Magnetic resonance imaging (MRI)

Published on 12/06/2015 by admin

Last modified 22/04/2025

Print this page

This article have been viewed 1406 times

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
Noise	Unwanted electrical signals causing grain on the image
Precession	Is 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 Sequence	The bursts of electromagnetic energy produced by the radio-frequency coils • Comprises, e.g. Saturated recovery Inversion recovery Spin echo
Radio-frequency (RF) Pulse	A burst of electromagnetic energy at right angles to the magnetic field
Relaxation Time	The time taken for the spinning protons to release the energy obtained and return to their original state
Repetition 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 Tesla
Resonance	When an object (a proton) responds to an alternating force (a radio-frequency signal) causing movement
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
Saturation	The maximum degree of magnetisation that can be achieved in a substance
Signal to Noise Ratio	Image quality = Signal (information required from image)/Noise (unwanted information on an image)
Signal to Noise Ratio	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 Encoding	The prediction of the strength of the magnetic field and the movement of the protons at a set point along a gradient
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
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

	Fig. 15.2 MRI coil position (diagrammatic).
Cryocooler	A closed container: • Condenses the liquid helium vapour • Prevents the need for topping up the liquid helium
Cryostat	A container for: • The coils of the superconducting magnet • A coolant, liquid helium
Gradient Coils (Secondary Magnetic Coils)	Are magnetic coils that are designed to increase or decrease the strength of the main magnetic field • They are usually positioned in three planes • Have a reducing strength along the field, e.g. 1.45 to 1.55 Tesla • The movement of the protons can be predicted • Are used to localise a slice and spatially encode slice information to show the position in the patient and therefore allow focusing on a particular area of the body.
Magnets	Produce the main magnetic field • Low-field strength permanent magnets up to 0.2 Tesla, usually ‘C’ shaped • Low- to mid-field strength resistive magnets up to 0.35 Tesla (no longer used) • Mid- to high-field strength superconducting magnets 0.5 to 2.0 Tesla, usually cylindrical Magnets have to have strength and straight, parallel lines of force in the iso centre
Phased Array Coils	• Receive coils • Produce an even signal covering a large area • Produce high spatial resolution images • Faster scan times Reduce patient movement both voluntary and involuntary
Radio-frequency (RF) Coils	• Produce a fluctuating magnetic field which causes the protons to precess • Detect the RF signal produced by the precessing proton when it turns to align itself with the external magnetic field
Solenoid Technology (ST) Coils	A type of open radio-frequency coil that encloses the area of interest, e.g. a neck coil giving: • Good signal to noise ratio • Better patient comfort • Anatomical coverage in the vertical magnetic field • Larger depth penetration • Imaging extends outside the edge of the coil Fig. 15.3 Vertical magnetic field.
Surface Coils	Small radio-frequency coils positioned close to the area of interest • Give a better signal to noise ratio • Can only detect a signal at a depth in the patient equal to the radius of the coil

Basic principles of image recording

Hydrogen	• Has one proton in its nucleus • Is common in the body in the form of water (10²³ hydrogen protons per ml)
Strong Magnetic Field Applied to the Patient	• Some of the hydrogen protons align to the new magnetic field • Field strength used: 0.15 to 1.5 Tesla • Earth’s magnetic field 5 × 10⁻⁶ Tesla Fig. 15.4 Proton alignment when a magnetic field is applied.
Gradient Coils	Determine the plane to be recorded
Pulses of Radio Waves	• Displace the protons from their new position • When the pulse ceases The protons realign They release the energy absorbed as a radio signal of the same frequency
Radio-frequency Coils	• Produce the pulsed radio waves • Detects the released energy from the protons Energy is proportional to the number of protons The energy produces a digital signal
Scanning	• To give a 256 × 256 matrix, 256 phase lines are needed • The slice position is determined by the gradient coils • The lines are determined by the measurement of the relaxation time of the protons following an RF signal
Image	• Signal is analysed by computer • Reconstructed image is displayed as a range of greyscale values
Contrast Agents	Examples • Water • aramagnetic, e.g. gadolinium compound • Superparamagnetic e.g. iron oxide nanoparticles
	Advantages of MRI • Does not use ionising radiation. • Can produce images in any plane • Gives good contrast resolution • Good for soft tissue imaging, e.g. tumours • Can be used for pregnant women but only when essential and without the use of contrast agents
	Disadvantages of MRI • Due to the magnetic field the presence of: Pacemakers Aneurysms clips Electrical implants Metal implants, e.g. stainless steel Ferromagnetic foreign bodies in the patient Ferromagnetic objects in the room Are usually contraindicated due to the potential movement of the object or the heating of the metal by induction, but note that titanium is not affected by the magnetic field • Claustrophobia if a closed tube is used • Hyperthermia can occur in obese patients due to the radiofrequency energy • Noise from the machine can be very high, therefore ear protection is necessary Use of ear protectors The use of noise cancellation systems

