Controls How Much Longitudinal Relaxation Takes Place Before the Protons Are Flipped Again
The body is largely composed of water molecules. Each water molecule has 2 hydrogen nuclei or protons. MRI takes reward of the high prevalence of hydrogen in the body and the magnetic properties of the proton in a hydrogen atom. Hydrogen atoms induce a modest magnetic field due to the spin of this atom's proton. When a person goes inside the powerful magnetic field of the scanner, the magnetic moments (the mensurate of its tendency to align with a magnetic field) of some of these protons changes, and aligns with the direction of the field.
The magnetic field in an magnetic resonance imaging (MRI) scanner is generated by surrounding a coil of wire with super cooling fluids (liquid helium and liquid nitrogen) lowering the temperature to about 10°K (-263°C or -441°F). Electrical current in the coil moves very fast creating the extremely large magnetic field.
Magnetic field strengths are measured in units of gauss (One thousand) and Tesla (T). I Tesla is equal to 10,000 gauss. The earth'southward magnetic field is nigh 0.5 gauss. The strength of electromagnets used to pick up cars in junk yards is about the field strength of MRI machines (one.five to 2.0T). The Bo in MRI refers to the master magnetic field and is measured in Tesla (T). The majority of MRI systems in clinical employ are between i.5T and 3T. Altering the field strength will affect the Larmour frequency at which the protons precess.
The protons placed in a magnetic field have the interesting property in that they will blot energy at specific frequencies, and so re-emit the energy at the same frequency. To measure the net magnetization in a encephalon scan, a curlicue tin can exist placed around the caput can be used to both to generate electromagnetic waves and measure out the electromagnetic waves that are emitted from the head in response.
Proton density (PD) is the concentration of protons in the tissue in the form of water and macromolecules (proteins, fat, etc). The T1 and T2 relaxation times define the way that the protons revert dorsum to their resting states subsequently the initial RF pulse. The nearly common outcome of period is loss of signal from rapidly flowing arterial claret.
And so when the patient is showtime placed in the static magnetic field that the machine creates, MRI takes advantage of that high prevalence of hydrogen in the body and the magnetic properties of the proton in a hydrogen atom. Hydrogen atoms induce a small magnetic field due to the spin of this atom's proton. Hydrogen protons within the patient'south body will and then align to the magnetic field which is typically 30 to 60 thou times stronger than the magnetic field of the earth.
A radio frequency (RF) pulse is then emitted from the scanner, tuned to a specific range of frequencies at which hydrogen protons precess. This results in some of the hydrogen protons being "knocked" 180° out of alignment with the static magnetic field and being forced into phase with other hydrogen protons. The echo time refers to time betwixt the application of RF excitation pulse and the peak of the bespeak induced in the coil and is measured in milliseconds.
As the energy from the RF pulse is dissipated, the hydrogen protons volition return to alignment with the static magnetic field. The MRI signal is derived from the hydrogen protons as they move back into alignment with the magnetic field, and autumn out of "phase" with each other. The actual process is much more complicated, but is broken down into T1 relaxation and T2 decay. The MRI signal is and so broken down and spatially located to produce images.
Magnetic Resonance
A MRI is an epitome from a scanner that actually measures "magnetic resonance." A stiff magnetic field is placed across the tissue along the direction of the bore of the magnet and is referred to as Bo . The magnetic moments inside the tissue will tend to align towards Bo , although because of molecular vibrations and collisions, they volition remain more often than not randomly distributed. After some time, the magnetic moments will accomplish an equilibrium with a small corporeality favoring the direction of Bo . While magnetic resonance tin apply to a large number of different atoms (or even molecules), in clinical MRI we are looking at the magnetic moments of the hydrogen nuclei (protons), in the tissue. Hydrogen is used, once again, because information technology has a very high abdundance in the body, among other characteristics.
Nuclei have an intrinsic quantum property chosen spin. When a magnetic field is imposed on the nucleus of an atom, this nuclear spin will orient itself according to this field, and so our z-axis can now be the direction of the magnetic field, for convenience.
