Current neuroradiologic techniques and their anaesthetic implications

Current Neuroradiologic Techniques and their Anaesthetic Implications

E. A. M. Frost and F. G. M o s e r

The variety and sophistication of neuroradiologic procedures has increased dramatically over the last twenty years because of the rapid technological advances in radiological tools. Since 1960, most of the studies which are now mainstays of neuro-imaging have been developed or have been radically altered. Previously direct carotid angiography and pneumoencephalography were the most useful procedures in the armamentarium of the neuro-radiologist. Magnetic resonance imaging (MRI) and computerised tomography, both of which have been developed in the last twenty years, are now the most prevalent diagnostic neuro-imaging procedures. Angiography has changed radically, as has myelography. Digital angiography and ultrasound techniques (carotid and transcranial doppler) are possible due to the increasing use of computer technology in medical imaging. Interventional neuroradiology, which was first discussed in 1960,1 has changed the notion that radiologists are simple diagnosticians by allowing the performance of endovascular surgery. Anaesthesia plays an important role in the performance of neuro-imaging procedures and as such, an accurate understanding of the commonly performed procedures is crucial.

administered by the radiologist. However, several groups of patients require the expertise of an anaesthesiologist to provide adequate pain relief, appropriate monitoring, optional cardiorespiratory support, or immobility (Table 1). Studies indicate that 7-10% of patients undergoing neuroradiologic studies require some form of anaesthesia. 2"4 Although many patients require only monitored sedation, all patients should be evaluated preoperatively and optimally prepared. The record must be reviewed and the patient questioned for signs and symptoms of intracranial hypertension (headache, nausea, vomiting, change in mental status). Any concurrent cardiovascular, pulmonary or renal disorders must be identified as well as prior reactions to contrast media and other allergens. Consent must be obtained; patients and their families must be aware that although the radiologic study may have minimal risk, there may be significant risk if the patient is in poor medical condition, and/or general anaesthesia is indicated. Goals for premedication include reduction of patient anxiety and secretions as well as prevention of nausea and vomiting. Goals specific for the neuroradiological patient include stabilisation of intracranial dynamics, elevation of the seizure threshold, and protection of the brain from ischaemia. Administration of medications such as anticonvulsants, osmotic diuretics, antihypertensives, antidysrhythmics, and steroids must be continued through the day of the procedure. Patients with a history of allergic reactions to contrast material should be given steroids and antihistamines prior to the procedure. Regardless of the type of anaesthetic planned, all patients should be fasted. In diabetics, the insulin doses must be adjusted, especially if the patient is receiving glucocorticoids.

General preparation Many radiologic tests are painless and require no anaesthesia or analgesia. Some patients require sedation to allay anxiety, which ofttimes is ably

Elizabeth A. M. Frost, MD, Department o f Anesthesiology, Franklin G. Moser MD, Department of Radiology, The Albert Einstein College of Medicine of Yeshira University, Montefiore Medical Center, 1300 Morris Park Avenue, Bronx, NY, 10461, USA CurrentAnaesthesia and Critical Care (1990) 1,90-98

(~)1990LongmanGroupUK Ltd

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CURRENT NEURORADIOLOGIC TECHNIQUES AND THEIR ANAESTHETIC IMPLICATIONS 91 Table 1 -- Anaesthetic expertise is required in several groups of patients to obtain optimal studies Extremes of age Mental retardation Lengthy procedures Neurologic diseases precluding immobility (eg. Parkinson's disease, status epilepticus) Major head injury Allergic history Other major medical diseases

The most important radiologic procedures are as follows:

