X_rays - Etsu

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15 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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ENTC 4390

PRODUCTION OF X RAYS

INTRODUCTION



To produce medical images with x rays, a
source is required that


1. Produces enough x rays in a short time


2.

Allows the user to vary the x
-
ray energy


3.

Provides x rays in a reproducible fashion


4.

Meets standards of safety and economy of

operation


Currently, the only practical sources of x
rays are radioactive isotopes, nuclear
reactions such as fission and fusion, and
particle accelerators.


Only special
-
purpose
particle accelerators known
as x
-
ray tubes meet all the
requirements mentioned
above.


In x
-
ray tubes, bremsstrahlung
and characteristic x rays are
produced as high
-
speed
electrons interact in a target.


A heated filament releases electrons that are
accelerated across a high voltage onto a
target.


The stream of accelerated electrons is referred to as
the
tube

current.


X rays are produced as the electrons interact
in the target.


The x rays emerge from the target in all directions but
are restricted by collimators to form a useful beam of x
ray’s.


A vacuum is maintained inside the glass envelope of the
x
-
ray tube to prevent the electrons from interacting with
gas molecules.

ELECTRON SOURCE


A metal with a high melting point is
required for the filament of an x
-
ray tube.


Tungsten filaments (melting point of tungsten
3370 C) are used in most x
-
ray tubes.


A current of a few amperes heats the filament,
and electrons are liberated at a rate that
increases with the filament current


The filament is mounted within a negatively
charged focusing cup.


Collectively, these elements are termed the
cathode assembly
.


The focusing cup, also called the
cathode block,
surrounds the filament
and shapes the electron beam width.


The voltage applied to the cathode block is
typically the same as that applied to the
filament.


This shapes the lines of electrical potential to focus
the electron beam to produce a small interaction
area (focal spot) on the anode.


A
biased
x
-
ray tube uses an insulated
focusing cup with a more negative voltage
(about 100 V less) than the filament.


This creates a tighter electric field around the
filament, which reduces spread of the beam and
results in a smaller focal spot width.



Although the width of the focusing cup slot
determines the focal spot width, the filament
length determines the focal spot length.


X
-
ray tubes for diagnostic imaging typically have two
filaments of different lengths, each in a slot machined
into the focusing cup.


Selection of one or the other filaments determines the
area of the electron distribution (small or large focal
spot) on the target.



The filament current determines the filament
temperature and thus the rate of
thermionic

electron emission.


As the electrical resistance to the filament current
heats the filament, electrons are emitted from its
surface.


When no voltage is applied between the anode and
the cathode of the x
-
ray tube, an electron cloud, also
called a
space charge cloud,
builds around the
filament.


Applying a positive high voltage to the anode with
respect to the cathode accelerates the electrons
toward the anode and produces a tube current.


Small changes in the filament current can produce
relatively large changes in the rube current
.



The existence of the space charge cloud shields the
electric field for tube voltages of 40 kVp and lower, and
only a portion of the free electrons are instantaneously
accelerated to the anode.


When this happens, the operation of the x
-
ray tube is
space
charge limited,
which places an upper limit on the tube
current, regardless of the filament current.


Above 40 kVp, the space charge cloud effect is overcome
by the applied potential difference and the tube current is
limited only by the emission of electrons from the
filament.


Therefore, the filament current controls the tube current in a
predictable way
(emission
-
limited
operation).


The tube current is five to ten times
less
than
the filament current in the emission
-
limited
range.


Higher kVp produces slightly higher tube current for
the same filament current;


For example, at
5
A filament current, 80 kVp produces
800 mA and


120 kVp produces ~1,100 mA, approximately as kVp
15
.



Beyond a certain kVp, saturation occurs whereby all of
the emitted electrons are accelerated toward the anode
and a further increase in kVp does not significantly
increase the tube current.

Anode


The anode is a metal target electrode that is
maintained at a positive potential difference
relative to the cathode.


Electrons striking the anode deposit most of
their energy as heat, with a small fraction
emitted as x
-
rays.


Consequently. the production of x
-
rays, in quantities
necessary for acceptable image quality, generates a
large amount of heat in the anode.


To avoid heat damage to the x
-
ray tube, the
rate of x
-
ray production must be limited.


Tungsten (W, Z = 74) is the most widely used anode
material because of its high melting point and high
atomic number.


A tungsten anode can handle substantial heat
deposition without cracking or pitting of its surface.


