receptor, the better the image quality as a result of
a reduction in the geometric unsharpness
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Lack of sharpness in the subject
• Not all structures in the body have well-defined
boundaries (superimposition essentially present in
most situations)
• The organs do not have square or rectangular
boundaries
• The fidelity with which details in the object are
required to be imaged is an essential requirement
of any imaging system
• The absence of sharpness in the object is
reflected in the image
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Lack of sharpness due to motion (1)
• Common and understandable blur in medical
imaging
• Patient movement :
• uncooperative child
• organ contraction or relaxation
• heart beating, breathing etc.
• Voluntary motion can be controlled by keeping
examination time short and asking the patient to
remain still during the examination
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Lack of sharpness due to motion (2)
• Shorter exposure times are achieved by the use of
fast intensifying screens
• N.B.: Faster screens result in loss of details
(receptor unsharpness)
• Further, the use of shorter exposure time has to be
compensated with increased mA
• This often requires use of large focal spot
(geometric unsharpness)
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Lack of receptor sharpness
• The intensifying screens or image receptors in
radiography have a crystal size which is finite and
reduces image sharpness
• The digital image pixel size significantly impacts
image sharpness, i.e., small pixels usually mean
improved image sharpness
• An image obtained without the screen will be
sharper than that obtained with the screen, but will
require much more dose
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 8: Image quality
Topic 4: Distortion and artifacts
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Distortion and artifacts
• Unequal magnification of various anatomical
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structures
Inability to give an accurate impression of the real
size, shape and relative positions
Grid artifact (grid visualized on the film)
Light spot simulating microcalcifications (dust on
the screen)
Bad film screen contact, bad patient positioning
(breast)
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Distortion and artifacts
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Distortion and artifacts
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 8: Image quality
Topic 5: Image noise
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Noise
• Defined as uncertainty or imprecision of the
recording of a signal
• Impressionist painting: precision of object
increases with number of dots
• X Ray imaging: when recorded with small
number of X-ray photons has high degree of
uncertainty, more photons give less noise
• Other sources of noise:
• Grains in radiographic film
• Large grains in intensifying screens
• Electronic noise of detector or amplifier
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Image Noise
• Noise is characterized by the standard
deviation (s) of the optical density or digital
signal in a uniform region of the image
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Image noise
• Information that is not useful is noise
• Snow in a TV image, speckles in an ultrasound image are
examples of noise
• Noise interferes with visualization of image features needed
for diagnosis
• Different components of noise are:
• Radiation noise due to low photon flux (quantum noise)
• Structure noise
• Receptor noise (non-uniform response to a uniform X Ray beam)
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Summary
• Different technical and physical factors may
influence the image quality by reducing the
detecdtability of the anatomical structures useful
for diagnosis (increasing the image unsharpness)
• Some factors depend on the receptor, some others are
more related to the radiographic technique
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Where to Get More Information
• Hendee WR, Riternour ER, eds. Medical
Imaging physics, 4th ed. Wiley-Liss, Inc.
Wilmington, DE, 2002
• Sprawls P. Physical principles of medical
imaging. http://www.sprawls.org/ppmi2/
• The Essential Physics of Medical Imaging. JT
Bushberg, JA Seibert, EM Leidholdt, JM
Boone. Lippincott Williams & Wilkins,
Philadelphia, 2011
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