Medical Imaging X-Ray Radiography PDF

Medical Imaging X-Ray Radiography X-rays  Half value layer Example : The thickness of material required to reduce the intensity of an x- or gamma-ray beam to one-half of its initial value. Example A narrow beam containing 2000...

Medical Imaging X-Ray Radiography X-rays  Half value layer Example : The thickness of material required to reduce the intensity of an x- or gamma-ray beam to one-half of its initial value. Example A narrow beam containing 2000 monoenergetic photons is reduced to 1000 photons by a slab of copper 0.01m thick. – What is the total linear attenuation coefficient of the copper slab for these photons? – What is the HVL? The HVL of a monoenergetic beam of x Solution or γ-rays in any medium is  Planer Radiography/Projection imaging The acquisition of a 2D image of the patient's 3D anatomy  Instrumentation of Planer Radiography Basic components of a planar X-ray radiography system are: – X-ray tube – a collimator reduce the patient dose and amount of Compton scattered X-rays, – an anti-scatter grid reduce further the contribution of scattered X-rays to the image, – a detector converts the energy of the transmitted X-rays into light  * Space Charge Electrons leave filament – filament becomes positive Negative electrons stay close Electron cloud surrounds filament Cloud repels new electrons from filament Limits electron flow from cathode to anode - + - -  Kilovoltage & Space Charge raising kilovoltage gradually overcomes space + + - + - charge ++ - – Higher fraction of electrons make it to anode as kilovoltage increases At high enough kilovoltage saturation results Tube Current (mA) – All electrons liberated by filament reach target Raising kilovoltage further has no effect on # electrons reaching anode Saturation kVp Voltage  Saturation Voltage + + - + - ++ - kilovoltage at which a further increase does not increase tube current – 100% of electrons already going to target Tube current said to be emission limited – tube current can only be increased by increasing filament temperature  Focal Spot portion of anode struck by electron stream Focal spot sizes affects and limits resolution +  Focusing Cup negatively charged focuses electron stream to target – overcomes tendency of + electrons to spread because of mutual repulsion Focusing Cup  Focal Spots Most tubes have 2 filaments & thus 2 focal spots only one used at a time small focus – improved resolution large focus – improved heat ratings – Electron beam strikes larger portion of target Cross Section of X-Ray Tube Dunlee Web Site: http://www.dunlee.com/new_tube_anatomy.html  Line Focus Principle Focal spot steeply slanted – 7-15 degrees typical + Focal spot looks small from patient’s perspective Actual FS – Imaging size Apparent FS Looks large from filament – better heat capacity Patient  Line Focus Principle Actual (true) focal spot + – as seen from filament Actual FS Apparent (effective, Apparent FS projected) focal spot – as seen from tube port or patient Patient  Target Angle – Angle between target & perpendicular to tube axis – Typically 7 – 15 degrees + Target Angle, Q  Line Focus (cont.) + Actual FS Apparent FS Target Angle, Q Apparent FS = Actual FS X sin Q  Target Angle (cont.) Large Small – poorer heat ratings – optimizes heat ratings – better field coverage – limits field coverage Large Target Angle Small Target Angle (Small Actual Focal Spot) (Large Actual Focal Spot) + +  Heel Effect Intensity of x-ray beam significantly reduced on anode side beam goes through more x - - target material exiting - the anode cathode side anode side  X-ray Detectors Traditional x-ray film – Screen-film radiography Digital detectors – Computed radiography – Digital radiography  Screen-film cassette The screen-film detector system used for general radiography consists of – a cassette, – one or two intensifying screens, – a sheet of film. The film is a sheet of thin plastic with a photosensitive emulsion coated onto one or both sides. A typical screen film cassette and its cross-section X-ray film Example: Optical densities of region A and B on a radiograph are 1.0 and 1.5, respectively.. Calculate the contrast between the two regions. Solution: