Anesthesia in the MRI Suite and for CT Scan


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)



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 16.3
    Ionizing radiation doses











































    Source

    Exposure

    Cross-country airplane flight

    3 mrem

    Dental bitewing X-Ray

    0.5–5 mrem

    Chest X-Ray

    5–15 mrem

    Abdominal X-Ray

    40–60 mrem

    Mammogram

    70 mrem

    Head CT scan

    200 mrem

    Chest CT scan

    800 mrem

    Abdominal CT scan

    1000 mrem

    PTCA

    500–5000 mrem

    Annual dose from smoking (1 pack/day)

    20 mrem/year

    Annual dose from natural sources

    300 mrem/year


    mrem millirem, PTCA percutaneous transluminal coronary angioplasty


  • 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.


  • There is little evidence from either in vitro or in vivo studies that there is any detrimental biological impact from exposure to clinically-relevant static 1.5 T or 3 T magnetic fields [4, 5].


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 16.4
    MRI zones




























    Zone

    Locations

    Comments

    I

    Areas outside of MRI suite

    Freely accessible to public

    No restrictions

    II

    Reception area

    Nursing station

    Interview & Waiting area

    Buffer between Zones I & III

    Patient interviews & Screening occur here

    Patients are supervised by nurses

    Ideal for anesthesia inductions

    III

    MRI Control room

    ± Adjoining spaces & Hallways

    Access is strictly controlled

    Only screened patients & personnel may enter

    Only screened equipment may enter

    Access must be restricted by barriers or locks

    IV

    MRI scanner room

    Hazardous environment!

    Access is strictly controlled

    Only screened patients & personnel may enter

    No ferromagnetic items may enter

    Constant direct supervision by MR personnel


    Based on the ACR Guidance Document on MR Safe Practices [6]




    • 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 suitewithin 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 iodinebased.


  • There is a high rate of adverse events (up to 5 %) from the administration of CT contrast agents [7, 8], including hypersensitivity/anaphylactoid reactions, anaphylaxis, thyroid dysfunction, and kidney injury.


  • 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.

Aug 26, 2017 | Posted by in Uncategorized | Comments Off on Anesthesia in the MRI Suite and for CT Scan
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