MRI Principles
MRI is based on the interaction between nuclei of hydrogen atoms occurring abundantly in all biological tissues and the magnetic fields generated and controlled by the MRI system's instrumentation. Hydrogen nuclei have a non-zero magnetic moment. When a body tissue is placed in the magnetic field of the MR scanner, the magnetic moments of the protons tend to align themselves with the main magnetic field of the scanner. Having aligned the hydrogen protons in a known direction, a pulsed radiofrequency field is applied to the body tissues, causing a number of hydrogen protons to flip or absorb energy. When the RF field is turned off, the protons gradually return to their previous positions , and in the process release the energy they absorbed in the form of a RF signal. It is this signal that is used to develop the MR images by the computer.
Main Indications
The current main indications for a MRI examination in Australia as determined by the Medicare rebate schedule are for the neurological and musculo-skeletal systems. However, as supported by the current literature, MRI has a proven role in assessing liver and breast lesions, in the assessment of breast implants ,in the staging of prostatic carcinoma, in the staging of gynaecological malignancies and in the assessment of the biliary tree and these examinations are being routinely undertaken despite the absence of a rebate.
Main Contra-indications
The main contra-indications to MRI examinations include cardiac pacemakers, cochlear implants, epidural electrodes, ferromagnetic aneurysm clips. Relative contra-indications include first trimester pregnancy and claustrophobia.
Use of Contrast Media
Gadolinium-DTPA is the most commonly used contrast agent in MRI. Its main use is to increase lesion conspicuity in situations where contrast uptake may be expected as in the case of break down in the blood brain barrier or in neovascularity in tumours. IV Gadolinium can also be used to increase vessel conspicuity in MR Angiography but should be used with caution in patients with renal failure due to its association with Nephrogenic Systemic Fibrosis (NSF) pending further study.
Rebatable versus Non-Rebatable Examinations
At present the Medicare rebatable examinations are mainly for neurological and musculo-skeletal indications and these too have a quota imposed on the number of examinations that can be performed per patient per year. There is no rebate available for most MRI examinations of the chest and abdomen (ie. body MRI). Referrals for MRI examinations have to be requested by medical specialists as defined by the Health Insurance Commission.
Consumer Information
Click on icon below to download consumer information in PDF format.
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Magnetic Resonance Imaging (MRI) Scans and Procedures (May 2009) |
Last reviewed May 2011.
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