NEUROIMAGING / MAGNETIC RESONANCE IMAGING

Magnetic Resonance Imaging (MRI)

Created 27/05/2022, last revision 10/01/2023

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; to depict extracranial arteries, a contrast agent should be used
- significant improvement in the acute stroke diagnosis using DWI, PWI, SWI, and GRE sequences
disadvantages of MRI compared to CT
- higher costs and longer duration of the examination
- difficulties with ventilated patients (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
- more MRI contraindications

Magnetic Resonance Imaging modalities

T1C+ (contrast-enhanced)

T1 fat supressed

T1C+ fat supressed

FLAIR

GRE/SWI

GRE

Cortical cerebral microbleeds and lobar hematomas in patient with cerebral amyloid angiopathy

SWI

Cerebral microbleeds on SWI (mixed pattern)

DWI/ADC

DWI

ADC

Diffusion tensor imaging (DTI) tractography

Flow sequences

CSF flow studies

MR angiography

Bright blood

Non-contrast MRA (TOF)

MR venography (TOF)

Contrast-enhanced MRA

Dark blood

Fast Spin Echo (FSE) Black Blood MRA

Inversion Recovery (IR) Black Blood MRA

Susceptibility-weighted (SW) Black Blood MRI

Vessel wall imaging

T1C+ black blood

Demonstration of ICA vasculitis using post-contrast high resolution MRI (HRMRI) (Obuses, 2014)

Others

PWI

ASL

MRS

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+
  - high-resolution MR T1 C+ dark blood sequence (see below) can be used to detect vasculitis
- fat suppression → 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, so 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	MR-T1	MR-T2	CT
bone, calcifications	dark	dark	bright
water, CSF	dark	bright	dark
fat	bright	dark	dark
infarction	dark	bright	dark
tumor	dark	bright	dark
gadolinium	bright	bright	–
air	dark	dark	dark

Diffusion-weighted images (DWI)

→ MR-DWI in the acute stroke diagnosis

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
- ADC <620×10-3 mm/s is likely to identify ischemia
- after recanalization, there is often an increase of signal on ADC maps in the ischemic area (visible on DWI and FLAIR)
DWI changes are not specific to ischemia – they can occur in any transport mechanisms impairment (edema), such changes are often reversible, and lesions are not hypointense in the ADC map)
artifacts – DWI depends 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 DWI)

Corpus callosum infarction and intraventricular hemorrhage

Perfusion weighted images (PWI)

PWI provides information about the current blood supply to the brain tissue
after bolus administration of gadolinium-based contrast agent (GBCA); the same parameters as for CT perfusion (CBV, CBF, MTT, TTP) can be obtained
the tissue with impaired perfusion (↑ MTT) comprises completed infarction (core, approx. corresponding to DWI lesion), 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)

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Blooming artifact

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Susceptibility weighted images (SWI)

acute thrombus detection
- thrombus is hypointense on MR SWI (deoxyhemoglobin)
- high sensitivity to detect thrombi in distal segments (unlike MRA)
detection of venous thrombosis
- hypointense signal in affected veins and vessels
- stasis and enlargement of the surrounding veins
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 anticoagulation therapy)
- early detection of hemorrhagic transformation (higher sensitivity compared to CT and MR GRE)

Arterial Spin Labeling (ASL)

ASL uses water in arterial blood as an endogenous contrast agent
enables non-invasive perfusion imaging with the creation of CBF maps
ASL seems to overestimate perfusion lesion compared to PWI
useful in these indications:
- acute stroke/TIA
- chronic cerebrovascular diseases
- degenerative diseases (AD, FTD)
- brain tumors
- migraine (hyperperfusion)
- posterior reversible encephalopathy (PRES)

Contrast-enhanced MRI

contrast agents depict or enhance structures poorly visible in the non-enhanced images
contrast agents facilitate proton relaxation, thus shortening the T1 and T2 relaxation times. Shortening the T1 relaxation time leads to enhancement of the T1-weighted image, whereas T2 leads to its attenuation
contrast agents can be divided into paramagnetic and super-paramagnetic agents
- paramagnetic substances amplify the magnetic field, which causes a shortening of the relaxation time. They have a wide range of applications and are often used in CNS examinations because they can penetrate damaged HEB – 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 highly effective

→ gadolinium-based contrast agents

MRI and acute stroke diagnosis

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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

Intracranial arteries

TOF (Time Of Flight) method, without contrast agent administration
- evaluates only flow, false-positive findings are common
- sufficient to show occlusions/stenoses of major arteries
- low sensitivity for peripheral occlusions
SWI dark blood sequence detects thrombi thanks to blooming artifact
T1C+ dark blood sequence helps to detect inflammatory changes in the vessel wall

Extracranial arteries

contrast-enhanced MRA (CE-MRA) for extracerebral vessels
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
- later, with the transformation of oxy-hemoglobin to deoxy-hemoglobin (max. day 2), a typical decrease in the T2 signal can be seen
- in the following days, methemoglobin levels increase, and the lesion becomes hyperintense (mainly in T1 and PD)
- after approx. 2-4 weeks, a rim of low signal in T2 corresponding to hemosiderin in macrophages appears
the gradient echo sequence (GRE) can also detect early bleeding with similar sensitivity to CT. Thus, MRI can be used as an initial imaging method in the acute stroke program [Lin, 2001]
- GRE is more sensitive than CT for detecting old bleeding and is equally liable for detecting hemorrhagic infarction [Renou, 2010]
DWI/ADC
- does not 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 becomes hyperintense around day 30; with hemorrhage, hypointensity persists for > 100 days [Ebisu, 1997]

DDx of a primary ICH and arterial hemorrhagic (HI) infarction

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DDx of a primary ICH and venous hemorrhagic infarction

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