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CT PhysicsLecture 3
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Data Acquisition: Major Components
Gantry Generator Xray Tube Collimator + Filter Detector Array DAS – Data Acquisition System
Patient table
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3rd Generation CT Discussed generations of CT scanners –
modern day scanner is 3rd generation with rotating x-ray tube and detectors.
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3rd Generation CT System
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What do we mean by cross section?
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Gantry Ring-shaped part of the CT scanner
Houses Slip rings Generator Cooling system X-ray source Filtration Collimation Detectors
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Gantry
Aperture (The Hole) Commonly 70 cm Large Bore – 80 – 90 cm (Toshiba Acquilon LB)
Tilt capabilities Varies from system to system but usually between
+/- 12 to +/- 30 degrees in 0.5 degree increments Laser
Positioning ISOCENTER
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Gantry
XY
Z
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Gantry
Important to recognize that especially in the imaging of a bariatric patient that despite a listed aperture size that it does not account for the table height for which has to advance into the bore
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Couch
Carbon Fiber Top Strong & rigid Low absorption Floats and Rests on
pedestal Vertical and Horizontal
movement
Weight limit Generally 450 lbs. Philips Brilliance Large
Bore = 650 lbs.
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Couch Typical Maximum Scan Range
Not comparable to the length of the couch Generally can cover approximately 162 cm of scan
length Most modern scanners do have a general increase
in scan length
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What is a Slip Ring?
Its what made helical scanning possible by providing continuous rotation without conventional cables
Enables the transmission of power and electrical signals from a stationary to a rotating structure.
Consists of conductive rings and brushes which facilitates the transfers
Allows for faster scan times and continuous acquisitions without cable worry
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What is a Slip Ring?
Brushes enable transmission of power by sliding in and out of grooves on the stationary ring 2 Brush designs
Wire Brush – Conductive wire as contact
Composite Brush – Conductive material as contact
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What is a Slip Ring?
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What is a Slip Ring?High Voltage Slip Ring
• AC power delivered to high voltage generator
• Supplies Slip Ring which powers tube
• GENERATOR DOES NOT ROTATE WITH TUBE
Low Voltage Slip Ring
• More Common today• AC Power is
transferred to slip rings by brushes
• Provides power to high voltage (step up) transformer then to tube
• POSITIONED TO ROTATE WITH TUBE
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Generator
Produce high voltage for the creation of x-ray photons
Modern scanners use High Frequency Generator
Parameters we can control:kVp (80, 100, 120, 140)mA (25-1000)Time (0.5 – 2 sec)
mA * time = mAs
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Heat Capacity Modern CT units can accommodate 3 – 5
million HU Heat Unit
Product of kVp, mA and seconds – the heat generated. Eg 75kVp X 100 mA x 2 s = 15000 heat units
When heated to capacity machinery will automatically compensate Decrease kV Decrease mA Decrease time
End result will result in imaging that is sub-optimal due to the increased presence of noise on the images
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X-Ray Source Tube:
Lead lined cast steel: NOT PYREX GLASS
Lead lined to further contain potential leakage or off focus radiation
Greater cooling properties More efficient isolation of
high voltages Getter:
Barium Helps ensure vacuum by
absorbing any air molecules potentially released by the target during operation
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CT Unique Components
Bow Tie Filter Shaped in order to
compensate for the attenuations within both the head and the body
Effectively hardens the beam and equalizes the amount of radiation reaching the image receptor
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CT Unique Components Collimator
2 kinds Beam Collimation - Single
Detector Detector Collimation – Muti-
Detector Pre-Patient
Adjustable; influenced by focal spot size (Penumbra)
Ensures constant beam width at the detectors
Detector (Pre-detector) Assist in shaping the beam Remove any scattered
radiation from reaching the detectors
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Collimation Restricts the x-ray beam to a specific area Reduces scatter radiation Improves contrast resolution Decreases patient dose
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Collimation Specified in number of detector rows x
detector width. Eg. 16 x 1.25 (20 mm)
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Detectors
Purpose – capture attenuated radiation from the patient and convert it into an electrical signal Signal then converted into
digital data
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Detector Characteristics Efficiency• How well the detector receive
attenuated photons Capture Efficiency
Efficiency in which detectors can obtain attenuated photons Influenced by detector size
and distance between detectors
Absorption Efficiency Number of photons absorbed by
detectors Atomic #; Density of material;
Size and Thickness of the face
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Detector Characteristics Stability
Detector response How often is calibration needed?
