Quality and Intensity of X-ray Beam PDF

Quality and Intensity of the x-ray beam Cletus Amedu Outline  Intensity of the x-ray beam  How to measure intensity ( i.e. Quantity)  Factors affecting intensity (i.e. Quantity)  Quality of an x-ray beam with particular relevance to radiography  Factors affecting the x-ray output ( kVp, mA...

Quality and Intensity of the x-ray beam Cletus Amedu Outline  Intensity of the x-ray beam  How to measure intensity ( i.e. Quantity)  Factors affecting intensity (i.e. Quantity)  Quality of an x-ray beam with particular relevance to radiography  Factors affecting the x-ray output ( kVp, mA, Filtration, Tube target material, Diastance from the source)  The invisible x-ray image X-ray beam  Photons of Electro-magnetic Radiation  Have high energy (and short wavelength)  Capable of causing ionization- they possess the ability to eject electrons from their atoms  They are called ionizing radiation  The energized electrons are –vely charged and the remaining atom which is +vely charged are called ions (or charged particles)  Intensity  Defined as the total energy carried by the beam per second per unit area at right angles to the direction of travel.  A measure of the quantity of radiation flowing in unit time  Similar to the brightness of light Units = Joules per second per square metre = J.s-1.m-2 =Watts per square metre (W m-2) Total Intensity of the heterogenous x-ray beam The sum of All its individual components (separate monochromatic components) How to measure intensity  Practical difficulties  X-ray causes ionisation of the air it travels through  The ionising effect on air = exposure  Measured in Coulombs per kilogram (C kg-1)  Ionisation chamber – an instrument used to detect exposure (charge created in the air it contains) in C kg-1  Intensity is a measure of the number of photons in the beam (i.e the higher the number of photons, the greater the intensity of the beam)  Exposure rate (cKg-1s-1) – is the intensity of a beam passing through a unit area in unit time Thimble Ionization Chamber C = cap (wall) W = central wire (electrode) I = insulator  Condenses air into a solid medium surrounding the central electrode.  The cap (wall) of the thimble chamber is said to be air equivalent (same atomic number as air Z= 7.6 e.g. graphite, bakelite, plastic) and so has same absorption properties as the same mass of air.  The central aluminium electrode has a fixed amount of positive charge put onto it from an external source.  When the chamber is irradiated, some of the more energetic electrons liberated in the cap (wall) will penetrate the air of the chamber and be attracted to the electrode. Thimble Ionization Chamber  There is also a small contribution due to ionisation resulting from absorption of the x-rays by the air in the chamber.  The electrode will lose some of its positive charge.  TIC is designed such that the ionisation produced is proportional to the exposure over a range of radiation qualities.  A TIC is connected by an electrically screened cable to an instrument (an electrometer) which measures the charge collected on the central electrode. Ionization  Very sensitive process  Only 34 eV is required to form an ion pair  If 100 keV photon is completely absorbed almost 3000 ion pairs will have been formed  Biological tissue is very sensitive because of ionisation Why Ionization in air?  It is readily available  Its composition is close to being universally constant Inverse square law  The relationship between intensity and distance obeys the inverse square law  ‘The intensity of the beam is inversely proportional to the square of the distance from its source’ Intensity  1/d2 Inverse square law Intensity1 = Distance22 Intensity2 Distance12 Inverse square law 1. Radiation at I2 is spread over four times the area of I1 2. so the intensity of I2 is one-quarter of the intensity at I1. 3. Therefore by doubling the distance between the point and the source of radiation we reduce the intensity by one quarter.  Twice distance – 22 = 4 (therefore ¼ intensity)  Three times distance – 32 = 9 (1/9 intensity) Note:  Note: 1. X-rays are not emitted from a point source - as the focal spot has a finite size. 2. They are not emitted equally in all directions - as the anode heel effect causes the intensity to vary across the beam. 3. Absorption and scattering of the x-ray beam occur as it passes through air. 4. These effects are, however, small for x-ray beams generated above 50 kVp, so the inverse square law can be applied to such beams. Other factors affecting Intensity  Material placed in the path of the X-ray beam will remove some of the photons – a process called attenuation  The higher the atomic number of that material, the greater the amount of photons removed  Scattering is the redirection of some of the photons in the beam Quality of an x-ray beam  Describes the penetrating power of the X-ray beam  Average photon energy or wavelength of beam  The higher the energy photons the higher the penetrating power  Higher quality beams require a greater thickness of material to reduce intensity  X-ray beams are heterogeneous In Diagnostic Radiography, we relate quality to:  Generating voltage (kVp)  Half value layer (HVL) - the thickness of a specified material (often aluminium) required to reduce the beam’s intensity by 50%. The higher the quality of the beam the greater the HVL.  