Myelography Myelography is a diagnostic test which involves placement of contrast material into the thecal sac to examine the spinal column. CT scanning may follow plain film myelography. Contrast is placed into the thecal sac, usually after a lumbar puncture, occasionally after lateral C1, C2 puncture. Myelography is a relatively safe procedure; however it involves a certain amount of discomfort for the patient. It is typically performed with the patient, prone, and positioned with a pillow or other bolster device under the abdomen. The increased curvature of the lower spine opens up the intra-spinal spaces. Typically an L2/3 puncture is performed for patients with degenerative disease. Lower lumbar punctures can also be performed. After placement of a twenty gauge spinal needle, 5 cc of cerebrospinal fluid for routine analyses of cells, protein and sugar, is removed followed by placement of 18 cc of contrast material. The patient is variously positioned for imaging to demonstrate different portions of the spinal column. The patient may even be placed in a very steep Trendelenberg angulation to concentrate the contrast material in the cervical spine. C1, C2 punctures are performed to demonstrate the top of a lesion blocking the spinal canal. Contrast materials used currently are non-ionic materials with very low complication rates. 5 Pantopaque is an ester-based material which is safe and easy to use. However, it requires the use of larger gauge needles since it is relatively viscous. While it has a fairly low rate of immediate complications, there is a very high rate of long term complications including arachnoiditis. 6 Metrizamide is a water soluble contrast material which has a very low rate of long-term complications. However the immediate complications include severe headaches, occasional seizures and, rarely, occipital cortical blindness. 7 It also has the disadvantage of diluting very rapidly in CSF. E E G changes may be seen within 24 to 48h after injection. 8 Examples of presently available non-ionic contrast materials are iohexol (Omnipaque) and iopamidol (Isovue, Niopam). These substances show a very low rate of cerebral irritation and systemic effects, and

can be used to obtain cysternograms (when contrast is placed in the basal cisterns) as well as myelograms. An advantage over metrizamide is that non-ionic materials maintain their integrity as a contrast agent in the CSF longer. A study conducted by the Committee on Contrast Media of the International Society of Radiology indicated a 2.33% incidence of non-fatal allergic reactions after intravenous contrast injection. 9 Another study has indicated a 5% incidence of adverse systemic reaction, of which one-third are severe, to A 3-fold increase in adverse reactions occur if previous allergic reactions to iodine compounds has occurred. Most anaphylactic reactions occur within 3 minutes of the initial injection. 11 Mild allergic reactions are characterised by itching, skin rash, or a vasomotor reaction of transient tachycardia and hypotension. Diphenhydramine 50mg intravenously is indicated. More pronounced urticaria and facial oedema may also occur, requiring hydration with isotonic fluids along with administration of an antihistamine and close observation. Bronchospasm is a more serious allergic reaction. Treatment consists of stopping the injection of contrast agent, subcutaneous epinephrine (adrenaline) 3txg/kg, and intravenous aminophylline 5-7mg/kg over twenty min followed by an infusion of 0.60.9mg/kg/h. A beta2-agonist inhaler may also be used, as well as subcutaneous terbutaline 0.250.5mg. Laryngeal oedema is another serious allergic reaction which presents with stridor. Immediate intubation is indicated. Cardiovascular collapse, the most severe form of anaphylaxis, presents with hypotension, tachycardia, dyspnea, cyanosis, confusion, and eventual loss of consciousness. Immediate and aggressive therapy is directed at maintaining oxygenation, adequate intravascular volume and cardiovascular status. Therapy includes volume expansion to counter capillary leakage of intravascular fluid by histamine release, adrenaline (0. l m g ' 0 . 2 5 mg boluses), diphenhydramine 1.0mg/kg and hydrocortisone (lgm). Myelograms have become less important as diagnostic procedures with the advent of the MRI. Many of the indications for myelography have been supplanted by MRI, including disk and degenerative disease, and spinal column metastasis and tumours. 12 Myelography still plays a role in diagnosis and surgical planning. It remains the most sensitive test for intra-dural extra-medullary disease although gadolinium enhanced MRI is making inroads in this area. 13 Anaesthetic considerations

Phenothiazines may predispose patients to metrizamide-induced seizures by lowering the seizure threshold. TM Other agents such as enflurane and ketamine should also be avoided. Phenobarbital and benzodiazepines are the premedicants of choice when

