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X-ray Physics and Technology Radiation Protection for X-ray Technologists Dr Tim Wood Clinical Scientist Overview • • • • • Atoms, electrons & radiation X-ray production (‘low’ kVp X-ray tubes) X-ray interactions with matter Factors affecting patient dose X-ray detectors – – – – Film CR DR Image intensifiers Atoms and Electrons: The basics Outer electrons (negatively charged) orbiting the nucleus An inner nucleus made up of protons (positively charged) and neutrons (zero charge), jointly known as nucleons Atomic Structure • Neutral Atom • Number of electrons = number of protons • +ve & -ve charges cancel out; the atom is said to be in its Ground State • Number of protons (Z = Atomic Number) and electrons determines the chemical nature of the atom • If the Atomic Number (Z) changes, so does chemical nature and behaviour of the atom Atomic Structure • Electrons carry a single negative charge and orbit around the nucleus at precise distances or levels • The electrons are held in place by the attractive forces generated by the positive protons in the nucleus • These outer levels are known as Orbits, Shells or Energy Levels Atomic Structure Properties of Electrons: Energy Level Rules • Electrons can move from shell to shell, but cannot exist in the Forbidden Zone between shells • The energy holding each electron in orbit in its shell is known as the Binding Energy (BE) and will vary from shell to shell • To remove an electron from its shell, it needs to receive an amount of energy greater than the BE Properties of Electrons: Ionisation - • If an electron is removed from a neutral atom, it is no = +1 0 charge charge longer neutral but + positively charged • A positively charged atom is known as a Positive Ion • The size of the positive charge equals the number of electrons removed • This process is known as Ionisation Properties of Electrons: Excitation • Where an electron in a neutral atom is raised to a higher level, by Excitation applying energy (less than the BE), the atom remains neutral but is in an Excited State • This process is known as Excitation • After a short time, the electron falls back to its original position and releases energy in the form of radiation Energy, The Electromagnetic Spectrum and X-rays Wave Properties • Energy may be transported (propagated) by waves, described as electromagnetic radiation • Waves are oscillations • Each wave as it moves generates an electrical and magnetic field; hence electromagnetic radiation The Electromagnetic Spectrum Visible Ionising Radiation Non-ionising Radiation Infrared Ultraviolet Near Radar Far X Rays FM TV Gamma Rays Short wave Cosmic Rays 10-14 Power Transmission Broadcast 10-12 10-10 10-8 10-6 10-4 10-2 1 102 104 106 108 Wavelength in Metres 1010 High 108 106 104 102 1 10-2 10-4 10-6 Energy – Electron Volts 10-8 10-10 10-12 10-14 Low Waves or Particles? • EM radiations spread like waves, over space. However, the way they give up their energy is distinctly not wavelike • Absorption of energy occurs in well-defined chunks of energy, known as wave packets or more correctly photons Radiation • Non-ionising • Electromagnetic – Ultraviolet, visible, infrared, microwaves, radio & TV, power transmission • Ionising • Capable for producing ions when interacting with matter • Natural & man-made • Electromagnetic – X-, gamma and cosmic rays • Particles – alpha, beta, neutrons, positrons, electrons Alpha particle () – Helium nucleus 0 + + 0 Types of Ionising Radiation Stopped by a piece of paper Beta particle ()/electrons Stopped by a layer of clothing or a few mm of a relatively low density material such as aluminium. Gamma rays ()/X-rays Stopped by cm thicknesses of lead or feet of concrete The Physics of X-ray Production X-ray tube design - basic principles • Electrons generated by thermionic emission from a heated filament (cathode) • Accelerating voltage (kVp) displaces space charge towards a metal target (anode) • X-rays are produced when fast-moving electrons are suddenly stopped by impact on the metal target • The kinetic energy is converted into X-rays (~1%) and heat (~99%) X-ray tube design Stationary anode – dental X-ray tube Rotating anode – general X-ray tube X-ray tube design • Evacuated glass envelope (allow electrons to reach the target) • Filament (cathode) is source of electrons, with a focussing cup around it to generate a narrow beam of electrons – Often dual focus to offer finer resolution on diagnostic sets Thermionic emission • Applying a current to the filament causes it to heat up to ~2200°C (‘white hot’ like a light bulb) • ‘Free’ electrons in the metal gain enough energy to overcome the binding potential – Can overcome the forces holding them in the metal and escape from the surface • Tungsten