An alloy of 10% rhenium and 90% tungsen provides
added resistance to surface damage.


The high atomic number of tungsten provides better
bremssrahlung production efficiency compared with low
-
Z
elements.


Molybdenum (Mo, Z = 42) and rhodium
(Rh, Z = 45) are used as anode
materials in mammographic x
-
ray tubes.


These materials provide useful characteristic
x rays for breast imaging.


Anode Configurations


X
-
ray tubes have stationary and rotating
anode configurations.


The simplest type of x
-
ray tube has a
stationary (i.e., fixed) anode.


It consists of a tungsten insert embedded in a
copper block.


The copper serves a dual role:


it supports the tungsten target, and


it removes heat efficiently from the tungsten target.


Unfortunately, the small target area limits
the heat dissipation race and
consequently limits the maximum tube
current and thus the x
-
ray flux.


Many dental x
-
ray units, portable x
-
ray
machines, and portable fluoroscopy systems
use fixed anode x
-
ray tubes.



Despite their increased complexity in design
and engineering, rotating anodes are used for
most diagnostic x
-
ray applications, mainly
because of their greater heat loading and
consequent higher x
-
ray output capabilities.


Electrons impart their energy on a continuously
rotating target, spreading thermal energy over a large
area and mass of the anode disk.


A bearing
-
mounted
rotor
assembly
supports the anode disk within the
evacuared x
-
ray tube insert.


The rotor consists of copper bars arranged
around a cylindrical iron core.


A series of electromagnets surrounding the
rotor outside the x
-
ray tube envelope makes
up the
stator,
and the combination is known
as

an
induction motor.



Rotation speeds are 3,000 to 3,600

(low
speed) or 9,000 to 10,000 (high speed)
revolutions per minute (rpm).


X
-
ray machines are designed so that the x
-
ray
tube will not be energized if the anode is not
up to full speed;


this is the cause for the short delay (1 to 2
seconds) when the x
-
ray tube exposure button is
pushed.


Rotor bearings are heat sensitive and
are often the cause of x
-
ray tube failure.


Bearings are in the high
-
vacuum environment
of the insert and require special heat
-
insensitive, nonvolatile lubricants.


A molybdenum stem attaches the anode to
the rotor/bearing assembly, because
molybdenum is a very poor heat conductor
and reduces heat transfer from the anode to
the bearings.


Because it

is thermally isolated, the
anode must be cooled by radiative
emission.


Heat energy is emitted from the hot anode as
infrared radiation, which transfers heat to the
x
-
ray tube insert and ultimately to the
surrounding oil bath.



The focal track area of the rotating anode is
equal to the product of the track length (2
p
r)
and the track width (Ar), where r is the radial
distance from the track to its center.


A rotating anode with a 5
-
cm focal track radius
and a 1
-
mm track width provides a focal track
with an annular area 3/4 times greater than
that of a fixed anode with a focal spot area of 1
mm x 1 mm.


The allowable instantaneous heat loading depends on
the anode rotation speed and the focal spot area.


Faster rotation speeds distribute the heat load over a
greater portion of the focal track area.


Anode Angle and Focal Spot
Size


The anode angle is defined as the angle of the target surface with
respect to the central ray in the x
-
ray field.


Anode angles in diagnostic x
-
ray tubes,
other than some mammography tubes,
range from 7 to 20 degrees, wirh 12
-

to
15
-
degree angles being most common.


Focal spot size is defined in two ways:


The actual focal spot size is the area on the anode
that is struck by electrons, and


it is primarily determined by the length of the
cathode filament and the width of the focusing cup
slot.


The effective focal spot size is the length
and width of the focal spot as projected
down the central ray in the x
-
ray field.


The effective focal spot width is equal to the
actual focal spot width and therefore is not
affected by the anode angle.


However, the anode angle causes the
effective focal spot length to be smaller than
he actual focal spot length.


There are three major tradeoffs to
consider for the choice of anode angle.


A smaller anode angle provides a
smaller
effective
focal spot for the same
actual
focal area.


A smaller effective focal spot size provides
better spatial resolution.



However, a small anode angle limits the size of the
usable x
-
ray field owing to cutoff of the beam.


Field
coverage
is less for short focus
-
to
-
detector distances.


The optimal anode angle depends on the
clinical imaging application.


