Magnetic Resonance Imaging (MRI) All Notes Flashcards
Name the 4 components to an MRI scanner
the magnet, the radiofrequency system, the gradient system, the host computer
What has the most impact on quality of the magnet?
magnetic field homogeneity
name the three types of magnets
superconducting magnets, permanent magnets and resistive magnets.
Name the two properties of superconducting magnets that make them the most popular choice.
high field strength (better SNR) and good homogeneity.
how is the magnet made superconducting?
by cooling it to -274 degrees Celsius
what is meant by superconducting?
when an electric current is applied it will keep flowing with virtually no loss of energy
what is the superconducting magnet made of?
Solenoid (coil of wire)
what is used to cool the magnet?
helium
what is the helium surrounded by to maintain the low temp
liquid nitrogen
What are some advantages for permanent MRI magnets?
can be used to make open magnets, magnetic field can be positioned horizontally or vertically, do not use cryogens or electricity
what are the disadvantages of permanent magnets?
magnetic field is unstable if ferrite not kept at the same temperature, cannot be quenched, very sensitive to external disruption by cars, elevators, etc.
what are resistive MRI magnets?
electrical coils wrapped around an iron core
what are the advantages of resistive MRI magnets
they can be designed as an open magnet and can be switched off in the evening
What is the radiofrequency system?
the RF system is needed to transmit RF into the patient at an appropriate Larmour frequency and to detect and gather the very tiny RF echo that the patient emits afterwards/
what does the host computer do
the computer will calculate the RF pulses for the sequence and these pulses are amplified along the RF transmitting chain
Describe (very basic) RF transmission
Pt in strong magnetic field, pump radiowaves into them, turn off radiowavse, pt sends radiowaves back out, frequency we send in is the same as what we get back
What is the relationship between field strength and RF requirement
as field strength increases, there is an increased need for RF power.
what is the gradient system
the gradients are used to alter the main magnetic field of the scanner in order to get spatial information about the patient being scanned
what causes the noise in an MRI scanner
the switching of the gradient coils
how is the digitised data from the patient echoes processes?
using the fast fourier transforms (FFFT)
What happens when an external magnetic field B0 is applied?
the proton spins that are normally randomly orientated with no net magnetisation vector now line up with or against the magnetic field
what has has higher energy; the spins with or against the magnetic field?
The spins antiparallel to the magnetic field have higher energy
What happens when a proton drops to a lower energy level?
it must give off energy in the form of an RF signal at the resonant frequency
What does the strength of the MR signal depend on?
Low temperature (cannot lower temp in vivo)
large magnetic dipole moment
Large N (1H is abundant)
B0 (this is the only thing we can control!!)
What is T1 =
Spin-lattice relaxation time
what is T2 =
Spin-spin relaxation time
what is T2* =
Actual transverse relaxation time, taking into account magnetic field inhomogeneities
What is the free induction decay (FID) signal?
the signal produced after a 90 degree pulse
what time for M0 to return to equillibrium?
4-5 x T1
T1 is similar to T2 for ____. T1»_space; T2 for _____.
T1 is similar to T2 for liquids. T1»_space; T2 for solids.
what is the magic angle?
NB in MSK shoulder and knee imaging as when a tendon makes this angle with the field, we get decreased signal and this mimics a tear in the tendon.
what is T1 relationship with B0
T1 is dependent on B0
what is T2 relationship with B0
T2 is independent of B0
what is the main contrast agent used in MRI?
gadolinium
What are the three sequences that most sequences used in modern scanners are now based on?
- Spin-Echo sequence
- Inversion Recovery sequence
- Gradient Echo sequence
What is the tip angle?
the term used to describe the angle that the RF pulse tilts the NMV. EG: A 90 degree RF pulse would tilt the NMV through 90 degrees from the Z-axis to the XY plane
What is the echo generated from
the echo is generated from the rephasing spins
how many minimum echoes to build an image?
256 echoes
What is Tr
time between successive repetitions of a pulse sequence
what is Te
echo time is the time between the first RF pulse and the middle of the echo
what does matrix size determine
no of pixels in an image
what does FOV mean
the field of view is the area you wish the image to cover
What is slice selection in MR based on?
that the resonant frequency depends on magnetic field strength, at the isocentre, strength is B0, its a little over B0 at the head, a little less than B0 at the feet
what is the slice thickness determined by in MR?