metal is ideal material Thermionic emission • Require two sources of electrical energy to generate X-rays – Filament heating current (~10 V, ~10 A) – Accelerating voltage of between 30-150 kV (30,000150,000 V); this results in a current of electrons between the anode and cathode (0.51000 mA) Electron production in the X-ray tube kV Applied voltage chosen to give correct velocity to the electrons mA - Filament (heats up on prep.) + Target The physics of X-ray production • Electron reaches the anode with kinetic energy equivalent to the accelerating potential (kVp) • Electrons penetrate several micrometres below the surface of the target and lose energy by a combination of processes – Large number of small energy losses to outer electrons of the atoms = heat – Relatively few, but large energy loss X-ray producing interactions with inner shell electrons or the nucleus Heat generating processes • When an electron (e-) strikes the target, most likely interaction is with loosely bound e-s that surround nuclei • Relatively weak interactions – slight deflection, ionisation or excitation • Small amount of energy transfer (per interaction) – observed as heat • However, accounts for ~99% of all energy dissipated from e- beam in the diagnostic range Bremsstrahlung Bremsstrahlung • If e- passes close to nucleus, strong electromagnetic interaction – decelerates, and deflected • Radiates energy in all directions as X-ray photons, up to a maximum equivalent to kVp = continuous spectrum • High energy cut-off (≡ kVp) due to release of all energy in head on collision with heavy nucleus • Low energy cut-off due to self-attenuation by target, X-ray window and additional filtration • >80% of X-rays produced are Bremsstrahlung (except for mammography) Bremsstrahlung Characteristic X-rays Characteristic X-rays • Interactions with tightly bound e- (typically K-shell) • If energy of e- exceeds binding energy (BE) of bound state → ionisation • Vacancy leaves atom unstable • e- from higher state drops down (most often from L- or M-shell), releasing X-ray photon (energy = difference in BE) • Gives characteristic peaks on X-ray spectrum that are specific to the target material (BE Z2) • For Tungsten target, Kα = 58 keV and Kβ = 68 keV – Not observed below 70 kVp The X-ray spectrum • Combination of these yields characteristic spectrum. 4.00E+05 60 kVp 80 kVp 120 kVp 3.50E+05 3.00E+05 Intensity 2.50E+05 2.00E+05 1.50E+05 1.00E+05 5.00E+04 0.00E+00 0 20 40 60 80 Energy (keV) 100 120 140 The X-ray spectrum • The peak of the continuous spectrum is typically one third to one half of the maximum kV • The average (or effective) energy is between 50% and 60% of the maximum – e.g. a 90 kVp beam can be thought of as effectively emitting 45 keV X-rays (NOT 90 keV) • Area of the spectrum = total output of tube – As kVp increases, width and height of spectrum increases – For 60-120 kVp, intensity is approximately proportional to kVp2 x mA Controlling the X-ray spectrum Exposure factors • Increasing kVp shifts the spectrum up and to the right – Both maximum and effective energy increases, along with the total number of photons • Increasing mAs (the tube current multiplied by the exposure time) does not affect the shape of the spectrum, but increases the output of the tube proportionately • kV waveform – three-phase or high frequency generators will have more high energy photons than single phase. Hence, output and effective energy are higher The X-ray spectrum 4.00E+05 60 kVp 80 kVp 120 kVp 3.50E+05 3.00E+05 Intensity 2.50E+05 2.00E+05 1.50E+05 1.00E+05 5.00E+04 0.00E+00 0 20 40 60 80 Energy (keV) 100 120 140 Quality & Intensity Definitions: • Quality = the energy carried by the X-ray photons (a description of the penetrating power) • Intensity = the quantity of x-ray photons in the beam • An x-ray beam may vary in both its intensity and quality Quality • Describes the penetrating power of the X-ray beam, and is governed by the kilo-voltage (kVp) • Usually described by the Half-Value Thickness – i.e. the thickness (in mm) of Al required to half the beam intensity for a given kVp • Typically >2.5 mm Al for general radiography • Changing the quality of the beam will change the contrast between different types of tissue. • A highly penetrating beam is referred to as ‘Hard’ and a poorly penetrating beam as ‘Soft’ Intensity • Intensity - is the quantity of energy flow onto a given area over a given time; the ‘brightness’ of an x-ray beam • The tube current (mA) is a measure of X-ray beam intensity • Intensity is directly proportional to mA. – i.e. Double the mA, double the dose (quality not affected) • Intensity is also affected by kVp X-ray tube design • Heat generation is a significant problem for Xray tubes, and is generally the limiting factor upon their