CT- the basic principles section 1 Flashcards
in Ct attenuated beam is detected by
specialised detectors
what do these detectors measure
the attenuated beam and these measurements are sent to a computer system and converted to create a ‘cross sectional’ image which is displayed on the monitor
what is a CT protocol
the protocol consists of scan parameters e.g. exposure factors. differs for each body part
what are the 7 main components of a CT scanner
1) the CT gantry
2) the high voltage generator
3) the x-ray tube
4) collimators and filters
5) the detector array
6) the data acquisition system (DAS) and Ct computer
7) the Ct table
what is the CT gantry
the vertical frame or ‘doughnut’ that surrounds the patient
what is the bore or aperture called in the gantry housing
the opening where the patient is positioned, usually 70cm in diameter
what is the mylar window
it is a narrow, transparent window that surrounds the bore and is where the x-ray beam will pass
what 3 things does the gantry housing contain
1) detectors
2) collimators
3) data Acquisition system (DAS)
what controls can be found on the gantry housing
gantry angulation, table movement, laser lights and emergency stop
why is it important to warn patient about laser lights for positioning
as can cause damage to the eye
what does the emergency stop button do
removes power to the table drives, gantry drives and x-ray system
where and when should the system emergency off (SEO) be used
located on the wall and in the event such as a fire or smoke
what 3 things are mounted on the rotating gantry
1) x-ray tube
2) detectors
3) CT generator
what does the high rotational speeds have the potential to do
create a massive centrifugal force
what are slip-rings
electro-mechanical devices consisting of parallel, electrically conductive rings and brushes attached to the rotating gantry
what is axial scanning also referred to as
rotate/translate
what happens during axial scanning
the x-ray tube and detectors rotate 360 degrees around the patient. The CT table is stationary during this. After each 360 degree rotation the gantry halts and the xray table moves or ‘translates’ into its next scanning position. the gantry then rotates 360 degrees in the opposite direction
what type of images does axial acquisition produce
discrete ‘packages’ of image data
what is the disadvantage of packages of data in axial scanning
slice thickness, position and orientation of each image are locked. the resultant images can only be manipulated in a very limited way
what happens in helical/spiral scanning
the table moves at a constant speed through the gantry as the x-ray beam rotates continuously around the patient
what is helical scanning also referred to as
rotate-rotate scanning
What are the three advantages of helical scanning
1) faster scan times and minimal interscan delay
2) data is acquired as a volume
3) increased temporal resolution
a major benefit of helical scanning is
that images in alternative planes can be produced, known as multiplanar reformation (MPR)
what type of generators are used by helical CT scanners
high-frequency, solid-state generators. These are small,compact and more efficient and provide a stable,high power output to the x-ray tube
what is the rotating anode x-ray tube mounted onto
the rotating gantry
what is the disadvantage of lengthy exposure times for CT
X-ray tube is subjected to far greater thermal load
what is heat capacity
the x-ray tube’s ability to withstand the heat production
what is the heat dissipation rate
the rate at which the x-ray tube is able to rid itself of the heat produced
what is the reason behind replacing a glass envelope by a metal envelope
this eliminates the risk of electrical arcing caused by tungsten deposits, due to vaporisation, on the glass envelope
what do ceramic insulators provide
superior high voltage insulation and isolate the metal envelope from the anode and cathode voltage
what is the effect of using large, rotating anode disks (typically 12-13cms or more in diameter)
this increases the heat storage capacity and have an improved heat dissipation rate allowing higher tube currents
what do brazed graphite anode discs consist of
tungsten-rhenium focal track brazed onto graphite based bodies
what is the purpose/benefit of graphite
increases the heat storage capacity because of its high heat capacity
if scans are requested that exceed the current capacity, what happens?
the system will not operate in order to protect the tube
If the scanner is left idle, what must you do and why
a tube warm up to prevent the cold x-ray tube having to deal with the heat generated. this maintains image quality and extends the x-ray tube life
what do collimators and filters do
modify the x-ray beam, filters the x-ray photons and guard the detectors from scattered radiation
what is the purpose of the additional collimators in front of the detectors
reduce signal generation from scattered radiation, thus improving contrast resolution and reducing dose
what is contrast resolution
the ability to differentiate between small differences in tissue density
what do the filters attached to the x-ray tube housing do
reduce radiation dose and improve image resolution by removing low energy photons from the x-ray spectrum to produce a more uniform beam
what do flat filters (often copper sheets) do
help to reduce the artefact known as beam hardening artefact
why are bow tie filters used
as they are shaped to reduce beam intensity at the periphery of the beam, to correspond with the thinner parts of the patients anatomy
what are bow tie filters usually made of
Teflon
where are the CT detectors located
on the rotating gantry opposite the x-ray tube
what happens when the attenuated beam enters the detector chamber
its energy is amplified and converted to an electrical signal which is then processed by the CT computer
what are the optimal detector characteristics (6 points)
1) high detector efficiency
2) high scatter suppression
3) high stability
4) low afterglow
5) low cross talk between detector elements
6) low cost and easy to service
what are the two types of detectors used in CT
1) ionisation chambers
2) scintillation detectors (also known as solid state detectors SSD)
what are ionisation chambers made out of
thin tungsten plates that sit within a hollow chamber that is filled with pressurised Xenon gas
what happens in an ionisation chamber
as the x-ray beam enters the chamber, the gas becomes ionised and the ions are collected by the electric field that exists between the plates, producing a signal that is amplified and processed by the computer as raw data
3 advantages of ionisation chambers
1) less expensive than SSD
2) easier to calibrate
3) stable
2 disadvantages of ionisation chambers
1) have to be kept under pressure in an aluminium casing but this filters the beam reducing the efficiency of the system
2) bulky
what is the scintillating material used in the form of crystals
caesium iodide and ceramic materials
what happens to the scintillation/SSD
these convert the energy absorbed from the x-ray beam to a light pulse. photo diodes detect this and convert it into a corresponding electrical signal which will be processed as raw data
what is desirable to have for the SSD and why
as little afterglow as possible as this will reduce contrast resolution
what is cross talk
this occurs when a photon strikes a detector and is partially absorbed. the same photon then enters the detector adjacent to it and is detected again. this produces two weak signals coming from 2 detectors and reduces image resolution
how is cross talk minimised
by using a crystal that is highly efficient at absorbing x-ray photons
advantage of SSD
absorb 100% of photons
disadvantage of SSD
more sensitive to temperature and moisture fluctuations
what are multi slice CT (MSCT) scanners
have multiple, parallel rows of detectors- known as detector array
what does multiple rows of detectors mean (6 points)
1) rapid scan speed
2) better coverage in single breath hold
3) reduced patient moving artefact
4) the ability to scan at peak contrast enhancement due to quicker scan speed
5) less requirement to sedate paed/agitated patients
6) good temporal resolution
where is the DAS located
close to the detectors
what does the DAS do
it samples each of the detector elements, amplifies the signal and converts it from an analogue signal to a digital signal. This sampled data is processed by the CT computer and is now known as image data. this image data is displayed on a screen monitor where it can be manipulated
where does the CT table sit and what does it allow
within a cradle and allows for vertical movement of the table for patient positioning and is to be motored in and out of the gantry bore
what is referencing and what does it allow
where the table position is set to zero when it has been centred over an anatomical landmark. allows for consistency between examinations
