Introduction

MRI (magnetic resonance imaging)

Introduction

Types of MRI scans

Usage and Advantadges

Limitations

MRI (magnetic resonance imaging)

Introduction

Neuroimaging can be divided into two main fields: structural (MRI) and functional neuroimaging (fMRI). The earliest technique was CT scans (Computed Tomography), but later radiation was replaced with powerful magnets, leading to the development of MRI.

MRI works through the protons of the hydrogen atoms in the water molecule. Initially, their spins align parallel to the magnetic field. One can then apply a radio frequency pulse (RF), to give energy to the aligned protons and knock over their alignment. When the field is removed, the spins realign, releasing the Energy they had previously absorbed. Depending on the environment, the protons will recover at different speeds. Therefore, with MRI one measures the amount of energy which is released and the speed at which the protons recover. A standard MRI image is like a water map: the grey areas indicate cells (containing a lot of water), whilst the white areas represent mostly fat.

Types of MRI scans

There are different kinds of MRI, here are the most known:

MRSI: magnetic resonance spectroscopic imaging, focuses on the Brain chemistry, like changes in brain metabolites.
T1 MRI: focuses on tissue relaxation time T1, where fat appears bright and water dark.
T2 MRI: water and fluid filled regions appear bright while fat appears darke. This type is helpful for identifying inflammation, edema, and fluid buildup.
PWI: perfusion weighted imaging, measures the flow of blood.
DTI: Diffusion Tensor Imaging (DTI), assesses the directional movement of water in tissues. This helps visualise pathways and identify abnormalities like trauma or multiple sclerosis.

Usage and Advantadges

In clinical settings, MRI scans detect abnormal brain structures, stroks, edema, tumors and irregular blood flow. In research studies, with computational methods it is possible to quantify group average differences in: brain structure, chemical properties and blood flow.

An advantadge of MRI is its high resolution for detectting and localizing pathologies. However, due to long imaging time, it is susceptible to blurring and small changes may not be easily detected.

Limitations

Several Artifacts can arise from air-tissue interfaces or metallic implants. Compared to CT, MRI is less effective at imaging areas with poor water content such as bones. Furthermore, scans can take 20-60 minutes, requiring patients to remain motionless, which is challenging for the elderly, children and patients in pain.

Compared to EEG or MEG, MRI has a lower temporal resolution, to improve signal-to-noise ratios, data is often smoothed across time, further reducing the temporal precision.

image 1: Brainscandude, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons2 Jan Ainali, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons3 Shivani Mattikalli/The Quantum Atlas, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons4 Wang Q, Yang J, Yu C, Deng Y, Wen Q, Yang H, Liu H and Luo R, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

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