Beam filtration  Effective photon energy (or effective wavelength) - Half Value Layer Exposure plotted against thickness of material for two x-ray beams of different qualities but with the same half-value layer in the material.  The quality of the x-ray beam can be controlled by the operator of the x-ray tube by changing the kV at which the tube operates 1. This can be demonstrated by increasing the kV which increases the quality of the beam, and makes the beam more penetrating. 2. This means that the thickness of aluminium required to reduce the ionisation chamber reading needs to be greater. Factors affecting x-ray tube output The Tube output is a combination of the quality and intensity  Tube kilovoltage (kVp)  Tube current (mA)  Filtration  Tube target material  Distance from the source Kvp  Affects quality because peak kV determines maximum photon energy  lncreases the penetrating power  Increases half value layer  For a tungsten target, kV must be at least 70 for K emissions to occur  Higher kV gives electrons more energy which makes x-ray process more efficient  Voltage waveform affects BOTH quality and intensity Intensity  kVp2 X-ray spectra Spectra obtained from an x-ray tube with a tungsten target when operated at 51, 63 and 100 kV. The tungsten Kα and Kβ emission lines are present on the spectrum of the beam generated at 100 kV. When a pulsating voltage is applied to the x-ray tube, the spectrum of the beam changes from moment to moment during the exposure. Changes in the spectra due to the change in kV: 1. The maximum photon energy of the beam increases as the tube voltage increases. 2. The average photon energy of the beam increases as the tube voltage increases. 3. The overall intensity of the beam increases as the tube voltage increases. 4. There are no K-series emission spectra when the tube voltage is at 63 kV and 51 kV mA  Affects ONLY intensity of beam  Current is the rate of flow of electric charge and therefore the amount of electrons flowing  1000 mA = 1C s-1 Intensity  tube current  For pulsating current, averages are used X-ray spectra 1. The diagram illustrates the effect on a 100keV x-ray beam when changing the tube current from 500 to 1000 mA. No other factors have bean changed. 2. The result is a doubling of intensity across all photon energies in the spectrum. This has the effect of doubling the area enclosed by the curve and therefore confirms that the total output has doubled. 3. Note that although the tungsten K-series emissions have doubled in intensity, they still appear at precisely the same photon energy. 4. There is no change in the quality of the beam (average photon energy remains the same). Beam filtration  Inserted in X-ray beam to improve quality  Reduces intensity  Consists of a thin sheet of metal such as aluminium (Z=13) or copper (Z=29), molybdenum or palladium  Absorbs low energy X-ray photons  Therefore, reduces patient’s skin dose X-ray Spectra 1. The effect of filtration on a 100 keV x-ray beam. 2. The low energy part of the beam has been preferentially filtered out, leaving a beam which, although reduced in intensity, has a higher average photon energy and is therefore of higher quality. 3. Because the spread of photon energies has been reduced, the beam has become less heterogeneous. 4. Adding extra filtration would further reduce the energy spread and the beam would become nearly monochromatic (homogeneous), although its intensity would be compromised. 5. Note that the value of Emax is unchanged. 6. Note, too, that the tungsten K-series emissions appear at the same photon energy levels, but their intensity has been reduced by the effect of filtration. Distance from source  As distance increases intensity is reduced  Adheres to inverse square law  This is due to the divergence of the beam  The passage of X-rays through air affects neither its intensity, nor its quality (when above 50 kVp) Intensity  1/d2 Target material  An increase in atomic number increases intensity  Atoms have greater positive charge  Using a different target material modifies characteristic radiation and therefore quality of beam  Target does not alter the quality of the Bremsstralung radiation, only its intensity Intensity  Z X-ray spectra by two different target materials Target materials chosen to suit operating conditions  Target focal track is normally tungsten- rhenium alloy  Mammography tubes operate at low kVp and therefore have molybdenum or rhodium target Left (mammography tube): the tube with a molybdenum target was operating at 28 kV (giving an Emax of 28 keV); its spectrum shows