INSTRUMENTATION AND SAFETY
1. Types of magnets
The design of MRI is essentially determined by the type and format of the main magnet, i.e. closed, tunnel-type MRI or open MRI.
The most commonly used magnets are superconducting electromagnets (figure 2.1). These consist of a coil that has been made superconductive by helium liquid cooling, and immersed in liquid nitrogen. They produce strong, homogeneous magnetic fields, but are expensive and require regular upkeep (namely topping up the helium tank).
In the event of loss of superconductivity, electrical energy is dissipated as heat. This heating causes a rapid boiling-off of the liquid Helium which is transformed into a very high volume of gaseous Helium (quench). In order to prevent thermal burns and asphyxia, superconducting magnets have safety systems: gas evacuation pipes, monitoring of the percentage of oxygen and temperature inside the MRI room, door opening outwards (overpressure inside the room).
Superconducting magnets function continuously. To limit magnet installation constraints, the device has a shielding system that is either passive (metallic) or active (an outer superconducting coil whose field opposes that of the inner coil) to reduce the stray field strength.
Low field MRI also uses:
• Resistive electromagnets, which are cheaper and easier to maintain than superconducting magnets. These are far less powerful, use more energy and require a cooling system.
• Permanent magnets, of different formats, composed of ferromagnetic metallic components. Although they have the advantage of being inexpensive and easy to maintain, they are very heavy and weak in intensity.
To obtain the most homogeneous magnetic field, the magnet must be finely tuned (“shimming”), either passively, using movable pieces of metal, or actively, using small electromagnetic coils distributed within the magnet.
Characteristics of the main magnet
The main characteristics of a magnet are:
• Type (superconducting or resistive electromagnets, permanent magnets)
• Strength of the field produced, measured in Tesla (T). In current clinical practice, this varies from 0.2 to 3.0 T. In research, magnets with strengths of 7 T or even 11 T and over are used.
• Homogeneity
2. Magnetic field gradients
Gradients components
They produce a linear variation in magnetic field intensity in a direction in space. This variation in magnetic field intensity is added to the main magnetic field, which is far more powerful. The variation is produced by pairs of coils, placed in each spatial direction.
The direction of the magnetic field is not modified. By adding them to B0, a linear variation is produced in the total magnetic field amplitude, in the direction to which they are applied (figure 2.3). Their action is considered as homogeneous on a plane perpendicular to the direction of application.
This modifies resonance frequency, in proportion to the intensity of the magnetic field to which they are submitted (in accordance with Larmor’s equation: the stronger the field, the faster they precess). This variation in Larmor frequency also causes a variation and dispersion of spin phases.
Gradient characteristics
Gradient performances are linked to:
• their maximal amplitude (magnetic field variation in mT/m), which determines maximal spatial resolution (slice thickness and field of view)
• their slew rate, corresponding to their switching speed: high slew rates and low rise time are required to switch gradients quickly and allow ultra-fast imaging sequences such as echo planar (EPI)
• their linearity, which must be as perfect as possible within the scanning area.
Eddy currents
The rapid switching of the gradients induces currents in the conducting materials in the vicinity of the gradient coils (cryogenic envelope, electric wires, antennas, homogenization coils, etc.). These induced currents (Eddy current) will oppose the gradient fields and cause a decay in their profile.
There are several methods to reduce the effects of these induced currents:
• Active gradient coil shielding
• Optimizing the electric current profile sent to the gradient coils while ascending and descending to offset the Eddy currents
Moreover, gradient switches produce Lorentz forces causing vibrations in the gradient coils and their supports. These vibrations are the main source of the characteristic MRI noise.
Radiofrequency system
3. Radiofrequency system components
The radiofrequency system comprises the set of components for transmitting and receiving the radiofrequency waves involved in exciting the nuclei, selecting slices, applying gradients and in signal acquisition.
Coils are a vital component in the performance of the radiofrequency system (figure 2.6). In transmission, the goal is to deliver uniform excitation throughout the scanned volume. On reception, the coils must be sensitive and have the best possible signal to noise ratio.
An MR scanner generally contains a « whole body » coil, located in the cylinder of the machine, homogeneously covering the entire scan volume. The sensitive volume of surface coils, being placed in direct contact with the zone of interest, has less depth and is more heterogeneous. However, surface coils offer a better signal to noise ratio and imaging capacity with higher spatial resolution. The homogeneity and sensitive volume of surface coils can be improved by combining them into a phased array. They still have the advantage of a better signal to noise ratio, but at the cost of more complex signal processing.
