During scanning patients are also exposed to the activation of magnetic field ramps or gradients for spatial localization of the signal and to RF impulses for signal generation and decoding. Specific effects that may constitute a source of patient risk during scanning have been documented in their connection.
Magnetic Field Gradients
Many of the effects ascribed to magnetic fields that vary over time are in fact related to the associated electric field. Switching the magnetic field gradients on and off may induce electric currents capable of affecting the cell membrane potential and, if sufficiently intense, of stimulating the peripheral nervous system and the cardiac muscle. The stimulation threshold of peripheral nerves may be painful but is reversible and is used as a safety reference indicator for cardiac stimulation, which in contrast carries the risk of fibrillation. In fact, with ramp durations of less than 1 ms, the former threshold is always lower than, and is reached before, the latter .
The intensity of these currents is proportional to the induced electric field and depends on the conductivity of biological tissues. The other critical factor of gradients, besides their amplitude, is the slew rate; the value of the induced electric field can be calculated using a model represented by a cylindrical object with radius r as follows:
For instance, if during an examination a localization ramp from 0 to 20 mT/m in 200 |is is launched from a 1 m coil, then the temporal variation of the magnetic fieldwillbe 100 T/s, andfor an abdomen 0.4min diameter the value of the electric field calculated using the above model will be 10 V/m, which exceeds the 6 V/m threshold of peripheral nerve stimulation [4, 13].
An ability to excite 20 mT/m gradients is shared by several last-generation MR units with static fields of 1.0 or 1.5 T; 30 mT/m gradients with ramps shorter than 100 |is have also been developed to improve acquisition time and spatial resolution.
At high magnetic fields, spatial resolution can be improved by strengthening the gradients; this results in an increased induced electric field that may easily exceed the safety threshold. To avoid this effect, the electric field increase could be offset by increasing gradient duration; however, an increase for instance from 0 to 40 mT/m in 400 |is would result in a value of ^ of 100 T/s, while based on the model mentioned above the electric field would still be 10 V/m. In addition, increasing gradient duration leads too close to the values where the thresholds of peripheral and cardiac stimulation converge, preventing peripheral nerve stimulation from being used as a safety indicator.
Radiofrequency Electromagnetic Fields
The RF impulses applied in MR imaging are always accompanied by an RF electric field, which in turn induces RF electric currents in patients.
Due to the Joule effect, these currents may induce a potentially adverse heating of tissues, or burns caused by conducting loops accidentally generated by the contact between the patient's limbs or by leads of auxiliary devices inadvertently left on the patient's skin [14,15].
At 3.0 T, the resonance frequency of protons is 128 MHz, i.e. twice the value at 1.5 T, the spatial distribution of the RF field generated by the coils becomes more complex with increasing frequency, and the temperature rise may become localized, generating hot spots.
At very high frequencies, the wavelength of the field is comparable to the size of the anatomical structure being scanned, a situation that may lead to generation of stationary waves that impair RF field homogeneity. In addition, as the frequency increases so do tissue conductivity and consequently the density of the induced current, resulting in greater power density being deposited in tissues.
Greater conductivity also involves impaired RF signal penetration, requiring greater power to obtain the same signal.
Prevention of accidental tissue heating requires careful evaluation of the energy deposited per unit of mass in unit of time, or specific absorption rate (SAR), which is measured in W/kg of exposed tissue. Between 1.5 and 3.0 T, this rate increases with B2, making the safety threshold more likely to be reached in the course of the same sequence performed with a 3.0 T than with a 1.5 T unit .
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