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 nucleons (protons and neutrons), such as hydrogen and carbon-13 (but not carbon-12!) inherently create a small magnetic field that can be excited by radio frequency irradiation, measured, then manipulated by MRI, then measured again as the tissue relaxes after the external field is turned off.[5]. In the absence of an external magnetic field, the individual nuclear magnetic fields point in random directions, resulting in no net magnetic field. However, in the presence of an external magnetic field, a fraction of the atoms align with the magnetic field while others align against the external field, resulting in a net magnetic field that can be measured. The observed signal is the small net magnetic field resulting from the population differences between the "up" and "down" nuclei. Because the population difference between the atoms aligned with or against the field is a function of the external magnetic field strength, increasing the magnetic field strength of MRI spectrometers enhances the observed signal-to-noise ratio.
| 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]
Adverse effects
Claustrophobia
Nephrogenic systemic dermopathy
The use of gadolinium-based contrast agents in patients with renal insufficiency may increase the risk of nephrogenic systemic dermopathy (nephrogenic systemic fibrosis).[11][12] Among patients on hemodialysis, the risk may be 1% after use of gadolinium-based contrast agents.[11]
In the United States, the Food and Drug Administration cautions against using gadolinium-based contrast agents if:[13]
- Glomerular filtration rate is less than 30 mL/min/1.73m2
- Hepatorenal syndrome or peri-operative liver transplantation period
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.
- ↑ 11.0 11.1 Lee CU, Wood CM, Hesley GK, Leung N, Bridges MD, Lund JT et al. (2009). "Large sample of nephrogenic systemic fibrosis cases from a single institution.". Arch Dermatol 145 (10): 1095-102. DOI:10.1001/archdermatol.2009.232. PMID 19841395. Research Blogging.
- ↑ Abujudeh HH, Kaewlai R, Kagan A, Chibnik LB, Nazarian RM, High WA et al. (2009). "Nephrogenic systemic fibrosis after gadopentetate dimeglumine exposure: case series of 36 patients.". Radiology 253 (1): 81-9. DOI:10.1148/radiol.2531082160. PMID 19709997. Research Blogging.
- ↑ Anonymous (5/23/2007) Information for Healthcare Professionals Gadolinium-Based Contrast Agents for Magnetic Resonance Imaging (marketed as Magnevist, MultiHance, Omniscan, OptiMARK, ProHance). Food and Drug Administration