Download 15.CT Physics Module C

Module C
Computed Tomography
Physics, Instrumentation,
and Imaging
Disclaimer

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Data Acquisition
Three step process:
1. Data Acquisition
2. Image Reconstruction
3. Image Display
CT components
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Gantry
Gantry Apertures range from
50 – 85 cm
Current table limit
450 lbs.
Diagnostic Ct tables are curved
Radiation Therapy tables are flat
CT components
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Tilt 30 degrees cephalad or caudal
Tilt capabilities absent using Positron
Emission Tomography (PET)
Table and scanning controls
Gantry
 table
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CT X-Ray Tube
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More than one filament
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tungsten
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Focusing cup
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Anode
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Stationary
rotating
Anode
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Rotating most common today
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Shorter exposure times
Small focal spots for greater detail
High heat load capacity
Produces heterogeneous beam from large
rotating anode
Large anode disc and small focal spot
facilitate spatial resolution
requirements
Anode
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Made of rhenium
Tungsten
Molybdenum alloy (RTM)
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Additional types of discs-
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All metal disk
Brazed graphite disk
Chemical vapor disk
Anode
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Target angle is generally 12 degrees
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Rotational speed is either
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3,600 rpm
10,000 rpm
Heat storage capacity
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2-5 million heat units (HU)
Anode
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Heat dissipation rates are in the area
of 400,000 HU/m.
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Achieved using
Oil cooled
 Liquid cooled
 Fan cooled
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X-ray Beam
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Polychromatic
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CT has to have Monochromatic
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Bowtie Filter
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Remove the long wavelength x-rays from
the beam
Shape the beam
Collimation
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Two sets of collimators
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Patient radiation dose
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Quality of the CT images
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Together called PRE-detector
Separately called PRE and POST patient
CT Detectors
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Positioned within the gantry
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Generation determines the type
Rotating
or
 Stationary
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Detector Characteristics
1.
2.
3.
4.
5.
Efficiency
Response time
Dynamic Range
High Reproducibility
Stability
Types of Detectors
 Gas
Ionization
 Solid-State
Xenon Detectors
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Inert gas
Fast rate of decay
Used in third generation scanners
Fourth generation scanners have nonrotating detectors. Fourth generation
detectors rock back and forth.
Continue…….
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Solid State Detectors
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Detectors
Also called Scintillation detectors
Made of:
Bismuth germinate
 Cesium iodide
 Cadmium tungstate
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Sampling
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Governed by Nyquist Theorem
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Increasing sampling methods
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Adjustments in slice thickness
Use of closely packed detectors
Quarter-shifted detectors
Array Processor
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Primary function – reconstruct the projected
raw data
Solves all mathematically complex problems
Used in retrospective reconstruction and
post processing
Number and type used affects the
reconstruction time of the scanner
DAS
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Data Acquisition system
A set of electronics that are located
between the detector array and the CT host
computer
DAS functions:
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Measures the transmitted beam
Logarithmic conversions
Changes the measurements into digital form
Sends the digital signals to the array processor
for processing