Transcript Lecture 8 (2) - Sources in diagnostic Rad. - X-rays - gnssn

Radiation Sources in medicine
diagnostic Radiology
X-ray Generation and Imaging
IAEA
International Atomic Energy Agency
Day 6 – Lecture 8(2)
Objective
• To become familiar with the technology and operation of
x-ray tubes and generators and their specific use in
medicine.
• To understand the specific radiation risks linked with
these devices.
• To become familiar with the various types of image
receptors.
• To be aware of the advantages and limitations of each
type of receptor.
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Contents
• Description and physical characteristics of x-ray tubes and generators.
Principles of operation.
• Radiation quality of an x-ray beam; x-ray tube potential, total filtration,
first half-value layer.
• Influence of radiation quality on patient dose and image quality.
• Equipment malfunction and Quality control.
• Physical characteristics of x-ray film and intensifying screens.
• Physical characteristics of digital imaging technologies.
• Equipment malfunctions affecting radiation protection.
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Introduction
X-ray equipment and accessories:
•
should be certified to be in compliance with the relevant
standards of the International Electro-Technical
Commission (IEC) or equivalent national standard(s);
•
should be designed and purpose built for the intended
imaging task;
•
shall indicate at the control panel all the important
parameters relevant to image quality and patient dose.
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Generation of x-rays
Three basic elements are needed for x-ray generation:
•
a source of electrons; a heated tungsten filament (cathode);
•
a metal target (anode);
•
a high electric field (kilovolts) to accelerate the electrons
between the source and the target;
The cathode and anode are held in an evacuated envelope (the
x-ray tube). A high voltage generator provides the potential to
accelerate the electrons from the cathode to the anode.
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Generation of x-rays (cont)
A stationary
anode x-ray
tube
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Generation of x-rays (cont)
Specific requirements for x-ray tubes:
•
as small a focal spot as practicable;
•
sufficient filament current (and therefore
electrons) to minimize exposure times;
•
an efficient method for dissipating the heat
generated in the target (anode);
•
appropriate material, area and angulation of the
anode;
•
a choice of either a rotating or stationary anode;
•
more than one filament (for different size focal
spots).
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Generation of x-rays (cont)
Rotating anode x-ray tube
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Generation of x-rays (cont)
Number of
photons
Bremsstrahlung radiation
produced by the x-ray tube has
a continuous energy spectrum.
•
•
Its properties are subject to
the anode material, the
peak x-ray tube voltage and
the filtration of the x-ray
tube.
This radiation is produced in
all directions from the
anode.
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Bremsstrahlung
Characteristic
kV peak
Photon Energy (keV)
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Generation of x-rays (cont)
X-ray tube assembly (x-ray tube,
housing and collimator)
The x-ray tube assembly has an
aperture to allow the useful x-ray
beam to emerge but it is also shielded
to restrict unwanted radiation.
•
•
X-RAY TUBE
HOUSING
(ASSEMBLY)
HIGH
VOLTAGE
CABLES
LIGHT
BEAM
COLLIMATOR
Leakage radiation through the
shielding must be minimized and
must comply with standards.
The tube housing also usually contains oil for electrical
insulation and heat dissipation.
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Generation of x-rays (cont)
X-ray tube housing and collimator
• The useful radiation beam is directed
at the patient, usually through an
adjustable collimator which allows
the operator to control the size and
shape of the x-ray beam.
• The location, size and shape of the x-ray
beam is usually (but not always) defined
by a light beam, hence the description
light beam collimator.
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Generation of x-rays (cont)
Generators
X-ray generators provide both
the current to the x-ray tube
filament (the source of
electrons) and the high
voltage required to accelerate
the electrons from the
cathode towards the anode.
However, some mobile x-ray equipment may use a capacitor
(typically 1 µF) to store the required electrical energy.
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Generation of x-rays (cont)
Generators
• Various type of generators are used in diagnostic
radiology - single phase, three phase or high frequency,
each of which produce characteristic waveforms.
However, they must ensure an accurate and consistent
high voltage and a stable radiation output.
• Most modern generators are microprocessor-controlled
with a high frequency inverter (effectively no voltage
ripple).
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Generation of x-rays (cont)
Generators
X-ray generators are rated on the basis of the maximum voltage
and electrical power they can deliver. The maximum current that a
generator (and x-ray tube) can withstand varies with the voltage at
which it is operating.
•
For medical uses, generators supply high voltages ranging from
25 to 150 kV peak (according to the application) together with an
appropriate current (e.g. 300 mA at 100 kV peak).
•
Generators are provided with circuits that can accurately control
exposure times, typically ranging from milliseconds to several
seconds.
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Filtration
A substantial part of the x-ray
spectrum emitted by the
anode is low energy radiation
which would be absorbed in
the human body and not
reach the image receptor.
Number of
photons
Bremsstrahlung
Characteristic
Appropriate filtration removes
low energy photons before
they reach the patient.
kV peak
Photon Energy (keV)
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Filtration (cont)
• Filtration is effected by the irremovable materials of the tube
assembly (i.e. the glass envelope, the cooling oil, and the x-ray
tube assembly port) through which the beam passes before
emerging from the housing. This is the inherent filtration.
•
Added filtration is used to further modify the spectrum.
Aluminium is typically used but for special purposes can include
Copper Molybdenum, Hafnium, etc.) The mirror in the light beam
collimator generally also acts as a filter.
•
The combination of the inherent and added filtration is the total
filtration and is expressed in terms of millimetres of Aluminium
equivalent.
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Filtration (cont)
The quality of the emergent x-ray beam (and therefore its
penetrating power) depends on the:
• applied x-ray tube potential (kV peak);
• total filtration;
• anode material (but this is not within the user’s control).
