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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

L 6: X Ray production

IAEAInternational Atomic Energy Agency

IntroductionA review is made of: The main elements of the of X Rays tube: cathode and anode structure The technology constraints of the anode and cathode material The rating charts and X Ray tube heat loading capacitiesIAEA

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Topics Basic elements of an X Ray source assembly Cathode structure Anode structure Rating chart X Ray generator Automatic exposure control

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Overview To become familiar with the technologicalprinciples of the X Ray production

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Part 6: X Ray productionTopic 1: Basic elements of an X Ray source assembly


Basic elements of the X Ray source assembly Generator : powercircuit supplying the required potential to the X Ray tube X Ray tube and collimator: device producing the X Ray beamIAEA

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X Ray tubes

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X Ray tube components Cathode: heated filament which is the sourceof the electron beam directed towards the anode tungsten filament

Anode (stationary or rotating): impacted by

electrons, emits X Rays Metal tube housing surrounding glass (or metal) X Ray tube (electrons are traveling in vacuum) Shielding material (protection against scattered radiation)IAEA

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X Ray tube componentshousing cathode

1: mark of focal spotIAEA

1: long tungsten filament 2 : short tungsten filament 3 : real size cathode 6: X Ray production 9


Part 6: X Ray production

Topic 2: Cathode structure


Cathode structure (I)Cathode includes filament(s) and associated circuitry tungsten material : preferred because of its highmelting point (3370C) slow filament evaporation no arcing minimum deposit of W on glass envelope

To reduce evaporation the emission temperature of the cathode is reached just before the exposure in stand-by, temperature is kept at 1500C so that2700C emission temperature can be reached within a second IAEA

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Example of a cathode

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Cathode structure (I)

Modern tubes have two filaments

a long one : higher current/lower resolution a short one : lower current/higher resolution

Coulomb interaction makes the electron beam divergent on the travel to the anodelack of electrons producing X Rays larger area of target used focal spot increased lower image resolution

Focalisation of electrons is crucial !IAEA

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Topic 3: Anode structure


X Ray tube characteristics Anode mechanical constraints Material : tungsten, rhenium, molybdenum, graphite Focal spot : surface of anode impacted by electrons Anode angle Disk and annular track diameter (rotation frequency from 3,000 to 10,000 revolutions/minute) Thickness mass and material (volume) heat capacity

Anode thermal constraints Instantaneous power load (heat unit) Heat loading time curve Cooling time curveIAEA

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Anode angle (I)

The Line-Focus principle

Anode target plate has a shape that is more rectangular or ellipsoidal than circular

the shape depends on : filament size and shape focusing cups and potential distance between cathode and anode

This conflict is solved by slantingthe target faceIAEA

Image resolution requires a small focal spot Heat dissipation requires a large spot

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Anode characteristic

1 : anode track 2 : anode track

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Anode angle (II) Angle Angle

Incident electron beam width

Actual focal spot size Incident electron beam width Apparent focal spot size Film

Actual focal spot size Increased apparent focal spot size Film

THE SMALLER THE ANGLE THE BETTER THE RESOLUTIONIAEA

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Anode heel effect (I) Anode angle (from 7 to 20) induces avariation of the X Ray output in the plane comprising the anode-cathode axis Absorption by anode of X photons with low emission angle The magnitude of influence of the heel effect on the image depends on factors such as : anode angle size of film focus to film distance

Anode aging increases heel effectIAEA

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Anode heel effect (II) The heel effect is not always a negativefactor It can be used to compensate for different attenuation through parts of the body For example: thoracic spine (thicker part of the patienttowards the cathode side of the tube) mammography6: X Ray production

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Focal spot size and imaging geometry Focal spot finite size image unsharpened Improving sharpness small focal spot size For mammography focal spot size 0.4 mm nominal Small focal spot size reduced tube output (longerexposure time)

Large focal spot allows high output (shorter exposuretime)

Balance depends on organ movement (fast movingorgans may require larger focus)IAEA

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Topic 4: Rating Chart


Heat loading capacities A procedure generates an amount of heat depending on: kV used, tube current (mA), length of exposure type of voltage waveform number of exposures taken in rapid sequence

