Physics

To produce an image, the MRI system must calculate how much signal is coming from each three-dimensional location in the patient. This location is called a _______.

Voxel

If current flows clockwise/same direction as B₀ through the gradient coil, would this increase or decrease the magnetic field strength relative to B₀?

Increase

If current flows counterclockwise/opposite direction to B₀ through the gradient coil, would this increase or decrease the magnetic field strength relative to B₀?

Decrease

Altering the amplitude (strength) of the magnetic field changes which properties?

• Precessional frequency (increases with increasing strength)

• Phase (gain phase with increasing strength)

When using a spin-echo sequence, when is the slice select gradient activated?

During the 90-degree excitation pulse and the 180-degree rephasing pulse

When using a gradient sequence, when is the slice select gradient activated?

During the excitation pulse only.

In a typical coronal image, which axis is the slice select gradient?

Y-axis

In a typical coronal image, which axis is the frequency encoding gradient?

Z-axis

In a typical coronal image, which axis is the phase encoding gradient?

X-axis

In an axial image of the head, which axis is the slice select gradient?

Z-axis

In an axial image of the head, which axis is the phase encoding gradient?

X-axis

In an axial image of the head, which axis is the frequency encoding gradient?

Y-axis

How does the system produce a thin slice?

• High gradient amplitude

• Narrow transmit bandwidth

How does the system produce a thick slice?

• Low gradient amplitude

• Wide transmit bandwidth

How is the amount of current passing through the gradient coil related to slice thickness?

Increased current passing through gradient coil —> increased gradient amplitude —> thinner slice

The transmit bandwidth is a small range of frequencies on either side of the ________ frequency.

Carrier

True or false: As the transmit bandwidth increases, the carrier frequency also increases.

False: The carrier frequency is the central frequency. Increasing the transmit bandwidth widens the range of frequencies around that center, but does not change the center frequency itself.

When is the frequency encoding gradient applied?

When the echo occurs.

How does the frequency encoding gradient affect TE?

The echo (TE) is usually centered in the middle of the frequency encoding gradient. If the sampling time increases, the readout period becomes longer, shifting the midpoint later and potentially increasing TE.

How would you increase FOV in the frequency direction?

Use a shallow (weak) frequency encoding gradient

How would you decrease FOV in the frequency direction?

Use a steep (strong) frequency encoding gradient

True or false: the frequency-encoding gradient affects the frequency of hydrogen atoms only, not the phase.

False: Altering the strength of the magnetic field affects both frequency and phase. In the frequency encoding axis, the system “pays attention to” the changes in frequency, whereas int he phase encoding axis, the system pays attention to changes in phase

When does phase encoding occur?

The phase-encoding gradient is usually switched on after the application of the RF excitation pulse

What happens when the phase encoding gradient is turned off?

The magnetic moments of hydrogen nuclei return to precessing at the Larmor frequency (because they all experience B₀ when the gradient is removed), but the phase shift remains.

True or false: The phase encoding gradient is the same amplitude each time it is switched on.

False: Unlike the frequency-encoding gradient, which is switched on to the same amplitude each time it is applied, the amplitude of the phase-encoding gradient (as well as the polarity) is changed throughout the pulse sequence. The number of times is determined by the phase matrix we select.

What happens when you choose a high phase matrix?

The phase-encoding gradient is applied many times during the pulse sequence.

What is the gyromagnetic ratio of Hydrogen?

42.58 MHz/T

The strength of the phase encoding gradient determines…

The pixel resolution (size of the pixel) in the phase encoding direction.

• Steep (strong) gradient = smaller pixels

• Shallow (weak) gradient = larger pixels

The number of pixels is determined by the amount of times the gradient is applied (phase matrix).

How often do the amplitude and polarity of the phase encoding gradient change throughout the scan?

Every TR, meaning that a different phase shift is produced every TR

Does T1 recovery and T2 decay happen before or after the phase encoding gradient?

Neither. These processes happen at the same time that the phase encoding gradient is applied. T1 recovery and T2 decay begin as soon as the RF pulse is switched off, and continue while the gradient creates position-dependent phase shifts.

What is the Larmor equation?

ω = γB₀: ω is the Larmor frequency (MHz), γ is the gyromagnetic ratio (MHz/T), & B₀ is the strength of the external magnetic field (T)

When placed in an external magnetic field, protons align either parallel to the magnetic field (called ___________) or antiparallel (called _________)

Spin up, spin down

What is resonance?

When a nucleus is exposed to an external force that has an oscillation similar to the natural frequency of its magnetic moment (its Larmor frequency), the nucleus gains energy from the external source.

What is the net magnetic vector (NMV)?

