Raiobiology 9

IAEAInternational Atomic Energy Agency

RADIOBIOLOGY

Prof.Dr.Tarek Elnimr

L 9Presented to the

Biology Departments

in Faculty of Sciences

on February 15 , 2009


L 9: X Ray production

IAEA 6: X Ray production 3

Introduction

A 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 capacities


Topics

• Basic elements of an X Ray source assembly

• Cathode structure

• Anode structure

• Rating chart

• X Ray generator

• Automatic exposure control


Overview

• To become familiar with the technological principles of the X Ray production


Part 6: X Ray production

Topic 1: Basic elements of an X Ray source assembly

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology


Basic elements of the X Ray source assembly

• Generator : power circuit supplying the required potential to the X Ray tube

• X Ray tube and collimator: device producing the X Ray beam


X Ray tubes


X Ray tube components

• Cathode: heated filament which is the source of 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)


X Ray tube components

1: long tungsten filament2 : short tungsten filament3 : real size cathode

1: mark of focal spot

housing cathode



Topic 2: Cathode structure



Cathode structure (I)

• Cathode includes filament(s) and associated circuitry

• tungsten material : preferred because of its high melting point (3370°C)

• 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 ± 1500°C so that 2700°C emission temperature can be reached within a second


Example of a cathode


• 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 anode• lack of electrons producing X Rays

• larger area of target used

• focal spot increased lower image resolution

Focalisation of electrons is crucial !

Cathode structure (I)



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 curve


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 cup’s and potential

• distance between cathode and anode

• Image resolution requires a small focal spot

• Heat dissipation requires a large spot

• This conflict is solved by slanting the target face


Anode characteristic

1 : anode track2 : anode track


THE SMALLER THE ANGLETHE SMALLER THE ANGLETHE BETTER THE RESOLUTIONTHE BETTER THE RESOLUTION

Anode angle (II)

Angle

Incident electron beam width

Apparent focal spot size

Actual focal spot size

Film

Angle

Incident electron beam width

Increased apparent

focal spot size

Actual focal spot size

Film

‘


Anode heel effect (I)

• Anode angle (from 7° to 20°) induces a variation 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 effect


• The heel effect is not always a negative factor

• It can be used to compensate for different attenuation through parts of the body

• For example:• thoracic spine (thicker part of the patient

towards the cathode side of the tube) • mammography

Anode heel effect (II)


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 (longer

exposure time)

• Large focal spot allows high output (shorter exposure time)

• Balance depends on organ movement (fast moving organs

may require larger focus)



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 : HU = kV x mA x s• 3 phase units, 6 pulse : HU = 1.35 kV x mA x s• 3 phase units, 12 pulse: HU = 1.41 kV x mA x s


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


• Manufacturers combine 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 would damage

the system operated from a 1-phase half wave rectified generator (unacceptable)

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

X Ray tube rating chart (II)


0.01 0.05 0.1 0.5 1.0 5.0 10.0

700

600

500

400

300

200

100

50 kVp

70 kVp

90 kVp120 kVp

Unacceptable

Exposure time (s)

Tu

be

curr

ent

(mA

)X Ray tube A

half-wave rectified3000 rpm 90 kV

1.0 mm effective focal spot

X Ray tube rating chart (III)


0.01 0.05 0.1 0.5 1.0 5.0 10.0

700

600

500

400

300

200

100

50 kVp

70 kVp90 kVp125 kVp

Acceptable

Exposure time (s)

Tu

be

curr

ent

(mA

)

X Ray tube B3 full-wave rectified

10.000 rpm 125 kV1.0 mm effective focal spot

X Ray tube rating chart (IV)

Unacceptable


Anode cooling chart (I)

• Heat generated is stored in the anode, and dissipated 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 on

indefinitely

• 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 completely


240

220

200

180

160

140

120

100

80

60

40

20

Elapsed time (min)

He

at

un

its

sto

red

(x

10

00

)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

500 HU/sec1000 HU/sec

350 HU/sec

250 HU/sec

Imput curve

Cooling curve

Maximum Heat Storage Capacity of Anode

Anode cooling chart (II)



