1
Q
which imaging techniques are ionising and non-ionising based?
A
ionisation radiation based: involves rays formed from EMR 1. CT or CTA 2. SPECT 3. PET non-ionisation radiation base: 1. ultrasound 2. MRI
2
Q
for which body part is MRI ideal?
A
ideal for looking at soft tissues (as opposed to bones) e.g. the brain, discs between the vertebrae
3
Q
what are the advantages of MRI?
A
- non-ionising radiation - uses magnetic field and radiofrequency pulses
- allows repeated studies
- non-invasive
- excellent soft tissue detail
- no problems concerning radiation dose
4
Q
what are the limitations of MRI?
A
- expensive hardware
- time consuming - scans (cf CT secs v 10 min)
- some participants cannot be scanned (with metal implants?)
5
Q
what are the characterstics of MRI?
A
- 2D slice through patient
- high quality images
- anatomical and functional information
- take really long time - this is because its a multi stage process unlike x rays
- different tissues have different characters in the rate they lose energy
6
Q
Outline the key steps in MRI working
A
It is pretty complicated but it involves:
- BIG MAGNET: the presence of magnetic field means nuclei can have different energy states
- BIG RADIOFREQUENCY GENERATOR: output of energy at the right frequency means nuclei can absorb energy and move from one state to another
- ANTENNAE - to detect the ‘excited’ sample signals
- SMALLER MAGNETS - whose strength varies with location (gradient magnets): allows to collect spatial information and make it homogenous across patient
- COMPUTER: to convert the signals to acceptable images
7
Q
Answer the following questions for magnets used in MRI:
- type of magnets are used in MRI?
- unit of measurement for magnetic field strength?
- strength of a. clinical, b. research, c. strongest whole body scanner available
- earth’s magnetic field
A
- strong magnets
- Tesla (T)
- a. clinical - 1.5T, b. research - 3.0T and c. strongest whole body - 7T
- ~ 3.2x10-5 T (2.5-6.5)
8
Q
what kind of magnets can be used in MRI?
A
- permanent ferromagnets
2. super conducting magnets
9
Q
why super conducting magnet are usually used in MRI?
A
because they do not need constant power source, once current is initially supplied meaning the magnet is always on
10
Q
why do we want a magnet in MRI?
A
according to physics law, a moving electric charge produces a magnetic field and the magnetic field then generates an electric field
11
Q
why hydrogen is used in MRI?
A
- has its own spin unlike carbon and oxygen
- abundant expression in body (density varies with tissue type) because:
hydrogen + carbon = fat
hydrogen + oxygen = water
12
Q
what are the two orientations of hydrogen in MRI? what does the system prefer in state of energy?
A
- parallel orientation - also called the ‘right’ way is low energy state
- antiparallel orientation - also called the ‘wrong’ way is the high energy state
system prefers low energy to high energy
13
Q
what is the preference of system in terms of energy?
a. high to low
b. low to high
A
low energy to high energy
14
Q
what is impact on the orientation of magnetic field when there is
- no external magnetic field
- with external magnetic field
A
- no external magnetic field - random orientation of proton H molecules
- with external magnetic field - parallel orientation with opposite spins and direction
15
Q
why aren’t all the proton the right way in parallel alignment?
A
because
- heat provides energy to move from low energy (parallel) to high energy state (anti-parallel) and energy gap is very small ???
- therefore, strong magnetic field is required/essential to overcome the effect of heat and produce a significant net alignment of protons
16
Q
why do we need a big radio frequency generator?
A
the energy between the two states (low and high) is very small and corresponds to a radio wave energy level
17
Q
what happens to the system in MRI once energy (RF) is supplied?
A
the absorbed RF energy is transmitted at the resonance frequency
18
Q
why do we need an antennae in MRI?
A
to collect the signal transmitted from the RF generator
19
Q
what is the antennae referred to in MRI?
A
they are called coils
20
Q
what is spatial localisation?
A
to effectively have signals arising from different location with different strength (quality)
21
Q
what does an MR image show?
A
it is a density map showing the spatial distribution of signals
22
Q
Define
- echo time (TE)
2. repetition time (TR)
A
ET - it is the time between RF energy supplied and signal collected
TR - it is the time between two consecutive RF energy supply
23
Q
what does the MRI consists of in terms energy in-signal collected?
A
it consists of a repeating sequence of energy in-signal collected
24
Q
how does the repetition sequence change with increasing resolution?
A
the higher the resolution, the more repetition of (energy in-signal collected)
25
how many repetitions are required if you are scanning something 10cm long and require a resolution of 1mm?
```
repetitions = required resolution x length of object
repetitions = 1 x 100 = 100
```
26
what are the three types of images obtained from MRI?
1. T1 weighted (T1W)
2. T2 weighted (T2W)
3. PD weighted (PDW)
27
what is relaxation in reference to MRI?
the rate of energy lost is called relaxation
28
why is it recommended to wait between the energy in-signal collection procedure?
- when energy is supplied to the system and the signal produced is measured immediately then there is little contrast between tissues
- however, if you wait those tissues that lose signal very quickly will appear dark in the image as compared to tissues that lose signal more slowly
29
what are the three types of relaxation and how are they processed?
1. T1-TR
2. T2-TE
3. T2*-TE
- they occur simultaneously but differ in how the energy is transferred
30
how to obtain the image contrast in MRI?
image contrast is mainly due to changes in relaxation behaviour between different tissue types
31
name one contrast agent used to modify the rate signal that is lost from the system?
gadolinium
32
which objects are not allowed into MRI scan room for safety purposes?
