2.5 Industrial, scientific & medical
2.5.1
Medical applications driven by RF power: From imaging to cancer treatment,
a flexible and versatile technology in the doctor’s toolbox
RF technology is making its way into all kinds of medical applications, ranging from the
well-known imaging techniques (MRI, EPRI) over low frequency, external heat treatment,
and electro-surgical tools, to minimally invasive endoscopic cancer treatment (RF ablation).
One clear trend is the increasing share of RF-based technologies for ablation. Another is the
trend towards higher RF frequencies (several GHz) and higher powers (> 100 W) in order to
Focus applications,
products & technologies
achieve higher spatial resolution, better control, and shorter treatment times.
RF radiation is not a new technology in medicine. It is currently
used for imaging purposes in MRI (magnetic resonance imaging)
and EPRI (electron paramagnetic resonance imaging), techniques
that employ frequencies from a few megahertz to about 500 MHz.
Other well-known external heat-treatments to rejuvenate skin or
relieve muscle pain make use of frequencies around 480 kHz – not
too demanding in terms of RF. Surgical equipment to cut and
simultaneously coagulate blood vessels runs off RF at about 5 MHz.
The latter application belongs to a class of treatment techniques
that is growing rapidly and uses RF radiation to deposit energy
locally at various parts of the body – in general to “ablate”
(remove) unwanted tissue. Inside the body, the RF energy heats
the surrounding tissue until it is desiccated and/or necrotized. The
damaged tissue will later be re-absorbed by the surrounding, living
tissue. Further application examples for RF ablation include cancer
treatment in the lung, kidney, breast, bone and liver, removal of
varicose veins, treatment of heart arrhythmia, and a growing list of
other applications that benefit from the high control and feedback
possible with RF.
Another advantage of RF in this context is the fact that it can be
applied via small catheters ending in antennas that deploy the RF
signal. Unlike older, direct-current techniques, the tissue is heated
only very locally around the antenna. Neighboring nerves (and the
heart) are not stimulated. This led to the development of a variety
of specialized catheters, used during minimally invasive surgery,
along with ultrasound or X-ray imaging to determine the exact
location of the RF-active part. During the treatment, the impedance
of the surrounding tissue can be monitored and the end-point
determined. With proper catheters, one can even achieve “self
limitation” due to the reduced uptake of RF energy in desiccated
tissue. Likewise, the RF frequency can be used to tune the energy
deposition zone around the catheter: the higher the frequency, the
smaller the penetration depth – and hence the volume to deposit
the RF energy – in the watery tissue.
With the trend towards higher RF frequencies and powers, the
complexity of RF generators and the requirements for the device
technology also increase. Above 10 MHz, say, up to 3.8 GHz, the
technology of choice for power amplifiers is Si LDMOS (laterally
diffused metal oxide semiconductor). This technology has proven
to be powerful, efficient, and rugged in base stations, radar
systems, broadcast transmitters, and other industrial, scientific,
and medical (ISM) applications. LDMOS is available from up to
50 V supply to achieve power levels up to 1,200 W per single
device, with outstanding ruggedness and high gain and efficiency.
To drive and control the LDMOS power amplifier stages, it
takes voltage-controlled oscillators, phase locked loops, and
medium power amplifiers. These parts of the RF signal chain are
conveniently available based on reliable and high-volume SiGe:C
(QUBiC) semiconductor technologies. Going a step further, one
can even use high-speed converters to drive the signal chain
entirely from the digital domain, for full and easy control over the
shape and modulation of the applied RF.
RF implications
These in-situ medical applications and, in general, most of the ISM
applications, usually form highly mismatched RF loads during some
part of the usage cycle. This in turn means that, without protection
or other measures, all of the "injected" RF power reflects back into
the final stage of the amplifier and needs to be dissipated in the
transistor(s), and most likely destroys the device(s) if this situation
lasts too long. LDMOS transistors are designed to be extremely
rugged and generally withstand these mismatch situations without
degrading over time.
This device ruggedness, or the ability to withstand “harsh” RF
conditions in general, be it mismatch or extremely short pulse rise
and fall times, is essential for reliable device performance. RF power
companies have gone to great lengths to achieve best-in-class
device ruggedness. The technologies have been hardened under
the most stringent ruggedness tests during development, which is
particularly true for the 50 V technology. Among other factors, the
base resistance of the parasitic bipolar and the drain extension of
the LDMOS device play key roles in this respect.
This ruggedness, combined with the power density and the high
efficiencies achievable, make LDMOS the preferred technology for
RF power amplifiers up to 3.8 GHz.
NXP Semiconductors RF Manual 16
th
edition
63
PHILIPS [ NXP SEMICONDUCTORS ]
