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The introduction of the first computed tomographic (CT) scanners in 1975 revolutionized the imaging of neurosurgical lesions. Since that time, the diagnostic armamentarium of the neuroradiologist in terms of neuroimaging has expanded progressively. Diagnostic procedures usually involve limited pain so that the neuroradiologist frequently administers sedation. The anesthesiologist is involved, however, in cases for which difficulty with sedation is anticipated for a myriad of reasons. These include extreme ages, claustrophobia, serious neurologic or systemic illness, and the inability of patients to remain motionless to be able to achieve high-quality images.
Many radiology departments have recently adopted the guidelines for the sedation of children formulated by the American Academy of Pediatrics Committee on Drugs. These guidelines address patient selection, dietary precautions, equipment, monitoring, and discharge criteria. Furthermore, strict selection criteria minimize sedation-related side effects. The screening of patients before the procedure often reveals those who have either anticipated airway difficulties or serious medical conditions for whom the presence of an anesthesiologist becomes advisable. In one study, 13.1% of patients were inadequately sedated for their procedure, resulting in failure of 3.7% of procedures. Older children and those undergoing magnetic resonance imaging (MRI) and CT scans experienced higher failure rates. In addition, Malviya found that 5.5% of children sedated by nonanesthesiologists experienced an untoward respiratory event. These events occurred more commonly in American Society of Anesthesiologists (ASA) status III and IV patients.

I. General considerations

Once involved in the neuroradiologic procedure, the anesthesiologist faces the usual problems of providing anesthesia in an environment foreign to and far from the operating room. Nonferromagnetic equipment and monitoring modalities are required in the MRI suite, and most neuroradiographic procedures require that the anesthesiologist be positioned at a distance from the patient. Additionally, the issue of contrast media with their attendant osmolar effects and potential for toxic and allergic reactions must be considered. Finally, the design of modern medical facilities often requires that patients be either recovered by qualified personnel in an area adjacent to the radiology suite or transported over relatively large distances through the medical center to the regular postanesthesia care unit.
Evaluation. A thorough preoperative evaluation should be performed for each patient scheduled for a neurodiagnostic procedure. In addition to elucidation of the patient's medical and surgical history, experience with sedatives and anesthetics, allergies, and medications, particular attention must be directed to the neurologic examination. Patients must be carefully evaluated for signs of increased intracranial pressure (ICP), preexisting neurologic deficits, and, in the case of the emergency patient, concomitant injury to the spine and other major organ systems.
Equipment. Equipment available in the radiology suite should include compatible anesthesia delivery systems, adequate supplies of medical gases, suction apparatus, age-appropriate drugs and airway equipment, and monitors for oxygen saturation, end-tidal carbon dioxide (CO₂), electrocardiogram (ECG), and blood pressure compatible with the imaging modality.

II. Computed tomographic scan

Basic considerations. Ionizing radiation is delivered to the patient during CT scanning. Radiation detectors are permanently fixed all of the way around the CT gantry. For each image, an x-ray tube rotates in a circle within the detector ring emitting a beam of ionizing radiation that passes through the body. The multiple detectors record the quantity of the x-ray beam that is attenuated or absorbed as it passes through the patient's tissues. This information from various angles is electronically integrated and the average attenuation value of each point in space is expressed in Hounsfield units.
Attenuation by a substance is directly related to its electronic density. Therefore, structures are hypodense (of lower attenuation), isodense (of similar attenuation), or hyperdense (of higher attenuation) relative to the brain parenchyma. The attenuation of vascular structures is increased by the use of iodinated contrast drugs. While CT axial images are displayed initially, high-resolution thin images can be transformed into coronal or sagittal sections through computer reformatting.
CT scanning is relatively insensitive for viewing structures within the posterior fossa because of image degradation by the artifact produced by the interface of bone and brain parenchyma. This modality remains the choice, however, for the detection of skull fractures and acute subarachnoid hemorrhage in the emergent setting. Spiral acquisition CT scan is also popular because larger anatomic regions can be imaged quickly, which is particularly useful in the trauma situation. For example, visualization from the aortic arch to the circle of Willis can be accomplished in <1 minute of scan time. With spiral acquisition scanning, unlike conventional CT scan, the patient is moved at a constant speed through the scanning field while the x-ray tube rotates continuously. CT angiography utilizes spiral acquisition scanning during the administration of iodinated contrast as an intravenous bolus. Complex intracranial aneurysms of <2 mm in diameter can also be visualized with CT angiography.
Management
Sedation. With the current rapid CT scanners, very few adults or school-aged children require sedation for CT scanning. Occasionally, either intravenous midazolam in titrated doses of 0.5 mg may be effective, or an infusion of propofol at a dose of 25 to 100 mcg/kg/minute will sedate an anxious adult patient for the brief procedure. If an adult or older child manifests symptoms of increased ICP or serious airway compromise, it could be better to proceed with general anesthesia rather than attempt sedation. Several suggested techniques for the sedation of small children are listed in Table-1. In addition to concerns about equipment, monitoring, and airway management, it must be remembered that the temperature of children within the cold radiology suite environment must be carefully maintained.
General anesthesia. In the clinical situation where increased ICP is a critical factor, the intravenous induction of general anesthesia can be accomplished in children through an indwelling catheter, the insertion of which has been facilitated by the prior application of EMLA (lidocaine 2.5% and prilocaine 2.5%) cream to the dorsum of the patient's hands. In adults, an intravenous induction utilizing thiopental, 3 to 4 mg/kg, or propofol, 1 to 2 mg/kg, followed by succinylcholine, 1 mg/kg, with the addition of a small dose of narcotic such as fentanyl, 50 to 100 mcg, and lidocaine, 1 mg/kg, to deepen the anesthetic is appropriate for endotracheal intubation. In small children, the issue of an undiagnosed muscular dystrophy mitigates against the use of succinylcholine. An induction utilizing either rocuronium, 0.6 to 1 mg/kg, or mivacurium, 0.2 to 0.25 mg/kg, for relaxation is often indicated. Anesthesia in children is maintained with an intravenous infusion of propofol, 25 to 100 mcg/kg/minute, or low concentrations of inhaled anesthetics. Rapid awakening at the end of the procedure is desirable. An infusion of remifentanil, 0.1 to 0.2 mcg/kg/minute, is an alternative in adults.
 

