The quality of an MR image depends on several factors:
- Spatial resolution and image contrast
- Signal to noise ratio (and contrast to noise ratio)
- Artifacts
La résolution spatiale correspond à la "finesse" de l’image, c’est-à-dire à la taille du plus petit détail que l’on pourra détecter. Ainsi, plus les voxels de signal enregistrés seront petits, plus la résolution spatiale sera élevée. Le volume du voxel est défini par la dimension de la matrice (256 x 256 ou 512 x 512 etc..), le champ de vue (10 cm, 20 cm, etc.…), et l’épaisseur de coupe.
Spatial resolution corresponds to the size of the smallest detectable detail. The smaller the voxels are, the higher the potential spatial resolution will be. Voxel volume is determined by the matrix size (256 x 256 or 512 x 512 etc..), the field of view (10 cm, 20 cm, etc....), and slice thickness.
Image contrast varies with the type of pulse sequence and its parameters. Tissue-contrast is also modified by pre-saturation pulses or contrast agents.
Image contrast and signal weighting have to be adapted to the imaging objectives: anatomy, edema, tissue-characterization (fat, hemorrhage, water), vascularization...
Signal-to-noise ratio
Noise is like interferences which present as a irregular granular pattern. This random variation in signal intensity degrades image information. The main source of noise in the image is the patient's body (RF emission due to thermal motion). The whole measurement chain of the MR scanner (coils, electronics...) also contributes to the noise. This noise corrupts the signal coming from the transverse magnetization variations of the intentionally excited spins (on the selected slice plane).The signal to noise ratio (SNR) is equal to the ratio of the average signal intensity over the standard deviation of the noise.
The signal to noise ratio depends both on some factors that are beyond the operator's control (the MR scanner specifications and pulse sequence design) and on factors that the user can change:
- Fixed factors : static field intensity, pulse sequence design, tissue characteristics
- Factors under the operator's control
- RF coil to be used
- Sequence parameters : voxel size (limiting spatial resolution), number of averagings, receiver bandwidth
RF coil
The smaller the sensitive volume of a coil, the lower the noise from the adjacent structures of the selected slice plane which it can detect, and the better the signal to noise ratio will be.A local coil, or better, a surface coil have a higher signal to noise ratio than a body coil.
Paramètres de séquence
Voxel volume
The signal comes from the excited protons on the selected slice plane. The number of spins in parallel state in excess is proportional to the static magnetic field intensity. The larger the field intensity is, the higher the excess number of spins in parallel state (available to make the MR signal) will be. Thus, the signal intensity varies almost linearly with the main field intensity.Assuming a uniform proton density, the number of excited spins is proportional to the voxel size and so is the signal intensity. The signal goes up linearly with the voxel size.
To sum up, MRI is a compromise between:
- Spatial resolution:limited by the voxel size which is determined by the matrix size, the field of view and slice thickness
- Signal to noise ratio:depending on the voxel size, the number of averagings and the receiver bandwidth
- Total scan time
Number of excitations
When the number of excitations (or averagings) for the same slice increases:- The signal is identical for each measure
- The noise is random and is not the same for each measure
In other words, the average signal remains constant, but the average noise goes down with the square root of the number of excitations.
The signal to noise ratio goes up with the square root of the number of excitations.
En cumulant les mesures pour une même coupe, le signal augmente de façon proportionnelle au nombre de mesure, alors que le bruit ne va augmenter que proportionnellement à la racine carré du nombre de mesures. En moyennant plusieurs mesures pour une même coupe, la moyenne du signal reste donc constante alors que la variabilité du bruit diminue.
Le rapport signal / bruit augmente proportionnellement à la racine carré du nombre de mesures.
Given a voxel size and static field strength, the number of excited spins is defined and so is the amount of MR signal. The readout of the MR signal can take more or less time, depending on the receiver bandwidth. The relation between the receiver bandwidth and the strength of the readout gradient is such that:
- a broad bandwidth corresponds to a fast sampling of the MR signal and a high-intensity readout gradient
- a narrow bandwidth corresponds to a slow sampling of the MR signal and a low-intensity readout gradient
To sum up, MRI is a compromise between
- Spatial resolution:limited by the voxel size which is determined by the matrix size, the field of view and slice thickness
- Signal to noise ratio:depending on the voxel size, the number of averagings and the receiver bandwidth
- Total scan time
Receiver bandwith
Given a voxel size and static field strength, the number of excited spins is defined and so is the amount of MR signal. The readout of the MR signal can take more or less time, depending on the receiver bandwidth. The relation between the receiver bandwidth and the strength of the readout gradient is such that:- a broad bandwidth corresponds to a fast sampling of the MR signal and a high-intensity readout gradient
- a narrow bandwidth corresponds to a slow sampling of the MR signal and a low-intensity readout gradient
To sum up, MRI is a compromise between:
- Spatial resolution:limited by the voxel size which is determined by the matrix size, the field of view and slice thickness
- Signal to noise ratio:depending on the voxel size, the number of averagings and the receiver bandwidth
- Total scan time
Pendahuluan.
