Class Notes - Biomedical Engineering

Report
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.

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