The X-ray Tube Cathode Assembly Continued

Transcription

1 The X-ray Tube Cathode Assembly Continued C. Focusing Cup: 1. Blooming 2. Saturation Current or Temperature Limited Region 3. Grid Biased X-ray Tubes

2 The Cathode Assembly 2

3 Focusing Cup The focusing cup is a shallow depression in the cathode assembly designed to house the filament. It is made of nickel or molybdenum. Filaments Focusing Cup

4 Purpose of the focusing cup: The purpose of the focusing cup is to narrow the thermionic cloud as they are driven from cathode to anode. Because the electrons are all negatively charged, they tend to spread out (repel) from each other. The focusing cup is given a negative charge which tends to focus the electrons to an area on the target.

5 The Focusing Cup

6 A Focused Electron Stream Results in Better Recorded Detail

7 Blooming Focal spot size increases or blooms, as milliamperage (ma) is increased. For example: 100 ma at 1 second = 100 mas 200 ma at ½ second = 100 mas 400 ma at ¼ second = 100 mas 800 ma at 1/8 second = 100 mas 1200 ma at 1/12 second = 100 mas

8 Focal spot size increases or blooms, as milliamperage (ma) is increased. Even though each of the technics on the previous slide produced the same quantity of x-rays, there were progressively more and more electrons produced by thermionic emission. It is harder to focus more electrons since they tend to repel as they travel from cathode to anode. This phenomenon is called blooming which will reduce recorded detail.

9 Blooming is a phenomenon that occurs as the ma is increased. The size of the focal spot will actually increase as the number of thermionic electrons increases. The radiograph with the best recorded detail would be one made with a technique using the smallest ma and longest time. 9

10 Saturation Current or Temperature Limited Region At any particular value of the filament current, an equilibrium is reached between the rate of electron emission by the filament and the rate of return to the filament from the space charge electrons. Thus, the space charge has a definite size for a given value of the filament current. A small change in filament current causes a large change in the size of the space charge.

11 Filament Filament Current determines Filament Temperature Which Determines the Number of Electrons Boiled Off

12 A Progressive Rise in ma as kvp is Increased (Not Good)

13 Space Charge Compensator To correct for the space charged limited region, a space charge compensator is placed in the filament circuit. The space charge compensator automatically lowers the filament current by just the right amount, as the kvp is raised, to keep ma constant over the useful kvp range. Now, the technologist has full control of tube operation, being able to change kvp without producing any change in ma.

14 Saturation Current or Temperature Limited Region

15 Grid-Biased Tube Grid biased tube is also called Grid pulsed tube or Grid controlled tube Has three electrodes (triode) instead of two: Anode, cathode, and wire grid Special type of X-ray tube that is used in: Digital angiography Capacitor-Discharge Mobile (Portable) Units Cardiac Catheterization (Cine radiography)

16 The purpose of a grid biased x-ray tube is to stop and start exposures quickly by regulating the flow of electrons (ma) from cathode to anode. Exposure is started by removing the negative charge on the grid to nearly zero; it is stopped by restoring a negative charge to the grid

17 Grid-Biased Tubes

18 I. The X-ray Tube 1. Anode Assembly 2. Stationary Anode 3. Rotating Anode

19 Anode Assembly The anode assembly is a complex device which consists of the: Anode (Anode Disk) Target, Focus, Focal Point, Focal Spot, Focal Track Stator Rotor Loosely speaking, the terms anode and target, are used interchangeably.

20 Anode Assembly

21 Anode The anode is the positive side of the x-ray tube and this major function: 1. it serves as the target surface for the high speed bombarding electrons and produces x-ray photons in the form of brems and characteristic radiation as discussed earlier.

22 There are two types of anodes: Stationary anode Anode is does not move (stationary) during the exposure Today, limited to low-power x-ray units such as dental units. Rotating anode Anode rotates during the exposure Diagnostic radiology uses rotating anodes because they are much more efficient in dissipating heat

23 Stationary Anode

24 X-RAY TUBES (Stationary vs. Rotating)

25 Anodes Stationary Rotating 90% - 95% Tungsten 5% - 10% Rhenium 25

26 Rotating Anode

27 Rotating Anode Composition

28 Tungsten (Z =74) Rhenium (Z = 75) In diagnostic radiology, tungsten is the primary metal of choice of x-ray production because of its: 1. High atomic number The percentage of the electrons energy appearing in the form of x rays, is directly proportional to the atomic number of the target/focal track. 2. High melting point Since more than 99% of the electrons kinetic energy changes to heat, the high melting point of tungsten at C makes it suitable as a target metal.

29 Molybdenum (Z = 42) In mammography, molybdenum is the primary metal of choice of x-ray photons because of its: Atomic number Molybdenum (42) because it has the ability to emit a more uniform range of lower energy photons (25 28 kvp) High melting point Tube window Composed of beryllium instead of Pyrex