Nuclear Magnetic Resonance
Summary
NMR
Learning objectives
After reading this chapter, you should be able:- To describe magnetic properties of hydrogen nuclei: spin, precession, Larmor frequency
- To present the origin of the net magnetization
- To explain the nuclear magnetic resonance phenomenon
- To differentiate spin-lattice relaxation from spin-spin relaxation
- To define relaxation times T1 et T2
Key points
Within a magnetic field B0, the sum of spins is a net magnetization aligned with B0. This macroscopic magnetization results from a slight excess of spins in parallel state and a null transverse magnetization due to spins being out of phase.Precession frequency (Larmor frequency) of protons is proportional to field strength intensity.
A RF pulse that matches the precession frequency affects the spin equilibrium : there is an exchange of energy and a tip down of the net magnetization vector. Flip angle depends on intensity, waveform and duration of RF pulse.
Relaxation is the dynamic physical process in which the system of spins returns to equilibrium. Relaxation can be broken down into:
- Recovery of longitudinal magnetization, aligned with B0, following an exponential curve characterized by time constant T1
- Decay of transverse magnetization, due to spins getting out of phase, according to an exponential curve characterized by time constant T2.
References
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Nuclear spin
Hydrogen nuclei (protons) have magnetic properties, called nuclear spin. They behave like tiny rotating magnets, represented by vectors.The sum of all the tiny magnetic fields of each spin is called net magnetization or macroscopic magnetization. Normally, the direction of these vectors is randomly distributed. Thus, the sum of all the spins gives a null net magnetization.
Within a large external magnetic field (called B0), nuclear spins align with the external field. Some of the spins align with the field (parallel) and some align against the field (anti-parallel).
Main nuclei imaged in human MRI
- In clinical MRI, Hydrogen is the most frequently imaged nucleus due to its great abundance in biological tissues.
- Other nuclei such as 13C, 19F, 31P, 23Na have a net nuclear spin and can be imaged in MRI. However, they are much less abundant than hydrogen in biological tissues and require a dedicated RF chain, tuned to their resonance frequency.
Precession and Larmor frequency
Spins wobble (or precess) about the axis of the BO field so as to describe a cone. This is called precession. Spinning protons are like dreidles spinning about their axis. Precession corresponds to the gyration of the rotating axis of a spinning body about an intersecting axis.The resonance frequency, called Larmor frequency (ω0) or precessional frequency, is proportional to the main magnetic field strength: ω0 = γ B0.
Net magnetization
The magnetic vector of spinning protons can be broken down into two orthogonal components: a longitudinal or Z component, and a transverse component, lying on the XY plane.
Precession corresponds to rotation of the transverse component about the longitudinal axis.
Within the B0 magnetic field, there are more spins aligned with the field (parallel - low energy state) than spins aligned against the field (anti-parallel - high energy state). Due to this slight excess of parallel spins, net magnetization (macroscopic magnetization) has a longitudinal component (along the Z axis) aligned with B0.
As spins do not rotate in phase, the sum of all the microscopic transverse magnetizations of each spin is a null transverse macroscopic magnetization.
Nuclear Magnetic Resonance
Exchange of energy between two systems at a specific frequency is called resonance. Magnetic resonance corresponds to the energetic interaction between spins and electromagnetic radiofrequency (RF).Only protons that spin with the same frequency as the electromagnetic RF pulse will respond to that RF pulse. There is a modification of spin equilibrium and absorption of electromagnetic energy by atomic nuclei, which is called excitation. When the system returns from this state of imbalance to equilibrium (relaxation), there is an emission of electromagnetic energy.
Excitation
Excitation modifies energy levels and spin phases. At the quantum level, a single proton jumps to a higher energy state (from parallel to anti-parallel). The consequence on the macroscopic net magnetization vector is a spiral movement down to the XY plane.In a rotating frame of reference, the net magnetization vector tips down during excitation. The flip angle is in function of the strength and duration of the electromagnetic RF pulse.
