The DWI and ADC maps show changes in ischemic brain within minutes to few The DWI and ADC maps show changes in ischemic brain within minutes to few hours
hours
The signal intensity of acute stroke on DW images increase during the first week The signal intensity of acute stroke on DW images increase during the first week after symptom onset and decrease thereafter, but signal remains hyper intense for after symptom onset and decrease thereafter, but signal remains hyper intense for a long period (up to 72 days in the study by Lausberg et al)
a long period (up to 72 days in the study by Lausberg et al)
The ADC values decline rapidly after the onset of ischemia and subsequently The ADC values decline rapidly after the onset of ischemia and subsequently increase from dark to bright 7-10 days later .
increase from dark to bright 7-10 days later .
This property may be used to differentiate the lesion older than 10 days from more This property may be used to differentiate the lesion older than 10 days from more acute ones (Fig 2).
acute ones (Fig 2).
Chronic infarcts are characterized by elevated diffusion and appear hypo, iso or Chronic infarcts are characterized by elevated diffusion and appear hypo, iso or hyper intense on DW images and hyperintense on ADC maps
hyper intense on DW images and hyperintense on ADC maps
DW MR imaging characteristics of Various Disease Entities MR Signal Intensity
Disease DW Image ADC Image ADC Cause
Acute Stroke High Low Restricted Cytotoxic edema
Chronic Strokes Variable High Elevated Gliosis
Hypertensive encephalopathy
Variable High Elevated Vasogenic edema
Arachnoid cyst Low High Elevated Free water
Epidermoid mass High Low Restricted Cellular tumor
Herpes encephalitis High Low Restricted Cytotoxic edema
CJD High Low Restricted Cytotoxic edema
MS acute lesions Variable High Elevated Vasogenic edema
Chronic lesions Variable High Elevated Gliosis
CLINICAL USES OF DWI &
ADC
Stroke:
Stroke:
Hyperacute Stage Hyperacute Stage:- within one hour minimal hyperintensity seen in DWI and ADC value decrease 30% or more below normal :- within one hour minimal hyperintensity seen in DWI and ADC value decrease 30% or more below normal (Usually <50X10
(Usually <50X10-4-4 mm mm22/sec)/sec)
Acute Stage:- Hyperintensity in DWI and ADC value low but after 5-7days of ictus ADC values increase and return to normal Acute Stage:- Hyperintensity in DWI and ADC value low but after 5-7days of ictus ADC values increase and return to normal value
value (Pseudonormalization)(Pseudonormalization)
Subacute to Chronic Stage:- ADC value are increased (Vasogenic edema) but hyperintensity still seen on DWI Subacute to Chronic Stage:- ADC value are increased (Vasogenic edema) but hyperintensity still seen on DWI (T(T2 shine effect)2 shine effect)
GRE
GRE
In a GRE sequence, an RF pulse is applied that partly flipsIn a GRE sequence, an RF pulse is applied that partly flips the NMV into the transverse plane (variablethe NMV into the transverse plane (variable flip flip angle).
angle).
Gradients, as opposed to RF pulses, are used Gradients, as opposed to RF pulses, are usedto dephase (negative gradient) and rephase (positive gradients) to dephase (negative gradient) and rephase (positive gradients) transverse magnetization.
transverse magnetization.
Because gradients do Because gradients do not refocus field inhomogeneities, GRE sequences with long TEsnot refocus field inhomogeneities, GRE sequences with long TEs are T2* weighted (because are T2* weighted (because of magnetic susceptibility) rather
of magnetic susceptibility) rather than T2 weighted like SE sequencesthan T2 weighted like SE sequences
GRE Sequences contd:
GRE Sequences contd:
This feature of GRE sequences is exploited- in detection of hemorrhage, as the iron in Hb becomes This feature of GRE sequences is exploited- in detection of hemorrhage, as the iron in Hb becomesmagnetized locally (produces its own magnetized locally (produces its own local magnetic field) and
local magnetic field) and thus dephases the spinning nuclei. thus dephases the spinning nuclei.
The technique is particularly The technique is particularlyhelpful for diagnosing hemorrhagic contusions such as those helpful for diagnosing hemorrhagic contusions such as those in the brain and in pigmented villonodular in the brain and in pigmented villonodular synovitis
synovitis..
SE sequences, on the other hand- relatively SE sequences, on the other hand- relatively immune from magnetic susceptibility artifacts, and also lessimmune from magnetic susceptibility artifacts, and also less sensitive in depicting hemorrhage sensitive in depicting hemorrhage and calcification.
and calcification.