The NMV reflects the relative balance between spin-up and spin-down nuclei. Since spin-down takes more energy, more than half of protons in an external magnetic field always spin-up. Therefore, NMV is the sum of all magnetic moments of excess spin-up nuclei and aligns in the z-axis.

What is a 90-degree RF excitation pulse?

The 90-degree RF excitation pulse produces an oscillating magnetic field (termed B₁) applied at 90° to B₀ (or a variable angle if GRE) at a defined range of frequencies (transmit bandwidth) which matches the precessional frequencies of the protons in the slice we want to image.

What is spin-lattice relaxation?

Anti-parallel protons flip back to parallel and the NMW in the z-axis ‘regrows’. Energy is dissipated into the environment (lattice). Also known as T1 relaxation.

What is the T1 recovery time?

The time for 63% of the longitudinal magnetization to recover

What is spin-spin relaxation?

Gradual loss of coherent transverse magnetization as energy is dissipated to neighboring molecules. Also called T2 decay.

What is the T2 decay time?

The time for 63% of transverse magnetization to dephase (37% left in phase)

What are the T1 and T2 properties of fat?

Short T1 recovery & T2 decay (since molecules are large and bulky, they spin slowly, therefore they return to normal faster)

What are the T1 and T2 properties of water?

Long T1 recovery & T2 decay (since molecules are small and spaced apart, they spin faster, therefore they take longer to return to normal)

What is free induction decay?

Free induction decay (FID) results from transverse magnetization losing phase coherence due to both:

1. Spin-spin relaxation (T2): Interactions between neighbouring spins cause them to dephase.

2. Magnetic field inhomogeneities (T2*): Variations in the magnetic field cause spins to precess at slightly different frequencies.

What are the properties of a permanent magnet?

• Use permanently magnetized material instead of a superconducting material

• Does not require an external power source or cooling system

• Typically only 0.1-0.3 T (maximum of 1.2T)

• Can be open configuration

• They are extremely heavy

• They cannot be turned off

• They have limited homogeniety

• They have a smaller fringe magnetic field

What are the properties of a resistive magnet?

• They use thousands of layers of aluminum or copper strips

• They don’t use a supercooling material like liquid helium, so resistance is present in the strips

• Therefore current must constantly be passed through the wire to generate B₀ (very high energy requirement)

• The resistance generates heat, so a water coolant is used

• They can be easily turned off

• Max field strength is only 0.3T

• They have limited homogeneity

• They have a smaller fringe magnetic field

What are the properties of a superconducting magnet?

• Most common type used in MRI

• Doesn’t need external power supply once its ramped up

• Uses a superconducting wire to generate B₀ (ex. NbTi)

• No electrical resistance when cooled below a certain temperature (ex. NbTi, 4K/-269 degrees Celcius via liquid Helium)

• Typically 1.5-3T

• In an emergency, the liquid helium would need to be boiled off (quenched) to eliminate B₀

What are the 5 MRI magnet types we learned about in lecture?

• Permanent

• Resistive

• Superconducting

• Hybrid MR/PET

• Niche

What factors control the strength of B₀?

• The amount of current

• The number of windings

• The diameter of the wire (greater = stronger)

What is found within the cryostat?

• Main magnet solenoid

• Liquid helium

• Active shims

What happens when the ‘persistent switch’ is turned on

Turning the persistent switch on generates heat, which introduces resistance to a superconducting wire and gradually reduces the magnetic field (ramping down).

What are the two types of shielding?

Passive - large steel plates in the walls of the magnet room

Active - ‘bucking coils’ which carrying current in the opposite direction to the main magnet solenoid to oppose (buck) the effect of the main magnet

What are the three types of shims?

• Passive - trays of ferromagnetic shims along the full length of the bore. Positioned at start-up.

• Active - performed by additional superconducting solenoids in the cryostat, in addition to passive shims. Performed at start-up.

• Gradient - involves applying a current to the gradient coils to offset minor field inhomogeneities during image acquisition

In which direction do gradients slope (from negative to positive) along each orthogonal axis?

• Z: infero-superior (foot —> head)

• Y: postero-anterior (bottom —> top)

• X: left to right

What is rise time?

How long it takes to get the gradient ‘up to’ the desired slope (typically <1000 microseconds)

What is slew rate?

A measure of how fast the gradient changes (T/m/s). Calculated by dividing the gradient amplitude by the rise time

What is a Faraday cage?

The aluminum panels that line the walls, floor and ceiling of the scan room to protect from electrical interference to the MRI signal from outside sources, and to prevent RF pulses from causing electrical interference to outside devices

What is the most important property of a receive coil?

SNR

What makes a material diamagnetic?

• All electrons are paired

• It is repelled by a magnetic field

• Flux lines diverge

• X < 0

What makes a material paramagnetic?

