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3 RESULTS
Before the testing of the automatic deactivation
circuitry we tuned and matched the coils of the
probe in the Larmor frequency (63.87 MHz) by
applying an average human pelvic region load on the
frames and adjusting the values of tuning
components. Using the network analyzer
(HP8719D) to measure the reflection coefficient we
managed to drop the S
22
parameter at -55 dB,
keeping the resonance frequency range below 250
KHz, which led to very accurate tuning. The
decoupling between the adjacent elements was also
successful, since the transmission coefficient S
12
drops below -30 dB.
The functionality of the analog switch was tested
in the laboratory using a Signal Generator (HP ESG-
4000A) and a Combiscope (FLUKE PM3380B). RF
pulses of the same power and length with those
transmitted from the MRI system were created in the
Signal Generator and were sent to the probe as input.
The RF pulses varied in length from 2-5 ms and in
power from 5-20 dBm.
The first set of measurements examined the log
detector’s functionality. Specifically, we measured
the DC signal produced from the RF pulse
rectification (Fig. 3B). The resulting DC pulse is
initiated and terminated almost immediately after the
beginning and the end of the signal generator’s RF
trigger respectively. Also, the correspondent DC
pulse’s amplitude is equal to the RF pulse’s
amplitude as expected, allowing accurate
comparison with the DC threshold.
The second set of measurements showed the
output of the probe and verified the turning off and
on of the analog switch during the activation and the
deactivation phase respectively (Fig. 3C). A
potential disadvantage of the method is that there is
an undesired latency of 10 μs in the switching
process that is capable of producing artifacts in the
imaging data (Fig. 3E). The latency is caused mainly
from the processing delays of the microcontroller
that triggers the analog switch and remains constant
without regard to the RF pulse length and power that
is triggered.
4 DISCUSSION
Certain improvements could be applied on the
automatic switching mechanism of the circuitry in
order to overcome the presence of latency in the
function of the analog switch. A way to reduce the
latency is to control the switch directly from the DC
signal that derives from the comparator’s output,
bypassing the time-consuming processing of the
microcontroller.
Also, a practical problem could potentially
appear in the clinical application of the automatic
deactivation circuitry. The probe detunes itself
automatically during the RF pulse transmission and
does not require a decoupling signal from the
scanner. However, many MRI scanners’ protocols
run primary tests on the connected probes by
sending pulse signals in the opposite direction for
software initialization. In that case, the switch would
cause compatibility issues and the probe would not
be recognized by the MR system.
Our prototype automatic deactivation design is a
robust and simplified mechanism that can be applied
on self-tunable MR coils. It was tested in various
conditions and found to be fully functional and able
to switch off or on the probe at all times.
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