Patent Application
20210252213
The following relates generally to the medical device arts, infusion pump arts, magnetic resonance imaging (MRI) arts, electric motor arts, and related arts.
Magnetic resonance imaging (MRI) is a powerful medical diagnostic and clinical assessment technique. However, MRI generates strong magnetic fields and radio frequency (RF) interference, and in turn MRI images are susceptible to degradation due to RF interference from nearby magnetic fields and/or RF emitting devices. In view of this, medical MRI systems are generally enclosed in an RF shielded room (sometimes referred to as the MRI room), that is, a room in which the walls (and possibly floor and/or ceiling) include a wire mesh sheeting or the like forming an enclosing Faraday cage. Patients undergoing an MRI examination procedure are evaluated pre-procedure to ensure they do not have excessive implanted ferromagnetic material—for example, any implanted cardiac pacemaker is required to be MRI compliant or MRI safe. Laboratory safety protocols prohibit items containing ferromagnetic materials. In general, it is prohibited to introduce or use ferromagnetic materials in the MRI room because the MRI field may cause large attraction forces, leading to dangerous situations, and because the ferromagnetic material may distort the MRI system's imaging.
This situation creates difficulties for using motorized devices such as infusion pumps, fans, motorized patient tables, or the like in an MRI room. An electric motor is an electromagnetic device, and employs interaction between electric and magnetic fields to convert input electrical power into motive (mechanical) force output, usually in the form of a rotating shaft whose rotation is driven by the motor. In such motors, windings are wrapped around a ferromagnetic core to form an electromagnet producing the magnetic field when the coil is electrically energized. These are arranged as stator windings which are mounted in a stationary fashion, and rotor windings mounted on a rotating element (rotor). Interaction between the stator and rotor magnetic fields produces the motive force. Alternatively, one of these magnetic fields may be provided by a permanent magnet comprising magnetized ferromagnetic material. In an induction motor, only one set of windings (usually the stator windings) is electrically energized using an input alternating current (a.c. current), and the resulting time-varying magnetic field induces a.c. current in the rotor windings thereby providing the interacting magnetic field generating the motive force on the rotor. An induction motor thus operates in a fashion akin to a transformer, except that the output is rotation of the secondary electromagnet in an induction motor, rather than the electrical current induced in the secondary electromagnet. In a variant inductor motor design, the rotor windings are replaced by short-circuited electrically conductive bars—this is referred to as a squirrel cage rotor.
Such motors are problematic when used in an MRI room. The ferromagnetic material presents a physical hazard if it is drawn into the MRI bore by the intense magnetic field generated by the MRI device. Furthermore, both the ferromagnetic material and the generated magnetic fields can interfere with operation of the MRI device, thereby leading to degraded clinical MRI images and potential for medical misdiagnosis.
Various approaches are employed to address the difficulty of using an electric motor in an MRI room. These approaches generally require employing a specially designed motor that is MRI compatible. For example, an electrostatic motor operating on the basis of attraction and repulsion of electric charge can be employed. However, electrostatic motors are a non-standard motor design, and generally require high operating voltages and provide low efficiency, and are more typically used for miniaturized devices, e.g. micro-electro-mechanical systems (MEMS). The high voltages can also introduce electrostatic discharges with concomitant RF noise. Piezoelectric motors have similar difficulties. Another approach is to locate the electric motor outside the MRI room and run the rotating shaft through the wall into the MRI room. This approach requires a long rotating shaft, complicates operation as the motor is located outside of the MRI room, and the shaft penetration compromises integrity of the RF shielding of the MRI room. In the case of dedicated devices that are used only in the MRI room, specialized motor designs are known that make use of the magnetic field generated by the MRI device itself in the motor operation. See, e.g. Roeck et al., U.S. Pub. No. 2010/0264918 A1. Such a motor is only usable inside the MRI room due to its reliance on the magnetic field generated by the MRI device. This means the infusion pump cannot go with the patient to and from the MRI room, which presents substantial practical difficulties.
