Patent Application
20060084866
The field of the invention is imaging probes, especially medical imaging probes adapted for use in blood vessels and in other cavities.
MRI has advantages over other medical imaging methods in that it does not use ionizing radiation, and can distinguish better between different kinds of soft tissue than x-rays or ultrasound. A disadvantage of conventional MRI is that it requires a large, expensive magnet with limited mobility. It also has limited spatial resolution, with voxels typically about 1 mm on a side, due to limits on the RF field strength that can be used (to avoid overheating of body tissue and peripheral nerve stimulation), the distance from the RF coils to the region being imaged inside the body, and the limited time that most patients can tolerate staying inside the bore of a magnet without moving. Even if the patient does not move, parts of the body move internally, for example with the cardiac cycle, and temporal gating cannot compensate perfectly for this motion. These limitations prevent conventional MRI from being used to detect plaque in the wall of the arteries, for example, where a resolution of the order of 0.1 mm is required. Conventional MRI also has limited capability of measuring diffusion, and can measure diffusion rates of medical interest only by pushing the limits of conventional gradient coils.
In order to overcome these limitations of conventional medical MRI, MRI probes which are inserted into the body of a patient have been designed, for example, probes which go into the blood vessels, into the digestive track, or into other body cavities. In most cases, these probes have only RF receiving antennas, and conventional external magnets and RF coils are still used to apply the static magnetic field and to transmit the RF magnet fields that are used to excite the nuclei. See, for example, E. Atalar et al, “High Resolution Intravascular MRI/MRS using a Catheter Receiver Coil,” MRM 36(4), 596-605 (1996). This may improve the spatial resolution possible with MRI, but does not eliminate the need for large, expensive magnets. In other cases, for example U.S. Pat. No. 5,572,132 to Pulyer, MRI probes have been designed that are fully self-contained, with their own magnets (usually permanent magnets), RF transmitting and receiving coils (often the same coil), and even gradient coils, eliminating the need for large magnets and RF transmitting coils.
Westphal et al, in U.S. Pat. No. 5,959,454, describes a an MRI probe with an external imaging region on one side, for use outside the body to examine skin, for example. Crowley, in U.S. Pat. Nos. 5,304,930 and 5,517,118, describes MRI probes used outside the body, for imaging a part of the body, in which the magnetic field gradient is large, and the nuclei do get out of phase very quickly. Prado et al, in U.S. Pat. No. 6,489,767, describes a palm-sized MRI probe with a planar imaging region on one side.
Golan, in WO 01/42807, and Blank et al in WO 02/39132 A1, describe self-contained intravascular MRI probes which use thousands of spin echoes to obtain high signal to noise ratio in a high gradient magnetic field. Other types of medical imaging probes used inside the body are also known. For example, U.S. Pat. No. 6,059,731 to Seward is one of many publications describing a phased array of ultrasound transducers which is inserted into a blood vessel. Another example is Yock, P. G. and Linker, D. T., “Looking Below the Surface of Vascular Disease,” Circulation 81(5): 1715-1718 (May 1990).
There are various types of non-imaging sensors which have been used in conjunction with intravascular probes. U.S. Pat. No. 4,752,141, for example, describes a probe with a contact temperature sensor, to detect the elevated temperature of inflamed plaque in arteries.
U.S. Pat. No. 6,475,159 describes one or more infrared temperature sensors for detecting plaque in arteries, in which a transparent balloon, surrounding the sensors, expands to make contact with the wall, and the infrared sensors view one or more locations on the wall through the balloon.
In other cases, for example in U.S. Pat. Nos. 5,265,606 and 5,284,138, a sensor (in this case a blood gas sensor) must be kept away from the blood vessel wall in order to provide accurate measurements of oxygen or carbon dioxide concentration, or pH level.
There are intravascular probes, some of them with sensors of various types, which expand against the wall of the vessels in order to perform therapeutic functions.
U.S. Pat. Nos. 6,306,141 and 6,533,805, both to Jervis, describe stents made of superelastic NiTi, which are mechanically manipulated to expand inside arteries that are partially blocked by plaque. U.S. Pat. No. 5,197,978 to Hess, and 5,466,242 to Mori, also describe stents made of NiTi, but using the shape-memory temperature effect instead of the superelastic effect. U.S. Pat. No. 6,053,873, to Govari and Fenster describes a stent which expands inside an artery, with pressure and blood-flow sensors, the latter using non-imaging ultrasound transducers to make Doppler measurements.
U.S. Pat. No. 6,036,689, to Tu et al, describes a catheter with electrodes that expand against the walls of an artery, using either a mechanically expanding basket or a balloon, and use RF energy to ablate plaque. The electrodes have a temperature sensor to allow control of the ablation process.
U.S. Pat. No. 4,841,977, to Griffith et al, describes a catheter which performs balloon angioplasty, and has an ultrasound transducer array, surrounded by the balloon, which images the procedure in real time.
U.S. Pat. No. 6,542,781, to Koblish et al, describes helical or loop structures, made either of NiTi or other materials, which expand to press against the inside of a pulmonary vein and produce a circular lesion going all the way around, for example by RF heating, in order to treat atrial fibrillation. Temperature sensors are used to provide feedback for the heating. The helical and loop structures can also be used to push other diagnostic or therapeutic elements against the wall of a blood vessel.
U.S. Pat. No. 6,152,899 describes a catheter for shrinking veins, which has expandable arms, each with an electrode and a thermocouple. The electrodes heat the wall of the vein from the inside, using the thermocouples for feedback control of the temperature, and shrink the vein, with the expandable arms collapsing as the vein shrinks.
All of the foregoing patents, applications, and other publications are incorporated herein by reference.
