MRI - Prefi 2

Proton density

Measure of the concentration of mobile hydrogen nuclei available to produce an MR signal

Proton density

MR signal intensity linked to hydrogen nuclei concentration

T1 relaxation time

The time constant that describes the rate at which MZ returns to M0

T1 relaxation time

The return of net magnetization along the Z axis to its normal equilibrium state

T2 relaxation time

Interaction between individual spins

T2 relaxation time

Loss of phase coherence in XY plane

Phase coherence

A phenomen where a constant phase difference exists between any two signals or waves of the same frequency

Phase

The position of a magnetic moment of a spin on its precessional path at any moment in time

Out of phase (incoherent)

Magnetic moments of hydrogen nuclei are at different places on the precessional path at a moment in time

In phase (coherent)

Magnetic moments of hydrogen nuclei are at same places on the precessional path at a moment in time

T2* relaxation time

Combines T2 relaxation with field inhomogeneities

T2* relaxation time

Rapid loss of phase coherence due to field variations

Pulse sequences

A technique to acquire images with different tissue contrast mechanisms

Pulse sequences

A set of specific instructions programmed into the computer with an expectation as to how the images should appear

Pulse sequences

A group of two or more pulses

Repetition time (TR)

Time from application of one RF pulse to the application of next pulse sequence

Echo time (TE)

Time from the application of RF pulseto the peak of sign induced in the coil

Echo time (TE)

Time between middle of exciting

Time from inversion (TI)

Time from the application 180 degree inverting pulse to 90 degree excitation pulse

Saturation recovery (SR)

Utilizes a single RF pulse for signal generation

Spin echo (SE)

Most widely used two-pulse echo in MRI

Spin echo (SE)

SE generated by 90°-180° sequence

Multi-echo spin echo (MESE)

Provides unique echo signals for contrast variations

Multi-echo spin echo (MESE)

90°-180°-180° sequence

Inversion recovery (IR)

Used to selectively null the signal of certain tissues (fat or fluid)

Inversion recovery (IR)

180°-90°-180° sequence

Short Tau Inversion Recovery (STIR)

Signal of fat is zero

Fluid Attenuated Inversion Recovery (FLAIR)

Suppresses signal from cerebrospinal fluid

Stimulated echo (SE)

Produced by rapid series of three RF pulses

Stimulated echo (SE)

Occasional use due to potential artifact generation

Signal

FID induced in the receiver coil

Spatial resolution

The ability to distinguish between two points as separate and distinct

Slice thickness

Influences the amount of signal, as well as the sharpness of an image

Field of view (FOV)

Determines how much of the patient we are going to see

Examination time

To minimize the unsharpness caused by voluntary and involuntary motion