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MRI versus CT

MRI and computed tomography (CT) are complementary imaging technologies and each has advantages and limitations for particular applications. CT is more widely used than MRI in OECD countries with a mean of 132 vs 46 exams per 1000 population performed respectively.[68] A concern is the potential for CT to contribute to radiation-induced cancer and in 2007 it was estimated that 0.4% of current cancers in the United States were due to CTs performed in the past, and that in the future this figure may rise to 1.5–2% based on historical rates of CT usage.[69] An Australian study found that one in every 1800 CT scans was associated with an excess cancer.[70] An advantage of MRI is that no ionizing radiation is used and so it is recommended over CT when either approach could yield the same diagnostic information.[71] However, although the cost of MRI has fallen, making it more competitive with CT, there are not many common imaging scenarios in which MRI can simply replace CT, although this substitution has been suggested for the imaging of liver disease.[72] The effect of low doses of radiation on carcinogenesis are also disputed.[73] Although MRI is associated with biological effects these are not thought to cause any measurable harm.[74]

File:PAPVR.gifIodinatedcontrast medium is routinely used in CT and the main adverse events are anaphylactoid reactions and nephrotoxicity.[75] Commonly used MRI contrast agents have a good safety profile but linear non-ionic agents in particular have been implicated in nephrogenic systemic fibrosis in patients with severely impaired renal function. [76]

MRI is contraindicated in the presence of MR-unsafe implants, and although these patients may be imaged with CT, beamhardening artefact from metallic devices, such as pacemakers and implantable cardioverter-defibrillators, may also affect image quality.[77] MRI is a longer investigation than CT and an exam may takebetween 20 – 40 mins depending on complexity.[78]


Safety issues, including the potential for biostimulation device interference, movement of ferromagnetic bodies, and incidental localized heating, have been addressed in the American College of Radiology‘s White Paper on MR Safety, which was originally published in 2002 and expanded in 2004. The ACR White Paper on MR Safety has been rewritten and was released early in 2007 under the new title ACR Guidance Document for Safe MR Practices.
In December 2007, the Medicines and Healthcare Products Regulatory Agency (MHRA), a UK healthcare regulatory body, issued their Safety Guidelines for Magnetic Resonance Imaging Equipment in Clinical Use.
In February 2008, the Joint Commission, a US healthcare accrediting organization, issued a Sentinel Event Alert #38, their highest patient safety advisory, on MRI safety issues.
In July 2008, the United States Veterans Administration, a federal governmental agency serving the healthcare needs of former military personnel, issued a substantial revision to their MRI Design Guide,[79] which includes physical and facility safety considerations.

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