The xray machine Shapes and sizes Xray machine

The x-ray machine

Shapes and sizes • X-ray machine function: • is to provide a sufficient intensity of electron flow in a controlled manner to produce an x-ray beam of desired quantity and quality. • They come in a different shapes and sizes. • They operate at maximum voltages ranging from 40 to 150 k. Vp and tube currents of 25 to 1200 m. A. • Where therapeutic x-ray machines operated at higher or lower voltages and at tube currents ≤ 20 m. A

• Every x-ray machine has three principal parts: 1) 2) The x-ray tube The high voltage section (generator) 3) Control console. Note: except dental and portable machine these are housed compactly.

Room design • Most rooms have the head of the x-ray tube located in one room. • The control console in an adjoining room • A protective barrier (with a window) • The high-voltage generator is housed in a cubicle container (3 ft) located in the corner. • Newly designed they are ceiled above the room

X-ray tube • It is rarely seen by radiologic technologist. • • - It is contained in a protective housing. It has two main parts: The cathode The anode Each of these is called an electrode Any tube with two electrode is called diode The x-ray is a diode.

The X-Ray Tube Development • Dr. Roentgen used a Crookes-Hittorf tube to make the first x-ray image. • There was no shielding so x-rays were emitted in all directions. • The Coolidge Hot cathode tube was a major advancement in tube Design. The radiator at the end of the anode cool the anode. • This is the variety of tube designs available in 1948. • The Coolidge tube was still available.

The X-Ray Tube Development • Two major hazards plagued early radiography. - Excessive radiation exposure - Electric Shock • The X-Ray Modern X-ray Tube: • There are two principle parts: • The rotating anode • The cathode • Any tube that has two electrodes is called a diode.

Protective housing • The tube is housed in a lead lines metal protective housing. • The x-ray photons are generated isotropically or in all directions. • The housing is designed to limit the beam to window. • Those emitted from the window useful x-rays • The others that penetrate through the housing are leakage radiation • Those result in unnecessary exposure of pt and technologist

Protective housing • The tube can not have more than 100 m. R at 1 m (26 µ C/kg) / Hour when operated at it maximum output. • The housing also provide mechanical support and protection from damage. • The housing incorporates specially designed high voltage receptacles to protect against electrical shock • On some tubes, the housing also contains oil that provides more insulation and a thermal cushion. • Some have a cooling fan to air-cool the tube or the oil in which the tube is bathed. • Never hold the tube during an exposure. • Never use the cables or terminals as handles for positioning the tube.

The X-Ray Tube Glass Envelope • X-ray tube is an electronic vacuum tube • The components of the tube are contained within the glass envelope. • The glass envelope is made of Pyrex to withstand the tremendous heat produced during x-ray. • The vacuum allows for more efficient x-ray production and longer life • If the tube filled with gas the e- flow from cathode to anode would be hindered less x-rays would be produced and more heat would be created. • The window is a 5 cm square with a thin section of glass where the useful beam is emitted. • Allow maximum x-ray emission and minimum absorption in the glass envelope.

The Cathode • The cathode is the negative side of the tube and contains two primary parts: • The filaments • The focusing cup

The Filaments • It is a coil of wire • It is usually about 2 mm in diameter and 1 to 2 cm long • The filament emits electrons. • When the filament current is sufficiently intense the outer shell e- of the filament atoms are boiled off and ejected from the filament. • this phenomenon is called thermionic emission. • Most tube have two filaments which provide a choice of quick exposures or high resolution. • The filaments are made of thoriated tungsten

The Filaments • Tungsten is used in x-ray tube because of it’s high melting point of 3410°C. • X-rays are produced by thermionic emission when a 4 A or higher current is applied. • Tungsten doesn’t vaporize easily if it did gassy and internal parts are coated with it tube failure. • 1%-2% of thorium increase the efficiency of thermionic emission and prolongs tube life.

Focusing Cup • Filament is embedded in metalic cup • Cause the negative beam tend to spread out • The focusing cup has a negative charge so that it can condense the electron beam to a small area of the anode. • - The effectiveness of the focus cup is determined by: Its size and shape Its charge The filament size and shape The position of the filament within the focusing cup.

Filament Current • When the x-ray machine is turned on, a low current flows through the filament to warm it and prepare it for the big thermal volt necessary for x-ray production. • Low filament current it is not hot enough for thermionic emission. • Once the current is high enough for thermionic emission a small rise in filament current will result in a large rise in tube current.

Space Charge • When emitted by the filament, the electrons form a cloud near the filament momentarily before being accelerated to the anode. • This is called a space charge. • Space charge effect is produced due to the electrostatic repulsion (e- are difficult to be emitted by the filament).

Dual-focus tube • Most diagnostic tubes have two focal spots • Two filaments are in the focal spot • Large focal spot (1 to 2. 5 mm) large filament large details when techniques that produce high heat are required. • Small focal spot (0. 3 - 1 mm) small filament fine details are required. • The selection is made with m. A selector on the control console

The anode • Two types : stationary and rotating • Stationary : portable x-ray machine where low m. A and power are the only needed • Rotating : produce high intensity x-ray beams in a short time. • The rotating anode allows the electron beam to interact with a much larger target area. • The heat is not confined to a small area.

The Rotating Anode • The anode serves three functions: • Receives the electrons emitted from the cathode. • It is a electrical conductor (e- from the tube to cables and back to the high-voltage section of the x-ray machine). • Mechanical support for the target.

The Rotating Anode • The Anode must also be a good thermal conductor. • When the electron beam strikes the anode more than 99% of the kinetic energy is converted to heat. • The heat must be conducted before melting the anode. • Copper is the most common anode material

Anode target • • Target is the area of the anode struck by the e. In rotating anode the target is a rotating disc. Tungsten-rhenium is used as the target for the electron beam. Tungsten is used for three reasons – High atomic number - Higher efficiency x-ray production and x-ray energy. – Heat conductivity – High melting point - During exposure anode is raised to 2000 C

The Rotating Anode The rotor is an electromagnetic induction motor. It spins at 3400 rpm. High speed anodes spin at 10, 000 rpm. Even with the anode rotating, some melting occurs. The heat must be rapidly dissipated. • Molybdenum and copper are used to rapidly transfer the heat from the target. • •

• When the exposure button is depressed, current is applied to the tube that produces a magnetic field by the stator outside the glass envelope • The magnetic field interacts with the rotor and starts the rotation of the anode. • That’s why we have to wait a sec before taking the exposure to allow the rotor accelerating to its designed revolution per minute. When the anode is spinning at the correct speed, the exposure can be made. • After the exposure is completed, it slows by reversing the motor.

• This is what can happen to an anode when the anode stops turning. • The anode actually melts.