III. MRI

MRIs are acquired when protons on water molecules within the patient's body are excited through a combination of a strong magnetic field and intermittent radiofrequency (RF) pulses. Additional magnetic fields are applied to create gradients and excite the protons to different orientations within the basic magnetic field. In the excited state, these protons gain energy and shift from a low energy state to a high-energy state. This process is termed resonance. When the RF pulse is turned off, the protons lose energy in two ways: T1, the longitudinal relaxation time, and T2, the transverse relaxation time. Fortunately, no ionizing radiation is employed with MRI, but a large, powerful, external magnetic field is necessary.
Standard MRI sequences include T1-weighted images, proton density, and T2-weighted images, as well as T1 images with the intravenous administration of gadolinium contrast material. Conventional double-echo T2-weighted imaging has largely been supplanted by fast-scan echo or hybrid rapid acquisition relaxation enhancement (RARE) sequencing. Additional imaging sequences include gradient echo, blind-attenuated inversion recovery, magnetic resonance angiography (MRA), perfusion imaging, and diffusion weighting. Through constant technologic improvement, MRI has achieved increased accuracy with difficult clinical scenarios. Currently, the areas optimally imaged by MRI include the limbic system, pineal gland region, sella and parasellar structures, cranial nerves, internal auditory canal, cerebellopontine angle, leptomeninges, and posterior fossa. Flair imaging can detect processes in the area of the periventricular and cortical surfaces.
While MRA provides images of arterial and dural sinus blood flow, functional MRI is being utilized with increasing frequency in patient evaluation. Precise three-dimensional anatomic localization and a contrast effect dependent on the level of blood oxygenation facilitate functional studies of sight and hearing. These permit the lateral identification of the areas responsible for language before epilepsy surgery. Diffusion-weighted imaging is useful in the detection of acute cerebral infarctions and is able to identify regions of ischemia within minutes. Perfusion-weighted imaging that utilizes intravenous gadolinium contrast can be added to a diffusion-weighted study to demonstrate the presence of cerebral tissue at risk for further ischemia.
The frequency of adverse reactions to gadolinium-based contrast of all types is much lower than the frequency of adverse reactions to iodinated contrast material. The reactions to gadolinium can, however, range from mild erythema of the skin to severe life-threatening reactions including periorbital edema, respiratory distress, and cardiovascular collapse. In patients who report a previous reaction to gadolinium-based contrast, pretreatment with corticosteroids, famotidine (Pepcid), and diphenhydramine (Benadryl) and substituting a different gadolinium-based contrast agent can be helpful.
Monitoring. The MRI suite presents a hostile environment to the anesthesiologist. The following summarizes several of the safety considerations:
Metallic objects within the patient can be affected, depending on their metallic properties. Implants (including vascular surgical clips, stents, cochlear implants, and intrauterine devices) are subject to magnetic flux, which can induce electrical currents and cause local heating and movement. The function of pacemakers, cardiac catheters, and insulin pumps can be altered with exposure to the magnetic field. The radiologist most commonly assesses the hazards of metallic implants.
External metal wires, such as the ECG leads and temperature probe, can burn the skin. It is therefore necessary to use nonferromagnetic monitoring modalities especially designed for and compatible with the magnetic resonance (MR) scanner. Pulse oximeters, even nonferrous and fiberoptically cabled ones, should be placed on a distal extremity as far from the scanner as possible. The occurrence of scan artifacts and local skin irritation and swelling has been reported in patients who have either permanent eye makeup or tattoos that contain ferromagnetic pigments.
Small loose metal objects can be pulled toward the magnet and are obviously dangerous. Large objects such as oxygen cylinders can also be pulled into the magnet with considerable force and can be a serious threat to both patients and health care personnel. In general, no metallic objects are allowed in the scanning room.
Noise is a problem in the MR scanner. The torque on the loops of wire in which gradient currents are induced during the RF pulses causes vibration and audible noise that can average 95 decibels in a 1.5-Tesla scanner. Newer 3T machines generate twice the noise, hampering communication and necessitating ear protection for patients regardless of whether they are awake, sedated, or asleep.
RF heating from induced currents is a potential problem, particularly in small infants. Periodic assessment of body temperature must be conducted. Infants can also become cold within the magnet because of the low ambient temperature in the scanning room.
Newer, more powerful 3T MR scanners can induce electric currents in patients that can stimulate the peripheral nervous system. Paresthesias, cephalgia, and muscle contractions have all been reported. The RF impulse-induced electric currents can also cause burns.
The anesthesiologist must find the point of least distortion by orienting monitors in various positions relative to the magnetic field. Most ECG artifacts cannot be eliminated and monitors should be MR-compatible and have good artifact suppression characteristics. It is important to twist the ECG cables, keeping the electrodes close together, and position the electrodes near the center of the imager with the cables padded to avoid direct contact with the skin, to get the best images and to reduce the possibility of burns.
Specialized equipment compatible with MRI is currently available for monitoring blood pressure, ETco₂, Doppler, oxygen saturation, and ECG. Compatible anesthesia machines and ventilators are available as well. Plastic laryngoscopes are also available although the batteries in the handle could still be pulled into the magnetic field. Problems remain in terms of patient accessibility, hypothermia, and the physical limitations of positioning obese patients within the magnet's bore.
Anesthetic management. Many sedative techniques have proven quite reasonable for MR scanning, including the scanning of infants shortly after a meal to take advantage of postprandial drowsiness. The anesthesiologist is called to assist with MRI, however, when a patient is complicated neurologically, manifests serious illness, or has significant airway difficulties. In general, securing the airway with an endotracheal tube and then supplying ventilatory assistance through hand ventilation, an MRI-compatible ventilator, or a long circuit ensure an optimum combination of monitoring, airway protection, and anesthetic delivery. Certain centers have utilized an intravenous infusion of propofol, either with or without a laryngeal mask airway, to achieve the same goals. Whatever the technique, the ability to maintain adequate control of the airway in the face of limited accessibility remains of paramount importance.

