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Medical Imaging > CT > Flashcards

CT Flashcards

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USMLE® Step 1 flashcards
1
Q

What is CT used for

A

head trauma
screening for lung cancer
blood flow

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2
Q

what is the sequence events of CT

A

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

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3
Q

CT image reconstruction

A

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

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4
Q

3rd generation CT

A

fan of beam that covers the head
rotate 360
slip ring technology - continous spiral round the patient as table moves during the scan

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5
Q

benefits of slip ring technology

A

makes it faster - goverend by time taken to move table

less radiation

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6
Q

multi slice CT

A

256 slice scanners
able to scan a whole body in up-to 90 seconds
2 diverging beams

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7
Q

describe the gantary

A

tube, heat dispenser

houses xray source, detector array, collinator supply and possible high voltage generator

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8
Q

xray tube

A

large diameter anode discs with layed alloy and thick graphite backing that aids heat sink

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9
Q

how do they manage heat

A

oil or air cooled housing
graphite backing
pulsed beam

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10
Q

life expectancy of CT scanner

A

5-6 years

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11
Q

what is acquistion

A

the process through which single continous set of spiral scan is acquired without an intervening pause

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12
Q

what is interpolation

A

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

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13
Q

what is collimation

A

radiation beam is double collimated
- tube exit - pre patient
controls patient dose
varied from 1.25mm to 8cm

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14
Q

what is the effect of having a smaller slice

A

more dose but less detail

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15
Q

what is detector array

A

solid state type, made from ultra fast ceramic material

efficiency is important as it determines tube loading and controls patient dose

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16
Q

what is beam attenuation

A

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

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17
Q

what is beam attenuation dependant on

A

different tissue densities

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18
Q

what is beam attenaution proportional to

A

the tube output

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19
Q

how is attenauation measured

A

by detectors sitauted opposite the xray tube

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20
Q

what should a ct detector ideally have

A

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

21
Q

how does the detector work

A

ceramic stinillators which respond to xray exposure by emiting pulses of light in proportion to the absorbed energy

22
Q

what is the transmitted intensity

A

the intensity of the xray beam after passing through the patient

23
Q

what is the transmitted intensity inversely proportional to

A

the attenuation of the xray beam

24
Q

what is the attenuation coefficient

A

the decrese in intensity following pentration into matter
expressed numerically
ie the % of xray remaining after passing through the object

25
Q

how is resultant data worked out

A

by subtracting the attenuate signal from the incident-beam intensity signal

26
Q

what does pre processing involve

A

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

27
Q

what does convolution involve

A

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

28
Q

what is back projection

A

a complicated process in which the convulted scan data is projected back into the image matrix
done 100s of times to create image

29
Q

hounsfield scale

A

each pixel assigned a numerical value, which is the avergae of all the attenuation values within that voxel
dispalyed on scale

30
Q

what is the houndfield unit of bone

A

+1000 HU

white

31
Q

what is th houndsfield unit of air

A

-1000 HU

black

32
Q

CT technology and MIPS/MPRS

A

2d images that are taken and the raw data is reconstructed or resued in another way to give us additainal images and info

33
Q

Multiplanar reformatting MPR

A

the original cross sectional images are used to produce image data in another body plane

34
Q

maximum intensity projection MIPs

A

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

35
Q

shaded surafce display SSD

A

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

36
Q

what is a scout view

A

planning scan

37
Q

what are the normal settings for kVp

A

80
120
140

38
Q

normal settings for mA

A

200
350
400
500

39
Q

why do a CT head

A 
head/brain truama
stroke 
cerebro-vascular accident 
infection/inflammatory disease
tumours
40
Q

Glasgow coma scale

A

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

41
Q

what does a score of 15 on the glasgow coma scale indicate

A

normal concious level

42
Q

what does a score of 13-14 on the glasgow coma scale indicate

A

may indicate mild TBI

43
Q

what does a score of 9-12 on the glasgow coma scale indicate

A

moderate injury and associated with a mortaility rate of 2-3%

44
Q

what does a score of <8 on the glasgow coma scale indicate

A

servere head injury

assoicated with a mortality rate of up to 36%

45
Q

what does a extradural head injury look like

A

convex shape, coming away from outside of the head

46
Q

what does a subdural head injury look like

A

blood between the dura
harder to spot
look for a midline shift of the ventricles

47
Q

intra cranical haemorrhage

A

bleeding inside the brain

bright white indicates fresh blood

48
Q

appearances of blood in CT

A

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

49
Q

ct guided biopsy technique

A

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

Medical Imaging (12 decks)

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