15.CT Physics Module C
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Transcript 15.CT Physics Module C
Module C
Computed Tomography
Physics, Instrumentation,
and Imaging
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Data Acquisition
Three step process:
1. Data Acquisition
2. Image Reconstruction
3. Image Display
CT components
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
Tilt 30 degrees cephalad or caudal
Tilt capabilities absent using Positron
Emission Tomography (PET)
Table and scanning controls
Gantry
table
CT X-Ray Tube
More than one filament
tungsten
Focusing cup
Anode
Stationary
rotating
Anode
Rotating most common today
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
Made of rhenium
Tungsten
Molybdenum alloy (RTM)
Additional types of discs-
All metal disk
Brazed graphite disk
Chemical vapor disk
Anode
Target angle is generally 12 degrees
Rotational speed is either
3,600 rpm
10,000 rpm
Heat storage capacity
2-5 million heat units (HU)
Anode
Heat dissipation rates are in the area
of 400,000 HU/m.
Achieved using
Oil cooled
Liquid cooled
Fan cooled
X-ray Beam
Polychromatic
CT has to have Monochromatic
Bowtie Filter
Remove the long wavelength x-rays from
the beam
Shape the beam
Collimation
Two sets of collimators
Patient radiation dose
Quality of the CT images
Together called PRE-detector
Separately called PRE and POST patient
CT Detectors
Positioned within the gantry
Generation determines the type
Rotating
or
Stationary
Detector Characteristics
1.
2.
3.
4.
5.
Efficiency
Response time
Dynamic Range
High Reproducibility
Stability
Types of Detectors
Gas
Ionization
Solid-State
Xenon Detectors
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…….
Solid State Detectors
Detectors
Also called Scintillation detectors
Made of:
Bismuth germinate
Cesium iodide
Cadmium tungstate
Sampling
Governed by Nyquist Theorem
Increasing sampling methods
Adjustments in slice thickness
Use of closely packed detectors
Quarter-shifted detectors
Array Processor
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
Data Acquisition system
A set of electronics that are located
between the detector array and the CT host
computer
DAS functions:
Measures the transmitted beam
Logarithmic conversions
Changes the measurements into digital form
Sends the digital signals to the array processor
for processing