The bandwidth of the RF pulse (range of frequencies) and the steepness of the slice selection gradient
what is the relationship between gradient strength and slice thickness?
the stronger the gradient, the narrower the slice
what effect does a bigger bandwidth have on slice thickness?
bigger bandwidth, thicker slice, we want slice thin as possible so bandwidth small as possible!!
what is the data acquisition time?
the time it takes for a sequence to run fully (depends on matrix size, Tr, and NEX)
Briefly describe spin echo sequence (know diagram also)
B0 magnetic field lines up proton spins with or against the magnetic field.
Sum of the magnetic dipole moments of protons gives the Net Magnetisation Vector (NMV) pointing in the Z direction.
We apply a 90 degree RF pulse to rotate the NMV to the XY plane.
RF pulse switched off, spins begin to relax.
Spins are in phase, then lose spin coherence because of T2* effects and also due to T1 and T2 relaxation
180 degree pulse applied this is the refocusing pulse. flips the spins through 180 degrees, some spins were fast some were slow, after refocussing pulse, slow spins now ahead of the fast spins. eventually, fast spins catch up to the slow spins and the echo is produced!
what is the fate of the echo once produced?
the echo (very weak RF signal) is detected by the MR coil, amplified and sampled by ADC and stored in the computer, image then formed by FFT.
T1 Weighting spin echo: Tr and Te
T1 weighting has a short Tr and short Te
T2 weighting spin echo: Tr and Te
Long Tr and long Te
What are some advantages to spin echo
not as sensitive to inhomogeneity of magnet, T2* effects eliminated by the refocussing pulse, hence, larger echo.
different types of contrast achieved by the same sequence (T1, T2, etc.)
Disadvantage of spin echo
takes a long time, esp T2 weighted images, can lead to motion blurring and reduced throughput in the scanner.
Give a reason for using IR instead of spin echo
if strong T1 weighted imaging is required
or if we need to suppress fat or CSF signal.
what is the main difference between the spin echo and IR sequences
IR has a 180 degree pulse before the 90 degree pulse
In inversion recovery sequence, what is meant by Ti?
the time between the initial 180 and 90 degree pulse is known as the inversion time (Ti)
In IR, what determines how strongly T1 weighted the sequence will be
Ti and Tr
What is the main purpose of the IR sequence’s 180 degree initial pulse?
180 RF pulse will tip the spins and NMV into the opposite direction to the main magnetic field, B0
Where does T1 relaxation occur in IR?
During the Ti
How can we reduce T2 effects in IR?
keeping the Te as short as possible.
How to suppress fat signal?
using a STIR sequence
what are the main advantages of IR sequences
can produce heavily T1 weighted images
can selectively suppress fat signal (STIR) or CSF (FLAIR).
disadvantages of IR
Long scan times
not possible to get as many slices per Tr as with other sequences
slice profile poor due to use of three RF pulses
Why were gradient echo sequences designed?
to reduce scan times
what are gradient echo sequences used for?
orthopaedic and angiography
what do gradient echo sequences require?
a very good magnetic field homogeneity
How is scan time reduced in gradient echo?
elimination of the 180 RF pulse and use of an RF pulse <90 degrees.
How does gradient echo refocus spins?
gradient echo refocuses by gradient switching
What is FLASH?
Fast Low Angle Shot is the most basic form of gradient echo, uses low tip angle between 10 and 70 degrees.
effect of GE sequences on SNR
GE has reduced SNR
name three parameters influencing image contrast in GE
flip angle, Tr, Te
Why was fast spin echo developed?
to speed up conventional spin echo
how is time saving achieved in fast spin echo
by acquiring more than one echo after the 90 degree pulse
below what T have no significant biological effects been seen
below 2.5 T no biological effects of significance have been recorded
What is the Echo Train Length
no of 180 degree pulses determines the number of echoes
what minor effects have been seen
10% change in nerve conduction at 24T
nausea, vertigo at 4T
Magnetophosphenes - flashes of light induced in rods >4T
what effect in the aorta and above what magnetic strength?
elevated T waves on ECG have been seen on ECG in magnets over 0.1T
what is the projectile effect
forces on ferromagnetic objects inside and outside of the patient
size of current depends on:
electrical resistance, cross sectional area and rate of change of the magnetic field
What is the specific absorption rate?
Specific Absorption Rate (SAR) is the RF power absorbed per kg of tissue
what can be caused by escaped helium during accidental quenching of the magnet?
asphyxiation
