MRI for Nurses

What Nurses should know about MRI

Radiology Nurse

The staff nurse in the Radiology department is a very important part of the clinical team. Usually the staff nurse is on rotational posting in various sections of Radiology. These are general X-Rays, Special procedure rooms like IVU, Fluoroscopy, CT scan, Angio suite, MRI etc. In various sections the role and responsibilities varies according to the procedures.

Radiology Nurse in MRI

Magnetic Resonance Imaging(MRI) equipment is a powerful diagnostic Imaging tool. An MRI image is often superior to a plain film x-ray for viewing and understanding soft tissue structure. An MRI image is formed by a large magnet which polarizes hydrogen atoms in the tissues, provides a summation of the spinning energies within the tissues' cells and produces an image representation of this process.

Precautions while sending a patient for MRI form Emergency department,ICU or ward

From an emergency department MRI scan is usually requested for spinal cord injury or acute stroke patients. Patients past medical history may not be known at the time of admission to the emergency department. Patient may be on immobilisers and with monitoring devices and life lines attached and it may not be MR compatible. Patient screening is the primary step before sending for an MRI scan. Make sure there is no cardiac pacemaker and implanted devices on the patient body.If patient had any previous history of surgery then get necessary details of the operation such as date and type of surgery and is there any metallic implants or devices in the patient body. Patient should be transferred to MR compatible monitoring devices and infusion pump before entering into the scan room. The attending Doctors and Nurses should screen by themselves and remove all metallic items from their body before entering into the scan room for patient management.Portable oxygen cylinders should not bring into the MR environment which carries a potential risk of injury to the patient and also damage to the equipment. Oxygen supply should be provided through a wall socket inside the MR room or through a long tubing connected to the oxygen tank placed outside the MR room. If the patient is on ventilator then transfer the patient to MR compatible ventilator before moving into the scan room.

Examples of items that may create a health hazard or image artifacts during an MRI exam include:

- Neurostimulator
- Aneurysm clip
- Cochlear implants
- Cardiac Pacemaker
- Implantable devices - eg. Implantable cardioverter defibrillator (ICD)
- Implanted drug infusion device
- Intravascular filters
- Metal implants such as orthopedic devices
- Metal objects, especially if in or near the eye
- Shrapnel or bullet wounds
- Dentures and bridges
- Any implants that involve materials sensitive to magnetic field
- Medication patch that contains metal(eg.transdermal patch)

Before sending the patient for MRI scan check with the MRI technologist or Radiologist if you have any questions about implanted object or condition that could impact the MRI procedure and patient safety.


Please note: This information is provided for the educational purpose only.You should contact the referring physician, Radiologist or MRI technologist for specific information on MRI safety to patients.

Reference Manual for Magnetic Resonance Safety,Implants and Devices can be available from Biomedical Research publishing group Los Angels,CA, by Frank G.Shellock Ph.D

http://www.magneticresonancesafetytesting.com/

MRI Fringe Field

Magnetic Resonance Imaging (MRI) Equipment


In an MRI equipment the stray magnetic field outside the bore of the main magnet is called fringe field. These fields are created by magnetic flux lines and the number of flux lines per unit area is called magnetic flux density. The flux density is higher at the magnetic isocenter and the density decreases as we move away from the isocenter of the magnet.


Magnetic Isocenter is the centre of the bore of the magnet in three planes (x,y,z). The magnetic field homogeneity is measured in the isocenter of the magnet. Magnetic field homogeneity is very important for clinical magnetic resonance imaging. Shimming is the process whereby the evenness of the magnetic field is optimized. Shim coils are used to make the magnetic field more homogeneous.

In a superconductive magnet the flux lines run parallel to the bore of the magnet and it is called main magnetic field. Main magnetic field is indicated by B0.

The strength of the magnetic field is measured in gauss(G) or tesla (T). Tesla is the SI unit of magnetic flux density. It was defined in 1960 in honor of inventor Nikola Tesla. The cgs unit of measurement of magnetic field is gauss . It is named after the German mathematician and physicist Carl Friedrich Gauss . One Tesla = 10,000 gauss.

Fringe field should be contained within the MRI scanning room. The typical requirement is to contain the 5G(0.5mT) flex line within the walls and ceiling of the scanning room. The stray field outside the scan room can affect patient monitoring devices and medical implants.

Fringe field is contained within the specification by shielding the flex lines.

There are two types of magnet shielding, active and passive shielding. Passive shielding is done using steel plates in the wall and ceiling. This will help the flux lines to contain within the room. An actively shielded magnet uses pairs of coils called bucking coils with equal but opposite current to the main magnetic field coils. Bucking coils counter the main field and the mechanical structure within the cryostat becomes more complex.

Effective magnet shielding in essential to prevent stray field extending outside the scan room. Shielding will also help to reduce the MR siting cost. Moving heavy metal objects may interfere with the main magnetic field if the shielding is not appropriate. Before any MRI installation an experienced engineer should conduct a site survey to find out the shielding requirement .