Magnetic resonance microscopy
Magnetic resonance microscopy (MRM, µMRI) is magnetic resonance imaging (MRI) at a microscopic level down to the scale of 5-10 µm³. The first definition of MRM was MRI having voxel resolutions of better than 100 µm³.
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
- Resolution: Medical MRI resolution is typically about 1 mm³; the desired resolution of MRM is 100 µm³ or smaller to 10 µm³, 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 (for nuclear magnetic resonance; electron magnetic resonance commonly operates at much higher frequencies). The major barriers for practical MRM include:
- Magnetic field gradient: High gradient focus of 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) 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.
Magnetic Resonance Force Microscopy (MRFM) has nm³-scale resolution. It improves the sensitivity issue by introducing microfabricated cantilevers 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 early phase of development. Because the specimen needs to be in a high vacuum at cryogenic temperatures, MRFM can be used only for solid state materials.
- R. Sharma, Microimaging of hairless rat skin by magnetic resonance at 900 MHz. Magnetic Resonance Imaging. 27(2):240-255, 2009
- P. Glover and P. Mansfield, Limits to magnetic resonance microscopy, Rep. Prog. Phys. 65 1489–1511, 2002
- R. Sharma, A. Sharma. 21.1 Tesla Magnetic Resonance Imaging Apparatus and Image Interpretation: First Report of a Scientific Advancement. Recent Patents in Medical Imaging.1(2):89-104, 2011
- R. Sharma, B.R. Locke. Jet Fuel Toxicity: Skin Damage measured by 900 MHz MRI Skin Microscopy and Visualization by 3D MRI Image Processing. Magn Reson Imaging.30(1):1013-1048, 2010
- R. Maronpot. Applications of Magnetic Resonance Microscopy, Toxicologic Pathology, 32(Suppl. 2):42–48, 2004
- R. Sharma,Physical Basis of Gadolinium Induced Skin Nephrofibrosis: Testing by Gadolinium-Protein Targeting Assay and Iron Oxide Nanoparticle Based Magnetic Resonance Microscopy. ISRN Dermatology. 1;---
- R.Sharma,Skin age testing criteria: characterization of human skin structures by 500 MHz MRI multiple contrast and image processing,Physics in Medicine and Biology.55, 14, 2010
- Introduction to Magnetic Resonance Microscopy Auditory Research Laboratory at the Univ. of North Carolina.