Quadrature RF coils (circularly polarized coils) consist of at least two coils that are oriented orthogonal to each over (and both are othogonal to B0 axis). They have a better signal to noise ratio than linear RF coils.
Depending on the manufacturers and the type of coil, certain coils can be transmitters, receivers or both.
The radiofrequency channel also comprises analog-digital converters and a spectrometer to receive and analyze the signal.
Optimizing the radiofrequency channel
Optimization of the radiofrequency channel is automated and carried out in several stages prior to an imaging sequence:
• the exact Larmor frequency is set, this being slightly modified by the patient’s presence in the magnetic field
• transmission power is adjusted according to the weight of the patient and the transmit coil, to obtain the desired flip angles
• the receiver gain is adjusted to avoid signal saturation or conversely, weak amplification resulting in a deteriorated signal to noise ratio.
Faraday cage
As the resonance frequency of protons is very close to that of the radio waves used in radio broadcasting and the FM band, the MR device is placed in a Faraday cage to insulate it from external RF signals which could alter the signal. The copper Faraday cage completely encases the MR scanner. Openings through this cage need to be carefully designed to avoid canceling out the shielding effect
4. Computer systems
Coordination of the various stages of the examination and sequences, the spectrometer, image reconstruction and post-processing are all controlled by an internal computer system and by data acquisition and post-processing consoles.
The main performance criteria for computer equipment for an MRI device are processing speed and ergonomics.
5. MRI Safety and precautions
Metal and magnetic field
Due to the presence of a strong magnetic field, certain materials may present a functional or even a vital risk:
• Projectile effect (attraction by a static magnetic field and acceleration, with speeds of up to several meters per second): ferromagnetic material (if in doubt about the ferromagnetic nature of a metal object, a test can be carried out using a small magnet)
• Displacement of intra-corporeal metallic foreign objects: Intraocular metallic foreign body (metal worker, history of ballistic orbit trauma, old intra-cranial aneurysm clips)
• Perturbed functioning of certain devices: cardiac pacemaker, neurostimulators, cochlear implant, derivation valves.
In regard to prostheses, non ferromagnetic materials with no electrical activity (titanium and its alloys, nitinol, tantalum, etc.) carry no particular risks in relation to magnetic field. For low magnetic prostheses (orthopedic material), a delay of 6 to 8 weeks after implantation is advised to avoid displacing the material.
Heart valves are generally MR compatible.
In all cases, it is advisable to check the MR compatibility of the material (see http://www.mrisafety.com/), particularly when operating in high fields: some devices carry no risks at 1.5 T but can be dangerous at a higher field.
Gradient strength and switching
Rapid switching of the magnetic field gradients can trigger peripheral nerve and muscular stimulation. Stimulation of the heart, which can be dangerous, occurs at a higher level than for the peripheral nerves.
Echo-planar sequences are those most likely to cause this type of adverse effect, as they put the greatest strain on the gradients, with ascents and descents at high frequencies and strengths.
RF and SAR
SAR corresponds to the amount of radiofrequency energy deposited in the patient, which may result in heating. It is measured in W/kg (which explains the need to specify the patient’s weight before the exam).
SAR is proportionate to the square of the strength of the static magnetic field and the square of the flip angle. It can be reduced:
• by using quadrature coils with lower transmission volumes
• by optimizing the sequence parameters (increasing TR, reducing the number of slices, flip angle, echo train length).
SAR standards exist to limit the maximum acceptable dose for patients under MR scanning (IEC 60601-2-33 standard). The safety standards are designed to ensure that no tissue is subjected to a temperature increase of over 1°C.
The other risk from RF exposure is that of skin burns provoked by the induced current in a conducting loop. These burns may occur in contact with electric leads forming a loop (ECG monitoring in particular), metal devices (skin patches, body piercing, dental appliances) or when there is skin contact (hands on the stomach, calves touching).
SAR (Specific Absorption Rate)
SAR value in W/kg is of the type:
with:
• B0 = static magnetic field amplitude
• B1 = RF pulse amplitude
• α = flip angle
• D = cyclic ratio (fraction of the duration of the sequence during which the RF waves are transmitted)
• ρ = density
1. Types of magnets
The design of MRI is essentially determined by the type and format of the main magnet, i.e. closed, tunnel-type MRI or open MRI.