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water soluble myelographic agents are use. 15 If an antisialogogue is used, glycopyrrolate is advantageous over atropine since it is a more potent drying agent without pronounced cardiovascular effects and will not cross the blood brain barrier. Droperidol provides antiemesis and does not increase cerebral blood flow. 16 Although in many instances, sedation is supervised by the radiologist, in certain situations (Table 1), monitored sedation by an anaesthesiologist is prudent - a situation that allows the radiologist to focus all his attention to the technical details of the procedure. Examples of such situations include patients with emphysema who may become hypoxic from normal doses of narcotics, patients with a history of allergy to contrast media or critically ill patients. The goal of monitored sedation is to provide a patient who is calm, yet cooperative. The premedicant drugs.already discussed may be safely titrated intravenously. The benzodiazepines work effectively as sedative-hypnotics and provide a moderate initial period of tranquilisation; nevertheless, their sedative effects may last several days in older patients. The benzodiazepines may be combined effectively with a narcotic (usually fentanyl) to provide a sedative-analgesic combination. Fentanyl may also be combined with droperidol to provide an effective neuroleptanalgesic technique. In fact, the neuroleptic combination of fentanyl and droperidol has been shown to maintain intracranial stability. 17 Intermittent boluses or a continuous infusion of thiopental may be useful in obtaining sedation, but repeated use must be closely monitored. Appropriate monitoring includes electrocardiographic, blood pressure and oximetric measurements, as well as a precordial stethoscope. Oxygen supplementation is advisable, and end tidal CO2 can be measured via an angiocath placed through a nasal cannula nostril prong. The anaesthesiologist must be prepared to convert to general anaesthesia if conditions under sedation are not adequate for the study. Also, should seizures or anaphylaxis occur, intubation may be needed. If large amounts of hyperosmolar contrast material are used, catheterisation of the bladder may be indicated. Also diabetics and patients with renal insufficiency are particularly susceptible to contrast induced renal failure. Vigorous hydration with nonsugar containing solutions is indicated. Depending on the amount of sedation administered and the risks of the procedure being performed, admission to a Post Anaesthetic Care Unit (PACU) may be warranted for continued observation. Administrative of general anaesthesia in a neuroradiology suite is often hampered by constraints of space, light, equipment (suction, gas supply, scavenging systems) and personnel, knowledgeable in anaesthesiology. Interdepartmental consultations can significantly decrease the problems by assisting the specialties in an understanding of their requirements (Table 2).

Table 2 - - Requirements for safe administration of anaesthesia

in a neuroradiology suite Regulation anaesthetic machine Suction Gas scavenging system Electrocardiograph Pulse oximeter Sphygmomanometer (preferably automatic) Defribillator Adequate lighting Resuscitation equipment (laryngoscope, tubes, cardiac drugs, AMBU bag) Central oxygen supply

Many techniques can provide adequate general anaesthesia. Patients can be intubated after a generous dose of thiopental, lidocaine (lignocaine), and a non-depolarising muscle relaxant. Larynogotracheal anaesthesia is useful to decrease tracheal reactivity to the endotracheal tube during the procedure. For maintenance, a pure inhalation technique may be used. However, since many radiologic procedures involve very little stimulation (unlike abdominal surgery), muscle relaxants are often employed to allow a lighter plane of anaesthesia without patient movement. Isoflurane has distinct advantages over other inhalation agents because it exerts a cerebral protective effect, has none of the epileptogenic potential of enflurane, and causes little change in cerebral blood flow (CBF) when used in moderate concentrations combined with hyperventilation, is If nitrous oxide is not used, air should be given to avoid toxic concentrations of oxygen and the increased cerebral blood flow which occurs at hyperoxic levels. Alternatively, a combination of pentothal, nitrous oxide, and muscle relaxant may be used. 4 To avoid atmospheric contamination by inhalation agents, many anaesthesiologists use total intravenous anaesthesia. One technique utilises spontaneous ventilation with thiopental and lidocaine infusion. 19 The lidocaine diminishes airway reactivity to the endotracheal tube. Thiopental reduces intracranial pressure by decreasing cerebral blood flow, and may protect the brain from ischaemia by decreasing the cerebral metabolic rate. Thiopental infusions have also been combined with fentanyl, droperidol, muscle relaxants and hyperventilation to diminish intracranial pressure while maintaining normotension and adequate cerebral perfusion pressure. Alphaxalone/alphadalone (Althesin) has been successfully used alone by infusion, or in combination with muscle relaxants and narcotics in intubated patients.Z° Although this drug was not available in the United States, it was used in many other countries. Complications included involuntary movements and a generalised flushing response. For these reasons, it was employed much less frequently, and has now been withdrawn. To avoid intubation, especially in children who are at risk of developing croup or laryngeal oedema,

