An introduction to magnetic resonance Imaging by Kerry Jackson

ABSTRACT: This article aims to provide a brief introduction to the subject of magnetic resonance imaging [MRI). which is a diagnostic tool utilised to create highly detailed images of the structures within the body. These images are created through the use of a strong magnetic field; therefore, staff working in this area must be fully aware of any protocols in place to ensure that personal and patient safety is not compromised.

Common applications for use in practice include the investigation of central nervous system disorders and diagnosis of joint disease. Patients undergoing MRI scanning require sedation or general anaesthesia, and there are a number of points that nursing staff can address to minimise the impact on the patient.

How MRI scanning works

Magnetic resonance imaging (MRI) is a diagnostic tool utilised to create highly detailed images of the structures within the body. In order to understand how the MRI scan is produced, it is important to be aware of atomic structure.

An atom is a cluster of protons and neutrons that form the nucleus, which is orbited by electrons. According to Squire and Novelline, ‘The MRI unit uses a powerful magnetic field to align the magnetization of atoms in the body. Radio frequency fields systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field which is detectable by the scanner’.1

Essentially, these protons align when placed in a magnetic field and when a radiofrequency is sent towards them they resonate. When the radio frequency signal is stopped, the protons return to their original position emitting energy. It is this energy that is picked up by the associated MRI computer to create an image.

This image can then be manipulated to provide sufficient detail of the area of interest. Since pathological processes change the properties of the affected tissues, these changes are then reflected on the cross-sectional 3-D image produced.

Until quite recently, MRI has had a limited application in the field of veterinary medicine, primarily owing to the expense of the imaging unit and associated requirements, such as the need for a specially constructed MRI suite to house the magnet and related equipment. Running an MRI suite requires specialist equipment to operate and produce the images, and experienced specialist staff to operate the equipment and interpret the images produced. The use of MRI scanning is becoming more common in the veterinary field as it provides good contrast between the soft tissues of the body, as well as detailed images of the skeletal structures (Figure 1).

Figure 1: A canine MRI scan

Types of MRI scanner

MRI scanners can be either low strength (0.2 Tesla) open field, which do not fully enclose the patient; or high strength (1.5 Tesla) closed field magnets, which fully enclose the patient (Figure 2). To put this into context, the Earths magnetic field has a strength of 0.00006 Tesla.

Figure 2: An open field MRI scanner

The high strength, closed field magnets have several advantages over the smaller open field magnets in that the images produced are generally of greater quality, they provide superior contrast resolution and are better able to distinguish small details. Because of this, the required sequences can be completed in a shorter time frame which has advantages for the anesthetised patient through the reduction of the time the procedure takes.

It is worth noting that the actual magnet in the high strength superconducting units (1.5 Tesla) cannot be turned off, as it is supercharged and cooled by cryogens (liquid helium). They are only turned off (quenched) in emergency situations. ‘Quenching’ or shutting down the magnet presents many health and safety issues because it involves the venting of helium.

The strong static magnetic field can attract magnetic items with lethal force, and websites such as www.metrasens.com/ ferroguard graphically demonstrate this. There are, therefore, several important health and safety considerations with regard to the use of MRI:

• the entrance to the MRI room must be clearly marked and all equipment entering the room must be MRI compatible – non magnetic, non ferrous – this including all anaesthetic and monitoring equipment and tables used to transfer patients to the scanner.

• patients should be routinely checked to ensure that all metallic items are removed before transfer into the scan room in order to prevent serious injury. This includes patient collars, all items used for induction, and staff should also ensure ET tubes have plastic connectors only.

• staff must also remove all metallic items (scissors/pens/watches/hair clips/ mobiles etc) as all of these items will be pulled into the magnet with force (Figure 3).

