Studied by 3 people
Diagnostics x-ray imaging systems
are usually operated at voltages of 25 to 150 kVp and at tube currents of 100 to 1200 mA.
to provide a controlled flow of electrons intense enough to produce an x-ray beam appropriate for imaging
What is the function of the x-ray imaging system?
Fixed X-ray Apparatus Mobile X-ray Apparatus Portable X-ray Apparatus Dental X-ray Machine Mammographic X-ray Machine Panoramic X-ray Machine
Types of X-ray Machine According to Purpose
Fixed X-ray Apparatus
The machines which fall into this group are characterized by the fact that they require transformers of such size and output that they have to be built into the room and provided with special electrical connections to the mains.
Fixed X-ray Apparatus
What type of x ray appratus when the X-ray tube is connected to the transformer by high-tension cables and is mounted on some form of gantry which allows only limited movement. Such machines are likely to be capable of an output of at least 300 mA and 120 kV and in some instances much higher (1000 mA and 200 kV ).
Mobile X-ray Apparatus
These machines have higher output than portable machines by virtue of their larger transformers and are mounted on wheels with output of 90-125kv and 40 to 300mA. Most machines are movable on smooth surface within the radiology section.
Mobile X-ray Apparatus
In machines of this type the transformers are larger to permit higher output and because of their increased weight are no longer located in the tube head but are mounted on wheels and form the best of the apparatus.
Mobile X-ray Apparatus
These sets cannot be taken apart and the tube stand and control panel are built into the apparatus.
Portable-X-ray Apparatus.
Commonly used in veterinary practice because of convenient transportation. The maximum output usually varies from 70-110 kv and 15-35 mA.
Portable-X-ray Apparatus.
In such machines the transformers are of small size and low weight and are located within the tube head immediately adjacent to the X-ray tube.
Portable-X-ray Apparatus.
The tube head itself is supported on a tube stand which may comprise either a small table top model or a considerably more substantial floor stand mounted on wheels.
Portable-X-ray Apparatus.
The apparatus also has a small control panel which is attached to the tube stand or the tube head or supported on a separate stand.
o Relatively cheap and require little maintenance. o They cost less than other types of machine. o They are strongly constructed and need little maintenance. o They can be operated from any 12 – or 15-A electrical point. o They can be easily transported for use on farms or other premises. o They are light and easily maneuvered.
Advantages of Portable-X-ray Apparatus.
Because of low electrical output, these machines are of limited value in radiographic examination.
Disadvantages of Portable-X-ray Apparatus.
Dental X-ray Machine
Apparatus manufactured for dental use is sometimes advocated for veterinary use because of its low price.
Dental X-ray Machine
Such machines are of low output (in the region of 10 m A and 70 k V) and are designed to only cover a small area of the patient.
• X-ray tube • Operating console • High voltage generator
3 main Components of An X-ray Imaging System
X-ray Tube
A vacuum tube containing electrodes that accelerate electrons and direct them to a metal anode, where their impacts produce x-rays.
• Support structure • Protective housing • Glass or metal enclosure
External Components of Xray Tube
Ceiling Support System
Consist of two perpendicular sets of ceiling-mounted rails.
Floor to Ceiling Support System
Has a single column with rollers at each end.
C-arm Support System
The image receptor is attached to the other end of the C-arm from the x-ray tube.
Protective Housing
Guards against excessive radiation exposure and electrical shock.
Glass or Metal Envelope
The glass is made up of pyrex to withstand the tremendous heat generated.
X-ray tube window
Is the area of the glass or metal enclosure, approximately 5 cm2
, that is thin and through which the useful beam
of x-rays is emitted.
the useful beam.
X-rays emitted through the window is called _
leakage radiation.
X-rays that escaped through the tube housing are called.
100 mR/hr at 1 m
Leakage radiation should not exceed ____when operated at maximum condition.
• Cathode • Anode
Internal Components of xray tube
Cathode
The cathode is the negative side of the x-ray tube and has two primary parts: a filament and a focusing cup.
Filament
-Usually approximately 2mm in diameter and 1 or 2 cm long.
-small and large filament
Thermionic Emission
When the current through the filament is sufficiently high, the outer-shell electrons of the filament atoms are “boiled off” and ejected from the filament.
