[LearnMRI] Principles and Characteristics of MRI imaging

26 Dec 2023     #bio #brainImaging #mri

I’d like to write posts summarizing various aspects of MRI and its modalities, as well as the Alzheimer’s disease-related features observable through MRI as I studied.

Magnetic Resonance Imaging (MRI)

In a device composed of magnets, high-frequency waves are directed at the human body, resonating hydrogen atomic nuclei in the body’s tissues, and converting the differences in signals emanating from each tissue into digital information, resulting in images.

Principles of MRI Imaging

Human tissues contain a significant amount of water. Hydrogen nuclei within water molecules possess magnetic properties. By emitting high-frequency waves, these hydrogen nuclei can be resonated. When a radiofrequency (RF) pulse is emitted and then turned off (RF pulse), the atomic nuclei absorb and subsequently release the high-frequency signal. Analyzing the differences in the signals returning to the MRI device and maximizing them, a two-dimensional image is formed, which is the essence of MRI.

The magnitude and waveform of the emitted signal vary depending on factors such as the concentration of water molecules, blood flow, and the binding state with surrounding chemical structures. Consequently, the relaxation times, T1 and T2, differ based on the composition of tissues and blood. Since the composition varies with different diseases, the signals obtained also differ accordingly. By capturing these signal variations, various types of MRI images can be obtained, including T1-weighted images (T1WI), T2-weighted images (T2WI), FLAIR, and others.

The T1 and T2 relaxation times are measured based on different criteria after applying a 90-degree RF pulse to the protons. When the magnetization of the protons is flipped from the longitudinal axis ($Mz$) to the transverse axis, an $Mxy$ vector is formed. The T1 and T2 relaxation times are measured from the moment when the $Mz$ vector reaches 0% and the $Mxy$ vector reaches 100%.

• T1 relaxation time: The time it takes for $Mz$ to recover up to 63%.
  • Recovery is faster in fat, brain tissue, and cerebrospinal fluid (CSF) in that order (shorter T1 relaxation time).
• T2 relaxation time: The time it takes for $Mxy$ to decay down to 37%, relatively unaffected by magnetic field strength.
  • Signal decay is faster in fat, brain tissue, and CSF.
  • Tissues with shorter T1 relaxation times also exhibit a rapid decline in the T2 curve. Water and fat have opposite signal intensities in T1 and T2 (opposite signal intensity).

Spin echo is a technique for acquiring images by manipulating the repetition time (TR) and echo time (TE) while applying RF pulses of 90 and 180 degrees. TR is the time from one 90-degree pulse to the next, while TE is the time until the signal is obtained after the 90-degree pulse. By repeating the pulse during image acquisition, various images can be obtained by adjusting TR and TE.

Pros and Cons of MRI

Pros

• Better contrast of soft tissues compared to CT.
• Ability to observe anatomical, physiological, and functional information.

Cons

• Ferromagnetic artifacts: Even small amounts of ferromagnetic materials in the body can disrupt the homogeneity of the magnetic field, causing distortion in the images.
• Presence of dental fillings or other inserted materials can reduce image quality.

Contraindications

• MRI should not be used for patients with implants or other materials inside the body that may be affected by the magnetic field.