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‫ن‬
Lec.2
.‫د‬
X – ray production and X – ray machine
X –ray machine consist of :♦ Tube-head.
♦ Holder.
♦ Base ( earth base or wall-mounted base).
♦ Control Panal.
The tube-head represent the major part in X –ray machine and it content
♦ X –ray tube.
♦ Radiator.
♦ Surrounding oil facilitates the removal of heat.
♦ Filter. (there are build in and added filters both of them made of AL )
♦ Collimator. (either circular or rectangular in shape , made of lead ? )
♦ Surrounded cover.
♦ Tube positioned device ( TPD)
X –ray tube ( Coolidge tube):It is also called Hot cathode tube. It was designed by William David Coolidge
in 1913. it consists of a double walled glass tube, which is exhausted to nearly
perfect vacuum. Glass tube is thinner or single walled at the site of x-ray
emissions. The basic apparatus consists of:
(a)
A Cathode (- ve) consists of filament of tungsten and a focusing
device "Molybdenum cup".
The filament is a coil of wire 0.2 cm in diameter when the current is applied,
the filament is heated and emits e- at a rate proportional to its temperature. The
emission of e- from cathode is known as " thermionic emission ". the current or
milliamperage control the quantity of e-, which in fact control the tube current.
The filament is fitted in a negatively charged concave reflector cup of
molybdenum. The cup focuses the e- emitted into a narrow beam. The high
negative charge of the cup repels the negatively charged electrons, which from
an e- cloud around the filament. The positive charge of the anode attracts them.
Complete vacuum in the tube facilitates the movement of the e- ( why ?).
(b) An anode (+ ve) composed of a copper stem to which is attached the
target (small piece of tungsten). The area where e- strike is known as
the focus ( focal spot).
♦ A high voltage connected between the cathode and anode. ( why ?)
♦ A surrounding lead luting the x-ray tube from inside. ( why ?)
♦ X-ray tube used a double walled glass tube (why ?)
♦ X-ray tube should be vacuumed (why ?)
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‫ ٭‬e- : mean electrons.
Properties of target metal:
♦ It should have higher atomic number. the higher atomic number, denser
is the metal ( to stop the high speed e-). it is 74 for tungsten.
♦ It should have low vapour pressure athigh temperature. Since e- beam is
directed into small area, some atoms may reach the vapour state, so
blisters may be found.
♦ It should have high melting point. Since most of energy converted to
heat. ( M.P. of tungsten is 3370◌ْ C).
♦ It should have high degree of thermal conductivity. Since most of heat
generated is passed to the radiator or other cooling device. The thermal
conductivity of the tungsten is low so it fitted in a copper stem.
Line focus principle:
The image sharpness increase When the focal spot size decreases, but the
generated heat will increase, and this will decrease the life of focal spot. To
take the benefit of the smaller focal spot the target is place at an angle of 45◌ْ (
some manufactory make 20◌ْ ) with respect to the central ray of x-ray beam. By
this angulation the actual focal spot remain large ( 1.0-3.0 mm), allowing the
dissipation of heat, while the effective focal spot is
small ( 0.1- 1.0 mm). which keeps the the x-ray image sharp.
A radiator:
It is a cooling device connect with the copper stem ( which is a good
heat conductor) to dissipate the heat that generated at the target.
Another method of dissipating heat is to used a rotating anode ( used
in few models of cephalostat machines). The insulating oil in the x-ray
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tube help in dissipating the heat.
X –ray machine also have some accessories :(a) Timer:
Used to control the exposure time.it is calibrated in fractions of second.
Some machines provide timers according to the impulses per exposure, the
no. of impulses divided by 60 gives the exposure time in fractions of a
second
(b) Transformers:
Proper current to heat the filament and the potential difference between
the cathode and anode are provided by step up and step down transformers.
Autotransformers are also used. They perform both step up and step down
function.
Working of the tube:
When filament is heated electrically, a cloud of electrons is produced
around the filament and these electrons are accelerated in very high speed
toward the anode by the high voltage (potential difference). Also the electrons
will be focused on the focal spot on the target by the present of rectangular or
C-shaped molybdenum focusing device. When these high velocity e- strike the
target focus, the x- rays are produced.
Adjusting the current flowing through the filament by means of a rheostat can
control the intensity of the x- rays. If the filament is heated to a high
temperature, it emit a large no. of e- and hence the intensity of the x-rays.
If the voltage across the tube is increased, the e- attain a greater energy before
striking against the target, so the high energy rays with better penetrating
power are emitted.
The electron bombard the target and are brought suddenly to rest and the
energy lost by electron is transferred into either heat (about 99%) or X-ray
(about 1%) which emitted in all directions from the target, there are small
window in the lead casing permit to X-rays to pass. And the heat is removed
and dissipated by the capper block and the surrounding oil.
Roentgenological accessories:
Diaphragm
It is a device absorb approximately 90% of stray and secondary radiations
emanating from the x-ray tube. this permit direct ray to pass through and act on
the film. Without the used of diaphragm, the picture may be foggy and poor in
contrast. Diaphragm is preferably used where superior contrast and detail are
required.
Intensifying screen
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It is a device used to intensify the photographic effect of x- rays thereby
shortening the exposure time.
When the x-rays strike the emulsion of the film, the absorption of energy is
only 1% and the rest of energy is wasted. It is well known that certain chemical
( e.g. phosphors) have the property of absorbing roentgen rays and emitting
their energy as ordinary light. This property is called " fluorescence".
Intensifying screen are used in combination with film of all extra oral
radiography including panoramic and cephalometric. Intensifying screen is
available in three speeds ( slow, intermediate and fast). They are not used with
intra oral films because of loss of resolution. It definitely serves the diagnostic
requirement, decreasing 85-90% of the patient's dose. Intensifying screen
compose of:
♦ a base: made up of plastic polyester of 0.25 mm thickness.
♦ a phosphor layer: consist of light sensitive phosphor crystals ( e.g. terbium
activated lanthanum oxybromide, calcium tungstate, zinc sulphide etc.).
♦ a protective coating: a plastic coat of about 8 µm to protect the phosphor layer
and keep the Intensifying screen clean of any debris, scratches etc.
Intensifying screen are mounted in pairs in a cassette which made of metal.
The cassette is light proof when closed. They are either rigid ( mostly) or
flexible ( very rarely).
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