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Stroke: Fogging phenomenon on CT – radiology video tutorial

Stroke: Fogging phenomenon on CT – radiology video tutorial

hello this is frank gaillard from and today in another episode in our stroke series we’re going to examine
the phenomenon of CT fogging. let’s start off by looking at a case. this patient presented two days following the onset of right side
of the weakness with a well demarcated low-density region in the superior division of the left
middle cerebral artery territory. note that the caudate head and lentiform nucleus are spared suggesting that the most likely embolus lodged in an M2
branch sparing the M1 branch from which the lenticulostriate perforators
originate. the patient was scanned once more seven
days later and much to the confusion of the treating
physicians the brain had returned to normal with
apparent return of normal gray white matter differentiation. despite the patient had not improved this is a typical timing and appearance
of CT fogging. when one sees it however it’s worthwhile
asking a couple of questions. firstly is this the right patient? after all if you
accidentally scanned the wrong patient then of course it would appear that they
had not had an infarct. another worthwhile question to ask is, are the
dates correct? if there has been a much longer delay or
more importantly if you’re looking at a CT obtained earlier than the stroke then
of course the appearances will be normal another trap is that in fact the
lesion did not represent an infarct but rather represented oedema from an adjacent tumour in which case if the patient had
received dexamethasone the oedema may have dramatically reduced, although you would expect still to
see some underlying abnormality. similarly the for the cause hypoattenuation was that of cerebritis then perhaps antibiotics could have
improved the appearances sufficiently to resulted in a near-normal CT scan. once you’ve satisfied yourself that these are not
viable options the diagnosis is that of CT fogging, which represents a transient return of the
infarcted brain to a near-normal appearance on CT. CT fogging is seen in up to
fifty percent of cases if serial scans are obtained however typically in the setting of
acute infarcts, scans are obtained over the first few days to evaluate for swelling
and secondly hemorrhage and then not again for quite some time usually missing the time period at which CT fogging occurs. classically CT fogging is seen
during the second and third weeks following an infarct. a number of
processes have been implicated including migration into the infarcted tissues of
lipid laden macrophages proliferation of capillaries a decrease in
the overall amount to the oedema and cortical laminar necrosis. if in doubt of the diagnosis the easiest
way to confirm that this represents CT fogging is to administer contrast as the cortex will usually usually demonstrate vivid ribbon-like cortical enhancement. alternatively an MRI can also
demonstrate similar features. This same patient was scanned a week later still
with MRI and demonstrates the vivid cortical enhancement on post
contrast T1 sequences. non contrast T1 sequences demonstrate some
intrinsic high T1 signal in keeping with cortical laminar necrosis and there
is some residual high T2 signal on FLAIR although note that there is
no significant positive mass effect. so in summary CT fogging occurs
typically two to three weeks following an ischaemic infarct and results in the return of the infarcted tissue to a near-normal appearance on non-contrast CT. it’s important to realize that this does
not represent recovery but rather is merely an artifact. when presented with this, especially if
clinicians to not believe your assessment, this can be confirmed with
contrast studies. you can of course read more about the foggin phenomenon on

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