Developments

Cylindrical System with Improved Solenoid Technology (Philips)	• Better signal to noise ratio • Anatomical coverage in the vertical field • Coils form a loop round the body • The anatomy of interest is inside the coil • Sensitivity is in the head to feet direction • 1.0 Tesla open system = 1.5 Tesla cylindrical system • Neck coil 12 cm ST coil does not touch the skin therefore is more comfortable for the patient • 3 × 12 cm ST coil, covers the circle of Willis to the aortic arch
Diffusion Weighted Imaging (DWI)	• Principle In normal tissue the protons in the water move freely Following injury, cells walls absorb water, swell and restrict the movement of the protons • An echo plane sequence is followed by two gradient episodes which cancel out plane shift in damaged areas but do not affect the image of normal tissue due to the free movement of the protons • The injured areas therefore have a high signal intensity • Application Early stroke Early head injury
Functional MRI (Echo Planar Imaging EPI)	• The production of a moving tomographic image Has improved strength and switching of the gradient system Higher resolution with thinner slice thickness Data is collected, processed and stored more quickly • The imaging sequence results in: A phase encoded gradient A frequency encoded gradient A phase encoded gradient A reversed frequency encoded gradient The image is recorded during the frequency encoded gradients
	Example applications
	• To measure blood flow response when the patient is at rest and during stimulation • Investigates the working of the brain Visual, auditory and motor functions Smell, speech, pain Conditions, e.g. epilepsy, neuropsychiatric problems
Magnetic Resonance Angiography	• Using a paramagnetic contrast agent or • Using flow-related enhancement The signal is due to the flow of blood into the imaging plane
MR Guided Therapy	Using MRI to assist with, e.g. • Aspiration cytology • Electrophysiology • Tissue removal using lasers • Cryotherapy
MRI PAT (Parallel Acquisition Techniques)	• Uses array coils (multicoil, phased array receiver coils) • Coils are arranged along the direction of the plane of interest and are used instead of the gradient coils • Less k-space lines are required to form the image with no loss in resolution • If in a 256 × 256 matrix, only half the lines are needed the scan time is halved • Reconstruction uses a coil sensitivity map to reduce artefacts • Therefore produces reduced scan times with no loss of image quality
Open Magnetic Resonance Imaging	• Principle No body coil Surface coils are used that transmit and receive • Allows scanning in any plane • More acceptable to patients, including children and claustrophobic patients • Allows guided therapy/intervention procedures • In radiotherapy planning, some patients can be scanned in the treatment position
Perfusion MRI	• Measurement of blood flow before, during and after the injection of a contrast agent • Can calculate the volume of blood and the time taken for it to pass through an organ • Application, e.g. Determine the viability of tissue after an infarct The blood flow associated with and through tumours
Three Dimensional Imaging	• Specialist computer software is used to produce the image • A radio-frequency pulse of a set bandwidth is used to give a slice through the body • Information from an even number of slices is obtained • The data is recorded on a three dimensional matrix • The data can be viewed in any plane Fig. 15.5 Three dimensional matrix.

Sources

	Holmes J E, Bydder G M 2005 MR imaging with ultrashort TE (UTE) pulse sequences: basic principles. Synergy, January
	Hussain Z, Roberts N, Whitehouse G 1995 The application of T₁ and T₂ and proton density measurements to optimise detection of hepatic metastases using MRI. Radiography Today 61(695):April
	Jones T 1998 Gradient echo pulse sequences decoded. Synergy, September
	Radiography Technical Support 2006 Solenoid technology improving imaging with open magnets. Synergy, November
	Talbot J 2000 MRI artefacts: the good, the bad and the ugly. Synergy, October
	Talbot J 2001 What is noise and why is it a problem? Synergy, January
	Weal P, Kilkenny J 2003 The practical applications of parallel imaging techniques using standard radio-frequency coils. Synergy, November
	Westbrook C 1998 MR advances – the future. Synergy, February