IV. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT)

These nuclear imaging modalities are used to measure glucose consumption (PET) and regional cerebral blood flow (SPECT), thereby providing an indirect measurement of tissue metabolism and oxygen utilization. PET scans use positron-labeled molecules of which the most common is 2-[¹⁸F] fluoro-2-deoxy-D-glucose (FDG). The FDG is injected intravenously and taken up into cerebral tissue. Because FDG cannot diffuse out of the brain after phosphorylation, it is well suited for imaging cerebral metabolism. PET scans are used in the investigation of malignancy because tumor cells have a much higher glucose metabolism. A PET scan of the brain takes 15 to 30 minutes.
SPECT scans use radiotracers that emit single photons. Three radiopharmaceutical agents have been approved by the United States Food and Drug Administration (FDA) for brain perfusion and SPECT scanning. Of the three, Tc-99m hexamethylpropyleneamine oxime ([^99mTc]HMPAO) is used most commonly. It reaches maximum uptake 10 minutes after injection but has a constant distribution for hours afterward, which makes scheduling a scan very flexible. SPECT scans are used to investigate malignancy, cerebral ischemia, neurodegenerative disorders, epilepsy, cortical visual loss, and migraine headaches. A SPECT scan of the brain takes 20 to 45 minutes. Almost all of the radiopharmaceuticals are cleared through the urinary tract. For both PET and SPECT scans, the radiation exposure for the patient is usually equivalent to a CT scan but may be as high as ten effective dose equivalents.
Anesthetic considerations. Many anesthetic drugs have been shown to alter cerebral blood flow and metabolism, but, because of the stability of the tracers after injection, anesthetizing a patient for a scan should not interfere with the results. Anesthetic considerations include the necessity to provide all appropriate monitoring modalities, medical gases, drugs, suction, and airway equipment in a remote location. Unlike MRI, PET scanners do not require nonferromagnetic monitors, and the visualization of and access to the patient are more easily accomplished.

