Mri terminology

Question 1

What is the definition of MRI?

Answer 1

A non-invasive technique that uses radio-frequency radiation in the presence of a powerful magnetic field to produce high-quality images of the body in any plane

MRI stands for Magnetic Resonance Imaging.

Question 2

What does dephasing refer to in MRI?

Answer 2

When the RF pulse is switched off, the spinning protons go out of phase resulting in a reduction in the received signal

Question 3

What is Echo Time (TE) in MRI?

Answer 3

Gives the quantity of dephasing that happens between the excitation pulse and the echo

The shorter the echo time, the lower the dephasing, the higher the proton density (T1), and the better the image.

Question 4

What is a Flip Angle (α)?

Answer 4

The degree by which the proton is tipped in relation to the main magnetic field when a RF pulse is applied to it

Question 5

What is the purpose of a Fourier Transform in MRI?

Answer 5

A method of mathematically changing data, e.g., changing image space to k-space

Question 6

What is a Gradient Echo?

Answer 6

A basic pulse sequence that only uses magnetic field gradient reversal to re-phase the transverse magnetisation and produce echoes of the magnetic resonance signal

Allows shorter repetition times and faster scanning, using flip angles between 0° and 90°.

Question 7

What is Image Space in MRI?

Answer 7

An MRI image

Question 8

What is Inversion Recovery (IR)?

Answer 8

A basic pulse sequence of 180°, 90° and 180° which inverts the magnetisation and measures the time taken for the nuclei to return to equilibrium

The rate of recovery depends on the relaxation time T1.

Question 9

What is k-space?

Answer 9

The Fourier transform of an MRI image which gives the frequency and the phase encoding directions

Question 10

What does the Larmor Equation calculate?

Answer 10

The frequency of the RF pulse at a given field strength, as the nuclei of different elements will precess at different frequencies

Question 11

What is Larmor Frequency?

Answer 11

The rate at which the protons spin when a magnetic field is applied

Question 12

What does the Magnetic Field Gradient determine?

Answer 12

The plane to be imaged

The stronger the gradient, the faster the scan or the higher the resolution.

Question 13

What is Noise in MRI?

Answer 13

Unwanted electrical signals causing grain on the image

Question 14

What is Precession in the context of MRI?

Answer 14

The circular movement of the magnetic axis of a spinning proton which is prescribed when an external magnetic field is applied to the proton

Question 15

What is a Pulse Sequence?

Answer 15

The bursts of electromagnetic energy produced by the radio-frequency coils

Comprises, e.g., Saturated recovery, Inversion recovery, Spin echo.

Question 16

What is a Radio-frequency (RF) Pulse?

Answer 16

A burst of electromagnetic energy at right angles to the magnetic field

Question 17

What is Relaxation Time?

Answer 17

The time taken for the spinning protons to release the energy obtained and return to their original state

Question 18

What does Repetition Time (TR) refer to?

Answer 18

The time between the beginning of one radio-frequency pulse sequence to the start of the next, e.g., 300 ms or 500 ms at 1.5 Tesla

Question 19

What is Resonance in MRI?

Answer 19

When an object (a proton) responds to an alternating force (a radio-frequency signal) causing movement

Question 20

What is Saturated Recovery (SR)?

Answer 20

When all the longitudinal magnetisation is measured before a 90° radio-frequency pulse is applied

Time-consuming procedure used for protein density weighted images; superseded by spin echo sequences.

Question 21

What is Saturation in the context of MRI?

Answer 21

The maximum degree of magnetisation that can be achieved in a substance

Question 22

What is the Signal to Noise Ratio?

Answer 22

Image quality = Signal (information required from image) / Noise (unwanted information on an image)

Can be improved by increasing the number of signal excitations, increasing the field of view, or increasing the strength of the main magnetic field.

Question 23

What is Spatial Encoding?

Answer 23

The prediction of the strength of the magnetic field and the movement of the protons at a set point along a gradient

Question 24

What is Spin Echo (SE)?

Answer 24

The reappearance of a magnetic resonance signal after the initial signal has disappeared following a 90° radio-frequency pulse followed by a 180° radio-frequency pulse

Used to detect localised pathology.

Question 25

What is Spin Polarisation?

Answer 25

The difference between the number of protons that have aligned with the magnetic field and those that have not

Gives the strength of the signal; the more protons that align, the stronger the signal.

Question 26

What is T1 Relaxation Time?

Answer 26

The time taken for the proton spins to release the energy obtained from the initial radio-frequency impulse and return to their natural state

It represents the time required for the longitudinal magnetisation (Mz) to go from 0 to 63% of its final maximum value.

Question 27

What is T2 Relaxation Time?

Answer 27

The time required for the transverse magnetisation to reduce to about 37% of its maximum value

Is the characteristic time constant for loss of phase unity amongst spins oriented at an angle to the static main magnetic field.

Question 28

What is Tesla in the context of MRI?

Answer 28

A unit for measuring the strength of a magnetic field

A magnetic flux density of 1 Tesla exists if the force on a 1 metre long straight wire, carrying a current of 1 ampere, is 1 Newton and the wire is placed at right angles to the direction of magnetic flux.