use • Hence, it is necessary to: – Ensure efficient cooling mechanisms – take the heat away so it doesn’t build up with multiple exposures – Have mechanisms to prevent over-heating – should it get too hot, have mechanisms in place to stop further exposures – Minimise heat generation on a single point of the anode (stop it melting!) X-ray tube cooling • Generally, the tungsten target is mounted on a copper block/rotor (either directly or indirectly) that extends out of the evacuated glass envelope • Heat is transferred from the target to the surrounding coolant (most often oil, but very occasionally water) via conduction and/or radiation, which in turn gives up its heat to the atmosphere (possibly through a heat exchanger) • Expansion bellows can detect when the coolant is getting too hot (or by other means) and prevent further exposures • BUT, what about spreading the heat generating processes over a larger area?... The rotating anode • Heat can be spread over a large area by rotating the anode during exposure • Tungsten annulus set in a Molybdenum disk attached to a copper rotor • The assembly is rotated via an induction motor • Full rotation ~20 ms • Takes about 1 s to get up to speed – The prep phase (push the exposure switch down to the first stop until you can hear it whirring) before pushing down all the way to expose X-ray tube design – The rotating anode The focal spot • To minimise geometric unsharpness, the smallest focal spot should be used… • BUT, this would be at the expense of excess heating and reduced tube life • The solution is to use an angled target as the source of X-rays – Angle allows broad beam of electrons to give a smaller apparent focal spot – Have multiple filaments for focal spot size selection – large focal spot for general use (tube lasts longer), and small focal spot where better resolution is required X-ray tube design – dual focus The focal spot Actual focal spot size Heat rating • kV, mA and exposure time should be such that the temperature of the anode does not exceed its safe limit – The control system is designed to prevent exposures that exceed the tube rating • Require much higher tube ratings for CT and interventional fluoroscopy units Shielding • X-rays are emitted from the target in all directions, not just towards the patient • Hence, Lead shielding is used in the tube housing to absorb X-rays not required for imaging of the patient • Legal requirements on how much ‘leakage’ radiation is emitted from the tube during operation – Medical Physics testing checks this during the Critical Examination of new installations The diagnostic X-ray tube X-ray Interactions with Matter X-ray Properties • • • • • • • • Electromagnetic photons of radiation Emitted with various energies & wavelengths not detectable to the human senses Travel radially from their source (in straight lines) at the speed of light Can travel in a vacuum Display differential attenuation by matter The shorter the wavelength, the higher the energy and hence, more penetrating Can cause ionisation in matter Produce a ‘latent’ image on film/detector X-ray interactions with matter • Contrast is generated by differential attenuation of the primary X-ray beam • Attenuation is the result of both absorption and scatter interactions • Scatter occurs in all directions, so conveys no information about where it originated – can degrade image quality, if it reaches film/detector • Scatter increases with beam energy, and area irradiated Pass through Absorption Attenuation Scatter Attenuation • For a mono-energetic photon beam: where, I = final intensity, I0 = incident intensity, µ = attenuation coefficient, x = thickness • Equal thicknesses of material reduce the intensity by the same fraction (half-value thickness). Attenuation • Attenuation coefficient, µ, decreases with increasing photon energy (except for absorption edges) • Increases with atomic number of material, Z • Increases with density of material, ρ • Transmission of radiation @ 70 kVp; – 1 cm of soft tissue 66% transmitted – 1 cm bone 17% transmitted – 1 cm tooth 6% transmitted Forward vs. Back-scatter • Forward scatter is most likely, but ... • Forward scatter is attenuated by the patient, and • Deeper layers receive a smaller intensity, so there are fewer scattering events • Overall, see more back scatter. • Advantage for image quality (less scatter, but more attenuation at the detector), but may pose a risk in terms of radiation protection Forward vs. Back-scatter Interaction Processes • • • • Elastic scattering Photoelectric effect Compton effect Pair production Elastic Scatter • • • • Photon energy smaller than BE Causes e- to vibrate – re-radiates energy No absorption, only scatter < 10% of total interactions in diagnostic range i.e. not significant 2 Z