Table 1.

The probes used in PET and their mechanisms, applications, and defects

Agent	Mechanism	Application	Defect
[18F]Fluoro-2-deoxy-2-glucose (FDG)	GLUT-1 and GLUT-3 are overexpressed in tumor, and the expression relates to malignant change	Diagnosis, evaluation of malignancy, differentiation of recurrent brain tumors from necrosis, indication of prognosis[2,110], differentiation of metastatic brain tumor from primary brain tumor[111]	Low sensitivity[2], cannot differentiate inflammation from cancer[112], cannot precisely determine the tumor boundaries[19]
l-[methyl-11C]Methionine (MET)	Increased protein synthesis, activated carrier-mediated transport systems[113]	Diagnosis[114], differentiation of gliomas from other non-tumoral brain lesions[115], delineation of the tumor extent[19,116,117], differentiation of tumor recurrence from radionecrosis	Overlap of several tumor grades makes the grading less useful
S-[11C]Methyl-l-cysteine (MCYS)	L-transport system	Diagnosis of solid tumors[118]	Not available
O-(2-[18F]Fluoroethyl)-l-tyrosine (FET)	Mediated by LAT2 (a subtype of the L-transport system) that is expressed only in tumor cells and not in inflammatory cells[119,120]	Diagnosis with high accuracy and sensitivity[121], grading gliomas[122], differentiation of gliomas from inflammation[123], biopsy guidance and treatment planning of cerebral gliomas[124]	Not available
α-[11C]Methyl-l-tryptophan (AMT)	Metabolized by the kynurenine pathway. The initial and rate-limiting enzyme of the kynurenine pathway, indoleamine 2,3-dioxygenase (IDO), widely expressed in low-grade gliomas and glio-neuronal tumors but restricted in high-grade gliomas[125,126]	Diagnosis of low-grade gliomas[126]	Not available
l-5-(2-[18F]fluoroethoxy)tryptophan (FEHTP)	Carrier-mediated transport systems	Diagnosis of tumor xenografts[127]	Limited number of clinical trials
3,4-Dihydroxy-6-[18F]-fluoro-l-phenylalanine (18F-FDOPA)	FDOPA can arrest in the basal ganglia but little in normal cerebral cortex and white matter[128]	Precise anatomic localization of brain gliomas[128], imaging of low-grade tumors and evaluation of recurrent tumors, distinguish tumor recurrence from radiation necrosis[129,130]	Cannot differentiate low-grade from high-grade gliomas[129]
124I-Labeled l-phenylalanine	Carrier-mediated transport systems	Diagnosis of glioma[131]	Not available
[11C]Choline	Increased lipogenesis in brain tumor requires more phospholipids, which results in the accumulation of the contents of choline-containing compounds[132,133]	Diagnosis, differentiation of low-grade from high-grade gliomas[134]	High uptake of [11C]choline in choroid plexus, venous sinuses, and the pituitary gland make it difficult to recognize the existence and/or the border of brain tumor around these structures[114]
3-Deoxy-3-[18F]fluorothymidine (FLT)	Accumulates in proliferating tissues through thymidine kinase[135,136]	Diagnosis, differentiation of high-grade from low-grade gliomas[135,137], prediction of the time to tumor progression and survival[136], monitor therapy response[138]	Fail in the evaluation of recurrent low-grade gliomas[130], increased accumulation in the normal brain with high-grade gliomas[137]
[11C]Acetate	Preferentially incorporated into the lipids of tumor cells and serves as an obvious carbon source[139]	Diagnosis of astrocytomas[140], differentiation of high-grade from low-grade gliomas[141]	Poor grading value#
18F-Labeled RGD	The receptors, αv-integrins (αvβ3, αvβ5), are highly expressed in activated endothelial cells and tumor cells but not in normal brain cells and quiescent endothelial cells[7]	Diagnosis of gliomas larger than 1.5 mm[10]	Limited number of clinical trials