Mức độ liên kết của radiopharmaceutical này trong thể vân là một dấu hiệu cho số lượng của các tế bào thần kinh dopaminergic còn nguyên vẹn trong khu vực não. Khi những hình ảnh này cho thấy, có ít ràng buộc của 18F-DOPA trong thể vân của các bệnh nhân mắc bệnh Parkinson | 84 ESSENTIALS OF NEUROIMAGING FOR CLINICAL PRACTICE Figure 3-7. Positron emission tomography studies with 18F-DOPA a radiopharmaceutical used to measure presynaptic dopamine synthesis. The degree of binding of this radiopharmaceutical in the striatum is a marker for the number of intact dopaminergic neurons in this brain region. As these images indicate there is far less binding of 18F-DOPA in the striatum of the patient with Parkinson s disease in comparison with the healthy volunteer. resting rCBF or rCMR across populations. To date these studies have been used only in research applications given that differences across populations are usually not detectable in an individual scan rather pooling of subjects is required. Neutral-state studies have demonstrated that groups of patients with major depression show decreased rCBF or rCMR in frontal regions compared with control populations Figure 38 and that groups of patients with obsessive-compulsive disorder demonstrate increased rCBF or rCMR in orbitofrontal cortex and the head of the caudate nucleus. Although neutral-state studies have provided a great deal of valuable information regarding the pathophysiology of numerous psychiatric illnesses studies that assess brain function during specific tasks may be a more powerful tool. Just as electrocardiogram data collected during a cardiac stress test may uncover cardiac abnormalities not detectable with a resting electrocardiogram functional neuroimaging studies that use activation paradigms may be more sensitive than neutral-state studies. Of course these studies may be conducted in patient populations and in healthy volunteers. SPECT is not as useful for these activation studies as PET or fMRI see Chapter 4 in this volume for a more detailed description of fMRI because generally only one image can be collected per day with SPECT. By comparison the use of 15O-labeled radiopharmaceuticals with PET permits investigators to conduct numerous studies in a single day. .
