Magnetic resonance microscopy

Magnetic Resonance Microscopy (MRM, µMRI) is Magnetic Resonance Imaging (MRI) at a microscopic level. A strict definition is MRI having voxel resolutions of better than 100µm³ ^[1].

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Differences between MRI and MRM

• Resolution: 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 to fit into a patient. 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 gradient: High gradient focus the magnetic resonance in a smaller volume (smaller point spread function), 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 (RF) coil used in MRM does not allow ultrahigh gradient.
• Sensitivity: Because the voxels for MRM can be 100,000 times smaller than those in MRI, the signal will be 100,000 times weaker ^[2].

Alternative MRM

Magnetic Resonance Force Microscopy (MRFM) is claimed to have nm³-scale resolutions. It improves the sensitivity issue by introducing microfabricated cantilever to measure tiny signals. The magnetic gradient is generated by a micrometre-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 at cryogenic temperatures, MRFM can be only used for solid state matters.

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