While operation in the MRI room, or in proximity to an MRI device, is an illustrative problem, there are other situations in which an electric motor can be problematic due to potential for detrimental magnetic interactions. For example, in positron emission tomography (PET) imaging, photomultiplier tube (PMT)-based radiation detectors are susceptible to magnetic interference. Electric motors in proximity to sensitive magnetometer devices such as superconducting quantum interference device (SQUID) devices can lead to erroneous magnetic field measurements. These are merely illustrative examples.
The following discloses a new and improved systems and methods.
In one disclosed aspect, an electric motor includes a stator comprising electrical windings, and a rotor magnetically coupled with the stator. The electric motor does not include ferromagnetic material, and the electric motor does not include any permanent magnet. The rotor optionally includes an outer rotor cylinder surrounding the stator. The rotor optionally further includes an inner rotor cylinder disposed inside the stator and connected to rotate with the outer rotor cylinder. The outer rotor cylinder may comprise a cylindrical sheet rotor. The electrical windings of the stator are, in illustrative embodiments, wound to form the stator as a three-phase stator. The electric motor may further comprise a fixed frequency motor driver operative to electrically power the stator at a fixed electrical frequency.
In another disclosed aspect, an infusion pump comprises an electric motor as set forth in the immediately preceding paragraph, along with a fluid delivery component comprising one of (i) a syringe receptacle and (ii) a fluid pump having an inlet configured to connect with an infusion fluid supply and an outlet configured to connect with a patient infusion delivery accessory. The electric motor is connected to operate the fluid delivery component by driving a plunger of an associated syringe mounted in the syringe receptacle or by operating the fluid pump.
In another disclosed aspect, a method of operating a medical device is disclosed. The method comprises operatively connecting the medical device to a patient, and operating an electric motor to apply motive force to the medical device to deliver a therapy to the patient. The electric motor does not include ferromagnetic material and does not include a permanent magnet.
One advantage resides in providing an electric motor with no ferromagnetic material.
Another advantage resides in providing an electric motor with no ferromagnetic material and no permanent magnet.
Another advantage resides in providing an electric motor which is compatible with an MRI device and with use inside an MRI room.
Another advantage resides in providing an electric motor with one or more of the foregoing benefits which retains a conventional induction motor design.
Another advantage resides in providing an electric motor with one or more of the foregoing benefits which retains a conventional induction motor design with a reduced number of component and/or reduced manufacturing cost.
Another advantage resides in providing an electric motor with one or more of the foregoing benefits which further provides intrinsic RF shielding.
Another advantage resides in providing an electric motor with one or more of the foregoing benefits which is further operable using a fixed frequency motor driver operative to electrically power the stator at a fixed electrical frequency to operate the electric motor.
Another advantage resides in providing an MRI-compatible infusion pump employing an electric motor with one or more of the foregoing benefits.
Another advantage resides in providing an electric motor or an MRI-compatible infusion pump employing such an electric motor, which is MRI-compatible but also usable outside of the MRI room and not in proximity to the MRI device.
A given embodiment may provide none, one, two, more, or all of the foregoing advantages, and/or may provide other advantages as will become apparent to one of ordinary skill in the art upon reading and understanding the present disclosure.
The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.
With reference to
The patient may require medical assistance or therapy during the MRI imaging procedure. For example, an infusion pump 10 may be employed to deliver an infusion fluid to the patient, e.g. a saline solution, an infused medication, or so forth. The illustrative infusion pump 10 is a syringe infusion pump including a syringe receptacle 12 into which a syringe 14 is inserted. (It is also noted that
The electric motor 20 includes a rotor/stator assembly 24 that drives a rotatable shaft 26 that is coupled with the drive arm 22 of the syringe infusion pump 10 (again, using gearing, clutches, or so forth, not shown; or, more generally, the shaft 26 is operatively mechanically coupled with a component of a medical device or the like that requires operative motive force). The rotor/stator assembly 24 includes a stator comprising electrical windings and a rotor magnetically coupled with the stator to define the electric motor 20. The illustrative motor has a stator that is not electrically driven, and is classified as an induction motor. As disclosed herein, the electric motor 20 does not include ferromagnetic material, and does not include any permanent magnet. The electric motor 20 further includes, or is operatively connected with (e.g. via suitable electrical wires or cable) a motor driver 28 that is operative to electrically power the stator at a fixed electrical frequency.
