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Anaesthesia for CT, MRI and Interventional Radiology 

Anaesthesia for CT, MRI and Interventional Radiology

Chapter:
Anaesthesia for CT, MRI and Interventional Radiology
Author(s):

Keith G. Allman

, Iain H. Wilson

, and Aidan O’Donnell

DOI:
10.1093/med/9780199584048.003.0030
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  • Pippa Dix

    Anaesthesia for CT and MRI scanning [link]

  • Anaesthesia for computerised tomography (CT) [link]

  • Anaesthesia for magnetic resonance imaging (MRI) [link]

  • Darryl Johnston

    Anaesthesia for interventional radiology (IR) [link]

Anaesthesia for CT and MRI scanning

The anaesthetist working in the medical imaging department may be expected to use unfamiliar equipment in a potentially hazardous environment. Ensure that trained assistance, monitoring, etc. are available and familiarise yourself with the surroundings. Locate the nearest resuscitation facilities (self-inflating bag/mask, portable oxygen, ‘crash’ trolley, and defibrillator)—confirm that your assistant and the radiographers also know where these are!

Indications for anaesthesia

  1. Infants and uncooperative children. Small babies (under 2 months) will often sleep through a scan if given a feed and wrapped up well.

  2. Older children or adults with psychological, behavioural, or movement disorders.

  3. Intubated patients such as acute trauma victims and patients receiving intensive care.

  4. Analgesia, sedation, or anaesthesia may also be required for interventional procedures performed under CT or possibly MRI guidance.

  5. Patients for elective scans commonly have a range of problems. Check the indications for scan and nature of the underlying pathology—developmental delay, epilepsy, malignancy, and psychiatric and movement disorders. Significant cardiovascular and respiratory problems are uncommon, but beware of ‘syndromes’ with CVS manifestations.

Anaesthesia—general points

  1. Choice of sedation or GA and type of GA depends upon the needs of the patient, the nature of the investigation, and the skills and experience of the anaesthetist (see later).

  2. Check whether the anaesthetic machines are using piped gases or cylinders. If using cylinders, confirm that a full spare oxygen cylinder is immediately available.

  3. Plan the location of the anaesthetic machine, suction and monitoring, and the configuration and routing of the breathing system in advance.

  4. Decide where to induce the patient—a dedicated induction area may not be available or may be very small. It is usual to induce on a tilting trolley, then transfer to the scanner when anaesthetised.

  5. Certain equipment configurations (e.g. anaesthetic machine in scan room and monitors in control room) may require two anaesthetists to manage the patient safely.

  6. Ensure satisfactory recovery facilities are available—i.e. appropriately equipped recovery bay and an experienced recovery nurse near the scanner, or arrangements for safe transfer of the patient to an operating department recovery room.

Anaesthesia for computerised tomography (CT)

  1. The CT scanning environment does not restrict the type of equipment used but space is often limited, so compact anaesthetic machines and monitors are more practical.

  2. Patient, anaesthetic machine, and monitors must all be visible from the control room.

  3. The patient's head is usually accessible during CT scanning so an LMA may be used if the patient does not require IPPV or airway protection.

  4. A variety of anaesthetic (and sedation) techniques can be used. The final choice should be determined by the equipment available and the patient's needs.

  5. Only ‘light’ anaesthesia to produce immobility and lack of awareness is required.

Hazards

  1. CT scanning generates potentially harmful ionising radiation so it is preferable for the anaesthetist to monitor the patient from outside the scan room. If it is necessary to remain near the patient, wear appropriate radiation protection.

  2. Cannulae, catheters, drains, and endotracheal tubes can be pulled out during transfers and by movement of the patient through the scanner—ask the radiographer how far the table will move and check that lines and breathing system do not snag other equipment.

Contrast media

  1. Modern intravascular contrast media for X-ray imaging utilise highly iodinated, non-ionic, water-soluble compounds.

  2. Common agents are iohexol (Omnipaque™), iopromide (Ultravist™) and iopamidol (Niopam™) which are monomeric, and the dimeric compound iodixanol (Visipaque™). Concentrations equivalent to 300–320mg iodine/ml are typically used.