Probability E Photoelectric Effect • Process of complete absorption • ~30% of interactions in diagnostic range • Energy is transferred to bound e-, which is ejected at a velocity determined by difference in photon and BE • e- dissipates energy locally, and is responsible for biological damage 3 Z Probability 3 E • Hence, main source of radiographic contrast (and dose), and why Lead is used in protection Photoelectric Effect Photoelectric Effect • Leaves atom in unstable state – electronic reconfiguration results in emission of X-ray or Auger electron • Auger emission more probable for low Z material – short range in tissue (= more biological damage) • Low energy X-rays reabsorbed locally • Rapid fall-off with increasing energy Compton Effect • Process of scatter and partial absorption – inelastic scattering • Photon collides with a free electron (photon energy >> BE) • Loses small proportion of its energy and changes direction • Energy loss depends on scattering angle and initial photon energy • Photon free to undergo further interactions until completely absorbed (Photoelectric) Compton Effect Compton Effect • Compton scatter mass attenuation coefficient almost independent of energy over diagnostic range Z Probability A • Ratio of Z/A similar for most elements of biological interest (~0.5) – offers little in terms of radiographic contrast Pair Production • A high energy photon, in the vicinity of a nucleus, is converted to a positron-electron pair • The positron annihilates with an electron to give two 0.51 MeV gamma photons • Promoted by heavy nuclei • Rapid increase with energy above 1.02 MeV (not important in diagnostic range) Probability Z Pair Production The Mass Attenuation Interaction Coefficient • Each process is independent – can add the interaction coefficients to give the total mass attenuation coefficient • Z dependence is the source of contrast in radiographic imaging The Mass Attenuation Interaction Coefficient K-edge Photoelectric Total Elastic Compton Pair production The Mass Attenuation Interaction Coefficient Maximising Radiographic Contrast • Maximise contrast due to Photoelectric absorption – use lower energy photon beams (note, it is the mean energy of the beam, not kVp that is important) • Use scatter rejection techniques such as scatter grids and air gaps • Limit beam to smallest area consistent with diagnostic task to minimise amount of scatter generated • BUT… Maximising Radiographic Contrast • More Photoelectric absorption means higher patient dose • Scatter rejection techniques attenuate the primary beam, so a higher patient dose is required for acceptable image receptor dose • NEED TO BALANCE IMAGE QUALITY WITH PATIENT DOSE!!! • Hence, the principle of ALARA (As Low As Reasonably Achievable) – Use the highest energy beam that gives acceptable contrast, consistent with the clinical requirements Factors affecting patient dose Factors Affecting Patient Dose • Tube Current (mA)/Exposure Factor (mAs) – Double the mA/mAs, double the intensity – Beam quality not affected • Tube Voltage (kVp) – Intensity α kVp2 – Penetrating power increases with kVp – Higher kVp reduces skin dose • Filtration (mm Al) • Focus-to-skin distance Patient dose reduction Filtration and beam hardening • ‘Soft x-rays’ contribute to patient dose without contributing to image production • Placing Al filters in the beam will increase beam quality – this is known as ‘Beam Hardening’ – Alternative materials may be used for filtration in specialised applications e.g. mammography (Mo, Rh, Ag) and fluoroscopy (Cu) • Lowest energy photons are most readily absorbed as photoelectric absorption dominates (proportional to the E3) • As the beam passes the Al, the proportion of low energy photons is reduced, and the average photon energy increases Filtration Patient dose reduction Filtration and beam hardening • Hence, Patient dose is reduced with little affect on the radiation reaching the detector • However; • Radiographic contrast is reduced due to the higher mean energy of the beam • Greater exposure factors required to yield satisfactory dose at film/detector (have to drive the tube harder, and hence tube life may be reduced) • The X-ray beam is also filtered by the target that they are produced in, the coolant oil and the window of the housing • ‘Inherent filtration’ equivalent to about 1 mm Al Focus-to-skin Distance: The Inverse Square Law • For a point source, and in the absence of attenuation, intensity decreases as the inverse of the square of the distance • This is a statement of the conservation of energy 2 2 2 1 D1 r D2 r The inverse square law • Patient dose can be significantly reduced by increasing the distance to the X-ray tube – FSD < 45 cm should not be used (<60 cm for chests – 180 cm used in practice) X-ray Detectors