Magnetic resonance imaging
Magnetic resonance imaging (also known as Nuclear Magnetic Resonance imaging or as an MRI scan) is a non-destructive imaging technique with a wide range of applications in the materials sciences and life sciences, including diagnostic imaging and neuroimaging. It employs the principle of nuclear magnetic resonance and is thus, in essence, a variant of NMR spectroscopy in which the exchange of information with the sample of interest is achieved by radiofrequency pulses at isotope-specific energy levels. The main difference between MR spectroscopy and MR imaging (the N is often dropped to avoid confusion with nuclear energy) is that the static magnetic field used in the former is supplemented in the latter by space-encoding magnetic field gradients which allow to combine the chemical information (or parts thereof) with spatial information to generate isotope-specific images.
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.[1][2][3]
- Magnetic resonance angiography
- Magnetic resonance spectroscopy[4]
- Magnetic resonance microscopy - concerned with imaging at resolutions around or below about 100µm
- Localized spectroscopy - combines MR spectroscopy and MR imaging by providing spectroscopic information from specific spatial locations within the sample
- Chemical-shift imaging - combines MR spectroscopy and MR imaging by providing information about the spatial distribution of spectroscopically visible chemical bonds within the sample
- 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.[5][6][7][8][9]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.[5]
| Pulse sequence | Description | Application |
|---|---|---|
| Standard pulse sequences | ||
| Spin echo | Proton density (water) | thoracic imaging |
| T1 relaxation time | Spin-lattice (longitudinal) relaxation time. Short repetition 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[7] |
| T2 relaxation time | Spin-spin (transverse) relaxation time. Long TR & TE | Water (including CSF, urine, cysts, abscesses) is bright[7] |
| Other pulse sequences | ||
| DWI (diffusion-weighted imaging) | Brain ischemia Tumor response to treatment | |
| ADC (apparent diffusion coefficient) | ||
| GRE (gradient echo) pulse sequences | Blood flow is bright | |
| PWI (perfusion-weighted imaging) | ||
Interpretation
The accuracy of interpretation depends on the quality of both the MRI machine used and the quality of the radiologist.[10]
References
- ↑ 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
- ↑ 5.0 5.1 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
- ↑ 7.0 7.1 7.2 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
- ↑ The Scan That Didn’t Scan - NYTimes.com.