Spin
In classical physics, a rotating object possesses a property known as angular momentum. Angular momentum is a form of inertia, reflecting the object's size, shape, mass, and rotational velocity. It'due south typically represented as a vector (L) pointing along the axis of rotation.
Spin is a quantum mechanical intrinsic property of unproblematic particles. It's very hard to imagine this property, and the notion of bodily rotation can be somewhat helpful. However, information technology's wise to separate this notion of a spinning particle from the breakthrough mechanical property we call spin. Although spin is a grade of angular momentum, an uncomplicated particle with spin doesn't mean it's rotating; particles with spin merely have spin. For example, although an electron has mass, it's indicated to be a "indicate particle", occupying no book of infinite at all.
How can we imagine an electron rotating? Diagrams and explanations of spin and its consequences can assist, merely nosotros must be careful not to misfile quantum mechanical (quantized athwart momentum) and classical (rotating particle) explanations of MRI.
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Atomic and subatomic particles posses a corresponding property known equally spin or spin angular momentum. Protons, neutrons, whole nuclei, and electrons all possess spin and are often represented as tiny spinning balls. Although inaccurate, this isn't a terribly bad way to think most spin as long as you lot don't take the analogy likewise far.
Several key differences should be recognized:
| • | The particle is not actually spinning or rotating. |
| • | Spin, similar mass, is a fundamental holding of nature and doesn't arise from more basic mechanisms. |
| • | Spin interacts with electromagnetic fields whereas classic angular momentum (L) interacts with gravitational fields. |
| • | The magnitude of spin is quantized, pregnant that it can simply take on a limited prepare of discrete values. |
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Precession
In MRI, nosotros are looking at the beliefs of millions and millions of proton-magnets. The net management of their moments is referred to equally the net magnetization vector Thousand. In equilibrium, since more than protons are pointing along Bo , One thousand points in the direction of Bo . This direction is typically referred to as the z-axis. In that location is no net polarization in the x- or y-axes. However, the protons really rotate circular that centrality (known as precession), and so that any one particular proton at any moment in time will be pointing in some management in the xy plane.
The simplest version of an MRI sequence involves a and so-called 90° pulse. This pulse of free energy is exactly enough to rotate the protons xc°, so the net magnetization is rotate from the z-axis, parallel to Bo , into the xy-aeroplane. At that point, Mz, the magnetization along Bo , is 0.
Annotation that you can put in less energy to give a rotation of less than ninety degrees, which is often used in gradient-echo sequences. Alternatively, y'all may want to utilize a 180° pulse to 'flip' the Thou vector into the -z direction; this pulse is twice as long (or strong) equally the 90° pulse and is used for inversion recovery sequences.
Nuclei precess around the magnetic field for essentially the same reasons that tops or gyroscopes precess around a gravitational field:
| • | Both gyroscopes and nuclei possess angular momentum. For the gyroscope, athwart momentum results from a flywheel rotating about its axis. For the nucleus, athwart momentum results from an intrinsic quantum belongings (spin). |
| • | Momentum is also sometimes called inertia. Objects possessing momentum have a tendency to maintain their motility unless acted upon by an external force, like a speeding truck has a cracking deal of (linear) momentum and tin can't easily be induced to modify its speed or management. Athwart momentum behaves similarly, conferring on the nucleus or gyroscope a strong resistance to changing its orientation or direction of rotation. |
| • | Static gravitational and magnetic fields create a torque or "twisting force" acting perpendicular to both the field and the management of the angular momentum. The gyroscope or nucleus doesn't "tip over" only is instead deflected into a circular path perpendicular to the field. |
| • | The resultant circular movement is called precession. Precession occurs at a specific frequency denoted either by ωo (called the angular frequency, measured in radians/sec) or fo (called the cyclic frequency, measured in cycles/sec or Hz). Since 2π radians = 360° = one bike (revolution), angular and cyclic frequencies can easily be converted by the equation: ωo = 2 π fo |
| • | The precession frequency of a gyroscope is a role of the mass and shape of the wheel, the speed of wheel rotation, and the strength of the gravitational field. The precession frequency of a nucleus is proportional to the strength of the magnetic field (Bo ) and the gyromagnetic ratio (γ), a particle-specific constant incorporating size, mass, and spin. This is embodied in the famous Larmor relationship, given by the equation: fo = γ * Bo |
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The spin is represented past the arrow. Notice the tip of the arrow precesses similar to the top of the gyroscope. This spin allows absorption of a photon of frequency νL, which is dependent on the strength of the magnetic field applied to the nulceus. The applied magnetic field is the management that the photons axis will marshal to when it's stiff enough.