An aspect of some embodiments of the invention concerns an imaging probe in a blood vessel, or another cavity, with two or more imaging sensors looking in different directions. As used herein, an imaging sensor is a sensor which distinguishes sensing data from a plurality of different directions or locations, optionally arranged in the form of pixels or voxels. The probe expands, pushing the imaging sensors in different directions against the walls of the blood vessel or other cavity, where they each image a different part of the wall, with a different range of azimuthal angles. In one embodiment, the probe is an MRI probe in an artery, and the sensors, each a self-contained MRI device with a magnet and at least one RF antenna capable of RF transmitting and receiving, to provide data for producing images of plaque. Alternatively, one or more of the sensors have separate transmitting and receiving antennas.
Alternatively, other RF coupling elements are used for receiving and transmitting RF fields, instead of or in addition to antennas, for example Hall effect, magneto-optical, piezoelectric and magnetostrictive sensors and actuators, and micromachined mechanical structures resonant at RF frequencies. As used herein, “RF coupling element” refers to any such means for receiving or transmitting RF electric and magnetic fields, including an antenna, while “antenna” refers specifically to an element which directly couples to an electric field, or couples inductively to a magnetic field, for example a whip antenna or a loop antenna. It should be understood that, generally, whenever antennas are mentioned herein, other RF coupling elements are optionally used instead or in addition, in other embodiments of the invention.
Even if each sensor has a limited azimuthal field of view, as in the MRI probes described by Golan and by Blank et al, a broad azimuthal range of the wall can be imaged, in an embodiment of the invention, possibly even a full 360 degrees, by combining the data from the different sensors. There is optionally no need to rotate the probe, or to use an azimuthal phase encoding gradient.
The different imaging sensors need not all be located at the same longitudinal position along the blood vessel. Plaque in arteries tends to extend longitudinally over a much greater distance than its azimuthal and radial extent. Thus, if there are two or more sensors, looking at different azimuthal ranges of the wall, and located at different longitudinal positions but not too far apart longitudinally, then these sensors can be used to produce an image of plaque in the r-θ plane almost as if all the sensors were located at the same longitudinal position.
For example, in an embodiment of the invention, there is a sub-probe comprising two sensors at one longitudinal position, looking at azimuthal ranges that are centered 180 degrees apart from each other (in the +x and −x directions), and a second sub-probe comprising two more sensors, at a different longitudinal position, which look at azimuthal ranges that are 180 degrees from each other and 90 degrees from the directions of the first two sensors (in the +y and −y directions). The four sensors together cover a large fraction or even all of 360 degrees around the artery wall. For example, if each sensor has an azimuthal field of view that is between 45 and 60 degrees wide (measured from the center of the artery, not from the center of the sensor) then together the four sensors cover between 180 and 240 degrees around the artery wall.
The present invention is not limited to MRI probes, but concerns other types of imaging probes, for example ultrasound probes, which are pressed against different sides of a blood vessel or other cavity.
Another aspect of some embodiments of the invention concerns a series of sub-probes, each comprising a pair of sensors, such as the sub-probes described in the previous paragraph. Each sub-probe is located at a different longitudinal position on a probe. Within each sub-probe, the pair of sensors is joined to an expansion mechanism which makes the sensors move away from each other to image opposite sides of a blood vessel or another lumen. A single control element, such as a control cable, incorporated into a catheter, is manipulated to make the different sub-probes expand simultaneously.
Optionally, each sensor is an imaging sensor, obtaining data from more than one voxel. Alternatively, each sensor does not obtain data from more than one voxel, and the data from the different sensors is not used to construct an image, but is used, for example, to find an average value, or a maximum or minimum value, or a distribution of values, of some parameter in the vicinity of the probe. Alternatively, even if each sensor only obtains data from one voxel,an image is constructed from the probe as a whole by combining data from the different sensors. For example, each sensor is a non-imaging NMR sensor, obtaining data from a single voxel, or each sensor is a thermal sensor, measuring the temperature of the wall of the blood vessel at its location.
Optionally, the control cable is coupled to the expansion mechanism for each sub-probe by means of an adaptive mechanism, such as individual springs, which allows each sub-probe to adapt to the inner diameter of the blood vessel at that longitudinal location, when it expands. If there were no adaptive mechanism and the lumen varies in diameter along its length, then all the sub-probes would only open as far as the narrowest part of the lumen, and the other sub-probes would not reach the wall. Or, possibly, the sub-probes in the narrower parts of the lumen would push against the wall so hard that they would distort the blood vessel (possibly breaking the plaque), enough so that these sub-probes could expand as much as the sub-probes in the wider parts of the lumen. With the adaptive mechanism, the sensors in different sub-probes press firmly enough against the wall to take reliable data, without pressing hard enough to distort the wall.
Another aspect of some embodiments of the invention concerns an intravascular probe, such as those described above with one or more pairs of sensors, which expands so that it touches two sides of a blood vessel. The outer surface of the probe comprises a sheath, to keep blood from coming into contact with components of the probe that may not be bio-compatible, and there is no passage by which blood can flow through the center of the probe. The probe is not in contact with the vessel wall over the entire circumference of the vessel, but only in two contact regions on opposite sides of the vessel. Between these two contact regions, there are contact-free regions through which blood can flow around the probe.
If the probe includes another pair of sensors further along longitudinally, which expands at right angles to the first pair of sensors, then the free regions and contact regions will be at different azimuthal angles at different longitudinal positions, but the free regions still form a continuous volume along which blood can flow.
An aspect of some embodiments of the invention concerns an MRI probe comprising a magnet or set of magnets in the shape of a cylinder (not necessarily a right circular cylinder), with slots carved out of the magnet for RF antennas, for example coils. The RF coils fit entirely into the slots, so that the magnet together with the coils fits into the smallest convex volume that contains the magnet, viz. the cylindrical shape of the magnet before the slots were carved out. This allows the probe to be inserted easily into a blood vessel, and also allows the surface of the magnet, with its high field, to be pressed against the wall of an artery being imaged, except perhaps for a thin sheath that covers the magnet, for example if the magnet is not bio-compatible. For probes which have high magnetic field gradients, it is potentially advantageous for the surface of the magnet to be close to the wall, in order to make the magnetic field as high as possible in the region of the wall that is being imaged, and in order to make the imaging region extent as far as possible into the wall.