• Some electrons are unpaired

• It is slightly attracted to a magnetic field

• Flux lines converge

• Ex. gadolinium (when >20 degrees Celcius)

What makes a material super-paramagnetic?

• Some unpaired electrons

• Very attracted to a magnetic field

• Flux lines converge

• X » 0

• Ex. iron oxide, hemosiderin, ferritin

What makes a material ferromagnetic?

• Half filled electron shells

• Very easily magnetized

• Retains magnetism

• X»»0

• Ex. iron, nickel, cobalt

What are extrinsic parameters?

Parameters that are under the control of the MRT and are used to manipulate intrinsic parameters.

Examples:

• TE

• TR

• Flip Angle

• Inversion time (TI)

• ETL

• b-value

TR & TE for a T1 weighted image:

• Short TR (vectors have less time to recover longitudinal magnetization; 300-700 ms)

• Short TE (not enough time for decay to occur, eliminates T2 weighting; 10-30 ms)

What does it mean for a vector to be saturated?

Whenever the NMV is pushed beyond 90°, it is said to be partially saturated. When the NMV is pushed to a full 180°, it is said to be fully saturated. If partial saturation of the fat and water vectors occurs, TI contrast is maximized.

TR & TE for a T2 weighted image:

• Long TR (vectors are allowed to fully recover, making them indistinguishable; 3,000-10,000 ms)

• Long TE (more time for T2 decay to occur which allows for contrast between tissues; 80-140 ms)

TR & TE for a PD weighted image:

• Long TR (eliminates T1 contrast)

• Short TE (eliminates T2 contrast)

What is TR?

Time of repetition: time (ms) between successive RF excitation pulses

True or false: tissues with shorter T1 recovery times will be hyperintense on T1 weighted images

True. Ex. fat has a short T1 recovery time, therefore is hyperintense on T1W

What is TE?

Time to echo: time (ms) between delivery of RF excitation pulse and peak of signal received

True or false: in PD images, tissues with high proton density appear hyperintense

True. Example fat and water both have high proton density so appear hyperintense, whereas bone, air, tendon, etc appear hypointense (low PD)

What does fMRI use to generate contrast?

fMRI (specifically BOLD fMRI) relies on the presence of deoxygenated haemoglobin (deoxy-Hb) to generate contrast. When a brain region becomes active, blood flow increases to the area, increasing the concentration of oxy-Hb but reducing the concentration of deoxy-Hb which is paramagnetic. This reduces local field inhomogeneities and increases the MRI signal.

High presence of deoxy-Hb = hypointense

Low presence of deoxy-Hb = hyperintense

How do contrast agents increase contrast?

• T1 agents (ex. Gad) reduce T1 recovery time of tissues which increases hyperintensity

• T2 agents reduce T2 decay time

What intensities are grey & white matter on T1 weighted images?

White matter hyperintense

Grey matter hypointense

What intensities are grey & white matter on T2 weighted images?

White matter hypointense

Grey matter hyperintense

What is the MT pulse in Magnetization Transfer Contrast?

• The MT pulse is an RF pulse that precedes the RF excitation pulse

• It specifically targets protons in the bound pool

• This saturates the bound pool with energy (i.e. vectors are tipped into the transverse plane)

• Some energy is transferred from the bound pool to the free pool (magnetization transfer coherence)

• This saturates some of the protons in the free pool

• When the RF excitation pulse is applied to the free pool, the saturated protons are pushed into the longitudinal plane and don’t contribute (or contribute less) to net transverse magnetization

• Therefore, the NMV is smaller from the free pool (hypointense) than the bound pool (hyperintense)

• This is called the magnetization transfer ratio

What is the magnetization transfer ratio?

The difference in signal between free and bound protons in Magnetization Transfer Contrast.

True or false: paramagnetic material appears hypointense in a susceptibility weighted image.

True. Paramagnetic and ferromagnetic materials cause increased inhomogeneities in the local magnetic field, which causes dephasing and a lack of signal.

What size flip angle is used for T1 weighted images?

Large. Allows a longer time for recovery of longitudinal magnetization. A shorter flip angle wouldn’t allow as great of a difference between tissues (ex. fat and water)

What size flip angle is used for T2 weighted images?

Short. Allows tissues to recover longitudinal magnetization faster (T1 recovery is not used for contrast, so this helps reduce scan time)

What size flip angle is used for PD weighted images?

Short. Allows faster T1 recovery (since it’s not used for contrast)

Describe how the 180-degree rephasing pulse produces a spin echo

After a 90° excitation pulse, transverse magnetization vectors begin to dephase, losing phase coherence.

• Faster-precessing spins form the leading edge, moving ahead in phase.