Resonance and Larmor Frequency
Protons in a magnetic field have a microscopic magnetization and act like tiny toy tops that wobble as they spin. The rate of the wobbling or precession is the resonance or Larmor frequency (νL). In the magnetic field of an MRI scanner at room temperature, there is approximately the same number of proton nuclei aligned with the master magnetic field Bo as counter aligned. The aligned position is slightly favored, equally the nucleus is at a lower energy in this position. For every one-one thousand thousand nuclei, at that place is about one extra aligned with the Bo field every bit opposed to the field. This results in a net or macroscopic magnetization pointing in the direction of the principal magnetic field. Exposure of private nuclei to RF radiation (B1 field) at the Larmor frequency causes nuclei in the lower energy state to leap into the higher energy state.
In magnetic resonance, the Larmor frequency and is determined past the gyromagnetic ratio γ of the item magnetic moment (in this case, we are looking at the hydrogen nucleus, and γ = 42.58 MHz/T):
νL = γ * Bo
Where νL is the frequency, γ is the gyromagnetic ratio γ/(2π) in units of hertz per tesla (Hz/T), Bo is the magnetic field.
At present if a really high magnetic field is nowadays, this precession is in the RF portion of the spectrum. The atoms are placed in a non-uniform magnetic field. The nuclei of these atoms will have a different Larmor frequency of spin as a upshot of the equation above. As the RF electromagnetic radiation is sent through the patient, the atomic nuclei in the body will blot the energy. This assimilation of energy causes nuclei to modify the management of their spin. You can intuitively understand this with the model shown in a higher place.
If you transmit energy into the arrangement at the resonant frequency, you can change plow protons abroad from pointing along Bo . Afterwards some time, these protons volition "relax" and give off energy to return to the lower energy state. This free energy will be given off at the same frequency, and it's this point that we measure.
Radiofrequency
On a macroscopic level, exposure of an object or person to RF radiation at the Larmor frequency, causes the net magnetization to spiral away from the Bo field. In the rotating frame of reference, the net magnetization vector rotate from a longitudinal position a distance proportional to the time length of the RF pulse. Subsequently a certain length of fourth dimension, the net magnetization vector rotates ninety° and lies in the transverse or x-y plane. It's in this position that the net magnetization tin be detected on MRI. The angle that the net magnetization vector rotates is commonly called the 'flip' or 'tip' angle. At angles greater than or less than 90° there volition yet be a pocket-size component of the magnetization that will exist in the ten-y aeroplane, and therefore be detected.
The recovery of longitudinal magnetization is chosen longitudinal or T1 relaxation and occurs exponentially with a time constant T1. The loss of phase coherence in the transverse plane is called transverse or T2 relaxation. T1 is thus associated with the enthalpy of the spin system, or the number of nuclei with parallel versus anti-parallel spin. T2 on the other hand is associated with the entropy of the system, or the number of nuclei in phase.
When the RF pulse is turned off, the transverse vector component produces an oscillating magnetic field which induces a small current in the receiver scroll. This signal is called the complimentary induction decay (FID).
