PICTORIAL REVIEW OF THE CLINICAL APPLICATIONS OF DWI IN PEDIATRIC IMAGING
The practical applications of DWI are in identifying regions and patterns of abnormal DWI signal and further characterizing them to low or high ADC’s values.
In this poster we will demonstrate various disease processes associated with diffusion restriction in pediatric neuroimaging.
CEREBRAL ISCHEMIA
Relatively rare disease,
having an estimated incidence of 2.5–13/100,000/year but remains one of the most common causes of death in childhood.
The pathophysiology of restricted diffusion is Cytotoxic edema causing cellular swelling and decreased extracellular space
a) Arterial ischemic strokes (Fig 3,4,5,7 & 8 )
There are several causes which include sickle cell disease,
cardioembolic,
Moya-Moya Syndrome,
cervical arterial dissection,
steno-occlusive cerebral arteriopathy and pro-coagulable states.
These are characterized by sharply marginated areas of restricted diffusion confined to arterial vascular territory3
b) Venous infarcts (Fig 9 )
Can occur secondary to infections,
extrinsic compression of the venous sinus by intracranial mass,
dehydration,
trauma,
and hypercoagable states.
There is predominance of vasogenic edema not confined to arterial territory associated with areas of haemorrhage3.
c) Global hypoxic ischemic injury ( Fig 24 )
Brain injury results from a combination of global cerebral hypoperfusion and hypoxia.
Conventional imaging may be negative until 3-7 days after the insult.
DWI provides a means of earlier detection of injury especially in the first hours after the insult4.
DISORDERS OF ALTERED AUTOREGULATION:
a) Posterior Reversible Encephalopathy Syndrome (PRES)
Secondary to the inability of the brain to autoregulate in response to acute changes in blood pressure. Hyperperfusion with resultant disruption of the blood brain barriers results in vasogenic edema, but usually not infarction. Areas of increased signal on ADC are seen due to increased diffusion.
Rarely,
there can be reduced diffusion and small hemorrhages in these areas.
b) Status epilepticus (Fig 16 )
Hypermetabolic state due to sustained seizure activity,
resulting in vasogenic and/or cytotoxic edema.
If severe enough,
cell metabolism will fail resulting in eventual cell death.
The involved regions are cerebral cortex and subcortical white matter,
hippocampi and mesial temporal lobes and thalamus,
particularly the pulvinar region.
These regions may demonstrate corresponding diffusion abnormalities,
with increased DWI signal and in some instances reduced ADC values5.
TRAUMA:
a) Diffuse axonal injury
It is the result of shearing forces,
typically from rotational acceleration (most often a deceleration).
DWI in addition to SWI sequences are exquisitely sensitive and may demonstrate small regions of restricted diffusion at the grey-white matter junction,
in the corpus callosum or the brain stem.
b) Cerebral fat embolism syndrome ( Fig 17 )
Cerebral fat embolism usually occurs in patients with long bone fractures ,
rarely it has been described as part of a sickle cell crisis with bone marrow fat necrosis and subsequent embolism.
On DIffusion weighted imaging it is seen as bright spots on a dark background (starfield pattern ) corresponding to the region of T2 signal abnormality.
Gradient can demonstrate multiple minute hypointense foci in the brain.
TOXIC AND METABOLIC DISORDERS:
a) Disorders of amino acid metabolism
i.
Canavan disease - ( Fig 24 )
ii.
Phenylketonuria
iii.
Maple syrup urine disease
b) Disorders of mitochondrial function (Fig 17 )
i.Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS)
ii.
Kearns-Sayre syndrome
iii.
Leigh disease
c) Lysosomal storage disorders
i. Krabbe disease
ii. Tay-Sachs disease
iii. Sandhoff disease
d) Hyperammonemia
f) Central pontine and extrapontine myelinolysis
DEMYELINATION:
a) Multiple sclerosis ( FIG 19 )
Multiple sclerosis is believed to result from a cell-mediated autoimmune response against one's own myelin components,
with loss of oligodendrocytes with little or no axonal degeneration in the acute phase,
however,
in later stages,
loss of oligodendrocytes results in axonaldegeneration.The areas which are involved are periventricular/perivenular,
calloso-septal interface,
subcortical U-fibers,
brachium pontis,
brainstem,
spinalcord and infratentoriallocations.
Majority of acute plaques demonstrate normal or increased diffusivity.Few acute MS plaques may show restricted diffusion often at the margins.Subacute/chronic plaques demonstrate increased diffusivity.
b) Acute Demyelinating encephalomyelitis ( Fig 23 )
Characterized by a monophasic acute inflammation and demyelination of white matter typically following a recent (1-2 weeks prior) viral infection or vaccination.
Lesions are usually bilateral but asymmetrical with involvement of cerebral cortex,
basal ganglia and thalami.
There can be peripherally restricted diffusion.
The center of the lesion,
although high on T2 and low on T1,
does not have increased restriction on DWI as one would expect with abscess nor does it demonstrate absent signal on DWI as one would expect from a cyst
1. PEDIATRIC BRAIN TUMOURS ( FIG 20 )
DWI provides information about tumor cellularity and thereby helps in differentiation of different tumor subtypes.
PNET are densely packed cellular tumors with decreased extracellular space and cause decreased diffusion in comparison to other tumors types.
Evolving ADC changes with treatment may also provide valuable information about tumor response.
Some of tumours which demonstrate restricted diffusion include medulloblastoma,
pineoblastoma,
retinoblastoma and ependymoma6.
2. INFECTIONS:
a) Abscess
Brain abscess is a potentially life-threatening condition requiring rapid treatment,
and prompt radiological identification.
These are characterized by peripherally enhancing lesions which demonstrate central restricted diffusion.
b) Encephalitis ( Fig 10,12,13,14 & 15 )
Brain parenchymal infection by a number of different pathogens.
These can have varying appearances based on the causative agent
MISCELLANEOUS:
1.
ACUTE ENCEPHALOPATHY WITH BIPHASIC SEIZURES ( Fig 21 ) :
This is a syndrome characterized by biphasic seizures and altered consciousness during acute phase and restricted diffusion in subacute stage predominatly in subcortical white matter on magnetic resonance imaging.
Viral infections like influenza A and human herpes virus 6 have been implicated as etiological factors.
2.
CSF SPREAD OF METASTASIS ( Fig 22 ):
This refers to spread of malignant cells through CSF from a primary CNS tumors resulting in drop metastasis.
CSF spread is most commonly seen in medulloblastoma ,
GBM ,
ependymoma ,
etc.
DIffusion imaging helps in identifying even the tiniest deposit in remotest corners which otherwise would have been overlooked on routine sequences.