The syringe infusion pump is disposed inside the MRI room 2, and is shown as an illustrative example of a motorized device that may be usefully used inside the MRI room 2 using an MRI-compatible electric motor 20 as disclosed herein. In other embodiments, the infusion pump may be of a non-syringe variety, in which the fluid delivery component (instead of being the syringe receptacle 12) includes a fluid pump having an inlet configured to connect with an infusion fluid supply (e.g. hanging from an IV stand) and an outlet configured to connect with the patient infusion delivery accessory 16. As another example, a motorized fan may be usefully deployed inside the MRI room 2. Moreover, as previously mentioned an embodiment of an electric motor 20 as disclosed herein may be employed in substantially any other type of motorized device that is used in a setting in which magnetic field interactions may be detrimental to operation of proximate equipment such as a PET imaging device, a SQUID or other magnetometer, or so forth.
The electric motor 20 does not contain any ferromagnetic parts, so it will not be attracted by the magnetic field generated by the MRI device 4. As another advantage, the electric motor 20 does not contain any ferromagnetic parts which might distort the MRI's imaging field. The electric motor 20 generates weak stray fields, which can be designed to be small enough as not to interfere with the MRI's imaging field. Optionally, any remaining stray fields can be shielded using e.g. electrically conductive sheet cover.
The electric motor 20 is an induction motor. (However, a different type of electric motor is alternatively contemplated, e.g. as illustrated in
The coil currents and induced currents produced during operation of the electric motor 20 will generate magnetic fields having the potential to disturb the imaging function of the MRI device 4. However, it is further recognized herein that at normal current levels and realistic distances of the motor from the MRI device (e.g., on the order of a half meter or larger) the fields and field gradients will be low, e.g. fields at or more likely below the milliTesla (mT) range, and gradients at or more likely below the mT/m range. In some illustrative embodiments, an outer sheet rotor is employed, which provides intrinsic shielding and consequent additional reduction of the fields that propagate outside the electric motor 20. Optionally, an additional shielding layer may be applied to further shield the stray fields.
The working principle of an induction motor is that an alternating current through a number of stator coils (typically 3-phase, but other coil distributions exist and are contemplated for the electric motor 20) creates a rotating magnetic field. This rotating magnetic field creates induced currents in the rotor, which in turn create a magnetic field that interacts with the stator field to provide motive force (e.g. torque) causing rotation of the rotor and of the shaft 26 connected to rotate with the rotor. The parts creating the motive force are the electrically conductive parts (coils and rotor). In a conventional induction motor, ferromagnetic material is added to increase the efficiency. However, as disclosed herein, the electric motor 20 does not include ferromagnetic material. With the ferromagnetic material omitted, the electric motor 20 still functions in the same way as a conventional induction motor, although at a significantly lower efficiency.
When the electric motor 20 is operated in a magnetic field environment such as that generated by the operating MRI device 4, there will be several disturbing forces. The external magnetic fields will interact with the currents in the motor coils, creating Lorentz forces. Because the coils of the stator are mechanically connected to a stationary support, this will not cause problems so long as the stator support is sufficient. The external field will also create eddy currents in the electrically conductive material of the rotor, which creates a damping torque proportional to the square of the field and also proportional to the square of the rotation frequency. To counter this effect, a large number of motor coils can be used. This reduces the damping torque because the electrical working frequency is much larger than the rotation frequency of the rotor. Conversely, the motor coils will create magnetic fields which could potentially distort the MRI field. However, because there are multiple coils, their resulting field will decrease very rapidly with distance. Further measures, such as the use of an external sheet rotor as in some embodiments disclosed herein, and/or the use of extra motor shielding, can ensure that the motor's external stray field will stay below the allowed (design-basis) disturbance field.
