BME 560 Medical Imaging: X-ray, CT, and Nuclear Methods X-ray Instrumentation Part 2 Today • Anti-scatter devices • X-ray screen-film systems • Other methods of X-ray detection X-ray System Produces X-rays from electrical energy Tailors X-ray spectrum Converts X-rays to light and records Source Restrictor (Collimator) Determines size and shape of beam Filter Subject Anti-scatter Selectively removes scattered photons Detector Total linear attenuation coefficient N x N 0e x At X-ray energies, most photons that interact in the patient are Compton-scattered. Rayleigh photoelectric compton pair Soft Tissue productin X-ray Scatter Tube Scattered radiation comes into detector from all directions. Result is a relatively uniform background “fog” that reduces dynamic range of the detector available to image true signal. Object Would like some way to reduce scattered radiation without blocking much direct radiation. Grid Detector Anti-scatter Strategies • • • • Collimation of the beam at the front end Air gaps Grids Scanning Slits Air Gap Moving the patient away from the detector reduces the scatter reaching the detector. Square-law Solid angle What price do we pay for this? Anti-scatter Grids Construct a device to collimate photons after they leave the patient. Thin lead strips must be precisely aligned. Performance depends on the grid ratio What is the price paid for a high grid ratio? Typical grid ratios: 5:1 to 16:1 (lower for mammography) Anti-scatter Grids • A stationary grid will leave line artifacts in the image. • A Potter-Bucky diaphragm is a movable grid that basically blurs the grid lines during exposure. • The grid also blocks some primary radiation in the system. Anti-scatter Grids Tradeoff between scatter penetrating the grid and primary radiation detected Anti-scatter Grids • Thickness of strips determines the likelihood of penetration. – Less scatter penetration = less primary radiation • Low-angle scatter may still get through. • Multiply-scattered photons may get through. Anti-scatter Grids • Additional exposure is needed to maintain same detector exposure level when using grid. • Grid Conversion Factor mAs with grid for exposure E GCF mAs without grid for exposure E • Typically 3 < GCF < 8 • May avoid grid for small body parts or low energies. Scanning Slits Moving source and collimator Move source, collimator, and slit together. Only takes one part of image at a time. Very high scatter reduction Slow Moving slit Stationary detector X-ray Detectors • Film-Screen • Image Intensifiers • Panel Detectors Film-Screen Detectors • Roentgen’s first X-rays exposed a photographic plate directly. – But photographic film has very low stopping power (a couple of percent). – To expose the film to its full dynamic range (contrast) would require high dose and most would be wasted. • Augment this with an intensifying screen that converts X-ray photons to visible light. Screen-film System • Double emulsion film sandwiched between pair of intensifying screens • Phosphor particles (high Z) covert Xray into light photons • Screen enhances contrast but lowers resolution • Engineering tradeoff: Phosphor thickness Reflective Layer Base Phosphor Film Coating Screen-film System • Reflective layer reflects light back into the film • Base for mechanical support • Phosphor layer material choice: – More fluorescent than phosphorescent – High linear attenuation coefficient = stopping power Reflective Layer Base Film • Conversion efficiency: total light energy per unit incident Xray energy (usually 5 – 20%) – Energy dependent Phosphor Coating Film • Very similar to photographic film; must be developed to fix the image • Two components: – Base: Plastic sheet, dimensionally stable (size and shape do not change under environmental and processing conditions) – Emulsion: Crystals of silver bromide suspended in gelatin substance; on one side (single-emulsion), or both sides (double-emulsion) of base. • Image is formed in the silver bromide crystals. Screen-film System • The screen-film combination usually has a speed quoted – More sensitive (= fewer Xray photons to result in a given image density) pairs have higher speed – At a particular energy! Exposure to get a standard level of film density Speed Sensitivity (mR) 1200 0.1 800 0.16 400 0.32 200 0.64 100 1.28 50 2.56 25 5.0 12 10.0 From Sprawls Radiographic Cassette • Ensures firm and uniform contact between intensifying screens and film sandwiched in between • Optical mirrors located outside screens to direct light towards film, maximize light conversion efficiency • Contains ID card and loaded only one way into X-ray machine Image source: The Essential Physics of Medical Imaging Film Density • Density describes the overall blackness of the radiograph Image source: http://www.nurseslearning.com/courses/fice/fde0030/Imaging_terms.htm Screen-film System Thicker screens result in higher sensitivity but increase image blur Other Detection Schemes • Detection is a result of radiation interaction with matter. Radiation interaction results in emission of by products, e.g. electrons, electromagnetic radiation, that can be sensed by instrumentation and recorded by data acquisition systems – – – – – Gas-Filled Detectors Scintillation Detectors Flat-panel detectors PSP plates Solid State Detectors Gas Filled Detectors • Radiation ionizes the gas. Charges freed by ionization produce a current. Positive Power Supply Radiation Ammeter Negative Gas Chamber Resistance R Gas Filled Detectors • Radiation interacts with gas and ionizes its atoms • Freed electrons interact with gas and ionize more atoms amplification Ionization Chambers Proportional Counters Geiger Mueller Counters Collected Charge – Ionization chamber: No amplification – Proportional counter: Amplification up to 106 times – Geiger-Muller counter: Very strong avalanche Voltage Spatial sensitivity is lacking – Not used for imaging Scintillation Detectors • Interaction of X-rays with some materials (CsI, cadmium zinc telluride - CZT) produces ‘scintillation’ or “flash of light”. scintillator X-ray Not capable of handling high photon flux. Visible light Electrical pulse photomulitplier The pulse can tell you about the energy of the incident photon. Photomultiplier Tube • Photomultiplier tube (PMT) converts light into electric current by photoelectric effect Dynodes Photocathode photons grid Anode Flat-panel Detectors Scintillator Light coupling Lightsensitive digital detector (CCD array) Varian Medical Systems Photostimulable Phosphor Plates • PSP plates • X-rays excite electrons which are trapped in the material lattice. • Later, the plate is scanned by a laser in a “plate reader” which frees the electrons locally and digitizes the image. • The plate can be reused. • Plugs in to the film-screen cassette slot. Solid State Detectors • They are compact semiconductors. Electrical conductivity of semiconductor is sensitive to impurities. The depletion layer is sensitive to radiation and electric current flow through, thus the measured current is a measure of + _ radiation. n-type ---------------------------------------- depletion layer ++++++++++ ++++++++++ ++++++++++ Radiation or incident particles p-type X-ray Image Detection • Screen-film: Still in use • PSP Plates: Displacing screen-film in many applications • Flat-panel: Increasing use but expensive • Solid state: Still in development for X-ray • Scintillation detectors: Not fast enough for X-ray imaging, but still important research tools. – SPECT imaging • Gas counter: Not useful for imaging but used for active measurement of patient exposure.