  3. You may be asked to administer IV contrast to anaesthetised patients. The volume required varies with the preparation, investigation, age, and body weight but may be up to 150ml (see table).

  4. Check the timing of injection with the radiographer because some ‘dynamic’ investigations (e.g. aortography) require contrast to be administered as the scan is occurring.

  5. Contrast is viscous and can be difficult to inject through small cannulae or injection ports (take the bung off and inject via the hub of the cannula).

  6. Automated contrast injectors should not be connected to central venous lines. The high pressure developed by the rapid injection of viscous medium down a long narrow lumen can burst the line.

  7. IV iodine-containing contrast media occasionally trigger allergic reactions (ask about iodine sensitivity).

  8. These agents may cause renal failure in patients who are dehydrated or who have impaired renal function so ensure adequate hydration in patients who have been starved for GA. Lactic acidosis can be precipitated in patients taking biguanides (metformin) and these should ideally be avoided for 48hr before and after the scan.

Investigation

Adult

Volume (ml)

Child

Volume (ml)

CT head

50–100

10 + 2ml/kg

CT body

100–150

(up to adult dose)

Aortography

100

Urography

2–3ml/kg

2–3ml/kg

Practical considerations

  1. Metal-containing objects (such as ECG leads, pressure transducer cables, and clips) lying in the X-ray beam can cause artefacts so route them away from the area to be scanned.

  2. Thoracic or abdominal scans may require ‘breath-holds’ to reduce respiratory movement artefacts. Both paralysed and spontaneously breathing patients can be ventilated manually and their lungs held in inspiration for the few seconds needed to perform each individual scan.

  3. The patient's arms usually need to be positioned above the head during thoracic or abdominal scans. Wide adhesive tape is useful for securing the limbs (keep a roll on the anaesthetic machine).

  4. Intensive care patients requiring CT scans should be managed like any inter-ICU transfer with full transport monitoring and ventilatory support. Ideally the ICU resident or consultant should supervise the patient and review the scan with the reporting radiologist. Getting such patients into and out of the scanner can be a slow process.

Anaesthesia for magnetic resonance imaging (MRI)

MRI is a versatile imaging tool free from the dangers of ionising radiation. A computer creates cross-sectional or three-dimensional images from minute radio-frequency signals generated as hydrogen nuclei are flipped in and out of alignment with a powerful magnetic field by high-frequency magnetic pulses.

  1. Non-invasive but can be unpleasant—subject has to lie motionless in a narrow noisy tunnel with the part of body to be imaged closely surrounded by an ‘aerial coil’ (a very claustrophobic environment).

  2. Typical sequence of scans lasts 15–25min but complex scans may take much longer.

  3. Up to 3% of adults cannot tolerate scanning without sedation or anaesthesia.

  4. Provision of safe anaesthesia for MRI requires specialised equipment and careful organisation—unlike CT you cannot simply take a standard machine and monitor to the MRI scanner.

Hazards

  1. Most scanners use a super-conducting magnet to generate a high-density static magnetic field, which is always present. Field strength is measured in Tesla (T)—most scanners use 0.5–1.5T magnets (about 10 000 times the Earth's magnetic field).

  2. Near the scanner (>5mT) the static field exerts a powerful attraction on ferromagnetic materials (e.g. scissors, gas cylinders, laryngoscopes) which can become projectiles. Electric motors (e.g. in syringe drivers) may run erratically and any information stored on magnetic media (credit cards, cassette tapes, or floppy disks) will be erased. The magnetic field decreases as distance from the scanner increases—beyond the 0.5mT boundary or outside the scan room can be considered safe.

  3. Devices (e.g. hypodermic needles) made from non-ferromagnetic stainless steel can be taken into the scan room. If you are unsure about an object don't risk it!

  4. Oscillating magnetic fields induce eddy currents in electrical conductors (e.g. ECG leads, metallic implants). These currents may disrupt or damage electronic equipment (including pacemakers) and cause heating effects that can result in burns.

