Biomedical Engineering Reference
In-Depth Information
100 pulses/min followed by 16 stimuli with a paced rate of
125 pulses/min. During this faster pacing, the output voltage
of the stimulus gradually decreased to zero. However, after
removal of the magnet, the output returned to the pro-
grammed value. If an implanted device is exposed to a strong,
external, static magnetic field, unwanted contact with the
magnetic switch poses a considerable danger, especially in
the case of defibrillators, when the therapy would be sup-
pressed. The reed relay is a mechanical part only. Its reliabil-
ity is lower than that of electronic systems. Therefore, a
magnet should be used with the highest caution.
the current mode repeatedly. The cardiac pacing systems
currently approved as MRI compatible have certain limita-
tions, for example they are compatible only with closed types
of MRI devices with an external field of 1.5 T or with changes
in amplitude gradient up to 200 T/m/s. That a pacemaker was
authorized for 1.5T devices means that it must not be used
for devices with either higher or lower external magnetic
fi elds [ 20 ] .
2.8
Device Construction and Materials
Materials used for the construction of implantable devices
must be biologically inert, nontoxic, sterilized, and capable
of long-term immunity against conditions in an organism's
internal environment. All parts of the implantable systems,
including the electronics and leads, must be produced from
biocompatible materials.
Solution of circuits uses custom-made microprocessors or
microcontrollers using complementary metal oxide semi-
conductor technology. For controlling the input and output
or program settings, read-only memory with a capacity of
1-2 kB and a word width of 8-32 bites are used. Long-term
diagnostic data, EGM, or sensor control output is saved to
random-access memory. Growing diagnostic possibilities of
the devices put more demanding requirements on the ran-
dom-access memory capacity.
From a mechanical point of view, the implantable devices
consist of a case and a header. The case is made of titanium
or a titanium alloy and contains all the electronics, battery,
capacitors, and output circuits (Fig.
2.4
). Data dealing with
2.7
Implantable Systems Compatible
with Magnetic Resonance Imaging
Originally, patients with an implanted pacemaker system
were not allowed to be imaged using magnetic resonance
(MRI). Nowadays, some innovations have occurred in this
area [19]. By launching into clinical practice a pacemaker
compatible with MRI together with leads compatible with
MRI, the biggest contraindication of a magnetic resonance
procedure was solved [ 20 ] .
Devices incompatible with MRI might cause interference
in the surgical implant by a static magnetic field, the gradient
of the magnetic field, electromagnetic waves, or a combina-
tion of these phenomena. Potentially, they might cause vibra-
tions; activate tensile and twisting forces; make contact with
the magnetic switch; or cause electromagnetic interferences,
failures of cardiac pacing, changes of programmed parame-
ters, or destruction of electronic circuits.
Changes to the construction and programming of systems
compatible with MRI constrain the possibility of the phenom-
ena stated above. To minimize the power that is induced on a
lead, the capacity at the lead's input to the device has been
changed. A classic magnetic switch that could have made
contact by the impact of a direct current magnetic field was
replaced by a Hall sensor. Because of the influence of attrac-
tion forces caused by a strong direct-current magnetic field,
usage of ferromagnetic parts was constrained considerably.
Additional protection of the internal feeding circuit forestalls
the power induced in the loop, avoiding the disturbance of the
feeding circuits by telemetry. From the perspective of pro-
gramming a special cardiac pacing mode, asynchronous pro-
gramming often is introduced. Furthermore, the collection of
diagnostic data and therapy of atrial arrhythmias is inter-
rupted. The construction and internal arrangement of leads
have been changed (number of conductors, gradient, diame-
ter, etc.) so that the interactions with gradient magnetic field
were eliminated and electrode heating was reduced.
Pacemakers are not automatically compatible with MRI.
First of all, before the imaging, it is necessary to program the
pacemaker to the MRI Safe mode; after the radiological pro-
cedure and during consequent follow-up, it must be reset to
Fig. 2.4
ICD (inside view)
