Magnetic Resonance Imaging Lecture 2 Flashcards
MRI uses ionizing radiation
TRUE/FALSE
False
MRI has become a vital tool for diagnosing:
Brain tumors and other disease of CNS.
Also for spotting soft-tissue injuries in muscles and ligaments
What is the fundamental property of matter in MRI?
Magnetism
In MRI, magnetism is generated by moving charges which are usually
Electrons
Atoms can have electron orbitals that are
paired (even # of electrons cancel magnetic field) or unpaired (magnetic field present)
Most materials do not show noticeable magnetic properties, except for
permanent magnets
The magnetic field exists as dipoles. There are two poles:
North pole: where magnetic field lines originate
South pole: where magnetic field lines return
Like poles repel, and opposite poles attract
Number of magnetic lines per unit area. It also decreases approx. with the square of the distance from source
Magnetic field strength (B)
also called magnetic flux density
The SI unit for magnetic field is
Tesla (T)
1T = 10,000 G (gauss)
Earth’s magnetic field=
0.00005T=0.05mT=0.5G
The magnetic field is induced by a
moving charge in a wire
What does the magnetic field direction depend on when it comes to the moving charge in the wire?
sign and direction
What is the right hand rule when it comes to the magnetic field?
fingers point in the magnetic field direction and thumb in the direction of a moving positive charge (opposite to the direction of electron movement)
Coiled current carrying wire increases the magnetic field strength inside the coil, with rapid falloff of field strength outside the coil
What does the amplitude of the current in the coil determine?
The overall magnetic field strength
Types of magnets in MRI
Air core magnet and solid core magnets
This magnet include wire-wrapped cylinders. It is also 1m in diameter and 2-3m long.
The magnetic field (B0) is generated by current flowing through the wire
B0 is horizontal (along z-axis)
Air core magnets
In air core magnets this is the peripheral magnetic field outside the magnet core.
It increases for larger bore diameters and higher field strengths.
Extensive fringe field
At what measurement are air core magnets superconducting?
=>1T
This magnet is constructed from permanent magnets or electromagnets.
B0 runs between magnet poles in the vertical direction.
Fringe fields are confined
Solid core magnets
Solid core magnets are commonly used in what type of MRI?
low-strength MRI
What kind of magnets are used in clinical MRI?
superconductive magnets with field strength of 1.5-3T
This is a magnetic property of materials. It is the extent to which a material becomes magnetized when placed in a magnetic field.
Susceptibility
What are the three types of susceptibility?
Diamagnetic, paramagnetic, ferromagnetic
This is a slightly negative susceptibility.
Oppose applied magnetic field: paired electrons the electron orbitals
Diamagnetic
Slightly positive susceptibility.
Enhance applied magnetic field: unpaired electrons in the electron orbitals
Paramagnetic
Supermagnetic-increase the applied magnetic field significantly
Can exhibit self-magnetism
Ferromagnetic
In an atom, these two have magnetic properties affected by spin and charge distributions intrinsic to _______ and _______/
Protons and neutrons
This part of the atom has a positive charge. The magnetic dipole produced by nuclear spin
Protons
This part of the atom is uncharged, but with subnuclear particles. The magnetic dipole produced in opposite direction of proton, and approximately same magnitude
Neutron
Magnetic moment of a nucleus that arises from the spin of the protons and neutrons
Represented by a
vector with magnitude and direction
With the nuclear magnetic moment, does the net magnetic moment exist with an odd or positive number of neutrons and/or protons?
Positive number
In the nuclear magnetic moment, the magnetic moment of many nuclei make up the MRI _____
signal
The protons in our bodies are typically oriented randomly and the magnetic moments cancel each other out ->
Net field=0
Putting a body in an MRI;
When placed in a strong field (the B0 field), the protons line up with (parallel) or against (antiparallel) the main field. In which direction are there slightly more protons?
Net magnetic moment in the direction of B0
Parallel direction
Putting a body in an MRI;
As the magnetic field strength increases, the energy separation of the low- and high- energy levels increases, and the number of protons in the low-energy state
Increases
Precession;
Bo=0
Spinning charge on proton generates magnetic dipole.
The spinning proton with magnetic moment u
Precession;
Bo is applied which imposes a torque on the magnetic moment u
The spin axis is tilted and the magnetic moment precesses about the axis of Bo
How can you figure out precession frequency?
Larmor equation
Larmor equation;
Gyromagnetic ratio is _______ for hydrogen protons
42.58 MHz/T
Net magnetic moment (Mo);
Component of that is Mo parallel to Bo
This is maximum at equilibrium
Longitudinal magnetization (Mz)
Net Magnetic Moment (Mo);
Component of that is Mo that is in the X-Y plane
This is zero at equilibrium
Transverse magnetization (Mxy)
What are the two parts of the net magnetic moment (Mo)?
