Magnetic Resonance Imaging (MRI)

MRI sequence attenuation in different tissues

	T1	T2	DWI
Fluid (Urine, CSF)	Dark	Light	Dark
Muscle	Grey	Grey
Fat	Light	Light
Brain – Grey matter	Grey	Grey	Grey
Brain – White matter	Light grey	Dark grey	Light grey
Methaemaglobin	Light	Dark or light
Melanin	Light
Proteinaceous fluid	Light

MRI in Ischaemic stroke

	T1	T2	DWI	ADC	Contrast	SWI
Early hyperacute	N	N	Bright	Dark	Nil	N
Late hyperacute (>6hours)	N	Bright +	Bright	Dark	Mild enhancement	N
Acute	Reduced (after 16hrs)	Bright ++	Bright	Returning to normal	Enhancement (>5days)	Microhaemorrhage
Subacute	Reduced	Bright ++ (May get fogging ~D10)	Bright	Normal (day 10-15) then Bright	Enhancement
Chronic	Reduced	Bright ++	Bright – returning to normal	Bright	Enhancement for 2-4 months

MRI and intracranial haemorrhage

Stage	Time	Blood products	CT	T1	T2	DWI	ADC	T2*
Hyperacute	<24hrs	Oxy-Hb	Hyper	Iso	Bright	Bright	Dark	Rim bloom
Acute	1-3days	Deoxy-Hb	Hyper	Iso	Dark	Dark	Dark	Bloom
Early subacute	>3d-1wk	Intracel. Met-Hb	Iso	Bright	Dark	Dark	Dark	Very dark
Late subacute	1wk-mths	Extracel. Met Hb	Hypo	Bright	Bright	Bright	Dark	Dark rim, variable centre
Chronic	>14days	Haemosiderin	Hypo	Dark	Dark	Dark	Dark	Dark

DWI and ADC

· Stroke

o Increasing intensity during first week then slow resolution occurs over up to 72days

o Almost 100% sensitive – rare cases of missed CVA, thought to be due to initial sub-occlusive thrombus.

o Almost 100% specific (see below)

o ADC

§ Initially dark

§ By 5-7 days returns to normal intensity (definitely by 7-10 days)

· Tumours – Glioma

o May be hyper, hypo or iosdense on DWI

o If increased signal it is usually minor and non-specific

· Tumours – Mets

o May be hyper, hypo or iosdense on DWI – rarely hyperintense

o If there is a necrotic core it is usually strongly hypointense on DWI

· Tumours – Meningiomas

o Benign – usually isointense

o Malignant – often hyperintense

· Tumours – Lymphoma

o Generally hyperintense

· Abscess

o Usually hyperintense

o ADC

§ Usually hypointense, often far more so than stroke which may help with differentiation

· Herpes Encephalitis

o Hyperintense with low ADC

· Haemorrhage

· MS

o New, contrast enhancing lesions are often - Hyperintense

o Old lesions are often isointense

o ADC values often increased indicative of T2 shine through

· Sustained seizure activity

o Cortical and media thalamic DWI abnormality can occur