Film-Screen Imaging • Traditionally, all X-ray image capture has been through X-ray film Emulsion Adhesive layer Film base Emulsion Protective layer Film • Film is actually much more sensitive to visible light and UV than it is to X-rays – Hence, use a fluorescent screen to convert X-ray photons to light photons – Enables lower patient dose! • A latent image is formed upon exposure, which cannot be seen unless the film undergoes chemical processing – Mobile silver ions are attracted to electrons liberated by light photons, forming a speck of silver metal on the surface Processing • The invisible latent image is made visible by processing • There are three stages to this process; – Development – Fixing – Washing Processing • First stage is development: – Film is immersed in an alkaline solution of a reducing agent (electron donor) – Reduces positive silver ions to metallic grain of silver (black specks) – Unexposed crystals are unaffected by the developer – bromide ions repel the electron donor molecules – However, given sufficient time, the developer will penetrate the unexposed crystals – The amount of background fog is dependent upon the time, strength and temperature of the developer Processing • Second stage is fixing: – If the film is exposed to light after the first stage, the whole film becomes black – To ‘fix’ the film, unaffected grains are dissolved by an acid solution, leaving the X-ray image in the form of black silver specks • Final stage is washing: – The film is washed in water and dried with hot air – Inadequate washing would result in a brown/yellow film over time (from excess acid) and smell Optical Density • Optical Density: the amount of blackening in the film • Defined as the log of the ratio of the intensities of the incident and transmitted light – log is used as the eyes response is logarithmic The characteristic curve Optical density Linear region, gradient = gamma • Plotting OD against log exposure gives Saturation the Characteristic Curve of the X-ray film Solarisation • Different types of film – subtle differences but all basically the Fog same Log exposure The characteristic curve • Depends on type of film, processing and storage • Fog: Background blackening due to manufacture and storage (undesirable) – Generally in the range 0.15-0.2 • Linear portion: useful part of the curve in which optical density (blackening) is proportional to the log of X-ray exposure • The gradient of the linear portion determines contrast in an image and patient exposures must lie within this region – Need to match this to the clinical task! • Hence, film suffers from a limited and fixed dynamic range Automatic Exposure Control (AEC) • Limited latitude of film makes it difficult to choose correct mAs – skill and experience of radiographer • Alternative is to use an AEC to terminate the exposure when enough dose has been delivered to the film • AEC is a thin radiation detector (ionisation chamber) behind the grid, but in front of the film (though in mammo it is behind to avoid imaging the chamber on the film) • Usually three chambers that can be operated together or individually Automatic Exposure Control (AEC) • When a predetermined level of radiation is detected, the exposure terminates • Choice of chambers determined by clinical task – e.g. left and right for lungs in PA chest, but central if looking at spine • Also has a density control that can increase or decrease exposure where necessary • AEC limited to exposures in the Bucky system Digital imaging What is a digital image? • A digital image can be thought of as an array of pixels (or voxels in 3D imaging) that each take a discrete value • The value assigned is dependent on the X-ray intensity striking it • Depending on its value, each pixel is assigned a shade of grey • Pixel size may determine the limiting spatial resolution of the system What is a digital image? Why bother with digital? • Film has been used since the beginning, so why are we changing to digital techniques? – Increased latitude and dynamic range – Images can be accessed simultaneously at multiple workstations – Viewing stations can be set up in any location – Uses digital archives rather than film libraries – Images quicker to retrieve and less likely to be lost – Post processing – Softcopy reporting – lower cost if do not print – No need for dangerous processing chemicals Disadvantages of digital • Initial cost • Problems with interconnectivity • Lack of information and set up of automatic exposure control (AEC) • Lack of link between exposure and brightness – Potential for dose creep (see following slides) • Human error in choosing exam type and speed class • Generally poorer limiting spatial resolution when compared with film Dynamic Range - Film • With conventional film, too low