NEUROIMAGING / MAGNETIC RESONANCE IMAGING
Magnetic Resonance Imaging (MRI)
David Goldemund M.D.
Updated on 05/01/2024, published on 27/05/2022
- MR imaging is based on the behavior of atoms (most often – H, P, and Na) in the magnetic field
- structures generating low signals are hypointense; structures generating high signals are hyperintense; tissues producing no signal are dark (blood, calcifications)
- advantages over CT
- high contrast and resolution, good visualization of pathologies in the posterior fossa
- possibility of imaging in any plane (coronal, sagittal, oblique) and 3D reconstruction
- possibility of non-contrast imaging of the cerebral arteries; contrast agents are used to visualize extracranial arteries
- significant improvement in the acute stroke diagnosis using DWI, PWI, SWI, and GRE sequences
- does not use ionizing radiation
- disadvantages of MRI compared to CT
- higher costs and longer examination times (typically lasting 30-60 minutes)
- can be noisy, so earplugs or headphones are usually provided
- difficulties with ventilated patients, as an MRI-compatible respirator must be available.
- young children and uncooperative patients must be sedated or examined under general anesthesia (the risk of movement artifacts is substantial)
- numerous MRI contraindications
- higher costs and longer examination times (typically lasting 30-60 minutes)
Magnetic Resonance Imaging modalities
Basic MRI sequences (T1, T2, PD)
- the principle of the method is the detection of T1 and T2 relaxation times (resulting in so-called T1 and T2-weighted images)
- T1 is used for accurate anatomical imaging
- water signal is low; fat is hyperintense
- T1 signal is stronger when the relaxation time is shortened (e.g., due to the contrast agent) → T1C+
- fat suppression techniques can be used to improve the contrast between different tissues by suppressing the signal from fat → see here
- CHESS (Fat-Sat)
- STIR
- T2 – better detection of lesions containing more water
- FLAIR sequence is used for water suppression (the CSF is dark, and periventricular lesions can be better distinguished)
- CHESS (Fat-Sat) is used to suppress fat
- PD (proton density) – image quality depends on the density of hydrogen protons in the tissues. Compared to other sequences, it is less useful in CNS diagnostics
tissue
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MR-T1
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MR-T2
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CT
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bone, calcifications
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dark
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dark
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bright
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water, CSF
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dark
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bright
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dark
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fat
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bright
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dark
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dark
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infarction
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dark
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bright
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dark
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tumor
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dark
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bright
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dark
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gadolinium | bright | bright | – |
air | dark | dark | dark |
Diffusion-weighted images (DWI)
- DWI visualizes impaired (restricted) diffusion of water molecules (or protons) caused by the energetic failure of Na+/K+ membrane pumps
- it is highly sensitive and specific (88-100%) for detecting acute cerebral infarction within minutes of its onset, with a maximum of 4-6 hours
- acute ischemia appears hyperintense (bright) on DWI (b factor around 1000 s/mm2) and hypointense (dark) on calculated ADC (apparent diffusion coefficient) maps
- DWI changes are not specific to ischemia – they can occur in any impairment of transport mechanisms (edema). Such changes are often reversible, and lesions are not hypointense on the ADC map)
- artifacts can occur on DWI due to its dependence on the T2 signal
- increased T2 signal can lead to T2 shine through and T2 washout
- reduced T2 signal leads to the T2 blackout phenomenon (hypointense on DWI)
Perfusion-weighted images (PWI)
- PWI provides information about the current blood supply to the brain tissue by measuring the passage of gadolinium-based contrast agent (GBCA) through the brain tissue
- after bolus administration of GBCAs, the same parameters as for CT perfusion (CBV, CBF, MTT, TTP) can be obtained
- tissue with impaired perfusion (↑ MTT) comprises infarction (core, approx. corresponding to DWI lesion), the penumbra and also the area of benign oligemia. The difference between the area of impaired perfusion and diffusion approximately determines the size of the penumbra (PWI/DWI mismatch)
Gradient-echo sequence (GRE)
- paramagnetic substances appear dark (calcification, blood, metals)
- GRE reliably detects both recent and old bleeding, including cerebral microbleeds
- can detect acute ICH (initially, only a dark rim may be visible), SAH, or subtle hemorrhagic complications in ischemic stroke ⇒ GRE should be included in emergency brain MR studies (Kidwell, 2004)
- characteristics of hyperacute hematoma:
- T1 isointense
- T2/FLAIR – isointense or mildly hyperintense
- GRE hypointense (initially only hypointense rim + core of heterogenous signal intensity due to the diamagnetic oxyhemoglobin)
- DWI hyperintense, ADC hypointense
- GRE outperforms CT in the detection of old hemorrhages
- GRE allows the detection of intra-arterial and intravenous thrombi (blooming artifact)
- in addition to dural sinus thrombosis, it can help to detect smaller veins thrombosis
- both the dense artery sign and the blooming artifact are common in erythrocyte-rich thrombi [Liebeskind, 2011]]
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Susceptibility-weighted images (SWI)
- SWI is particularly sensitive to paramagnetic substances, such as oxyhemoglobin, deoxyhemoglobin, and methemoglobin, as well as iron deposits
- advantages and disadvantages:
- high-quality images with excellent spatial resolution outperforming GRE (may detect small abnormalities that may not be visible on other MRI sequences)
