Basic Principles Flashcards
What is the Mylar window?
Narrow transparent window though which X-ray beam passes
Ct gantry contains…
Detectors, collimators, data acquisition system ( DAs )
System emergency off (seo)
Located on wall
What are slip rings attached to gantry of helical ct?
Electro - mechanical devices made of parallel electrically conductive rings and brushes
What is helical/spiral scanning?
Continuous rotation of tube and detector elements around the pt. And continuous movement of table. Rotate- rotate scanning
Axial scanning?
’ Tube moves 360°, scanning halts, tube moves 360° opposite, scans again
Known as rotate/ translate scanning
Axial acquisition produces…
Discrete packages of image data, with locked thickness, position and orientation - manipulation is Ltd
Helical advantages?
Speed
Data acquired as volume- not broken into slices
Increased temporal resolution
MPR - multiplanar reformation + 3D possible
What is a data set?
A. Volume of data
X-ray generation in helical scanners?
High frequency solid state generators - stable high power output to tube
Describe Ct tube
Stating anode tube mounted on gantry
Subjected to great thermal loads
Design helps heat, storage and dissipation
Why do Ct s have metal envelope?
no risk of electrical arcing caused by tungsten deposits vaporising
Ct rotating anode are…
’ Thick ,increase heat storage, dissipation = higher tube currents possible
Brazed graphite anode with tungsten - rhenium focal track
Graphite = high heat capacity
Contrast resolution?
Ability to differentiate between small differences in tissue density
Beam hardening…
Artefact reduced by copper flat filters
Bow tie filters…
Teflon , Shaped to reduce beam intensity at periphery _ correspond with thinner parts of pt. Anatomy
Ct detectors located on…
Rotating gantry
Ct attenuated beam enters the…
Detector chamber and energy is amplified, converted to electrical signal
Optimal Ct detector is…
High efficiency, scatter suppression ‘ stability
Low after glow, crosstalk between elements cheap & serviceable
2 types of Ct detector …
Ionisation chamber
scintillation detector / solid state detector
Ionisation detectors are…
Thin tungsten plates, - Many detectors (chambers) used, pressurised xenon ‘ more gas molecules/ unit path length ,
Ions collected by electric field between plates - signal produced and amplified - processed by comp as raw data
Ct Ionisation chamber advantages…
Cheaper than ssd
Easier to calibrate
Stable
. Ct Ionisation chamber disadvantages..
Kept under pressure in Al casing
This reduces system efficiency by filtering beam
Bulky
Ct scintillation detector…
X-ray photons absorbed by caesium iodide or ceramic Crystal
Beam energy converted into light pulse
Detected by photo diode & converted into signal
Low afterglow reduces contrast resolution!
