Protocols & Procedure Week 4

Anatomy Essentials

• Most-tested midline & inferior structures

  • Corpus callosum (genu = anterior bend, splenium = posterior bend)

  • Optic chiasm

  • Pons & brain-stem landmarks (aqueduct of midbrain, 4th ventricle, sagittal sinus)

    
    

  • Ventricular system: lateral → 3rd → 4th ventricles

  • Cerebellum and cerebellar tonsils

    
    

• Lobes (know but quizzed less): frontal, parietal, temporal, occipital

• Cranial nerves

  • Frequently examined; very small → practice on axial images

  • Example shown: trigeminal nerve

    
    

• Misc.

  • Nasal septum / deviated septum, paranasal air cells

  • Extra-ocular muscles (e.g.
    inferior rectus)

• Internal carotid arteries (ICA) identified as paired flow voids

  
  

  Clinical Reasons for a “Routine” Brain MRI

  • Demyelinating disease (multiple sclerosis)

  • Primary CNS tumour or known metastases

  • Opportunistic infection (AIDS)

  • Acute / chronic stroke, CVA, infarction, haemorrhage

  • Hearing loss, visual disturbances

  • Head trauma

  • Pre-operative or radiation-therapy planning

Coil & Equipment Considerations

• Use dedicated head coil

  • Variants: HD, transmit/receive, vascular-focused, open-face, bar-style, “full mask”

• Immobilisation → high SNR

  • Foam pads, straps, earplugs ± earbuds, mirror attachment for claustrophobia

• Innovative workaround: slimmed headphones before earbud systems arrived (not manufacturer-approved!)

Patient Preparation & Positioning

• Supine in head coil; midsagittal plane centred (longitudinal midline)

• Horizontal landmark ≈ nasion/glabella (difference < $0.5\ \text{in}$ → registry will not split)

• Inter-pupillary line (IPL) parallel to couch

  • Use angled sponge under head/coil if pt. cannot lie flat – adjust slice angulation accordingly

• Comfort measures: knee sponge, skin-to-magnet pads, emergency squeeze ball, verbal check before leaving room

Scout / Localizer & System Calibrations

• 3-plane localizer (aka scout) $\approx 10{-}30\ \text{s}$

  • If only $2$ planes appear → cable/board disconnected

• Possible auto-calibration (e.g. ASSET) $\approx 5{-}10\ \text{s}$

Slice Prescription Fundamentals

• Coverage: vertex → foramen magnum (routine brain)

• General philosophy: “skin-to-skin” (overslice) vs protocol minimum – follow site rule

• Typical order built from sagittal reference, then copy/paste to other sequences

Sagittal Setup

• Start midsagittal → fan outward or drag left→right; medium slice/gap; ensure whole cerebrum/cerebellum

Coronal Setup

• Usually vertex → frontal sinuses → posterior skull base

• Angle parallel to brain-stem axis / clivus or simply keep vertical; often reserved for trauma, paediatrics, post-contrast

Axial Setup

• Angle parallel to AC–PC line or just superior to frontal sinuses

• Brain axials prescribed inferior→superior (common) but check local convention

• Must include foramen magnum to cortical surface

  
  

Routine Brain MRI – Core Sequences & Rationale

(Names differ by vendor; copy/paste geometry once set.)

• Axial Fast Spin Echo (FSE) $T2$

• Axial $T2$ FLAIR (suppresses CSF) → exquisitely sensitive for MS plaques

• Axial $T1$ (spin echo) – favoured in paediatrics for grey/white contrast

• Axial $T1$ Fat-Sat pre + post gadolinium (tumour search)

• Axial/Coronal/Sagittal 3D incoherent spoiled gradient echo $T1$ (e.g. SPGR, T1-BRAVO) – high-resolution, reformatting

• Axial Gradient Echo $T2^*$ (susceptibility) – micro-bleeds, haemorrhage

• Echo-Planar Imaging (EPI) Diffusion Weighted Imaging (DWI)

  • $b$-values chosen by system; stroke detection (restricted diffusion)

• Perfusion (Dynamic Susceptibility Contrast or ASL)

  • Primarily tumours; also acute stroke

    
    

Quick / “Fast Head” Protocols

• Goal < $10\ \text{min}$ entire exam (e.g. acute stroke, paediatric motion)

• Trade-offs

  • Longer echo trains → blurring; mitigate by shortening ETL

  • Parallel imaging + rectangular FOV to shorten time

  • Slightly thicker slices acceptable

    
    

Artefacts & Mitigation

• Motion (voluntary/involuntary)

  • Use pads, sedation, single-shot FSE or single-shot EPI when necessary

• Flow (carotids, vertebrals, venous sinuses)

  • Saturation bands inferior to FOV; consider superior band as well (see diagram)

  • Gradient Moment Nulling (GMN) effective on $T2$ techniques

• Ghosting (phase axis) → swap phase/freq direction

• Susceptibility rises with field strength; GRE & EPI more vulnerable

• Distortion (chemical-shift, off-resonance)

  • Reduce phase-encodes, adjust bandwidth

    
    

Contrast (Gadolinium) Indications

• Tumour (primary/metastatic), infection, active MS lesion, vascular abnormality, dating infarct

• Always include matched $T1$ Fat-Sat pre + post

• Perfusion series immediately post-bolus for cerebral blood volume (CBV) mapping

  
  

Temporal Lobe–Focused Imaging

• Indications: epilepsy focus, temporal lesion

• Same setup; target slab only inferior TL → superior corpus callosum

• Sequences: thin-slice axial, coronal perpendicular to axial (Sylvian fissure reference), 3D volume $T1/T2$ for small-structure visualisation

  
  

Internal Auditory Canal (IAC/ IAM) Protocol

• Clinical: sensorineural hearing loss, tinnitus, vertigo, facial palsy (rule-out vestibular schwannoma)

• Coverage box: foramen magnum → superior corpus callosum BUT slice group tightly around petrous temporal bones

• Key sequences

  • Axial thin-slice FSE $T2$ or $T2^*$ (cisternography-style)

  • Coronal FSE $T2$

  • 3D grad-echo $T1$ pre + post (isotropic $<1\ \text{mm}$) → nerve enhancement

• Imaging tricks

  • Volume 3D eliminates slice gap → multiplanar reformats; isotropic voxels

  • Use balanced GRE to tame pulsatile flow in labyrinthine artery

    
    

Image Optimisation & Parameter Pearls

• High intrinsic SNR in brain → exploit

  • Reduce Receive Bandwidth (RBW) to boost SNR

  • Fine matrix ≥ $512\times512$ for spatial resolution

  • Increase $NEX/NSA$ if SNR still low

• Volume (3D) acquisition: “very thin slices + 0 gap” mantra; allows MPR & quantitative volume tools

• Rectangular/asymmetric FOV + parallel imaging = shorter TR/scan-time without major SNR penalty

  
  

Common Registry Memory Aids

• Axials show cranial nerve origins – scrutinise during review

• GMN ↔ $T2$ sequences; Sat band ↓ Flow artefact

• Contrast questions → think “$T1$ post + fat-sat”

• Volume imaging (3D) → “thin, no gap, isotropic, reformat”

• IAC box = “raccoon mask” around petrous bones

• Coverage constants

  • Routine brain: vertex ↔ foramen magnum

  • Temporal focus: inferior TL ↔ superior corpus callosum

• Matrix benchmark $512$, scout $10{-}30$ s, calibration $5{-}10$ s, overslice $99.9\%$ certainty rule