Transonic V-Series Probes |
Flow Measurement in Microvessels - Acute Studies |
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V-Series Micro-Probes
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Transonic Systems' V-Series probes for
microcirculation with Transonic flowmeters produce repeatable high resolution volumetric blood flow measurements on vessels as small
as 250 micron diameter, but also in vessels as large as 1000 microns. This designated diameter range overlaps with our 1 mm R-Series flowprobe. Choosing the appropriate
series, size and style probe for your application will enhance the absolute accuracy of the flow measurement.
Acute Protocols: V-series probes are designed to overcome the
difficulties in obtaining stable, small volume measurements in vessels ranging from 250 – 900 microns diameter. Micro-vessels
are positioned in the V-groove of the reflector, the position of highest sensitivity. Stainless steel handles are recommended to
help stabilize the probe with a micromanipulator for lengthy acute procedures. Suitable acoustic couplant (Surgilube) is used to transmit the ultrasound.
Choosing Probe Size: Probe size should be chosen so that the full diameter of the
vessel nestles within the triangle formed by the V-reflector (see diagram below). It is also important that the vessel can be isolated a sufficient length along the
vessel without side-branching to fit the width of the reflector (see probe dimensions and required vessel segment lengths). For acute measurements in
larger vessels (750 microns and larger) R- & S-Series probes may be used.
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V-Series Reflective Path |
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If you look into a 90ş corner consisting of two
mirrors, you see a coherent picture of yourself (but flipped over, so your right ear is on the left side of the image). This is the reflective path of the V-probes; light
scattered off your face first hits one mirror, then the other, before it returns to your eye. In V-probes, these rays intersect the flow four times: the incident beam
intersects the flow; then, on the first reflection off the mirror surfaces, part of the beam traverses the vessel from right to left, another traverses it from left to
right; the flow is sampled a fourth time when these two halves of the beam, are reflected back to the receiving transducer. This 4X sampling of the flow enhances
resolution for micro-sized vessels.
Affect of Vessel Diameter on Flow Measurement Accuracy
With all Transonic probes, flow sensitivity is
optimized for a specific range of vessel diameters. In V-probes, the permissible vessel diameter is small compared to the physical vessel window. Substantial errors result
when the vessel fills the full probe window.
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Sensitivity
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Flow Sensitivity
Profile within V-Probe Window
For proper flow
readout, the vessel must be positioned within the reflector bracket's high-sensitivity triangle. For easy alignment, the preferred position is
in the bottom of the V-groove.
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Accurate Selection of V-Probes |
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Before flow measurements are made, vessel diameter must be measured in
order to select the proper probe size. For instance, it is possible to fit a 1 mm diameter vessel inside a 0.5V probe. However, this would lead to erroneous measurement
results since most of the vessel would be positioned outside the high intensity triangle. Optical pieces (2.5 to 4X magnification) and a calibrated microsurgical scale are
necessary to do this accurately.
Probe size is selected according to the following chart.
- 250 - 500µm diameter vessels --- 0.5V
- 350 - 700µm diameter vessels --- 0.7V
- 500µm - 1 mm diameter vessels --- 1.0V
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V-Series Probes with Flexi-Handles |
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1 mm V-Probe with
Flexihandle
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The new flexi-handle on V-Series probes allow easier
application of the V-probe on a vessel. The stainless steel handle is positioned on the cable 5 mm from the probe head.
This extra cable segment allows the probe tip to be
flexible and compliant on tissues without sacrificing the stability provided by the rigid handle. Probes resist twisting and are easily mounted in a micromanipulator.
V-probes with the new flexi-handle style deter
tugging on the micro-vessel and its consequent vessel spasm and shutdown. Positioned so that the probe opening faces up permits:
- better visualization of the vessel;
- easier application of acoustic gel;
- easier elimination of trapped air bubbles.
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V-Series Probes with Rigid Handles
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The rigid handle style ensures the maximum amount of control when using a
micromanipulator. The stainless steel handle is fixed to the probe head for rigidity and ultra-fine positioning.
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Positioning a V-Probe on a Vessel
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Accurate flow measurement requires the micro-vessel is carefully
positioned in the high sensitivity portion of the probe. The vessel should be "suspended" in the notch of the V-reflector, without causing a constriction of the
vessel. Use of a micro-manipulator for this is helpful.
- Prefill the probe with acoustic couplant to insure that all air
pockets in the probe are removed. (Transonic flowmeters will indicate good received test signal when all air bubbles are filled).
- Mount the probe handle in the micro-manipulator and position the
vessel approximately in the notch of the "V". Add couplant, if necessary.
- Gently "pull" the probe into position using the
micro-manipulator. If the flow reading on the flowmeter starts to decrease during this procedure, the reflector may now be constricting the vessel. Back off slightly
to restore full flow.
- Verify position of vessel with the optical magnification of an eye
piece or microscope.
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