The most commonly used magnets are superconducting electromagnets (figure 2.1). These consist of a coil that has been made superconductive by helium liquid cooling, and immersed in liquid nitrogen. They produce strong, homogeneous magnetic fields, but are expensive and require regular upkeep (namely topping up the helium tank).
In the event of loss of superconductivity, electrical energy is dissipated as heat. This heating causes a rapid boiling-off of the liquid Helium which is transformed into a very high volume of gaseous Helium (quench). In order to prevent thermal burns and asphyxia, superconducting magnets have safety systems: gas evacuation pipes, monitoring of the percentage of oxygen and temperature inside the MRI room, door opening outwards (overpressure inside the room).
Superconducting magnets function continuously. To limit magnet installation constraints, the device has a shielding system that is either passive (metallic) or active (an outer superconducting coil whose field opposes that of the inner coil) to reduce the stray field strength.
Low field MRI also uses:
• Resistive electromagnets, which are cheaper and easier to maintain than superconducting magnets. These are far less powerful, use more energy and require a cooling system.
• Permanent magnets, of different formats, composed of ferromagnetic metallic components. Although they have the advantage of being inexpensive and easy to maintain, they are very heavy and weak in intensity.
To obtain the most homogeneous magnetic field, the magnet must be finely tuned (“shimming”), either passively, using movable pieces of metal, or actively, using small electromagnetic coils distributed within the magnet.
Characteristics of the main magnet
The main characteristics of a magnet are:
• Type (superconducting or resistive electromagnets, permanent magnets)
• Strength of the field produced, measured in Tesla (T). In current clinical practice, this varies from 0.2 to 3.0 T. In research, magnets with strengths of 7 T or even 11 T and over are used.
• Homogeneity
2. Magnetic field gradients
Gradients components
They produce a linear variation in magnetic field intensity in a direction in space. This variation in magnetic field intensity is added to the main magnetic field, which is far more powerful. The variation is produced by pairs of coils, placed in each spatial direction.
The direction of the magnetic field is not modified. By adding them to B0, a linear variation is produced in the total magnetic field amplitude, in the direction to which they are applied (figure 2.3). Their action is considered as homogeneous on a plane perpendicular to the direction of application.
This modifies resonance frequency, in proportion to the intensity of the magnetic field to which they are submitted (in accordance with Larmor’s equation: the stronger the field, the faster they precess). This variation in Larmor frequency also causes a variation and dispersion of spin phases.
Gradient characteristics
Gradient performances are linked to:
• their maximal amplitude (magnetic field variation in mT/m), which determines maximal spatial resolution (slice thickness and field of view)
• their slew rate, corresponding to their switching speed: high slew rates and low rise time are required to switch gradients quickly and allow ultra-fast imaging sequences such as echo planar (EPI)
• their linearity, which must be as perfect as possible within the scanning area.
Eddy currents
The rapid switching of the gradients induces currents in the conducting materials in the vicinity of the gradient coils (cryogenic envelope, electric wires, antennas, homogenization coils, etc.). These induced currents (Eddy current) will oppose the gradient fields and cause a decay in their profile.
There are several methods to reduce the effects of these induced currents:
• Active gradient coil shielding
• Optimizing the electric current profile sent to the gradient coils while ascending and descending to offset the Eddy currents
Moreover, gradient switches produce Lorentz forces causing vibrations in the gradient coils and their supports. These vibrations are the main source of the characteristic MRI noise.
Radiofrequency system
3. Radiofrequency system components
The radiofrequency system comprises the set of components for transmitting and receiving the radiofrequency waves involved in exciting the nuclei, selecting slices, applying gradients and in signal acquisition.
Coils are a vital component in the performance of the radiofrequency system (figure 2.6). In transmission, the goal is to deliver uniform excitation throughout the scanned volume. On reception, the coils must be sensitive and have the best possible signal to noise ratio.
An MR scanner generally contains a « whole body » coil, located in the cylinder of the machine, homogeneously covering the entire scan volume. The sensitive volume of surface coils, being placed in direct contact with the zone of interest, has less depth and is more heterogeneous. However, surface coils offer a better signal to noise ratio and imaging capacity with higher spatial resolution. The homogeneity and sensitive volume of surface coils can be improved by combining them into a phased array. They still have the advantage of a better signal to noise ratio, but at the cost of more complex signal processing.
