28.2 Radiation Therapy, Diagnostic Imaging & MRI-Suite Anesthesia
Key Takeaways
- The MRI magnet is never turned off, so every person and object entering Zone IV must be screened every time, and the ferromagnetic projectile effect is the leading cause of MRI-related death.
- A helium quench rapidly displaces oxygen from the scanner room and is an asphyxiation emergency requiring immediate evacuation, and all resuscitation is performed outside the scanner room.
- Radiation therapy requires total patient isolation with monitoring by closed-circuit camera and remote telemetry, so the airway must be independently secure before staff leave the room.
- Pediatric radiotherapy typically requires 20 to 30 daily anesthetics over four to six weeks, making an existing central port the practical solution for vascular access and daily fasting a real dehydration risk.
- The 2016 FDA warning about repeated or prolonged anesthesia in children under 3 does not justify delaying medically necessary procedures, a position endorsed by the ASA and ACOG.
Why This Topic Matters on the NCE
Domain IV.A.11 lists diagnostic imaging and radiology, radiation therapy, and endoscopy among the NORA sub-topics. Radiation therapy in particular is a setting most students never see, and it has the most extreme requirement in all of anesthesia practice: total patient isolation with no one in the room.
1. MRI Anesthesia
The magnetic resonance environment is the only anesthetizing location where the equipment itself can kill.
The four zones
| Zone | Access |
|---|---|
| Zone I | Freely accessible public area |
| Zone II | Interface where patients are screened and interviewed |
| Zone III | Restricted; only screened personnel; the control room |
| Zone IV | The scanner room itself; the magnet is always on |
The magnet is never turned off between cases. Every person and object entering Zone IV must be screened, every time.
The three fields and their hazards
| Field | Hazard |
|---|---|
| Static magnetic field (1.5 to 3 tesla, and higher) | Ferromagnetic projectile effect. Oxygen cylinders, laryngoscopes, infusion pumps, scissors, and stethoscopes become lethal missiles. This is the leading cause of MRI-related death |
| Gradient (time-varying) fields | Acoustic noise exceeding 90 to 110 dB — hearing protection is required for the patient and any accompanying staff. Peripheral nerve stimulation |
| Radiofrequency field | Tissue heating and burns. No looped cables, no coiled ECG leads, no wire-containing tubing in contact with skin, and remove transdermal patches containing metallic backing |
Practical management
- MR-conditional equipment is required: non-ferromagnetic anesthesia machine, monitors, laryngoscopes, and infusion pumps, or standard equipment kept outside Zone IV with long circuits and infusion lines through a waveguide.
- The patient is inaccessible during scanning. Secure the airway and all lines before entering the bore, and plan how the patient will be extracted in an emergency — resuscitation happens outside the scanner room.
- Implanted device screening: pacemakers and implantable cardioverter-defibrillators (many modern devices are MR-conditional under a strict protocol), cochlear implants, neurostimulators, some older aneurysm clips, and retained metallic foreign bodies. A patient with a history of metal work or ocular injury needs an orbital radiograph to exclude an intraocular fragment.
- Programmable ventriculoperitoneal shunt valves must be re-verified after scanning.
- Quench: an emergency dump of liquid helium. It produces a roaring noise and rapidly displaces oxygen from the room, creating an asphyxiation hazard, plus cryogenic burn and barometric risk. Evacuate the room immediately if a quench occurs.
- Hypothermia in a cold scanner, and prolonged immobility.
- Gadolinium reactions and nephrogenic systemic fibrosis risk in severe renal impairment.
Who needs anesthesia
Children, patients with claustrophobia or movement disorders, patients with intellectual disability or dementia, and critically ill patients requiring ventilation. Sedation with dexmedetomidine or propofol and a natural or supraglottic airway is common, with the key requirement being absolute immobility for image acquisition.
2. Anesthesia for Radiation Therapy
The unique constraint
External beam radiation therapy delivers ionizing radiation that no one else can be in the room for. The anesthetized patient is alone.
