<s>[INST] context: PET > Biopsy "Positron emission tomography (PET) and single-photon emission tomography (SPECT) have ancillary roles in the imaging of brain tumors, primarily in distinguishing tumor recurrence from tissue necrosis that can occur after irradiation." -Harrison I7/e p2602 "MRI or CT scans are often unable to distinguish radiation necrosis from recurrent tumor, but PET or SPECT scans may demonstrate the increased glucose metabolism typical of tumor tissue or the decreased metabolism of necrotic tissue. Magnetic resonance spectroscopy may demonstrate a high lactate concentration with relatively low choline concentration in areas of necrosis. Biopsy is frequently required to establish the correct diagnosis." - Harrison 17/e p2610 PET and SPECT belong to family of Radionuclide imaging. Radionuclide imaging tests differ from most other imaging modalities in that diagnostic tests primarily show the physiological function of the system being investigated as opposed to traditional anatomical imaging such as CT or MRI. The images are obtained by mapping the distribution of an administered radiopharmaceutical within the body. These radiopharmaceuticals, once administered to the patient, can localize to specific organs or cellular receptors. These radiopharmaceuticals emit radiations. This propey of radiopharmaceuticals allows nuclear medicine to image the extent of a disease-process in the body, based on the cellular function and physiology, rather than relying on anatomical changes in the tissue anatomy. Thus, as for X-ray radiography and computed tomography, radionuclide imaging uses ionizing radiation. However, the radiation is emitted from within the patient and subsequently detected in the imaging device, rather than transmitted through the patient from an external X-ray source. Compared with CT or MRI, the spatial resolution of radionuclide scans is relatively poor, but the overall diagnostic accuracy may be greatly improved over that of anatomic/morphologic images if a sufficiently specific probe is available. It has the potential ability to diagnose and stage a malignancy, and identify distant metastases with a single scan of the entire body. The specific organ function depicted is determined by the biological behaviour of the radiopharmaceutical. For example, a radiopharmaceutical excreted by the kidney will evaluate renal function whilst one excreted in the bile will depict the biliary tree. The biological behaviour of radiopharmaceuticals can be traced to paicular cellular or molecular mechanisms. Thus, by using different radiopharmaceuticals, a range of cell types can be imaged within the same organ, or different molecular mechanisms with the same cell type Radionuclide imaging devices are highly sensitive and can detect relatively small changes in functional activity. Thus, a change in tissue function is frequently detectable before any change in structure has occurred or can be detected confidently by anatomical imaging. For example, increased glucose metabolism of cancer cells within lymph nodes can be detected using positron emission tomography (PET) before the size of the lymph node exceeds the threshold to be diagnosed as abnormal on computed tomography (CT). Similarly, successful treatment may return functional activity to normal before anatomical abnormalities have resolved and thus radionuclide imaging can be advantageous in monitoring treatment (e.g. for patients with cancer). Diagnostic tests in nuclear medicine exploit the way that the body handles substances differently when there is disease or pathology present. For example, the ligand methylene-diphosphonate (MDP) can be preferentially taken up by bone. By chemically attaching technetium-99m to MDP, radioactivity can be transpoed and attached to bone the hydroxyapatite for imaging. Any increased physiological function, such as due to a fracture in the bone, will usually mean increased concentration of the tracer. This often results in the appearance of a 'hot-spot' which is a focal increase in radio-accumulation, or a general increase in radio-accumulation throughout the physiological system. Some disease processes result in the exclusion of a tracer, resulting in the appearance of a 'cold-spot. Another advantage of radionuclide imaging is that quantification of the amount of radiophamiaceutical within a paicular organ can be achieved through counting the amount of radiation emitted within a specific time. By obtaining repeated images, the change in concentration of radiopharmaceutical over time can be measured and displayed as time--activity curves, giving additional insights into the functional status of tissues With appropriate mathematical modelling, these changes in concentration can be used to quantify paicular physiological processes, such as glomerular filtration rate (GFR) or left ventricular ejection fraction. [/INST] question: Radiation induced necrosis can be diagnosed by: | options: A: PET B: CT C: MRI D: Biopsy | answer: D </s>