>FIG. 1D) resulting in an approximate 11 inch swing diameter (FIG. 1E); 3) providing an additional 27° take-off of the tube set and cable to provide more clearance to the patient's upper leg; and 4) providing a distal location, symmetric design, and electric activation of the retract button. Previous versions of the device utilized a lever on the back of the device requiring in excess of 10 lbf to activate frequently resulting in an unstable device during this step. Due to its symmetry, the new design is easily activated with the index finger of either the right or left hand with an approximate force of ½ lbf leaving the device completely stable during its use.

FIGS. 2-5 describe features and functionality of the solenoid needle driver of the vapor delivery device. FIG. 2 is a cross-sectional schematic diagram illustrating the functionality of solenoid needle driver 110. The solenoid needle driver includes the vapor delivery needle 106, a pull winding 116, a push winding 118, a magnet 120, a needle holder 122, a needle tube 124, a flexible tube 126, and a magnetic field sensor 128. In FIG. 2, the solenoid needle driver is shown in its fully advanced position. The vapor delivery needle 106 can be rigidly attached to magnet 120 via needle holder 122. The magnet can then be moved laterally by generating magnetic fields in the push and pull windings 116 and 118, as will be described in more detail below. Magnetic field sensor 128 senses the intensity of the magnetic field produced by the push and pull windings.

FIGS. 3A-3C show an exploded view of additional features of the solenoid needle driver. FIG. 3A shows a solenoid coil holder 130, which holds the push and pull windings of FIG. 2. The magnet 120 of FIG. 3B slides within the solenoid coil holder 130 of FIG. 3A, depending on the magnetic field generated by the coil windings. The needle holder 122 of FIG. 3C attaches the vapor delivery needle to the magnet.

In one embodiment, the magnet can be made from grade N48 Neodymium-Iron-Boron, having residual induction of about Br≈1.4 Tesla. The magnet of FIG. 3 has an inside surface that is shaped to fit over and snap onto the needle holder. Because the magnet is an oriented material having a high coercive force, the entire magnet is uniformly magnetized along its axis. The force on the needle driver is therefore proportional to the volume of the magnet, and the extra magnet material that forms the sides of the inside surface increases the needle driver force. The needle holder can include holes configured to receive adhesive to rigidly attach the vapor delivery needle to the holder.

Referring to FIG. 2, the solenoid includes push and pull windings that are configured in a push/pull configuration relative to the magnet and needle holder. In the fully retracted needle position, the back end of the magnet 120 is aligned with the back end of the push winding 118. The front end of the magnet extends into the pull winding 116 in this fully retracted needle position. To advance the needle into the advanced needle position, current is passed in opposite directions in the push and pull windings, as shown in FIG. 4. The push winding sets up a magnetic field that repels the same polarity magnet out of the winding. Because of the repulsion, the magnet is not in stable equilibrium along the push coil axis, and is prone to lateral movements that could increase contact between the magnet and its surroundings, and increase frictional resistance to axial advancement. The pull winding creates a magnetic field that attracts the magnet into the pull winding. The pull winding attracts the magnet to the axis of the coil, and thereby removes the instability of the push winding. The combination of push and pull windings approximately doubles the force exerted by a single winding. The push/pull pair of windings also makes the retract force identical to the advance force simply by reversing the direction of current to the coil pair, as seen in FIG. 4.

In one embodiment, the push and pull windings are each wound with approximately 400 turns of AWG #30 magnet wire, each coil having a DC resistance of about 10 Ohms. Current can be supplied to the solenoid coils by a 24 volt DC power supply that is activated for about 0.05 sec during advance or retract. Since the push and pull windings are connected electrically in parallel, the resistance of the coil pair in this example is 5 Ohms and the solenoid current is about 24/5=4.8 amps. The ON time is so brief that the winding temperature does not increase significantly during activation. The needle advances/retracts through its full range of about 11 mm in less than 0.020 seconds.

The axial force exerted by the solenoid windings on the magnet may be computed from the expression for the force on a magnetic dipole exposed to a field gradient. The magnetic field from the coils and its gradient may be computed from the law of Biot and Savart. The force on a point dipole may be integrated over the magnet volume to give the net force acting on the needle driver. This calculation is plotted in FIG. 5 for a magnet that is 10 mm inner diameter by 15 mm outer diameter by 20 mm long, and push/pull windings having 408 turns of #30 copper wire. The overall range of movement of the magnet is indicated in the figure. In this range the force is in the 2.5-5 pound range, and peaks at the mid-point of its travel range. The net force on the needle driver is this force minus the frictional forces encountered along its travel. The entire curve in FIG. 5 scales up and down in proportion to the solenoid current. Many forcing scenarios are possible, including varying the force along the needle trajectory, and making the retract force different from the advance force.