a dose will results in a ‘thin’ film • Too high a dose results in a very dark film • Fixed and limited dynamic range – must match exposure parameters to the film being used – Gives a measure of control over patient dose! Dynamic Range - Digital • With digital, too low a dose will still produce a recognisable image (just a bit noisy!) • Similarly, too high a dose will produce a recognisable image (but with very little noise!) • Consequences: – Less retakes = GOOD – Dose creep = BAD – must pay special attention to digital imaging to ensure doses are optimised Computed Radiography Computed Radiography (CR) • Was the first digital technique available commercially • Exploits storage phosphors which emit light that is proportional to the intensity of the X-rays that hit it, when they are stimulated by a laser beam • The most common digital technique as it is the cheapest (at least in the short term) – Old X-ray sets used for film-screen radiography can be used, provided exposure factors and AECs are adjusted for the new type of detector CR Components Physical Principles of Computed Radiography (CR) • Fluorescence describes the immediate release of low energy light photons after the absorption of X-ray photons (exploited in traditional filmscreen radiography) • Phosphorescence describes the delayed release of light photon energy. This is the principle of CR CR Stage 1: Image Capture • Image receptor is a laser stimulable phosphor, known as an image plate (IP) • Capture image by irradiating an IP in the same way as conventional film – Does not need a new X-ray system when replacing film-screen (just make sure automatic exposure controls are re-calibrated) • Typically ~40% of X ray photons are absorbed • IPs retain majority of absorbed X-ray energy as a pattern of electrons in meta-stable energy states – The spatial distribution of stored electrons is equivalent to the pattern of absorbed x rays – latent image CR Stage 2: Image Read Out • A red Laser is used as this matches the energy gap between Colour Centre and conduction band • Light in the blue end of the visible spectrum is emitted • Hence, optical separation of input and output light photons – Means a colour filter can be used to prevent laser photons contaminating the output signal • Blue light photons are collected via a photomultiplier tube and digital image is produced Dynamic Range • Film dynamic range defined by characteristic curve 10:1 • CR dynamic range > 10,000:1 • Linear relationship between log(signal) and log(dose) Körner M et al. Radiographics 2007;27:675-686 Detector Dose Indicators (DDI) • The DDI has been introduced for digital imaging as an indication of the level exposure on a broad region of the detector • Analogous to the OD of film • The definition of DDI is manufacturer specific! – – – – Some manufacturers have high DDI = underexposure Some the other way round Some are a function of the log of dose Some are linear… • Manufacturers will provide an indication of acceptable range of DDI, but local departments must validate these – DRLs and OPTIMISATION • Operators should monitor DDI of patient exposures to ensure doses remain acceptable Digital Radiography Digital Radiography • Directly acquire the data in digital format (no separate read-out phase like with CR) – Improves throughput of X-ray systems – could be important in chest clinic, mammo, etc • Most expensive method, as it requires complete dedicated X-ray system • Main technologies: – Phosphor coupled to a read out device – Indirect conversion – a-Se/TFT array – Direct conversion flat panels Indirect conversion • Indirect conversion involves converting the X-rays into visible light (in a phosphor), and detecting the resulting light photons (akin to film-screen radiography!) • Either amorphous silicon (a-Si) photodiode TFT array, or CCD for readout • Sharpness limited by both pixel pitch of readout array, and spread of light in phosphor – Usually CsI(Tl) needle phosphors to focus light down to the detector (like mini-fibre optics to minimise spread) – Needle phosphors can be thicker (more efficient) Indirect flat panels • Phosphor => X-ray to light photons • Light photons detected in photodiode array => light photons to electrical charge • Read out by the amorphous silicon TFT array (discussed after direct conversion) • Can be manufactured as a single panel up 45 x 45 cm2, but in practice tend to be made up of four smaller detectors ‘stitched’ together – Tiled detectors – Requires image processing and interpolation to cover the join between panels CCD detectors • CCD light detectors (like in a camera) can only be manufactured in relatively small sizes – Usually need multiple CCDs to cover image area (‘tiled detector’), or slot scanning technique – Also thicker than flat panels due to the