The slots and coils optionally do not extend over the entire length of the magnet, but each slot extends over less than the length of the magnet. This may allow the static magnetic field to be higher in the imaging region than if the slots and coils extended over the whole length of the magnet.
There is thus provided, in accordance with an exemplary embodiment of the invention, an imaging probe for imaging inside a cavity surrounded by a wall, the probe comprising:
Optionally, when the probe body is in the expanded state, the at least two imaging sensors are displaced from each other toward the wall, from their position when the probe body is in the contracted state.
Optionally, the probe is adapted for inserting into a blood vessel and using the blood vessel as the cavity.
Optionally, the probe includes a biocompatible sheath which covers the probe.
Optionally, the sheath keeps blood from coming into contact with and flowing through the probe body.
In an embodiment of the invention, when the probe body is in the expanded state, the probe touches the wall of the blood vessel in two contact regions on two opposite sides of the vessel, while leaving at least one free region, where the probe is not in contact with the wall, between the contact regions, thereby allowing blood to flow around the probe through the at least one free region.
Optionally, the probe has a diameter between 1 and 2 mm in its contracted state.
Optionally, the probe has a diameter between 2 mm and 6 mm in its expanded state.
Optionally, the diameter of the probe in its expanded state is at least 1.5 times the diameter of the probe in its contracted state.
In an embodiment of the invention, the imaging sensors are MRI sensors, each sensor comprising:
Optionally, at least one of the at least one RF coupling elements comprises an antenna.
Optionally, the antenna comprises a coil.
Alternatively or additionally, at least one of the at least one RF coupling elements uses the Hall effect.
Alternatively or additionally, at least one of the at least one RF coupling elements uses the magneto-optical effect.
In an embodiment of the invention, for at least one of the MRI sensors:
Optionally, for each of the MRI sensors, the at least one static magnetic field source comprises a permanent magnet.
Optionally, the magnets of the two MRI sensors repel each other.
Optionally, the magnets of the two MRI sensors are both magnetized in directions that are more than 45 degrees away from an axis along which the sensors move apart from each other when the probe body expands, and the magnets are magnetized in directions less than 90 degrees away from each other.
Alternatively, the magnets of the two MRI sensors are both magnetized in directions that are less than 45 degrees away from an axis along which the sensors move apart from each other when the probe body expands, and the magnets are magnetized in directions more than 90 degrees away from each other.
Optionally, the time-varying magnetic field created by the at least one RF coupling element of each MRI sensor is oriented at an angle between 45 and 135 degrees from the direction of the static magnetic field created by the static magnetic field source of said MRI sensor, at at least one location in the field of view of said MRI sensor.
Alternatively or additionally, the imaging sensors are ultrasound imaging sensors.
Optionally, the at least two imaging sensors comprise exactly two imaging sensors.
Alternatively, the at least two imaging sensors comprise at least three imaging sensors.
In an embodiment of the invention, the probe body comprises a plurality of expansion mechanisms, each expansion mechanism attached to at least two but not all of the imaging sensors, such that when each expansion mechanism causes the imaging sensors to which it is attached to move apart from each other, the probe body expands.
Optionally, the imaging sensors are arranged in a circle, and one of the expansion mechanisms is located between, and attached to, each pair of adjacent imaging sensors in the circle.
Optionally, at least one expansion mechanism comprises a pair of leaf springs.
Optionally, at least one of the expansion mechanisms comprises shape memory alloy.
Alternatively, the probe body comprises a single centrally located expansion mechanism which is attached to all the sensors, and causes the sensors to move apart from each other, expanding the probe.
Optionally, the expansion mechanism comprises a basket comprising a plurality of arms, each arm attached to exactly one sensor and each sensor attached to exactly one arm.
Optionally, the expansion mechanism comprises shape memory alloy.
Optionally, raising the temperature of the shape memory alloy above its transition temperature causes said expansion mechanism to expand.
Alternatively or additionally, said expansion mechanism operates using a superelastic effect of the shape memory alloy.
In an embodiment of the invention, the probe body comprises an expansion mechanism which causes the two sensors to move apart from each other, expanding the probe.
Optionally, the expansion mechanism comprises a pair of leaf springs joined at both their ends and free in their middle portions, and each sensor is attached to the middle portion of a different one of the leaf springs, and not attached to the other leaf spring.
There is further provided, in accordance with an exemplary embodiment of the invention, an imaging system comprising an imaging probe as described, and a catheter adapted for inserting the imaging probe into the cavity.
Optionally, the catheter comprises a control cable, and manipulating the control cable causes the probe body to expand and contract.
Optionally, the imaging system comprising a plurality of sub-probes, each sub-probe being an imaging probe as described, and a catheter adapted for inserting the sub-probes into the cavity.
Optionally, the catheter comprises a control cable, and manipulating the control cable causes the probe body of at least two of the sub-probes to expand and contract.
Optionally, the control cable is coupled to the sub-probes in a manner such that manipulating the control cable causes the probe bodies of a plurality of the sub-probes to expand simultaneously, and to contract simultaneously.
Optionally, for each sub-probe in said plurality, one or both of said sub-probe and its coupling to the control cable is sufficiently flexible so that, when the control cable is manipulated, each sub-probe in said plurality expands to an extent that depends on the distance to the walls of the cavity, at the location of that sub-probe.