- long acquisition time
- limited contrast
- SWI is used to detect a variety of abnormalities in the brain (similar to GRE):
- acute thrombus detection
- thrombus is hypointense on MR SWI (deoxyhemoglobin)
- SWI offers high sensitivity to detect thrombi in distal segments (where it often outperforms MRA)
- detection of venous thrombosis
- prediction of hemorrhagic transformation
- detection of cerebral microbleeds – up to 6x more sensitive than conventional gradient sequences (their presence increases the risk of bleeding during thrombolysis or anticoagulant therapy)
- early detection of hemorrhagic transformation (higher sensitivity compared to CT and MR GRE)
- acute thrombus detection
Arterial Spin Labeling (ASL)
- ASL is a technique that measures cerebral blood flow (CBF) using water in arterial blood as an endogenous contrast agent; CBF maps are created
- this is done by using a magnetic field gradient to temporarily alter the direction of the protons’ spin, which makes them visible to the MRI scanner. After a short period, the labeled protons pass through the brain and are detected by the MRI scanner
- advantages:
- non-invasive perfusion imaging
- high sensitivity (because it measures the passage of labeled protons through the brain rather than relying on the accumulation of a tracer)
- reproducibility – ASL results are highly reproducible, making it a valuable tool for research and clinical practice
- disadvantages:
- ASL seems to overestimate perfusion lesion compared to PWI
- lower spatial resolution compared to PET/SPECT (may not be able to detect small changes in CBF)
- susceptibility to motion artifacts
- technical challenges (ASL requires a high-field MRI scanner and specialized software)
- useful in these indications:
Contrast-enhanced MRI
- contrast agents are substances injected into the bloodstream to enhance the visibility of certain structures in MRI images
- may improve the visibility of lesions that are not visible on non-enhanced images
- may help distinguish between different types of tissues
- contrast agents facilitate proton relaxation, which shortens the T1 and T2 relaxation times
- a shortening of the T1 relaxation time leads to an increase in signal intensity, while a shortening of the T2 relaxation time leads to a decrease in signal intensity
- contrast agents can be divided into paramagnetic and super-paramagnetic agents
- paramagnetic substances amplify the magnetic field, resulting in a shortening of the relaxation time. They have a wide range of applications and are often used in CNS examinations because of their ability to penetrate damaged BBB – Magnevist (gadopentetate), Omniscan (gadodiamide), Dotarem (Gd-DOTA, gadoteric acid)
- super-paramagnetic agents are solid substances that are introduced into the body as suspensions and are very effective in shortening the relaxation times of protons
- contrast agents are associated with a risk of adverse events and increased costs
MRI and acute stroke diagnosis
- DWI – essential in the early detection of cerebral ischemia, can be included at the beginning of the examination protocol
- T1, T2, FLAIR – usually negative in the acute stage of stroke, changes on FLAIR (cytotoxic edema) appear within 3-4 hours
- DWI/FLAIR mismatch can guide the management of patients with an unclear time of onset of stroke (WAKE-UP, MR WITNESS)
- DWI/FLAIR mismatch can guide the management of patients with an unclear time of onset of stroke (WAKE-UP, MR WITNESS)
- GRE
- detection of bleeding, including recent hemorrhagic infarction
- detection of venous or arterial thrombi, characterized by a “blooming artifact“
- MRA – time-of-flight (TOF) technique is used for examination of intracranial arteries to detect occlusion (exclude artifacts)
- MR perfusion – helps in identifying the penumbra, the potentially salvageable tissue surrounding an ischemic core
- MR SWI – highly sensitive in detecting early hemorrhagic infarction or thromboembolism (high sensitivity even for peripheral segments)
Acute (0-7 days) |
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Subacute (7-21 days) |
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Chronic ( >3 weeks) |
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MRI angiography (MRA)
MRA techniques
Dark Blood MRA → see here
(vascular imaging strategy where the signal from flowing blood is suppressed – rendering it “black”)
- Fast Spin Echo (FSE) Black Blood MRA
- Inversion Recovery (IR) Black Blood MRA
- Susceptibility-weighted (SW) Black Blood MRI
Bright Blood MRA
- Contrast-enhanced MRA
- Non-contrast MRA
- Time-of-Flight (TOF)
- Phase-contrast (PC)
- Steady-state Free Precession (SSFP)
- Fast Spin Echo (FSE)
- Arterial Spin Labeling (ASL)
MRA applications
Extracranial arteries
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the stenosis should be measured according to NASCET criteria
- MRA may overestimate stenosis compared to DSA (pseudo-occlusion)
MR venography
- dural sinuses and veins thrombosis
- use non-contrast TOF or SWI
MRI and dissection diagnosis
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MRI and intracranial hemorrhage
- the paramagnetic properties of hemoglobin derivatives are responsible for the MRI signal changes
- imaging depends on the degree and extent of hemoglobin conversion within the coagulum (see the table)
- in the acute stage of hemorrhage, the finding in T1,2 is non-specific
- subsequently, as oxyhemoglobin converts to deoxyhemoglobin (peaking around day 2), a characteristic decrease in T2 signal intensity is observed
- in the following days, methemoglobin levels rise, causing the lesion to become hyperintense (mainly in T1 and PD)
- ~ 2-4 weeks later, a rim of low signal intensity appears in T2, corresponding to hemosiderin in macrophages
- gradient echo sequence (GRE) can also detect early bleeding with similar sensitivity to CT, allowing MRI to be used as an initial imaging method in the acute stroke setting [Lin, 2001]
- GRE is more sensitive than CT for detecting old bleeding and is equally effective for detecting hemorrhagic infarction [Renou, 2010]
- DWI/ADC
- does not effectively differentiate between hemorrhagic infarction and ICH
- hemorrhage is hyperintense in the hyperacute and late subacute stages, hypointense in the acute, early subacute, and chronic stages [Kang, 2001]
- mean ADC ratio is 0.70-0.73 in early stages and 2.56 in chronic stages [Kang, 2001]
- in ischemia, the ADC lesion typically becomes hyperintense around day 30; with hemorrhage, hypointensity persists for over 100 days [Ebisu, 1997]
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