Chapter 2 The Generation of Xray Xray Tubes

Chapter 2 The Generation of X-ray: X-ray Tubes

The Generation of X-ray • X-rays are produced whenever electrons, traveling at high speeds, collide with matter in any form.

The Generation of X-ray • There are three essentials that must be fulfilled before x-rays can be produced. They are: • 1. A source of electrons • 2. A means of accelerating & controlling their movement (via a difference in potential) • 3. A place to stop them with great suddenness (a point of impact)

The Generation of X-ray • The Law of Conservation of Energy energy can neither be created or destroyed. • Binding Energies - The forces that hold electrons in orbit around the nucleus. The nucleus (+) attracts the electron (-), but the spin of the electron keeps them from collapsing toward the nucleus.

The Generation of X-ray • Ground State - Inner shells have more energy available due to their proximity to the nucleus (stronger interaction). Outer shells have less energy available because they have weaker interaction with the nucleus.

The Coolidge Hot-Cathode Tube In modern x-ray tube the glass bulb is exhausted to as complete a vacuum as is possible to attain. The cathode (-) is composed of a small spiral filament of tungsten wire, about 1 cm long & 0. 2 to 0. 3 cm in diameter, which is housed in a focusing cup. The anode (+) is a solid rod of copper & molybdenum in the opposite end of the tube.

The Coolidge Hot-Cathode Tube The surface of the anode (+) facing the cathode (-) filament is beveled at between 7 and 17 degrees& has a block of tungsten set into it; it is only a few centimeters from the filament.

How The Three Essentials are Fulfilled in the Coolidge Hot-Cathode Tube • The source of electrons in modern xray tubes is the tungsten filament. This is connected to a step-down transformer & heated to incandescence by current from it. The heating of the wire alters its atomic stability in that the electrons are less firmly combined with the nucleus of the atom.

How The Three Essentials are Fulfilled in the Coolidge Hot-Cathode Tube • These loosely bound electrons hover about the cathode like a cloud. The greater the heat to the filament, the more electrons available at the face of the cathode. • This is called Thermionic Emission or Boiling Off of Electrons (the source of electrons)

How The Three Essentials are Fulfilled in the Coolidge Hot-Cathode Tube • If a high voltage current is applied to the tube (negative to the cathode) these electrons will be repelled from the cathode (like charges repelling) towards the anode (unlike charges attracting) with one-third to one-half the speed of light (a means of acceleration).

How The Three Essentials are Fulfilled in the Coolidge Hot-Cathode Tube • These electrons will strike the anode with great force (a point of impact) & be converted into x-rays & heat. • The energy of the speeding electrons is converted into two type of energy: • Greater than 99% Heat • Less than 1% X-Ray (exothermic reaction - heat > energy)

The Focal Spot of the X-Ray Tube • Most of the electrons bombard the target over a small area near its center. This is the actual focal spot of the tube. The actual focal spot has an area nearly equal in size to the overall dimensions of the filament.

The Focal Spot of the X-Ray Tube • The smaller the focal spot, the greater is the detail produced on the radiograph, but the smaller is the capacity of the tube to produce x-rays. This is explained by the fact that all the energy is expended at the focal spot. The smaller the focal spot, the more intense will be the heat developed. Longer exposure time is necessary with such tubes.

The Focal Spot of the X-Ray Tube • To control the size of the focal spot a sleeve of molybdenum can be placed around the filament & given a negative charge which repels the electrons from all directions, this will form a more narrow stream.

The Focal Spot of the X-Ray Tube • The principle of line focus is a method used to give a smaller focal spot with a larger target area. The actual focal spot is a rectangle approximately 3 times as long as it is wide.

The Focal Spot of the X-Ray Tube • When the beveled anode (+) face is viewed from the patients point of view, the focal spot appears to be nearly square. This is the effective projected focal spot. It serves to bring the x-ray source closer to being one point while still maintaining the larger area for impact.

Double Focus Tubes • Are those tubes having 2 focal spots - one fine (0. 3 mm) focus for maximum detail & one large (2. 0 mm) for heavier exposures. A mechanical switch includes the focus of choice in the circuit. Most x-ray machines have this built in.