Longitudinal Magnetization (Mz) Transverse magnetization (Mxy)
What are some frames of reference?
Laboratory frame and rotating frame
Magnetic field B0 is parallel to the z-axis
Frames of reference
This is one type of frame of reference.
Observer is detached from the system
M0 rotating about the z-axis at the Larmor frequency
Laboratory frame
This is one type of frame of reference
Observer is part of the system and rotating about the z-axis at the Larmor frequency
M0 rotating about the z-axis appears stationary
Rotating frame
Make small adjustments to make B0 uniform throughout the volume
Shim coils
Placed within the main bore of the magnet to transmit energy to the patient and receive returning signals
Radiofrequency (RF) coils
Placed within the main bore to produce linear variation of the magnetic field strength across the magnet volume
Gradient coils
Magnetic component of the RF excitation pulse (B1 field) matches the precessional frequency of the protons
Displaces the equilibrium magnetization if the frequency matches the Larmor frequency
Resonance
Result of exciting the tissue sample with RF radiation.
Causes displacement of the equilibrium magnetization by causing antiparallel and parallel spins to flip.
Excitation
RF signal is emitted by the rotating magnetic moment.
__ _______ quantifies the rate of the decay of the magnetization within the xy plane (Mxy).
At equilibrium Mxy=0
After a 90-degree RF pulse, the Mz converted to Mxy
Initially the spins are in phase with each other.
With time, the spins begin to dephase gradually and Mxy=0
T2 relaxation
Also called the transverse or spin-spin relaxation
In T2 relaxation, Mxy follows what kind of decay?
exponential
In T2 relaxation; this is the transverse magnetization at time t
Mxy(t)
At T2 relaxation; this is the initial transverse magnetization at t=0
Mo
___ is the time it take for Mxy to decay to 1/e or 37% of its initial magnitude
At t=T2, Mxy=0.37Mo
T2
When returning to equilibrium, this is ___ ________.
Mz begins to recover immediately after the RF pulse, simultaneous with transverse decay
___ ________ is the process by which Mz grows to its initial maximum value
T1 relaxation
also called longitudinal and spin-lattice relaxation
This is the process by which Mz grows to its initial maximum value
T1 relaxation
In T1 relaxation, Mz regrows
exponentially
In T1 relaxation:
Mz(t)= longitudinal magnetization at time t
M0 = initial transverse magnetization at t = 0
T1 is the time needed for the recovery of 63% of Mz
At t = T1, = Mz= 0.63 M0
T1 relaxation
___ is the time needed for the recovery of 63% of Mz
T1
What are the basic acquisition parameters?
Time of Repetition (TR)
Time of Echo (TE)
Time of Inversion (TI)
Because of this, there are differences in T1 and T2 relaxations times
WE WANT TO ACCENTUATE THESE DIFFERENCES IN T1 & T2 by changing how frequently we put in RF pulses (Trep) and how frequently we listen for a return signal (Techo)
This together is a
Pulse sequence
Acquiring an MRI relies on the repetition of a sequence of events in order to sample the volume of interest and to build a complete dataset over time
___ is the time between the RF excitation pulses.
Time of repetition (TR)
During the TR interval, T2 decay and T1 recovery occur in ______ tissue
the same
Initial 90° RF pulse produces maximum Mxy
Signal decays exponentially
At time TE/2 following the initial 90° pulse, a 180° RF inversion pulse is applied.
induces rephasing of Mxy
At time ___, peak amplitude of the echo is reached due to rephasing of the spins.
A gradient polarity reversal can also induce echo formation (gradient echo).
Time of echo (TE)
At time TE, peak amplitude of the echo is reached due to
rephasing of the spins
Time between an initial inversion/excitation (180) RF pulse that produces maximum tissue saturation, and a 90 RF pulse.
During the __, Mz recovery occurs.
