for superconducting mri equipment should be equipped with pressure relief mechanisms and an exhaust fan, in addition to the required quench pipe. because a quench results in rapid loss of cryogens from the magnet, recommissioning the magnet is expensive and time-consuming. spontaneous quenches are uncommon, but a quench also may be triggered by an equipment malfunction, an improper cryogen fill technique, contaminants inside the cryostat, or extreme magnetic or vibrational disturbances. pregnancy main article: magnetic resonance imaging in pregnancy no effects of mri on the fetus have been demonstrated. as opposed to many other forms of medical imaging in pregnancy, mri avoids the use of ionizing radiation, to which the fetus is particularly sensitive. as a precaution, however, many guidelines recommend pregnant women only undergo mri when essential, especially during the first trimester. the concerns in pregnancy are the same as for mri in general, but the fetus may be more sensitive to the effects—particularly to heating and to noise. the use of gadolinium-based contrast media in pregnancy is an off-label indication and may be administered only in the lowest dose required to provide essential diagnostic information. despite these concerns, mri is rapidly growing in importance as a way of diagnosing and monitoring congenital defects of the fetus because it is able to provide more diagnostic information than ultrasound and it lacks the ionizing radiation of ct. mri without contrast agents is the imaging mode of choice for pre-surgical, in-utero diagnosis and evaluation of fetal tumors, primarily teratomas, facilitating open fetal surgery, other fetal interventions, and planning for procedures (such as the exit procedure) to safely deliver and treat babies whose defects would otherwise be fatal. claustrophobia and discomfort although painless, mri scans may be unpleasant for those who are claustrophobic or otherwise uncomfortable with the imaging device surrounding them. older closed bore mri systems have a fairly long tube or tunnel. the part of the body being imaged must lie at the center of the magnet, which is at the absolute center of the tunnel. because scan times on these older scanners may be long (occasionally up to 40 minutes for the entire procedure), people with even mild claustrophobia are sometimes unable to tolerate an mri scan without management. some modern scanners have larger bores (up to 70 cm) and scan times are shorter. a 1.5 t wide short bore scanner increases the examination success rate in patients with claustrophobia and substantially reduces the need for anesthesia-assisted mri examinations even when claustrophobia is severe. alternative scanner designs, such as open or upright systems, may be helpful where these are available. although open scanners have increased in popularity, they produce inferior scan quality because they operate at lower magnetic fields than closed scanners. commercial 1.5-tesla open systems have become available recently, however, providing much better image quality than previous lower field strength open models. mirror glasses may be used to help create the illusion of openness. the mirrors are angled at 45 degrees, allowing the patient to look down their body and out the end of the imaging area. the appearance is of an open tube pointing upward (as seen when lying in the imaging area). even though one is able to see around the glasses and the proximity of the device is very evident, this illusion is quite persuasive and relieves the claustrophobic feeling. for young children who cannot hold still or would be frightened during the examination, chemical sedation or general anesthesia are the norm. some hospitals encourage children to pretend the mri machine is a spaceship or other adventure. certain hospitals with children's wards have decorated scanners for this purpose, such as that at the boston children's hospital, which operates a scanner with a special casing designed to resemble a sandcastle. obese patients and pregnant women may find the mri machine a tight fit. pregnant women in the third trimester also may have difficulty lying on their backs for an hour or more without moving. 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. 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. 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, however, this substitution has been suggested for the imaging of liver disease. the effect of low doses of radiation on carcinogenesis also are disputed. although mri is associated with biological effects, these have not been proven to cause measurable harm. 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, also may affect image quality. mri is a longer investigation than ct and an exam may take between 20 and 40 minutes depending on complexity. guidance 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 originally was 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 u.s. 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, that includes physical and facility safety considerations. the european directive on electromagnetic fields this directive (2013/35/eu – electromagnetic fields) covers all known direct biophysical effects and indirect effects caused by electromagnetic fields within the eu and repealed the 2004/40/ec directive. the deadline for implementation of the new directive was 1 july 2016. article 10 of the directive sets out the scope of the derogation for mri, stating that the exposure limits may be exceeded during "the installation, testing, use, development, maintenance of or research related to magnetic resonance imaging (mri) equipment for patients in the health sector, provided that certain conditions are met." uncertainties remain regarding the scope and conditions of this derogation.