1. pacemakers
2. metallic foreign bodies in eye or elsewhere in body
3. loose metal objects
4. electrical or mechanical devices
33
what are the safety concerns associated with the use of liquid helium?
- the safety concerns associated are
- deliberate or accidental release of liquid helium
1. removes the magnetic field
2. risk of frostbite
3. suffocation - due to replacement of oxygen
4. or trapping of personnel - due to increased air pressure
34
what are the future directions for MRI?
1. to make it bigger because bigger the magnet -> bigger the difference -> more resolution -> better image quality
2. combining modalities - PET-MRI scanning modalities for higher resolution and tissue contrast and metabolic indicators
35
image weightings is based on which parameter?
relaxation time
36
what are the two parameters you can manipulate to control the relaxation process represented in an image?
1. repetition time (TR)
2. echo time (TE)
relaxation is the rate of energy lost
37
why is fat and water (body fluids) are used as reference tissues for image weighting?
because fat and water have the fastest and slowest relaxation respectively
38
what is the signal range in T1 (longitudnal) and T2 (transverse) for body fluids and fat?
```
WATER
T1 - low signal
T2 - high signal
FAT
T1 - high signal
T2 - low signal
intermediate - high signal on T2 weighted fast spin echo images
T1 = fluid is bright + fat is dark
T2 = fluid is dark and fat is bright
```
39
What is the impact of TE (echo time) and TR (repetition time) on T1 and T2 respectively?
short TR = maximise T1 contrast
long TR = minimise T1 contrast
short TE = minimise T2 contrast
long TE = maximise T2 contrast
40
addition of which RF can result in suppression of substances?
1. fat suppression
2. fluid suppression
1. fat suppression
- STIR
- Spectral fat suppression
2. fluid suppression
- FLAIR
41
what are the two main features of MRA?
1. time of flight
| 2. phase contrast
42
what are the uses of perfusion scanning?
1. image demonstrates the perfusion rate
| 2. may also be used in brain to evaluate salvageable cerebral tissue from stroke
43
what are the features of MR spectroscopy?
1. quantitive technique
2. uses chemical shift (based on nuclear spin)
3. spectral peaks indicate proportion of chemical present in voxel
44
# Define the following
1. Lamore (resonant) frequency
2. precession
3. DWI
4. fMRI
5. echo time
1. Lamore (resonant) frequency - it is the frequency at which the radio frequency energy absorbed is transmitted
2. precession - when a magnetic field is applied to the spinning nucleus, it starts rotating about its axis, this is called precession
3. DWI - diffusion weighted imaging
4. fMRI - functional MRI, emerging modality which allows the psychological as well as physical and functional imaging
5. echo time - it is the time between the RF energy supplied and collected
45
why superconducting magnets are used instead of permanent magnets in MRI?
- permanent magnets can be used for the magnetic field in MRI however, to generate the required magnetic field it would have to be massive
- therefore, superconducting magnets are used
- superconducting magnets are made of superconducting coils (wires) that create magnetic field when electric current is passed through it
- once current is initially supplied, the magnet is always on thus creating the permanent magnetic field
46
why liquid helium is used in MRI?
1. liquid helium is used to cool down the heat generated from constant electric current in the coils.
2. it also serves to reduce the resistance to zero as current is transmitted through coils.
3. Thus, it maintains the superconductivity of the magnet coil.
47
why radio frequency coil is needed in MRI?
the coils are tuned to a particular frequency to produce a magnetic field perpendicular to the magnetic field generated by superconducting magnets
48
how do hydrogen atom act as magnets to generate magnetic signals in MRI?
1. hydrogen atoms consists of a proton and has its own intrinsic spin
2. therefore, they can carry electric field and create magnetic field
3. this means hydrogen atom can act as tiny magnets and will be affected by any magnetic field applied to them
4. similarly, a magnetic field induces movements of electrons and creates an electric current
5. thus, magnetic signals from the tissue can be measured as an induced electric current in RF coils of MR machine
49
precession results in transverse or longitudinal magnetisation?
results in net longitudinal magnetisation (Mz) in the Z-axis direction.
50
why is transverse magnetisation needed and how is it generated?
- the magnetisation cannot be measured in the longitudinal axis (Mz)
- therefore, the magnetisations needs to flipped 90 degrees to be able to measure the magnetisation and create MR signal
- in order to flip, RF pulse is applied and this flips it into the transverse plane (Mxy)
- in order to do this, the RF pulse needs to oscillate at the same frequency as precessing nuclei called the resonant frequency
- the resonant frequency ensures the most efficient transfer of energy to nuclei
51
what is relaxation in MRI?
- As long as the RF pulse is applied the nuclei, they continue to precess in the transverse plane phase creating a net transverse magnetisation (large Mxy).
- As soon as the RF is switched off, the transverse magnetisation begins to disappear and the nuclei relax back to their resting state of net longitudinal magnetisation (B0, large Mz)
- the shift of nuclei from transverse to longitudinal plane
52
what is T1 and T2 signal in MRI?
1. T1 is the rate of which longitudinal relaxation is governed
- longitudinal relaxation is the state when the nuclei precessing in traverse plane after being hit by their surrounding molecules, give up energy to them and return back to their longitudinal axis
- in simpler words, T1 relaxation is the recovery of longitudinal relaxation
2. T2 is time constant when magnetisation is lost in transverse decay
- transverse decay is the rate at which transverse magnetisation is lost, depends on the local magnetic field
MI
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