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Investigation of Transcranial Doppler (TCD) Power
Output for Potentiation of Tissue Plasminogen Activator (tPA)
Therapy in
Stroke
MA Moehring,
AH Voie,
MP Spencer, DW
Amory, AV Alexandrov
BACKGROUND:
One hour 2 MHz TCD monitoring of the
residual flow signals at presumed occlusion site during intravenous tPA infusion may lead
to 10-15% improvement in recanalization and earlier recovery in stroke patients1.
Literature reports suggest that ultrasound-enhanced thrombolysis increases with lower
frequencies (10-102 kHz) and acoustic intensities well above 720mW/cm2.
We aimed to investigate power levels and frequencies used for clinical monitoring of tPA
therapy1.
METHODS: The study unit (Multigon 500M) was evaluated with wet tank
testing using low(0-150kHz, B&K 8103) and high(.5-2.5MHz, NTR TNU100A) frequency
hydrophones. The frequency response of the transducer (Spencer Technologies) was measured
using an arbitrary waveform generator (LeCroy 410 Wave Station). All measurements were
done at 100% diagnostic power allowed by the source unit.
RESULTS:
The probe had a stop band in the frequency regime below 150kHz. In
this band, signals were attenuated by >70dB relative to the center of the pass band at
2MHz. Pulse repetition frequency was found to be 10.2kHz, with 11.8mm sample volume(~30
cycles of 2MHz carrier per pulse repetition). Study monitoring was done at depths 44-58mm,
and the derated spatial peak temporal average intensity (ISPTA) at 45mm depth was measured
at 739mW/cm2. The ISPTA measured in the low frequency pass band (< 150kHz)
hydrophone did not exceed 1 nanoWatt/cm2.
CONCLUSIONS:
Primary power output used for tPA monitoring was at diagnostic
power levels and in the vicinity of 2MHz, thereby making unlikely mechanisms of
thrombolysis enhancement such as cavitation or mechanical disruption of clot. Reversible
changes in fibrin structure within clot and microstreaming at the clot/blood flow boundary
are hypothesized as the primary mechanisms of 2MHz potentiation of tPA lysis. A randomized
clinical trial is planned.
1. AV Alexandrov, et al, High rate of complete recanalization and dramatic
clinical recovery during tPA infusion when continuously monitored with 2-MHz transcranial
Doppler monitoring, Stroke, 2000;31:610-614.
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