• Tungsten have high melting point (3410 o C)
• Does not vaporize easily • Addition of 1% to 2% thorium to tungsten increases the efficiency of thermionic emission.
Filaments are usually made of thoriated tungsten because:
Focusing Cup
Where the filaments are embedded.
small focal spot
is used when better spatial resolution is required. Ranges from 0.1 to 1 mm
Large focal spot
used when large body parts are imaged and when other techniques that produce high are required.
anode
is the positive side of the x-ray tube; it conducts electricity and radiates heat and contains the target.
• Electrical conductor • Mechanical support • Thermal dissipator
Functions of anode assembly
Target
Area of the anode struck by the electrons from the cathode.
• Stationary anode • Rotating anode
Types of Anode
Stationary Anode
Are used in dental x-ray imaging systems, some portable imaging systems, and other-purpose units in which high tube current and power are not required.
Rotating Anode
-Capable of producing high intensity x-ray beam.
-Heat capacity can be further improved by increasing the speed of anode rotation.
-Rotates 3400 rpm and 10,000 rpm
stationary
In _ anode tubes, the target is consists of tungsten alloy embedded in a copper anode. About 4 mm2
Rotating
In _ anode tubes, the entire rotating disc is the target. About 3159 mm2
• Atomic number • Thermal conductivity • High melting point
Tungsten is the material of choice for the target for general radiography for 3 main reasons
Mammographic X-ray Tubes
it has molybdenum or rhodium targets principally because of their low atomic number and low K characteristics x-ray energy.
Alloying tungsten
(usually with rhenium) gives it added mechanical strength to withstand the stresses of high speed rotation.
Molybdenum and graphite
have lower mass density than tungsten, thus allowing the target easier to rotate.
The rotating anode
is powered by an electromagnetic induction motor.
the stator and the rotor
An induction motor is consist of 2 parts:
Focal Spot
Is the actual x-ray source
Line-Focus Principle
Results in an effective focal spot size much less than the actual focal spot size.
GOETZE PRINCIPLE
Line-Focus Principle is also known as
The line-focus principle
allows high anode heating with small effective focal spots. As the target decreases, so does the focal spot size.
Heel Effect
Radiation intensity is greater on the cathode side of the x-ray field than that on the anode side.
larger
The smaller the anode angle, the __ is the heel effect.
45%.
The difference in the radiation intensity across the useful beam of an x-ray field can vary by as much as _
Off Focus Radiation
Electrons bounce off the focal spot and then land on other areas of the target, causing x-rays to be produced from outside of the focal spot.
• X-ray tube life may be extended by using minimum radiographic factors of ma, kVp, and exposure time appropriate for each examination. • Use of faster image receptors. • Excessive heat results in reduced x-ray tube life.
Ways How to Extend X-ray Tube Life Span
• Radiation • Conduction • Convection
3 Ways to Dissipate Heat
Radiation
Transfer of heat by the emission of infrared radiation.
Conduction
Transfer of energy from one area of an object to another.
Convection
Transfer of heat by the movement of a heated substance from one place to another.
• Vaporized tungsten • Rotor bearings breakage • Anode pitting • Cracked anode
4 causes of X-ray Tube Failure
• Radiographic rating chart • Anode cooling chart • Housing cooling chart
3 types of x-ray tube rating charts significant to radiologic technologist
Radiographic Rating Chart
-Conveys radiographic techniques which are safe and unsafe for tube operation.
-Used to determine the length of time required for complete cooling after any level of heat input.
Anode Cooling Chart
Contains the thermal capacity of an anode and its heat dissipation characteristics.
Thermal energy
is conventionally measured in units of calories, British thermal units (BTU or joules.
heat units (HU)
In x-ray applications, thermal energy is measured in
heat units.
The capacity of the anode and the housing to store heat is measured in
1 kVp, 1 mA, and 1 s.
One heat unit is equal to the product of
Single-Phase
HU = kVp x mA x s
Three-phase/High-Frequency
HU= 1.4 x kVp x mA x s
• 1 HU = 1.4 J • 1 J = 0. 7 HU
Heat Units conversion to Joules
several million heat units.
Radiographic x-ray tube housings usually have maximum heat capacities in the range of
1 to 2
Complete cooling after maximum heat capacity requires from _ hours.
Note
Flashcard