Optionally, for each sub-probe in said plurality, one or both of said sub-probe and its coupling to the control cable is sufficiently flexible so that, if the cavity is any artery the inner diameter of which varies between 2 mm and 4 mm at the locations of the sub-probes in said plurality, then all of the sub-probes in said plurality will touch the inner walls of the artery when the control cable is manipulated to cause said plurality of sub-probes to expand, without exerting a pressure of more than 1 atmosphere on the wall of the artery.
There is further provided, in accordance with an exemplary embodiment of the invention, a method of producing images of the walls of a cavity, comprising:
Optionally, introducing an imaging probe into the cavity comprises introducing the imaging probe into a lumen.
Optionally, introducing the imaging probe into a lumen comprises introducing the imaging probe into a blood vessel.
Optionally, causing the imaging probe to expand comprises causing the imaging probe to touch the wall of the blood vessels at a contact region, and leaving a free region where the imaging probe does not touch the blood vessel wall, allowing blood to flow around the imaging probe.
Optionally, causing the imaging probe to expand comprises causing each of a plurality of sub-probes to expand by different amounts, depending on the inner diameter of the lumen at the location of each of said sub-probes.
Optionally, introducing the imaging probe into the cavity comprises using a catheter.
Optionally, causing the imaging probe to expand comprises manipulating the catheter.
Optionally, generating imaging data comprises transmitting electrical power to the imaging probe through the catheter.
Optionally, generating imaging data comprises receiving imaging data from the imaging probe through the catheter.
Optionally, reconstructing an image comprises analyzing data by a data analyzer, and including transmitting the imaging data from the imaging sensors to the data analyzer, wherein the data from at least two of the sensors is transmitted on a same cable.
Optionally, the sensing data from said two sensors is transmitted at different times.
Alternatively or additionally, the sensing data from said two sensors is transmitted in different frequency bands.
In an embodiment of the invention, the method includes digitally encoding the data from said two sensors into different digital channels before transmitting it, and decoding the data from said two sensors after transmitting it, before analyzing it.
There is further provided, in accordance with an exemplary embodiment of the invention, a probe adapted for inserting into a lumen, comprising a plurality of sub-probes, each having a contracted state, and a plurality of expanded states in each of which the sub-probe expands to a different extent, wherein each sub-probe is adapted to expand to an extent that depends on an inner diameter of the lumen, at the location of that sub-probe.
Optionally, the probe also includes a control cable, coupled to each of the sub-probes, which control cable, when it is manipulated, causes each of the sub-probes to expand, wherein for each sub-probe, one or both of said sub-probe and its coupling to the control cable are sufficiently flexible so that, when the control cable is manipulated, each sub-probe expands to the extent that depends on the inner diameter of the lumen at the location of that sub-probe.
Optionally, each sub-probe has a distal end and a proximal end, and manipulating the control cable shortens the distance between the distal end and proximal end of each sub-probe, thereby causing a middle portion of each sub-probe between the distal and proximal ends to bow outward, expanding that sub-probe.
In an embodiment of the invention, manipulating the control cable to expand the sub-probes allows the center of each sub-probe to remain in substantially a fixed position along the blood vessel.
Optionally, the control cable comprises:
Optionally, every expanded sub-probe returns to its contracted state when no pulling force is applied to the first portion relative to the second portion.
Optionally, a force between 0.5 and 2 newtons pulling on the first portion relative to the second portion is necessary and sufficient to fully expand all the sub-probes when there is no external force on the sub-probes resisting their expansion.
Optionally, a force between 0.5 and 2 newtons pulling on the first portion relative to the second portion is necessary and sufficient to expand all the sub-probes by a factor of 2 in diameter, when there is no external force on the sub-probes resisting their expansion.
Optionally, the second portion comprises a cable sheath surrounding the first portion which comprises an inner cable.
Optionally, the cable sheath includes a hole adjacent to the distal portion of each sub-probe, through which hole the inner cable is coupled to said distal portion.
Optionally, for at least one sub-probe, the first portion is coupled to the distal portion of that sub-probe through a distal adaptive spring, whereby, when the cable is manipulated, that sub-probe expands to an extent that depends on the inner diameter of the lumen, at the location of that sub-probe.
Alternatively or additionally, for at least one sub-probe, the second portion is coupled to the proximal portion of that sub-probe through a proximal adaptive spring, whereby, when the cable is manipulated, that sub-probe expands to an extent that depends on the inner diameter of the lumen, at the location of that sub-probe.
Optionally, at least one sub-probe comprises a pair of leaf springs.
Alternatively or additionally, at least one sub-probe comprises a basket structure.
Optionally, at least one sub-probe comprises shape memory alloy.
Optionally, the shape memory alloy is superelastic.
Optionally, at least one of the sub-probes has a diameter between 1 and 1.5 mm in its contracted state.
Optionally, said sub-probe has a diameter between 1.7 mm and 6 mm in its maximally expanded state.
Optionally, at least one of the sub-probes has a diameter between 1.7 mm and 6 mm in its maximally expanded state.
Optionally, the probe includes a plurality of sensors attached to at least one of the sub-probes, which sensors each generate sensing data from a different portion of the wall of the lumen, when said sub-probe is expanded sufficiently so that said sensors are adjacent to the wall.
Optionally, at least one of the sensors is a non-imaging NMR sensor.
Alternatively or additionally, at least one of the sensors is a thermal sensor.
Optionally, the plurality of sensors comprises sensors attached to at least two of the sub-probes.
Optionally, at least two of the sub-probes each have at least two of the sensors attached to them, and each of said sub-probes is adapted to expand to an extent such that each of the two sensors is adjacent to a different portion of the wall.
There is further provided, in accordance with an exemplary embodiment of the invention, a method of obtaining sensing data from an extended region of the wall of a lumen, comprising:
Optionally, the method includes transmitting the sensing data from the sensors to a data analyzer, wherein the sensing data from at least two of the sensors is transmitted on a same cable.