the molybdenum Kα and Kβ emissions (at 17.5 and 19.6 keV, respectively), which make a valuable contribution to the total output. Lower binding energy of K-shell electrons. Right: the tube with a tungsten target was operating at 100 kV (giving an Emax of 100keV); its spectrum shows the tungsten Kα and Kβ emissions (at 59 and 67 keV): their contribution to total output is minimal. Changing factor Quality Intensity Raising tube voltage (kVp) Increased (increase in average Increased (kVp2) energy of photons) Reducing tube voltage ripple (rectification Increased Increased - nearer to constant potential) Raising tube current (mA) No effect Increased (mA) i.e. no. of electrons flowing Increasing beam filtration Increased (by selective removal Reduced (related to of low-energy photons) thickness and Z of filter) Increasing distance from source No effect Reduced Using target with higher proton (atomic) Higher energy characteristic Increased (Z) number radiation only Invisible x-ray Image Invisible x-ray Image  X-rays emerge from the patient – intensity varies according to attenuation at different parts  Attenuation varies according to tissue type (density and atomic number) and thickness  Reflects the anatomy through which it is penetrated  An invisible image is formed  Following processing a negative visible image formed (reversed in tone or grey scale)  The amount of blackening is caused by the amount X-ray exposure X-ray absorption depends on:  Atomic number  Density (weight of 1 cm3 of substance)  Thickness  Wavelength (or Energy) of radiation Formation of the invisible image  If X-ray absorption is high, less radiation reaches the recording system less optical density/signal on visible image  Different tissue types and thickness vary the absorption  Latent image formed on the image receptor  Made visible following processing Characteristics of the invisible image  Subject contrast  Sharpness  Noise  Resolution – depends on all of the above  It is the ability to see closely placed small structures as separate structures  Unit = line pairs per millimetre Subject contrast Radiation contrast in the subject due to – Differential attenuation which is dependent on:-  Thickness of tissue  Atomic number  Physical density  Presence of contrast agent  kV  Scattered radiation reduces subject contrast How do we minimise the effects of scatter on subject contrast 1. Collimation due to less volume of tissue being irradiated (also reduces dose) 2. Reduce forward scatter by reducing kV if possible. 3. Compression/tissue displacement to reduce volume. 4. Remove or protect any objects placed near to the patient or detector which may act as a source of scatter e.g. the other limb, lead backing of cassette. 5. Secondary radiation grid for larger volumes of tissue 6. Air gap for high kV techniques because more forward scattering  We would not see bony trabeculae if kV is increased due to the reduction in PE absorption (beam more penetrating)  There is high contrast between bone and soft tissue  BUT it is difficult to differentiate soft tissue structures  We sometimes use positive (iodine and barium) or negative contrast (air) agents  Image sharpness  Geometric unsharpness  Movement unsharpness  Photographic unsharpness Geometric unsharpness is a direct consequence of the finite size of the X-ray source. Therefore by reducing the apparent or effective focal spot size we can reduce geometric unsharpness (less penumbra). Small target angle. Increasing the object-film distance increases geometric unsharpness and an increase in FFD will reduce the effects Edge penetration – gradual change in intensity at edges Image Noise 1.Low signal to noise ratio –(grainy appearance on image) 2.Fog (due to scatter) 3.Quantum mottle (due to random nature of X-ray photon production, producing non-uniform exposure on the detector)- usually seen when low kV is selected 4.Electronic noise (due to electronic processing of the image – over amplification or problems with electronics) Quantum mottle 1 2 3 1. Represents ideal world where X-ray quanta arrive on the film evenly. 2. Real situation where X-rays arrive randomly but fairly evenly due to enough radiation. 3. Half the exposure, film records individual quanta and therefore grain appears X-ray production  An increase in mA increases optical density on film/signal on digital detector  Effect of distance from source obeys the inverse square law  An increase in kVp reduces contrast and increases optical density on film/signal on digital detector Change in kVp In the above diagram: 1. Same object with same characteristics but x-ray densities vary if kV changed. 2. Densities on film – imagine this is a section of film showing densities. 3. mA constant. 4. Increase kV – increase densities (increase penetration0 – BUT decrease contrast ANY QUESTIONS?  [email protected] City, University of London Northampton Square London EC1V 0HB United Kingdom T: +44 (0)20 7040 5060 E: [email protected] www.city.ac.uk/department