CURRENT NEURORADIOLOGICTECHNIQUESAND THEIR ANAESTHETIC IMPLICATIONS 93 ketamine has been used in patients with normal intracranial pressure. Ketamine produces dissociative anaesthesia rapidly. Excessive salivation necessitates premedication with an antisialogogue. Cardiovascular support is good and there is little respiratory depression. Observation after the procedure in a PACU setting is highly recommended as ketamine can augment brain electrical activity and cause hallucinations. 21 Regardless of the anaesthetic technique chosen, it is important that the patient rapidly returns to consciousness so that the neurological status may be immediately assessed. All patients with a previous history of allergic contrast reaction should be prepared with prednisone 50mg orally, every 6h for 24h. Diphenhydramine 50mg and cimetidine should be incorporated in the premedication. Using these precautions, the incidence of mild reactions can be reduced to 4% .22

Diagnostic angiography Diagnostic angiography involves placement of catheters intravascularly and injection of contrast in an effort to opacify vascular circulations for diagnostic purposes. The typical cerebral angiogram requires passage of a catheter from the right femoral artery up the aorta selectively to the carotid or vertebral circulation. Arch aortograms can also be performed with digital technology. Digital angiography refers to a technique of recording information in digital form rather than by conventional X-ray films and screens. Thus, greater sensitivity to different densities is obtained. In addition, image processing and subtraction can be performed almost in real time. Digital techniques can be performed as part of catheter cerebral angiography (intra-arterial digital subtraction angiography, or IA-DSA). a3 The dose of contrast material can be decreased because of the increased sensitivity. The rapid processing of information facilitates complic a t e d diagnostic and interventional procedures. Also, the sensitivity of the system to small differences in X-ray attentuation makes feasible the technique of digital intravenous angiography (DIVA), otherwise known as intravenous digital subtraction angiography (IV-DSA). 24 These studies are usually obtained after percutaneous central venous or right atrial catheterisation, a technique which averts some of the risks of accidental embolisation inherent in catheter cerebral angiography. Disadvantages of IV-DSA compared to selective catheterisation techniques include (1) superimposition of multiple arterial branches, (2) patient motion during the relatively long time it takes contrast material to traverse the venous system, and the pulmonary circulation, to the arterial system, (3) the requirement for relatively large amounts of contrast material. Angiography has been performed from the femoral location for approximately thirty years. Prior to this

