Summary
A new method of positioning detachable coils is presented. It does not rely on markers, but on the decrease of electric resistance that occurs when the platinum-stainless steel junction emerges from the microcatheter tip.
Key words: coils, detachment, aneurysms, technique
Introduction
Detachable coils for brain aneurysms have a complex structure with different shapes, lengths, diameters, softness, detaching mechanisms, circular memory and 2D and 3D configurations. As to the positioning mechanism, all coils rely on markers. "Proximal" markers on both the coil and the microcatheter,were incorporated in the GDC technique with the purpose of increasing the safety of the embolization procedure 1. In fact, after detachment of the first coil, it is difficult to visualize the platinum-stainless steel junction of the second and subsequent coils within the intra-aneurysmal coil mesh. The proximal radiopaque markers (in the parent artery, away from the aneurysm coil mesh) allow precise placement of the junction within the aneurysm even if the actual junction cannot be visualized. It is imperative that the junction emerge from the microcatheter for no more than 1 to 2 mm: In fact, because of its relative stiffness, the stainless steel delivery wire could potentially perforate the aneurysm if advanced too far. A 0.5-cm long "proximal" platinum radiopaque marker is part of the technology of the GDC coil and other types of detachable coils. This marker is located in the stainless steel delivery wire, 3 cm proximal to the platinum-stainless steel junction. Another platinum radiopaque marker is part of the GDC microcatheter, 3 cm proximal to its tip 1. Alignment of the proximal radiopaque markers on the microcatheter and the GDC (in the parent artery) insures that the junction is just 1 to 2 mm beyond the microcatheter tip, no more and no less.
However, even if these markers are extremely useful, they pose some problems. In particular, the elimination of the proximal markers on both the microcatheter and the coil would allow:
1) Easier catheterization of the aneurysm. It is knownthat a microcatheter with a proximal marker is somewhat stiffer than a microcatheter having only the distal marker. In difficult aneurysms, elimination of the proximal marker could be a significant advantage, making endovascular navigation easier especially during negotiation of narrow curves.
2) More accurate positioning of the detachment zone. It is imperative that the detachment zone of the coil be advanced just beyond the tip of the microcatheter, no more and no less. To allow aneurysm catheterization, it is almost always necessary to steam-shape the microcatheter distal end However, this maneuver may change the distance between the markers due to the shrinkage of the plastic material, leading to improper positioning of the detachment zone of the coil.
3) Softer delivery wire. The proximal marker on the delivery wire of the coil makes the wire somewhat stiffer and this may increase the likelihood of moving the microcatheter tip (or even forcing the microcatheter tip out of the aneurysm), while pushing a coil. A softer delivery wire could be particularly advantageous in small aneurysms.
4) Easier and simpler construction of coils and microcatheters, with a possible decrease in price.
Obviously, the solution to these problems would be to eliminate the markers (and their inherent drawbacks).
Material and Methods
The concept 2 of electrically determining the positioning of the coil was tested in the following in vitro research.
An alternate, sine wave, electric current generator (Hewlett Packard 33120A) was connected to the proximal end of a GDC delivery wire. The other (ground) pole of the generator was connected to the ground pole of a digital multi-meter (Tektronix DMM252). The other pole of the multimeter was connected to a stainless steel bowl, filled with 150 ml of saline (figure 1). GDC coils were utilized for these five experiments. The GDC was slowly advanced into the saline-filled bowl via a single-marker Tracker microcatheter, under continuous infusion of saline (figure 1) under digital microscope (Scalar UM02-SUZ-01) observation (figure 2).
Figure 1.
Picture showing the experimental setting utilized to determine the electronic positioning of a GDC coil. A saline filled metallic bowl is connected (arrowhead) to one of the two poles of an alternated current generator. A GDC coil (arrow) is delivered into the bowl. The proximal end of the GDC delivery wire is connected to the other pole of the AC generator (not shown).
Figure 2.
