MRI Flashcards
What is Spin Echo
- 90 degree pulse applied
- 180 degree pulse applied at TE/2
- Echo taken at TE
180 degree pulse is applied to remove magnetic field inhomogeneities
In Spin Echo - T1 weighted, T2 weighted and Proton density image can all be produced (by altering TR adn TE)
What is an ECHO train?
- Type of spin echo
- 90 degree followed by successive 180 degree each with an increasing TE
- There is a limit to the echo train length (as signal is decaying via T2*)
- PD Image initially produced on first TE followed by T2 weighted images which get stronger
*
What is a turbo spin echo sequence?
- Spin echo sequence
- Echo train is first created (90 followed by succesive 180 degree pulses)
After each echo a phase encoding gradient is applied (reduces time massively)
- Better spatial resolution
Which sequence is quicker?
Gradient echo is quicker than spin echo
Why does Spin Echo take a long time?
- TR is the most time consuming part of the pulse sequences
- A 180 degree pulse also takes time to deliver
- If we shorten TR
- there wont be sufficient time between 90 degree pulses
- there will be very little longitudinal magnetisation and as a result very little transverse magnetisation
The solution is to:
- Find a way of refocusing the dephasing protons without a slow 180 degree pulse
- Find a way to preserve longitudinal magnetisation before flipping (therefore maintining signal)
What happens in Gradient echo?
Initially the same steps as a spin echo apply
- RF pulse (of any flip angle compared to 90 degrees in spin)
- Slice select gradient
- Phase encoding gradient
- Frequency encoding gradient
A magnetic field gradient is applied (instead of a 180 degree pulse)
- After the frequency encoding gradient
- Negative magetic field gradient applied causing protons to dephase and quicker loss of Mxy signal
- Positive magetic field gradient applied causing partial rephasing creating a signal
Gradient Echo - flip angle used?
Can use any flip angle
Using a smaller flip angle means quicker recovery and another RF pulse can then be applied generating 2 echoes
What type of images do Gradient echoes produce?
T2*
Cannot produce a true T2 weighted image as no 180 degree pulse to discount field inhomogeneities
In Gradient echo how do we overcome small Mxy signal caused by short TR?
Use a small flip angle (10-35 degrees)
Why are gradient echo sequences faster?
3 resons
- Shorter TR
- Small flip angles
- No time consming 180 degree pulse
What does a large flip angle in gradient echo produce?
T1 weighted images
Advantages and disadvantages of permanent magnets?
Max strength 0.2 - 0.3
Usually made from rare earth metals
- neodymium-iron-boron
- samarium-cobalt
Fringe field is fully contained within the permanent magnets
Advantages:
- always on
- dont require an energy input
Disadvantages
- thermal instability
- limited magnetic field strength
- very heavy
What are resistive magnets?
Otherwise known as electromagnets where electric current is passed through a wire
Max strength is 0.6T field strength
Made out of copper or aluminium wrapped around an iron core
Disadvantages
- Are only magnetic when a current is flowing
- generates alot of heat so impracticale at high field strengths
What are superconducting magnets?
Most common magnet used in MRI scanners
Electric current is passed through a superconducting material Has zero electrical resistance
Coils of a supermagnet are made from Niobium titanium inside a copper matrix
Once an electrical current is sent through the material, it flows permanently, creating a magnetic field
Considerations
- need to be cooled to 4 degrees kelvin (-269 degrees celsius) - liquid helium or nitrogen used to do this
- liquid helium or nitrogen otherwise known as cryogens
- Magentic field only turned off by quenching
- superconducting material warms up, electrical resistance increases and magnetic field disappears
ADVANTAGES
- High magnetic field strengths (30,000 times higher than earths field)
- Excellent magnetic field homogeneity
very expensive however
What coils are used in MRI?
3 types
- Gradient coils
- RF coils
- Shim coils
RF Coils
- Send and receive the RF pulse to excite the protons (cannot do both simultaneously however)
- Should be as close to the region of interest as possible
- Size of coils do not affect spatial resolution
- Produce magentic field at right angles to main field
- Body coil
- usually a permanent part of the scanner
- can transmit RF pulse and receive MR signal
- Head coil
- transmits and receives
- used for brain scanning
- Surface coils
- only receive
-
improve
- SNR
- Resolution
- Smaller FOV
- are not used in ‘Whole Body Imaging’
- result in LESS UNIFORMITY
Can cause heat damage !!
Gradient Coils
Gradient coils change field strength in a LINEAR fashion
Three sets of gradient coils
- Slice selecting gradient coil
- Frequency encoding gradient coil
- Phase encoding gradient coils
MRI scanner noise is due to switching on/off of GRADIENT COILS
Can induce currents in patients!!