- The provider monitors by closed-circuit camera and remote telemetry from a shielded control room, and must be able to reach the patient within seconds when the beam pauses.
- The airway must be absolutely secure and independently maintained before staff leave.
- Immobility must be complete — the treatment field is measured in millimeters, and many patients are in a custom thermoplastic immobilization mask that further restricts airway access.
The pediatric course
A child receiving radiotherapy for a central nervous system tumor, neuroblastoma, or rhabdomyosarcoma typically requires daily anesthetics, five days a week, for four to six weeks — 20 to 30 anesthetics in a month.
- Vascular access is the practical crux. Most of these children have an implanted port or tunneled central line placed for chemotherapy, and it should be used to avoid daily venipuncture.
- Technique: propofol infusion or sevoflurane with a natural airway, laryngeal mask, or nasal cannula, chosen for rapid onset and rapid recovery. Each treatment lasts only minutes.
- Daily fasting in a small child undergoing weeks of treatment risks dehydration and hypoglycemia; schedule the treatment early and encourage clear fluids to 2 hours.
- Parental presence at induction and a consistent team reduce distress across a long course.
- Total body irradiation before hematopoietic stem cell transplantation is longer, may require multiple fractions, and involves an immunosuppressed, often pancytopenic patient.
The FDA warning in context
In December 2016 the FDA issued a warning that repeated or lengthy (more than 3 hours) exposure to general anesthetic and sedation drugs in children under 3 years, or in the third trimester of pregnancy, may affect the development of children's brains. The professional response, endorsed by the ASA and by ACOG for pregnancy, is that medically necessary procedures should not be delayed or avoided on this basis, that elective procedures in this age group may reasonably be deferred, and that the discussion should be had with families. Radiation therapy for a malignancy is unambiguously medically necessary.
Brachytherapy
Radioactive sources are placed within or adjacent to the tumor, most commonly gynecologic. Placement requires anesthesia, frequently neuraxial for prolonged applicator comfort, and staff must observe radiation safety precautions during and after loading.
3. Endoscopy and Diagnostic Imaging Suites
Gastrointestinal endoscopy
- Shared airway, left lateral position, and a high volume of cases with variable patient acuity.
- Propofol sedation is standard. Deep sedation without a secured airway is the most common technique and the most common source of NORA airway complications.
- ERCP is a specific challenge: prone or semi-prone position, long duration, an elderly and often septic or jaundiced population, and limited airway access, so tracheal intubation is frequently preferred.
- Duodenal insufflation, hypoxemia, and aspiration are the recurring problems.
Computed tomography
Brief but requires immobility and often breath holds; contrast considerations as previously described; trauma CT frequently involves an unstable patient in a poorly equipped location.
Cardiac catheterization and electrophysiology laboratory
- Long procedures with radiation, contrast, and a cardiac-compromised population.
- Electrophysiology ablation may require deliberate periods of apnea or high-frequency jet ventilation for catheter stability, and sedation depth affects arrhythmia inducibility — coordinate with the electrophysiologist.
- Cardioversion, transesophageal echocardiography, and structural interventions such as transcatheter aortic valve replacement all take place here.
Exam Traps
- The MRI magnet is always on. Screen every person and object entering Zone IV.
- A quench displaces oxygen — evacuate immediately.
- Resuscitation happens outside the scanner room.
- Radiation therapy means total isolation — the airway must be secure before you leave.
- The FDA anesthetic neurotoxicity warning does not justify delaying medically necessary surgery.
A 3-year-old with medulloblastoma requires daily anesthesia for six weeks of cranial radiotherapy. Which management approach is most appropriate?
During an MRI under general anesthesia, a loud roaring noise begins and a white vapor cloud fills the scanner room. What is happening and what is the correct action?
Which patient finding most requires an orbital radiograph before MRI?
Why is tracheal intubation frequently preferred over deep sedation for endoscopic retrograde cholangiopancreatography?