MRI versus CT
Is MRI bad?
Magnetic resonance imaging
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. 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. An Australian study found that one in every 1800 CT scans was associated with an excess cancer. 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. 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. The effect of low doses of radiation on carcinogenesis are also disputed. Although MRI is associated with biological effects, these have not been proven to cause measurable harm. In a comparison of possible genotoxic effects of MRI compared with those of CT scans, Knuuti et al. noted that although previous studies have demonstrated DNA damage associated with MRI, "the long-term biological and clinical significance of DNA double-strand breaks induced by MRI remains unknown". Iodinated contrast medium is routinely used in CT and the main adverse events are anaphylactoid reactions and nephrotoxicity. 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.MRI is contraindicated in the presence of MR-unsafe implants, and although these patients may be imaged with CT, beam hardening artefact from metallic devices, such as pacemakers and implantable cardioverter-defibrillators, may also affect image quality. MRI is a longer investigation than CT and an exam may take between 20 - 40 mins depending on complexity.