CT Tube Flashcards
- high x-ray tube voltages
- tube currents of hundreds of milliamperes, and
- scan times between 0.5 and 2 seconds.
CT x-ray technique
provide a very stable tube current and voltage.
High-frequency power supplies
used at high power ratings (up to 60 kW)
Large focal spots (1 mm)
used at low power ratings <25 kW
Small focal spot (0.6 mm)
used with CT scanners typically produces
a beam with an aluminum HVL of up to 10 mm
heavy filtration
used to minimize the dynamic range of
exposures at the detector
Bow-tie filter
- defines the section thickness and reduces scatter.
- restriction of the useful x-ray beam to reduce patient dose and improve image contrast
Collimation
- are located in the x-ray tube as well as the X-ray detectors.
- used to control the width of the fan beam between 1.0 and 10mm, which controls the width of the image slice
Collimators
Allow section thickness to range between 1 and 10 mm
Adjustable collimators
largest component of CT Machine
Gantry
The Gantry contains:
- x-ray tube,
- collimators,
- detector array,
- DAS (Data Acquisition System)
- the opening within the center of the gantry
- 28 inches(71.1cm) wide - to accommodate a variety of patient sizes
Aperture
- Generates X-ray beam
- Power requirement: typically 120 KV at 200-500 MA
- Anode heating capacity: 1MHU- 4MHU
- High-Speed Rotors for heat dissipation
- Focal spot size: small for spatial resolution
X-ray Tube
one of the most important issues that impacts X-ray tube performance is
heat management
Tube heat management
specification is described in
heat units (HU)
1 heat unit is equal to?
0.74 J
- is mounted on the tube housing
or adjacent to it - limits the area of the patient that intercepts the useful beam
- determines the slice thickness and patient dose
Pre-patient collimator
- restricts the x-ray field of view by the detector array and reduces scatter radiation incident
- does not contribute to patient dose
post- patient collimator/ pre-detector collimator
- Absorbs radiation and converts some or all of the absorbed energy into a small electrical signal
- Measures the intensity of radiation transmitted through the patient.
- Should have a high overall efficiency to minimize the patient radiation dose
- Must have a large dynamic range
- Must be very stable with time
- Must be insensitive to temperature variations with the gantry
Detector/X-ray Detector
what detector is an electric signal that is proportional to the incident radiation intensity is digitized and stored in a computer
CT detectors
consist of a gas-filled
chamber with anodes and cathodes maintained at a
potential difference.
Xenon gas ionization detectors
Incident x-ray photons ionize the gas, producing what?
electron-ion pairs
are more stable than solid-state detectors and have a wide linear response with no lag
Gas detectors
produce light when x-ray photons are absorbed, and are coupled to a light detector (photomultiplier tube or photodiode)
Scintillation crystals
The most common material used in solid-state detectors is BLANK which is an efficient X-ray detector
Cadmium tungstate (CdW04),
Only BLANK are used for fourth-generation scanners, which require thin detectors because of the detection geometry
Solid-state detectors
2 General Classification of
Detectors
- Solid-state detectors/ Scintillation Detector
- Gas Detector
- requires power supply
- have very high quantum and conversion efficiencies
- large dynamic range
Solid-state detectors
first crystal used in the earliest scanner and was
replaced by Solid-state scanner
Sodium Iodide (Nal)
- Bismuth Germanate (BGO)
- Cadmium Tungstate (CDWO4)
- Cesium Iodide (CSI)
- Calcium fluoride
examples of…
Solid State Crystals
- are constructed of a large metallic chamber with baffle space at approximately 1mm intervals
- a high voltage is applied to tungsten septa between two chambers to collections produced by radiation.
- contains inert gas (eg. Xenon gas)
- have excellent stability and a large dynamic range
- has 45% intrinsic detection efficiency
- have lower quantum efficiency than solid-state detectors
Gas Detector
3 Important Factors Contributing to
Detector Efficiency
- Geometric Efficiency-
- Quantum Efficiency
- Conversion Efficiency
refers to the area of the detectors sensitive to radiation as a fraction of the total exposed area.
Geometric Efficiency
refers to the fraction of incident x-rays on the detector that are absorbed and contribute to the measured signal
Quantum Efficiency
refers to the ability to accurately convert the absorbed X-ray signal into an electric signal
Conversion Efficiency
interfaces the x-ray detectors to the system computer and may consist of Preamplifier, Integrator, Multiplexer, Logarithmic amplifier, and Analog-digital converter
Data Acquisition System (DAS)
Operates in three phases or High-frequency power
Generator
- located at the rear of the CT gantry to ensure more room for CT operators at the front of the gantry
- works in cooperation with a trolley means having a slightly inclined tabletop holding a stretcher carrying the patient
- an automated device linked to the computer and gantry.
- should be a low Z material
- should not cause artifact
- 300-600 lbs.(136-272 kg) patient weight limit
Support Couch/ Patient Couch/PSA
Attached to the table used for CT examinations, helps to hold the head still
Cradle
refers to the movement of the patient couch
Indexing
is an image distortion caused by table indexing and respiration
Image Misregistration
Has special requirements: 30% relative humidity /below 200C room
temperature
Computer
temporary memory that stores information while the software is used
RAM(Random Access Memory)
for storage only; data on this memory cannot be overwritten
ROM(Read Only Memory)
- performs calculations and logic operations under the control of software instructions.
- Heart of the Computer.
CPU(Central Processing Unit)
- contains meters and controls for selecting proper radiographic
technique factors - For proper mechanical movement of the gantry and patient couch,
and for commands that allow image reconstruction and transfer - the point from which the technologist controls the scanner
- has two television monitors, patient data, view result image
Operating Console/ Control console
term for heat dissipation
Revolution per minute (RPM)
The most commonly used in General conventional x-ray RPM is?
3500 RPM
Most ideal RPM in CT scan and xray is
10,000 RPM
unit of energy
Joules
relationship of collimation and contrast is?
directly related/proportional
difference between black and white
radiographic contrast/opaqueness
- any charged particle
- positive or/and negative
ion
- the removed electron from the atom from which it was separated
- combination of positive and negative
ion pair
- has no charge
- no. of protons is equal to no. of electrons
stable atom
collection of proton and neutron
nucleons
three things in the photomultiplier tube
- photocathode
- dynode
- collecting node
convert light to electrical signal
photocathode
multiple the electrical signal
dynode
collect electrical signal
collecting anode
process of producing light when stimulated by external force
scintillation
Enumerate the parts of the Data Acquisition System (DAS)
- preamplifier
- integrator
- multiplexer
- logarithmic amplifier
- analog-digital converter
in CT x-ray technique it is used to reduce x-ray beam hardening
effects
Copper or aluminum filters