The forces on the magnet can be altered by selective placement of magnetic materials along its path of travel. For example, a steel ring or washer may be placed at the distal end of the solenoid needle driver to provide a holding force on the magnet after the solenoid current is turned OFF. The emergency needle release ring shown in FIG. 1A may serve the same purpose when it is made of magnetic steel. The initial force exerted by the solenoid windings during retraction must be strong enough to overcome the holding force of the washer.

The solenoid needle driver of FIGS. 2-5 provides the ability to precisely sense the position of the vapor delivery needle. In one embodiment, the magnetic field sensor 128 can be placed in the proximity of the needle driver magnet that will provide a voltage output that is proportional to the magnetic field produced by the magnet. The magnetic field sensor can be located such that the magnetic field has a monotonic relationship to the magnet position (for example adjacent the proximal end of the magnet when in the retracted position, as seen in FIG. 2). Since the magnetic field of the magnet is large compared to the magnetic field of the solenoid windings and other stray magnetic fields, the position of the magnet, and thus the position of the needle, can be uniquely determined by the magnetic field sensor at any time before, during, and after needle deployment. Delivery of vapor to an under deployed needle can thereby be prevented by a controller of the vapor delivery system. The magnet position versus time during needle deployment may also be monitored by the controller. Complete but slow or jerky movement of the magnet may indicate excessive friction in the system or binding of the magnet or needle.

In another embodiment, an indication of magnet/needle position can be provided by the back EMF that the magnet exerts on the solenoid windings. When the solenoid is energized with a constant current source, the back EMF appears as a dip in the voltage across the solenoid whenever the magnet moves. The current/voltage/back EMF can be measured with a current sensor in the solenoid, or alternatively, in the generator that provides power to the solenoid. When the solenoid is energized with a constant voltage, such as a 24 Volt DC power supply, the back EMF manifests as a dip in the current flowing through the solenoid circuit. FIG. 6 shows a normal solenoid current 132 (including the dip in current) when the needle driver magnet deploys normally, and a failed solenoid current 134 (showing a constant current) when the magnet is prevented from moving. The back current only occurs when the magnet is moving, indicating that the magnet and needle driver are fully deployed in about 10 msec. The current remains ON for one sampling cycle of the computer, or 50 msec.

To isolate the back EMF, the current sensor output can be band pass filtered to eliminate the DC level associated with non-deployment, and eliminate fast transients when the current turns ON and OFF. Amplification of the filter output results in a pulse that may be interpreted as an ON bit when sampled by the controller. If no back current pulse is measured after solenoid power is switched ON, the magnet has not moved, and the operator is alerted. Further analysis of the back current wave form may provide a refined diagnostics of the magnet and needle movement. For example, the duration of the back current indicates how long the magnet was moving, which is indicative of how far the magnet has moved. Noise or jerkiness of the wave form may indicate points of increased friction along its travel.

FIG. 7 shows a detailed view of the vapor generator 112 of the vapor delivery system of FIG. 1C. The vapor generator 112 can include an inner vapor coil 136 and an outer RF coil 138 of conductive wire that surrounds the inner vapor coil. Sterile water can be introduced into the inner vapor coil, and RF current applied by a RF generator 139 to leads 140 of the outer RF coil can inductively heat the inner vapor coil and generates a steam or vapor. The inner vapor coil can be connected to a supply of sterile water through a plastic tube that extends from the inner coil to a fluid source.

In a preferred embodiment, the windings of the vapor coil can be constructed from Inconel, a nickel/chrome stainless steel metal tubing. For example 18 gauge RW Inconel 625 tubing or 18 gauge thin wall (TW) Inconel 625 tubing are preferred. Alternating, RF frequency currents flowing in the outer RF coil of FIG. 7 induce circumferential current flow in the body of the inner vapor coil. It is important to have good electrical contact between the windings of the inner vapor coil.

