Positron Emission Tomography and Single Photon Emission Tomography

Positron emission tomography (PET) exploits the annihilation of positrons and electrons into photons to achieve the nuclear imaging analog of X-ray computed tomography (CT). In the decay of a positron-emitting radionuclide, the positron interacts with an electron, yielding two photons that travel in (nearly) opposite directions. By detecting those photons in coincidence, the projection data required for tomographic reconstruction are obtained. 18F-Fluorodeoxyglucose (FDG) is the most commonly used tracer in the clinic. Similar to glucose, FDG is transported into cells by a glucose transporter, but remains trapped within the cell, thus reflecting the energy metabolism within tissues. Highly malignant brain tumors usually show increased FDG uptake in comparison to the surrounding brain parenchyma (Figure 27.10). However, because of inherent limitations, such as low resolution and high background glucose metabolism of normal gray matter structures, 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) use is not a part of the routine diagnostic evaluation of brain tumor patients. Other potential uses for PET in brain tumor evaluation include grading, localization for biopsy, differentiating radiation necrosis from tumor

figure 27.9. Fiber tracking in brain tumors with diffusion tensor imaging (DTI). The three-dimensional (3-D) relationship of the corona radiata with the tumor can be clearly appreciated. The corona radiata of the first patient (A) surrounds the surface of the tumor because of mechanical compression rather than infiltration. In the second patient's case (B), the trajectory of the corona radiata was not changed. Instead, it projected into the core of the infiltrative tumor. (From Mori et al.,57 by permission of Annals of Neurology.)

figure 27.9. Fiber tracking in brain tumors with diffusion tensor imaging (DTI). The three-dimensional (3-D) relationship of the corona radiata with the tumor can be clearly appreciated. The corona radiata of the first patient (A) surrounds the surface of the tumor because of mechanical compression rather than infiltration. In the second patient's case (B), the trajectory of the corona radiata was not changed. Instead, it projected into the core of the infiltrative tumor. (From Mori et al.,57 by permission of Annals of Neurology.)

figure 27.10. (A) CT scan, (B) 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) scan, and (C) fused CT and PET scan images of left frontal lobe recurrent GBM. Note increased FDG uptake in the periphery of the necrotic mass (black arrows), compatible with tumor recurrence rather than radiation necrosis. Note the presence of another focus of increased uptake (white arrow) in the right parietal region consistent with the diagnosis of multifo-cal GBM.

figure 27.10. (A) CT scan, (B) 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) scan, and (C) fused CT and PET scan images of left frontal lobe recurrent GBM. Note increased FDG uptake in the periphery of the necrotic mass (black arrows), compatible with tumor recurrence rather than radiation necrosis. Note the presence of another focus of increased uptake (white arrow) in the right parietal region consistent with the diagnosis of multifo-cal GBM.

necrosis, assessing response to therapy, predicting survival, and assessing malignant transformation of low-grade gliomas. In 47 patients with different brain tumors, the sensitivity of FDG-PET for differentiating tumor from radiation necrosis was 75% and the specificity was 81%.61 In another study yielding even lower sensitivity and specificity values, the authors concluded that the ability of 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) to differentiate recurrent tumor from radiation necrosis is limited.62 FDG-PET was found to be of prognostic importance in multiple studies.63,64 However, in the assessment of response to chemotherapy or radiotherapy, the role of FDG-PET remains of limited clinical utility.65

Newer tracers such as methyl-[11C]-l-methionine (MET) for measurement of amino acid transport and incorporation and 18F-3-deoxy-3-fluorothymidine (FLT) for evaluation of DNA synthesis, among others, seem to be promising in further characterization of brain tumors; however, they remain of limited clinical use currently.

The use of single-photon emission tomography (SPECT) in brain tumors is limited. Thallium (Tl) is the most studied radiotracer with the longest track record. Some studies have shown a relationship between 201Tl uptake and tumor grade.66 Due to the overlap between tumor uptake and his-tologic grades, 201Tl cannot be used as the sole noninvasive diagnostic or prognostic tool in brain tumor patients.67,66 However, it may help in differentiating a high-grade tumor recurrence from radiation necrosis. 99mTc-Sestamibi is theoretically a better imaging agent than 201Tl, but it has not convincingly been shown to differentiate tumors according to grade.68

method was immediately recognized and allowed its widespread use. In a large series of 325 cases, the use of the frame-less stereotactic viewing system was associated with minimal additional effort or time spent in setting up the procedure. The system was found to be reliable, achieving a useful registration in 95.4% of cases.69 In another case-control study, the impact of neuronavigation on glioblastoma surgery regarding time consumption, extent of tumor removal, and survival was evaluated, with and without the use of neuronavigation, in 52 cases and in 52 corresponding controls. Radical tumor resection based on radiologic evaluation was achieved in 31% of navigation cases versus 19% in conventional operations. The absolute and relative residual tumor volumes were significantly lower with neuronavigation. Radical tumor resection was associated with a highly significant prolongation in survival (P less than 0.0001). Survival was longer in patients who underwent surgery using neuronavigation (median, 13.4 versus 11.1 months).70

Similarly, stereotactic techniques have improved the efficiency of postoperative radiation of brain tumors. In stereo-tactic conformal radiotherapy, a computer-generated plan guides the use of a variable collimator to distribute the radiation field in such a way that the tumor may receive a very large dose, a surrounding area a moderate dose, and radiosensitive structures a minimal dose of radiation.71

Problems arise with techniques that use preoperative image coregistration, however, because the brain tends to move during the procedure due to swelling or to the introduction of air. Intraoperative acquisition of data sets eliminates the problem of brain shift in conventional navigational systems.

Herbal Remedy Secret Uncovered

Herbal Remedy Secret Uncovered

Discover How To Use Herbal Medicine Effectively To Heal Away Disease amp illnesses That Most Of The Herbalist Do Not Want You To Know About. If You Have Never Know What Is All About Herbal Medicines amp The Correct Way Of Using Herbs To Build A Healthier Life, Then This Guide Is About To Reveal All Just That.

Get My Free Ebook


Post a comment