MRI Flashcards
The typical range for T1 is… (in ms)
200-2000ms
T1 (increases/decreases) with increasing magnetic field strength
T1 increases with increasing magnetic field strength
True/False:
For any tissue T1 is always longer than T2
True. T2 < T1.
The T2 of any tissue is the time taken (in msec) for the MR signal to decay to (?%) of its full value
The T2 of any tissue is the time taken (in msec) for the MR signal to decay to 37% of its full value
True/False:
The T1 relaxation time influences the repetition rate of a spin echo pulse sequence
False.
Spin Echo: Protons lose phase coherence after an RF pulse due to…?
magnetic field inhomogeneities
Spin Echo: how are the proton spins rephased?
Using the 180 degree pulse
Can Spin Echo generate both T1 and T2 weighted images?
Yes
How do we create different weighted images with Spin Echo?
By varying the TR and TE values
In Fast spin echo sequences, we get faster imaging at the expense of what…?
Contrast
What flip angle does Gradient echo use?
Usually 10-35 degrees
What 2 differences does gradient echo have compared to spin echo?
- Small flip angles
- Gradient field used instead of 180 degree RF pulse
Can Gradient echo sequences generate T2 images?
No, only T2*, or T1
What type of MR pulse sequence is often used in MR Angiography?
Gradient echo sequences
Can Gradient echo sequences compensate for any magnetic field inhomogeneities?
Yes, only the extrinsic field inhomogeneities (not intrinsic)
What is the resonant frequency of Hydrogen nuclei known as?
Larmor frequency
What is the Larmor equation?
Larmor frequency = Gyromagnetic ratio x Magnetic field strength (Bo)
In slice selection: How do we define the slice to be imaged?
Using a magnetic field gradient
How does Gadolinium contrast affect the T1 and T2 relaxation times of tissues?
Gadolinium contrast decreases both T1 and T2 relaxation times
Do hydrogen nuclei in fat and water precess at the same frequency? Why/why not?
No
There are differences in the magnetic field of their orbital electrons.
Spatial resolution of MR images is improved by (increasing/ decreasing) slice thickness
Decreasing slice thickness
This reduces voxel size for finer spatial sampling
Spatial resolution of MR images is improved by: (increasing/ decreasing) matrix size
Increasing matrix size
A larger matrix size (eg 128x128 to 256x256) -> more pixels covering the same field -> smaller pixel size -> spatial resolution will improve
Spatial resolution of MR images is improved by: (increasing/ decreasing) field of view
Decreasing field of view
A smaller field view means the same number of pixels are used to sample a smaller region of the patient. Which means smaller pixels -> better spatial resolution
Is Spatial resolution of MR images
improved by changing main magnetic field strength?
No effect.
Increasing Bo will increase the SNR, but will not have a direct effect on spatial resolution
Spatial resolution of MR Images is improved by changing which 4 factors?
Decreasing Slice thickness
Increasing matrix size
Decreasing Field of view
Increasing phase encoding steps
MRI Signal: Noise ratio is improved by changing which 2 factors?
Increasing Bo magnetic field strength
Decreasing Echo Time
The STIR sequence minimises signal from what tissue?
Fat. It’s a fat suppressed sequence.
How is Chemical shift artefact affected by a higher field strength?
More artefact produced
Higher field heighten the artefact, commonly in fat vs water.
Why does the chemical shirt artefact occur between fat and water protons?
The artefact occurs because of differences in the intrinsic magnetic properties of protons in water and fat molecules
MRI Signal: Noise ratio (increases/decreases) when increasing slice thickness?
Increases, as thicker slices contain more proton nuclei giving more signal
MRI Signal: Noise ratio (increases/decreases) when increasing echo time (TE)?
Decreases, because as TE increases there will be greater decay of the signal prior to readout
MRI Signal: Noise ratio (increases/decreases) when increasing receiver bandwidth?
Decreases, because this will open the receiver coil to more bandwidths of noise. Signal has a much narrower spectrum than noise.
MRI Signal: Noise ratio (increases/decreases) when increasing field of view?
Increases, because pixel size will also increase (the same number of pixels are sampling a larger area). Therefore, voxels get bigger and generate larger signals.