V. Cerebral angiography

Cerebral angiography continues to be important in the evaluation of subarachnoid hemorrhage and carotid artery disease. The safest and most widely used arterial access is the common femoral artery so that the use of a transfemoral catheter has replaced direct puncture of the carotid artery. Digital subtraction angiography reduces the volume of intra-arterial contrast required and the overall duration of the procedure. Unfortunately, neurologic problems related to the angiography itself still occur. In a prospective study of 1,002 angiograms, the overall rate of ischemic events within 0 to 24 hours of the procedure was 1.3% and 2.5% in patients studied for cerebrovascular disease. Complications from the catheters include transient global amnesia, cortical blindness, multiple cholesterol emboli syndrome, and vascular complications such as hematoma and pseudoaneurysm at the puncture site.
Anesthetic management. Although anesthesiologists are asked to participate in diagnostic angiography on rare occasions, they must bear in mind that contrast media cause vasodilatation and a burning discomfort. Often the degree of sedation needs to be increased in anticipation of an intra-arterial injection of contrast. As always, there is the possibility that a reaction to the contrast material can occur.

VI. Myelography

Myelography is utilized to define the contents of the thecal sac and any intrinsic or extrinsic impressions. Contrast agents are introduced directly into the subarachnoid space, thereby bypassing the blood-brain barrier. The newest myelographic contrast agents are nonionic, of low osmolarity, and mix well with cerebrospinal fluid. The main complications related to myelography include headache, contrast-related complications, either subdural or epidural contrast injection, hematoma of the spinal canal, meningitis, seizures, and various forms of neurologic injury. The incidence of adverse events is higher in women and increases with cervical versus lumbar puncture and with the use of 22-gauge bevel-tipped versus 24-gauge Sprotte needles. Among the anesthetic considerations is the need to carefully position infants and children while the myelogram table is rotated to achieve good flow of the contrast material.

VII. Contrast material

In addition to CT scan, iodinated contrast is used for catheter angiography and myelography. High-osmolar contrast agents are ionic monomers at concentrations ranging from 60% to 76% by weight. This material possesses five to eight times the osmolality of human serum (280 mOsm/kg H₂O). Nonionic monomers and dimers as well as ionic dimers are considered low-osmolar contrast agents (LOCAs) when they have from two to three times the osmolality of human serum. The LOCAs demonstrate more hydrophilia and less tendency to bind to tissue and are therefore more biologically inert. Despite their higher cost, LOCAs are used for most neurodiagnostic procedures.
Iodinated contrast agents are nephrotoxic. After an initial mild vasodilatation, the renal vascular tree undergoes prolonged vasoconstriction. Patients who have preexisting renal insufficiency, diabetes mellitus, and low cardiac output syndromes are at risk for developing contrast agent-induced nephrotoxicity. Renal insufficiency can be ameliorated or prevented with adequate hydration and the withholding of any other nephrotoxic medications before the procedure. The use of fenoldopam, N-acetylcysteine, and dopamine has consistently failed to preserve renal function more effectively than hydration alone. Patients at risk should receive limited amounts of iodinated contrast media. Diabetic patients who have nephropathy and who receive metformin should be carefully monitored for the development of metformin-induced lactic acidosis after receiving iodinated contrast.
The hypertonicity of iodinated contrast media is responsible for the side effects that occur after injection. These include pain, flushing, nausea, and vomiting. Patients' responses to iodinated contrast material can range from a warm flushing and a metallic taste during contrast injection to an anaphylactoid reaction. Large multi-institutional studies have demonstrated that patients given nonionic contrast material have a 1 in 10,000 chance of having a severe reaction. Patients likely to have a problem with contrast include those who have had previous idiosyncratic contrast reactions, patients who have either asthma or multiple food and medication allergies, and patients who have other illnesses including preexisting azotemia and cardiac disease. The use of corticosteroids such as oral methylprednisolone, 32 mg, both 6 to 24 hours before and 2 hours before the injection of the contrast material, markedly reduces the chance of a severe reaction to contrast. Additionally, some centers add antihistamines and H₂ blocking drugs to the steroid regimen.
Not infrequently, a few hives about the face, neck, and chest are the sole reaction that a patient manifests to contrast. Often the only therapy necessary in such cases is reassurance with perhaps a dose of diphenhydramine, 25 mg to 50 mg intravenously (i.v.). A severe reaction to contrast, manifested by generalized skin irritation, respiratory difficulties, and hypotension, requires treatment with epinephrine, 100 mcg i.v., and glucagon for refractory hypotension. Equipment must be available in the radiology suite for immediate airway control and the administration of additional adjuncts such as beta2-agonists including albuterol for treating bronchospasm. Occasionally, large volumes of fluid as well as vasoactive drugs are necessary to support the blood pressure. A prolonged stay in the intensive care unit could be necessary to stabilize the patient.
Delayed reactions can be seen in 1% to 3% of patients receiving x-ray contrast material. These are usually mild and consist of rashes and hives.