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Magnetic resonance imaging

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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 focus is on providing information regarding the distribution of nuclei in space. The information regarding the spatial distribution of nuclei is usually provided in the form of a plot showing the variation of the density (number of nuclei of interest per unit volume) as a function of the position. In the case of medical MRI, the most commonly used nucleus is the nucleus of the hydrogen atom. Most biomedical MR images are essentially plots showing the distribution of water in the body because water constitutes about 70% of the total body weight of human beings. The technology developed as a result of research on the effect of gravity on light by Robert Pound.

Classification

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!) possess an intrinsic degree of freedom called nuclear angular momentum or Spin. When atoms are exposed to an external magnetic field, the spins align themselves with the direction of the magnetic field and precess in relation to the field. Applying a radio-frequency pulse perpendicular to this field causes them to move in phase. The tissue relaxes after the external radio-pulse is turned off.[5] Different tissues have different relaxation times. These relaxation time differences can be used to generate image contrast. 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 (the macroscopic measure of many spins) 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.

For clinical applications, MRI units range in field strengths from 0.05 T to 3.0 Tesla.[10]

MRI pulse sequences
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.[11]

Therapeutic Application

The principle of magnetic resonance is also used therapeutically. It is referred to as Magnetic Resonance Therapy. Supporters of the therapy claim a broad indication spectrum in nonconservative orthopedics. [12] [13]

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).[14][15] Among patients on hemodialysis, the risk may be 1% after use of gadolinium-based contrast agents.[14]

In the United States, the Food and Drug Administration cautions against using gadolinium-based contrast agents if:[16]

References

  1. 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
  2. Gilman S. Imaging the brain. First of two parts. N Engl J Med. 1998 Mar 19;338(12):812-20. PMID 9504943
  3. Gilman S. Imaging the brain. Second of two parts. N Engl J Med. 1998 Mar 26;338(13):889-96. PMID 9516225
  4. 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. 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
  6. Hendee WR, Morgan CJ. Magnetic resonance imaging. Part II--Clinical applications. West J Med. 1984 Nov;141(5):638-48. PMID 6516335
  7. 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
  8. Edelman RR, Warach S. Magnetic resonance imaging - Second of Two Parts. N Engl J Med. 1993 Mar 18;328(11):785-91. PMID 8369029
  9. Berger A. Magnetic resonance imaging. BMJ. 2002 Jan 5;324(7328):35. PMID 11777806
  10. Dominik Weishaupt, How Does Mri Work? An Introduction to the Physics And Function of Magnetic Resonance Imaging Springer, 2006
  11. The Scan That Didn’t Scan - NYTimes.com.
  12. Salomonowitz G. et al. (2011) Impact of magnetic resonance therapy on sickness absence of patients with nerve root irritation following a lumbar disc problem, Z Orthop Unfall, Oct; 149(5):575-81. PMID: 21984428
  13. A. Levers, M. Staat, W. van Laack (2011), Analysis of the Long-term Effect of the Nuclear Magnetic Resonance Therapy on Gonarthrosis, Special edition from Orthopedic Practice 11/2011
  14. 14.0 14.1 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.
  15. 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.
  16. 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