Which MRI techniques are used in the diagnosis and management of acute stroke?

Updated: Dec 19, 2018
  • Author: Souvik Sen, MD, MPH, MS, FAHA; Chief Editor: Helmi L Lutsep, MD  more...
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Answer

Answer

Some nuclei in the human body become excited when positioned in a strong magnetic field; they absorb the radiofrequency energy of the magnetic field and then release it until they relax completely. The energy is released from the excited tissue over a short period according to 2 relaxation constants known as T1 and T2, and the emitted energy signals are converted into images. The contrasts in the images result from different intensities of these emitted signals, which in turn result from different concentrations of the nuclei in different tissues in the body.

Hydrogen (ie, protons) is the most common magnetic resonance (MR)–observable nucleus in the human body and has the advantage of being present in many different tissues in different concentrations. Other organic particles have been tried but have demonstrated less spatial resolution than hydrogen. The biochemical compounds lactate and N -acetyl aspartate are under trial to increase understanding of the significance of the different concentrations of these compounds in different pathologic conditions (ie, MR spectroscopy).

The following are commonly used MRI techniques:

  • T1-weighted imaging (T1-WI) in which cerebrospinal fluid (CSF) has a low signal intensity in relation to brain tissue

  • T2-weighted imaging (T2-WI) in which CSF has a high signal intensity in relation to brain tissue

  • Spin density–weighted imaging in which CSF has a density similar to brain tissue

  • Gradient echo imaging, which has the highest sensitivity in detecting early hemorrhagic changes

  • Diffusion-weighted imaging (DWI) in which the images reflect the microscopic random motion of water molecules

  • Perfusion-weighted imaging (PWI) in which hemodynamically weighted MR sequences are based on passage of MR contrast through brain tissue


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