General Considerations
䊏 There are multiple and varied uses for nuclear medicine imag-ing.
䊏 It allows both anatomic/pathologic and functional imaging.
䊏 It requires the administration of radioactive material (admin-istration routes vary and include IV, intrathecal, and subcuta-neous around cutasubcuta-neous lesions).
䊏 It has poor anatomic resolution, making precise localization of disease difficult.
䊏 The advent of PET/PET CT allows evaluation of metabolism and perfusion with the added benefit of correlation with anatomic detail (PET CT).
IMAGING LIMITATIONS 䊐 Patient factors:
r Body habitus: Patients >350 lbs cannot be imaged on conventional tables.
r Patient positioning: Patients with contractures or inabil-ity to remain stationary are difficult to image.
r Patient stability: Due to the relatively long length of some studies, patients may not be candidates for nuclear medicine studies if they require monitoring or intensive care therapy.
䊐 Technologic factors:
r Different isotopes (radiotracers) have different energies and different decay times. This limits which studies can be performed in close temporal proximity to each other and dictates the order in which studies can be performed.
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This is particularly important in cardiac imaging where rest and stress studies may require 2 days of imaging.
r Radioactive material decays at specific rates (half-life).
Due to the variable half-lives of the agents used for clin-ically applied nuclear medicine, only a certain number of studies may be performed on a daily basis. Addition-ally, the agents that are used for the study (e.g. diseida for HIDA scans) are often scarce, limiting the number of studies that may be performed at any given time. There are strict quality-control measures in place for the agents used in nuclear imaging. If an agent does not pass qual-ity control, its production may be halted for a significant amount of time, thus rendering it impossible to perform studies using that agent.
r Due to the rate at which different isotopes decay and the extended periods of time that are often required for the agent to be taken up by the tissues, there are lim-itations on when studies can be performed. For exam-ple, it takes approximately 2–4 hours for technetium-99m (99mTc) methylene diphosphonate (MDP) (the agent used for bone scans) to be cleared from the background tis-sues and to bind to the bones. This requires injection of the agent early in the morning so that image acquisition can be performed in the afternoon (after a 4-hour delay).
Other agents, such as gallium (which is most often used for lymphoma imaging) may require as many as 2–6 days to clear background and bowel activity to an acceptable level that allows pathology to be recognized.
r Nuclear studies often require multiple days of imaging;
patients must return on multiple consecutive days for completion of the imaging (this is particularly true for gallium studies).
r With all nuclear imaging studies, metallic devices in or on a patient will attenuate (stop) the radiation from reach-ing the detector and cause an artifact, which can obscure disease.
r Safety issues:
r The agents used for nuclear medicine imaging are radioactive and often have long half-lives or decay to isotopes with long half-lives. This poses safety and
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180 CNS Imaging
disposal concerns. Strict guidelines are in effect for the handling and disposal of radionuclides. Conversely, the agents used for PET imaging have extremely short half-lives, requiring rapid imaging. Some PET isotopes have such short half-lives that on-site cyclotrons are required to produce the isotopes and allow for
imag-r ing.Certain types of nuclear medicine studies (e.g. white
blood cell [WBC] and tagged red blood cell [RBC] stud-ies) require ex vivo (i.e. outside the patient) labeling of blood with the agent. This requires blood to be withdrawn from the patient and manually mixed with the tracer to enable binding. This has obvious draw-backs, which place technologists and patients at risk for blood-borne pathogens.
CNS Imaging
䊏 Nuclear medicine studies allow for physiologic and anatomic imaging. So-called ictal (injection during seizure activity) and interictal (between seizure) studies are performed to evaluate the seizure focus. Due to its properties, rapid injection of tracer is required during seizures in patients undergoing ictal SPECT imaging. Therefore, patients must be on a ward floor where isotopes and technicians are rapidly available to inject the agent once a seizure occurs.