The net magnetization vector can be broken down into a longitudinal component (along the Z axis, aligned with B0), and a transverse component, lying on the XY plane.
During excitation, longitudinal magnetization decreases and a transverse magnetization appears (except for a 180° flip angle).
Longitudinal magnetization is due to a difference in the number of spins in parallel and anti-parallel state. Transverse magnetization is due to spins getting into phase coherence.
If we consider an excitation with a 90° flip angle, when the RF transmitter is turned off:
- There is no longitudinal magnetization (equal proportion of parallel and anti-parallel spins)
- A transverse magnetization exists (all spins are in phase : complete phase coherence)
- The net magnetization vector tips down during excitation but the microscopic spin magnetization vectors do not. Modifications of the energy state and phase of spins depend on intensity, waveform and duration of RF pulse.
- Longitudinal magnetization is due to a difference in the number of spins in parallel and anti-parallel state.
- Transverse magnetization is due to spins getting more or less into phase.
Relaxation and its characteristics: T1 and T2 times
Return to equilibrium of net magnetization is called Relaxation. During relaxation, electromagnetic energy is retransmitted: this RF emission is called the NMR signal. Relaxation combines 2 different mechanisms:- Longitudinal relaxation corresponds to longitudinal magnetization recovery
- Transverse relaxation corresponds to transverse magnetization decay
Longitudinal relaxation is due to energy exchange between the spins and surrounding lattice (spin-lattice relaxation), re-establishing thermal equilibrium. As spins go from a high energy state back to a low energy state, RF energy is released back into the surrounding lattice.
The recovery of longitudinal magnetization follows an exponential curve. The recovery rate is characterized by the tissue-specific time constant T1. After time T1, longitudinal magnetization has returned to 63 % of its final value. With a 1.5 T field strength, T1 values are about 200 to 3000 ms. T1 values are longer at higher field strengths.
Transverse relaxation results from spins getting out of phase. As spins move together, their magnetic fields interact (spin-spin interaction), slightly modifying their precession rate. These interactions are temporary and random. Thus, spin-spin relaxation causes a cumulative loss in phase resulting in transverse magnetization decay.
Transverse magnetization decay is described by an exponential curve, characterized by the time constant T2. After time T2, transverse magnetization has lost 63 % of its original value.
T2 is tissue-specific and is always shorter than T1. Transverse relaxation is faster than longitudinal relaxation.
T2 values are unrelated to field strength.
MRI instrumentation and MRI safety
Summary
Instrumentation and safety
- Magnets
- Gradients
- Radiofrequency system
- Computer system
- MR safety
- Directive européenne sur les champs magnétiques
Learning objectives
After reading this chapter, you should be able:- To present the different types of main magnet, their advantages and drawbacks
- Describe the role of liquid Helium and its safety implications (quench)
- Characterize a magnetic field gradient, its specifications and performance
- Describe the Eddy current effects
- Explain the origin of acoustic noise during MRI scanning
- Specify the components of the radiofrequency channel and the different types of antenna
- List the materials at risk and the precautions prior to an MRI examination
- Explain the origin of peripheral nerve stimulation during an MRI examination
- Present the factors affecting SAR and how to reduce it
Key points
In an MRI device at 1.5 T, the magnet is superconducting, cooled by liquid Helium. Quench corresponds to liquid Helium evaporation after heating. The large volume of gaseous Helium thus given off causes a risk of thermal burns and asphyxia. An adapted evacuation system and safety arrangements in the examination room should prevent such risks in any MRI installation.Gradients are characterized by amplitude, slew rate and linearity. Switching gradients cause induced currents (Eddy currents) and the acoustic noise of the MRI. They can provoke peripheral nerve stimulation, especially in echo-planar sequences.The quality of the radiofrequency system and coils is crucial. There are coils for each type of exploration and organ studied.Ferromagnetic materials carry a projectile risk effect inside the MR scanner.Intra-ocular metallic foreign bodies, intracranial aneurysm clips, cardiac pacemakers and cochlear implants are generally counter-indicated for MRI. It is advisable to check their MR compatibility and for any counter-indications prior to the examination.The SAR limit must not be exceeded, and sequences must be adapted accordingly (coil, TR, number of slices, flip angle, echo train).References
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MRI Main magnet
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.
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
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
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.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.
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.
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).
