MRI EXAM
MRI (Structural) — Revision Notes
1. Basic Principles (Conceptual)
MRI is a non-invasive imaging technique that produces detailed images of brain anatomy using:
A strong magnetic field
Radiofrequency (RF) pulses
Hydrogen protons in water and fat
Hydrogen protons behave like tiny magnets.
When placed in a magnetic field, they align and precess.
An RF pulse temporarily disrupts this alignment.
When the RF pulse stops, protons relax back to equilibrium and emit a signal that is detected by the scanner.
Image contrast depends on how tissues differ in their relaxation behaviour, not on tissue density.
MRI does not use ionising radiation.
2. MRI TERMINOLOGY — EXAM CORE
Fundamental MRI Terms
Magnetic field (B₀)
The strong, static magnetic field of the MRI scanner that aligns hydrogen protons.
Radiofrequency (RF) pulse
Energy applied at the resonance frequency to excite protons and generate signal.
Resonance
Condition in which RF energy matches proton precession frequency, allowing energy absorption.
Precession
Wobbling motion of protons around the magnetic field axis.
Magnetisation Terms
Longitudinal magnetisation
Net magnetisation aligned with B₀.
Transverse magnetisation
Magnetisation in the plane perpendicular to B₀; source of measurable MRI signal.
Relaxation Terminology
Relaxation
Return of excited protons to equilibrium after RF pulse.
T1 relaxation (spin–lattice)
Recovery of longitudinal magnetisation through energy loss to surrounding tissue.
T2 relaxation (spin–spin)
Decay of transverse magnetisation due to loss of phase coherence between protons.
Image Contrast and Signal
Signal intensity
Brightness of a structure on MRI.
High signal
Bright appearance.
Low signal
Dark appearance.
Isointense
Similar signal to surrounding tissue.
Hyperintense
Brighter than expected.
Hypointense
Darker than expected.
Weighting and Sequences
Sequence
A specific set of RF pulses and timing parameters used to generate images.
Weighting
Dominant tissue property contributing to contrast (T1-weighted, T2-weighted).
T1-weighted image
Image where contrast reflects differences in T1 relaxation.
T2-weighted image
Image where contrast reflects differences in T2 relaxation.
Key Structural MRI Sequences
FLAIR (Fluid-Attenuated Inversion Recovery)
T2-based sequence that suppresses CSF signal to highlight pathology.
Diffusion-Weighted Imaging (DWI)
Sequence sensitive to water molecule movement.
Restricted diffusion
Reduced water movement, appearing bright on DWI.
ADC (Apparent Diffusion Coefficient)
Map used to confirm true diffusion restriction.
Gradient-echo / Susceptibility-weighted imaging
Sequences sensitive to magnetic susceptibility differences (blood, calcium).
3. Main Structural MRI Sequences (What They Show)
T1-Weighted Imaging
Best for:
Anatomy
Brain structure
Myelination
Cortical malformations
Appearance:
White matter → bright
Grey matter → grey
CSF → dark
T2-Weighted Imaging
Best for:
Pathology
Oedema
Inflammation
Injury
Appearance:
CSF → bright
Grey matter → grey
White matter → darker
FLAIR (Fluid-Attenuated Inversion Recovery)
A T2-based sequence
Suppresses signal from CSF
Best for:
Periventricular pathology
White-matter disease
Cortical and subcortical lesions
Diffusion-Weighted Imaging (DWI)
Reflects movement of water molecules.
Restricted diffusion occurs when water movement is limited (e.g. acute injury).
Used for:
Acute hypoxic-ischaemic injury
Stroke
Highly cellular pathology
Gradient / Susceptibility Sequences
Sensitive to magnetic susceptibility differences.
Used for detecting:
Blood
Blood products
Calcium
Blood and calcium appear dark.
4. Clinical Applications and Interpretation
MRI is used to:
Assess brain anatomy
Identify structural abnormalities
Evaluate brain development
Distinguish normal variation from pathology
Diagnose genetic and acquired disorders
Guide clinical management and prognosis
Interpretation always requires:
Multiple sequences
Knowledge of patient age
Clinical context
No single MRI sequence is diagnostic on its own.
5. Normal vs Abnormal MRI Findings
Normal Brain
Clear grey–white matter differentiation
Symmetry between hemispheres
Age-appropriate signal appearance
Predictable myelination pattern in children
Abnormal Findings
Loss of grey–white differentiation
Abnormal signal intensity (especially high T2 signal)
Cortical thickening or thinning
Abnormal folding (gyral pattern)
Ventricular enlargement
Asymmetry
Unexpected signal for age
Abnormal signal often reflects:
Excess water
Tissue injury
Delayed or abnormal development
6. Key Developmental Features (Very High Yield)
Normal Development on MRI
Newborn brains have high water content
White matter appears:
Dark on T1
Bright on T2
Myelination progresses over time
Grey–white contrast gradually becomes adult-like
Myelination follows a predictable pattern:
From deep to superficial
From posterior to anterior
From caudal to cranial
By ~2 years:
MRI appearance is broadly adult-like
Why Development Matters
Normal infant brains can look abnormal if adult criteria are used
Delay or arrest of expected changes suggests pathology
MRI helps distinguish:
Normal delay
Genetic myelination disorders
Acquired injury
7. MRI in Developmental Disorders
Structural MRI can identify:
Microcephaly (reduced brain size)
Malformations of cortical development
Abnormal myelination
White-matter injury of prematurity
Hypoxic-ischaemic injury patterns
MRI patterns help determine:
Timing of injury
Severity
Likely developmental outcome
8. Strengths of MRI
Excellent soft-tissue contrast
Non-invasive
No ionising radiation
Sensitive to brain development
Multiple complementary sequences
Gold standard for paediatric neuroimaging
9. Limitations of MRI
Expensive
Time-consuming
Motion-sensitive
Young children may need sedation
Signal changes are often non-specific
Interpretation depends heavily on age and experience