[LearnMRI] The Types of MRI Modalities and Observable Brain Patterns

26 Dec 2023     #bio #brainImaging #mri

According to the Spin echo technique, T1-weighted images (T1WI) and T2-weighted images (T2WI) can be obtained. By manipulating these images, various MR modality images can be created. Since the signal intensity of lesion tissue varies in each image, the types of lesions emphasized are different.

T1-weighted Image (T1WI)

• Spin echo: Both the repetition time (TR) and the echo time (TE) are set short.
  • When TR is shortened, the recovery time (T1 realxation time) of $Mz$ varies depending on the tissue, emphasizing the difference. Some tissues have fully recovered, while others have not fully recovered by the time of the second pulse, leading to varying influences from the second pulse. This difference is reflected in the image.
  • TE should be set short to minimize its influence on the T2 relaxation time values.
• Signal intensity
  • Signal intensity is higher than T2 → anatomical structures are more clearly distinguished.
  • Subcutaneous fat and blood appear hyperintense, which means brighter, while muscles appear intermediate, and water appears hypointense, which means darker.
  • Marrow, being rich in fat, appears hyperintense, while cortex, having less water, appears hypointense.
  • Lesions: Lipoma, acute hemorrhage, lesions containing high protein content (e.g., mucocele)
• Observation: Cortical morphology (anatomical detail), vascular changes, blood-brain barrier integrity
• Feature: Cortical thickness, choroid plexus (ChP)
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T2-weighted image (T2WI)

• Spin echo: Both TR and TE are set long.
  • Lengthening TR minimizes its impact on T1 relaxation time.
  • Longer TE emphasizes the contrast in the extent of $Mxy$ decrease, resulting in different tissue representations in the image.
• Signal intensity:
  • Water appears hyperintense, aiding in the detection of pathological tissues with higher water content, such as lesions.
  • Most lesions appear as low signal intensity(hypointense) on T1 and high signal intensity(hyperintense) on T2.
  • The brightness of water in T2 images varies, with cysts appearing brightest, followed by edema, and then normal tissue.
  • Muscles, fat, and blood appear hypointense.
  • Cerebrospinal fluid (CSF) also appears hyperintense, making it challenging to distinguish lesions, such as Perivascular space (PVS).
• Observation: Lesions, hypointense lesions (such as acute hematomas, fungal balls, etc.), arteries (veins show varying signal intensities due to differing blood flow rates)
• Feature: Perivascular space (PVS)

FLAIR (Fluid Attenuation Inversion Recovery)

• CSF is rendered black in T2 images.
• Non-free-flowing water appears hyperintense, while fat appears hypointense.
• Observation: Lesions around the ventricles, edema (which appears bright due to stanant fluid), grey-white matter differentiation.
• Feature: Lesions, white matter hyperintensity (WMH)

GRE (Gradient Echo; T2*)

• Paramagnetic substances such as blood, calcium, and metal appear hyperintense, allowing for the observation of iron deposition.
• Observation: Excellent for detecting microbleeds in early and late brain hemorrhages, as well as diffuse axonal injury.
  • *Diffuse axonal injury: One of the components of brain trauma, characterized by axonal damage leadidng to a coma state after trauma.
• Feature: bleeding