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Magnetic resonance imaging
From Citizendium, the Citizens' Compendium
In diagnostic imaging, magnetic resonance imaging (commonly known as an MRI scan) has been described as a "non-invasive method of demonstrating internal anatomy, based on the principle that atomic nuclei in a strong magnetic field absorb pulses of radiofrequency energy and emit them as radiowaves - which can be reconstructed into computerized images. The concept includes proton spin tomographic techniques."[1] MRI is commonly used in neuroimaging.
Classification
- Echo-planar imaging allows much faster acquisition of images.
- Functional magnetic resonance imaging uses echo-planar imaging and measures changes in oxygenation status of hemoglobin in response to specific sensory or motor stimulation.[2][3][4]
- Magnetic resonance angiography
- Magnetic resonance spectroscopy[5]
- Cine magnetic resonance imaging is primarily used in cardiology.
- Diffusion magnetic resonance imaging usually uses echo-planar imaging and measures changes in the apparent diffusion coefficient (ADC).
Physical principles
In contrast to x-ray computed tomography which is based on the density of electrons in tissues, MRI is based on several properties of protons.[6][7][8][9][10]Atoms with an odd number of protons, such as hydrogen, inherently create a small magnetic field that can be measured, then manipulated by MRI, then measured again as the tissue relaxes after the external field is turned off.[6]
| Pulse sequence | Description | Application |
|---|---|---|
| Standard pulse sequences | ||
| Spin echo | Proton density (water) | thoracic imaging |
| T1 relaxation time | Spin-lattice (longitudinal) relaxation time. Short repitition time (TR) & echo time (TE) | More solid and less mobile molecules (including lipids, cerebral white matter, yellow bone marrow) are bright. T1 images can be obtained faster. T1 images better display gadolinium contrast medium[8] |
| T2 relaxation time | Spin-spin (transverse) relaxation time. Long TR & TE | Water (including CSF, urine, cysts, abscesses) is bright[8] |
| Other pulse sequences | ||
| DWI (diffusion-weighted imaging) | ||
| ADC (apparent diffusion coefficient) | ||
| GRE (gradient echo) pulse sequences | Blood flow is bright | |
| PWI (perfusion-weighted imaging) | ||
References
- ↑ Anonymous, (2008) Magnetic resonance imaging (English). Medical Subject Headings. U.S. National Library of Medicine.
- ↑ Le Bihan D, Jezzard P, Haxby J, Sadato N, Rueckert L, Mattay V. Functional magnetic resonance imaging of the brain. Ann Intern Med. 1995 Feb 15;122(4):296-303. PMID 7825767
- ↑ Gilman S. Imaging the brain. First of two parts. N Engl J Med. 1998 Mar 19;338(12):812-20. PMID 9504943
- ↑ Gilman S. Imaging the brain. Second of two parts. N Engl J Med. 1998 Mar 26;338(13):889-96. PMID 9516225
- ↑ Fisher M, Prichard JW, Warach S. New magnetic resonance techniques for acute ischemic stroke. JAMA. 1995 Sep 20;274(11):908-11. PMID 7674506
- ↑ 6.0 6.1 Hendee WR, Morgan CJ. Magnetic resonance imaging. Part I--physical principles. West J Med. 1984 Oct;141(4):491-500. PMID 6506686
- ↑ Hendee WR, Morgan CJ. Magnetic resonance imaging. Part II--Clinical applications. West J Med. 1984 Nov;141(5):638-48. PMID 6516335
- ↑ 8.0 8.1 8.2 Edelman RR, Warach S. Magnetic resonance imaging - First of Two Parts. N Engl J Med. 1993 Mar 11;328(10):708-16. PMID 8433731
- ↑ Edelman RR, Warach S. Magnetic resonance imaging - Second of Two Parts. N Engl J Med. 1993 Mar 18;328(11):785-91. PMID 8369029
- ↑ Berger A. Magnetic resonance imaging. BMJ. 2002 Jan 5;324(7328):35. PMID 11777806