time, direct carotid and vertebral angiography was performed almost exclusively. Beginning in the 1960's catheter techniques improved and the femoral approach became more common. It is now used almost exclusively in the United States as the means of approach for cerebral angiography. Occasionally an axillary approach may be performed if the approach from the femoral artery is not favourable (i.e. hindquarter amputation). It is only on very rare occasions that a direct carotid approach is used. Angiographic techniques involve placing the sedated patient on a specially designed fluoroscopy table. The indications for angiography are varied, and risks and complications depend on the underlying disease and the patient's medical condition including hypertension, and cerebrovascular disease. The most feared complication of angiography is stroke. Death occurs, although very rarely. Many studies have been performed on the rate of complications in angiography. The highest risks are for those patients being investigated for strokes. Stroke or stroke-like events occur in about 1% of these patients after angiography. 2s The contrast materials used are either an ionic contrast material such as Conray, Hypaque or Hexabrix or (more recently) non-ionic compounds (Omnipaque). Ionic contrast materials are very safe and inexpensive but carry a slightly higher risk of contrast reactions and are more destructive of the blood brain barrier in patients in whom large amounts of material are required. Non-ionic contrast materials are tolerated better by high-risk patients and if large volumes are to be used. Angiographic suites have evolved over the years. However, they still involve use of a fluoroscopy unit either integral to or separate from a film changer sequence. Most angiography is performed with a rapid sequence film changer and X-ray film. As digital angiography becomes more accepted as a standard technique, procedures in general will take less time and require less contrast material. Spinal angiography, that is angiography of the vessels which feed the spinal cord, is still performed, although much less frequently than in the past. MRI and, to a lesser extent, CT scanning has filled many of the functions for which spinal angiography was used previously. Angiography of the supplying vessels to the spinal cord is usually reserved for the evaluation of vascular lesions. The study presents certain problems in that it may be prolonged requiring multiple injections of contrast. Although patients usually suffer no ill effects, there are anecdotal reports of catastrophic complications of spinal cord angiography, including paraplegia. Renewed interest in angiography in the last three to five years is due partly to the development of interventional neuroradiology and partly to the failure of digital venous angiography and carotid doppler studies to supplant angiography as the standard for the evaluation of atherosclerotic disease.

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F i g u r e - Digital arteriogram. Done during embolisation of parietal AVM. Micro-catheter is in callosal marginal artery and several coils are in place in the feeding vessels to the AVM.

Angiography is rarely performed for indications related to turnouts or trauma such as subdural haematomas. It is the preferred method to diagnose cerebral vasculititis (asid e from biopsy). In the future MRI angiography may be accurate enough to accurately outline carotid plaques. 26 Angiography will probably still be required in the future to diagnose intra-cerebral aneurysms. It will also be necessary for the surgical planning of arteriovenous malformation ablation.

Interventional neuro-angiography Interventional neuroradiology is the application of radiologic techniques for the treatment of lesions in the central nervous system, head neck and spinal column. This sub-specialty has also been called surgical neuro-angiography because of the primarily surgical nature of the procedures as opposed to more straightforward diagnostic radiologic techniques. Through catheters, materials may be precisely placed to obliterate aneurysms or block blood flow to specific areas. Chemotherapeutic as well as destructive substances may be infused directly through vessels supplying a malignancy. The major lesions that are dealt with intracranially are arteriovenous malformations and fistulae (Figure).27 Interest in treating intracranial aneurysms as well as more aggressive approaches to intracranial atherosclerotic disease and vascular spasm after subarachnoid hemorrhages by these techniques has increased. 2s'29 Lesions that are treated in the extracranial area include; vascular tumours, such as juvenile angiofibromas and paraganglionomas, vascular malformations such as dural fistulae, arteriovenous fistulae and vascular metastases to the spinal column. 31-32 In some cases these procedures are performed as the primary mode of treatment; however, most are presurgical techniques. In that these are surgical procedures treating

