Original link: https://laffitto.xyz/archives/mrisequence
Some knowledge of MRI is used in the work. Here, we will do some simple sorting first, and we will continue to sort and update them in the future.
term description
Intensity
When describing most MRI sequences, the word intensity is often used to refer to the gray shading (intensity) of tissue or fluid. Hence the following term descriptions:
Absolute term description:
- high signal intensity = white
- intermediate signal intensity = grey
- low signal intensity = black
Relative term description:
- hyperintense = brighter than the thing we are comparing it to
- isointense = same brightness as the thing we are comparing it to
- hypointense = darker than the thing we are comparing it to
Absolute terms are preferably used, or if relative terms are used, the organizational structure of the validation comparison needs to be stated
Tips: Use density to describe the intensity in CT to avoid confusion!
Diffusion
The term intensity is also used when describing diffusion weighted sequences. The terms “restricted diffusion” and “facilitated diffusion” are used to denote whether water can move more easily (restricted) or more easily (facilitated) than intended tissue.
Sequence classification can refer to the classification of this article: MRI sequence
MRI sequence
MRI can only detect transverse macroscopic magnetization vectors. Such vectors can be generated when hydrogen protons enter a high-energy state.
Different classifications can be made according to the energy that can be generated in different directions by nuclear magnetic relaxation.
T1 weighted/T1 weighted
Also called longitudinal relaxation, it is mainly the release process (time) of the high energy of hydrogen protons.
The smaller the T1 of the tissue, the higher the signal intensity on the MRI image (white); the larger the T1, the lower the signal intensity on the image (black).
The signal strengths of different tissues are:
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Fat: white high signal intensity
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Muscle: gray intermediate signal intensity
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Fluids (urine, cerebrospinal fluid): dark low signal intensity
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Flowing blood: dark low signal intensity
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Bone: Dark
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air: dark
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brain:
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Gray matter: gray
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White matter: hyperintense compared to grey matter (white-ish)
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T1 is best for evaluating anatomy because the image macroscopically resembles tissue.
T1 Contrast Enhancement/T1+Contrast (gadolinium)
Injecting a contrast agent (gadolinium) increases the T1 signal of the flowing blood, allowing detection of hypervascular lesions.
Can be used to evaluate hypervascular lesions (eg, hemangioma, lymphangioma).
T1 fat suppressed
The aim is to detect edema in soft tissues that often contain a large fatty component. Suppressing the signal from fat can make fluid components with high signal more apparent.
T2 weighted/T2 weighted
Also called transverse relaxation, it is mainly the hydrogen proton dephasing process (time).
The smaller the T2 of the tissue, the lower the signal intensity on the MRI image (black); the larger the T1, the higher the signal intensity on the image (white).
Generally speaking, T1 is much larger than T2.
The signal strengths of different tissues are:
- Fat: white high signal intensity
- Muscle: gray intermediate signal intensity
- Fluids (urine, cerebrospinal fluid): white high signal intensity
- Flowing blood: dark
- Bone: Dark
- air: dark
- brain:
- Gray matter: gray
- White matter: hypointense compared to grey matter (dark-ish)
T2 fat suppressed
Similar to T1 fat suppression
Fluid Attenuation Inversion Recovery (FLAIR)
This sequence attenuates signals from fluids (such as CSF “cerebrospinal fluid”), thus helping to detect lesions (in the brain and spinal cord) that are normally covered by CSF.
susceptibility weighted imaging (SWI)
Capable of distinguishing calcium from blood, SWI is more sensitive than conventional gradient echo sequences in showing intracerebral venules and hemorrhages.
Short Tau Inversion Recovery (STIR)
Similar to the FLAIR sequence, STIR inhibits signaling from adipose tissue.
STIR cannot be used after gadolinium injection because the T1 of gadolinium is in the same fat range and eventually the signal from it will be attenuated.
- Fat: Dark
- Muscle: darker than fat
- Fluid: very bright
- Bone: Dark
- air: dark
- brain:
- Gray matter: gray
- White matter: darker than gray matter
Most useful when evaluating fluid-filled spaces.
Proton density weighted imaging (PDWI)
An intermediate sequence that shares some features of T1 and T2. It mainly depends on the density of protons. Therefore, denser tissue emits a brighter signal.
Excellent in evaluating joints as they can differentiate between fluid, hyaline cartilage and fibrocartilage.
- Fat: white
- Muscle: gray
- Fluid: White
- Bone: Dark
- air: dark
- hyaline cartilage: grey
- Fibrocartilage: dark
Diffusion Weighted Sequence
Diffusion-weighted imaging assesses the ease with which water molecules move within tissues (representing primarily fluid in the extracellular space) and provides insights into cellular architecture (eg, tumors), cellular swelling (eg, ischemia), and edema.
The main signal strengths of different tissues are:
- Fluids (eg, urine, cerebrospinal fluid): do not limit spread
- Soft tissue (muscles, solid organs, brain): Intermediate spread
- Fat: weak signal due to lack of water
Three sets of images are typically found when performing diffusion-weighted imaging: DWI, ADC, and B=0 images.
Traditional DWI/diffusion weighted image (DWI)
DWI specifically detects the motion of protons in water molecules. It represents the combination of the actual diffusion value and the T2 signal.
- Fat: weak signal due to lack of water
- Muscle: gray
- Fluid (CFS): Dark
- brain:
- Gray matter: gray
- White matter: dark (slightly lower compared to gray matter)
Acute pathology (ischemic stroke, cellular tumor, pus) usually presents with increased signal, indicating restricted diffusion. And importantly, because a portion of the image comes from the T2 signal, some tissue that is bright on T2 will appear bright on the DWI image without abnormal restricted diffusion.
Apparent diffusion coefficient maps (ADC)
The ADC represents an image of the actual diffusion value of the tissue without T2 effect. And an objective measure of the diffusion value can be obtained, essentially displayed as a grayscale inverted DWI image.
- CSF: High signal (white)
- Fat: weak signal due to lack of water
- Other soft tissue: Moderate signal strength (grey)
- brain
- Gray matter: Intermediate signal strength (gray)
- White matter: slightly higher compared to gray matter
Acute pathology (ischemic stroke, cellular tumor, pus) usually presents with diminished signal, indicating limited spreading.
B=0
Essentially T2-weighted images with some susceptibility effects.
Diffusion tensor (DTI)
Used to assess tumor-induced white matter deformation, reduced fractional anisotropy may indicate dementia.
Functional MRI/Functional MRI (fMRI)
The brain controls its blood flow very tightly locally. Active tissue exhibits elevated blood flow
Perfusion weighted ( PWI )
The amount of blood flowing into the tissue can also be detected and relatively quantified, yielding values such as cerebral blood volume, cerebral blood flow, and mean transit time. These values are useful in many clinical situations, including defining the ischemic penumbra in ischemic stroke, assessing the histological grade of certain tumors, or distinguishing radiation necrosis from tumor progression.
Reference article
- https://radiopaedia.org/articles/mri-sequences-overview#:~:text=An MRI sequence is a, the particulars of each sequence
- https://radlines.org/MRI_sequences
- https://www.wikidoc.org/index.php/MRI_sequences
- https://case.edu/med/neurology/NR/MRI Basics.htm
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