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Radiation Sources in medicine diagnostic Radiology

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1 Radiation Sources in medicine diagnostic Radiology
X-ray Generation and Imaging Day 6 – Lecture 8(2)

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.

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.

4 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. Important parameters include kVp, mA and exposure time (or mAs). Where radiographic exposures are controlled by an automatic exposure device, the exposure delivered (in mAs) should also be displayed. For fluoroscopic equipment, the accumulated fluoroscopic exposure time should be displayed and include an audible warning to the operator when a pre-determined time has been reached. The selection of “high dose rate” mode for fluoroscopy should be clearly displayed. (Ideally, the system should default to “normal” if the equipment is switched off and if unused for some specified time. This minimizes the risk of the “high dose” setting being accidentally used on subsequent patients.)

5 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.

6 Generation of x-rays (cont)
“Stationary” in comparison to a “rotating” anode x-ray tube. Typical of early x-ray tubes but can have severe limitations on loading (and therefore on radiation output) due to the heat generated during exposures A stationary anode x-ray tube

7 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). The detail in a radiographic image largely depends on geometry. i.e. a physically small source of x-rays (the focal spot), moderately large distances to the patient (e g 100 cm) with patient-image receptor distances as small as possible. The higher the filament current (amps), the higher the tube current (milliamps) and the greater the radiation output of the x-ray tube. Short exposure times are necessary to minimise blurring of the image due to patient movement. (Movement of the x-ray tube will also cause blurring but for general radiography the tube should be rigidly supported and stationary). Compare with tomography where the x-ray tube and image receptor move. The anode may be made of different materials. For general radiography tungsten is commonly used. However, for mammography where specific, low energy photons are necessary, Molybdenum and/or Rhodium is used. Rotating anodes are required for most general purpose radiography so that high tube currents can be utilized. Unless specially cooled, stationary anode x-ray tubes generally have significant power limitations (e.g. less than 100 mA) and because of the longer exposure times may only be suitable for simple x-ray examinations such as of the chest and extremities.

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

9 Generation of x-rays (cont)
Bremsstrahlung radiation produced by the x-ray tube has a continuous energy spectrum. Bremsstrahlung Characteristic kV peak Number of photons Photon Energy (keV) Removed by filtration 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.

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

11 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.

12 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.

13 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).

14 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 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.

15 Filtration Bremsstrahlung Characteristic kV peak Number of photons Photon Energy (keV) 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. Removed by filtration Removed by filtration

16 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.

17 Filtration (cont) The quality of the emergent x-ray beam (and therefore its penetrating power) depends on the: applied x-ray tube potential (kV 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.

18 Filtration (cont) Half Value Layer (HVL)
Ion chamber Filters 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.

19 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, 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). Excessive radiation leakage (in the direction of the useful x-ray beam) when capacitor discharge x-ray equipment is fully charged (but before the exposure is made); The high voltage in a capacitor discharge unit , although stored in a capacitor (e.g. 1µF) is simultaneously applied across the high tension cables and the x-ray tube. Electron flow is prevented by a voltage biased grid positioned between the cathode and anode. This voltage is dropped when the exposure is initiated. However, if the bias voltage is too low, electrons will pass from the cathode to the anode and generate x-rays even though the exposure may not have been initiated. In modern CD equipment, a lead shutter is fitted outside the x-ray tube assembly as a safeguard to prevent avoidable exposure of the patient and operator when the capacitor is charged. The shutter is removed when the exposure is initiated. However, some early CD equipment did not have a lead shutter and, if the bias voltage is incorrect, high exposure rates (emitted in the direction of the useful x-ray beam) can occur.

20 Image Production

21 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

22 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. X-ray film is used alone for some procedures e.g. intraoral dental radiography. In such cases, the efficiency of converting x-ray photons to an image largely depends on the amount of silver halide in the emulsion of the x-ray film. This efficiency is reflected in the different speed groupings of intraoral film (i.e. ‘D’, ‘E’ and ‘F’ speed film, each roughly twice the speed of the previous film). i.e. the image is the result of the film being directly exposed to x-rays. Film used with intensifying screens is manufactured to be sensitive to the wavelength of the light emitted by the intensifying screens. Such film should never be used without intensifying screens.

23 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).

24 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. Density (D) = - log (transmitted light intensity / incident light intensity). e.g. film which transmits 1/100th of the incident light has a density of 2; 1/1000th a density of 3, etc. The density of unexposed but processed film (base density) may be around The density of an exposed, processed radiographic image will range from the base density to a density of 2.0 or more.

25 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.

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.

27 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.

28 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. We will go on to look at some of these in more detail.

29 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;

30 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;

31 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.

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

33 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

34 Problems that may affect radiation protection (cont)
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)

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