MRI
CT
Seizures
Hypotonia
Failure to thrive
Developmental delay
Hydrocephalus, suspected shunt malfunction
Sensorineural hearing loss
Tumors & staging
Skeletal abnormalities
Metabolic disease
Vascular anomalies (aneurysms, vascular malformations, hemangiomas)
ENT issues
Nerve lesions
Spinal cord compression
Meningomyelocele
Tethered cord
Back pain
Osteonecrosis
Cardiac or aortic disease
Head or body trauma
Suspected intracranial hemorrhage
Seizures
Mental status changes/encephalopathy
Focal neurological findings
Vertigo, apraxia, headache, visual field defects
Increased ICP
Hydrocephalus, suspected shunt malfunction
Tumors & staging, mass effect
Differentiation of solid, cystic, inflammatory, vascular, and fatty lesions
Bone lesions
Mediastinal mass
Thoracic and abdominal masses or fluid collections, abscesses, and cysts
Spinal cord disease, e.g. meningomyelocele
Foreign body localization
In addition to the usual concerns for out-of-the-OR anesthesia, the MRI and CT environments present some unique complexities and hazards for anesthesiologists and their patients, including ionizing radiation, strong magnetic fields, cryogens, awkward patient access, the need for the anesthesiologist to remain disconcertingly far away from the patient, and cumbersome but necessary safety processes. All anesthesiologists should be educated regarding the unique safety aspects of the MRI and CT environments [1]. Table 16.2 lists websites that may be of interest.
Table 16.2
Websites of interest
|
Name |
URL |
Comments |
|---|---|---|
|
ASA Statement on Nonoperating Room Anesthetizing Locations |
Sets minimal guidelines for anesthesia care in out-of-the-OR sites | |
|
APSF Clinical Safety – MRI |
Information from the Anesthesia Patient Safety Foundation on MRI safety | |
|
Joint Commission Sentinel Event Alerts |
Alerts intended to draw attention to specific current safety & care concerns of national import (see SEA#38 & #47) | |
|
MRISafety.com |
Has large database of equipment & devices tested for MR environments | |
|
Simply Physics |
Lots of MRI information, including an excellent collection of photographs of MR projectile incidents | |
|
ACR Guidance Document for Safe MRI Practices |
American College of Radiology’s MR safety guidelines | |
|
OSHA Ionizing Radiation website |
U.S. Dept of Labor Occupational Safety & Health Administration; Health effects and standards information |
Physics
CT
-
X-Rays are high-frequency, high-energy electromagnetic radiation that carries sufficient energy to displace electrons from their atoms (ionization).
-
Ionizing radiation can directly or indirectly damage DNA, causing radiation-induced cell damage or death, leading to a wide range of problems including cataracts, sterility, radiation burns, teratogenesis, and cancer.
-
The radiation dose from a CT scan is much higher than for plain X-ray films (see Table 16.3). High radiation exposures are of particular concern in pediatric patients, since CT scans are known to increase the risk of leukemia and brain tumors [2].Table 16.3Ionizing radiation dosesSourceExposureCross-country airplane flight3 mremDental bitewing X-Ray0.5–5 mremChest X-Ray5–15 mremAbdominal X-Ray40–60 mremMammogram70 mremHead CT scan200 mremChest CT scan800 mremAbdominal CT scan1000 mremPTCA500–5000 mremAnnual dose from smoking (1 pack/day)20 mrem/yearAnnual dose from natural sources300 mrem/year
-
Radiation exposure is inversely proportional to the square of the distance from the source (inverse-square law).
-
CT scanners employ an X-ray tube that rotates axially around the patient gantry. Emitted radiation passing through the patient is sensed by a detector array.
-
CT measures electron density, differentiating between high-density tissues (calcium, bone, iron, and contrast-enhanced areas) and lower-density tissues (air, muscle, fat, water).
MRI
-
Nuclear magnetic resonance (MR) refers to the phenomenon whereby atomic nuclei exposed to magnetic fields absorb and emit electromagnetic radiation.
-
Magnetic resonance scanners have three interacting electromagnetic fields (static, gradient, and radiofrequency fields) that perturb the orientation and magnetic dipole moment of hydrogen nuclei, causing them to release energy that is detectable by the MR scanner [3].
-
The main static magnetic field is generated by large electric currents flowing through loops of wires immersed in superconducting cryogenic fluid.
-
Current clinical MR scanners use static magnetic fields of either 1.5 or 3 Tesla (T). These magnets are approximately 50,000 times the strength of the Earth’s natural magnetic field. In other words, these are enormously powerful magnetic fields! 3 T scanners have superior sensitivity and resolution.
