Magnetic Resonance Microscopy (MRM, µMRI) is Magnetic Resonance Imaging

Magnetic resonance imaging

Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...

 (MRI) at a microscopic level. A strict definition is MRI having voxel

Voxel

A voxel is a volume element, representing a value on a regular grid in three dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap...

 resolutions of better than 100 µm³ .

Nomenclature

Magnetic resonance Microscopy is a term to refer to very high resolution MRI images (<10 micrometres in some cases). The term is most widely used by the High Resolution Magnetic Resonance Imaging group from Duke University, headed by Dr. G. Allan Johnson.

Differences between MRI and MRM

• Resolution
  Angular resolution
  Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
  
  : Typical medical MRI resolution is about 1 mm³; the desired resolution of MRM is 100 µm³ or smaller.
• Specimen size: Medical MRI machines are designed so that a patient may fit inside. MRM chambers are usually small, typically less than 1 cm³.

Current status of MRM

Although MRI is very common for medical applications, MRM is still developed in laboratories. The major barriers for practical MRM include:

• Magnetic field
  Magnetic field
  A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
  
   gradient
  Gradient
  In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
  
  : High gradient focus the magnetic resonance in a smaller volume (smaller point spread function
  Point spread function
  The point spread function describes the response of an imaging system to a point source or point object. A more general term for the PSF is a system's impulse response, the PSF being the impulse response of a focused optical system. The PSF in many contexts can be thought of as the extended blob...
  
  ), 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
  Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
  
   (RF) coil
  Coil
  A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
  
   used in MRM does not allow ultrahigh gradient.
• Sensitivity
  Sensitivity (electronics)
  The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.Sensitivity is...
  
  : Because the voxels for MRM can be 1/100,000 of those in MRI, the signal will be proportionately weaker .

Alternative MRM

Magnetic Resonance Force Microscopy

Magnetic Resonance Force Microscopy

Magnetic resonance force microscopy is an imaging technique that acquires magnetic resonance images at nanometer scales, and possibly at atomic scales in the future. MRFM is potentially able to observe protein structures which cannot be seen using X-ray crystallography and protein nuclear...

 (MRFM) has nm³-scale resolutions. It improves the sensitivity issue by introducing microfabricated cantilever

Cantilever

A cantilever is a beam anchored at only one end. The beam carries the load to the support where it is resisted by moment and shear stress. Cantilever construction allows for overhanging structures without external bracing. Cantilevers can also be constructed with trusses or slabs.This is in...

 to measure tiny signals. The magnetic gradient is generated by a micrometre

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

-scale magnetic tip, yielding a typical gradient 10 million times larger than those of clinical systems. This technique is still in the beginning stage. Because the specimen need to be in high vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

 at cryogenic temperatures, MRFM can be only used for solid state

Solid

Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...

matters.

External links

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