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Anaesthesia for MRI
Dr. S. Parthasarathy
MD., DA., DNB, MD (Acu),
Dip. Diab.DCA, Dip. Software statistics
PhD (physio)
Definition
• MRI is a noninvasive diagnostic technique that
uses magnetic properties of atomic nuclei to
produce high-resolution, multi planar cross- sectional images of the body.
• Hydrogen is the atom most often used for
imaging.
The various parts of an MRI
Monitor and software
Technology
generates a high-density static magnetic field,
which is always present. Field strength is
measured in Tesla (T)
most scanners use 0.5-1.5 T magnets (about
10,000 times the Earth's magnetic field).
Physics
• some atoms within human body possess an
unpaired proton, eg hydrogen atom
• alignment of these nuclei are random
• subjected to a strong electromagnetic field,
they align themselves with the field
• The rate of the alignment depends on the type
of nucleus/ element
• emitted signal depends on the molecular
properties of the tissue
Various equipments and diffwerent place
MRI – lumbar, knee, neck
• The magnetic field decreases as distance from
the scanner increases—beyond the 0.5 mT
boundary or outside the scan room can be
considered safe. This boundary is known as the
5-Gauss Line. 1,000 Gauss equals 1 T.
• MRI is a valuable non invasive imaging
technique.
• MRI produces a high quality images of the
body in cross-section and in three dimension
• MRI is particularly useful for the imaging of
soft tissues eg CNS
• Revolutionized ortho, neuro practice
Hiccups with MRI
• Noisy and unfamiliar environment
• Motion interferes with images quality
• MRI scan takes up to an hour
• Hence patients may need anaesthesia
few indications for anesthesia
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Infants and children
Patients with learning difficulties
Patients with seizure disorders
Patients with claustrophobia
Critically ill-patients
• SL CCC – pneumonic
strong magnetic field- hazards
• the attraction of ferromagnetic objects to the
magnetic field of an MRI.
• risk of dislodgement of implanted metallic
objects (i.e., pacemakers, vascular clips,
automatic implantable cardioverter
defibrillators, mechanical heart valves, and
implanted infusion pumps),
• Orthodontic braces and dentures and tattoos
can degrade the image quality significantly.
Other problems
• Injury to patients, personnel, and equipment
-- from propelled ferromagnetic objects
brought into the magnetic field.
• What are those objects ??
• gas cylinders, pens, keys, laryngoscopes,
scissors, stethoscopes, paper clips, vials, and
needles
• malfunction of electronic equipment (such as
monitors and infusion pumps)
MRI - four zones:
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Zone I is the public zone
Zone II is the reception
Zone III – control room
Zone IV – scanner room
Personnel safety
• no significant deleterious effects to patients or
health care professionals from exposure to the
static magnetic field of an MRI.
• Pregnant patients have undergone MRI safely
during all stages of pregnancy. Nevertheless,
caution is advised.
• Think USG only
• the simple rule that ‘nothing enters the scan
room except the patient.’
MR safe’ or ‘MR compatible’.
• it presents no safety hazard to patients or
personnel. guarantee that it will function
normally and not interfere with the correct
operation of the MR imaging equipment, with
degradation of image quality.
• MR compatible is MR safe, functions
normally in the MR environment
Monitors
• MR compatible pulse oximeters must use
fibreoptic cabling to avoid burns
• Special ECG electrodes and cables are
required.
• Padding should be placed between cables and
the patient’s skin and the avoidance of loops in
cables within the scanner
Some hiccups
• Aortic blood flow in a magnetic field generates
currents that result in significant artefact in the
ST-T region of the ECG complex.
• Delay of up to 20 seconds in obtaining the
capnograph signal due to the length of the
• sampling tubing.
• The need for acoustic protection during MR
imaging will necessitate the use of ear-plugs or
ear defenders.
Separate machine
• A MR compatible anaesthetic machine should
be located within the scanning room.
• Only MR compatible vaporisers and gas
cylinders must be used on anaesthetic
machines within the scanner room.
What next ??
MRI compatible machines
Anaesthesia team
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Consultant
Team
Staff
Radio staff
Protocol
Beware -- credit cards, cassette tapes, or
floppy disks ??
MRI room is for radiologists – not for
anaesthesiologists
• MRI suites have been designed without the
• consideration for anesthetic needs, such as pipeline
gases, suction,
• bulkiness of the unit,
• the patient is often far from the anesthesiologist,
• access to the airway is limited.
• Intravenous lines, anesthesia circuits, oxygen
tubings, monitoring cables must be of sufficient
length to reach the patient deep within the scanner.
Acoustic problems
• Noise levels above the safe level of 85 decibels can
be produced during MRI due to the rapid switching
of the gradient.
• Staff working in MRI units should protect themselves
by remaining in the MR control room during
sequence acquisition, or by wearing earplugs
• All patients should be given ear protection,
regardless of if they are awake or anaesthetised.
Practical considerations
• Induction area adjacent to but outside the scan
room (beyond the 0.5 mT boundary - 5-Gauss
Line) equipped with a compact conventional
anesthesia machine and monitoring.
• Piped gases, scavenging, and suction in both
the induction area and the control room.
• Nonmagnetic gurney for patient transfer into
scanner. – stretcher routine ??
Practical considerations
• Respiratory gas/agent side-stream analyzer
with capnograph display fitted with an
extended sampling tube (increases the
response time by 5-10 s).
