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35 Cards in this Set

  • Front
  • Back
What is meant by data acqusition?
the method by which the patient is scanned to obtain enough data for the image reconstruction
How is scanning defined?
by the beam geometry, which characterized the particutlar CT system and also plays a central role in spatial resolution and artifact production
What is beam geometry?
refers to the size, shape and motion of the beam and its path
What are the components of beam geomtry?
the components refer to those physical drives that shape and define the beam, measure its transmission through the patient an convert the info into digital data for input into the computer
What do detectors do?
They convert the xray photons into and electrical signal (analog data)
What is a Ray in CT?
A ray is a part of the beam that falls on One detector
In CT what is a view?
A collection of rays for one translation across the object
What is Projection Data?
projection data is collected by the detector, each ray is attenuated by the patient and transmitted and projected onto the detector
What is a data sample?
Each transmission measurement is refferred to as a data sample
How do you find the total number of data samples?
DStotal = number f detectors X number of data samples
DStotal = number of data samples per view X number of views
What are the 3 types of acquisition geometry?
1. parallel beam geometry
2. fan beam geometry
3. spiral geometry
First Generation scanners
parallel beam, (pencil beam) translate-rotate motion, 1-2 detectors, 4-5.5 mins a scan
fixed anode, oil-cooled xray tube
Ex: EMI brain scanner
Second Generation scanners
fan beam (rectilinear multiple pencil beam), linear detector array (30 detectors), divergent rays, translate-rotate motion, scan times of 20 sec - 3.5 mins
fixed anode, oil-cooled xray tube
Third Generation scanners
fan beam, complete continuous rotation of tube and curved detector array, scan time seconds
Fourth Generation scanners
2 beam types; wide fan beam, complete rotation of tube around a stationary ring of detectors and nutating fan beam in which the apex of the fan is located outside a nutating ring of detectors, xray tube is stationary, very short scan time
Fifth Generation Scanners
developed for high speed CT scanning 9milliseconds)
EBCT -fan beam, tungstun target rings
DSR- dynamic spatial reconstructor, 3D images
What is Spiral- Helical Geomety?
The data acquisition of continous rotation scanners
data is collected in volumes, slip ring technology, continuous rotation,
What are slip rings?
they are electromechanical devices consisting of circular electrical conductive rings and brushes (wire or composite)that transmit energy across a rotation suface
Advantages of Slip Ring, Contiuous Rotation, Volume CT are?
Faster scan times, minimal interscan delay, no start-stop process, no wrap around cables
What type of power is used for CT?
CT uses three-phase power for effiecency of xrays, along with high frequency generators (located inside the scanner)
What are the two factors that the CT radiation source depend on?
1. radiation attenuation
2. the quantity of radiation required for transmission
What has caused the increased heat generation Spiral CT and what been done to remedy it?
Contiuous tube rotation for longer periods of time, increased power levels.
Tube enevelope with vacuum, insulation; larger, thicker, brazed graphite (tungstun-rhenium), rotating anode, improvement of heat dissepation rates
What purpose does filtration have in CT?
Filtration removes long-wavelength xrays, increasing beam energy (beam hardening), and shapes the energy distribution across the radiation beam = uniform beam
What is prepatient collimation for?
It improves axial resolution, defines the slice thickness.
What characteristics are needed in a detector for CT?
efficency, response time, dynamic range, high reproductability and stability
What is detector efficency?
the ability to capture, absorb, and convert xray photos to electrical signals
What is detector stability?
the steadiness of the detector response, keep signals useful without excess calibrations
What is meant by detector response time?
the speed in which the detector can detect an xray event and recover to detect another.
Should be very short to decrease afterglow and pile up
What is detector dynamic range?
the ratio of largest signal to be measured to the precision of the smallest signal to be discriminatied
usually 1 million to 1
What are the two types of electrical enery conversion?
Which is more efficient?
1. Scintillation- detectors convert xray energy to light, and the light is converted into electrical energy
2. Gas-Ionication- detectors convert xray energy directly to electical energy
Solid-state detectors are 94-100%, and only 50-60% or xenon gas
What are Scintillation Detectors made of and how do they work?
solid state photodiode multiplier scintillation crystals are made of calminum tungstate and creamic material (rare earth); they convert xray energy to light, and the light is converted into electrical energy with a photomultiplier tube,
What are Gas-Ionication Detectors made of and how do they work?
made up of gas chambers, usually separated by tungsten, enclosed by a realitively thick ceramic substrate material, filled with xenon gas
What is the DAS? What 3 functions does it preform?
Data acquisition system (DAS) refers to the detector electronics positioned between the detector array and the computer.
1. measures the transmitted radiation beam
2. encode the measurement to binary data
3. transmits binary data to computer
What is logarithmic conversion? What is it preformed by?
the transmission measurement data is changed into attenutaion and thickness data
Attenutation = log of transmission X thickness
the logarithmic amplifier
What has been done to improve streaking (aliasing) artifacts due to inadequate sampling?
Imaging of thinner slices, closely packed detectors, quarter-shifted detector arc, double-dynamic focus system