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Safety Concerns

According to our current knowledge, the health risks of an MRI study are minimal; however, it is important to know the factors that affect the patient. A tiny part of the radio pulses used for exciting the spin system during the examination are absorbed in the patient’s body. Nonetheless, since the RF amplifier emits several kilowatts of power, the absorbed energy cannot be neglected. The energy of the radio frequency radiation is low, it is non-ionizing, and it does not change the chemical composition of matter; it only heats the patient’s body. Nevertheless, the core body temperature is constant, which could be changed by a long sequence that is composed of many radio pulses; therefore, limitations were introduced concerning the energy absorbed by the patient’s tissues called specific absorption rate (SAR, unit of measurement: W/kg). Absorption is dependent on the frequency of the excitation (it is higher at higher frequencies) and on the qualities of the tissues. Biological effects are highly influenced by the circulatory system, which transports heat. Usually the determination of the local RF field is very complex, it often can be carried out by special measurements only. The rapidly changing gradient fields induce a voltage. In case of strong and rapid gradients these fields can be so high that they can stimulate the peripheral nerves. This results in involuntary twitching and pain, thus it is forbidden to expose the patient to this. The gradients are usually limited by the manufacturers to a safe value corresponding to a specific protocol.
The homogeneous magnetic field $B_{0}$ that generates the polarization in the MRI device is very high compared to the fields used in everyday life. In clinical practice coils with a magnetic field of 1.5 T and 3 T are used, while e.g. the magnetic field of a door magnet is typically 15-20 mT. In such high magnetic fields any ferromagnetic object can start moving and fly into the opening of the magnet with the power of a bullet. Such an accident would pose a deadly threat to the patient lying in the magnetic field, and it would also severely damage the device.
Besides the obvious mechanical injuries, the larger ferromagnetic objects close to the magnets pose different threats as well. The high magnetic field in the MRI device is produced by superconducting coils. These coils, which are made of special materials, do not have electrical resistance at low temperatures, thus the currents that are needed to produce the magnetic field can be maintained without feeding in energy. Liquid helium is used as a coolant for the coils. As a result of external disturbances a small part of the superconducting wire can return to its normal state. (It is analogous to the appearance of a vapour bubble in a pot of nearly boiling water.) In matter (in its normal state) the current over 100 A in the coil will start to dissipate, which generates heat. Consequently, the coil rapidly returns to normal state and the energy stored in the magnetic field dissipates, which results in the boiling away of the coolant. As the liquid gas boils away, it expands nearly a thousand-fold. This process is known as quenching. Although it occurs rarely, it is important to ensure that the extremely cold gas can escape safely, should quenching take place. The value of the evaporating coolant is tens of millions of forints, and quenching can easily damage the coils as well.

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