omi term 2 adaptive optics and mri

Question 1

how do u remove optical aberations from an image

Answer 1

detect optical aberration

quantify aberration (what type, how big)

localise aberrations- how is aberration distributed across image

be able to rapidly adjust optical system to compensate for the optical aberrations

Question 2

what is adaptive optics

Answer 2

it refers to an optical system which adapts to the optical effects which are introduced by medium between objects and its image in order to improve image quality

Question 3

resolution limits

Answer 3

resolution limit of optical system is limited only by the diffraction limit and limit is calculated using wavelength and aperature size of system

Question 4

what is resolution of fully dilated human eye,why this not achieved

Answer 4

0.3 arcmin,

due to eye constantly moving and optical elements of the eye are not aligned precisely along optic axis thereofre meaning res limit of 1 arcmin

when using standard imaging systems (OCT) we can only ontain practical res of approx 5 microns due to these aberrations and to get best image of back of eye we need to remove aberrations

Question 5

what do adaptic optic systems do

Answer 5

compensate for eyes aberrations using rapidly deformable mirror (at least 100Hz)

Question 6

how do adaptive optic system remove aberrations

Answer 6

1. laser spot foccused on retina
2. reflected light creates aberrated wavefront when leaving the eye
3. light analysed for aberrations
4. this infoo used to send commands to actuators
5. actuators change surface of a deformable mirror
6. provides necessary compensations to correct wavefront which removes aberrations from image

Question 7

what r 3 imaging planes

Answer 7

coronal
saggital
axial

Question 8

MRI

Answer 8

magentic resonance imaging and it uses bodys natural magnetic properties in order to image any part of the body

magnetic resonance is measured in tesla and typical mri scanners are 1.5T-3T

Question 9

how does mri work

Answer 9

hydrogen atom consist of single proton in atomic nucleues which have positive charge

human body is mainly made of water whihc has 2 h per molecule

being able to use h to image body is very useful as its available in abundance

proton of h spins on axis with a north and south pole

usually axes f protons are randomly aligned

when body is placed in magnetic field the h atoms line up along axis of field

they start to precess at diff rates depending on type of tissue

if radiofreq. pulse is applied to atoms from rf coil they absorb energy and start to precess in synchrony and this is excitation

the rate at which energy is rleased can be detected by rf coil and release is called relaxation or reception

Question 10

simplied mri routine

Answer 10

1- normal conditions are that protons are randomly aligned

2- in magnetic field protons line up and precess(rate depends on tissue)

3- RF pulse- prtons absorb energy and precess at same time (excitation)

4- reception meaning protons release energy at rate that dpeneds on tissue- detected by rf coil

Question 11

types of images from MRI

Answer 11

proton density- conc of hydrogen protons tissues rich in protons produce strong signals and have bright appearance

T1- weighted- based on relaxed (resting state) of deflection of magnetic vector (grey looks grey and white looks white)

T2- weighted- based on relaxation rate of returning to precessing at usual rate (grey looks white and white looks grey)

can reconstruct desnity of tissue and produce 3d image from info

Question 12

function of mri

Answer 12

we can use mri to measure functional brain activity and this used to measure magnetic differences between oxygenated blood and deoxygenated blood

brain requires energy from glucose via sent oxygenated blood

Question 13

mri safety

Answer 13

radiation used in radio part of em spectrum and its very long wavelength so low energy

radio is not known to damage organic tissue

any metal in ur body or in clothing can be extremely dangerous as strength of magnetic field can make it move or heat up

safety protocols are used when having a scan to avoid the potential damge from metal

Question 14

clinical applications of mri scan

Answer 14

used in opthalamology and optom

used to determine shape of eye

can visualise visual pathway

can investigate extent of cortical damage in people with scotomas

Question 15

MRI visual pathway

Answer 15

haemorrhage or tumour in brain can damage nerve fibres in visual pathway

this damage can be functionally measured by carrying out visual field test