Mri terminology Flashcards
What is the definition of MRI?
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.
What does dephasing refer to in MRI?
When the RF pulse is switched off, the spinning protons go out of phase resulting in a reduction in the received signal
What is Echo Time (TE) in MRI?
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.
What is a Flip Angle (α)?
The degree by which the proton is tipped in relation to the main magnetic field when a RF pulse is applied to it
What is the purpose of a Fourier Transform in MRI?
A method of mathematically changing data, e.g., changing image space to k-space
What is a Gradient Echo?
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°.
What is Image Space in MRI?
An MRI image
What is Inversion Recovery (IR)?
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.
What is k-space?
The Fourier transform of an MRI image which gives the frequency and the phase encoding directions
What does the Larmor Equation calculate?
The frequency of the RF pulse at a given field strength, as the nuclei of different elements will precess at different frequencies
What is Larmor Frequency?
The rate at which the protons spin when a magnetic field is applied
What does the Magnetic Field Gradient determine?
The plane to be imaged
The stronger the gradient, the faster the scan or the higher the resolution.
What is Noise in MRI?
Unwanted electrical signals causing grain on the image
What is Precession in the context of MRI?
The circular movement of the magnetic axis of a spinning proton which is prescribed when an external magnetic field is applied to the proton
What is a Pulse Sequence?
The bursts of electromagnetic energy produced by the radio-frequency coils
Comprises, e.g., Saturated recovery, Inversion recovery, Spin echo.
What is a Radio-frequency (RF) Pulse?
A burst of electromagnetic energy at right angles to the magnetic field
What is Relaxation Time?
The time taken for the spinning protons to release the energy obtained and return to their original state
What does Repetition Time (TR) refer to?
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
What is Resonance in MRI?
When an object (a proton) responds to an alternating force (a radio-frequency signal) causing movement
What is Saturated Recovery (SR)?
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.
What is Saturation in the context of MRI?
The maximum degree of magnetisation that can be achieved in a substance
What is the Signal to Noise Ratio?
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.
What is Spatial Encoding?
The prediction of the strength of the magnetic field and the movement of the protons at a set point along a gradient
What is Spin Echo (SE)?
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.
What is Spin Polarisation?
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.
What is T1 Relaxation Time?
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.
What is T2 Relaxation Time?
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.
What is Tesla in the context of MRI?
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.