Quality is characterized by:
• the first half value layer (HVL) which typically is
measured in millimetres of Aluminium.
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Filtration (cont)
Half Value Layer (HVL)
The HVL is the thickness of material which
attenuates the output (air kerma) of a
collimated x-ray beam by 50%.
Filters
It is measured under conditions which
minimize scattered radiation.
Ion chamber
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Some malfunctions that can compromise safety
•
excessive radiation leakage through the x-ray tube housing and
collimator;
•
x-ray tube voltage inaccuracy and inconsistency;
•
timer, tube current, mAs inaccuracy and inconsistency;
•
x-ray tube output inconsistency;
•
incorrect or inappropriate filtration;
•
poor congruency of the collimator’s light and x-ray beams;
•
for capacitor discharge equipment, excessive radiation leakage
(in the direction of the useful x-ray beam) when the capacitor is
fully charged (but without an exposure initiated).
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Image Production
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International Atomic Energy Agency
Principles
X-ray photons transmitted
through the structures
under examination
comprise the “x-ray (or
radiological) image”.
The photons are then
converted into a visual
image by interaction with
an appropriate detector
(image receptor)
The Fundamentals of Radiography. Kodak
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Image Receptor
An image receptor is a device that converts a pattern of x-ray
photons into an image. The image may be viewed directly (for
dynamic imaging such as fluoroscopy), recorded on an x-ray film
or other hard copy device, or converted to electronic form for
digital processing.
This conversion can be carried out by different methods,
separately or combined:
•
x-ray film and intensifying screen technology;
•
luminescent screens and electronic image intensifiers;
•
computed and digital imaging technology.
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Image Receptor (cont)
X-ray film and Intensifying screens
Radiography using film and intensifying screens as the
image receptor (in light tight cassettes) remains the most
common modality for recording x-ray images.
• However, the sensitive emulsion on x-ray film is not
particularly sensitive to direct x-ray exposure.
• Therefore, except for intraoral dental radiography,
intensifying screens are used to convert the x-ray energy to
light (blue, green, UV).
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Image Receptor (cont)
X-ray film and Intensifying screens
• The exposed film (bearing the latent image) is then
chemically processed to create a visible image. Film
processing may be manual or automatic.
• The sensitivity (or the speed) of a film or film-screen
system is the reciprocal of the radiation dose required
to produce a given density on the film.
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Digital imaging technology
Digital methods for processing and displaying x-ray images
were first introduced with the advent of computed tomography
(CT) in 1972.
Continuing advances in computer technology have promoted the
general development of image acquisition in digital form (CCD
cameras), most commonly from image intensifiers (digital
fluoroscopy) or from storage phosphor plates (computed
radiography).
Other detector systems such as ‘flat-panel’ technology for indirect
or direct digital radiography are now available for general purpose
equipment.
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Digital imaging technology (cont)
The technique of digital
subtraction angiography
(DSA), based on digital
image processing, allows
enhanced visualization of
blood vessels by
electronically subtracting
unwanted parts of the image.
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Digital imaging technology (cont)
At this time, there is no consensus on the best technology
for balancing dose and image quality. Digital imaging
potentially can provide lower doses than the film-intensifying
screen method.
• However, through post-exposure manipulation of the data,
satisfactory diagnostic images can be produced even when
unnecessarily high patient radiation doses are used.
• Proper quality assurance procedures are essential.
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Main characteristics of an image receptor
The selection of an imaging system should involve a thorough
evaluation and analysis of its complete characteristics together
with consideration of the technical and human environment in
which the system will be used.
The main characteristics to be considered when selecting an
image receptor are:
• spatial resolution; contrast resolution; dose efficiency;
Modulation Transfer Function; detector size; possibilities of
image storage and transfer; and qualities such as weight,
robustness, fast image access, etc.
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Main characteristics of an image receptor (cont)
•
Spatial resolution: determines the minimum size of
detail visualized;
•
Contrast resolution: determines the size of detail that can
be visibly reproduced when there is only a small
difference in density relative to the surrounding area. It is
the smallest exposure change that can be detected;
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Main characteristics of an image receptor (cont)
•
Dose efficiency: quantifies the balance between the
radiation dose absorbed by the receptor (and by the
patient) and the resulting image quality;
•
Modulation Transfer Function (MTF): is the measure of the
ability of an imaging system to preserve signal contrast as
a function of spatial frequency;
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Fluoroscopy: dynamic (real time) imaging
X-ray energy is converted into electromagnetic radiation in
the visible or near visible range by means of luminescent
(fluorescent) screens.
• Direct viewing of an image on a fluorescent screen with the
naked eye should not be permitted because of the
potentially higher radiation dose rates that may be required,
particularly if the user fails to properly dark adapt.
• Fluoroscopy should now only be performed using an
electronic image intensifier.
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Fluoroscopy: dynamic (real time) imaging (cont)
Light amplifier tubes,
in combination with a
television camera,
are the most widely
used image
intensification
systems.
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Problems that may affect radiation protection
Film-Intensifying screens
•
Unsatisfactory film storage (causing fogging); damaged
cassettes or intensifying screens.
•
Light sources within, or leaking into, the dark room.
•
Cassette pass hatch or storage container not provided or
improperly shielded.
•
Inappropriate developer chemistry (e.g. wrong type,
improperly diluted and / or replenished, wrong temperature).
Note: Ventilation is also an important occupational safety issue
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Problems that may affect radiation protection (cont)
Film-screen technology
•
Failure to follow the film manufacturer’s prescribed timetemperature development procedures (manual development)
or to properly maintain automatic film processors.
Fluoroscopy and Digital systems
•
Direct fluoroscopy (inefficient fluorescent screen)
•
Image intensified fluoroscopy (low efficiency, poor
resolution and contrast of the image intensifier TV
chain)
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