Heat Unit (HU) [joule] :unit of potential x unit of tube current x unit of time

The heat generated by various types of X Ray circuits are: 1 phase units : 3 phase units, 6 pulse : 3 phase units, 12 pulse:IAEA

HU = kV x mA x s HU = 1.35 kV x mA x s HU = 1.41 kV x mA x s 23

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X Ray tube rating chart (I) Tube cooling characteristics and focal spot size {mA - time} relationship at constant kV intensity decreases with increasing exposure time intensity increases with decreasing kV

Note: higher power reduced exposure time reduced motion unsharpness

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X Ray tube rating chart (II) Manufacturerscombine heat loading characteristics and information about the limits of their X Ray tubes in graphical representations called Tube Rating Charts Example: Tube A: a 300 mA, 0.5 s, 90 kV procedure woulddamage the system operated from a 1-phase half wave rectified generator (unacceptable)

Tube B: a 200 mA, 0.1 s, 120 kV procedure comply withthe technical characteristics of the system operated from a 3-phase fully rectified generator (acceptable)

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X Ray tube rating chart (III)X Ray tube A 1 half-wave rectified 3000 rpm 90 kV 1.0 mm effective focal spot

700 600

Tube current (mA)

500 400 300 200 100 0.01 0.05 0.1 0.5 1.0 5.0 10.0970

kV p

120 k 0 kVp Vp

Unacceptable

50 kVp

Exposure time (s)

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X Ray tube rating chart (IV)

700

Tube current (mA)

600 500 400 300 200 100 0.01 0.05 0.1

70 kV p 90 kV p 125 kV p

X Ray tube B 3 full-wave rectified 10.000 rpm 125 kV 1.0 mm effective focal spotUnacceptable

50 kV p

Acceptable

0.5

1.0

5.0

10.0

Exposure time (s)IAEA

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Anode cooling chart (I) Heat generated is stored in the anode, anddissipated through the cooling circuit A typical cooling chart has: input curves (heat units stored as a function of time) anode cooling curve

The following graph shows that: a procedure delivering 500 HU/s can go onindefinitely if it is delivering 1000 HU/s it has to stop after 10 min if the anode has stored 120.000 HU, it will take 5 min to cool down completelyIAEA

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Anode cooling chart (II)240

Maximum Heat Storage Capacity of AnodeHU 00 10500

Heat units stored (x 1000)

220 200 180 160 140 120 100 80 60 40 20

c /seec

Imput curve

/s HU

U/ 350 H

sec

ec 250 HU/s

Co oli

ng c

urv e

1

2

3

4

5

6

7

8

9

10

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12

13

14

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Elapsed time (min)

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Topic 5: X Ray generator


X-ray generator (I) It supplies the X-ray tube with : Current to heat the cathode filament Potential to accelerate electrons Automatic control of exposure (power application time) Energy supply 1000 X-ray beam energy (of which 99.9% is dissipated as thermal energy)IAEA

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X-ray generator (II) Generator characteristics have a strong influence onthe contrast and sharpness of the radiographic image The motion unsharpness can be greatly reduced by a generator allowing an exposure time as short as achievable Since the dose at the image plane can be expressed as: D = k0 . Un . I . T

U: peak voltage (kV) I: mean current (mA) T: exposure time (ms) n: ranging from about 1.5 to 3 6: X Ray production 32

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X-ray generator (III) Peak voltage value has an influence on thebeam hardness It has to be related to medical question What is the anatomical structure to investigate ? What is the contrast level needed ? For a thorax examination : 140 - 150 kV is suitable tovisualize the lung structure While only 65 kV is necessary to see bone structure

The ripple r of a generator has to be as low aspossibleIAEA

r = [(U - Um )/U] x 100% in6: X Ray production 33

Tube potential wave form (I) Conventional generators single single three three 1-pulse (dental and some mobile systems) 2-pulse (double rectification) 6-pulse 12-pulse

Constant potential generators (CP) HF generators (use of DC choppers to convert50Hz mains into voltages with frequencies in the kHz range) Inverter technologyIAEA

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Tube potential wave form (II)kV ripple (%) 100% 13% Three phase 6-pulse 4% Three phase 12-pulse Single phase single pulse Single phase 2-pulse

Line voltage

0.01 s 0.02 s

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The choice of the number of pulses (I) Single pulse : low power (

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