Magnetic resonance imaging: Difference between revisions
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==Physical principles== | ==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.<ref name="PMID6506686">Hendee WR, Morgan CJ. [http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=6506686 Magnetic resonance imaging. Part I--physical principles]. West J Med. 1984 Oct;141(4):491-500. PMID 6506686</ref><ref name="PMID6516335">Hendee WR, Morgan CJ. [http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=6516335 Magnetic resonance imaging. Part II--Clinical applications]. West J Med. 1984 Nov;141(5):638-48. PMID 6516335</ref><ref name="PMID8433731">Edelman RR, Warach S. [http://content.nejm.org/cgi/content/full/328/10/708 Magnetic resonance imaging - First of Two Parts]. N Engl J Med. 1993 Mar 11;328(10):708-16. PMID 8433731</ref><ref name"PMID8369029">Edelman RR, Warach S. [http://content.nejm.org/cgi/content/full/328/11/785 Magnetic resonance imaging - Second of Two Parts]. N Engl J Med. 1993 Mar 18;328(11):785-91. PMID 8369029</ref><ref name="PMID11777806">Berger A. [http://www.bmj.com/cgi/content/full/324/7328/35 Magnetic resonance imaging]. BMJ. 2002 Jan 5;324(7328):35. PMID 11777806</ref>Atoms with an odd number of protons, such as [[hydrogen]] | 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.<ref name="PMID6506686">Hendee WR, Morgan CJ. [http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=6506686 Magnetic resonance imaging. Part I--physical principles]. West J Med. 1984 Oct;141(4):491-500. PMID 6506686</ref><ref name="PMID6516335">Hendee WR, Morgan CJ. [http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=6516335 Magnetic resonance imaging. Part II--Clinical applications]. West J Med. 1984 Nov;141(5):638-48. PMID 6516335</ref><ref name="PMID8433731">Edelman RR, Warach S. [http://content.nejm.org/cgi/content/full/328/10/708 Magnetic resonance imaging - First of Two Parts]. N Engl J Med. 1993 Mar 11;328(10):708-16. PMID 8433731</ref><ref name"PMID8369029">Edelman RR, Warach S. [http://content.nejm.org/cgi/content/full/328/11/785 Magnetic resonance imaging - Second of Two Parts]. N Engl J Med. 1993 Mar 18;328(11):785-91. PMID 8369029</ref><ref name="PMID11777806">Berger A. [http://www.bmj.com/cgi/content/full/324/7328/35 Magnetic resonance imaging]. BMJ. 2002 Jan 5;324(7328):35. PMID 11777806</ref>Atoms with an odd number of [[nucleon]]s (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.<ref name="PMID6506686"/>. 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. | ||
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Revision as of 22:00, 31 October 2009
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]
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.