5. Magnetic Resonance Imaging Flashcards
What is it
MRI requires the generation of very strong magnetic fields,
typically up to 3.0 tesla.
The devices use superconducting magnets which are cooled by immersion in liquid helium at a temperature of 4.2 Kelvin.
It complements computerized tomography
(CT) in providing high-quality images of soft tissue.
Principle
cell nucleus with an unpaired proton is exposed to an electromagnetic field, it becomes aligned along the axis of that field.
A charged and spinning nucleus generates a magnetic field
and acts itself like a small magnet.
The aligned nuclei can then be displaced by brief exposure to another
magnetic field,
generated at right angles to the first.
This provokes the phenomenon of nuclear precession,
in which the nuclei rotate around an axis different from that
around which they are spinning
When the electromagnetic field is removed, the
nucleus resumes its original position, and as it relaxes to this position it emits low
radiofrequency (RF) radiation.
When the electromagnetic field is removed, the
nucleus resumes its original position, and as it relaxes to this position it emits low radiofrequency (RF) radiation.
T1 + T2
MR reports usually refer to T1 and T2 views. ‘T’ is a relaxation time constant,
T1 being the image generated a few milliseconds after the electromagnetic field is removed, and
T2 is an image generated slightly later.
Nuclei in hydrogen take longer to decay to their original position.
In practice, this means, for example, that in a T1
view, fluid will be dark (as minimal signal is generated),
whereas in the T2 view, fluid will be white.
Tesla
The tesla is the unit of magnetic flux density. Should you be asked, 1 tesla (T) is equal to 1 weber m
The Earth’s magnetic field is
approximately 1 gauss. 10,000 gauss equal 1 tesla
Gad
Intravenous contrast agents are sometimes used, typically gadolinium chelates, which
shorten nuclear relaxation rates, particularly in T1 imaging. In patients with impaired
renal function these chelates (and other exposure to gadolinium) uniquely can cause
the rare condition of nephrogenic systemic fibrosis (nephrogenic fibrosing dermopathy),
which is characterized by fibrosis of skin and internal organs
Practical problems
Practical problems:
there are practical difficulties in relation to the physical environment.
The patient is enclosed within a narrow tube to which access is limited.
The scanner is noisy (>85 decibels) and some patients may be very claustrophobic.
Scanning may be prolonged, with complex examinations lasting as long as 1–2 hours.
Magnetic field:
Magnetic field: at a magnetic field strength of approximately 50 gauss (indicated within the scanning room as a contour marked as the ‘50-gauss’ line),
all ferromagnetic items will be subject to movement and will also interfere with the generated image.
Items typically affected include hypodermic needles, watches, pagers, mobile telephones, stethoscopes, anaesthetic gas cylinders and ECG electrodes.
If these items are close to the field they will become projectile objects
Machines
Anaesthesia delivery: anaesthetic machines which contain ferrous metals (there are
non-magnetic machines and cylinders available) must remain outside the 50-gauss
line. The machine requires very long anaesthetic tubing and long leads
Patient access:
Patient access:
this is very restricted, and in particular the head and airway are
completely inaccessible during scanning.
All airway devices must be checked for the presence of any ferromagnetic material: the one-way valve on the pilot balloon of some cuffed endotracheal tubes contains a small spring, as do laryngeal mask airways.
Anaesthetic monitoring:
the field may induce current within electric cabling.
The consequent heating may lead to thermal injury.
Long sampling leads for gas analysis extends delay.
Standard ECG electrodes cannot be used.
An oesophageal stethoscope may be useful.
Pulse oximetry probes are non-ferrous,
but a distal site should be used and cable should be insulated.
(Severe burns due to induction heating have been
reported with standard pulse oximeters.)
Non-invasive blood pressure cuffs must have plastic connections as well as long leads to the machines, which must be outside
the 50-gauss line.
Gas analysis, airways pressure and respiratory indices are usually
displayed at the anaesthetic machine, and so again the main problem is delayed sampling time (up to 20 seconds) owing to long tubing.
Pacemakers
Pacemakers: cardiac pacemakers and implantable defibrillators require special consideration, as they will malfunction in fields greater than 5 gauss.
Infusion pumps:
Infusion pumps: these may fail if the field strength exceeds 30 gauss.
Implants and foreign bodies
Implants and foreign bodies: most patient implants (such as orthopaedic prostheses) are non-ferrous.
Surgical clips and wires may be magnetic,
but their presence does not usually contraindicate MR scanning,
as they become embedded in fixed fibrous tissue.
Exceptions are intracranial vascular clips.
Metal foreign bodies are likely to be ferrous. Non-ferrous items may heat.
Generic problems
Generic problems: there are the generic problems of anaesthetizing patients in
remote, unfamiliar and isolated areas. Many more children than adults require
general anaesthesia for MR scanning.
Generic problems
Generic problems: there are the generic problems of anaesthetizing patients in
remote, unfamiliar and isolated areas. Many more children than adults require
general anaesthesia for MR scanning.
