Question: With MRI magnets, what measurements are used to define the field strength?
About the article "Magnetic resonance imaging", section "How MRI works".

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In addition, the greater strength of neodymium magnets has inspired new applications in areas where magnets were not used before, such as magnetic jewelry clasps, children's magnetic building sets (and other neodymium magnet toys) and as part of the closing mechanism of modern sport parachute equipment.
They also are the main metal in the formerly popular desk-toy magnets, "Buckyballs", though some retailers have chosen not to sell them due to child-safety concerns.
The strength and magnetic field homogeneity on neodymium magnets has also opened new applications in the medical field with the introduction of open magnetic resonance imaging (MRI) scanners used to image the body in radiology departments as an alternative to superconducting magnets that use a coil of superconducting wire to produce the magnetic field.
Neodymium magnets are used in the surgically placed Linx anti-reflux system. 
The magnet is the largest and most expensive component of the scanner, and the remainder of the scanner is built around it.
The strength of the magnet is measured in teslas (T).
Clinical magnets generally have a field strength in the range 0.1–3.0 T, with research systems available up to 9.4 T for human use and 21 T for animal systems.
In the United States, field strengths up to 4 T have been approved by the FDA for clinical use.
Just as important as the strength of the main magnet is its precision.
The straightness of the magnetic lines within the center (or, as it is technically known, the iso-center) of the magnet needs to be near-perfect.
This is known as homogeneity.
Fluctuations (inhomogeneities in the field strength) within the scan region should be less than three parts per million (3 ppm).
Three types of magnets have been used:  Permanent magnet: Conventional magnets made from ferromagnetic materials ("e.g.
", steel alloys containing rare earth elements such as neodymium) can be used to provide the static magnetic field.
A permanent magnet that is powerful enough to be used in an MRI will be extremely large and bulky; they can weigh over 100 tonnes.
Permanent magnet MRIs are very inexpensive to maintain; this cannot be said of the other types of MRI magnets, but there are significant drawbacks to using permanent magnets.
They are only capable of achieving weak field strengths compared to other MRI magnets (usually less than 0.4 T) and they are of limited precision and stability.
Permanent magnets also present special safety issues; since their magnetic fields cannot be "turned off," ferromagnetic objects are virtually impossible to remove from them once they come into direct contact.
Permanent magnets also require special care when they are being brought to their site of installation.
Resistive electromagnet: A solenoid wound from copper wire is an alternative to a permanent magnet.
An advantage is low initial cost, but field strength and stability are limited.
The electromagnet requires considerable electrical energy during operation which can make it expensive to operate.
This design is essentially obsolete.
Superconducting electromagnet: When a niobium-titanium or niobium-tin alloy is cooled by liquid helium to 4 K (−269 °C, −452 °F) it becomes a superconductor, losing resistance to flow of electrical current.
An electromagnet constructed with superconductors can have extremely high field strengths, with very high stability.
The construction of such magnets is extremely costly, and the cryogenic helium is expensive and difficult to handle.
However, despite their cost, helium cooled superconducting magnets are the most common type found in MRI scanners today.
Most superconducting magnets have their coils of superconductive wire immersed in liquid helium, inside a vessel called a cryostat.
Despite thermal insulation, sometimes including a second cryostat containing liquid nitrogen, ambient heat causes the helium to slowly boil off.
Such magnets, therefore, require regular topping-up with liquid helium.
Generally a cryocooler, also known as a coldhead, is used to recondense some helium vapor back into the liquid helium bath.
Several manufacturers now offer 'cryogenless' scanners, where instead of being immersed in liquid helium the magnet wire is cooled directly by a cryocooler.
Magnets are available in a variety of shapes.
However, permanent magnets are most frequently 'C' shaped, and superconducting magnets most frequently cylindrical.
However, C-shaped superconducting magnets and box-shaped permanent magnets have also been used.
Magnetic field strength is an important factor in determining image quality.
Higher magnetic fields increase signal-to-noise ratio, permitting higher resolution or faster scanning.
However, higher field strengths require more costly magnets with higher maintenance costs, and have increased safety concerns.
A field strength of 1.0–1.5 T is a good compromise between cost and performance for general medical use.
However, for certain specialist uses ("e.g.
", brain imaging) higher field strengths are desirable, with some hospitals now using 3.0 T scanners. 
To perform a study the patient is positioned within an MRI scanner which forms a strong magnetic field around the area to be imaged.
Most medical applications rely on detecting a radio frequency signal emitted by excited hydrogen atoms in the body (present in any tissue containing water molecules) using energy from an oscillating magnetic field applied at the appropriate resonant frequency.