䊏 Physiologic imaging may be performed in different ways in the CNS, depending upon the information that is required. Perfu-sion imaging may be performed, as may imaging to evaluate metabolism. The tracer is different for both types of study.
Technetium-99m Hexamethylpropyleneamine Oxime CNS Imaging
䊏 This is an agent used for perfusion imaging in the brain. It does not provide information about brain metabolism.
䊏 Imaging is performed on a typical nuclear medicine gamma camera (it is NOT a PET agent).
䊏 It may be used as an ictal or interictal agent (i.e. injecting while the patient is actively seizing or between seizures).
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䊏 The radiotracer has a 6-hour half-life; however, in brain imag-ing, rapid imaging after tracer injection is required (within 2–6 hours).
INDICATIONS
䊐 Evaluation of patients with seizures with the intent being to identify the seizure focus. It is often interpreted in conjunc-tion with a concurrent MRI of the brain.
䊐 Evaluation of brain death
CONTRAINDICATIONS:None
LIMITATIONS
䊐 False-negative results can occur for the evaluation of seizures with this technique.
䊐 As with all nuclear medicine imaging studies, there is poor spatial resolution of this technique; therefore, it is difficult to precisely localize the area of abnormal uptake.
F-18 Fluorodeoxyglucose CNS Imaging
䊏 This is an agent used for evaluation of brain metabolism. It also evaluates brain perfusion.
䊏 Imaging is performed on a dedicated PET camera, a modified gamma camera, or a dedicated combined PET CT camera.
䊏 Due to the short half-life (approximately 90 minutes) and the often remote location of a PET camera, it is typically used for interictal (between seizures) imaging.
䊏 As with all fluorodeoxyglucose (FDG) imaging, tracer uptake is dependent upon serum glucose levels and insulin levels.
Patients should be fasting for the studies in order for uptake to occur in the brain.
INDICATIONS
䊐 Evaluation of patients with seizures to identify the seizure focus. It is often interpreted in conjunction with MRIs and perfusion imaging (99mTc hexamethylpropyleneamine oxime [HMPAO]).
䊐 Evaluation of patients with suspected Parkinson’s, Pick’s disease
䊐 Evaluation of patients with cerebrovascular accident to eval-uate for potential residual function
CONTRAINDICATIONS
䊐 Patients actively seizing
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䊐 Unstable patients (Long imaging times are required and the camera is often remote from immediate medical assistance.)
LIMITATIONS
䊐 The short half-life of the tracer limits the locations of patient injection and imaging.
䊐 Brain uptake is dependent upon factors such as the serum glucose and insulin levels. If the serum levels are not appro-priate, uptake will occur in structures outside the brain.
䊐 Skeletal muscle activity at the time of tracer injection will result in uptake in the skeletal muscles, which can compli-cate image interpretation.
Cerebrospinal Fluid Leak
䊏 This is an invasive procedure that involves a multidisciplinary approach. The procedure involves nuclear medicine technol-ogists and physicians, ear nose throat (ENT) specialists, and neuroradiologists.
䊏 It is performed as follows. Pledgets are placed into the nasal cavity (near the cribriform plate) by ENT. The patient is then transferred to the fluoroscopy suite where a neuroradi-ologist performs a lumbar puncture. In conjunction with the nuclear medicine technologist, the neuroradiologist admin-isters intrathecal radiotracer. The pledgets are subsequently removed by ENT after approximately 24 hours and radioac-tivity assessed with a Geiger counter. If there is radioacradioac-tivity on the pledgets, a cerebrospinal fluid (CSF) leak is present.
INDICATIONS
䊐 Assessment of clinically suspected CSF leak (e.g. following intracranial surgery, trauma)
CONTRAINDICATIONS:None
LIMITATIONS
䊐 If the pledgets are not appropriately positioned, a leak may not be identified.
䊐 Small leaks that allow only minimal leakage of radiotracer may not be detectable.
Obstructive Hydrocephalus
䊏 It may be necessary clinically to differentiate between com-municating and noncomcom-municating hydrocephalus in order to