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49 Cards in this Set

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who was CT developed by




what year

1970




Godfrey Hounsfield

what kind of beam is direted onto patient

well collimated xray beam

what is attenuated radiation measured by

solid state image receptor

where does this info go to?

a computer to reconstruct the cross sectional image on the monitor

how does the xray source and source detetor move synchronously?

they are connected

what how is CT operated

1. xray moved across patient to obtain a single projection


2. assembly rotates then moves across the patient again obtaining a second projection


3. repition


4. images superimposed to reconstruct the volume of the area imaged

how can CT be formatted

by computer to show


1. axial


2. coronal


3. sagittal slices of the area imaged

early design has improved (2)

1. imaging times (decreased it)


2. imaging detail

CT has not decreased what?

patient dose

The Hounsfield scale (grey scale)




air =


fat =


water =


blood =


soft tissue =


clotted blood =


bone calcification =


dense bone =


metal =

air = -1000


fat = -50


water = 0


blood = 12


soft tissue = 35


clotted blood = 40 -60


bone calcification = 800


dense bone = 1000


metal = 3000

contrast and brightness




brightness is known as




contrast is known as

window level




window width

brightness/window level will:




contrast/window width will:

picks midrange HU




picks the range of HU btw white and black

the lowest cut off will visualise anything below it as:

black

the highest gut off will visualise anything above it as

white

calculate the lowest cutoff




calculate the highest cutoff

WL - (WW/2)




WL + (WW/2)

a soft tissue window is around




a bone window is around

WW: 350 HU


WL: 40HU




WW: 1900 HU


WL: 300HU

what are CT advantages (4)

soft tissue attenuation increased by 10x


- can visualise brain liver, disc




able to view axial images of spine




reformatting to view coronal and sagittal slices and 3D representations




Improved bone diagnosis

what are the disadvantages of CT (60

1. radiation dose!


2. high cost


3. artifact from metal


4. long exposure time


5. small seze of gantry


6. claustrophobia

Effective doses in CT vs radiographic exam




Head


Chest


Ab


Pelvis

2 vs 0.07


8 vs 0.02


10-20 vs 1


10-20 vs 0.7

do other organs receive uneccessary dose in CT even though they arent the target of imaging?

yes


breast = 20-50mGy




Eye and thyroid in brain or throax imaging




gonads in pelvic imaging



CT is the large dose despite being on of the --- frequencies

smallest

Ct is one of the most important examinations




CT frequency is increasing rapidly in Aus




GPs have a hard time referring MRI but not Ct

true




true




true

when and who developed MRI

fleix bloch and Ed Purcell




1946

does MRI use ionising radiation

no

what is the basic method of obtaining an image

magnetic field + RF --> mathematically reconstructed image

what is the function of the magnetic field

align magnetic moments of atoms

what us the function of the Radiofrequency field

alter alignment of the magnetisation which causes nuclei to produce rotating magnetic field specific to each tissue type

what does an MRI image represent

differences among tissues in the number of nuceli and the rate at which nuclei recover from stimulation by radiowaves in presence of magnetic field

radiography involves interaction of ---- with ----




MRI involves interation of ---- (and static magnetic fields) with ----

xrays with electrons




radiwaves with nuclei alone

not all nuclei respond to magnetic fields. what is required of the nuclei to respond?

must have odd number of protons

what are nuclei suitable for MRI

K


H


Fl


Na


P

what is atom used for MRI

H

what is precession

spinning of protons around the magnetic field direction when they are aligned to magnetic field




the wobble is due to gravity and occurs whenever a spinning object is acted apon by outside force

when will rate of precession increase

with increasing magnetic field strenght

how else will precession increase

after static magnetic field is applied radiowaves cause precession of nucleus at greater angle

the longer the radiowave is applied, the ---- the angle of precession

greater

what is resonance




what is H resonance frequency

the aborption or emission of energy only at certain frequencies




Larmor frequency (42.3 megahertz per tesla 42.3MHz/T)

H proton in 1-T field will strongly absorb energy only from ---- RF

42.3MHz

what is the result of a lamor frequency on H (2)

1. rotates protons out of longitudinal alignment and into tranverse alignment




2. cause nuclei to move in phase with each other (phase coherence)

why can an antenna/receiver coild pick up MRI signal

nuclei are tiny magents and emit EM waves as it rotates. these are picked up when they are in transverse alignment




and sent to computer

relaxation occurs when the ---- pulse is over and the protons --- again. At this point the signal ---




what gives information about normal tissue and pathological process in tissue

RF


precess


diminishes






the rate of relaxation

relaxation is divided into 2 categories

T1 relaxation




T2 reaxation

what is T1 relaxation

time it takes from a proton to precess back into alignment with external magneitc field (back into longitudinal alignment)

T1 relaxation shows high intesity with ----



epidural fat will show ---



you can visualise

fat



white



large extruded disc herniation

T2 relaxation of tissue

is time for protons to lose their phase.




when spin begin to precess out of phase with each other when RF stops: loss of phase coherence


(result is referred to as transverse or spin-spin type relacation

T2 time is always --- than its T1 relaxation time

shorter

T2 has high intesity with


hydrated dic and CSF show



a herniated disc will show low signal because

water


white


there isnt much water in it

what are the advantages of MRI (3)

1. 100x better soft tissue attenuation than Ct


2. best for disc disease


3. no known harmful effects

what are the disadvantages of MRI (4)

1. high cost


2. unsuitable if ferrous metals present in patient


3. poor for bony pathology


4. small gantry