Optionally, at least one of the sensors is an imaging sensor.
Optionally, the imaging sensor is an MRI sensor.
There is further provided, in accordance with an exemplary embodiment of the invention, an imaging system for imaging the walls of a lumen, comprising:
Optionally, the imaging system includes a catheter which holds together the control cable, the transmitting channel, and the receiving channel.
Optionally, the catheter is adapted for inserting the probe into the lumen.
There is further provided, in accordance with an exemplary embodiment of the invention, a probe adapted for inserting into a blood vessel, comprising:
Optionally, the sheath comprises silicone.
Alternatively or additionally, the sheath comprises polyurethane.
Alternatively or additionally, the sheath comprises SCBS.
Alternatively or additionally, the sheath comprises a composite material.
Optionally, the sheath is between 10 and 100 micrometers thick.
Optionally, when the probe touches the wall of the blood vessel in the first and second contact regions, it leaves a second free region on an opposite side of the blood vessel from the first free region, thereby allowing blood to flow around two sides of the probe.
Optionally, the probe includes a second sub-probe body, having a contracted state and an expanded state, located at a different longitudinal location from the first sub-probe body, wherein the sheath also covers the second sub-probe body and keeps blood from coming into contact with and flowing through the second sub-probe body, and wherein, when the second sub-probe is in its expanded state, the probe comes into contact with the wall in a third contact region and a fourth contact region, on opposite sides of the blood vessel, leaving a third free region between the third and fourth contact regions, thereby allowing blood to flow around the probe at the longitudinal location of the second sub-probe body.
Optionally, when the probe touches the wall of the blood vessel in the third and fourth contact regions, it leaves a fourth free region on an opposite side of the blood vessel from the third free region, thereby allowing blood to flow around two sides of the probe at the longitudinal location of the second sub-probe body.
Optionally, the direction from the first contact region to the second contact region, and the direction from the third contact region to the fourth contact region, excluding any longitudinal components, differ from each other by more than 10 degrees and less than 170 degrees.
Optionally, the free regions connect to form a continuous passage within which blood can flow past the entire length of the probe.
Optionally, the surface of the sheath does not have pockets where blood stagnates.
In an embodiment of the invention, the probe includes a second sub-probe body, located at a different longitudinal location from the first sub-probe body when the probe is inserted in the blood vessel, the second sub-probe body having a contracted state and a plurality of expanded states, and the first sub-probe body has a plurality of expanded states, the first and second contact regions are at the longitudinal location of the first sub-probe body, and the two sub-probe bodies are adapted so that when the first sub-probe body is in the expanded state in which the probe touches the wall of the blood vessel in the first and second contact regions, then the second sub-probe body is in an expanded state in which the probe touches the wall in a third contact region and a fourth contact region, on opposite sides of the blood vessel, at the longitudinal location of the second sub-probe body.
Optionally, the probe also includes two imaging sensors mounted on the sub-probe body and having fields of view in different directions, and, when the sub-probe body is in the expanded state, the fields of view of the imaging sensors respectively comprise portions of the wall on different sides of the blood vessel.
There is further provided, in accordance with an exemplary embodiment of the invention, an imaging system comprising a probe according to an embodiment of the invention, and a catheter adapted for inserting the probe into the blood vessel.
There is further provided, in accordance with an exemplary embodiment of the invention, an imaging system for imaging the walls of a blood vessel, comprising:
There is further provided, in accordance with an exemplary embodiment of the invention, a method of obtaining sensing data from an extended region of the wall of a blood vessel, the method comprising:
There is further provided, in accordance with an exemplary embodiment of the invention, a method of producing images of the wall of a blood vessel, the method comprising:
There is further provided, in accordance with an exemplary embodiment of the invention, a magnetic resonance sensor comprising:
NMR signals from said excited nuclei and generating NMR electrical signals therefrom; wherein a smallest convex volume which includes all of the at least one magnet is substantially cylindrical, the at least one magnet substantially reaches all of the radial surface of the convex volume, except for at least one slot, each slot being less than the length of the convex volume, and one of the at least one RF coupling elements is located in one of the at least one slots, substantially entirely within the convex volume.
Optionally, at least one of the at least one RF coupling elements comprises an antenna.
Optionally, the at least one magnet comprises a sintered material whose skin depth, at the proton nuclear magnetic resonance frequency at the greatest field at the surface of the magnet, is at least twice the largest dimension of the magnet perpendicular to the cylindrical axis of the convex magnet volume.
Optionally, the at least one magnets substantially comprise only a single magnet, uniformly magnetized in a single direction.
Optionally, at least one of the at least one slots with at least one RF coupling element in it runs substantially perpendicular to the cylindrical axis of the convex magnet volume.
Optionally, the at least one RF coupling element in said slot comprises a coil.
Optionally, the time-varying magnetic field at the center of the coil is oriented substantially perpendicular to the direction of the slot and to the cylindrical axis.
Optionally, the magnet is magnetized substantially parallel to the direction of the slot, adjacent to the slot.
Optionally, the slot is less than half the length of the convex magnet volume.
There is further provided, in accordance with an exemplary embodiment of the invention, an imaging probe for imaging inside a cavity surrounded by a wall, the probe comprising:
There is further provided, in accordance with an exemplary embodiment of the invention, an NMR system comprising:
Optionally, the NMR probe is adapted for use inside the body.
Optionally, the NMR probe is adapted for use as an intravascular NMR probe.
In an embodiment of the invention, the NMR system is adapted for imaging a a wall surrounding a cavity, the NMR data comprises imaging data, the NMR probe has a contracted state and an expanded state, the at least one magnetic resonance sensor comprises at least two magnetic resonance sensors, adapted for MRI, mounted on the NMR probe and having fields of view in different directions, and when the NMR probe is in the expanded state inside the cavity, the fields of view of the magnetic resonance sensors respectively comprise portions of the wall of the cavity on different sides of the cavity.