Figure 3: Warning poster

Use of MRI in practice

Common applications for MRI use in practice include investigation and diagnosis of disease or injury of the spine and central nervous system, investigation of joint disease and the diagnosis and staging of neoplastic disease. Indeed, spinal MRI is considered to provide superior images and carry less associated risk for the patient than myelography.

MRI is especially useful in the treatment of neoplastic disease as it allows for the affected area to be mapped with great accuracy, which assists with surgical and treatment planning for radiotherapy. Patients undergoing MRI scans require general anaesthesia – or at the very least sedation for shorter procedures. The process is particularly noisy and patients must lie absolutely still or the resulting images will be poor and the sequence will have to be repeated.

The procedure varies in length according to the area under investigation and the sequences required. Some procedures can take over an hour – not including induction and recovery – particularly if contrast studies are required.

Patient preparation and recovery will largely depend on the individual, their associated condition and the number of sequences required. The clinician responsible for the patients care will select the most appropriate sedation or anaesthetic protocol; but there are several steps that nursing staff can take to facilitate the smooth running of the procedure and minimise the impact on the patient:

• gaining informed consent from the client as well as a complete history and any relevant radiographs

• ensuring patients are fasted

• completing a specific questionnaire relating to patient’s previous surgical history in order to assess if the patient has any surgical implants and the location of any microchip. (Surgical implants can produc
e heat during the scan process which presents a risk of thermal injury at the site of the implant; microchips will leave an artefact on the image produced)

• ensuring the area to be scanned is clearly detailed on the patient records and these records stay with the patient at all times

• packing the patients ears with cotton wool will dampen the noise experienced within the scanner and can be particularly beneficial for patients that are sedated (Just remember to remove them when the patient is in recovery!).

• ensuring the bladder is empty before transfer, as patients may lose control of this organ while under anaesthesia or sedation and the presence of a liquid (urine) adjacent to the electrical coil could cause injury.

Repeated imaging can cause an increase in the temperature of the magnet and associated equipment, which is compensated for by the circulation of cold air. This then has a negative effect on the temperature of the patient. The use of additional items such as bubble wrap and blankets to insulate the patient while being scanned will help to reduce risk of hypothermia. Heat sources which have a metal component will not be suitable for use in this area.

Additional considerations

Patients undergo MRI scanning for a variety of reasons; those presenting with suspected spinal injuries must be moved with utmost care to prevent further compromise. Patients suffering brain injuries may present as high-risk candidates for anaesthesia. Brain injury or abnormal pathology may impact on respiratory centres or be a cause of increased intracranial pressure.

Monitoring patients undergoing MRI can be challenging as most conventional equipment contains some ferrous material and is unsuitable for use in this area. If MRI compatible equipment is not available, an oesophageal stethoscope (with plastic ear pieces) can be used in conjunction with direct monitoring if patients are undergoing MRI in an open field magnet.

It may also be possible to utilise a capnograph situated outside the magnetic field (scan room) by using an additional extension set to the sample line. This allows the person monitoring the anaesthetic to assess the patient’s ventilation, circulation and metabolism since it may not be possible for staff to access the patient and assess the depth of anaesthesia visually.

Once the scan is complete the patient is transferred to a suitable area for recovery and is monitored as any patient would be recovering from sedation or anaesthesia.

Author

Kerry Jackson RVN

Kerry qualified as a veterinary nurse in 2002 and has worked in both general and referral practice, including the Royal Veterinary College, where she worked in the Queen Mother Hospital for animals and subsequently in the Clinical Investigation Centre.

Kerry is currently teaching on the Veterinary Care Assistant course and looking forward to studying for her diploma in teaching.

To cite this article use either

DOI: 10.1111/j.2045-0648.2011.00112.x or Veterinary Nursing Journal Vol 26 pp 390-392

Reference

1. SQUIRE. L. F„ and NOVELLINE. R. A. (1997). Squire's Fundamentals of Radiology [15th ed.l] Harvard University Press.

Popular Keywords

Categories

VNJ Articlesclinicalimagingresonance