3 Introduction to Brain Imaging
61
In addition to FDG-PET, other radiopharmaceuticals have been used in assessment of refractory epilepsy. C-11 flumazenil PET and F-18 flumazenil PET
are markers for GABA-A receptor concentration in the brain. Flumazenil is a
benzodiazepine receptor ligand, which is otherwise used therapeutically to treat
benzodiazepine toxicity. When labelled with the PET radioisotope, it can be used
for imaging benzodiazepine receptor concentration in the brain. Benzodiazepine
receptors are ubiquitous and surrogate markers for neuronal integrity. Reduced
flumazenil binding on PET scans, reflecting neuronal injury, is seen in epileptogenic
foci. Asymmetrically reduced flumazenil PET signal in the medial temporal lobe
has been recently shown to more precisely delineate the seizure focus in refractory
epilepsy cases, as compared to FDG-PET scans. The degree of abnormality on
flumazenil PET has been shown to correlate with seizure frequency in refractory
epilepsy [44, 61, 113]. C-11-labelled alpha-methyl tryptophan (AMT)-PET enables
assessment of cellular incorporation of amino acids in brain tissue. AMT-PET
has been studied in tuberous sclerosis and cortical developmental malformations.
Increased uptake of AMT is seen in epileptogenic tissue and can be used to guide
surgical planning in these conditions [57, 105, 113].
In brain tumour patients, FDG-PET can be used for grading of gliomas and for
identifying hypermetabolic lymphomas. High-grade gliomas, particularly glioblastoma multiforme, demonstrate a high degree of FDG uptake. Hypermetabolic foci
within lesions can be used to guide biopsy and can contribute to decision-making
when distinguishing tumour recurrence and radiation necrosis. However, low-grade
gliomas may demonstrate reduced tracer uptake as compared to the surrounding
brain parenchyma because there is high background FDG uptake in the normal
brain tissue. Therefore, accurate delineation of tumour margins for surgical therapy
planning purposes is suboptimal with FDG-PET in brain tumours. However, other
radiopharmaceuticals have been used to circumvent this problem. C-11 methionine
and F-18 fluoroethyl tyrosine are radiolabelled amino acid analogues that have low
uptake in the normal brain tissue but demonstrate significantly greater accumulation
in both low- and high-grade gliomas [113–115]. Increased concentration of Ltype amino acid transporter in the neoplastic tissue has been shown to correlate
with tracer uptake [114]. The sharper contrast between abnormal neoplastic tissue
and the surrounding normal brain enables more accurate assessment of tumour
margins for surgical and radiotherapy planning. In multiple studies, abnormal C11 methionine uptake areas were found to extend beyond contrast-enhanced MRI
lesions in the majority of brain tumour patients [114] (Fig. 3.3). Similarly, use of
FET-PET increases the specificity of biopsy planning in brain tumour patients as
compared to MRI alone [82, 98].
3.3 Summary and Conclusions
MR imaging at high spatial resolution is used for noninvasive localisation of brain
tumours and seizure-causing lesions and for the mapping of functional brain areas
and white matter fibre tracks with respect to structural abnormalities. The goal of
61
In addition to FDG-PET, other radiopharmaceuticals have been used in assessment of refractory epilepsy. C-11 flumazenil PET and F-18 flumazenil PET
are markers for GABA-A receptor concentration in the brain. Flumazenil is a
benzodiazepine receptor ligand, which is otherwise used therapeutically to treat
benzodiazepine toxicity. When labelled with the PET radioisotope, it can be used
for imaging benzodiazepine receptor concentration in the brain. Benzodiazepine
receptors are ubiquitous and surrogate markers for neuronal integrity. Reduced
flumazenil binding on PET scans, reflecting neuronal injury, is seen in epileptogenic
foci. Asymmetrically reduced flumazenil PET signal in the medial temporal lobe
has been recently shown to more precisely delineate the seizure focus in refractory
epilepsy cases, as compared to FDG-PET scans. The degree of abnormality on
flumazenil PET has been shown to correlate with seizure frequency in refractory
epilepsy [44, 61, 113]. C-11-labelled alpha-methyl tryptophan (AMT)-PET enables
assessment of cellular incorporation of amino acids in brain tissue. AMT-PET
has been studied in tuberous sclerosis and cortical developmental malformations.
Increased uptake of AMT is seen in epileptogenic tissue and can be used to guide
surgical planning in these conditions [57, 105, 113].
In brain tumour patients, FDG-PET can be used for grading of gliomas and for
identifying hypermetabolic lymphomas. High-grade gliomas, particularly glioblastoma multiforme, demonstrate a high degree of FDG uptake. Hypermetabolic foci
within lesions can be used to guide biopsy and can contribute to decision-making
when distinguishing tumour recurrence and radiation necrosis. However, low-grade
gliomas may demonstrate reduced tracer uptake as compared to the surrounding
brain parenchyma because there is high background FDG uptake in the normal
brain tissue. Therefore, accurate delineation of tumour margins for surgical therapy
planning purposes is suboptimal with FDG-PET in brain tumours. However, other
radiopharmaceuticals have been used to circumvent this problem. C-11 methionine
and F-18 fluoroethyl tyrosine are radiolabelled amino acid analogues that have low
uptake in the normal brain tissue but demonstrate significantly greater accumulation
in both low- and high-grade gliomas [113–115]. Increased concentration of Ltype amino acid transporter in the neoplastic tissue has been shown to correlate
with tracer uptake [114]. The sharper contrast between abnormal neoplastic tissue
and the surrounding normal brain enables more accurate assessment of tumour
margins for surgical and radiotherapy planning. In multiple studies, abnormal C11 methionine uptake areas were found to extend beyond contrast-enhanced MRI
lesions in the majority of brain tumour patients [114] (Fig. 3.3). Similarly, use of
FET-PET increases the specificity of biopsy planning in brain tumour patients as
compared to MRI alone [82, 98].
3.3 Summary and Conclusions
MR imaging at high spatial resolution is used for noninvasive localisation of brain
tumours and seizure-causing lesions and for the mapping of functional brain areas
and white matter fibre tracks with respect to structural abnormalities. The goal of