principles of magnetic resonance imaging (MRI)
Introduction
The image in MRI results from different radiofrequency signal intensities of different tissues contained within a magnetic field. Proton magnetic resonance results from the absorption of a pulse of radiofrequency electromagnetic radiation & resultant excitation (disalignment of proton spin with the magnetic field). The excitation energy depends upon the molecular environment of the proton. Spin echo measures emission of radiation associated with relaxation of the excited proton spin state.
In clinical magnetic resonance imaging, spin echo transformed by Fourier analysis is used to form an MRI image. White in the image is a high signal intensity. Most of the signal comes from water.
The intensity of the signal & thus the image contrast can be modulated by altering:
- the interval between radiofrequency pulses (TR)
- the time between the radiofrequency pulse & the signal reception (TE)
T1 & T2 relaxation times:
The rate of relaxation of the excited proton spin states is the relaxation rate. The relaxation rate is different for different tissues & for pathologic vs normal tissue. Two relaxation rates T1 & T2 are measurable in human tissue.
T1 images:
The T1 relaxation time is the lifetime {63% or (1-1/e*) decay} of protons in the excited spin state. T1-weighted images are produced by keeping the TR & TE relatively short.
high-intensity T1 signal
- fat
- subacute hemorrhage
T2 images:
The T2 relaxation time is the lifetime of proton spin states becoming dephased owing to interactions among neighboring protons. T2 images are produced by using longer TR & TE times.
high-intensity T2 signal
*the number e is the basis of the natural logarythm (ln) ~ 2.3
Principle of MRI contrast agent: (gadolinium)
- relatively greater delivery of contrast to areas of increased vascularity produce an increase in T1-weighted signal over baseline
More general terms
Additional terms
References
- ↑ Introduction to Clinical Imaging, Radiology Syllabus, UCSF, 1993
- ↑ Medical Knowledge Self Assessment Program (MKSAP) 11, American College of Physicians, Philadelphia 1998
- ↑ Harrison's Principles of Internal Medicine, 14th ed. Fauci et al (eds), McGraw-Hill Inc. NY, 1998, pg 2289
- ↑ UpToDate 14.1 http://www.utdol.com