Shim Coils
Used to make magnetic field as homogenous as possible
Has nothing to do with fringe fields
Gadolinium
- Shortens T1 and T2 (time taken for recovery of both)
- Gadolinium cheleated with DTPA as it is toxic on its own
- Does not pass blood brain barrier
Post contrast images obtained are T1 weighted (shortening of T1 causes higher signal)
TR can also be reduced when using contrast (resulting in faster scan times)
Can result in nephrogenic systemic fibrosis
- starts as fibrosis of the skin and connective tissue of extremeties
- no known cases in people with normal kidney function
- most cases assocaited with Gadiodiamide
Gad can be given even in severe renal impairment but with consideration
MR Angiography
What do spin echo sequences demonstrate?
- Flow void
- seen in fast flowing blood
- protons in blood get flipped by 90 degrees
- since this blood then flows onwards, it is replaced by blood that is not in transverse plane. A 90 degree pulse hits the new blood but no signal as BOTH 90 AND 180 DEGREE PULSES REQUIRED TO GIVE A SIGNAL IN SPIN ECHO
- Flow enhancement (seen in both spin echo and gradient echo)
- seen in veins (slow flowing blood)
- protons in SLICE get flipped to 90 degrees
- this slice slowly gains longitudinal magnetisation
- unexcited blood from outside slice moves in and posseses full longitudunal magnetisation
- after further RF pulse there is high transverse signal in blood vs tissue surrounding it
Spin echo is better at assessing vessel wall
Gradient Echo in MR Angio (Time of flight)
Gradient echo used
- Tissue band subjected to multiple TRs which saturate tissue giving no signal (small flip angle followed by gradient echo early on leaving very low longitudinal magnetisation)
- Unsaturated blood flowing inwards then generates strong signal
- Slow flowing blood near to the slice can become saturated leading to loss of signal
Pulsatile flow seen as ghosting artefacts
BLOOD AND CSF BRIGHT ON GRADIENT ECHO
Diffusion weighed imaging
- T2 weighted imaging
- Uses spin echo planar imaging
Two large gradient pulses are applied either side of the 180 degree pulse in a spin echo planar imaging sequence
- for moving protons (normal) - a gradient will cause a reduction in signal
- for stationery protons (in restricted diffusion) - a gradient will create a high signal (with low ADC signal)
HIGH SIGNAL FOR TISSUES THAT ARE DIFFUSION RESTRICTING
ADC
- Measures the degree of diffusion weighting
- high B value = heavily diffusion weighted
Removes the effect of T2
- Restricted diffusion = low ADC signal
- Normal Diffusion = high ADC
What is Aliasing?
Causes wrap around artefact (2 noses sign)
Due to too small FOV
Does not occur if the whole body part is in FOV
Most commonly in phase encoding gradient (but can occur in both)
Reduced by:
- increasing FOV
- using surface coil
Motion Artefact
Due to patient movement
Occurs in phase encoding direction
Chemical Shift Artefact
Larmour frequency for hydrogen within fat and water is slightly different (results in Chemical Shift artefact)
- Chemical shift artefact produces a black or white band at the fat tissue interface. (Signal void usually).
- Occurs in the FREQUENCY ENCODING direction and is a result of spatial misregistration of water and fat molecules
- Chemical shift is more pronounced at higher magnetic field strength. Chemical shift artefact can be combated using STIR sequences as this suppresses fat.
Reduced by:
- HIGHER BANDWIDTH FOR PIXEL AND WIDER RECEIVER BANDWIDTH
- using steeper phase encoding gradient and reducing the field of view
- Chemical shift is exploited in Dixon sequences where fat can be nulled
Truncation Artefact
Known as:
- Ringing artefact
- Gibbs artefact
Parallel stripes at high contrast interfaces
Seen in:
- CSF and spinal cord
- brain edges
Due to undersampling in phase encoding direction
- corrected by increasing number of phase encodings
- reducing FOV
MRI Safety - Gradient Coil FIelds
Electrical fields produced perpendicular to gradient fields. Cause Eddy currents
- Nerve stimulation
- Arrythmias
Pregnancy and MRI
When should it be avoided?
Should be avoided during first trimester of pregnancy
What noise do MRI scanners produce in Db?
Most dont exceed 120Db
Hearing protection required if >90Db
Radiofrequency Fields
What do they cause?
Burns due to microwave heating
Specific absorption ratio
- this is the RF energy deposited per unit mass of tissue
- unit is W/kg
- SAR is greater for larger body parts and for 180 degree pulses