Ten times magnification view of the tip of a Tracker microcatheter and of a GDC coil. Both are in saline. A) The platinum-stainless steel junction (arrow) is inside the microcatheter. B) The junction is more distal and is now covered by the microcatheter marker. C) The junction (arrow) is now just distal to the microcatheter tip. This has an effect on the electrical resistance of the system (see figure 5). Detachment of the coil can now be performed by applying a direct current (like the standard GDC detachment). D) The junction (arrow) is now 1 mm beyond the microcatheter tip. E) The junction is several mm beyond the microcatheter tip. This is an unsafe position, and the delivery wire might perforate the aneurysm.
After several tests with various frequencies and voltages, it was established that 90 KHz was the best frequency and 300 mV the best voltage.
Results
As soon as the detachment zone emerged from the microcatheter tip (figure 2C) the electric current jumped from 0.011 mA to 1.122 mA (more than a 100-fold increment, figure 3): according to Ohm's law 3, when the electric resistance decreases, the current must increase (if voltage is constant). When the platinum-stainless steel junction is inside the microcatheter, it is "covered" by the plastic material of the microcatheter that acts as an electric insulator (high electric resistance of the system). As soon as the junction emerges from the microcatheter tip, it is no longer insulated from the external environment and the electric resistance of the system decreases. This leads to an increase in the electric current. This is shown in figure. 4 and 5.
Figure 3.
Photograph of the multimeter utilized to perform the experiments. The multimeter is set to detect the value of the alternated electric current of the system (see also text). If the junction is inside the microcatheter (positions A and B of figure 2), the multimeter (top) signals 0.011 milliamperes. As soon as the junction emerges from the microcatheter tip (positions C,D, and E of figure 2), the multimeter (bottom) signals 1.122 milliamperes (an increment of more than 100 times).
Figure 4.
Schematic drawing showing the setting of the experiments. The long arrow points to the saline filled metallic bowl. The short arrow points to the platinum-stainless steel junction. Single arrowhead points to the alternate current generator. Double arrowheads point to the multimeter. In this drawing the junction is still outside the bowl. Due to the fact that the platinum coil is electrically insulated from the delivery wire, almost no current flows through the system.
Figure 5.
Same as figure 5. In this drawing, however, the junction (arrow) is inside the saline filled bowl. Due to the fact that the junction is electrically uninsulated from the delivery wire, electric current now flows through the system (see also text).
Discussion
The in vitro research demonstrated the basic theoretical principle. Animal testing will need to be performed to confirm the basic principle in vivo. Animal testing will also be necessary to assess the safety of delivering the alternate current in the body
In the clinical setting the metallic bowl is equivalent to the needle (ground) electrode in the patient's body. The generator and the electric current detection systems can be miniaturized. For instance, they could be contained in a small plastic box that has a hole where the proximal end of the delivery wire is grasped. A minibuzzer (in the box) will then signal that the detachment zone is out of the microcatheter. The box can be powered by a small, rechargeable battery and can have a LED signaling low battery. Pushing two additional holes in a battery charger can recharge the battery.
Alternate electric current was utilized in this research: direct electric current cannot be utilized in that it would detach the coil.
Prerequisite for the correct functioning of the system is that the platinum coil has to be electrically insulated from the delivery wire (like the currently available GDCs). However, given the fact that platinum has a very high intrinsic electric resistance (compared to stainless steel), the AC positioning system could still work even if the platinum portion is not insulated from the delivery wire. This was tested in a previous, unpublished, research.
The schematic diagram of a simple detection circuitry is shown in figure 6. It acoustically signals to the operator when the platinum-stainless steel junction emerges from the microcatheter tip. It implies the use of the well-known electric balance for measuring the electrical resistance of a conductor, which goes under the name of Wheatstone's bridge 4. The body of the patient constitutes one of the four arms of the Wheatstone bridge (figure 6).
Figure 6.
Electric circuit of a detection system that signals when the GDC junction emerges from the microcatheter tip. Arrow points to an alternate current generator. The entire circuitry is based on the so-called "Wheatstone bridge" (see also text).
As pointed out in the material and methods section, when the junction is inside the microcatheter, a low current (0.011 mA) still flows through the system. This constitutes a fundamental safety mechanism. If the detection system does not "read" this small current it means that there is a bad connection with the delivery wire and that it is not safe to continue advancing the delivery wire (this could be signaled by a blinking red light).
References
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