optics between the phosphor and detector Direct conversion flat panels • Amorphous Selenium (aSe) is a photoconductor – Converts X-rays directly to electrons • Deposited directly onto amorphous silicon TFT array • No phosphor, hence no light spread • Resolution governed by effective pixel pitch The TFT array • Amorphous Silicon thin-film transistor array • Transistors amplify electrical signals • Electrical charge is stored in the TFT array until release by applying a high potential • Each row of detectors is connected to the same activating potential (gate-line control), and each column to a charge measuring device (read-out electronics) • The activating potential is applied row-by-row, so the timing of the detected signal determines the position of the pixel from which it originates • Each pixel ~100 μm The TFT array Modern flat panels Advantages of DR over CR • Image displayed immediately to operator in room • Faster – Greater throughput of patients as no intermediate read-out phase • Slightly better resolution (CR limited by laser spot size and scatter) • Harder wearing imaging device (as long as you don’t drop it!) – Or at least that’s the theory… Disadvantages of DR over CR • Much more expensive • Need to refurbish X-ray room The Image Intensifier The image intensifier • The X-ray image intensifier (II) captures the time varying image • Today, images are viewed via a remote display (image capture via CCTV (old units) or CCD) • Computerised image processing is used to enhance presentation The image intensifier • Three main components: – The input screen • The input window • The input phosphor • The photocathode – The electron-optics – The output screen Gain • Extent to which the II has intensified the light output from the system • Ratio of the brightness of the output phosphor to that of the input phosphor • Generally, 1 light photon from the input = 1 electron from the photocathode • After acceleration to 25 keV, 1 electron = many light photons in output phosphor • This is the flux gain – Typically ~50 Gain • Minification gain is intensification from reducing the image size at the output • Equal to the ratio of the areas of the two screens – For a 300 mm input and 30 mm output, minification gain would be (300/30)2 = 100 • Overall gain is the product of the two – ~5000 for this case! • Gain is not measurable – use the conversion factor as more practical descriptor – Ratio of brightness out to dose rate at the input (typically 25-30 Cd m-2 (μGys-1)-1 • Conversion factor deteriorates with time and usage (loss of detection efficiency in the phosphor) The ABC • In fluoroscopy, manual control of exposure factors is not practical • ABC takes a measurement of light intensity of the output, or signal from the camera (most common on modern systems) and feeds back changes required to the X-ray generator to maintain adequate light intensity • Generally, only the central region of the image is used by the ABC • Can increase kV and/or mA • The way it changes exposure factors can be quite complex, and will depend on the clinical application The ABC Brightness controlled by simultaneous adjustment of kVp and mA Brightness controlled mainly by adjustment of kVp The tube current is maximised at 3 mA Tube heating kV x mA (99% energy goes to heat) X-ray intensity kV2 x mA Penetrating power with kV Contrast with kV Flat Panel Fluoroscopy Flat Panel Fluoroscopy • As with general radiography, flat panels are now used on modern fluoroscopy systems – Phosphor coupled to a TFT – Indirect conversion – a-Se/TFT array – Direct conversion flat panels • High quality dynamic and static image capture • No distortions like the II (as long as set up correctly) • Not able to do genuine magnification – Resolution fixed by pixel pitch • Use Automatic Exposure Control (AEC) – set level of exposure required for adequate image – Detector used to control the system – Doses generally (but not always) lower on modern flat panel systems compared with II Flat Panel Fluoroscopy • Increased dynamic range – II has limited contrast ratio ~30:1 – Flat panel may use full 14-bit depth • Superior limiting spatial resolution to II – 3 lp mm-1 compared with 1-1.2 lp mm-1 for II (largest field size) • Directly acquire the data in digital format • But, more expensive • Can be used for Cone Beam CT (essentially a very wide multi-slice CT) – Poorer images compared with a ‘proper’ CT scanner CT How do we get the images? • Tube and detector rotate smoothly around the patient • X-rays are produced continuously and the detectors sample the X-ray beam approx 1000 times during one rotation How do we get the picture? • Back Projection – Reverse the process of measurement of projection data to reconstruct image – Each projection if smeared back across the reconstructed image