Magnetic resonance imaging (MRI)

15 Magnetic resonance imaging (MRI)

Definition of magnetic resonance imaging

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

Terminology

Acquisitions (Number of Excitations, Signal Averages)	The gathering of enough information to spatially encode one complete data set
Dephasing	When the RF pulse is switched off, the spinning protons go out of phase resulting in a reduction in the received signal
Echo Time (TE)	• 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
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
Fourier Transform	• A method of mathematically changing data, e.g. changing image space to k-space
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 • Uses flip angles between 0° and 90°
Image Space	An MRI image
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 the equilibrium • The rate of recovery depends on the relaxation time T₁
k-space	• The Fourier transform of an MRI image • Gives the frequency and the phase encoding directions
Larmor Equation	At a given field strength, the nuclei of different elements will precess at different frequencies, the equation is used to calculate the frequency of the RF pulse
Larmor Frequency	The rate at which the protons spin when a magnetic field is applied
Magnetic Field Gradient	The loss or increase of magnetic strength over distance controlled by the electrical current passing through the coil

• Determines the plane to be imaged

• The stronger the gradient the faster the scan or the higher the resolution

NoiseUnwanted electrical signals causing grain on the imagePrecessionIs the circular movement of the magnetic axis of a spinning proton which is prescribed when an external magnetic field is applied to the proton

Fig. 15.1 Precession.

Pulse SequenceThe bursts of electromagnetic energy produced by the radio-frequency coils

• Comprises, e.g.

Saturated recovery

Inversion recovery

Spin echo

Radio-frequency (RF) PulseA burst of electromagnetic energy at right angles to the magnetic fieldRelaxation TimeThe time taken for the spinning protons to release the energy obtained and return to their original stateRepetition Time (TR)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 TeslaResonanceWhen an object (a proton) responds to an alternating force (a radio-frequency signal) causing movementSaturated 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

SaturationThe maximum degree of magnetisation that can be achieved in a substanceSignal to Noise RatioImage 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

Spatial EncodingThe prediction of the strength of the magnetic field and the movement of the protons at a set point along a gradientSpin 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

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

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 (M_z) to go from 0 to 63% of its final maximum value

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 orientated at an angle to the static main magnetic field

Tesla

• 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

Hardware