The 90 degree RF pulse converts the recovered Mz into Mxy, which is then measured with the formation of an echo at time TE as shown earlier
Time of inversion (TI)
Basic Pulse Sequences
Spin-echo (SE) Inversion recovery (IR) Gradient Echo (GE)
90 RF pulse flips the spins onto the transverse plane, and spins begin to dephase with time
At TE/2, a 180 RF pulse inverts the spins and re-establishes phase coherence to produce an echo at TE
The signal from the echo is captured and recorded to produce the image
Spin Echo (SE)
Contrast proportional to the difference in signal intensity between adjacent pixels in an image
Signal intensity variations in different tissues depend on TR and TE settings
(look at powerpoint for equation)
SE contrast weighting
Signal intensity differences due to differences of T1 of the tissues Short TR (400-600ms) and short TE (10-30ms)
SE sequence T1-weighting
This maximizes the differences in M_z recovery during the return to equilibrium
400-600ms
Short TR
This minimizes the T2 decay during signal acquisition
10-30ms
Short TE
In T1-weighted MRI scan
Tissues with short T1=
higher signal intensity
In T1-weighted MRI scan
Tissues with long T1=
Lower signal intensity
In T1-weighted MRI scan
CSF is
dark
In T1-weighted MRI scan
Fat is
bright
In T1-weighted MRI scan
white matter is _____ than gray matter
brighter
Signal intensity differences due to differences in proton density Long TR (1,500-3,000ms) and shirt TE (10-30ms)
SE sequence: proton density (PD)-weighting
In SE sequence: PD-weighting
this minimizes T1 relaxation differences of tissues
1,500-3,000ms
Long TR
In SE sequence: PD-weighting
this preserves PD differences without allowing significant T2 decay
10-20ms
Short TE
In a PD-weighted MRI scan
CSF is
gray
In a PD-weighted MRI scan
Fat is
bright
In a PD-weighted MRI scan
gray matter is _____ than white matter
brighter
Signal intensity differences due to differences of T2 of the tissues Long TR (1,500-3,000 ms) and long TE (60-150 ms)
SE sequence T2-weighting
In SE sequence T2-weighting
this minimizes T1 relaxation differences of tissues
1,500-3,000ms
Long TR
In SE sequence T2-weighting
this emphasizes T2 decay differences of tissues
60-150ms
Short TE
In T2-weighted MRI scan
Tissues with large T2 =
higher signal intensity
In T2-weighted MRI scan
Tissues with low T2 =
lower signal intensity
In T2-weighted MRI scan
CSF is
bright
In T2-weighted MRI scan
Fat is
dark
In T2-weighted MRI scan
Gray matter is _____ than white matter
brighter
Null signal from certain tissues
Initial 180° inverting pulse flips Mz into the –z direction and starts to recover with a time dependent on the tissues T1
At TI, a 90° RF pulse is applied to remove the signal from a specific tissue
Since there is no Mz, there will be no Mxy to create a signal
In MRI we only collect data from Mxy
Fat and fluid have different T1, so Mz reaches 0 at different times
By selecting when to apply the 90° RF pulse, we can select which tissue to null
Inversion Recovery (IR)
Look at powerpoint for this equation
IR signal intensity
S∝ρ_H [1−〖2e〗^((−TI)⁄T1) ] [1−e^(−(TR −TI)/T1) ] [e^(−TE/T2) ]
Used to null fat signals
Uses shorter TI
Bounce point is when MZ = 0
90o RF pulse applied during the bounce point
If MZ = 0 at TI, maximum possible signal is 0
Selection of appropriate TI can suppress tissue signals depending on their T1 relaxation times
Short TAU inversion recovery (STIR)
Used to null CSF signals
Uses longer TI
TI selected at the bounce point of CSF to allow null the signal from CSF and provide a better visualization of the surrounding anatomy
Fluid Attenuated Inversion Recovery (FLAIR)
No 180 degree refocusing RF pulse, but use gradients to rephase the spins
Small flip angle < 90o (α) and short TR
(Shorter scan times)
Transverse magnetization decays at a rate denoted by T2*
(T2* results from inhomogeneities in the B0 field due to the use of gradients)
Gradient Echo (GE)
This gradient is (produced by special coils within the magnet housing) applied to change the local magnetic fields, which changes the resonance frequencies across the patient. This leads to an accelerated dephasing of the spins.
Dephasing gradient
This gradient is applied with the same magnitude but opposite polarity to the dephasing gradient to reverse the phase shift. This leads to the formation of a gradient echo.
Rephasing Gradient
This is is important for creating MR images and for determining the location of the sample volumes
Spatial localization
How is spatial localization achieved?
Achieved by superimposing linear magnetic field variations on the B0 field to generate corresponding position-depended variations in the precessional frequency of the protons
Simultaneous application of the RF pulse excited only the protons in resonance withing the frequency bandwidth of the RF pulse
Three sets of gradient coils along the x, y, and z axis produce magnetic field variation based on the magnitude of the current applied in each coil
What is each one used as?
