Brain Perfusion SPECT

Transcription

1 APPROVED BY: Director of Radiology Page 1 of 5 Brain Perfusion SPECT Primary Indications: Brain perfusion SPECT is most commonly performed (1) to aid in identification of the epileptogenic focus in patients with medically refractory epilepsy (usually partial complex seizures) in whom surgical treatment is being considered; and (2) to evaluate the adequacy of collateral cerebral blood flow in patients who are being evaluated before planned surgical sacrifice of an internal carotid artery. It is also occasionally used as an adjunctive diagnostic technique to assess cerebral blood flow patterns in patients with suspected cerebral vasculitis, dementia, or focal neurologic disease with normal CT or MRI. Rationale: Brain perfusion SPECT is performed following the injection of a lipophilic radiopharmaceutical that is taken up and retained within the brain in proportion to regional cerebral blood flow (to a first approximation). Because there is little washout/redistribution of the radiopharmaceutical from the brain, imaging can be performed up to 4 hours after injection. Ictal SPECT for seizure focus localization is based on the observation that the epileptogenic focus appears as a region with increased blood flow if the radiopharmaceutical is injected promptly after the onset of a seizure. Interictal SPECT will demonstrate decreased blood flow in the epileptogenic focus and often in other portions of the affected lobe. Interictal PET with F-18 fluorodeoxyglucose is generally considered to be more sensitive for seizure localization than is interictal SPECT, possibly because metabolic changes are more marked than perfusion changes.) In patients who have partial complex seizures, the epileptogenic focus is usually located within the temporal lobe. In patients who are being studied to assess the adequacy of collateral cerebral blood flow, the radiopharmaceutical is injected during balloon occlusion of the vessel of interest. If asymmetric relative cerebral perfusion (>10%) is seen on the subsequent SPECT images, a baseline brain SPECT study is generally obtained for comparison. Interfering Conditions: Ideally, the radiopharmaceutical should be injected while the patient is in a quiet, dimly lit room since stimulation may change the pattern of cerebral perfusion. However, when a patient is studied during balloon occlusion or during a seizure, the ambient environment cannot usually be controlled. Precautions: Patients with neurologic deficits or dementia must be closely monitored during SPECT imaging. If sedation is required to perform the study, it should be given after the radiopharmaceutical.

2 APPROVED BY: Director of Radiology Page 2 of 5 Radiopharmaceutical: Tc-99m bicisate (ECD; Neurolite ) or Tc-99m exametazime (HMPAO; Ceretec ) Note: Tc-99m exametazime must be injected through a filter, which will be attached to the syringe by the radiopharmacy when dispensed. Adult Dosage: For an ictal study where the radiopharmaceutical is kept at the bedside until the patient has a seizure, the dispensed dosage is 45 mci for Tc-99m bicisate or 42 mci for Tc-99m exametazime. Tc-99m bicisate may be used for up to 7 hours after compounding, while Tc-99m exametazime must be used within 6 hours after compounding. Thus for ictal studies, the dosage ranges are as follows. Tc-99m bicisate: mci Tc-99m exametazime: mci For all other studies, the dosage is 35 mci for both radiopharmaceuticals. Pediatric Dosage: For an ictal study with Tc-99m bicisate, where the radiopharmaceutical is kept at the bedside until the patient has a seizure, the dispensed dosage should be 500 µci/kg and this may be used for up to 7 hours after compounding. Therefore, the dosage range is µci/kg (minimum dosage 3.0 mci; maximum dosage 35 mci). For an ictal study with Tc-99m exametazime, where the radiopharmaceutical is kept at the bedside until the patient has a seizure, the dispensed dosage should be 600 µci/kg and this must be used within 6 hours after compounding. Therefore, the dosage range is µci/kg ( minimum dosage 3.0 mci; maximum dosage 42 mci). For all other studies, the dosage is 350 µci/kg, with a minimum dosage of 3.0 mci for both radiopharmaceuticals. Radiation Dosimetry: Tc-99m ECD Adult 45 mci: critical organ (gallbladder) 4.5 rem; effective dose 1.9 rem. Infant 4.8 mci: critical organ (gallbladder) 4.1 rem; effective dose 0.7 rem. Tc-99m HMPAO Adult 42 mci: critical organ (gallbladder) 8.6 rem; effective dose 2.4 rem. Infant 4.8 mci: critical organ (gallbladder) 7.8 rem; effective dose 1.4 rem. Route of Administration: Intravenous Patient Scheduling: Requests for brain SPECT studies should be directed to the attending nuclear medicine physician or a nuclear medicine resident. This indi-vidual should obtain the relevant clinical history, determine whether it is appropriate to perform this examination, determine the availability of Tc-99m ECD or HMPAO for the time of study request, and schedule order with radiopharmacy staff. All

