IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

L 6: X Ray production

IAEA International Atomic Energy Agency

Introduction

• • • A review is made of: The main elements of the 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

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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 technological principles of the X Ray production

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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 6: X Ray production

Topic 1: Basic elements of an X Ray source assembly

IAEA International Atomic Energy Agency

Basic elements of the X Ray source assembly

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

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X Ray tubes

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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, > 99% of electron energy is dissipated as heat Metal tube housing surrounding glass (or metal) X Ray tube (electrons are traveling in vacuum) Shielding material (protection against extra focal spot radiation from anode)

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X Ray tube components

housing cathode 1: mark of focal spot

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

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 6: X Ray production

Topic 2: Cathode structure

IAEA International Atomic Energy Agency

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 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 causes the electron beam to diverge on the way to the anode • larger area of target used • focal spot increased  lower image resolution Focusing of electrons is crucial !

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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 6: X Ray production

Topic 3: Anode structure

IAEA International Atomic Energy Agency

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

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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 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

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Anode characteristic

IAEA 1 : anode track 2 : anode pits caused by electron beam being stationery on the anode

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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 Actual focal spot size Increased apparent focal spot size Film Film THE SMALLER THE ANGLE THE BETTER THE RESOLUTION IAEA

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Anode heel effect (I)

• • • Anode angle (from 7 ° to 20°) induces a variation of the X Ray output in the imaging plane parallel to 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 IAEA

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Anode heel effect (II)

• • • 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

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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 (longer exposure time) • Large focal spot allows high output (shorter exposure time) • Balance depends on organ movement (fast moving organs may require larger focus)

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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 6: X Ray production

Topic 4: Rating Chart

IAEA International Atomic Energy Agency

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

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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)

• 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 rectified generator (unacceptable) 1-phase half wave •

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 )

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X Ray tube rating chart (III)

IAEA 700 600 500 400 300 200 100 0.01

X Ray tube A

1 f

half-wave rectified 3000 rpm 90 kV 1.0 mm effective focal spot Unacceptable 0.05

0.1

0.5

1.0

Exposure time (s) 5.0

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10.0

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X Ray tube rating chart (IV)

IAEA 700 600 500 400 300 200 100 0.01

3

f

X Ray tube B full-wave rectified 10.000 rpm 125 kV 1.0 mm effective focal spot Unacceptable Acceptable 0.05

0.1

0.5

1.0

Exposure time (s) 5.0

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Anode cooling chart (I)

• • • Heat generated is stored in the anode and dissipated by radiative cooling to the x-ray tube, oil, and housing 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

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Anode cooling chart (II)

IAEA 240 Maximum Heat Storage Capacity of Anode 220 200 180 160 140 120 100 80 60 40 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Elapsed time (min)

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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 6: X Ray production

Topic 5: X Ray generator

IAEA International Atomic Energy Agency

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)

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X-ray generator (II)

• • • 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 = k 0 . U n . I . T

• • • •

U

: peak voltage (kV)

I

: mean current (mA)

T

: exposure time (ms)

n

: ranging from about 1.5 to 3

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X-ray generator (III)

• • 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

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Tube potential wave form (I)

• • • • • • • Conventional generators single f single f three f three f 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 convert 50Hz mains into voltages with frequencies in the kHz range)  “Inverter technology”

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Tube potential wave form (II)

kV ripple (%) 100% 13% 4% Single phase single pulse Single phase 2-pulse Three phase 6-pulse Three phase 12-pulse Line voltage IAEA 0.01 s 0.02 s

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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

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The choice of the number of pulses (II)

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

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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 6: X-ray production

Topic 6: Automatic Exposure Control (AEC)

IAEA International Atomic Energy Agency

Automatic exposure control

• Optimal choice of technical parameters (kV, mA) to optimize patient dose and image quality • 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

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Automatic exposure control

X Ray tube Collimator Beam

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Patient Table Grid

AEC detectors

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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 6: X-ray production

Topic 7: X-ray equipment operation and mode

IAEA International Atomic Energy Agency

X-ray equipment operation mode and application (II)

Radiography and Tomography • Single and 3 f • • 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 f 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)

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• • • capable of pulsing at 30, 15, 7.5 fps or less priority given to contrast automatic settings of kV 6: X Ray production 42

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  • • • output : 30 kW @ 0.4 focus size output : 80 kW @ 0.8 focus size frame rate : up to 120 fr/s 1kW

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Summary

• The x-ray system: • provides the required source of power • delivers an appropriate X Ray spectrum • assures the optimum adjustment of exposure to optimize image quality

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Where to Get More Information

• The Essential Physics of Medical Imaging. JT Bushberg, JA Seibert, EM Leidholdt, JM Boone. Lippincott Williams & Wilkins, Philadelphia, 2011

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