Positron emission tomography (PET) scans provide two- and three-dimensional pictures of brain activity by measuring radioactive isotopes that are injected into the bloodstream. PET scans of the brain are used to detect or highlight tumors and diseased tissue, measure cellular and/or tissue metabolism, show blood flow, evaluate patients who have seizure disorders that do not respond to medical therapy and patients with certain memory disorders, and determine brain changes following injury or drug abuse, among other uses.
PET may be ordered as a follow-up to a CT or MRI scan to give the physician a greater understanding of specific areas of the brain that may be involved with certain problems. Scans are conducted in a hospital or at a testing facility, on an outpatient basis. A low-level radioactive isotope, which binds to chemicals that flow to the brain, is injected into the bloodstream and can be traced as the brain performs different functions.
The patient lies still while overhead sensors detect gamma rays in the body’s tissues. A computer processes the information and displays it on a video monitor or on film. Using different compounds, more than one brain function can be traced simultaneously.
PET is painless and relatively risk-free. Length of test time depends on the part of the body to be scanned. PET scans are performed by skilled technicians at highly sophisticated medical facilities.
The French investigators performed F-18 fluoro-L-dopa (FDOPA) positron emission tomography (PET) in patients with temporal lobe epilepsy (TLE) and compared the findings to those with fluorodeoxyglucose (FDG) PET This study was motivated by a prior observation that F-18 FDOPA uptake in basal ganglia is decreased in patients with refractory TLE. A correlation was observed between the localization of seizure focus hypometabolism and decreased FDOPA uptake in both the basal ganglia and the substantia nigra, suggesting that there is a dopamine involvement in TLE. In another study the impact of TLE duration on brain FDG PET pattern was investigated. A negative correlation was demonstrated between epilepsy duration and ipsilateral glucose metabolism and a positive correlation with the contralateral glucose metabolism. This observation suggested that duration of TLE correlates with asymmetric glucose metabolism in the temporal lobes possibly reflecting the compensatory increased metabolism in the more normal temporal lobe over time.
The Brazilian researchers compared the FDG PET brain glucose metabolism with Tc-99m ethylcysteinate dimer (ECD) single-photon emission computed tomography (SPECT) brain regional cortical blood flow in patients with Alzheimer dementia. This study showed that similar functional cerebral regions are involved on PET and SPECT in Alzheimer's disease, although PET seemed to be more powerful in depicting the extent and severity of the functional impairments.
The impact of FDG PET on the diagnosis and clinical management of patients with dementia was reported by the investigators from the Kettering Medical Center in Kettering, Ohio. FDG PET scans were performed on 96 patients with suspected dementia or mild cognitive impairment. Referring physicians were also surveyed to report on whether and how results of the PET scan influenced patient management. The survey response rate was 76%. The survey indicated that PET changed the initial diagnosis to Alzheimer dementia in 26% and disease management in 27% of patients leading to initiation of anticholinergic drug therapy.
There has also been a growing interest in imaging beta-amyloid deposits directly with PET in Alzheimer dementia. An Australian study evaluated the relationship between amyloid burden as assessed by Pittsburgh Compound-B (PIB) PET and cognitive decline in predominantly normal elderly population (age 73 ± 6 years). These investigators observed that subjects with declining cognition were more likely to show cortical PIB retention than in stable subjects, suggesting that amyloid deposition is not a part of normal aging and likely represents preclinical Alzheimer's disease (described as those up to 30% of otherwise normal persons who are over 75 years but show amyloid deposition at autopsy). The researchers from the University of Pennsylvania compared the amyloid imaging agents [F-18]3'-F-PIB and [C-11]PIB in patients with Alzheimer's disease and in healthy subjects. The F-18-labeled compound showed uptake and retention characteristics similar to those of C-11-labeled compound in the more important cortical brain regions with SUV in the range of 3.1 to 4.5. Another similar study corroborated this finding and concluded that the F-18-labeled PET ligand might permit wide application of the compound due to longer half life and more ease of distribution in comparison to the C-11-labeled compound.
The potential benefit of contrast-enhanced PET-CT was compared to non-enhanced PET-CT for differentiation of brain tumor recurrence from radiation necrosis. In this study, 29% of recurrent tumors were missed on non-enhanced PET-CT and were identified correctly on the enhanced scan. Additionally contrast enhancement revealed enhancing lesions in the other areas of brain away from the immediate region of interest. However, the clinical significance of these findings was unclear. The authors concluded that contrast-enhanced PET-CT is more suited in this clinical setting than the more common current procedure of using non-enhance scans.
The Belgian researchers evaluated the diagnostic utility of PET with F-18 fluorotyrosine (FTYR), as a marker of amino acid transport, for detection of skull base meningioma. All tumours showed high FTYR uptake with an average tumor to cortex ratio of 2.53 ± .35. The metabolic abnormality on PET extended beyond the magnetic resonance imaging (MRI) abnormality in 38% and was smaller in 8% of cases. The authors concluded that FTYR PET was useful for the detection of residual meningiomas of the skull base with almost in half of the cases the tumor extent as depicted on PET differing from that on MRI, clinical significance of which is currently undetermined.
Non-FDG-based PET was highlighted in several presentations. The UCLA group evaluated PET with F-18 fluorothymidine (FLT), as a marker of cell proliferation, in a group of patients with brain tumor who were treated with anti-angiogenic agents such as bevacizumab.] A 25% reduction in tumor FLT uptake was considered as metabolic response which was then compared to MRI and survival data. A multivariate analysis showed that FLT response was the most powerful predictor of survival with a lack of reduction of FLT uptake at 6 weeks after treatment initiation increasing the hazard ratio of death by 5-fold. In another corollary study with FLT, the same group of investigators employed a 3-comparment, 2-tissue kinetic model with metabolite and partial volume corrections to estimate the rate constants during treatment in patients with high grade brain tumors. Changes in the influx rate K and the SUV were linked to treatment response which in turn was correlated to patient outcome.
The Japanese investigators compared C-11 acetate, C-11 methionine (MET), and FDG PET in 16 patients with primary and recurrent brain gliomas (5 low grade, 11 high grade). The sensitivities for low-grade and high-grade gliomas were, respectively, 60% and 100% for MET, 40% and 91% for acetate, and 0% and 40% for FDG. The mean SUV for low grade and high grade tumors were, respectively, 1.4 ± 0.4 and 3.1 ± 1.6 for acetate and 4.2 ± 0.2 and 8.3 ± 3.9 for FDG. There was no significant difference in the MET uptake between the low-grade and high-grade tumors. The authors concluded that although MET might not be able to differentiate the histologic grades, it is more sensitive than both FDG and acetate for tumor detection. Therefore this study suggested that multi-tracer PET may be helpful for a more comprehensive evaluation of patients with brain glioma, although the very low rate of FDG localization in the high-grade tumors in this study is atypical.
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