The RF signal has a frequency equal to the unique resonant frequency of the nuclei, the Larmor frequency. In one case the RF signal is turned off, three basic processes occur:
| • | Absorbed RF Energy is Emitted Nuclei that have absorbed the radio frequency energy volition not remain in their excited state for a long fourth dimension. They return to their initial state, emitting a radio frequency signal to their surroundings. These signals are picked upwards by detectors that are placed all around the trunk. The signals are so compiled using Computed Tomography (CT) techniques into an image. The post-obit two nuclear processes are used to get together a MRI. |
| • | Spin-Lattice Relaxation When a nucleus absorbs a photon at its Larmor frequency, its spin state changes. Yet the nucleus will not stay in this state. Information technology will return to its original state after emitting a photon. The time it takes to exercise this is called the spin-lattice relaxation time, and is given by the constant T1. |
| • | Spin-Spin Relaxation Some other type of relaxation used in MRI is spin-spin relaxation. Because the magnetic field varies, the nuclei's Larmor frequency will vary. Since they spin at unlike frequencies, the nuclei will gradually end upwardly out of phase, or spin at unlike times. MRIs use the loss of betoken due to the phase-difference between these nuclei to aid in creating the image. |
Relaxation
The diverse types of MRI scans that are used (near ordinarily the T1-weighted scan and the T2-weighted browse) measure this relaxation time differently. Computer programs translate the information into cantankerous-exclusive pictures of the h2o in homo tissue. The layer of myelin that protects nerve-cell fibers is fatty and therefore repels water. In the areas where the myelin has been damaged by MS, the fat is stripped away. With the fatty gone, the expanse holds more water, and shows up on an MRI scan as either a brilliant white spot or a darkened area depending on the type of scan that is used. Gadolinium (gd) tin can exist injected intravenously to further enhance the sensitivity of the T1-weighted MRI scan.
MRI image contrast is influenced by several characteristics of tissues and other materials including: T1, T2 and T2* relaxation as well equally spin density, susceptibility furnishings and catamenia effects. Relaxation is the process in which spins release the free energy received from a RF pulse.
T1 and T2 relaxation rates affect the SNR in an prototype. Improvement in the SNR is seen when the TR is increased significantly to nearly three to 5 T1 times. Changing the TR fourth dimension will also affect the T1 weighting of the prototype and the acquisition time. T1 weighting occurs in a brusk TR spin echo sequence because of incomplete recovery of longitudinal magnetization.
The T1 relaxation fourth dimension, besides known equally the spin-lattice relaxation time, is a measure of how rapidly the internet magnetization vector (NMV) recovers to its ground land in the direction of Bo . The return of excited nuclei from the high energy state to the depression energy or ground state is associated with loss of energy to the surrounding nuclei. Nuclear magnetic resonance was originally used to examine solids in the class of lattices, hence the proper noun "spin-lattice" relaxation. Two other forms of relaxation are the T2 relaxation time (spin-spin relaxation) and T2* relaxation.
Gradients
A MRI sequence produces bespeak from all the tissue in the scanner that's within the transmit/receive coils. Without a ways of spatial localization, all you would go is a single number for the entire body. Drs. Lauterbur and Mansfield discovered a way to separate betoken from different parts of the body. In order to understand how they did information technology, and how MR scanners work, you demand to empathize magnetic gradients.
While the main magnetic field of the scanner (Bo ) can't modify, additional smaller magnetic fields tin can exist added with changing electrical fields. If you recollect physics classes, a irresolute electric field produces a magnetic field which is the ground of electromagnets. Each MR scanner has three sets of spatial encoding electric coils to produce magnetic fields in the x, y, and z directions. These coils can exist adjusted to produce not a constant field only a gradient, in other words a magnetic field that changes in strength depending on your position.
These magnetic fields are much weaker than Bo and vary linearly beyond the x, y, or z direction. They can even be turned on in combinations to create a linear gradient in whatever capricious direction in space.
Piece-Selection
At present that nosotros accept gradients, nosotros can split up different parts of anatomy past frequency. Nosotros will showtime with the simplest type of separation: the imaging slice. Call back that protons but exchange energy efficiently if the frequency of the free energy matches their precession frequency. Thus, the 90° and 180° pulses must be sent at the Larmor frequency of the proton. We can combine this with gradients to select a piece of the trunk to epitome.