  5. Large masses of metal (e.g. anaesthetic machines, gas cylinders) near the scanner or small amounts of non-ferrous metals within the three-dimensional volume being scanned can distort the magnetic fields, causing poor quality images.

  6. The scan room is usually shielded to prevent external electrical interference from swamping the MR signals. All electrical equipment within the scan room must also be fully shielded and electrical conductors entering the room (e.g. monitoring cables) require special radio-frequency filters.

  7. Rapidly changing magnetic fields cause mechanical vibrations and extremely loud ‘knocking’ noises, which can potentially damage hearing.

Equipment

Two alternative approaches are feasible:

  1. Specialised ‘MRI compatible’ equipment within the scan room, or

  2. Conventional equipment outside the scanner's magnetic field in the control room.

Standardising on one option keeps the anaesthetist, anaesthetic machine, and monitors together. Choice depends upon space, funds, frequency of general anaesthesia, and individual preference. Using conventional equipment at a distance avoids crowding the scanner, is less expensive, and allows faulty monitors to be substituted. The anaesthetist can regulate the anaesthetic and monitor the patient without being in the scan room and hazards can be reduced by applying the simple rule that ‘nothing enters the scan room except the patient and the trolley’.

A typical set-up is as follows:

  1. Induction area adjacent to but outside the scan room (beyond the 0.5mT boundary) equipped with a compact conventional anaesthetic machine and monitoring.

  2. Piped gases, scavenging, and suction in both the induction area and the control room.

  3. Non-magnetic tipping trolley for patient transfer into scanner.

  4. Compact (e.g. wall-mounted) anaesthetic machine and ventilator in the control room with a 10m co-axial (Bain) breathing system.

  5. Respiratory gas/agent side-stream analyser with capnograph display fitted with an extended sampling tube (increases the response time by 5–10s).

  6. MRI-compatible pulse oximeter (fibreoptic patient probe and shielded cable).

  7. ECG with MRI-compatible (carbon fibre) patient leads and electrodes.

  8. NIBP machine with an extended hose, non-metallic connectors, and a range of cuffs.

  9. Recent technology allows a monitor unit within the scan room, with a slave unit in the control room, improving patient monitoring during transfers.

Practical considerations and techniques

  1. Physically and ‘magnetically’ restricted access makes patient observation and treatment difficult so a secure airway is a priority.

  2. Neonates and young babies (<2 months)—will often sleep through a short scan if fed, wrapped up, and placed on their side in scanner.

  3. Babies and small children (<15kg), and any patient with an intracranial space-occupying lesion, suspicion of raised ICP, or needing a protected airway—use intubation and IPPV.

  4. Larger children and adults (if no risk of raised ICP)—use spontaneous ventilation and a standard LMA (not a flexible one with a wire spiral).

  5. If intubating a patient for a head scan use an RAE tube—it keeps the connectors and breathing system clear of the head coil.

  6. Tape the valve on the pilot tube of a cuffed ETT or LMA outside the aerial coil or the metal of the spring will distort the images.

  7. Sedation with oral or IV benzodiazepines may be used by radiologists for healthy but claustrophobic adults. Patients with severe back or root compression pain may also require strong analgesia to tolerate positioning for a scan.

  8. The role of sedation for MRI scanning in children is unclear. Some children's centres have reported successes with structured sedation programmes run by dedicated sedationists. However, the safety of having heavily sedated children in the medical imaging department without direct anaesthetic supervision has been questioned.

Tips for IPPV through a 10m breathing system1:

  1. Use a system that functions as a ‘T-piece’ (Mapleson D or E) so dead space is unaffected by length. Ayre's T-piece and co-axial Bain systems work well and are both suitable for ventilating babies and small children.

  2. Airway pressures measured near the ventilator may not accurately represent distal pressures at the endotracheal tube.

  3. Tidal volume delivered to the lungs will be reduced by ‘compression losses’ of the gas within the system and by expansion of the tubing during inspiration, making it difficult to compensate for significant leaks round uncuffed tracheal tubes—change to a slightly larger tube so the leak is minimal.

  4. As a result of these effects IPPV using a simple pressure generator (e.g. Penlon Nuffield 200 with a Newton valve) may not be effective in children weighing more than 15kg.

