MRI 3

Question 1

Magnetic moment (μ)

Answer 1

The torque exerted on a magnet, moving electrical charge, or current-carrying coil when it is placed in a magnetic field.

Question 2

Torque

Answer 2

Force that induces rotational motion.

Question 3

Angular momentum (J)

Answer 3

A quantity given by multiplying the mass of a spinning body by its angular velocity.

Question 4

Right-hand rule

Answer 4

A method used to determine the direction of a magnetic field generated by a moving electrical charge or electrical current.

Question 5

Spin system

Answer 5

A collection of atomic nuclei that possess the NMR property within a spatial location.

Question 6

Net magnetization (M)

Answer 6

The sum of the magnetic moments of all spins within a spin system.

Question 7

Flux

Answer 7

A measure of the strength of a magnetic field over an area of space.

Question 8

B (magnetic field)

Answer 8

The strong static magnetic field generated by an MRI scanner.

Question 9

Precession

Answer 9

The gyroscopic motion of a spinning object in which the axis of the spin itself rotates around a central axis, like a gyroscope.

Question 10

Larmor frequency

Answer 10

The resonant frequency of a spin within a magnetic field of a given strength.

Question 11

Resonant frequency

Answer 11

The frequency of oscillation that provides maximum energy transfer to the system.

Question 12

Parallel state

Answer 12

The low-energy state in which an atomic spin processes around an axis that is parallel to that of the main magnetic field.

Question 13

Antiparallel state

Answer 13

The high-energy state in which an atomic spin processes around an axis that is antiparallel (opposite) to that of the main magnetic field.

Question 14

Longitudinal

Answer 14

Parallel to the main magnetic field, or the z-direction of the scanner (i.e. into the bore).

Question 15

Transverse

Answer 15

Perpendicular to the main magnetic field of the scanner, in the x-y plane.

Question 16

The Zeeman effect

Answer 16

The energy difference (△E) between the parallel and antiparallel states increases linearly with the strength of the static magnetic field.

Question 17

90-degree excitation pulse

Answer 17

A quantity of electromagnetic energy that when applied to a spin system during MR excitation, results in equal numbers of nuclei in the low- and high- energy states.

Question 18

180-degree excitation pulse

Answer 18

A quantity of electromagnetic energy that when applied to a spin system during MR excitation, results in a flipping of the usual net magnetization such that there are now more nuclei in the high-energy state than the low-energy state.

Question 19

Reception

Answer 19

The process of receiving electromagnetic energy emitted by a sample at its resonant frequency (also called detection).

Question 20

MR signal

Answer 20

The current measured in a detector coil following excitation and reception.

Question 21

Relaxation

Answer 21

A change in the net magnetization over time.

Question 22

Transverse relaxation (spin-spin relaxation)

Answer 22

The loss of net magnetization within the transverse plane due to the loss of phase coherence of the spins.

Question 23

Longitudinal relaxation (spin-lattice relaxation)

Answer 23

The recovery of the net magnetization along the longitudinal direction as spins return to the parallel state.

Question 24

T2 (decay)

Answer 24

The time constant that describes the decay of the transverse component of net magnetization due to accumulated phased differences caused by spin-spin interactions.

Question 25

T2* (decay)

Answer 25

The time constant that describes the decay of the transverse component of net magnetization due to both accumulated phase differences and local magnetic field inhomogeneities. T2* is always shorter than T2.

Question 26

T1 (recovery)

Answer 26

The time constant that describes the recovery of the longitudinal component of net magnetization.

Question 27

Scalar

Answer 27

A quantity that has magnitude but not direction (i.e. mass, charge, length).

Question 28

Vector

Answer 28

A quantity with both magnitude and direction (i.e. velocity, force, electromagnetic fields).

Question 29

Dot product

Answer 29

The scalar product of two vectors. Summing the products along each dimension creates it (A! .B).

Question 30

Cross product

Answer 30

The vector product of two vectors. It’s direction is perpendicular to the plane defined by those vectors and its magnitude is given by multiplying their product times the sin of the angle between them (AxB).

Question 31

Gyromagnetic (magnetogyric) ratio

Answer 31

The ratio between the charge and mass of a spin. It is a constant for a given type of nucleus.

Question 32

Nutation

Answer 32

The spiraling change in the precession of the net magnetization during an excitation pulse.

Question 33

On-resonance equation

Answer 33

The presentation of an excitation pulse at the resonant frequency of the sample, resulting in maximum efficiency.

Question 34

Off-resonance excitation

Answer 34

The presentation of an excitation pulse at a frequency other than the resonant frequency of the sample, resulting in reduced efficiency.

Question 35

Bloch equation

Answer 35

An equation that describes how the net magnetization of na spin system changes over time in the presence of a time-varying magnetic field. This equation provides the mathematical representations of magnetization during steady state, excitation, and relaxation. Thus, the Bloch equation (named after Felix Bloch) provides the theoretical foundation of MRI.