• MRI-compatible pulse oximeter (fiber-optic
patient probe, and shielded cable).
Practical considerations
• Compact (e.g., wall mounted) anesthesia
machine and ventilator in the control room
with a 10-m co-axial (Bain) or circle breathing
system.
• ECG with MRI compatible (carbon fiber)
patient leads and electrodes.
• NIBP machine with an extended hose,
nonmetallic connectors, and a range of cuffs.
It is painless !!
• Remember
• MRI is painless procedure
• Preop investigations
• Nothing special depends upon the disease
• Plates, pacemaker etc
Children
• oral midazolam (0.5mg/kg, maximum 10 mg)
in a small amount of cherry or strawberry
syrup.
• Disabled children do not need higher doses of
sedatives but are three times more at risk of
hypoxia under sedation
Anaesthetic options
• Options
• Sedation ,TIVA, Controlled GA
Alternative to fasting
• A very safe and simple technique for newborns
is the ‘feed and scan’ technique in which
children are fed and one has to wait until the
young patient falls asleep.
• unpredictable ‘induction times’ and the high
failure rates of the scanning procedure
Sedation – options
• Other sedative options
• Oral chloral hydrate
• Dose - 25 and 100 mg/kg
• nausea and vomiting, long recovery times and
postoperative agitation
• Oral melatonin
Other sedative techniques
• Pentobarbital – oral or rectal – 3-6 mg/kg
• Dosing is 1–1.5 mg/kg when applied
intravenously or 4–5 mg/kg when injected
intramuscularly. Onset time is 1–3 min, and
duration is 15–30 min
• What is this drug ??
• Ketamine
Propofol
• Propofol seems to be a perfect drug for
sedation because it is effective, has a short
recovery time and can easily be titrated to the
required sedation level.
• Dosing is normally 2–5 mg/kg/h intravenous
Other sedatives
• Dexmed
• A loading dose of 2–3mic.g/kg over 10 min
followed by 1–2mic. g/kg/h as an infusion for
sedation maintenance is recommended.
• Midazolam used alone is not suitable for MRI
sedation as its duration is too short for a
successful procedure of 20–30 min
• Can be combined – be careful
When sedation, when GA ??
• > 3 years , 10 kg , no comorbidities
• Consider sedation
• < 3 years < 10 kg , lot of co illness – GA
• Some may sleep without drugs - students !!
GA
• Control room :: Prior to induction, patients should be
placed on a non-ferrous trolley (without oxygen
cylinders) so that they can be safely transferred to the
MR room once they are anaesthetised.
• Induction agents includes sodium thiopental (5mg/kg) ,
propofol (2-3mg/kg)
• Secure airway with LMA
• Second metal check ; remove stethoscope, needles ,
oxygen cylinder from the trolley
• Transfer patient to MRI room
Procedure
• Transfer patient onto MRI scanner
• Check to ensure the airway is secured
• Maintenance of anaesthesia may be either
inhalational or intravenous
• TIVA is an acceptable technique for MRI
TIVA
• Propofol infusion ( 6-10mg/kg/hr) for patient
• Infusion pump should be keep in control room,
while connecting lines are passed through
waveguide.
• Ear protection before commencing scanning
• Anaesthetists monitor the patient from control
room
Recovery
• Once study has been completed, patient should
be remove from the scanner, and be woken up
and recovered in a suitable recovery area
• Usually there is no need for analgesics
• Patients for MRI scan are usually day case
Contrast agents
• The most commonly used intravenous MR
contrast agent is gadolinium dimeglumine.
• contrast-enhanced MR angiography.
• doses of 0.2 ml/kg and has minor side effects
including nausea, vomiting and pain
• on injection.
• Nephrotoxic ??
Maintenance of body temperature
• More in infants
• No active heating necessary.
• Once the study has been completed, the patient
should be removed from the scanner and be
woken up and recovered in a suitable recovery
area.
• Analgesia is unlikely to be required for MRI
scanning,
EMERGENCIES IN THE MRI SUITE
• the presence of a strong magnetic field
• risk of projectiles,
• restricted access imposed by the MRI scanner.
The patient should be removed from the magnetic
field as quickly as possible and transferred to the
induction room, which should be close to the
scanner and will contain the necessary
anaesthetic and resuscitation equipment and
drugs
INTENSIVE CARE PATIENTS
REQUIRING MRI
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multiple drug infusions
extensions of adequate length or wave guides
higher standard of monitoring.
MR compatible monitors – must
Pulmonary artery catheters with conductive wires
in contact with heart muscle and epicardial pacing
wires – removal plannned
INTENSIVE CARE PATIENTS REQUIRING
MRI
• Many tracheostomy tubes are not MR
compatible and will need to be changed prior
to the examination.
• pre-MR X-ray screen – history vague
• The pilot balloons of cuffed tracheal tubes may
contain a small ferromagnetic spring will need
to be taped securely away from the area being
scanned
Health staff
• long-term repeated exposure to strong magnetic fields
has a harmful effect on the human body.
• current recommendations suggest that a timeweighted average of 200mT over any 8-hour
period should not be exceeded by healthcare
personnel.
• Ideally all staff should vacate the MRI
examination room whilst the scan is in progress.
Summary
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MRI – introduction and physics
Which patients need anesthesia ??
Preop
Anesthesia
Emergence
• Special !!
Thank you all