Optionally, for at least one slot, a same RF coupling element both excites nuclei in the excitation region, and receives NMR signals from said excited nuclei and generates NMR electrical signals therefrom.
Alternatively or additionally, for at least one slot, a first RF coupling element excites nuclei in the excitation region, and a second RF coupling element receives NMR signals from said excited nuclei and generates NMR electrical signals therefrom.
Optionally, the at least one slots comprise a plurality of slots, each with at least one RF coupling element, all of said plurality of slots running substantially in a same direction perpendicular to the cylindrical axis, and spaced apart in the direction of the cylindrical axis.
Optionally, the at least one RF coupling element in each of said plurality of slots comprises a coil.
Optionally, the time-varying magnetic field at the center of the coil in each of said plurality of slots is oriented substantially perpendicular to the direction of the slot and to the cylindrical axis.
Optionally, the magnet is magnetized substantially parallel to the direction of the slot, adjacent to the slot, for each of said plurality of slots.
Optionally, each of said plurality of slots is less than half the length of the convex magnet volume, in a direction parallel to the cylindrical axis.
Optionally, for at least two of the slots, the NMR electrical signals produced by the RF coupling elements in those slots are lumped together in the receiving channel.
Alternatively or additionally, for at least two of the slots, the NMR electrical signals produced by at least one RF coupling element in a first one of the two slots, and the NMR electrical signals produced by at least one RF coupling element in a second one of the two slots, are sent through the receiving channel in a manner that allows the controller to distinguish the two sets of signals.
Optionally, the controller uses the two sets of signals to reconstruct an image comprising separate pixels adjacent to the first slot and the second slot.
There is further provided, in accordance with an exemplary embodiment of the invention, a method of analyzing NMR signals from a viewing region which is extended in a longitudinal direction, comprising:
Optionally, analyzing the NMR data comprises reconstructing an image of the viewing region with a plurality of pixels in the longitudinal direction.
Alternatively or additionally, analyzing the NMR data comprises obtaining an NMR spectrum of the viewing region.
Optionally, exciting nuclei in the excitation region comprises creating the time-varying magnetic fields at different times by the RF coupling elements that are in slots that are selected to be treated separately.
Alternatively or additionally, exciting nuclei in the excitation region comprises creating the time-varying magnetic fields in different frequency bands by the RF coupling elements that are in slots that are selected to be treated separately.
Optionally, the NMR electrical signals from RF coupling elements in slots that are selected to be treated separately are obtained from said RF coupling elements using separate cables.
Alternatively or additionally, the NMR electrical signals created by the RF coupling elements in slots that are selected to be treated separately are transmitted at different times.
Alternatively or additionally, the NMR electrical signals created by the RF coupling elements in slots that are selected to be treated separately are transmitted at different frequencies.
Exemplary, non-limiting, embodiments of the invention are described in the following sections with reference to the drawings. The drawings are generally not to scale and the same or similar reference numbers are used for the same or related features on different drawings.
FIGS. 3 is a perspective side view of the probe shown in
As shown most clearly in
Although the field of view of sensors 102 and 104 is at a different longitudinal location than the field of view of sensors 106 and 108, this may not matter very much, if the sensors are used to image plaque, and if plaque tends to extend longitudinally over a distance greater than the distance between the two pairs of sensors.
Data from the four sensors is optionally collected simultaneously. Alternatively, data is collected simultaneously from only some of the sensors, or data is collected serially, one sensor at a time. Data collected simultaneously from two or more sensors is optionally transmitted on a single cable (not shown) going through catheter 115, for example an electrical cable or an optical fiber, using different frequency bands for different sensors, or using any method known in the art of multiplexing to transmit more than one channel on a single cable, or the data is transmitted by one or more RF channels. However, whether the data is collected simultaneously or serially, it is not necessary to rotate the probe each time a different azimuthal field of view is imaged.
Optionally, instead of two pairs of sensors as shown in
Optionally, instead of or in addition to sub-probes comprising pairs of sensors, the sub-probes comprise sets of three or more sensors, and the sensors in each set expand outward in different directions. The expansion mechanism is, for example, a basket mechanism, or any other expansion mechanism known to the art of intravascular probes. Optionally, the expansion mechanism includes any of the features described herein for the leaf springs with a pair of sensors, for example pulling on a single wire causes more than one sub-probe to expand.
The mechanism by which inner wire 114 and sheath 116 make the pairs of leaf springs expand is shown more clearly in
Optionally, pulling back on inner wire 114 relative to sheath 116 allows the center of each sub-probe to remain substantially in a fixed position in the blood vessel. This can be accomplished, for example, if sheath 116 is sufficiently rigid and resistant to buckling so that, when inner wire 114 is pulled and sheath 116 is pushed, the distance between proximal ends 120 and 124 remains constant. There is also optionally a mechanism, not shown in the drawings, near the proximal end of the catheter outside the body, which allows a doctor to make a single manipulation which pulls inner wire 114 back and simultaneously pushes sheath 116 forward by the same distance. This allows the mid-point between distal end 118 and proximal end 120, and the mid-point between distal end 122 and proximal end 124, to both remain fixed in place when the probe is expanded.