Use one to select slice, one as the phase encode, and one as the frequency encode
Net gradient= √(G_x^(2 )+G_y^(2 )+G_z^(2 ) )
This gradient is applied along the z-axis and RF pulse turned on simultaneously
Determines the slab of tissue to be imaged
Localizes a proton to a slice by changing the frequency of the protons
Applied gradient and the bandwidth of the RF pulse determines the slice thickness
Slice select gradient
The slice select gradient determines what?
The slab of tissue to be imaged
Linearly increasing frequency encoding gradient applied along the x-axis to specify position within a slice
Localizes the protons in a slice along the x-axis by changing the frequency
Effective field at any point (x) along the x-axis is: B(x)=Bo+xGf
From Larmor equation, we can see the resonant frequency f(x) varies linearly with position x along the x-axis:
Each column has unique frequency
Frequency encoding gradient (also known as the readout gradient)
Localizes the protons along the y-axis by changing the phase of the protons
Causes dephasing of the protons according to their position along the gradient
Once gradient is turned off, the protons go back to their original frequency, but have a different phase
Applied repeatedly, at different gradients, to each slice
Each row has a unique phase.
Phase encoding gradient
This artifact is due to patient motion (breathing, cardiac, swallowing, involuntary muscle movement,…)
Seen strongly in the phase encode direction
Motion artifact
This artifact is due to local distortions in magnetic field
Tissue interfaces (air-fat, air-water interfaces)
Metal (fillings, braces, implants,…)
Signal void and geometric distortion
Metal and susceptibility artifact
This artifact Appears as a series of lines in the MR image parallel to abrupt and intense changes in the object at this location
Ex: CSF-spinal cord and skull-brain interface
Fine lines visible in an image are due to undersampling of high spatial frequencies because of incomplete digitization of the spin echo signal
Truncation artifacts
This type of artifact Appears as an edge enhancement between regions containing predominantly water molecules and those containing predominantly fat molecules
Fat and water have different resonance frequencies
Occurs in frequency encode direction
One side there is signal enhancement where the fat and water signals are superimposed, while on the other there is signal nulling because fat has shifted away from where it is supposed to be
Chemical shift artifact
Chemical shift artifact occur in
frequency encode direction
motion artifact is seen strongly in the
Phase encode direction
This artifact occurs when the field of view is too small for size of imaged object
Mismapping of anatomy outside the FOV to inside the FOV
Aliasing (wrap around) artifact
For MRI safety, there are some things you must do before you enter because the magnet is ALWAYS ON:
Patient Safety Questionnaire
Posted Signs
Only MRI approved equipment can go into the room (crash carts, needles, catheters, stethoscopes, wheelchairs, oxygen tanks, etc.
On slide 70 in powerpoint, there are the approved limits for human imaging
Magnet- device interference 4.0T
Gradients: nerve stimulation-<6T/s
deafness 105dBA over 1 hr or 200 pascals peak
RF coils- tissue heating <0.4W/kg (body) and <3.2W/kg (head)
Do we use shielding in an MRI?
Yes
For shielding in an MRI, we are worried about outside RF interacting with our RF pulses and interfering with our pulse sequences
What lines the whole room?
This basically creates a faraday cage of the scanner room
Copper paneling
The magnet is cooled by liquid helium
this happens when the liquid cryogen boils off rapidly
this results in helium escaping very rapidly from the cryogen bath
coils cease to be superconducting
accompanied by a loud bang
might activate the STOP magnet, but it might not
it can be a partial ______ and the magnet is still active…proceed with caution
Quenching
Encloses body part to be imaged
Increases signal by being close to body part
Does not detect noise from the rest of the body
RF coils
What are the two types of RF coils?
Birdcage coils and Surface coils
In MRI, we measure the magnetization that is _______ to the main B0 field
Perpendicular
What is the time of repetition
Time between successive 90 degree RF pulse
What is the correct order of a spin echo pulse sequence?
2 Mxy is max and Mo is 0
3 Spins begin to dephase and Mxy decays according to T2
1 90 degree RF pulse flips spins onto xy plane
4 At TE/2, a 180 degree RF pulse is applied to invert the spins and reestablish phase coherence
5 Spin echo is produced at TE
What is the direction of the Bo field in a cylindrical air core scanner?
Parallel to the long axis of the cylinder
Spatial localization in MRI depends on?
Varying magnetic field across the patient through the use of gradients
Slice selection is performed by
Turning on the slice select gradiatient during Rf excitation
FLAIR is used to null signal from ____ and STIR is used to null the signal from ____
CSF, fat
Which is true about T1-weighted image
Shirt TR and short TE
Which is true about T2 weighted image?
Long TR and long TE
Which is true about PD weighted image
long TR and short TE
Contrast in MRI relies on
Proton density, T1 and T2 relaxation time