Response Time How quickly the detector can detect a
photon and how quick can it recover in order to record the next event
Dynamic Range Ratio of the largest signal to
measured to the precision of the smallest that is discriminated; …Accuracy…
Product of Capture Efficiency, Absorption Efficiency, and Conversion Efficiency
Afterglow continuous luminescence after event
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Types of Detectors
1.) Scintillation Detectors
• Scintillation crystal coupled to a photodiode tube1. X-Ray falls onto crystal which
upon interaction creates photons of light
2. Light gets directed to the photomultiplier increasing the light
3. Light strikes photocathode which emits electrons
4. Electrons pass through dynodes which are arranged and maintained resulting in a signal
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Scintillation Detector
Solid State Photodiode Scintillation Crystals (Solid State) Photodiode – calcium tungstate and ceramic to
which the crystals are bonded by fiber optics Allows current flow when exposed to light Current is proportional to the amount of light Extremely fast response time Conversion Efficiency = 99 % Capture Efficiency = 99 % Dynamic Range = 1,000,000 to 1
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Types of Detectors2.) Gas Ionization Chamber• Think AEC…• Series of individual highly pressurized gas
(Xenon) filled chambers which are separated by metal plates (Tungsten)
1. X-Ray strikes gas filled chambers2. Gas becomes ionized3. Plates are charged + and -, e- migrate to the
positive plate, positive ions to the negative plate
4. As ions move produces a small signal depending on the number of photons
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Gas Ionization Detectors Pressure 30 Atm to
increase probability of event
Excellent stability Zero Afterglow
Detection efficiency less than scintillation detector 50 – 60 % compared
to 95 – 100 % No longer utilized…
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Multi-slice DetectorsDual Row• Introduced in 1992• Allowed for faster volume
coverage than single slice• Dynamic Focal Spot –
position of focal spot switched by a computer system during each scan in order to double sampling and measurements• Produced two contiguous
slices with great resolution
con·tig·u·ous adjective \kən-ˈti-
gyə-wəs, -gyü-əs\ —used to describe things that touch each other or are immediately next to each other
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Multi-slice DetectorsMulti-row(slice) Detectors• Purpose = increase volume
coverage speed of both single and dual slice scanners• IE. 16 slice scanner will be 16
times faster than single slice• Commonly used for machines
which acquire from 4 to 320 slices per rotation
Consists of multiple separate detector rows which can image simultaneously up to the maximum amount of slices per 360 degree rotation 16 slice scanner = 16 slices per
rotation…Duh
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ARRAY TYPES
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Multi-slice Detectors
Two Different Types
Matrix Array Detectors• AKA Fixed ArrayAll detector cells are equal in dimension along
an array• Perfect Cubes one right after the other
Adaptive Array DetectorsDetector cells have different sizes along an
array
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Multi-slice Detectors
Number of slices is dependent on configuration used Data collection “channels”
Configuration can use each individual detector to produce that size slice Ie. 16-1.25 mm detector can
produce 16-1.25 mm slices Or….can use combination
to produce larger slices Ie. 16-1.25 mm detector can
produce 8-2.5 mm slices or 4-5 mm slices
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Multi-slice Detectors
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DAS Data Acquisition System Refers to all the electronic components which
lie after the detectors but before the computer
Performs 3 functions1. Measures attenuated radiation2. Converts measurements to digital3. Transmits digital data set to computer
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DASDAS Components:• Pre-amplifier – takes the weak electrical signal from
the detectors and boosts it so that it can be more easily converted
• Logarithmic amplifier – performs the conversion of attenuated transmission data to logarithmic data which is sent across to ADC• Conversion of transmission to attenuation and thickness
Attenuation = log transmission * thickness…orµ1 + µ2 + µ3 = (ln I0 / 1) (I / x)
µ = linear attenuation coefficient I0 = Original intensityI = Transmitted intensityx = Thickness of Object
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DAS ADC
Analog to Digital Converter – divides the electrical signals into multiple portions
The more portions the better the digital signal…now how do we get more portions again? Hmmmmmm… Help determine the bit depth (grey scale) of image
Optoelectronics Series of lens and light diodes which moves data
transmissions from the ADC to the computer at tremendously high speed
50 million bits / sec.
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Data AcquisitionIn order to ensure that an adequate amount of
measurements (samples) are obtained for optimal quality…
1. Utilize thin slices to reduce sample artifact…2. Closely Packed Detector Array – closer they
are, the more are available to receive radiation, the more samples obtained…
3. Quarter Shifted Detector Arc – provides 2 sets of data which can be individually reconstructed• Siemens developed
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References Image from of Sprawls.com Stewart Bushong “Radiologic Science for
Technologists” Bushberg et al., “The Essential Physics of
Medical Imaging” Wikipedia Impact.org