Data Aquisition Concepts

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Terms in this set (...)

Ray
Part of an x-ray beam that falls on one detector
View
Collection of rays for one transmission across the patient
Profile
An electrical signal which represents a signature of the attenuation as the ray moves across the slice
Data Sample
Each transmission measurement
X-ray Systems
X-ray generator
X-ray tube
X-ray beam filter
Collimators
X-ray Generator
Three phase power
High frequency
Located inside the gantry
Can rotate with the tube
X-ray tube in 1st and 2nd generation scanners
Fixed Anode
Oil cooled X-ray tubes
Rotating Anode
Produces a heterogenous beam
Large diameter disk
Multiple focal spot sizes for spatial resolution
Made of rhenium, tungsten, and molybdenum
Small target angle of 12 degrees
Rotation spees 3600 rpm to 10000 rpm
Heat Issues
Tube rotates for long periods of time so it needs to withstand high temperatures
Tube envelope is now METAL
Cathode - one of more tungsten filaments
Anode- thicker and made of braze graphite
Bearing Assembly
Provide and ensure smooth rotation of the anode disk
Lubricated with silver
Conducts heat away from tube
Filtration
Restricts the passage of low energy x-ray photons
Bow tie shaped
Hardens the beam
Poly chromatic beam
Special filter needed to make beam appear monochromatic for reconstruction purposes
What is total filtration
Sum of inherent filtration and added filtration
Inherent filtration
3mm
Added filtration
0.1 to 0.4 mm
Collimation
Restricts path of x-rays
Defines slice thickness
Affects patient dose and image quality
CT detector technology
Captures beam from patients and converts it to electrical signals
For a photon to generate a signal....
1. Enter the detector
2.Collide with detector
3.Produce a measurable conversion
4.Be amplified
Efficiency
Ability to capture, absorb, and convery photons to electrical signals
Capture Efficiency
Efficiency of the detectors to obtain photons transmitted through the patient
Absorption Efficiency
Number of photons absorbed by the detector
Stability
Steadiness of the detector response
Response Time
Speed at which the detector can detect an x-ray event and recover to detect another
Should be very short to avoid afterglow
Dynamic Range
Ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated
Usually 1 million to 1
Total Detector Efficiency
Also known as dose efficiency
Product of the capture, absorption, and conversion efficiencies
Scintillation Detectors
Solid state detectors
Most common in CT scanners today
More sensitive to incoming rays from various angles
X-ray hits crystals, flashes of light are produced, light is directed to the PM tube where is hits the photocathode which releases electrons
QDE is 95 to 100%
Gas Ionization Detectors
In 3rd generation scanners
X-rays hit the chambers, gas gets ionized, that produce positive and negative ions, migration of ions cause a small signal current that varies with number of photons absorbed
Not used in 4th generation scanners because detectors must be sensitive to photons from a variety of angles
Matrix Array Detectors
Also known as fixed array detectors
Cells that are equal in all dimensions
Isotropic in design
Adaptive array detectors
Improve spatial resolution in both planes
Anisotropic in design