dangerous lesions, there is a high complication rate. The treatment of intracranial arteriovenous malfunctions carries a defined rate of intracranial haemorrhage and death. Some centres measure intracranial vascular pressures and perform mapping EEG during procedures. Additionally intracranial amytal testing is frequently performed - that is injections of small amount of sodium amytal into a vascular pedicle to determine if normal brain is supplied by an identified vessel. As practitioners have become more sophisticated and techniques have improved, complication rates have declined. Hieshima recently reported a major complication rate of 3% and a death rate of 3% in the treatment of intracranial aneurysms. 33 Intracranial haemorrhage during neuroradiologic procedures may be characterised by severe hypertension, cardiac dysrhythmias and loss of consciousness. Therapy includes lidocaine and attempts to regain cardiovascular stability. A case report indicates better outcome with barbiturates than nitroprusside. 34 These procedures should be treated as surgery. They should be done with necessary monitoring equipment and operating room availability.35 Anaesthetic considerations M a n y of the anaesthetic principles outlined for patients undergoing myelography apply also to the patient presenting for angiography. Because of the higher complication rate associated with interventional neuroangiography, anaesthetic support should be routinely sought, especially for monitoring. Changesin level of consciousness can be followed by maintaining verbal contact. Changes in the motor, sensory and speech centres may be detected through simple tasks. The Wada test is used to determine the side of the speech centre, 36 prior to operations in the potential vicinity of the speech area in left-handed and ambidextrous patients. It is also utilised in the right-handed and ambidextrous patients when doubt exists as to which cerebral hemisphere is dominant for speech. An internal carotid artery is selectively cannulated. The patient is asked to hold up both hands and begin counting. Sodium amytal 150200mg is then injected into the carotid artery. Only the contralateral arm should fall; if the patient also becomes aphasic, then the speech centre has been localised. Anaesthesiologists are requested to monitor these patients because apnea has been noted after injections in some animals. 36 Complications of angiography include cerebral embolisation due to dislocation of thrombus from the catheter, atheromatous plaque from vessel walls, or air from loose catheter fittings, intramural-injection with compression of vessels (dissection), and arterial spasm. Transient or permanent neurological impairment may occur. After direct carotid puncture there is the risk of a haematoma within the neck which can lead to respiratory obstruction. Transient pain and

CURRENT NEURORADIOLOGIC TECHNIQUES AND THEIR ANAESTHETIC IMPLICATIONS

burning behind the eye may occur during injection of contrast media into the internal carotid artery, due to the high osmolality of the contrast material relative to blood. Hypotension may occur during injection of contrast agent, probably from a baroreceptor response. Bradycardia may be associated and respond to atropine. Other commonly seen dysrhythmias include premature ventricular contractions, nodal rhythms, and transient asystole. Contrast agents can damage the central nervous system by a direct necrotising effect associated with hypoxic microvascular damage. 37 The effect is enhanced by vasoconstrictors and ameliorated by vasodilators. 3s Thus vasopressors should be avoided during angiography. If necessary, ephedrine, with its predominantly beta effects, should be administered. Hyperventilation may improve the quality of cerebral angiography by slowing transit time of contrast media. Theoretically at least, small abnormal blood vessels which fill only transiently can be visualised. T u m o u r vessels are more clearly defined with hyperventilation since these arteries, unlike the normal cerebral circulation, usually will not vasoconstrict. 39 Mechanical ventilation by itself increases cerebral venous pressure and slows cerebral circulation.

CT scan A C T scan is currently the most common neuroradiologic test worldwide. The CT scan is a computerised x-ray machine producing cross-sectional images with high spatial and contrast resolution. It involves the use of an x-ray beam and a set of detectors. By the use of Fourier transformation, a series of images are obtained. CTs are performed for many indications and provide a minimal dose of radiation. Additionally, they are becoming more commonly used as guiding tools for intracranial stereotactic biopsies. 4° In that the CT scan uses X-radiation, lead shielding is all that is required to protect patients and observers. The gantry is mobile and the patients can be moved in and out rapidly. C T will remain as a ubiquitous neuro-imaging technique for many years. Although it is not as sensitive as MRI, in many instances it is an excellent test for the cerebrovascular diseases, neoplasms (both primary and secondary), cerebral degenerative disease and spinal column disease. 4~ It is still the study of choice for acute trauma and bone disease. CT is considered by some to be the test of choice for disease of the head and neck, with MRI reserved for equivocal problems. 42 Anaesthetic considerations The most important factor in providing a diagnostic test in CT scanning is that the patient remains