• Slower-precessing spins form the lagging edge, falling behind in phase.

The 180° rephasing pulse:

• Flips the transverse magnetization vectors by 180°, reversing their positions relative to one another in the transverse plane.

• The leading-edge spins become the lagging-edge spins, and vice versa.

• The spins continue precessing at their respective frequencies. The spins that were previously ahead are now behind, while the spins that were behind are now ahead.

• The faster-precessing spins catch up with the slower-precessing spins, restoring phase coherence.

A spin echo occurs when the transverse magnetization vectors rephase (superimpose), producing a maximum in the detected signal.

What images are produced in a CSE using one 180° rephasing pulse?

T1W (short TR & short TE)

What images are produced in a CSE using two 180° rephasing pulses?

First pulse: PD (long TR & short TE)

Second pulse: T2W (long TR & long TE)

What two time periods does tau represent?

• Time between 90° excitation pulse and 180° rephasing pulse (also called inversion time, or TI)

• Time between 180° rephasing pulse and peak of signal (spin echo)

What is tau equal to?

TE = 2 x Tau

Tau = TE/2

How does FSE differ from CSE?

• Uses multiple 180° rephasing pulses to create an ‘echo train’

• Fat is hyperintense on T2W imaging due to J-coupling

• Shorter scan time (increased ETL = decreased scan time)

• Reduced magnetic susceptibility artefacts

• Multiple lines of k-space filled with 1 TR

How does inversion recovery work?

• A 180° inversion pulse flips the net magnetization vector from the positive z-axis to the negative z-axis.

• The vector begins recovering toward the positive z-axis through T1 relaxation.

• After a selected inversion time (TI), a 90° excitation pulse is applied.

• If the tissue's longitudinal magnetization is zero at that moment, there is no longitudinal magnetization to tip into the transverse plane. Therefore, that tissue produces no signal.

What are the TI times of different tissues (ms)?

• Fat: 150-180

• CSF: 2200-2500

• White matter: 400

• Grey matter: 650

How does a gradient echo work?

• An RF pulse, often with a variable flip angle, tips the NMV into the transverse plane.

• A dephasing gradient is applied, causing protons at different positions to precess at different frequencies.

• A gradient of opposite polarity is applied. Protons that precessed faster during the first gradient now accumulate less phase relative to slower-precessing protons, allowing them to catch up in phase.

• When the accumulated phase differences from the gradients cancel, the protons rephase and the signal reaches a maximum. This produces a gradient echo.

• After the echo, the protons continue to dephase as the effects of the gradients no longer cancel and intrinsic T2 relaxation continues.

What are the differences between SE and GRE?

• GRE uses a variable flip angle, often less than 90°. Conventional SE typically uses a 90° excitation pulse followed by a 180° rephasing pulse.

• GRE uses a rephasing gradient rather than a 180° RF pulse to generate the echo. This generally reduces RF energy deposition and SAR compared with SE sequences that use repeated 180° refocusing pulses.

• GRE is more affected by magnetic field inhomogeneities because gradient rephasing does not correct for their effects. SE largely refocuses dephasing caused by static field inhomogeneities.

• GRE sequences sensitive to transverse decay produce T2*-weighted images because signal loss from both spin-spin interactions (T2) and magnetic field inhomogeneities contributes to decay. Conventional SE can produce T2-weighted images because the 180° pulse refocuses much of the dephasing caused by static field inhomogeneities.

• GRE is generally more sensitive to magnetic susceptibility effect & chemical shift artefacts.

• GRE is generally faster

What is the most common alloy used in permanent magnets?

ALNICO (aluminum, nickel & cobalt)

How is energy deposited into the patient’s body?

The RF pulse has a magnetic component and an electric component. The magnetic component of the RF pulse does the flipping of NMV, and the electric component deposits heat (larger B₀ = more energy deposition)

Clinical magnets require a homogeneity of ____ or better

10ppm

What is this describing: Copper plated cylinder with etchings through which a current is passed creating a magnetic field

Gradient coil

True or false: gradient coils are located in the cryostat and are cooled by liquid helium

False. Gradients do generate heat and need to be water-cooled, but are not located in the cryostat

What are the requirements required to switch gradients quickly?

• High slew rate

• Low rise time

The transmit coil is located ________ to B₀

Perpendicular

The depth of a coil (how deep we can see) is equivalent to what?

The radius of the coil

When is the filling factor optimal?

When the entire sensitive volume of coil contains tissue

For a volume coil, filling factor tends to be higher or lower than that of a surface coil?

Lower (coil not totally filled byvanatomy and contains empty space, reducing SNR)

True or false: In a phased array coil, to maximize signal, each loop is weighted differently (different sensitivity)

True