Download 19.CT Physics Module D

Module D
Computed Tomography
Physics, Instrumentation,
and Imaging
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CT Imaging Systems
 High Voltage Generator
 3 Phase system - more efficient production
of x-rays are possible
 The high voltage cables were eliminated
with helical technology.
CT Imaging Systems
 Generators today are high-frequency and triphasic
 In contrast to earlier generators today's’ are:
 Small and compact
 More efficient
 Take low voltage 5 – 50 Hz
 Flow frequency current is converted into high
voltage, high frequency of between 500 –
25,000 Hz.
 Power ratings for CT generators range
between 30-60 kilowatts (KW)
Power ratings for CT generators range between
30-60 kilowatts (KW) allowing for a wide range
of exposure techniques
kVp selections
80
100
120
130
140
Milliamp selections
30
50
65
100
125
150
175
200
CT Imaging Systems
 Generators are mounted
 On the orbital scan frame with the tube
 In the corner of the gantry (stationary)
 Slip ring technology make this possible.
Slip Ring Technology
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Electromechanical devices
Transmit energy across a rotating surface
Rings and brushes
Made spiral or volume scanning possible
Two slip ring designs
 Disks
 Cylinders
 PAGES 81 and 82 in Seeram
CT Imaging Systems
 Wire brushes are used to transmit
power to the CT components
– Brushes glide in contact grooves along the
slip rings
– Two types of brushes
– Composite
– Wire
Low voltage slip ring scanners
 480 AC current
 Slip-ring provides power to highvoltage transformer then to
radiographic tube
 X-ray generator and tube are
positioned on the orbital scan frame
High-Voltage Slip-Ring Scanners
 AC delivers power to the high-voltage
generator
 High-voltage generator then supplies highvoltage to the slip-rings
 High-voltage from the slip ring is transferred
to the x-ray tube
 High-voltage generator does not rotate with
the x-ray tube
CT Imaging Systems
 Main difference is the way in which the images are
gathered and how the reconstruction algorithms
are used.
 Conventional CT
– One row of detectors
– SDCT
 Spiral CT
– 164 rows of detectors
– MDCT
Slice by Slice Scanning
 Also called “step and shoot”
 Tube rotates around the gantry
 Attenuated radiation from the patient is
captured by detectors
 Tube stops and the couch moves
 Process repeated until the entire exam is
completed
Z-Axis
 CT is often referred to as Axial CT
 This is incorrect
 CT is an axial scanning TECHNIQUE only
 CT images are acquired along the Z-axis of
the patient’s body
 The Z-axis is along the transverse or axial
plane
Volume CT
 Tube rotates continuously while making an
exposure as the patient moves through the
gantry
 The continuous rotation of the x-ray tube
and the couch top movement equal the
spiral geometry acquisition
 THUS- the name - Spiral CT
Original spiral CT scanners
 Had only 1 row of detectors
 (SDCT) single detector computer
tomography
 Used different algorithms than what are
used today with 164 rows of detectors
CT computer systems
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Processes information for the DAS
windowing
Image enhancement
Image magnification
ROI
Quantitative measurements
MPR or multiplanar reconstruction
3-D imaging
MIP maximum intensity projections (vascular imaging)
Volume rendering
Surface rendering
Tasks surrounding image manipulation
CT Computer System
 Minicomputer
 Capable of performing complex computations
while receiving high-level input and out put
 Used in MRI also
 Computer processing architecture
 Pipeline
 Parallel
 distributed
Array Processor
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?most important part?
Dedicated electronic circuitry
Rapid calculations
Receives information from the detectors
Measurements from hundreds of projections
are used to piece information back together
Array Processor
 Microprocessors assist
 Speeds of 1 nanosecond
 Number of array processors determines the
speed of reconstruction
 Retrospective
 Post-processing
Scanning Process
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Voltage causes electrons to “boil off”
Electrons are accelerated striking the cathode
X-ray production
Beam travels through tube and the “bow tie” filter
 Filter shapes and
 Defines the beam
 After filtration comes collimation
 (Filtration occurs twice)- prior to entering the
patient and again after it is attenuated by the
patient.
Attenuation
Depends on:
1. electrons per gram
2. affective atomic density of the absorber
3. atomic number of the absorber
4. energy of the transmitted photons
What does the “Z” number of the absorber
mean?
Lambert-Beer Law
 -is “an exponential relationship that explains
what happens to x-ray photons as they
travel through body tissue”.
 Lambert-Beer Law incorporates the
principles of:
– Photoelectric effect
– Compton Scattering
Lambert-Beer Law
Iin = Iout e
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-μx
I out = the transmitted intensity
I in = the original intensity
e = Euler’s Constant (2.718) (Base of the Natural Logarithm)
Φ = Linear Attenuation Coefficient
x = thickness of the object
Continue…..
Lambert-Beer Law equation
Values are known for the transmitted and
original beam intensities and for the
thickness of the object.
Because these values are known, the linear
attenuation can be derived.
Linear attenuation coefficient
- Linear attenuation coefficient represents the
rate that x-rays are attenuated
(diminished) as they travel through the body.
Image Display
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CRT (Cathode Ray Tube)
Display matrix
Pixel size determinant
Bit depth
CT number value scale
Window width and level
Primarily a factor related to the DFOV (display field of view)
(DFOV has the GREATEST IMPACT on image resolution
and image noise)
 Protocol
 Technologist controls or Selectable scan factors
Monitor Matrix
 Resolution is based on the size of the matrix
– 512 x 512
– 1024 x 1024
– 2048 x 2048
Newer scanners allow for the selection of these
larger maxtrices during scanning.