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motionless. New units produce a section in under 2 seconds, thus greatly reducing the time required for study. Intravenous anaesthesia often provides adequate sedation; nevertheless, general anaesthesia is required in 30% - 5 0 % of all children, in 17% of acute head injured patients 4 and in adults unable to cooperate because of dementia or movement disorder. As the scanner environment is kept cool to protect the integrity of the electronic circuitry, when anaesthetising patients, especially infants, temperature should be measured continuously. The incidence of hypothermia exceeds 40% in children less than 12 months of age. 4 Infants with severe hydrocephalus present problems of head stabilisation and visualisation of the posterior fossa. Even in adults, visualisation of this area may be difficult and require extreme head flexion, causing discomfort in the conscious patient, or kinking of the endotracheal tube in an anaesthetised patient. Brainstem compression in the presence of a large infratentorial tumour has been described.43 Use of contrast agents adds further risk to CT scanning techniques. A large amount of hypertonic iodinated contrast media may be given intravenously to identify breakdown of the blood brain barrier, as occurs with infarcts, neoplasms, and abscesses. Complications include allergic reactions as well as renal compromise and volume overload. There is a 1 - 2 m r a d per h radiation exposure. 4 Anaesthesiologists should wear lead aprons and use a lead glass screen.

Magnetic resonance imaging Magnetic resonance imaging has added even greater dimensions to neuro-imaging. It involves the use of a high field strength magnet and radio-frequencies to produce an image in any plane. In some instances the MRI is rapidly becoming a more important test than the CT scan in the evaluation of some neurological disease. When MRI is available it has become the test of choice for many diseases. It is felt by some that MRI should be the screening test for patients with epilepsy. 44 The great sensitivity of MRI to changes in water content has made it the test of choice in the study of diseases of the white matter. MRI demonstrates vascular diseases of the posterior fossa in a fashion not previously possible. Other indications for MRI include spinal column and cord disease, cranial nerve abnormalities, neoplasia and base of skull t u m o u r s . 45,46

The major component of the MRI image is the hydrogen proton which is the main component of water and most biologic materials. In the brain however, it is water in its various physical forms which creates the MRI image. Additionally, characteristic signals can be obtained from high-water content blood and from blood break-down products: fat, calcium and ferric iron. Recently a contrast enhancement material (gadolinium) has become

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available which affects the magnetic spin of the hydrogen proton resulting in a characteristic image on one of the acquisition sequences (T1). The appearance of the MRI image is dependent on the type of pulse sequence used to derive that image. Typically images are derived using either exponential time constants (T1 & T2), balanced (proton density) or gradient echo pulse sequences. An inversion recovery method is also available. T1 emphasises anatomic detail and gives a relatively high spatial resolution. It is With the T1 pulse sequence that gadolinium is effective as a contrast material. T2 predominately emphasises free water. These images are highly sensitive for detecting pathology but result in loss of spatial resolution. Balanced or proton density has increased sensitivity as compared with T1 but less than that of T2. Gradient echo which employs a lower flip angle attempts to give a T2-type image in a shorter period of time. T1, T2 and the balanced intermediate are all spin echo pulse sequences. Inversion recovery differs in that a heavily weighted T1 specific image results. MRI studies usually require 30 min to an hour. The patient is usually completely encased in a large cylindrical magnet. The magnet gantry weighs anywhere from 6-27 thousand pounds. Some permanent magnets can weigh up to 100 tons. In that these are very high field magnets all magnetic equipment and all electronic equipment must either be specially shielded or kept at a distance. Thus patients with pacemakers and cochlear implants, and those with cerebral aneurysm clips made of ferromagnetic magnetic materials should not be studied by M R I . 47-49

Anaesthetic considerations As during CT scanning, immobility is necessary for precise imaging. The patient is placed in a narrow cylinder and is relatively inaccessible. Claustrophobia is a frequent problem as the patient's head may be 2 metres or more from the opening. Access to the airway is limited. However, since there is no risk of radiation to the anaesthesiologist, he may remain close by to monitor the patient. The most important features of MRI for the anaesthesiologist are the presence of the powerful electromagnetic field and the use of radiofrequency pulses. Any ferromagnetic device may be moved toward the centre of the magnet or heat up as it absorbs energy. Ferromagnetic objects such as keys, stethoscopes, pens, scissors or safety pins can be accelerated into the scanner by the powerful magnetic field. Recordings from monitors are distorted by radiofrequency pulses. 5° Entirely non-ferromagnetic heart monitors, respirators and other monitoring equipment are now being manufactured. Standard blood pressure cuffs with lengthened rubber tubing, mercury manometers, Dopplers, electrical oscillotonometers, capnographs and pulse oximeters may be used within

