IAEAInternational Atomic Energy Agency

X-ray Generation and Imaging

Radiation Sources in medicine diagnostic Radiology

Day 6 – Lecture 8(2)

IAEA 2

Objective

• To become familiar with the technology and operation of x-ray tubes and generators and their specific use in medicine.

• To understand the specific radiation risks linked with these devices.

• To become familiar with the various types of image receptors.

• To be aware of the advantages and limitations of each type of receptor.

IAEA 3

Contents

• Description and physical characteristics of x-ray tubes and generators. Principles of operation.

• Radiation quality of an x-ray beam; x-ray tube potential, total filtration, first half-value layer.

• Influence of radiation quality on patient dose and image quality.

• Equipment malfunction and Quality control.

• Physical characteristics of x-ray film and intensifying screens.

• Physical characteristics of digital imaging technologies.

• Equipment malfunctions affecting radiation protection.

IAEA 4

X-ray equipment and accessories:

• should be designed and purpose built for the intended imaging task;

• shall indicate at the control panel all the important parameters relevant to image quality and patient dose.

Introduction

• should be certified to be in compliance with the relevant standards of the International Electro-Technical Commission (IEC) or equivalent national standard(s);

IAEA 5

• a source of electrons; a heated tungsten filament (cathode);

• a metal target (anode);

• a high electric field (kilovolts) to accelerate the electrons between the source and the target;

The cathode and anode are held in an evacuated envelope (the x-ray tube). A high voltage generator provides the potential to accelerate the electrons from the cathode to the anode.

Three basic elements are needed for x-ray generation:

Generation of x-rays

IAEA 6

A stationary anode x-ray

tube

Generation of x-rays (cont)

IAEA 7

Specific requirements for x-ray tubes:

• as small a focal spot as practicable;

• sufficient filament current (and therefore electrons) to minimize exposure times;

Generation of x-rays (cont)

• an efficient method for dissipating the heat generated in the target (anode);

• appropriate material, area and angulation of the anode;

• a choice of either a rotating or stationary anode;

• more than one filament (for different size focal spots).

IAEA 8

Rotating anode x-ray tube

Generation of x-rays (cont)

IAEA 9

Bremsstrahlung radiation produced by the x-ray tube has a continuous energy spectrum.

Generation of x-rays (cont)

Bremsstrahlung

Characteristic

kV peak

Number of photons

Photon Energy (keV)

Remov

ed

by fi

ltrat

ion

• Its properties are subject to the anode material, the peak x-ray tube voltage and the filtration of the x-ray tube.

• This radiation is produced in all directions from the anode.

IAEA 10

X-ray tube assembly (x-ray tube, housing and collimator)

Generation of x-rays (cont)

• Leakage radiation through the shielding must be minimized and must comply with standards.

• The tube housing also usually contains oil for electrical insulation and heat dissipation.

LIGHT BEAM

COLLIMATOR

HIGH VOLTAGECABLES

X-RAY TUBE HOUSING

(ASSEMBLY)

The x-ray tube assembly has an aperture to allow the useful x-ray beam to emerge but it is also shielded to restrict unwanted radiation.

IAEA 11

X-ray tube housing and collimator

• The useful radiation beam is directed at the patient, usually through an adjustable collimator which allows the operator to control the size and shape of the x-ray beam.

Generation of x-rays (cont)

• The location, size and shape of the x-ray beam is usually (but not always) defined by a light beam, hence the description light beam collimator.

IAEA 12

X-ray generators provide both the current to the x-ray tube filament (the source of electrons) and the high voltage required to accelerate the electrons from the cathode towards the anode.

Generators

Generation of x-rays (cont)

However, some mobile x-ray equipment may use a capacitor (typically 1 µF) to store the required electrical energy.

IAEA 13

• Various type of generators are used in diagnostic radiology - single phase, three phase or high frequency, each of which produce characteristic waveforms. However, they must ensure an accurate and consistent high voltage and a stable radiation output.

Generators

Generation of x-rays (cont)

• Most modern generators are microprocessor-controlled with a high frequency inverter (effectively no voltage ripple).

IAEA 14

X-ray generators are rated on the basis of the maximum voltage and electrical power they can deliver. The maximum current that a generator (and x-ray tube) can withstand varies with the voltage at which it is operating.

• For medical uses, generators supply high voltages ranging from 25 to 150 kV peak (according to the application) together with an appropriate current (e.g. 300 mA at 100 kV peak).

• Generators are provided with circuits that can accurately control exposure times, typically ranging from milliseconds to several seconds.

Generators

Generation of x-rays (cont)

IAEA 15

A substantial part of the x-ray spectrum emitted by the anode is low energy radiation which would be absorbed in the human body and not reach the image receptor.

Appropriate filtration removes low energy photons before they reach the patient.

Filtration

Bremsstrahlung

Characteristic

kV peak

Number of photons

Photon Energy (keV)

Remov

ed

by fi

ltrat

ion

Remov

ed

by fi

ltrat

ion

IAEA 16

• Filtration is effected by the irremovable materials of the tube assembly (i.e. the glass envelope, the cooling oil, and the x-ray tube assembly port) through which the beam passes before emerging from the housing. This is the inherent filtration.

Filtration (cont)

• Added filtration is used to further modify the spectrum. Aluminium is typically used but for special purposes can include Copper Molybdenum, Hafnium, etc.) The mirror in the light beam collimator generally also acts as a filter.

• The combination of the inherent and added filtration is the total filtration and is expressed in terms of millimetres of Aluminium equivalent.

IAEA 17

• applied x-ray tube potential (kV peak);

The quality of the emergent x-ray beam (and therefore its penetrating power) depends on the:

Filtration (cont)

Quality is characterized by:

• the first half value layer (HVL) which typically is measured in millimetres of Aluminium.