In this case, the heating element may be modeled as a transformer having an N turn outer RF coil and one turn inner vapor coil. The current flowing in the inner coil may be approximately N times the current flowing in the RF coil. The induced inner coil current produces heat in the inner coil via Ohmic (I²r) heating that converts water flowing in the inner coil to steam. As described above, it is advantageous for the individual coil windings of the inner vapor coil to be in physical contact, which can be achieved by soldering or welding the individual windings together to ensure good electrical contact between windings. One distinct advantage of Inconel 625 coiled tubing is that the oxide layer that forms on its surface is sufficiently thin to freely pass RF current between adjacent windings of the inner vapor coil, with no requirement for soldering.

Another advantage of Inconel is that it is an intrinsically non-magnetic material. While magnetic permeability can enhance the coupling between the RF coil and inner coil, the magnetic properties of stainless steel tubing are not consistent from lot to lot of tubing. Because consistency in calorie output from device to device is very important, non-magnetic tubing is preferred in this application. Stainless steels like 304 may be annealed to eliminate magnetic properties. Another advantage of Inconel 625 tubing is that its electrical properties are almost independent of temperature over the range (room temperature to 350° C.) experienced by inductive heating of the inner coil. Stainless steels such as 304 or 316 not only have a much more substantial change in electrical properties with temperature, but due to the multi-phase make up of these materials, temperature cycling can produce history dependent and unpredictable changes in electrical properties, a small but significant effect.

The calorie output of the vapor delivery device of this disclosure is related to the power input of the RF generator during therapy delivery through an efficiency coefficient. Calorie output will be consistent from shot to shot if the power delivered is a constant, independent of changes in component values due to the thermal cycling of the device. Calorie output will be consistent from device to device if the input power is always the same for a given therapy, and if the efficiency coefficient is consistent from device to device. Device to device consistency is achieved through consistency of device manufacturing. In addition, consistency improves as the power coupling efficiency approaches 100%, provided that the input power is held constant. In other words, variations in device parameters have a diminishing effect on output as the percentage of the constant input power delivered to the output approaches 100%.

In one embodiment, the RF generator is designed to servo the electrical power delivered at its output to a set value by measuring voltage and current at the output, computing the output power, and adjusting output voltage in real time to keep the output power equal to the set power. The percentage of the input power that is delivered to the vapor coil as Joule heating can be computed by analyzing an equivalent circuit of the vapor delivery system, in which current is inductively coupled from the RF coil to the vapor coil through a mutual inductance, M.

The elements can be defined as:

- V=rf generator voltage amplitude in volts, delivered at frequency f in MHz
- I₁=rfcurrent flowing into the delivery device in amps
- I₂=rf current flowing in the vapor delivery coil in amps
- R_cand R₁are the ac resistances of the cable and rf coil respectively in Ohms
- L_cand L₁are the inductances of the cable and rfcoil respectively in pH
- R₂is the circumferential ac resistance of the vapor delivery coil in Ohms
- L₂is the inductance of the vapor delivery coil in pH
- M is the mutual inductance between the rf coil and vapor delivery coil in pH
- where M²=c L₁L₂, 0<c<1
- c is the transformer coupling coefficient between the rf and vapor coils

The electrical power coupling efficiency is defined as the ratio of Ohmic heat generated in the vapor delivery coil to the input power:

η=<I₂²R₂>/<I₁V>, 0<η<1

And the brackets < > represent the average over one cycle of the sine wave input at frequency “f”. The design goal is to make the electrical power coupling efficiency, η, as close as possible to unity (100%). Using standard mathematical analysis, the circuit equations for FIG. 7 may be solved to yield an expression for η in terms of the other circuit parameters:

η=xQ₂/(1+xQ₂+Q₂²)

- where x=2πfcL₁/(R_c+R₁)
  
  Q₂=2πfL₂/R₂

Where x comprises parameters of the input circuit and coupling coefficient, and Q₂comprises parameters solely of the secondary vapor delivery coil. Efficiency is plotted in FIG. 10 as a function of Q₂for fixed values of x. It is seen that efficiency has a peak value when Q₂=1 for all values of x, and that efficiency increases with increasing x. One way of appreciating this fact is that when the electrical resistance if the inner coil is too high, induction of eddy currents will be small with little heat produced. Conversely, when the electrical resistance of the inner coil is too small, the I₂²R₂Ohmic heat will be small. The peak in the curve is expected.