the MRI suite provided the patient cables are at least 20 feet in length. All cathode ray tubes should be kept far away to prevent distortion of their displays. Plastic precordial or esophageal stethoscopes should be employed. The scanner emits a loud, rhythmical, drum-like noise which tends to obscure auscultated heart and breath sounds. Fibreoptic or telemetric ECG monitoring circumvents decreased image quality, caused by radiofrequency signals detected by conventional ECG waves. An Ohmeda analyser functions well. Aneuroid chest wall sensors have been successfully used to monitor respiratory movements which are then displayed on a distant oscilloscope screen. Some modification of equipment for general anaesthestic techniques is also necessary. Plastic endotracheal tubes, connectors, and hoses are acceptable and anaesthesia is usually established in an adjacent room. Although the laryngoscope itself is not ferrous, the batteries inside are highly ferrous. Plastic- or paper-coated batteries should alleviate this problem. The Jackson-Rees modification of the Ayre's T-piece has been successfully used; the anaesthesia machine being wall-mounted three metres from the scanner. 51 Compensation must be made for the significant compression volume in an extended circuit. A Forreger Model BC anaesthesia machine has been modified to be devoid of ferromagnetic parts and successfully used within 2 feet of the MRI machine, s2 A Narco Air-Shields ventilator (Model VC 20-1) is non-ferromagnetic and may be used concomitantly. Intravenous anaesthesia may also have a place. Methohexital or thiopental has been successfully combined with spontaneous respiration. 53'54 Also, a thiopental, muscle relaxant, and narcotic combination has been used in managing acutely ill patients using a nonferrous 225/SIMVR ventilator (Monaghan Medical Corp., Plattsburg, NY). Resuscitation poses a special problem as equipment malfunctions near the MRI unit. If the unit has a resistive magnet, it may be quickly turned off and will not affect resuscitation. Several hours are required to re-establish a stable magnetic field for subsequent studies. However, magnetic fields produced from superconduction magnets cannot be eliminated. It would thus be necessary to mo.ve the patient from the vicinity of"the magnet.

Emission computed tomography There are two major emission computed tomography techniques used for cerebral blood flow and metabolism studies. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) both use radiation emitted from the patient as a result of radioactive decay. The external configuration of the PET scanner is similar to that of CT, with a circumferential array of detectors to collect photons emitted from the patient in the centre. An

CURRENT N E U R O R A D I O L O G I C TECHNIQUES AND THEIR ANAESTHETIC IMPLICATIONS

on site cyclotron is required to form positron emitters (usually carbon-11, oxygen 15 and flourine-18) because of the extremely short half lives of the isotopes. PET permits the study of regional cerebral blood flow with tracer doses of carbon dioxide (C1502) and oxygen (O150). Regional metabolic activity can be determined with the use of the positron emitting analog of glucose, 18-flourine 2-deoxyglucose (18FDG). At present, PET is used primarily as a research tool. More widespread clinical use depends on the future development of less costly and cumbersome equipment, particularly smaller and cheaper cyclotrons. SPECT uses conventional radionuclides which decay by emission of a single photon, such as 99mTc and 123I. Transaxial images are obtained through the use of multiple circumferential detectors. The sensitivity of SPECT is less than that of PET systems, resulting in relatively inferior images. However, SPECT systems are far less costly than PET units because an on-site cyclotron is not necessary.

Pneumoencephalograms Pneumoencephalograms remain a diagnostic test in some areas of the world. A technique similar to myelograms for examining the brain, but not the spine is used. Air instead of contrast material is injected. As CT scans and MRIs with their improved diagnostic ability become readily available, the number of pneumoencephalograms performed will be reduced. Rarely, CT-air cysternograms are still performed for evaluation of small acoustic neuromasY. These, although safe, can cause severe headaches. General anaesthesia is frequently required for pneumoencephalography because of patient discomfort. However, as this technique is almost obsolete, the reader is referred to other reviews. 56'57

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