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)


• Generator characteristics have a strong influence on the 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

X-ray generator (II)


• Peak voltage value has an influence on the beam 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 to

visualize the lung structure • While only 65 kV is necessary to see bone structure

• The ripple “r” of a generator has to be as low as possible

r = [(U - Umin)/U] x 100%

X-ray generator (III)


Tube potential wave form (I)

• Conventional generators• single 1-pulse (dental and some mobile systems)

• single 2-pulse (double rectification)

• three 6-pulse

• three 12-pulse

• Constant potential generators (CP)

• HF generators (use of DC choppers to convert 50Hz mains into voltages with frequencies in the kHz range) “Inverter technology”


100%

13%

4%

Line voltage

Single phase single pulse

Single phase 2-pulse

Three phase 6-pulse

Three phase 12-pulse

0.02 s

0.01 s

kV ripple (%)

Tube potential wave form (II)


The choice of the number of pulses (I)

• Single pulse : low power (<2 kW)

• 2-pulse : low and medium power

• 6-pulse : uses 3-phase mains, medium and high power (manual or automatic compensation for voltage drop)

• 12-pulse : uses two shifted 3-phase system, high power up to 150 kW


• CP : eliminates any changes of voltage or tube current • high voltage regulators can control the voltage

AND switch on and off the exposure• voltage can be switched on at any moment

(temporal resolution)• kV ripple <2% thus providing low patient exposure

• HF : combines the advantages of constant potential and conventional generator• reproducibility and consistency of tube voltage • high frame rate possible

The choice of the number of pulses (II)


Part 6: X-ray production

Topic 6: Automatic Exposure Control (AEC)



Automatic exposure control

• Optimal choice of technical parameters in order to avoid repeated exposures (kV, mA)

• Radiation detector behind (or in front of) the film cassette (with due correction)

• Exposure is terminated when the required dose has been integrated

• Compensation for kVp at a given thickness

• Compensation for thickness at a given kVp



X Ray tube

Collimator

Beam

Soft tissue

BoneAir

Patient

Table

Grid

Cassette

AEC detectors



• Optimal choice of technical parameters in order to avoid repeated exposures (kV, mA)

• Radiation detector behind (or in front of) the film cassette (with due correction)

• Exposure is terminated when the required dose has been integrated

• Compensation for kVp at a given thickness

• Compensation for thickness at a given kVp


Part 6: X-ray production

Topic 7: X-ray equipment operation and mode



X-ray equipment operation mode and application (II)

Radiography and Tomography• Single and 3 generators (inverter technology)

• output : 30 kW at 0.3 focus spot size

• output : 50 - 70 kW at 1.0 focus spot size

• selection of kV and mAs , AEC

Radiography and Fluoroscopy• Under couch equipment, three generator (inverter

technology) - continuous output of 300 - 500 W• output : 50 kW at 1.0 focus size for spot film

• output : 30 kW at 0.6 for fluoroscopy (high resolution)

• priority given to contrast

• automatic settings of kV


X-Ray equipment operation mode and application (III)

• Radiography and Fluoroscopy• Over couch equipment, three phase generator (inverter

technology) - continuous output of at least 500 W

• output : 40 kW @ 0.6 focus size for spot film

• output : 70 kW @ 1.0 for fluoroscopy (high resolution)

• priority given to contrast

• automatic settings of kV

• Cardiac angiography • Three phase generator - continuous output 1kW

• output : 30 kW @ 0.4 focus size

• output : 80 kW @ 0.8 focus size

• frame rate : up to 120 fr/s


Summary

• The main parts of the system contributing to

the desired X Ray production:

• provide the required source of power

• deliver an appropriate X Ray spectrum

• ensure the optimum adjustment of exposure to

warrant the image quality


Where to Get More Information

• Equipment for diagnostic radiology, E. Forster, MTP Press, 1993

• IPSM Report 32, part 1, X-ray tubes and generators

• The Essential Physics of Medical Imaging, Williams and Wilkins. Baltimore:1994

• Manufacturers data sets for different X Ray tubes

Raiobiology 9

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Transcript of Raiobiology 9