The orientation of the image is controlled by varying the main magnetic field using gradient coils.
As these coils are rapidly switched on and off they create the characteristic repetitive noises of an MRI scan.
The contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state.
Exogenous contrast agents may be given intravenously, orally or intra-articularly.MRI requires a magnetic field that is both strong and uniform.
The field strength of the magnet is measured in tesla – and while the majority of systems operate at 1.5T commercial systems are available between 0.2T–7T.
Most clinical magnets are superconducting which requires liquid helium.
The lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners for claustrophobic patients.
Principle: Magnetic Resonance Imaging (MRI) exploits the nuclear magnetic alignments of different atoms inside a magnetic field to generate images.
MRI machines consist of large magnets that generate magnetic fields around the target of analysis.
These magnetic fields cause paramagnetic atoms such as hydrogen, gadolinium, and manganese to align themselves in a magnetic dipole along the magnetic fields, created by the radiofrequency (RF) coils inside the MRI machine.
What the machine captures from the subject is the relaxation of the atoms as they return to their normal alignment when the RF pulse is temporarily ceased.
With this data, a computer will generate an image of the subject based on the resonance characteristics of different tissue types.
Strengths: The advantage of micro-MRI is that it has good spatial resolution, up to 100 µm and even 25 µm in very high strength magnetic fields.
It also has excellent contrast resolution to distinguish between normal and pathological tissue.
Micro-MRI can be used in a wide variety of applications, including anatomical, functional, and molecular imaging.
Furthermore, since micro-MRI’s mechanism is based on a magnetic field, it is much safer compared to radiation based imaging modalities such as micro-CT and micro-PET.
Weaknesses: One of the biggest drawbacks of micro-MRI is its cost.
Depending on the magnetic strength (which determines resolution), systems used for animal imaging between 1.5 and 14 teslas in magnetic flux density range from $1 million to over $6 million, with most systems costing around $2 million.
Furthermore, the image acquisition time is extremely long, spanning into minutes and even hours.
This may negatively affect animals that are anesthetized for long periods of time.
In addition, micro-MRI typically captures a snapshot of the subject in time, and thus it is unable to study blood flow and other real-time processes well.
Even with recent advances in high strength functional micro-MRI, there is still around a 10-15 second lag time to reach peak signal intensity, making important information such as blood flow velocity quantification difficult to access.
Cancer research: Micro-MRI is often used to image the brain because of its ability to non-invasively penetrate the skull.
Because of its high resolution, micro-MRI can also detect early small-sized tumors.
Antibody-bound paramagnetic nanoparticles can also be used to increase resolution and to visualize molecular expression in the system.
However, micro-MRI needs to be used in conjunction with other “true” molecular imaging modalities, such as micro-PET and micro-SPECT, in order to image down to the molecular level. 
Superconducting magnets have a number of advantages over resistive electromagnets.
They can generate magnetic fields that are up to ten times stronger than those generated by ordinary ferromagnetic-core electromagnets, which are limited to fields of around 2 T. The field is generally more stable, resulting in less noisy measurements.
They can be smaller, and the area at the center of the magnet where the field is created is empty rather than being occupied by an iron core.
Most importantly, for large magnets they can consume much less power.
In the persistent state (above), the only power the magnet consumes is that needed for any refrigeration equipment to preserve the cryogenic temperature.
Higher fields, however can be achieved with special cooled resistive electromagnets, as superconducting coils will enter the normal (non-superconducting) state (see quench, above) at high fields.
Superconducting magnets are widely used in MRI machines, NMR equipment, mass spectrometers, magnetic separation processes, and particle accelerators.
One of the most challenging use of SC magnets is in the LHC particle accelerator.
The niobium-titanium (Nb-Ti) magnets operate at 1.9 K to allow them to run safely at 8.3 T. Each magnet stores 7 MJ.
In total the magnets store 10.4 GJ.
Once or twice a day, as the protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting bending magnets will be increased from 0.54 T to 8.3 T. The central solenoid and toroidal field superconducting magnets designed for the ITER fusion reactor use niobium-tin (Nb3Sn) as a superconductor.
The Central Solenoid coil will carry 46 kA and produce a field of 13.5 teslas.
The 18 Toroidal Field coils at max field of 11.8 T will store 41 GJ (total?
They have been tested at a record 80 kA.
Other lower field ITER magnets (PF and CC) will use niobium-titanium.
Most of the ITER magnets will have their field varied many times per hour.
One high resolution mass spectrometer is planned to use a 21 Tesla SC magnet.