Transcript Lecture 05 Image quality and patient dose - gnssn

RTC on RADIATION PROTECTION OF
PATIENTS FOR RADIOGRAPHERS
Accra, Ghana, July 2011
Image Quality and Patient Dose
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International Atomic Energy Agency
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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Imaging quality
• Efficient diagnosis requires
• acceptable noise
• good image contrast
• sufficient spatial resolution
• These factors are linked
• “Objective” measurement of quality is
difficult
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Factors affecting image quality
Blur or
Unsharpness
Contrast
Image quality
Distortion
& artifact
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Noise
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Image contrast
Low
Contrast
Medium
Contrast
High
Contrast
Image contrast refers to the fractional difference in optical
density of brightness 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 kVp
X Ray spectrum (filtration)
Scatter rejection
• Collimator
• Grid
• …
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• The radiographic contrast
plus :
• Film characteristics
• Screen characteristics
• Windowing level of CT and
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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 be
investigated?
• What is the contrast level needed?
• For a thorax examination : 110 - 120 kV is suitable to
visualize the lung structure
• However 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 contrast is high
(chest)
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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
bring up the intensity and optical density
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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 a pronounced effect on
fog and contrast
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Image Contrast
• Difference in signal – pixel value, film
density
High
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Low
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Video monitor
• The video monitor is commonly used in
fluoroscopy and digital imaging
• The display on the monitor 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 level or
grey levels of a digital image
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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 “fuzziness” 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
• Film/screen: 0.01 mm
• CT: 0.5 mm
• Other definition: “Point-spread” function
• Characteristic of a “point” object
• Point object expected to be point in image
• Blurring due to imperfections of imaging system
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Factors affecting image sharpness
Object
Unsharpness
Geometric
Unsharpness
Image
Unsharpness
Motion
Unsharpness
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Subject
Unsharpness
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Resolution and Focal Spot Size
Penumbra
More blur
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Appearance of image
Less blur
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Measuring Resolution
{
Line pair test object
One line pair
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Geometric blur
• If the focal spot is infinitesimally small, the blur is
minimized because of minimal geometric bluntness
• As the focal spot increases, the blur in the image
increases
Small focal spot
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Large focal spot
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Geometric blur
• Another cause of lack of geometric sharpness 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 closer the
contact between the object and the film (or
receptor), the better the image quality as a result
of a reduction in the geometric sharpness
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Lack of sharpness in the subject
• Not all structures in the body have well-defined
and separate 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 subject/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 sharpness)
• Further, the use of shorter exposure time has to be
compensated with increased mA to achieve a
good image
• This often implies use of large focal spot
(geometric sharpness)
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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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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- 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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Noise
Decreasing radiation intensity
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Increasing noise
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Contrast & Noise
 Contrast
Noise 
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Radiography Issues
• Correct positioning
• Improves diagnosis and reduces retakes
• PRE-exposure collimation
• Minimises unnecessary tissue dose
• With CR/DR, there is a temptation to postexposure (electronically) collimate – RESIST
THIS!!
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Summary
• Different technical and physical factors may
influence the image quality by impairing the
detection capability 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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Patient dose assessment
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Overview
• To become familiar with the patient dose
assessment and dosimetry instrument
characteristics.
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Parameters influencing patient exposure
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Essential parameters influencing
patient exposure
}
Tube voltage
Tube current
Effective filtration
Exposure time
Field size
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Kerma rate
[mGy/min]
[min]
}
Kerma
[Gy]
[m2]
}
Area exposure
product
[Gy m2 ]
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Factors in conventional radiography:
beam, collimation
• Beam energy
• Depending on peak kV and filtration
• Regulations require minimum total filtration to absorb
lower energy photons
• Added filtration reduces dose
• Goal should be use of highest kV resulting in acceptable
image contrast
• Collimation
• Area exposed should be limited to area of CLINICAL
interest to lower dose
• Additional benefit is less scatter, better contrast
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Factors in conventional radiography:
grid,patient size
• Grids
• Reduce the amount of scatter reaching image receptor
• But at the cost of increased patient dose
• Typically 2-5 times: “Bucky factor” or grid ratio
• Patient size
• Thickness, volume irradiated…and dose increases with
patient size
• Except for breast (compression), no control
• Technique charts with suggested exposure factor for
various examinations and patient thickness helpful to
avoid retakes
• Use of AEC exposure
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Factors affecting dose in fluoroscopy
• Beam energy and filtration
• Collimation
• Source-to-skin distance
• Inverse square law: maintain max distance from patient
• Patient-to-image intensifier
• Minimizing patient-to- I I will lower dose
• But slightly decrease image quality by increased scatter
• Image magnification
• Geometric and electronic magnification increase dose
• Grid
• If small sized patient (les scatter) perhaps without grid
• Beam-on time!