Inner wire 114 is exposed at distal end 118, which is the distal end of the whole probe, because sheath 116 ends before distal end 118, as may be seen in
Alternatively, instead of sheath 116 being coupled to the proximal ends, and inner wire 114 being coupled to the distal ends, sheath 116 is coupled to the distal ends, and inner wire 114 is coupled to the proximal ends, and is pushed, rather than pulled, to expand the probe. Alternatively, instead of a sheath and an inner wire, there are two wires side by side, one of them coupled to the distal ends of the leaf springs, and one of them coupled to the proximal ends. A potential advantage of having a sheath coupled to the proximal ends, and an inner wire coupled to the distal ends, is that the sheath, which pushes when the leaf springs are expanded and is therefore subject to buckling, has a higher buckling limit than the inner wire. A low buckling limit for the inner wire is generally not significant because the inner wire pulls when the leaf springs are expanded. In fact, the inner wire under tension helps to stabilize the sheath against buckling. It is noted that the sheath would not be as effective at stabilizing the inner wire against buckling, if the inner wire were pushing against the proximal ends and the sheath were pulling against the distal ends, because the inner wire could buckle through an opening in the sheath, in the regions where the inner wire is coupled to the leaf springs.
If the inner wire and sheath are both coupled rigidly to the leaf springs, then the degree of expansion of leaf springs 110 will have a fixed relationship to the degree of expansion of leaf springs 112. For example, if the two pairs of leaf springs have the same geometry, then one pair of leaf springs will always expand by the same amount as the other pair of leaf springs. The expansion would depend only on how far inner wire 114 is pulled relative to sheath 116. However, this may not be desirable, since the blood vessel may not have the same diameter everywhere along the length of the probe. For example, suppose the blood vessel is narrower at the location of sensors 102 and 104, than it is at the location of sensors 106 and 108. If one pair of leaf springs always expands by the same amount as the other pair, and if they do not exert enough force on the wall of the blood vessel to significantly deform it, then when sensors 102 and 104 reach the blood vessel wall they will stop, and sensors 106 and 108 will not be able to reach the blood vessel wall. Sensors 106 and 108 may not be able to obtain good images of the wall if they are too far away from it. If the leaf springs at the narrow portion exert enough force on the blood vessel wall to significantly distort the blood vessel, then it may be possible for sensors 102, 104, 106 and 108 to all touch the blood vessel wall, by deforming the blood vessel where it is narrow, next to sensors 102 and 104. However, this could be dangerous if there is fragile plaque in the walls of the blood vessel.
In order to allow each sub-probe to adapt to the diameter of the blood vessel at the location of that sub-probe, without distorting the blood vessel, one or both of inner wire 114 and sheath 116 are optionally not rigidly coupled to the leaf springs. Instead, at least one of them, for example inner wire 114, is coupled flexibly to the leaf springs, by means of coil springs for example. This is shown schematically in
In
A block 208 is attached to inner wire 114 to the left of distal end 122. When inner wire 114 is pulled to the right relative to sheath 116, block 208 pushes to the right against spring 210, which pushes against distal end 122.
If there were no spring 210, and neglecting any stretching, compression, or buckling of inner wire 114 and sheath 116, the amount that one pair of leaf springs is open would fix the amount that the other pair of leaf springs is open, even if the inner diameter of the blood vessel were different for the two pairs of leaf springs. Including spring 210 between block 208 and distal end 122 of leaf springs 112, and/or a similar spring at the distal end of leaf springs 110, makes it possible for the two pairs of leaf springs to open by different amounts, depending on the forces they encounter.
The more spring 210 is compressed, the greater force it will exert on distal end 122, and the greater force leaf springs 112 will exert on the wall of the blood vessel. The force that leaf springs 112 exert on the blood vessel wall depends on the difference in the inner diameter of the blood vessel at the axial positions of the sensors, and on the spring constant of spring 210.
Optionally, the spring constant of spring 210 is chosen to have a value so that the pair of leaf springs at the narrower diameter will push sensors 106 and 108 against the blood vessel wall with a great enough force to keep them firmly in place when they collect imaging data, but with a force that is not so great that the sensors will significantly deform the wall of the blood vessel. In particular the force is preferably not great enough to break any plaque, which could be dangerous. For example, the probe does not press on the wall with a pressure greater than 1 atmosphere, or 0.5 atmospheres, or 2 atmospheres. Thus, in addition to allowing the different leaf springs to open by different amounts, spring 210 also serves a safety function.
As shown in
Optionally, there is a spring between ridge 204 and proximal end 124, instead of or in addition to spring 210 between block 208 and distal end 122. The springs need not be coil springs as shown in the drawing, but could be any kind of flexible coupling. The springs, if they are found only on one end (distal or proximal) of each leaf spring, need not be found on the same end of each leaf spring.
A hole 206 at the bottom of sheath 116, extending some distance to both sides of distal end 122, exposes inner wire 114 of the catheter. Hole 206 need not extend very far azimuthally around sheath 116, as it seems to do in
Flexible coupling between the catheter and the expansion mechanism, as exemplified by spring 210, is also optionally used if a different expansion mechanism is used. For example, instead of a pair of leaf springs, the expansion mechanism could be a basket, or any other expansion mechanism known to the art.
Optionally, instead of or in addition to there being a flexible coupling such as spring 210 between the catheter and the leaf springs, the leaf springs themselves are sufficiently flexible so that different pairs of leaf springs can open by different amounts, and so that the force exerted by the sensors on the blood vessel wall is not too great.
Optionally, leaf springs 110, and 112 (or whatever expansion mechanisms are used) are made of a superelastic material, such as superelastic NiTi. Superelastic materials are shape-memory materials that are used not too far above their martensite to austenite transition temperature, so that they revert to martensite, and undergo a large strain, when a relatively small stress is applied to them. Alternatively, one or more of the leaf springs are made of a material that is not superelastic, such as 304 or 316 stainless steel, or other biocompatible materials such as alloys based on cobalt, titanium or tantalum which are used in stents. It should be noted, however, that stents are generally made of materials with a low yield stress, so that they remain in an expanded state when the expanding force is removed, while for the leaf springs it is preferable to use a material which does not exceed its yield stress in the course of expanding, so that they will contract again when the expanding force is removed. Hence, some materials that are useful for stents may not be useful for the leaf springs, and vice versa.