Quadrature RF coils (circularly polarized coils) consist of at least two coils that are oriented orthogonal to each over (and both are othogonal to B0 axis). They have a better signal to noise ratio than linear RF coils.
Depending on the manufacturers and the type of coil, certain coils can be transmitters, receivers or both.
The radiofrequency channel also comprises analog-digital converters and a spectrometer to receive and analyze the signal.
Optimizing the radiofrequency channel
Optimization of the radiofrequency channel is automated and carried out in several stages prior to an imaging sequence:
• the exact Larmor frequency is set, this being slightly modified by the patient’s presence in the magnetic field
• transmission power is adjusted according to the weight of the patient and the transmit coil, to obtain the desired flip angles
• the receiver gain is adjusted to avoid signal saturation or conversely, weak amplification resulting in a deteriorated signal to noise ratio.
Faraday cage
As the resonance frequency of protons is very close to that of the radio waves used in radio broadcasting and the FM band, the MR device is placed in a Faraday cage to insulate it from external RF signals which could alter the signal. The copper Faraday cage completely encases the MR scanner. Openings through this cage need to be carefully designed to avoid canceling out the shielding effect
4. Computer systems
Coordination of the various stages of the examination and sequences, the spectrometer, image reconstruction and post-processing are all controlled by an internal computer system and by data acquisition and post-processing consoles.
The main performance criteria for computer equipment for an MRI device are processing speed and ergonomics.
5. MRI Safety and precautions
Metal and magnetic field
Due to the presence of a strong magnetic field, certain materials may present a functional or even a vital risk:
• Projectile effect (attraction by a static magnetic field and acceleration, with speeds of up to several meters per second): ferromagnetic material (if in doubt about the ferromagnetic nature of a metal object, a test can be carried out using a small magnet)
• Displacement of intra-corporeal metallic foreign objects: Intraocular metallic foreign body (metal worker, history of ballistic orbit trauma, old intra-cranial aneurysm clips)
• Perturbed functioning of certain devices: cardiac pacemaker, neurostimulators, cochlear implant, derivation valves.
In regard to prostheses, non ferromagnetic materials with no electrical activity (titanium and its alloys, nitinol, tantalum, etc.) carry no particular risks in relation to magnetic field. For low magnetic prostheses (orthopedic material), a delay of 6 to 8 weeks after implantation is advised to avoid displacing the material.
Heart valves are generally MR compatible.
In all cases, it is advisable to check the MR compatibility of the material (see http://www.mrisafety.com/), particularly when operating in high fields: some devices carry no risks at 1.5 T but can be dangerous at a higher field.
Gradient strength and switching
Rapid switching of the magnetic field gradients can trigger peripheral nerve and muscular stimulation. Stimulation of the heart, which can be dangerous, occurs at a higher level than for the peripheral nerves.
Echo-planar sequences are those most likely to cause this type of adverse effect, as they put the greatest strain on the gradients, with ascents and descents at high frequencies and strengths.
RF and SAR
SAR corresponds to the amount of radiofrequency energy deposited in the patient, which may result in heating. It is measured in W/kg (which explains the need to specify the patient’s weight before the exam).
SAR is proportionate to the square of the strength of the static magnetic field and the square of the flip angle. It can be reduced:
• by using quadrature coils with lower transmission volumes
• by optimizing the sequence parameters (increasing TR, reducing the number of slices, flip angle, echo train length).
SAR standards exist to limit the maximum acceptable dose for patients under MR scanning (IEC 60601-2-33 standard). The safety standards are designed to ensure that no tissue is subjected to a temperature increase of over 1°C.
The other risk from RF exposure is that of skin burns provoked by the induced current in a conducting loop. These burns may occur in contact with electric leads forming a loop (ECG monitoring in particular), metal devices (skin patches, body piercing, dental appliances) or when there is skin contact (hands on the stomach, calves touching).
SAR (Specific Absorption Rate)
SAR value in W/kg is of the type:
with:
• B0 = static magnetic field amplitude
• B1 = RF pulse amplitude
• α = flip angle
• D = cyclic ratio (fraction of the duration of the sequence during which the RF waves are transmitted)
• ρ = density
