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
Relaxation times T1 and T2
T1 is always longer than T2
T2 relaxation time is always longer than T2*
T1 times
TE
TR
Short TE (15ms)
Short TR (300-800ms)
T2 times
TE
TR
Long TE (90-140ms)
Long TR (1000-2000ms)
TE cannot be longer than TR
Do protons that are subject to a strong magnetic field precess in phase?
No. They align with magentic field but precess in different phases.
Phase syncronisation happens after 90 degee pulse
Can free induction decay occur at any flip angle?
Yes all apart from a 180 degree pulse.
Any angle that results in tranverse magnetisation can result in FID.
Does external magnetic field strength alter T1 time?
Yes…..it lengthens the T1 relaxation time
What signal does fat return on turbo spin echo sequences?
Hyperintense signal
Results from multiple refocusing pulses that suppress J coupling and lengthen T2 giving larger signal
Cortical bone T1 and T2 times?
Long T1 and very short T2
Appears BLACK
Substances with low T2 signal?
- Melanin
- Calcification
- Fibrous tissue
- High protein content
- Flow void
In what dimension is phase encoding applied?
Usually applied along smallest dimension
e.g. laterally in brain imaging or AP in abdominal imaging
This allows shorter phase encoding steps and cuts down on scan time
K Space
Centre = contrast and low spatial frequencies
Periphery = spatial resolution and high spatial frequencies
Can half of K space be acquired to produce an image?
Yes
Acquiring half of K space is sufficient to obtain an image.
- known as zero filling
- may result in a blurry image
- ‘mirroring’ can also take place where acquired half can be mirrored to missing half. Known as half or fractional NEX
Single Shot FSE (Fast/turbo spin echo)
Also known as Echo planar imaging
HASTE (Half fourier single short turbo spin echo)
Type of turbo spin echo where whole of K space is filled with one 90 degree pulse and echo train of 180 degree pulses.
90 degree followed by repeated 180 degree pulses and phase encoding gradients (reversal gradients applied in frequency endoing direction in between phase encoding gradient and next 180 degree pulse)
Produces highly T2 weighted images. (initial images are PD then as echo train lengthens, images at the end are more T2 weighted).
Used in MRCP
Echo planar imaging can produce:
- T1
- T2
- T2*
Only half of K space is filled in HASTE (much quicker than turbo spin echo)
What is Ernst Angle?
The excitation flip angle that provides maximal MR signal for T1
(NOT CONTRAST BETWEEN TISSUES)
What is Spoiling?
Is a type of Gradient echo sequence
Is an Incoherent sequence
Spoiling elimintates residual transvere magnetisation (T2 signal)
Can be achieved by random variations in the phases of excitation pulse
What do Coherent or Rewound gradient echo sequences do?
They preserve signal by preserving residual magnetisation
- use very short TR
- water is very bright on these sequences
-
used for
- myelography
- angiography
Are very sensitive to motion and flow
- this destroys the signal
What is opposed phase imaging?
(Chemical shift imaging)
Are paired gradient echo sequences:
- in phase
- out of phase to differntiate water and fat e.g dixon sequences
Main purpose is to identify microscopic fat content of tissues by showing signal drop out on out of phase imaging
- It is a gradient echo sequence
- Difference in Larmour freq between fat and water is 220Hz at 1.5T
Uses different TE but same TR
- TE should be at point where fat and water have a 180 degree phase difference
India Ink appearance in out of phase imaging
Using a higher magnetic field strength results in increased suppression of fat in out of phase imaging
FLAIR
Suppression of fluid. Fat will appear bright
Combines inversion with a T2 weighted spin echo sequence
Inversion Recovery
Not susceptible to magnetic field inhomogenetities as they work on T1 relaxation times
Lowers overall SNR in tissues
Are STIR sequences sensitive??
NO
Any tissue with a T1 close to fat will also be nulled e.g.
- haemorrhage
- protein rich fluid
- gad
What is SNR in MRI?
SNR is inversely proportional to the square root of bandwidth
SNR is proportional to the SQUARE ROOT of the number of sequence repetitions
Increasing bandwidth increases noise
Decreasing bandwidth increases SNR
- Spin echo returns higher signal than gradient echo
Using smaller coils improves SNR but NOT spatial resolution
What are superparamagnetic iron oxide particles used for?
Used in hepatic imaging to diagnose hepatic tumours
Perfusion Brain Imaging
Based on the T2* shortening upon first passage of an exogenous contrast agent
- Performed using T2* imaging
What are phased array coils?
Are smaller coils that can be used as surface coils
- Allow for larger FOV and increased SNR
Can patients with intracranial clips be images in MRI?
Yes if they are titanium.
Metallic heart valves are also safe
MRI Fields Controlled Areas
MR Controlled area is where stray fields are greater than 0.5mT
Inner MR controlled area 3mT
T1 and T2
FAT
WATER
Fat
T1: 250ms
T2: 80ms
CSF
T1: 2000ms
T2: 150ms
Water
T1: 3000
T2: 3000
What controls T1 weighting in spin and gradient echo?
TR
Gradient echo
What flip angle used in T1?
What flip angle used in PD weighting?
T1 - large flip angle
PD - small flip angle
What determines slice thickness?
Transmit bandwidth
- Increasing the transmit bandwidth increases the slice thickness
Increased slice thickness = reduced resolution
When is Slice selection gradient switched on?
During initial RF pulse and during 180 degree pulse (in spin echo)
How does phase encoding gradient affect spatial resolution?
Steeper phase encoding gradients results in increased spatial resolution
Does coil size affect spatial resolution?
NO
Dedicated surface coils increase SNR though
Summary of gradients
Effect of TE and TR on SNR
Increasing TE results in more LOSS of transverse magnetisation = lower SNR
Increasing TR results in more RECOVERY of longitudinal magnetisation = higher SNR
What determines scan time?
TR x number of phase encoding steps/NEX
FOV
The steeper the phase/freq encoding gradient = smaller FOV
Higher gradient strengths are needed for smaller FOV
Can gadolinium alone be excreted by the body?
Manganese
No
Gad has 7 unpaired electrons and is paramagnetic
High T1 signal
Iron Oxide contrast
Normal tissues are low signal
Excreted by liver
Liver lesions will appear as high signal on T2
Known as a T2 enhancing agent
Faraday cage
Protects from external RF fields (not fringe field)
Surround the scanner to protect from external electromagnetic (RF) fields
Usually copper shielding
What nuclei can be used in MRI?
Only nuclei with an odd number of PROTONS or NEUTRONS
Ferrous artefacts cause….
Signal drop out
Causes rapid dephasing of Mxy signal
Affects Gradient echo more than spin echo
Can T1 and T2 images be produced simultaneously?
NO
They would cancel eachother out
What causes Noise in MR?
Due to induction of the RF coils
What is magnetic susceptibilty?
Where tissues magnetise to different degrees and so have different precessional frequency
Most noticed between boundaries with a large difference
e.g metal implants
