5. Magnetic Resonance Imaging

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.

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.

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.

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

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: 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: 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

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: 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.

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: cardiac pacemakers and implantable defibrillators require special consideration, as they will malfunction in fields greater than 5 gauss.

Infusion pumps: these may fail if the field strength exceeds 30 gauss.

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: 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: 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.