True/False: MRI produces permanent biological effects > 3T
False.
there is no evidence of permanent biological effects < 8 T
Are all metal objects attracted to the scanner magnetic field?
No, only ferromagnetic objects (ie those containing iron, nickel or cobalt) will be strongly attracted towards the scanner
Biological effects: Which MR Technique can cause peripheral nerve stimulation?
Rapid gradient switching can cause nerve stimulation
Biological effects: What is specific absorption rate?
a measure of tissue heating due to RF pulses
What 5 materials are bright on T1 images?
Blood, Fat, Melanin and proteinaceous fluid. Also Contrast agents.
What materials are bright on T2 sequences?
Fat and fluid
MRI Safety: what strength of gauss line must be contained within the inner controlled area?
3mT Gauss line
True/False: Specific Absorption Rate depends on patient weight
True
MRI: a smaller field of view gives increased or decreased?:
1. Resolution
2. Signal strength
3. Gradient strengths needed?
- Higher resolution
- Lower signal strength
- Higher gradient strengths needed
What is aliasing?
Wrap around artefact from when body parts being images are bigger than the field of view
What is Nyquist limit?
Signal being sampled must be over twice the wavelength of the highest frequency
MRI Rectangular field of view: does this have a higher or lower signal to noise ratio?
Lower SNR
What are the 2 problems with rectangular field of view (in MRI)
- Reduced SNR
- Aliasing can occur
Gadolinium contrast (for MRI) is diamagnetic, parramagnetic or ferromagnetic?
Paramagnetic
Mri sequence acquisition times equation?
Acquisition time = No. of excitations x No. of phase encoding steps x TR
The centre of K space gives what info?
Low frequencies, gross contrast
The outer K space gives what info?
High spatial frequencies, fine detail
Fast spin echo description?
90 degree pulse followed by a train of 180 degree pulses
Fast spin echo advantage?
Saves time, depending on echo train length
Fast spin echo disadvantage?
Contrast is worse
Why is an inversion recovery sequence useful?
The signal is flipped and if the 90 degree pulse applied at the right time, you can nullify the signal from a chosen tissue (fat - STIR, fluid - FLAIR)
Inversion recovery: what is the formula for Inversion time?
TI= 0.693 x T1
Gradient echo disadvantages? (3)
Lower Signal noise ratio
Can’t do T2
Susceptibility artefacts
Gradient echo advantages? (3)
Fast acquisition
Sensitive to blood flow (angiography)
Low power/less heating side effecfs
Magnetic susceptibility artefact: explain
Boundary between two tissues of different magnetic properties leads to signal voids
Specific absorption rate in MR is proportional to flip angle: true or false?
True
2 relative contraindications to Gadolinium contrast for MR?
Low eGFR
Pregnancy
Breastfeeding is SAFE
With fast spin echo, the longer the echo train the more T1 weighted or T2 weighted?
More T2 weighted
Movement artefact is much worse in T1 or T2 weighted sequences?
T2 weighted because of the longer time scale
How do we minimise chemical shift artefact?
Increase receiver bandwidth
Motion artefact occurs in which type of encoding?
Phase encoding only
Fat: T1 and T2 times?
260, 80ms
CSF: T1 and T2 times?
2400, 160ms
Kidney: T1 and T2 times?
760, 30ms
Diffusion weighted MRI:
- T1 or T2?
- restricted water appears bright/dark?
- restricted = long/short T2? ADC? b-900 image?
T2
Bright
Long T2, low ADC, bright b-900
A short Tr is less than…
500ms
A long Tr is more than…
1500ms
A short Te is…
Less than 30ms
A long TE is more than
90ms
MRI image contrast can be improved with (3)
Using contrast agents
Increasing SNR
Fat suppression
Hydrogen has a gyro magnetic ratio of…
42MHz per Tesla
Proton density images had long/short TE and TR?
Long TR
Short TE
Spectroscopy: choline is pathological if high/low
High
Spectroscopy: NAA is pathological if high/low
Low
Spectroscopy: peaks correspond to
Shifts of the resonant frequencies
Rephrasing during a Gradient echo is achieved by…
Frequency encoding Gradient
When these two things go down, SNR goes up. What 2 thugs.
Bandwidth
TE
