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 Converts X-rays to light and records Produces X-rays from electrical energy Tailors X-ray spectrum Source Filter Restrictor (Collimator) Subject Anti-scatter Detector Determines size and shape of beam Selectively removes scattered photons

Total linear attenuation coefficient At X-ray energies, most photons that interact in the patient are Compton-scattered. Soft Tissue

X-ray Scatter Tube Object Grid Detector 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. Would like some way to reduce scattered radiation without blocking much direct radiation.

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 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 Reflective Layer Double emulsion film sandwiched between pair of intensifying screens Phosphor particles (high Z) covert X-ray into light photons Screen enhances contrast but lowers resolution Engineering tradeoff: Phosphor thickness Base Film Phosphor 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 Conversion efficiency: total light energy per unit incident X-ray energy (usually 5 – 20%) Energy dependent Base Film 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 Exposure to get a standard level of film density The screen-film combination usually has a speed quoted More sensitive (= fewer X-ray photons to result in a given image density) pairs have higher speed At a particular energy! 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.

Gas Filled Detectors Radiation interacts with gas and ionizes its atoms Freed electrons interact with gas and ionize more atoms - amplification Ionization chamber: No amplification Proportional counter: Amplification up to 106 times Geiger-Muller counter: Very strong avalanche 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 Not capable of handling high photon flux. Visible light X-ray 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

Flat-panel Detectors Scintillator Light coupling Light-sensitive 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.

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.