Neuroradiology 2 2007

Neuroradiology 2
Comparing modalities

Graduate Entry Medicine Programme
Daniel Bulte


Objectives
 Review imaging modalities
 Understand their features
 Learn their limitations
 See how they can complement each other
 Ask more questions


Phrenology

Gall and Spurzheim
(1810)
Functional localization


Brodmann’s anatomical 
areas


Important point

 We do not only use 10% of our brains!

Functional Localisation
 Brain injury and surgical cases
 Direct electrode stimulation/recording
 PET ­ blood flow and metabolism
 EEG ­ scalp potentials from neuronal firing
 fMRI ­ blood oxygenation
 MEG ­ magnetic field from neuronal firing


Measurement
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X­Rays
CT Scans
MRI/MRS 
Ultrasound/Sonography
PET/SPECT
EEG
MEG
NIRS

Early X­Ray


Advanced CT Imaging


X­Ray and CT
 Uses ionising radiation
 Only sees hard tissue without 
contrast agents
 Fantastic spatial resolution
 No spatial distortions
 Readily available & “cheap”
 No functional/metabolic information


How Tomography Works
y

Oblique
x


How Tomography Works
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Magnetic Resonance 
Imaging
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Good spatial resolution
OK temporal resolution
Safe

Readily available
Gives both structural and functional images
Functional measurements are indirect
Can be spatially distorted
Can be used with or without contrast agents
Best with soft tissues


Magnetic Resonance 
Spectroscopy MRS
 Looks at signals other than H2
 A chemical profile of the voxel
 Poor spatial resolution
 Very poor temporal resolution
 Many compounds overlap
 Difficult to quantify
 Very safe


MRS


Electroencephalography 
EEG


EEG
 Measures electrical activity directly
 Fantastic temporal resolution
 Very poor spatial resolution

 Very safe (passive)
 Limited spatially
 Cheap and easy, and portable
 Can be difficult to interpret


EEG


Magnetoencephalography 
MEG


MEG
 Poor spatial resolution, but better 
than EEG
 Great temporal resolution
 Very sensitive, but susceptible to 
noise (blinking, breathing etc)
 Expensive, not often available
 Limited depth profile
 Can be difficult to interpret
 Very safe (passive)
 Direct measurement of activity


MEG


MEG vs EEG

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Transcranial Doppler TCD
 Detecting intracranial stenosis/thrombosis/ 
occlusion/dissection
 Detecting collateral flow secondary to 
obstruction proximal to the circle of Willis
 Detecting vasospasm in patients with 
subarachnoid haemorrhage
 Determining hyperaemia in head trauma
 Evaluating feeding vessels to arteriovenous 
malformations circle of Willis


TCD