Radiation Sources in medicine diagnostic Radiology Xray Generation

Radiation Sources in medicine diagnostic Radiology X-ray Generation and Imaging IAEA International Atomic Energy Agency Day 6 – Lecture 8(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 2

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 3

Introduction X-ray equipment and accessories: • should be certified to be in compliance with the relevant standards of the International Electro-Technical Commission (IEC) or equivalent national standard(s); • 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. IAEA 4

Generation of x-rays Three basic elements are needed for x-ray generation: • 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. IAEA 5

Generation of x-rays (cont) A stationary anode x-ray tube IAEA 6

Generation of x-rays (cont) Specific requirements for x-ray tubes: • as small a focal spot as practicable; • sufficient filament current (and therefore electrons) to minimize exposure times; • 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 7

Generation of x-rays (cont) Rotating anode x-ray tube IAEA 8

Generation of x-rays (cont) Number of photons • 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. R by em filt ove ra d tio n Bremsstrahlung radiation produced by the x-ray tube has a continuous energy spectrum. Bremsstrahlung Characteristic k. V peak Photon Energy (ke. V) IAEA 9

Generation of x-rays (cont) X-ray tube assembly (x-ray tube, housing and collimator) 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. X-RAY TUBE HOUSING (ASSEMBLY) HIGH VOLTAGE CABLES LIGHT BEAM COLLIMATOR • 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. IAEA 10

Generation of x-rays (cont) 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. • 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 11

Generation of x-rays (cont) Generators 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. However, some mobile x-ray equipment may use a capacitor (typically 1 µF) to store the required electrical energy. IAEA 12

Generation of x-rays (cont) Generators • 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. • Most modern generators are microprocessor-controlled with a high frequency inverter (effectively no voltage ripple). IAEA 13

Generation of x-rays (cont) Generators 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 k. V peak (according to the application) together with an appropriate current (e. g. 300 m. A at 100 k. V peak). • Generators are provided with circuits that can accurately control exposure times, typically ranging from milliseconds to several seconds. IAEA 14

Filtration R by em fi l ov t ra ed tio n 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. Number of photons Bremsstrahlung Characteristic Appropriate filtration removes low energy photons before they reach the patient. k. V peak Photon Energy (ke. V) IAEA 15

Filtration (cont) • 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. • 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 16

Filtration (cont) The quality of the emergent x-ray beam (and therefore its penetrating power) depends on the: • applied x-ray tube potential (k. V peak); • total filtration; • anode material (but this is not within the user’s control). Quality is characterized by: • the first half value layer (HVL) which typically is measured in millimetres of Aluminium. IAEA 17

Filtration (cont) Half Value Layer (HVL) The HVL is the thickness of material which attenuates the output (air kerma) of a collimated x-ray beam by 50%. Filters It is measured under conditions which minimize scattered radiation. Ion chamber IAEA 18

Some malfunctions that can compromise safety • excessive radiation leakage through the x-ray tube housing and collimator; • x-ray tube voltage inaccuracy and inconsistency; • timer, tube current, m. As 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). IAEA 19

Image Production IAEA International Atomic Energy Agency

Principles 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) The Fundamentals of Radiography. Kodak IAEA 21

Image Receptor 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. This conversion can be carried out by different methods, separately or combined: • x-ray film and intensifying screen technology; • luminescent screens and electronic image intensifiers; • computed and digital imaging technology. IAEA 22

Image Receptor (cont) 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. • 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). IAEA 23

Image Receptor (cont) X-ray film and Intensifying screens • The exposed film (bearing the latent image) is then chemically processed to create a visible image. Film processing may be manual or automatic. • 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 24

Digital imaging technology Digital methods for processing and displaying x-ray images were first introduced with the advent of computed tomography (CT) in 1972. 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 25

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 26

Digital imaging technology (cont) 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. IAEA 27

Main characteristics of an image receptor 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. 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 28

Main characteristics of an image receptor (cont) • 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; IAEA 29

Main characteristics of an image receptor (cont) • 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; IAEA 30

Fluoroscopy: dynamic (real time) imaging X-ray energy is converted into electromagnetic radiation in the visible or near visible range by means of luminescent (fluorescent) screens. • 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 31

Fluoroscopy: dynamic (real time) imaging (cont) Light amplifier tubes, in combination with a television camera, are the most widely used image intensification systems. IAEA 32

Problems that may affect radiation protection 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). Note: Ventilation is also an important occupational safety issue IAEA 33

Problems that may affect radiation protection (cont) Film-screen technology • Failure to follow the film manufacturer’s prescribed timetemperature 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) IAEA 34