By turning on the magnetic field gradient, the protons at each position in the body experience a slightly different magnetic field - slightly more or less than Bo . Thus, we go a slope of precession frequencies along the body that differ. By then altering the frequency of our 90° and 180° pulses, nosotros will excite dissimilar protons. The magnetic field gradients are shut to the lodge of several hundredths of a percent over a couple centimeters, so an extremely pocket-sized frequency change will movement the position for the next image slice.
The scanner tin select the particular slice to image by turning on the piece-select gradient and and then altering the frequency of the excitation pulses (90, 180, and whatsoever inversion pulse) to friction match the frequency at the desired slice position. Protons not in the slice will non go excited since their Larmor frequency volition not match the frequency of the pulse, thus they won't efficiently receive energy from the pulse.
Fourier Transform
In social club to understand how to determine spatial localization within a slice (frequency and phase encoding) we need to look at the Fourier transform. Fourier, a French mathematician, realized that all signals, or oscillating functions, can be represented equally a combination of unproblematic sine and cosine waves. Each sine and cosine corresponds to a detail frequency in the betoken. High frequencies correspond to chop-chop irresolute features, while low frequencies (including zero, a constant signal) correspond to slowly changing features in the original betoken. Fourier devised a method for transforming a signal in time, such as music, into the set of frequencies that compose it, and this is known as the Fourier transform.
MRI Measurement
The MRI Measurement consists of the following:
| • | Alignment of the protons in the body with the big magnetic field of the MRI scanner. After a few seconds in the scanner the protons in the patient are aligned with the magnetic field. |
| • | A RF pulse is used to tip the protons out of alignment with the scanner'southward magnetic field. |
| • | In one case out of alignment the magnetic moment of the hydrogen protons can be measured as they rotate past measurement coils (loops of wire) inducing an electrical current. |
| • | The protons are pulled back into alignment with the primary magnetic field decreasing the measurable bespeak. The charge per unit at which this occurs determines the T1 backdrop of a tissue. If the protons in a tissue render to alignment faster than all other tissues then this tissue will exist brightest on a T1-weighted scan. |
| • | While rotating the protons gradually become out of stage with 1 some other decreasing the measurable signal. The rate at which this dephasing occurs determines the T2 properties of a tissue. If the protons in a tissue remain in phase with one another longer than all other tissues then this tissue will be brightest on a T2-weighted browse. |
| • | A proton density (PD) scan minimizes both T1 and T2 contrast to produce an image in which brightness is determined past the number of protons in a voxel. |
Tissue Contrast
The contrast on the MRI can be manipulated past irresolute the pulse sequence parameters. A pulse sequence sets the specific number, force, and timing of the RF and gradient pulses. The two well-nigh important parameters are the TR and the TE. The TR is the time between consecutive 90° RF pulse. The TE is the time between the initial 90° RF pulse and the echo.
Ii controls determine tissue contrast: TR (repetition time) and TE (echo time) of the scan.
| • | Repetition fourth dimension (TR) is the time from the application of an excitation pulse to the application of the adjacent pulse or the time betwixt successive RF pulses. A long repetition time allows the protons in all of the tissues to relax back into alignment with the chief magnetic field. A short repetition fourth dimension will result in the protons from some tissues not having fully relaxed back into alignment earlier the side by side measurement is made decreasing the signal from this tissue. Information technology determines how much longitudinal magnetization recovers between each pulse. It's measured in milliseconds. |
| • | Echo fourth dimension (TE) is the fourth dimension between the application of RF excitation pulse and the peak of the point induced in the scroll. A long echo time results in reduced signal in tissues like white affair and grayness matter since the protons are more probable to become out of phase. Protons in a fluid volition remain in phase for a longer time since they are not constrained by structures such as axons and neurons. A short echo time reduces the amount of dephasing that tin can occur in tissue similar white affair and gray matter. Information technology's measured in milliseconds. |
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