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:
- 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
Directive européenne sur les champs magnétiques (2004/40/EC) et IRM
La directive 2004/40/CE1 concerne les prescriptions minimales de sécurité et de santé relatives à l'exposition des travailleurs aux risques dus aux agents physiques (champs électromagnétiques).Elle vise à introduire des seuils pour protéger les travailleurs exposés, dans le cadre de leur travail, aux risques dus aux champs électromagnétiques.
Or il n’a pas été tenu compte dans cette directive de l’utilisation des champs électromagnétiques en médecine et notamment en Imagerie par Résonance Magnétique. Dans les limites fixées par la directive, les médecins et le personnel soignant qui utilisent des appareils d’IRM ne pourraient plus travailler dans des conditions légales.
Dans la plupart des pays européens, les organismes scientifiques et sociétés savantes dans le domaine de l’imagerie et de la santé ont demandé un report de la transposition de la directive dans le droit national (dont la date limite est avril 2008), ainsi qu’une modification de cette directive pour prendre en compte les particularités et bénéfices de l’IRM.
NMR signal and MRI contrast
Summary
MRI signal contrast
- Signal recording
- 90° pulse
- 180° pulse
- Spin echo, TR, TE
- TR and T1-weighting
- TE and T2-weighting
- Signal weighting
- Tissue contrast
Learning objectives
- To describe how the NMR signal is recorded
- To define the free induction decay
- To explain differences between T2 and T2*
- To describe the role of the 180° rephasing pulse
- To state the stages of a spin echo sequence
- To understand the relation between TR, TE and T1-, T2-, and DP signal weightings
- To give common values of T1, T2, TR and TE times
Key points
- Coils are only sensitive to variations of transverse magnetization vector. After a 90° RF pulse, the Free Induction Decay (FID) signal is oscillating at resonance frequency and signal enveloppe is a decay curve described as an exponential curve, depending on tissue-specific spin-spin relaxation and static field inhomogeneities. This decay is characterized by time constant T2*. T2* is always shorter than T2.
- The 180° RF pulse reverses dephasing due to static field inhomogeneities (T2* effects) but not random spin-spin relaxation (T2 effects, tissue-specific). Spin Echo sequence requires an excitation pulse (90° RF pulse) and a 180° rephasing pulse. Time between 90° pulse and 180° pulse is TE/2. MR Signal is acquired at echo time TE, when signal of the echo is the strongest. The signal enveloppe joining maximums of echos after 180° RF pulses is corresponding to the pure T2 decay curve.
- The 90° - 180° RF pulses sequence must be repeated as many times as the number of lines in the data matrix. The time between each 90° RF pulse (excitation pulse) is called Repetition Time (TR).
- You must keep in mind that tranverse relaxation (transverse magnetization decay, producing MR signal) and longitudinal relaxation (longitudinal magnetization recovery) are simultaneous. The longer the TR is, the more longitudinal magnetization will recover.
- With a spin echo sequence :
- TR modifies T1-weighting : the longer is the TR, the more T1-weigthed the image is
- TE modifies T2-weighting : the shorter is the TE, the less T2-weigthed the image is
- A short TR court and a short TE court give a T1-weighted image.
- A long TR long and a long TE long give a T2-weighted image.
- A long TR and a short TE give a PD-weighted image.
References
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Signal recording
A magnet is a magnetic dipole and it can be represented by a magnetic vector. A moving magnetic field induces a current in a loop of wire. For example, the rotating magnet below induces a sinusoidal current that can be recorded.MRI coils can be used for transmitting and/or receiving. As it is not possible to receive RF signal in the same axis as B0, coils are only sensitive to variations of transverse magnetization vector. 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.
90° RF pulse
After a 90° RF pulse, net magnetization tips down so that longitudinal magnetization has disappeared and transverse magnetization has appeared.
Once the RF transmitter is turned off, relaxation happens :
- transverse magnetization decays
- longitudinal magnetization recovers
- protons re-radiate the absorbed energy
In absence of any magnetic gradient, this signal is called Free Induction Decay (FID). FID signal decays faster than T2 would predict and decreases exponentially at characteristic time constant T2*.
T2* takes into account :
- tissue specific spin-spin relaxation (random interactions between spins) responsible for pure T2decay
- static inhomogeneities in magnetic fields which accelerate spins dephasing
180° RF pulse
A 180° RF pulse can rephase spins and reverse static field inhomogeneities.After a 90° RF pulse, spins dephase and transverse magnetization decreases. If we apply a 180° RF pulse, spins rephase and transverse magnetization reappears.