In a practical design of a vapor delivery system, the heating element may comprise nested solenoid coils, where the inner vapor coil is a single turn winding. Formulas for the inductance of solenoid coils and the mutual inductance of nested solenoid coils may be found in the literature, completing the solution for efficiency. These formulas require numerical integration. However, with the nested coils are long and thin, closed form solutions are available. In this approximation, Q₂is:

Q₂=2πfμ₀μ₂Dt/(4ρ) for a single turn solenoid coil

- where μ₀=permeability of free space=4π×10⁻⁷Henries/meter
- μ₂=relative permeability of the vapor coil
- D=vapor coil diameter in meters
- t=smaller of the vapor coil tube wall thickness or the skin depth at frequency f in m.
- ρ=electrical resistivity of the inner coil tubing in Ohm-meters

The combination of the parameters comprising Q₂are chosen, in a preferred embodiment, to force Q₂≈1, thereby optimizing power coupling efficiency. The relative permeability of the inner coil, μ₂, is equal to unity for non-magnetic materials such as Inconel and annealed 300 series stainless steels.

While the operating frequency may be adjusted to set Q₂=1, common medical RF generators have operating frequencies in the 400 to 500 kHz range. Complications such as power being radiated from the heating element occur at higher frequencies, and efficiency starts to droop at lower frequencies. Diameter and tubing wall thickness may be adjusted within the system constraints, and materials having appropriate electrical resistivity may be chosen to bring Q₂as close to unity as possible. In a preferred embodiment, the outside diameter of the inner coil is 9 mm, the wall thickness is 0.2 mm, the material is Inconel 625 having resistivity of 1.32 μOhm-m, and the operating frequency is 440 kHz, yielding a computed Q₂of 1.2, and a measured Q₂close to unity. Numerical evaluation of the inner coil inductance brings the computed and measured values of Q₂to even closer agreement.

Parameters on the input side of the heating element circuit may be chosen to maximize the value of x, within practical constraints. For example, is easy to increase L₁by using many turns of fine wire in a single layer RF coil, or by creating a multiple layer coil of a larger diameter wire. In practice, heating powers of 100 Watts or more may be needed to deliver adequate vapor therapy. AC currents in the range of 5 to 20 amps may be required to deliver adequate power, so that the wire chosen for the RF coil must have a current carrying capacity in this range. Wire that is too small in diameter for a given RF current will show a dramatic increase in resistance and temperature over time during therapy. From the formula for x, an increase in resistance R₁drives the value of x back down.

For example, in one embodiment it was found that a single layer coil made with AWG #22 Litz wire gave higher calorie output than coils of the same length made from either #20 or #24 Litz wire. Multiple layer coils may be practical to a point. The coupling constant, c, falls off as the separation between the RF and vapor coils increases. In an example of long solenoid coils, c is equal to the ratio of the cross sectional areas of the inner and outer coils, so falls off with the square of the diameter of the outer coil for a fixed vapor coil diameter. Another practical limitation is that the impedance of the RF coil circuit increases with increasing RF coil inductance L₁, requiring higher voltages to produce a given current in the RF coil. High voltage has practical and regulatory limitations in medical applications. Cable resistance also has practical limitations, as bulky cables made with large diameter copper wire are not tolerated in clinical practice. Working vapor delivery systems having x>15 and efficiency between 75%-90% have been achieved.

The overall efficiency of the vapor delivery system is less than 100%, even when the electrical power coupling efficiency approaches 100%. This is due to heat lost to conduction, convection and radiation from the heating element and delivery tubing and needle, and to Ohmic heating by currents in the RF coil, which is partially conducted or radiated away from the inner coil. The thermal design of the delivery device minimizes these losses to about 8% of the heat generated in the vapor coil. The overall efficiency of the vapor delivery system is defined by the equation:

Calories output from the needle×(4.186 Joules/calorie)/treatment time=εP_in

- where ε=overall power coupling efficiency
- P_in—constant power input from the Rezūm generator

The calorie output from the vapor delivery needle is readily measured by delivering the vapor to a known quantity of water in a calorimeter. For delivery devices used in BPH therapy the mean measured calorie output was 208 calories. The BPH treatment therapy time for these devices is 9 seconds. Delivery devices constructed in our lab have achieved 208 calories output with as little as 115 Watts input power, for an overall efficiency (computed from the above equations) of ε=84%. The first commercial delivery devices achieved 208 calories with an input power of 132 Watts for an overall efficiency of ε=73%.