A potential advantage of using a superelastic material is that the leaf springs can undergo a large displacement, as much as several percent for NiTi, without undergoing plastic deformation. Non-superelastic alloys, by contrast, have yield strains typically less than 0.2%. Although in principle a leaf spring made of a non-superelastic material could also undergo a large displacement without plastic deformation, if the leaves of the leaf spring are thin enough, such thin leaves may be difficult to manufacture, and the leaf spring might not exert enough force on the blood vessel wall to hold the probe in place, unless the leaves are made of a material with very high elastic modulus. With superelastic materials, the leaves may be made thicker, and such high elastic modulus is not needed. Optionally, the probe shown in
Note that the probe in
The coils are located in slots 408, which are cut out of the magnet. Alternatively, there is one long slot with a single long coil, instead of two separate slots each with its own coil. Having two slots each with its own coil has the potential advantage that, for the same longitudinal extent of the field of view of the sensor, less of the magnet volume is missing, especially near the imaging region close to the surface of the probe, so the magnet produces a greater magnetic field in the imaging region. Having one long slot with a single long coil has the potential advantage that, for the same ohmic heating, the RF field is greater.
Optionally there is more than one coil in each slot. For example, if there are separate transmitting and receiving coils, then optionally there is a transmitting coil and a receiving coil in the same slot. The part of the magnet that is removed to make the slots would contribute relatively little to the static magnetic field in the imaging region, which is just above the coils. Using this volume for the coils, rather than putting the coils outside a cylindrical magnet without slots as in the prior art, allows the magnet to be brought closer to the imaging region, more than making up for any loss in magnetic moment of the magnet due to the slots.
Allocating a larger volume for the coils enables stronger RF fields to be produced with a same amount of ohmic heating, or the same RF fields with less ohmic heating. Strong RF fields in short pulses (i.e. high bandwidth) are potentially advantageous, especially if the static magnetic field is very inhomogeneous, because they make it possible to excite a larger volume, and to obtain more spin echoes within a given time, producing a higher signal to noise ratio. Strong RF fields also make it possible to refocus the magnetic moments of the nuclei quickly, and, particularly with a high RF bandwidth, the nuclei do not diffuse away from the resonant region before they can be refocussed. Ohmic heating of the RF coils, which can affect the magnetization of permanent magnets as well causing heating of body tissue, may be the factor which limits how strong the RF fields are. In other cases, however, direct RF heating of body tissue is the limiting factor.
If the slot takes up too large a fraction of the magnet volume, however, then the static magnetic field will be weaker because the magnet volume will be smaller, for a given envelope of the probe, and a lower static magnetic field may result in lower signal to noise ratio. Optionally, the shape and size of the slot and coils are optimized, for a given probe envelope, in order to maximize some measure of probe performance, for example the signal to noise ratio that can be obtained in a given data acquisition time. Such optimization of the design may be done, for example, using software to simulate the probe performance and to calculate the RF and static magnetic field distribution.
Optionally, the magnet is made of a sintered material which has a relatively high resistivity, so the RF coils will not induce significant eddy currents in the magnet. Alternatively, the magnet is a good conductor, but using a magnet which is a good conductor has the potential disadvantage that eddy currents may partly cancel the RF magnetic field in the imaging region, and may heat the magnets to an undesirably high temperature.
The imaging region is above the probe, in the orientation of the probe shown in
The use of slots for RF coils in an MRI probe is not limited to the magnet configuration shown in
Note that, over a fairly broad imaging region near each of the magnets and coils, the RF magnetic field is approximately perpendicular to the static magnetic field. Only the component of RF field that is perpendicular to the static magnetic field contributes to excitation of nuclei in NMR, so the configuration shown in
Magnets 602 and 604 repel each other, which is an advantage when the magnet is expanding, because, assuming the probe uses the expansion mechanism shown in
The configuration in
Two other examples of a probe with four MRI sensors, with an expanding structure, but with the sensors all at different longitudinal locations, are shown in
In
A potential disadvantage of the configuration in
In
The configurations shown in
Optionally, any of the probes described here have radio-opaque markings, which are used to precisely locate the probe, for example with a fluoroscope, and thus to correlate the images made by the probe with the position of the probe in the body.
In describing the geometry of probes, sensors, or other bodies, “substantially” as used herein, in particular in the claims, means “to within 10% of the diameter” of the probe, sensor, or other body being described. As used herein, “cylinder” and cylindrical” do not necessarily refer to a right circular cylinder, unless explicitly stated as such.
In describing probes covered with a sheath, the sheath is considered part of the probe. Hence, describing a probe as touching a wall does not necessarily mean that a part of the probe such as an imaging sensor touches the wall directly, but it also includes the case where only the sheath touches the wall directly.
Describing the at least one magnets of an MRI sensor as “substantially comprising only a single magnet, uniformly magnetized in a single direction,” means that any lack of uniformity in the direction or magnitude of magnetization of the at least one magnets does not change the magnetic field in the imaging region enough to substantially affect the operation of the sensor, and any discontinuity in the magnets, for example if the magnets comprise two magnets touching each other or separated by a thin layer of glue, does not cause the operation of the probe to differ substantially from what it would be if the magnets comprised a single continuous magnet.
The invention has been described in the context of the best mode for carrying it out. It should be understood that not all features shown in the drawings or described in the associated text may be present in an actual device, in accordance with some embodiments of the invention. Furthermore, variations on the method and apparatus shown are included within the scope of the invention, which is limited only by the claims. Also, features of one embodiment may be provided in conjunction with features of a different embodiment of the invention. As used herein, the terms “have”, “include” and “comprise” or their conjugates mean “including but not limited to.”
This application is related to a patent application titled “Magnetic Coil Configurations for MRI Probes,” attorney's docket number 334/03511, filed on even date, at the US Patent and Trademark Office, the disclosure of which is incorporated herein by reference.