-
Clinical MRI scanners measure at least three different properties of tissue samples: T1 relaxation, T2 decay, and proton density.
-
Due to the complex spatial encoding of MR signals, any patient movement significantly degrades image quality. Anesthesia facilitates patient immobility for the scan.
-
Cryogens, most commonly liquid helium, are utilized inside the scanner to cool the wire bundles to superconducting temperatures.
-
A “quench” is the rapid release of cryogen from the scanner in the form of a gas. Quenching rapidly eliminates the static magnetic field (1–3 min), and is used only for emergencies. Liquid helium expands at a 750:1 ratio as it boils into gas; thus it must be vented to the outside via special escape valves and ventilation system. If the expanding helium gas escapes into the scanner room, hypoxia and asphyxiation may result. Quenching is used only in emergencies to remove a patient or equipment from the magnet.
-
MR angiography and venography (MRA/MRV) are MRI imaging techniques that study blood vessels and vascular flow, in particular to evaluate stenoses or aneurysms.
-
Three decades of clinical MRI use has revealed no known significant physiological impacts from MRI, unlike X-Ray or CT.
Physical Layout
CT
-
CT scanner rooms are lined with lead to restrict ionizing radiation.
-
Large lead-infused windows in the control room allow direct visualization of patients and monitors in the scanner.
-
For anesthesia cases, the CT scanner room should have oxygen (preferably built-in wall gas lines), vacuum suction, and available electric outlets.
MRI
-
The MRI suite is designed around the MR scanner and its extremely powerful magnetic fields. Of paramount concern is the ability to restrict access to the scanner room and its magnetic fields.
-
Faraday cage – The walls of MRI scanner rooms are sheathed in metal (copper or aluminum) forming a complete box around the scanner, shielding it from external radiofrequency interference.
-
Large windows in the control room wall facilitate direct visualization of patients in the scanner. They are lined with fine copper mesh to maintain Faraday cage continuity.
-
The American College of Radiology (ACR) sets standards for MRI suite design and access as well as safe practices in MR environments [6]. A major concern is ensuring screening and controlling access to the powerful magnetic fields, as a component of large-scale efforts to minimize the risk of harm from ferromagnetic object projectiles, or adverse effects related to implanted medical devices.
-
The ACR guidance documents define the conceptual division of all MR suites into four Zones [6] (See Table 16.4).Table 16.4MRI zonesZoneLocationsCommentsIAreas outside of MRI suiteFreely accessible to publicNo restrictionsIIReception areaNursing stationInterview & Waiting areaBuffer between Zones I & IIIPatient interviews & Screening occur herePatients are supervised by nursesIdeal for anesthesia inductionsIIIMRI Control room± Adjoining spaces & HallwaysAccess is strictly controlledOnly screened patients & personnel may enterOnly screened equipment may enterAccess must be restricted by barriers or locksIVMRI scanner roomHazardous environment!Access is strictly controlledOnly screened patients & personnel may enterNo ferromagnetic items may enterConstant direct supervision by MR personnel
-
Access to Zones III & IV is strictly controlled, due to the hazards of ferromagnetic objects in strong magnetic fields.
-
Only trained, approved personnel and properly screened patients may enter Zones III & IV.
-
-
The scanner room should have built-in wall gases (oxygen, air, and nitrous oxide) and vacuum (suction) lines available for patient care.
-
Direct observation of the patient may be at least somewhat compromised during MRI scans. The anesthesiologist’s workstation in the control room should be positioned to optimize visualization of the patient and anesthesia monitoring equipment.
-
Anesthesia care can and does occur in all four Zones.
-
It is preferable to set up an “anesthesia induction suite” within Zone II, for the induction and emergence of anesthetized patients, away from the magnetic field hazards of Zones III & IV.
Contrast Enhancement
CT
-
Radiocontrast agents are often employed during CT scans to improve tissue differentiation and visualization of vascular structures.
-
CT contrast agents are typically iodine–based.
-
Adverse reactions are most common with older, high-osmolar contrast agents. Newer, low-osmolar and iso-osmolar agents are considerably safer.
-
Adverse reaction management: Call for assistance as appropriate. Respond with monitoring, oxygen, fluid resuscitation, antihistamines, bronchodilators, steroids, epinephrine, advanced airway management, and ACLS/PALS as needed.
-
Radiocontrast agents can be nephrotoxic, causing iatrogenic acute renal failure, aka contrast-induced nephropathy, especially in patients with pre-existing renal disease (GFR <30 mL/min/1.73 m2). Renal function should be checked pre-operatively.
-
Oral contrast enhancement can be achieved with diatrizoate agents such as Gastrografin.
MRI
-
Gadolinium-based contrast agents are often employed for the enhancement of MRI images.
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