X-ray Tube -2

Line focus principleHeal effectRatingsTube failure

Line-Focus Principle

The Effective Focal Spot is the beam projected onto the patient.

As the anode angle decreases, the effective focal spot decreases.

Diagnostic tube target angles range from 5 to 15°.

Advantages & limitations of LFP

Advantages limitationsThe advantage of Line

focus is it provides the sharpness of the small focal spot with the heat capacity of the large large focal spot.

Smaller target angles will produce smaller effective focal spots and sharper images.

Area covered by the beam reduces with target angle To cover a 17” the angle

must be 12° To cover 36” the angle must

be 14° Anode heel effect (next

slide)

Anode Heel Effect

Due to the absorption of x-rays by the anode heel the radiation intensity on the cathode side of the x-ray tube is higher than the anode side.

This effect is increased when the target angle is reduced

Influence of Anode heel effectThe difference in the

intensity can vary by as much as 45%.

If the center is 100% the anode side of the beam can as low as 75% and the cathode as much as 120%.

The heel effect should be considered when positioning areas of the body with different thickness or density.

The cathode side should be over the area of greatest density.

Anode Heel Effect on Focal spot size and ResolutionThe effective focal

spot size and shape change across the projected field

The sharpness of the image can be dependent upon which area of the beam coverage you are looking at.

Similar to the shape distortion when the tube is not centered.

Off Focal Radiation

The electrons can rebound and interact with other areas of the anode other than the focal area.

These interations can produce x-rays too.

This is called Off-Focal Radiation.

Control of Off Focal Radiation

A diaphragm is placed between the tube and the collimator to reduce off focus rays.

But the off focus radiation completely

The percentage of off focus radiation increases with kV

X-ray Tube Rating ChartsThe that provide

information on the safe exposures and safe operating conditions of x-ray tubes are called tube rating charts

With careful use, the x-ray tube can provide long periods of service.

Inconsiderate or careless operation can lead to shortened life or abrupt failure.

X-ray tubes are very expensive. Cost varies from $2,000 to $20,000.

Types of rating chartsRadiographic rating chartAnode cooling chartHousing cooling chart

Radiographic rating chartThis is the most important of the three chartsIt conveys which radiographic exposures are safe

and which are unsafeThe chart shows a family of curves for different mAThe two axes X & Y show scales of Time and kV

respectivelyFor a given mA, any combination of kVp and time

that lies below the curve is safeAny combination that lies above the curve of

desired mA is unsafeModern x-ray systems have a microprocessor

control that does not allow unsafe exposure to be made.

Use of Series of radiographic rating chartsImportant to use the correct rating chart

e.g

Rating charts for different filament sizes (focal spot sizes)

For different anode rotation speedsFor different anode anglesFor the type of high voltage rectification

Radiographic rating charts can be used to check the proper operation of microprocessor control protection circuit

QuestionRadiographic examination of the abdomen

with a tube that has a 0.6 mm focal spot and anode rotation of 10,000 rpm requires technique factors of 95 kVp, 150 mAs.

Calculate using the correct rating chart, the shortest possible exposure time for this examination.

Anode cooling chart Anode cooling charts contain the

information about the thermal capacity of an anode and its heat dissipation characteristics.

It does not depend on the filament size and the speed of rotation

Usually the cooling is rapid at first and slows as the anode cools

In addition to knowing the maximum heat capacity the chart is used to determine the length of time required for complete cooling after any level of heat input.

Anode cooling chart

Anode thermal capacity, HU & Joule

The maximum amount of heat that can be stored in the anode without thermal damage

In x-ray applications, It is measured in Heat Units (HU)1HU = 1kVp x 1 mA x 1s (for single phase)

Actual heat energy is measured in Joules (J)1 J = 1 volt x 1 amp x 1s = 1 kV x 1 mA x 1s (here the kV is the

direct or the RMS voltage)For single phase kVp = 1.4 kV (rms)For 3 phase or high frequency kVp = kV

(rms)Therefore for three phase/high frequency, HU=1.4 x kVp x mA x s

Questions1. Radiographic examination of the lateral

lumber spine with a single-phase imaging system requires 98 kVp, 120 mAs. How many heat units are generated by this exposure?

2. A fluoroscopic examination is performed with a single-phase imaging system at 76 kVp and 1.5 mA for 3.5 minutes. How many heat units are generate?

3. Six sequential skull films are exposed with a three-phase generator operate at 82 kV, 120 mAs. What is the total heat generate?

Questions1. A particular examination results in

delivery of 50,000 HU to the anoe in a matter of seconds. How long will it take the anode to cool completely? (use the anode cooling chart given in slide 16)

2. How much heat energy (in joules) is produce during a single phase mammographic exposure of 25 kVp, 200 mAS?

Housing Cooling ChartThe cooling chart for the housing of the x-

ray tube has a similar shape as the anode cooling chart.

The maximum heat capacity of the housing is in the range of several million heat units.

Complete cooling after maximum heat capacity requires from 1 to 2 hours

X-ray tube failureAll causes of tube failure relate to the thermal

characteristics of the tube.When the temperature of the anode during a

single exposure is excessive, localized melting and pitting occurs.

These surface irregularities lead to variable and reduced radiation output.

If the melting is severe, the tungsten vaporizes and can plate the port. This can cause added filtering or interference with the flow of electrons.

If the temperature of the anode increases to rapidly, the anode can crack and then become unstable in rotation.

Maximum radiographic techniques must never be applied to a cold anode.,

During long exposures (1 to 3 seconds) the anode may actually glow like a light bulb.

The heat may cause a failure of the bearing for the anode or a crack in the glass envelope.

Because of the high heat of the filament, the inside of the glass envelope. This will tungsten atoms are slowly vaporized and plate eventually lead to arcing and tube failure.

Continuous high mA radiography will actually lead to the filament breakage.

Anode faults

a. New

b. Surface damage due to repeated over load

c. Pitting due to slow rotation

d. Surface damage due to Exceeding of heat capacityC d

Tube Warm-up ProceduresBy warming the anode through a series of

exposures and increasing kVp settings, the anode will build up heat that is needed to avoid fracture of the anode.

This process takes a little over one minute put will add to the life of the tube.

Close shutters of collimator.Make exposure of 12 mAs @ 70 kVpWait 15 secondsMake exposure of 12 mAs @ 80 kVpWait 15 secondsMake exposure of 12 mAs @ 90 kVpTube warm up is now complete.

END

V.G.WimalasenaPrincipal

School of radiography