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 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

• 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.
• Magnetic resonance angiography (perfusion-weighted imaging)
• NMR spectroscopy
• Magnetic resonance microscopy - concerned with imaging at resolutions around or below the resolution limit if the human eye (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 NMR spectroscopy and MR imaging by providing information about the variation in the spatial distribution of nuclei having spectroscopically distinguishable values of chemical shifts
• Cine magnetic resonance imaging is primarily used in cardiology.
• Diffusion magnetic resonance imaging (diffusion-weighted 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.

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. 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.

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
T2 relaxation time	Spin-spin (transverse) relaxation time. Long TR & TE	Water (including CSF, urine, cysts, abscesses) is bright
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.

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.

Adverse effects

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

In the United States of America, the Food and Drug Administration cautions against using gadolinium-based contrast agents if:

• Glomerular filtration rate is less than 30 mL/min/1.73m²
• Hepatorenal syndrome or peri-operative liver transplantation period