3 APPROVED BY: Director of Radiology Page 3 of 5 Patient Preparation: None required scheduling information should be entered on the Scheduling Form for Miscellaneous Studies Needing Radiopharmacy Notification. Equipment Setup: Gamma camera: Dual-head LFOV SPECT using a noncircular orbit to maintain the detectors as close to the patient s head as possible. Collimator: Low-energy high-resolution Energy window: 140 kev with a 20% window The Philips Vertex and the Siemens E-Cam or Symbia cameras are preferred because they allow for data acquisition over an orbit closer to the patient s head. The same camera should be used for both the ictal and interictal portions of a study done to evaluate for an epileptogenic focus. Patient Positioning: The patient must be positioned so that he/she will be able to remain motionless for the SPECT acquisition. Arm boards, positioning wedges, and support straps must be effectively used to eliminate patient discomfort. The patient should be positioned as far as possible toward the narrow head-holder portion of the table. The patient s head rests in a cushioned plastic head holder and is positioned so that the orbitomeatal line is perpendicular to the axis of rotation. Every effort should be made to minimize the patient-to-detector distance during the SPECT acquisition. To achieve this, move the detectors toward the head end of the table until the lower portion of the brain is just barely included in the field of view which will allow the detectors in the lateral position to be inside the radius of the shoulder (Note that, during acquisition, the camera will automatically step out about 1 cm from the marked positions; thus it is appropriate to move the detectors quite close to the patient for marking.) Procedure: Since cerebral function and, therefore, cerebral blood flow may be altered in unpredictable ways by external stimuli, the patient should be exposed to minimal external stimuli several minutes prior to, during, and after the injection of the radiopharmaceutical. The venous line is placed and/or tested for patency when the patient arrives in the department. Ideally, the patient should be resting comfortably in the supine position, in a quiet, dimly lit room for 15 minutes prior to the injection of the radiopharmaceutical and during the uptake phase for 30 minutes after the injection. If sedation is needed for imaging, sedation should not be initiated until 5-10 minutes after the injection. For ictal SPECT studies, the injection will be performed by a staff nurse or resident physician of the Department of Neurology after proper instruction from nuclear medicine personnel in the safe handling of radioactive material for the procedure. The

4 APPROVED BY: Director of Radiology Page 4 of 5 injection of the radiopharmaceutical should be performed as soon as possible after the onset of the seizure. The injection should not be performed if the seizure activity has stopped. Administering the tracer more than one minute after the onset of the seizure is discouraged, particularly if the seizure becomes more generalized with time. The times from the onset of electrical seizure activity and clinical seizure activity until the injection of the radiopharmaceutical should be recorded and this information provided to the nuclear medicine physician. To facilitate the prompt injection of the tracer, the nurses and/or physicians observing the patient have been appropriately trained in radiation safety and are permitted to inject the radiopharmaceutical. The syringe containing the tracer will be kept in a shielded container in the patient's room and will be secured when it is not under the direct supervision of the responsible nurse. During balloon occlusion studies, it is more difficult to control external stimuli at the time of radiopharmaceutical injection. To the extent possi-ble, the patient should be injected in a quiet, dimly lit room and resting comfortably, as described above. The radiopharmaceutical should be injected about 2-3 minutes before the anticipated deflation of the bal-loon. The injection will be performed by nuclear medicine staff who will properly dispose of the syringe and any other radioactive waste after completion of the injection. Image acquisition may begin as soon as minutes following the injection of the tracer. The patient should be made as comfortable as possible in order to minimize patient movement. The initial transaxial images should be reconstructed on the acquisition computer using a ramp filter. These transaxial images should be trans-ferred to the DEC workstation. Additional filtering should be performed using a brain filter. For paired brain SPECT images (e.g., ictal/interictal, baseline/ballon occlusion, baseline/acetazolamide), subtraction images are helpful. The reconstructed and projection images from the Vertex should be exported to interfile and kept online until the second study is performed. For studies originating from Children s (acquired on ECAM), projection images and reconstructed images should be sent to NUCMACP. Richard Laforest (medical physicist: voice ; pager ) should be notified when subtracted images are required. View Digital Acquisition Film Display (if

5 APPROVED BY: Director of Radiology Page 5 of 5 SPECT See document SPECT Acquisition and Filtering Guidelines To be filmed by physician Items Required For Complete Study: 1. Projection images, transaxial images reconstructed with a Butterworth filter using a frequency cutoff of 0.50 and an order of 5.0, filtered transaxial images, volume-rendered images. 2. Digital images successfully transferred to the archival computer. 3. The physician should print the appropriate orthogonal images of the SPECT study.