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Factors affecting dose in CT
• Beam energy and filtration
• 120-140 kV; shaped filters
• Collimation or section thickness
• Post-patient collimator will reduce slice thickness
imaged but not the irradiated thickness
• Number and spacing of adjacent sections
• Image quality and noise
• Like all modalities: dose increase=>noise decreases
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Factors affecting dose in spiral CT
• Factors for conventional CT also valid
• Scan pitch
• Ratio of couch travel in one rotation divided by
slice thickness
• If pitch = 1, doses are comparable to
conventional CT
• Dose proportional to 1/pitch
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Patient dosimetry methods
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Patient dosimetry
• Radiography: entrance surface dose ESD
• Output factors
• Dose – area product (DAP)
• Fluoroscopy: Dose - Area Product (DAP)
• CT:
• Computed Tomography Dose Index (CTDI)
• Dose – length product (DLP)
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From ESD to organ and effective dose
• Except for invasive methods, no organ doses can be
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measured
The only way in radiography: measure the Entrance
Surface Dose (ESD)
Use mathematical models to estimate internal dose.
Mathematical models based on Monte Carlo
simulations
Dose to the organ tabulated as a fraction of the
entrance dose for different projections
Since filtration, field size and orientation play a role:
long lists of tables (See NRPB R262 and NRPB
SR262)
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From DAP to organ and effective dose
• In fluoroscopy: moving field, measurement of
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Dose-Area Product (DAP)
In similar way organ doses calculated by Monte
Carlo modelling
Based on mathematical model
Conversion coefficients estimated as organ doses
per unit dose-area product
Again numerous factors are to be taken into
account such as projection, filtration, …
Once organ doses are obtained, effective dose is
calculated following ICRP103
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Dose measurements: how to measure dose
indicators ESD, DAP,CTDI…
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Measurements of Radiation Output
X Ray tube
Filter
SDD
Ion. chamber
Table top
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Lead slab
Phantom (PEP)
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Measurements of Radiation Output
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Operating conditions
Consistency check
The output as a function of kVp
The output as a function of mA
The output as a function of exposure time
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Measurement of entrance surface
dose
Includes backscatter (~30%)
TLD, solid state
dosimeter or
ion chamber
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Dose Area Product (DAP)
Transmission
ionization
chamber
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Dose Area Product (DAP)
0.5 m
1m
2m
Air Kerma: 40*103 Gy 10*103 Gy
Area:
2.5*10-3 m2 10*10-3 m2
Area
100 Gy m2 100 Gy m2
exposure product
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2.5*103 Gy
40*10-3 m2
100 Gy m2
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Calibration of a Dose Area Product
(DAP)
Ionization
chamber
Film cassette
10 cm
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10 cm
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Levels of Dosimetry
• Level 1 - published tables
• Level 2 - Monte Carlo tables using known
data
• Level 3 - direct measurement of skin dose
• Level 4 - humanoid phantom measurements
with TLD
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Level 1 - Published Dose Tables
• ICRP, NCRP and various books have
tables of “typical” doses for various x-rays
• Tables show organ doses and sometimes
effective dose
• The data is usually old, from x-rays made
with slower film/screen systems than in
current use
• Still useful as a guide
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Level 2 - Monte Carlo Systems
• Provide organ doses, and effective dose
• Use calculated data but with user entry of
various parameters :
• HVL, kVp
• skin dose or free-in-air exposure at skin distance
• FSD, field size and position
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Monte Carlo Systems
• Various computer programs and lookup
tables, eg. TISSDOSE, XDOSE, PCXMC
• Most users do not know the actual values
for input variables, so often must use
assumed values
• HVL, kVp, FSD, field size easy to assume
– kVp/mAs and field size not always easy
to assume
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Typical Radiology Doses (Melbourne Data)
ESD (mGy)
Eff. Dose (mSv)
Abdomen (AP)
2.5
0.35
Chest (PA)
0.15
0.023
3.2/4.0
0.23/0.11
Pelvis (AP)
1.7
0.29
Thoracic spine (lat.)
2.6
0.097
Mammography (4 views)
4.4
0.44
Procedure
Lumbar spine AP/Lat
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XDose
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PCXMC
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ImPACT CT Dose
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Summary
• In this lesson we learned the factors
influencing patient dose, and how to have
access to an estimation of the detriment
through measurement of entrance dose,
dose area product or specific CT dosimetry
methods.
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