functional metabolic imaging of the brain using fluorine-18 fluorodeoxyglucose ( fdg ) positron emission tomography ( pet ) is one of the most useful tools in distinguishing the patient 's symptoms from the various types of dementia by elucidating global and regional deficits in cerebral glucose metabolism .
magnetic resonance imaging ( mri ) often plays a role as well , as it is more widely available , and is used to evaluate structural abnormalities within the brain parenchyma .
we present a case where hybrid imaging of pet / mri was used to diagnose a patient with semantic dementia ( sd ) , which is one of the lesser known subtypes of frontotemporal lobe dementia ( ftd ) .
sd is characterized by a loss of language and verbal skills with late stage behavioral changes .
it is associated with predominantly temporal lobe atrophy ( left greater than right ) and is often referred to as temporal variant ftd .
a 61-year - old , right - handed male presented with a sudden change in mental status preceded by a significant decline in functionality over the past 2 years .
he was previously a highly functional person and worked as an executive for a fortune 500 company .
his change in mental status was characterized by several behavioral changes ; including cruelty toward family members and pets , promiscuity , adultery , drug abuse , wild spending habits , and lack of empathy .
his family had also noticed a change in his speech patterns in terms of phonation and pronunciation of words .
his initial diagnosis was bipolar disorder ; however , he did not respond to mood stabilizing drugs . after losing his job ,
the mri [ figure 1 ] demonstrated diffuse parenchymal atrophy , which was slightly more extensive in the temporal lobes . there was a lacunar infarct in the right thalamus that was suggestive of vascular dementia .
61-year - old male with change in mental status and decline in function diagnosed with semantic dementia .
( a and b ) the magnetic resonance imaging ( mri ) transaxial views of the t1-weighted and ( c and d ) t2-weighted images with proton density and fluid suppression sequences demonstrate diffuse parenchymal atrophy , slightly more substantial in the temporal lobes ( arrows on a and c ) .
these non - specific findings do raise the possibility of frontotemporal dementia ( ftd ) .
a thalamic infarct ( arrows on b and d ) , along with subtle white matter abnormalities are also seen , which are suggestive of vascular dementia , though inconsistent with the negative magnetic resonance angiography findings .
the images were co - registered with the mri using the institute of electrical and electronics engineers ( ieee ) mutual information algorithm .
the resultant pet / mr images [ figure 2 ] demonstrated a pattern of decreased fdg uptake that was predominantly confined to the temporal lobes and was particularly severe in the amygdalae and hippocampi .
the decrease in activity was greater on the left side and extended superoposteriorly toward the parietal lobe and wernicke 's area .
the other atrophic areas and white matter changes seen on the mri had intact glucose metabolism .
the lacunar infarct also had decreased fdg uptake , but that was considered to be an incidental finding .
the decreased glucose metabolism affecting only the temporal lobes ( worse on the left side ) with the concordant areas of atrophy on the mri were highly specific for semantic dementia and allowed for a definitive diagnosis to be made .
61-year - old male with change in mental status and decline in function diagnosed with semantic dementia .
the fluorine-18 fluorodeoxyglucose ( f-18 fdg ) positron emission tomography ( pet ) of the brain was obtained and coregistered with the t1-weighted sequences of the mri .
two separate triangulated cross - sections of the reconstructed images in ( a and d ) axial views , ( b and e ) coronal views , and ( c and f ) sagittal dimensions show significantly reduced uptake corresponding to atrophy in the temporal lobes ( arrows on a ) , involving the amygdalae ( arrows on b ) , hippocampi ( arrows on e ) , and wernicke 's area ( arrow on c ) .
the function is well preserved in the remaining areas of global atrophic change and white matter changes .
there is also reduced activity in the location of the suspected right thalamic infract ( arrow d ) .
although each type of dementia has its own constellation of symptoms , there is significant overlap and most patients present with features of several different psychiatric and neurologic disorders . the majority of patients present with some type of memory loss with a variety of non - specific behavioral changes .
anatomic imaging is often not useful , other than to exclude physical causes such as stroke or hydrocephalus .
there may be subtle clues to a diagnosis , but nothing definitive . functional imaging provided by fdg pet evaluation of brain glucose metabolism
alzheimer 's dementia ( ad ) is characterized by decreased glucose metabolism in the temporal and parietal lobes with sparing of the frontal lobes .
ftd , as the name suggests , demonstrates decreased activity in the frontal and temporal lobes with sparing of the parietal lobes .
dementia with lewy bodies ( dlb ) has pattern of decreased activity similar to ad , but also demonstrates decreased activity in the occipital lobe .
however , many early stages of dementia may have atypical or variable appearances on fdg pet .
for example , semantic dementia has involvement confined to the temporal lobe , but the deficits are often not as striking as in the other types of dementia on fdg pet . on mri
, the aforementioned types of dementia may show global atrophy , with more extensive changes in the involved lobes ( such as more frontal lobe atrophy in ftd ) .
however , these structural changes often occur late in the disease progression and can be non - specific .
additionally , there is no typical structural abnormality in dlb . often at times , the fdg pet will show metabolic deficits before atrophic changes become apparent on mri . in this case
, we were able to combine metabolic deficits with the anatomic information provided by mri .
this allowed us to corroborate the decreased activity confined to the temporal lobe with specific structures or areas within the brain that were involved .
the amygdale , hippocampi , and wernicke 's area all showed focal decreases in activity compared with the overall reduction of radiotracer activity in the temporal lobes .
the fdg pet , on the other hand , showed that the decreased activity was greater on the left side , which is classic for semantic dementia , whereas the atrophy on the mri seemed relatively symmetric .
since the affected areas of the brain are involved in empathy ( amygdala ) , meaningful word usage ( wernicke 's area ) , memory ( hippocampus ) , and conceptual knowledge ( amygdala and hippocampus interaction ) ; the patient 's symptoms could be easily explained .
this case demonstrates an example of the value of combined pet - mri in clinical practice .
co - registration of images provides essential information that is needed to make a diagnosis in challenging cases .
thus , clinical symptoms can be related to lesions in specific parts of the brain .
 we report a case of a 61-year - old male who presented with a sudden change in mental status . from a psychiatric standpoint , 
 his symptoms were consistent with a bipolar disorder . 
 a neurology consult raised suspicion for vascular dementia , given the sudden onset of symptoms ; however , the magnetic resonance angiography ( mra ) was unremarkable . 
 the magnetic resonance imaging ( mri ) had findings that were suggestive of both vascular and frontotemporal lobe dementia based on parenchymal atrophy and a lacunar infarct near the thalamus . 
 however , by co - registering the magnetic resonance images with a subsequent fluorine-18 fluorodeoxyglucose positron emission tomography ( f-18 fdg pet ) , and combining the functional data with the anatomic appearance , the diagnosis was narrowed to semantic dementia , which is one of the lesser known subtypes of frontotemporal lobe dementia ( ftd ) . 