X RAY TUBE

Functions of protective housing Prevent leakage of radiation Prevent accidental electric shock Mechanical support to the x ray tube

CATHODE ASSEMBLY

Filaments are usually made of thoriated tungsten because

1.Its melting point is very high2.Does not vaporize easily

ANODE

Common anode materials areCoppermolybdenum graphite

ANODE ASSEMBLY

STATIONARY ANODE ROTATING ANODE

ROTATING VS STATIONARY ANODE Rotating anodes let you have higher tube

current and shorter exposure times. Rotating anodes are powered by induction

motors.

INDUCTION MOTOR

Tungsten is used as target material because of atomic number is high (74) make good x-ray

production. Thermal Conductivity is very good in

tungsten. High Melting point for tungsten, 3400C.

FOCAL SPOT

This is where the high-voltage electrons hit the anode.

Line-focus Principle

the process of making the angle between the anode surface and the central ray of the X-ray beam such that the effective focal spot size is made small as compared to the actual focal spot size.

Heel Effect

Reduction of the X-ray intensity at the anode side of an X-ray field. In an X-ray field it can be observed that the film density is lower on the anode side than on the cathode side. This phenomenon is termed the heel effect

space charge The excess of electrons or ions in a given

volume.

Space Charge Effect

When a low voltage is applied across the tube, the current is small due to space charge effects.

The electron cloud surrounding the filament has a tendency to repel other electrons back to the filament unless sufficient accelerating voltage is applied to pull them away from the filament as fast as they are produced.

With increasing kV, the effects of space charge are gradually overcome and the current (mA) increases until all electrons that are released at the filament reach the anode.

X RAY TUBE FAILURE

1. Tungsten target (anode) pitting2. Bearing failures/glass envelope breaking3. Heat4. Filament failure5. arcing

A.) The vaporized tungsten from both filament and anode.

B.) Breaking of the filament itself.C.) Holding the rotor switch prior to making

exposuresD.) Single excessive exposures exceeding tube

limits.E.) Long exposure times.

Open Filament It is the condition where in the filament wire

becomes thinner and eventually breaks. The excessive heating of the filament causes the filament to vaporize. Occurs due to prolonged periods of high mA operation.

Arcing It is a sustained luminous discharge of

electricity across a gap in a circuit or between electrodes.

Methods of heat transfer

Radiation - transfer of heat through emission of infared radiation.

Conduction - transfer of energy by contact. Convection - transfer of heat by movement of

heated air, oil from one place to another.HEAT IS THE MOST IMPORTANT CAUSE OF

TUBE FAILURE

VOLTAGE RECTIFICATION

Process of converting AC into DC

Half wave rectification

Shortcomings of half wave Waste of supply Increase exposure time

Full wave rectification

Diods

A diode is a device which only allows current to flow through it in one direction. In this direction, the diode is said to be 'forward-biased'

During the positive half cycle of input voltage, the diode is forward biased and it conducts for all volts which is greater than its barrier potential.

During negative half of the cycle, the diode is reverse biased hence it does not conduct

Q. What will happen if unrectified current is passed through x ray tube?

Bremsstrahlung Radiation

When an electron is propelled towards the nucleus of an atom, its velocity increases then decreases as it moves away from the nucleus due to electrostatic attraction of positively charged protons in the nucleus.

The energy of the E= mc2 has a continuous spectrum up to a maximum equal to the maximum kinetic energy of the electron.

The production of Bremsstrahlung radiation increases with the atomic number of the target atom; thus, it increases electrostatic attraction

Characteristic Radiation The second method of X-Ray production is by

orbital electron transitions. Characteristic X-Rays are emitted at specific

energies dependent on the type of target material.

High energy electrons interact with an electron in an inner orbital shell and if the energy is high enough may eject that electron, producing a vacancy in the inner orbital shell.

The vacancy is filled by an outer orbital electron, the excess energy is emitted as an X-Ray with discrete energy equal to the difference between the energy states of the inner and outer electron shell

X-RAY Tube Output The combined output of Bremsstrahlung and

Characteristic X-Rays results in an X-Ray spectrum

X RAY INTERACTION WITH MATTER

The higher the energy of the x-ray, the shorter the wavelength.

Low energy x-rays interact with whole atoms. Moderate energy x-rays interact with

electrons. High energy x-rays interact with the nuclei.

Five forms of x-ray Interactions Classical or Coherent Scattering Compton Effect Photoelectric Effect Pair production Photodisintegration

Two Forms of X-ray Interactions Important to Diagnostic X-ray

Compton Effect Photoelectric Effect

Changes in X-Ray Beam Quality and Quantity An increase in: Results in:

Current (mAs) Increase in quantity

Voltage (kVp) Increase in quantity and quality

Added Filtration decrease in quantity, increase in

quality

Increased target Z increase in quantity and quality

Increased voltage ripple decrease in quantity and quality

BRAIN STORMING SESSION

1.When filtration is added to an x-ray tube, radiation__________ increases

2.The quality of the x-ray beam is controlled by the ...

3.Filtration is added to the x-ray beam in order to remove___________x-rays.

4.The unit of measurement for the patient absorbed dose is the---------------

5.The wavelength of a bremsstrahlung x-ray is _________ proportional to it's frequency.

6.As the atomic number of an atom increases, the chances of a photoelectric effect occurring ------------

7. A _____________ interaction involves a photon collision with a loosely bound electron and formation of a scattered photon

8.___________________ must be known in order to calculate the energy of the x-ray photon

9.Adding filtration is sometimes called _______________________the beam.

10What versus what is used to plot the x-ray emission spectrum?

11.The target of an x-ray tube is often made from: Balsa woodCobaltTungstenAluminum

12.What are the two types of radiation? Particulate and electromagnetic

Alpha and BetaAlpha and GammaPositive and negative

13.Who discovered natural radioactivity? Wilhelm Roentgen

Marie CurieHenri BecquerelNone of the above

13.The primary risk from occupational radiation exposure is the increased risk of:

CancerBlindnessAbrasionsNone of the above

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