  5. Increased expiratory resistance of some long systems (e.g. Ayres T-piece) generates a positive expiratory pressure which increases with the fresh gas flow.

Intensive care patients:

  1. Same considerations apply as for CT scanning (see [link]) but potential hazards are greater so risk/benefit balance should be assessed carefully.

  2. Do not scan patients who are haemodynamically or otherwise unstable.

  3. Electronic pressure transducers, metal-containing ICP ‘bolts’, temporary pacing wires, and conventional ECG leads must be removed before the patient enters the scan room.

  4. Full checks (and if necessary plain radiographs) must be performed to confirm there are no hazardous metallic implants or foreign bodies present.

  5. Patients who are stable on inotrope infusions can be scanned, but infusion pumps must remain at a safe distance from the magnet—ideally outside the scan room. Prepare duplicate pumps in the control room with extended infusion lines threaded with breathing system into the scan room. Connect patient to running infusions while outside the room, check they are stable, then move into the scanner.

Patient and staff safety

  1. To avoid accidental injury all patients having an MRI scan must complete a screening/consent form. In case of children or sedated ICU patients these must be completed on their behalf by relatives or staff.

  2. To prevent injury and property damage all staff must similarly complete a screening questionnaire and leave metallic objects, pagers, credit cards, etc. outside the room.

  3. Greatest dangers arise from ferromagnetic implants and foreign bodies—certain types of artificial heart valves, old cerebral aneurysm clips, steel splinters in the eye, where movement could disrupt valve function or precipitate intracranial or vitreous haemorrhage, respectively.

  4. Patients and staff with cardiac pacemakers must remain outside the 0.5mT boundary.

  5. Anaesthetised and sedated patients should have their ears protected to prevent noise-induced auditory damage.

  6. IV MRI contrast media are paramagnetic but do not contain iodine and have a high therapeutic ratio. Side effects include headache, nausea and vomiting, local burning, and wheals (2.4%). Severe hypotension/anaphylactoid reactions are rare (approximately 1:100 000).

  7. Commonly used agents are gadopentetate (Magnevist™) at a dose of 0.2–0.4ml/kg and gadodiamide (Omniscan™) at 0.2ml/kg. More recently gadobutrol (Gadovist™) at 0.1ml/kg and Gadoteric acid (Dotarem™) at 0.2ml/kg have been used in patients with a reduced GFR or an unknown GFR (most children). These agents are associated with a lower risk of nephrogenic systemic fibrosis.

Cardiac arrest

  1. Do not attempt advanced life support in the scan room.

  2. Do not allow the cardiac arrest team into the scan room.

  3. Start basic life support with non-metallic self-inflating bag and chest compressions.

  4. Remove patient from scan room on non-magnetic trolley and continue resuscitation outside 0.5mT boundary.

Further reading

Association of Anaesthetists of Great Britain & Ireland (2002). Provision of Anaesthetic Services in Magnetic Resonance Units. London: AAGBI.

Hatch DJ, Sury MRJ (2000). Sedation of children by non-anaesthetists. British Journal of Anaesthesia, 84, 713–714.

Menon DK, Peden, CJ, Hall, AS, Sargentoni J, Whitwam, JG (1992). Magnetic resonance for the anaesthetist. Part I: physical principles, applications, safety aspects. Anaesthesia, 47, 240–255.

Peden, CJ, Menon DK, Hall, AS, Sargentoni J, Whitwam, JG (1992). Magnetic resonance for the anaesthetist. Part II: anaesthesia and monitoring in MR units. Anaesthesia, 47, 508–517.

Shellock FG (2001). Pocket guide to MR procedures and metallic objects: update 2001. Lippincott Williams & Wilkins.

Anaesthesia for interventional radiology (IR)

In this subspeciality minimally invasive procedures are performed under image guidance, usually in the X-ray department. Procedures are often performed to avoid open surgical procedures to reduce post-procedure pain and recovery time. They may be diagnostic or therapeutic. The imaging utilised may involve radiation exposure, e.g. fluoroscopy and computerised tomography (CT), or may be ultrasound or magnetic resonance imaging (MRI).

