Magnetic resonance microscopy (MRM, μMRI) is

magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and ...

(MRI) at a microscopic level down to the scale of microns. The first definition of MRM was MRI having

voxel In computing, a voxel is a representation of a value on a three-dimensional regular grid, akin to the two-dimensional pixel. Voxels are frequently used in the Data visualization, visualization and analysis of medical imaging, medical and scient ...

resolutions of better than 100 μm.

Nomenclature

Magnetic resonance microscopy refers to very high resolution MRI imaging (down to nanometer scale, in some cases comparable with histopathology). The term MR microscopy is most widely used by the High Resolution Magnetic Resonance Imaging department at Duke University, headed by Dr. G. Allan Johnson, and the National High Magnetic Field Lab group at AMRIS, University of Florida/Florida State University.

Differences between MRI and MRM

* MRM represent a higher evolution of MRI * MRM employs a much stronger magnetic field, which is conducted on a much smaller scale. * Resolution: Medical MRI resolution is typically about 1 mm; the desired resolution of MRM is 100 μm or smaller to 10 μm, comparable with histology. * Specimen size: Medical MRI machines are designed so that a patient may fit inside. MRM chambers are usually small, typically less than 1 cm³ for the imaging of rats, mice and rodents. BrukerBio Spin Company, Billerica, MA specialises in the supply of different microimaging probes (5 mm – 75 mm) for ex vivo/in vivo imaging of excised biological samples.

Current status of MRM

Although MRI is very common for medical applications, MRM is still developing in laboratories up to resonance frequencies of 1000 MHz^{/sup> (for nuclear magnetic resonance; electron magnetic resonance commonly operates at much higher frequencies). The major barriers for practical MRM include:
* Magnetic field

A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
gradient

In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...
: High gradient focus of magnetic resonance in a smaller volume (smaller point spread function

The point spread function (PSF) describes the response of a focused optical imaging system to a point source or point object. A more general term for the PSF is the system's impulse response; the PSF is the impulse response or impulse response ...
), results in a better spatial resolution. The gradients for MRM are typically 50 to 100 times those of clinical systems. However, the construction of radio frequency

Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
(RF) coils used in MRM does not allow ultrahigh gradients.
* Sensitivity: Because the voxels for MRM can be 1/100,000 of those in MRI, the signal is proportionately weaker.

Alternative MRM

Magnetic resonance force microscopy (MRFM) has nm-scale resolution. It improves the sensitivity issue by introducing microfabricated cantilevers to measure tiny signals. The magnetic gradient is generated by a micrometre

The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...
-scale magnetic tip, yielding a typical gradient 10 million times larger than those of clinical systems. This technique is still in the early phase of development. Because the specimen needs to be in a high vacuum

A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
at cryogenic

In physics, cryogenics is the production and behaviour of materials at very low temperatures.

The 13th International Institute of Refrigeration's (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a univers ...
temperatures, MRFM can be used only for solid state materials.

References

External links

Introduction to Magnetic Resonance Microscopy
Auditory Research Laboratory at the Univ. of North Carolina.

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Magnetic resonance imaging}