How can a 180° RF pulse rephase spins? Consider the following race:
- Once the race starts (the relaxation begins), the turtle and the rabbit are at the same place (the starting line): they are in phase.
- As the rabbit runs faster, there is a distance between him and the turtle: they dephase.
- Then both have to turn around and go back (180° RF pulse)
- Assuming they are both going at the same speed as before, they arrive at the same time at the starting (finish) line : they rephase.
The 180° RF pulse restores phase coherence:
- After the 90° RF pulse spins dephase (during a time defined as TE/2)
- After the 180° RF pulse, spins are back in phase at time TE after the 90° RF pulse
- Then they dephase again.
The signal enveloppe joining maximums of echos after 180° RF pulses is corresponding to the pure T2 decay curve.
Spin echo, TR, TE
Spin Echo sequence is based on repetition of 90° and 180° RF pulses. Spin Echo sequence have two parameters:- Echo Time (TE) is the time between the 90° RF pulse and MR signal sampling, corresponding to maximum of echo. The 180° RF pulse is applied at time TE/2.
- Repetition Time is the time between 2 excitations pulses (time between two 90° RF pulses).
Each tissue has a specific proton density, T1 and T2 time. The NMR signal depends on these 3 factors.
After time T1, longitudinal magnetization has returned to 63 % of its final value. T1 defines the recovery rate of longitudinal magnetization.
For example, here are longitudinal magnetization recovery curves for 2 tissues A and B with different T1s.
For example, here are transverse magnetization decay curves for 2 tissues A and B with different T2s.
Let's consider 2 tissues A and B with different T1s. If TR is very long, even if tissue A has a longuer T1 than tissue B, the longitudinal magnetization of both tissues will recover completely before the next excitation.
Thus, the transverse magnetization amplitude will be the same for both tissues after each excitation.
TR and T1-weighting
If TR is short and if tissue A has a longer T1 than tissue B, the longitudinal magnetization of tissue A will recover less than the longitudinal magnetization of tissue B.Thus, the transverse magnetization amplitude of tissue B will be higher after the next excitation.
In the graphs below :
- The first part of the following curves corresponds to the longitudinal magnetization recovery after the first excitation.
- TR is the delay between excitations.
- The second part corresponds to the transverse magnetization decay after the second excitation.
- The MR signal is acquired at time TE after excitation.
TE and T2-weighting
By setting the TR to long values, the T1 effect on tissue contrast will be reduced. If TE is long enough, differences in transverse relaxation will alter tissue contrast (the T2 effect).(But if TE is too long, the signal will have disappeared !)
Signal weighting (T1, T2, PD) and sequences parameters : TR, TE
- A long TR and short TE sequence is usually called Proton density -weighted
- A short TR and short TE sequence is usually called T1-weighted
- A long TR and long TE sequence is usually called T2-weighted
For example:
- A tissue with a long T1 and T2 (like water) is dark in the T1-weighted image and bright in the T2-weighted image.
- A tissue with a short T1 and a long T2 (like fat) is bright in the T1-weighted image and gray in the T2-weighted image.
- Gadolinium contrast agents reduce T1 and T2 times, resulting in an enhanced signal in the T1-weighted image and a reduced signal in the T2-weighted image.
T1 (ms)
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T2 (ms)
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Water
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3000
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3000
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Gray matter
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810
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100
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White matter
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680
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90
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Liver
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420
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45
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Fat
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240
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85
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Gadolinium
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Reduces T1
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Reduces T2
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- TE is always shorter than TR
- A short TR = value approximately equal to the average T1 value, usually lower than 500 ms
- A long TR = 3 times the short TR, usually greater than 1500 ms
- A short TE is usually lower than 30 ms
- A long TE = 3 times the short TE, usually greater than 90 ms
Don't forget: a good MRI sequence gives high tissue contrast but lasts the shortest time possible!
Basics of tissue contrast in MRI
To distinguish different tissues, we need to obtain contrast between them. Contrast is due to differences in the MR signal, which depend on the T1, T2 and proton density of the tissues and sequence parameters.The higher the signal is, the brighter it will appear on the MR image. Interpretation is based on analysis of tissue contrast, for given signal weightings (T1, T2, T2* or PD).
MR image could be compared to the representation of a painting with only 2 colors. For example, red would correspond to the T1 effect, yellow to the T2 effect, and pigment density to proton density. If we change the TR and TE, we can see better the red or yellow part of the painting better.
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