A small anode angle (approximately 7 to 9 degrees) is
desirable for small field
-
of
-
view image receptors, such
as cineangiographic and neuroangiographic
equipment, where field coverage is limited by the
image intensifier diameter (e.g.,
23
cm).


Larger anode angles (approximately 12 to 15 degrees)
are necessary for general radiographic work to
achieve large field area coverage at short focal spot
-
to
-
image distances.


The nominal focal spot size (width and
length) is specified at the central ray of
the beam.


The central ray is usually a line from the focal
spot to the image receptor that is
perpendicular to the A
-
C axis of the x
-
ray rube
and perpendicular o he plane of a properly
positioned image receptor.


In most radiographic imaging, the central
ray bisects the detector field.


X ray mammography is an exception.



Tools for measuring focal spot size are


The pinhole camera,


The slit camera,


The star pattern, and


The resolution bar pattern.


The
pinhole camera
uses a very small
circular aperture (10 to 30
m
m diameter) in
a disk made of a thin, highly attenuating
metal such as lead, tungsten, or gold.


With the pinhole camera positioned on the
central axis between the x
-
ray source and
the detector, an image of the focal spot is
recorded.



The
slit camera
consists of a plate made of a
highly attenuating metal (usually tungsten) with a
thin slit, typically 10
m
m wide.


In use, the slit camera is positioned above the image
receptor, with the center of the slit on the central axis and
the slit either parallel or perpendicular to the A
-
C axis.


Measuring the width of the distribution and correcting for
magnification yields one dimension of the focal spot.


A second radiograph, taken with the slit perpendicular to
the first, yields the other dimension of the focal spot.


The
star pattern T
est tool contains a radial
pattern of lead spokes of diminishing width and
spacing on a thin plastic disk.


Imaging the star pattern at a known magnification and
measuring the distance between the outermost blur
patterns (areas of unresolved spokes) on the image
provides an estimate of the resolving power of the focal
spot in the directions perpendicular and parallel to the A
-
C axis.


A large focal spot has a greater blur diameter than a small
focal spot.


The effective focal spot size can be estimated from the blur
pattern diameter and the known magnificacion.


A resolution bar pattern is a simple tool for
in
-
the
-
field evaluation of focal spot size.


Bar pattern images demonstrate the effective
resolution parallel and perpendicular to the A
-
C
axis for a given magnification geometry.



The focal spot is the volume of target
within which electrons are absorbed and
are produced.


For radiographs of highest clarity,
electrons should be absorbed within a
small focal spot.


To achieve a small focal spot, the electrons
should be emitted from a small or “fine”
filament.


Radiographic clarity is often reduced by
voluntary or involuntary motion of the
patient.


This effect can be decreased by using x
-
ray
exposures of high intensity and short duration.



The smaller, fine filament is used when
radiographs with high detail are desired
and short, high
-
intensity exposures are
necessary


If high
-
intensity exposures are needed to
limit the blurring effects of motion, the
larger, coarse filament is used.

TUBE VOLTAGE AND VOLTAGE
WAVEFORMS


The intensity and energy distribution of x rays
emerging from an x
-
ray tube are influenced by
the potential difference (voltage) between the
filament and target of the tube.


The source of electrical power for radiographic
equipment is usually alternating (ac).


This type of electricity is by far the most common form
available for use, because it can be transmitted with
little energy loss through power lines that span large
distances.


X
-
ray tubes are designed to operate at a single
polarity, with a positive target (anode) and a
negative filament (cathode).


X
-
ray production is most efficient

(more x rays are
produced per unit time) if the potential of the target is
always positive and if the voltage between the filament
and target is kept at its maximum value


In most x
-
ray equipment, ac is converted to direct
current (dc), and the voltage between filament and
target is kept at or near its maximum value.


The conversion of ac to dc is called
rectification.


Two electrical currents flow in an x
-
ray
tube.


The
filament current

is the flow of electrons
through the filament to raise its temperature
and release electrons.


The
tube current
flows from the filament to the
anode across the x
-
ray tube.


The figure illustrates
the influence of tube
voltage and filament
current upon tube
current.


One of the factors is
space charge
.


At low tube voltages,
electrons are released
from the filament more
rapidly than they are
accelerated toward the
target


The cloud accumulates
around the filament.



The useful beam of an x
-
ray tube is
composed of photons with an energy
distribution that depends on four factors.


Bremsstrahlung x rays are produced with a
range of energies even if electrons of a single
energy bombard the target.