CT Flashcards
What is CT used for
head trauma
screening for lung cancer
blood flow
what is the sequence events of CT
tube and detector rotate around patient
radiation is attentuaed as it passes through the patient, remnant beam is measured by detectors
beam converted into an electrical signal which is amplfied then adjusted
data sent to computer
data processed
data displayed, manipulated and recorded
CT image reconstruction
computed system calculates using mathematical formulae the CT number of each voxel
basic assumption of CT reconstruction is that the imaged object is completly stationary during the course of the scan
3rd generation CT
fan of beam that covers the head
rotate 360
slip ring technology - continous spiral round the patient as table moves during the scan
benefits of slip ring technology
makes it faster - goverend by time taken to move table
less radiation
multi slice CT
256 slice scanners
able to scan a whole body in up-to 90 seconds
2 diverging beams
describe the gantary
tube, heat dispenser
houses xray source, detector array, collinator supply and possible high voltage generator
xray tube
large diameter anode discs with layed alloy and thick graphite backing that aids heat sink
how do they manage heat
oil or air cooled housing
graphite backing
pulsed beam
life expectancy of CT scanner
5-6 years
what is acquistion
the process through which single continous set of spiral scan is acquired without an intervening pause
what is interpolation
modification of scan data before reconstruction in order to synthesis a set of data that yeilds a clincally acceptable represenation of a cross section of anatomy
what is collimation
radiation beam is double collimated
- tube exit - pre patient
controls patient dose
varied from 1.25mm to 8cm
what is the effect of having a smaller slice
more dose but less detail
what is detector array
solid state type, made from ultra fast ceramic material
efficiency is important as it determines tube loading and controls patient dose
what is beam attenuation
patient stationary as tube rotates in a circular orbit around the patient in a plane perpendicular to the long axis of the patient
intensity of beam striking the pateint is known as incident intenisty
what is beam attenuation dependant on
different tissue densities
what is beam attenaution proportional to
the tube output
how is attenauation measured
by detectors sitauted opposite the xray tube
what should a ct detector ideally have
high detection efficiency, absorbing all the emergent xray beam energy
high conversion efficiency, enabling all the detected energy to contibute to the image formation
a wide dynamic range so all intenties are contverted
how does the detector work
ceramic stinillators which respond to xray exposure by emiting pulses of light in proportion to the absorbed energy
what is the transmitted intensity
the intensity of the xray beam after passing through the patient
what is the transmitted intensity inversely proportional to
the attenuation of the xray beam
what is the attenuation coefficient
the decrese in intensity following pentration into matter
expressed numerically
ie the % of xray remaining after passing through the object
how is resultant data worked out
by subtracting the attenuate signal from the incident-beam intensity signal
what does pre processing involve
raw data is produced by processing scan data
dose measurements with computed knowing table and detector position
allows it to know the voxels the beams passes through
what does convolution involve
algorithms applied to raw data to provide a sharp or smooth image to enhance the appearance of tissue of interest
created using the different image dentities and enhancing them to create a grey scale
what is back projection
a complicated process in which the convulted scan data is projected back into the image matrix
done 100s of times to create image
hounsfield scale
each pixel assigned a numerical value, which is the avergae of all the attenuation values within that voxel
dispalyed on scale
what is the houndfield unit of bone
+1000 HU
white
what is th houndsfield unit of air
-1000 HU
black
CT technology and MIPS/MPRS
2d images that are taken and the raw data is reconstructed or resued in another way to give us additainal images and info
Multiplanar reformatting MPR
the original cross sectional images are used to produce image data in another body plane
maximum intensity projection MIPs
to seperate superimposed structures/vessels
select voxels along row/column in a volume of interest with a specific range of CT numbers
basis for CTA
shaded surafce display SSD
provides a 3D image of the surface of a structure
very useful in osteopedics as the image can be rotated and viewed from various angles
what is a scout view
planning scan
what are the normal settings for kVp
80
120
140
normal settings for mA
200
350
400
500
why do a CT head
head/brain truama stroke cerebro-vascular accident infection/inflammatory disease tumours
Glasgow coma scale
point system for responsivness for stimuli that inclused testing of the eye openig, motor response and verbal response
have to score less than 13 to have a CT
what does a score of 15 on the glasgow coma scale indicate
normal concious level
what does a score of 13-14 on the glasgow coma scale indicate
may indicate mild TBI
what does a score of 9-12 on the glasgow coma scale indicate
moderate injury and associated with a mortaility rate of 2-3%
what does a score of <8 on the glasgow coma scale indicate
servere head injury
assoicated with a mortality rate of up to 36%
what does a extradural head injury look like
convex shape, coming away from outside of the head
what does a subdural head injury look like
blood between the dura
harder to spot
look for a midline shift of the ventricles
intra cranical haemorrhage
bleeding inside the brain
bright white indicates fresh blood
appearances of blood in CT
fresh blood - bright white
after 2-3 weeks - ct will show haemotome which is a similar density to the brain
over time will appear as cerbral spinal fluid
ct guided biopsy technique
patient positioned to faciliate needle access to the lesions and ideally also ensure as much comfort
prelimarily axial CT scanning
most appropraite slice is selected to plan route