• total filtration;

• anode material (but this is not within the user’s control).

IAEA 18

The HVL is the thickness of material which attenuates the output (air kerma) of a collimated x-ray beam by 50%.

It is measured under conditions which minimize scattered radiation.

Filtration (cont)

Half Value Layer (HVL)

Ion chamber

Filters

IAEA 19

• excessive radiation leakage through the x-ray tube housing and collimator;

• x-ray tube voltage inaccuracy and inconsistency;

• timer, tube current, mAs inaccuracy and inconsistency;

• x-ray tube output inconsistency;

• incorrect or inappropriate filtration;

• poor congruency of the collimator’s light and x-ray beams;

• for capacitor discharge equipment, excessive radiation leakage (in the direction of the useful x-ray beam) when the capacitor is fully charged (but without an exposure initiated).

Some malfunctions that can compromise safety

IAEAInternational Atomic Energy Agency

Image Production

IAEA 21

X-ray photons transmitted through the structures under examination comprise the “x-ray (or radiological) image”.

The photons are then converted into a visual image by interaction with an appropriate detector (image receptor)

Principles

The Fundamentals of Radiography. Kodak

IAEA 22

An image receptor is a device that converts a pattern of x-ray photons into an image. The image may be viewed directly (for dynamic imaging such as fluoroscopy), recorded on an x-ray film or other hard copy device, or converted to electronic form for digital processing.

Image Receptor

• x-ray film and intensifying screen technology;

• luminescent screens and electronic image intensifiers;

• computed and digital imaging technology.

This conversion can be carried out by different methods, separately or combined:

IAEA 23

• However, the sensitive emulsion on x-ray film is not particularly sensitive to direct x-ray exposure.

• Therefore, except for intraoral dental radiography, intensifying screens are used to convert the x-ray energy to light (blue, green, UV).

X-ray film and Intensifying screens

Radiography using film and intensifying screens as the image receptor (in light tight cassettes) remains the most common modality for recording x-ray images.

Image Receptor (cont)

IAEA 24

• The exposed film (bearing the latent image) is then chemically processed to create a visible image. Film processing may be manual or automatic.

X-ray film and Intensifying screens

Image Receptor (cont)

• The sensitivity (or the speed) of a film or film-screen system is the reciprocal of the radiation dose required to produce a given density on the film.

IAEA 25

Digital methods for processing and displaying x-ray images were first introduced with the advent of computed tomography (CT) in 1972.

Digital imaging technology

Continuing advances in computer technology have promoted the general development of image acquisition in digital form (CCD cameras), most commonly from image intensifiers (digital fluoroscopy) or from storage phosphor plates (computed radiography).

Other detector systems such as ‘flat-panel’ technology for indirect or direct digital radiography are now available for general purpose equipment.

IAEA 26

Digital imaging technology (cont)

The technique of digital subtraction angiography (DSA), based on digital image processing, allows enhanced visualization of blood vessels by electronically subtracting unwanted parts of the image.

IAEA 27

At this time, there is no consensus on the best technology for balancing dose and image quality. Digital imaging potentially can provide lower doses than the film-intensifying screen method.

• However, through post-exposure manipulation of the data, satisfactory diagnostic images can be produced even when unnecessarily high patient radiation doses are used.

• Proper quality assurance procedures are essential.

Digital imaging technology (cont)

IAEA 28

The selection of an imaging system should involve a thorough evaluation and analysis of its complete characteristics together with consideration of the technical and human environment in which the system will be used.

Main characteristics of an image receptor

The main characteristics to be considered when selecting an image receptor are:

• spatial resolution; contrast resolution; dose efficiency; Modulation Transfer Function; detector size; possibilities of image storage and transfer; and qualities such as weight, robustness, fast image access, etc.

IAEA 29

• Spatial resolution: determines the minimum size of detail visualized;

• Contrast resolution: determines the size of detail that can be visibly reproduced when there is only a small difference in density relative to the surrounding area. It is the smallest exposure change that can be detected;

Main characteristics of an image receptor (cont)

IAEA 30

• Dose efficiency: quantifies the balance between the radiation dose absorbed by the receptor (and by the patient) and the resulting image quality;

• Modulation Transfer Function (MTF): is the measure of the ability of an imaging system to preserve signal contrast as a function of spatial frequency;

Main characteristics of an image receptor (cont)

IAEA 31

X-ray energy is converted into electromagnetic radiation in the visible or near visible range by means of luminescent (fluorescent) screens.

Fluoroscopy: dynamic (real time) imaging

• Direct viewing of an image on a fluorescent screen with the naked eye should not be permitted because of the potentially higher radiation dose rates that may be required, particularly if the user fails to properly dark adapt.

• Fluoroscopy should now only be performed using an electronic image intensifier.

IAEA 32

Fluoroscopy: dynamic (real time) imaging (cont)

Light amplifier tubes, in combination with a television camera, are the most widely used image intensification systems.

IAEA 33

Film-Intensifying screens

• Unsatisfactory film storage (causing fogging); damaged cassettes or intensifying screens.

• Light sources within, or leaking into, the dark room.

• Cassette pass hatch or storage container not provided or improperly shielded.

• Inappropriate developer chemistry (e.g. wrong type, improperly diluted and / or replenished, wrong temperature).

Problems that may affect radiation protection

Note: Ventilation is also an important occupational safety issue

IAEA 34

Film-screen technology

• Failure to follow the film manufacturer’s prescribed time-temperature development procedures (manual development) or to properly maintain automatic film processors.

Fluoroscopy and Digital systems

• Direct fluoroscopy (inefficient fluorescent screen)

• Image intensified fluoroscopy (low efficiency, poor resolution and contrast of the image intensifier TV chain)

Problems that may affect radiation protection (cont)