Double Focus Tubes • Penumbra - blurring of the edge of an organ or bone due to the size of the focal spot. • Today the large focal spots are 0. 8 mm - 1. 0 mm, and are actually smaller than the old small focal spots. This is accomplished via new ways to cool the tubes.

Double Focus Tubes Summary • Large Filament = larger effective focal spot - used for larger body parts looses some detail. • Small Filament = smaller effective focal spot - used for small body parts such as extremity fractures (hair line) and fine detail. (this causes incredible heat build up)

Methods of Cooling the Anode • Sufficient heat is generated in the operation of an x-ray tube to melt the tungsten target (3370 C) that methods had to be developed to dissipate the heat & protect the tube. • Construction of the anode with two metals one with a very high melting point (tungsten) & the other with a high conductivity for heat (copper) - was an important first step.

Methods of Cooling the Anode • The main cooling methods that are used or have been used are as follows: • Natural Radiation - In this method heat is lost via the glass tube into the air. Low capacity tubes may be used for short periods without any means of cooling.

Methods of Cooling the Anode • Air Cooling by Radiation - A radiator (series of metal discs) may be attached to the extreme end of the anode to increase the surface area which can give off heat into the air. • Water Cooling - Obsolete today - water was carried away through a hollowed out anode stem.

Methods of Cooling the Anode • Oil Cooling - Almost all x-ray tubes in use today are surrounded by oil. The oil insulates as well as cools. oil & air cooling may be combined. • The Rotating Anode Tube - As the name implies the anode target rotates during the exposure. This allows us to increase the exposure because of the tremendous ability to dissipate heat.

The Rotating Anode • The anode in the tube is a beveled tungsten disc attached to a rotor that revolves when the tube is on. The cathode filament is offset to one side so that the electron stream hits near the edge of the revolving disc. • The rotating anode continually presents a different area on the target to the electron stream.

The Rotating Anode • The focal spot remains fixed in space while the circular anode rotates during the exposure to provide a cooler surface for the electron stream to strike. • The heat is distributed over a broad band, thus maintaining the temperature rise well within safe limits. As the capacity of the tube to withstand heat is increased, the capacity of the tube to produce x-rays is increased.

The Rotating Anode • It also permits manufacturers to produce tubes with smaller effective focal spots. • The disadvantages are: » the tube is very delicate » special lubricants are necessary for the motor which will not produce volatile gases

Tube Capacity • The tube capacity or their ability to produce x-rays is affected by the rotating anode. • Tubes are rated in terms of: • Kilovoltage (k. V) • Miliamperage (MA) • Time of exposure (S)

Tube Capacity • These factors are dependent on the rectification system, cooling method and focal spot size. • k. V - capacity is determined by the distance from the filament to the target. • MA - capacity is determined by size of the focal spot & the rectification system used. • S - capacity is determined by the anode & tube cooling rates.

The Heel Effect • Heel effect is the term applied to the fact that x -ray radiation does not exit the long axis of the tube in uniform intensities. • The intensity of the beam is equal to the number of rays & diminishes fairly rapidly from the central ray to the anode side of the patient, while increasing slightly toward the cathode side of the patient.

The Heel Effect • We can use this to our advantage in radiography if we remember to position the tube with the anode end of the tube towards the more easily penetrated body part. • Originally it was thought that the heel effect was due to the angulation of the anode & was based upon theory of refraction.

The Heel Effect • Today we believe that the electrons which are traveling at high speeds, bombard the target and xray is produced in 360 degrees of direction. 180 degrees of this x ray produced is absorbed by the anode itself. • Therefore, only 180 degrees of x-rays leave the anode. • Of those, some are absorbed by the lead shutters of the collimator, allowing only those travelling in the desired direction to exit the tube.

The Heel Effect • X-ray is not uniform along the film surface. Therefore the effects of the heel effect are seen toward the edges of the film. • To decrease the heel effect, increase the collimation & decrease the film size. • With increased collimation you get less heel effect.

The Heel Effect • For example, in taking a radiograph of the cervical-thoracic area, the neck area should receive the rays from the anode portion of the beam as the neck is thinner than the thoracic region.