The induction motor 20 does not include ferromagnetic material. The induction motor 20 (and more particularly the rotor/stator assembly 24) includes a rotor, which may for example comprise a thin-walled electrically conductive cylinder (although a cage-shaped rotor such as a squirrel cage rotor is also contemplated), and a stator comprising a set of coils, e.g. a multiple of three when employing 3-phase input electrical power) arranged at a small distance around or inside the rotor. It is contemplated to exchange the rotating and stationary parts (so that the cylinder is stationary and the coils rotate around or inside it), but this is generally not preferred because this will complicate the electrical connection of the coils.
With reference now to
With reference to
With reference to
With reference to
The embodiments of
The inner rotor cylinder 40 is, in some embodiments, a cylindrical sheet rotor, that is thin sheet of metal shaped to from the cylinder of the rotor. Likewise, the outer rotor cylinder 50 is, in some embodiments, a cylindrical sheet rotor. This design enhances the shielding provided, especially in the case of an outer cylindrical sheet rotor 50. In other embodiments, the inner and/or outer rotor cylinder 40, 50 may be dielectric cylinder(s), e.g. printed circuit boards (PCBs) with a conductive loop pattern printed or otherwise formed on or in the dielectric cylinder(s). In yet other embodiments, the inner and/or outer rotor cylinder 40, 50 may be squirrel cage rotor(s).
The embodiments of
As previously mentioned, it is generally considered necessary in the art to include ferromagnetic material in an induction motor in order to provide sufficient magnetic flux to enable the achievement of high torque. However, it is recognized herein that the disclosed induction motor 20 with no ferromagnetic material can provide sufficient torque for many applications, such as driving an infusion pump, mechanical fan, or so forth.
With reference to
With returning reference to
The illustrative embodiments of
The magnetic field of the MRI device 4 may induce currents in the conducting parts of the rotor when it is moving, resulting in a damping torque. More particularly, a voltage is induced according to Lenz' law, which results in a current when there is an electrically conductive path. The electrical power dissipated by this current has to be delivered and is added to the mechanical input power of the rotor. Because the mechanical power is expressed as the product of torque and rotation speed, this additional power is observed as a torque proportionally to the rotation speed, so it appears as a pure damping. The magnitude of the induced currents depends on several factors: (i) the magnitude of the magnetic field component that is radially aligned with the rotor; (ii) the rotation speed of the rotor; and (iii) the electrical resistance of the conductive path. Magnetic field components that are axially aligned with the rotor axis will have negligible effect. Therefore, if the rotor is oriented such that the rotor axis is not aligned with the local MRI (stray) field, additional damping will occur. Under unfavorable conditions (high B field, high rotation speed), this additional damping torque may significantly limit the performance of the motor.
To prevent this, the illustrative rotor 60 is not shaped as a closed sheet (that is, not a sheet rotor) but rather comprises one or more conducting loops 62A, 62B, 62C. These loops are shaped such that the induced voltage in one half of the loop (indicated as half-loop HL1) cancels the effect of the induced voltage in the other half-loop HL2. In the illustrative example, this is achieved by the conducting loops 62A, 62B, 62C having a pattern resembling a figure-eight. (At the crossing points the conductors should be isolated from each other, e.g. by using different PCB layers with interposed electrically insulating dielectric layers). Multiple loops can be constructed in this way, such that the rotor is efficiently filled with these conductors. The loops on different layers may overlap each other, provided that they are not connected electrically. The illustrative conducting loops 62A, 62B, 62C are a set of three phases, and each conducting loop comprises a closed contour such that the enclosed areas that have opposite current direction (indicated with arrows only for the conducting loop 62A for illustrative purposes) are equal in size. In contemplated variants, the number of phases may vary, the coil ends can be overlapping in different ways, and/or the loop shape may be varied (while ensuring that the enclosed areas having opposite current direction are equal). A design with more than two loop parts is also contemplated, provided that the sum of all enclosed areas that have clockwise current direction equals the sum of all areas with counterclockwise current direction.
To accommodate the opposing orientations of the loop halves HL1, HL2, the stator is split into two halves, electrically driven at 180 degrees phase difference (generally, at such a phase difference that the induced currents in the loop halves corresponding with the stator excitation are in phase) so that the two loop halves combine their contributions to the torque. In
With reference to
The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/066922 | 6/26/2019 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62691955 | Jun 2018 | US |
