Biomedical Engineering Reference
In-Depth Information
In a silicon semiconductor, ionizing radiation excites
electron into what is referred to as a conduction band. If the
semiconductor is in an electrical field, the conductibility
increases. Interactions of neutrons with silicon nuclei are
significant. Upon collision of a fast neutron with a silicon atom,
the atom is forced from its lattice position, and the silicon
atoms thus make imperfections in the crystal lattice [91, 92].
For patients with pacing or defibrillation systems, radiol-
ogy diagnostic or therapeutic devices in medical centers are
usually the most common sources of ionizing radiation. The
ionizing radiation effects are influenced by several factors,
including, among others, the distance between a radiation
beam and the device, the beam type and energy, the dose
rate, the total dose received over the period of the device
longevity, and the device shielding. The effects of ionizing
radiation may also differ depending on individual types
of devices and manufacturers. During ionizing radiation
treatment (linear accelerators, brachytherapy, and betatrons),
it is recommended that the device be shielded by a pro-
tective shield no matter its distance from the radiation
beam. The center of the beam should not point directly at the
device. After therapeutic irradiation, the function of the
device, including sensing, pacing thresholds, and capacitor
re-formation, must be checked. The majority of diagnostic
display methods, for example, X-ray skiagraphy and
fluoroscopy or computed tomography (CT), are not consid-
ered as dangerous. If a patient is examined using CT and the
device is not directly in the beam of the CT image, it is not
affected. If it is placed directly in the main CT pencil beam,
excessive sensing may occur throughout the period of expo-
sure to the beam.
If an implanted device is exposed to intensive ionizing
radiation or other extreme conditions, the device may be
reset or switched into a safety mode, which devices are
equipped with today. As a consequence, the device is
switched into an operation mode with basic parameters that
are considered safe for the majority of patients [93].
Because of stochastic effects, it is impossible to determine
a safe radiation dose or guarantee the proper function of a
device upon exposure to ionizing radiation. Recommendations
specify a maximum recommended safe dose of 2 Gy.
Experiments dealing with monitoring the in vitro operation
of implantable systems irradiated by ionizing radiation from
a linear accelerator proved the operation, even under consid-
erably higher repeated doses; however, the patient's safety
must always be the fi rst concern.