Common interventional procedures

  1. Angioplasty/stenting/coiling: vascular, neuro and cardiac.

  2. Embolisations: blocking vessels to reduce bleeding in a planned surgical operation, to stop bleeding post surgically, following trauma, or stopping tumour growth.

  3. Chemo-embolisation: combination of delivering cancer treatment directly to a tumour and then blocking its blood supply.

  4. Radiofrequency ablation: local destruction of tissue by heating.

  5. Cryoablation: local destruction of tissue by freezing.

  6. Thrombolysis.

  7. Biopsies.

  8. Vertebroplasty/cementoplasty: injection of cement into bone to reduce pain in tumours and fractures.

Indications for anaesthesia

  1. Patient may be required to be very still for long periods of time.

  2. Procedure may be very painful.

  3. Paediatric patients.

Anaesthesia for IR: general points

  1. As previously described for CT and MRI it is vital for the anaesthetist and their assistant to familiarise themselves with the equipment available in this ‘isolated’ environment (see [link] and [link]). Monitoring and anaesthetic machine must be fully checked and the location of resuscitation equipment checked. Scavenging is often not possible so TIVA may be useful or an Aldosorber may be used. Depending on the patient and procedure, you may be in or outside the scan room. Induction generally occurs within the radiology suite. Before starting check you have all the drugs drawn up that you anticipate using for anaesthesia, and those you may want in an emergency (metaraminol, ephedrine, atropine). After the procedure the patient is woken up in radiology and then generally transferred to main theatre recovery.

Angioplasty/stenting/coiling

  1. A balloon-tipped catheter is inserted into a narrow or blocked vessel and the balloon inflated. A stent may be placed to keep it open. Vascular and cardiac procedures often do not require a general anaesthetic.

  2. Endovascular repair of AAA (see [link]) is associated with a lower mortality and is favoured in those patients with poor left ventricular function. This may be done under regional (epidural and sedation) or general anaesthesia.

  3. Intracranial angioplasty and stenting are used for the treatment of intracranial aneurysms. A general anaesthetic is required because the patient must be completely still. A similar anaesthetic technique should be used as for craniotomy ([link] and [link]). Induction should be cardiovascularly stable, avoiding any drop in cerebral perfusion pressure, and the airway secured with an endotracheal tube. Invasive monitoring should be used. Care should be taken to ensure normocapnia and normothermia.

Embolisations

  1. Procedures that are superficial, involve an arteriovenous malformation, or involve the use of alcohol for the embolisation are very painful and require sedation, or a general anaesthetic.

  2. Depending on the position of the patient choose ETT or LMA.

  3. In obstetrics uterine artery embolisation is now included in the NICE guidelines for massive obstetric haemorrhage. Balloon catheters can be placed in the uterine artery and inflated to stop the pelvic bleeding. This can be done in an emergency, or can be inserted before a Caesarean section in a case that is anticipated to bleed.

Radiofrequency ablations (RFA)

  1. In this procedure the tumour is destroyed by heating. Depending on the size of tumour it may take as long as 40min, is painful, and requires a general anaesthetic.

  2. RFA is commonly used to treat hepatic and renal tumours—either metastases, difficult to reach tumours, or tumours in those patients who are too frail for an open procedure.

  3. Depending on the position of the tumour the patient may need to be prone and so requires an ETT; otherwise a laryngeal mask is often sufficient.

Cryoablation

  1. These procedures tend not to be painful and often sedation is all that is required.

Thrombolysis

  1. Minimally invasive treatment that dissolves blood clots and improves blood supply. This can be used to treat arteries in diseased vascular beds, deep vein thrombosis, coronary emboli, pulmonary emboli, and thrombosis in fistulae.

  2. Contrast media help define the clot. This is then dissolved by either medication delivered directly to it or a mechanical device.

  3. General anaesthesia is rarely required.

Notes:

1 Sweeting CJ, Thomas PW, Sanders DJ (2002). The long Bain breathing system: an investigation into the implications of remote ventilation. Anaesthesia, 57, 1183–1186.