Systems and methods for dynamic optical imaging

A medical system includes a catheter having an elongated tubular member and an inner core slideably received within the elongated member. The inner core includes an imager on a distal end and is coupled with a control system and an imaging system. The inner core is configured to scan the interior of a lumen by radially rotating around a center axis and axially translating along the center axis while within the elongated member. The medical system is configured to dynamically image a body lumen at a high speed in order to allow for optical imaging in a safe manner without long durations of blood sequestration and displacement. The medical system is configured to obtain three dimensional images of the body lumen with as little as one dimensional scanning of the lumen. Images of the lumen can be stored and viewed at a desired rate after scanning.

FIELD OF THE INVENTION

The field of the invention relates generally to optical imaging, and more particularly to a systems and methods for dynamic optical imaging in a medical environment.

BACKGROUND INFORMATION

The ability to image within a living body is fundamental to the proper diagnosis and treatment of medical conditions. Typically, a medical device such as a catheter or endoscope is used to gain access to and image remote regions of the body otherwise reachable only with invasive surgery. These systems use a variety of imaging techniques such as acoustical and optical imaging.

Acoustical imaging systems generally place either a phased array or single rotating transducer at the distal end of the medical device. The transducer emits acoustic pulses, i.e., mechanical sound waves, and receives the acoustic reflections that are created by the impact of these pulses with the surrounding tissues. The acoustic imaging system can then generate an image of the internal tissue based on the information provided by these reflections. The acoustic imaging system is able to produce images despite the presence of blood or other fluids surrounding the tissue. This makes acoustic imaging ideal for applications which require scanning large regions of internal tissue. For instance, when scanning internal body lumens such as blood vessels, the acoustic system can image the vessel both radially around the vessel circumference as well as longitudinally along the length of the vessel, typically referred to as “pull back.” Scanning of large regions within the blood vessel can take place without seriously impeding the flow of blood.

Optical imaging systems are similar to acoustic systems in that they typically include an optical imager at the distal end of the medical device. However, optical imaging systems use the transmission and receipt of optical energy, e.g., light, to create images of tissue within the body. Optical imaging systems typically employ a type of optical coherence domain reflectometry (OCDR), such as optical coherence tomography (OCT), to generate high quality images of internal tissue. Optical imaging systems are typically faster than acoustic imaging systems and can provide a higher degree of resolution. However, because optical imaging is dependent on the propagation of light, the presence of fluids or materials that impede light propagation can prevent proper imaging. For instance, when an optical imaging system is used to image the interior of a blood vessel, the flow of blood through that vessel must be sequestered, either by introducing saline to dilate the blood within the vessel or by stopping the flow of blood altogether. Sequestration for extended periods of time, in some case for less than sixty seconds, starves the tissue of oxygen and can result in serious adverse effects and is not desired. Because optical systems require obstruction of blood flow, they can only scan for limited periods of time and accordingly, optical imaging systems are not suited for scanning large regions of tissue in a safe manner.

Thus, there is a need for improved systems and methods of imaging internal tissue.

SUMMARY

An improved medical system preferably includes a catheter and an imaging system and is configured to dynamically image large regions of body tissue in a short time period.

Described next is an example embodiment of a method of optically imaging with an improved medical device. First, a substantially transparent fluid is introduced to a body lumen with an elongated tubular member. Then, a wide band light pulse is generated from a light source and the light pulse is split into a tissue pulse and a reference pulse. The tissue pulse is then directed towards a first location of the body lumen or other body tissue with an imager and a reflected tissue pulse is received from the body lumen at the imager. Next, the reflected tissue pulse is mixed with the reference pulse in a time gate to generate a light wave corresponding to a temporal duration of the reference pulse and a spatial profile of the tissue pulse, and the light wave is detected with a plurality of detectors. Finally, the inner core is preferably radially rotated around a center axis and axially translated along the center axis to image a second location of the body lumen.

In another example embodiment, the improved medical system includes a catheter having a proximal and a distal end, the distal end insertable into a living body and configured to optically image a body lumen. The catheter includes an elongated tubular member including an opening for introducing a fluid to the body lumen and an inner core configured to slide within the elongated member. The system also preferably includes a control system configured to rotate the inner core within the elongated member radially around a center axis and also configured to translate the inner core axially along the center axis, as well as a light source optically coupled with the inner core and configured to generate a wide band light pulse. The system further includes an interferometric optical imaging system optically coupled with the array and the inner core and configured to split the light pulse into a tissue pulse and a reference pulse, wherein the inner core is configured to direct the tissue pulse towards a body lumen. The imaging system includes a time gate configured to mix a tissue pulse reflected from the body lumen with the reference pulse and output a light wave corresponding to a temporal duration of the reference pulse, and a spatial profile of the reflected tissue pulse, and also an imager comprising a plurality of light detectors configured to detect the light wave.

DETAILED DESCRIPTION

The systems and methods described herein provide for the dynamic optical imaging of a body lumen or other body tissue inside a living body. In a preferred embodiment, a medical device, such as catheter, is inserted into a living body and used to image a body lumen at a high speed. For the sake of convenience, reference is made to the example embodiment of a catheter; however, such catheter embodiments can be adapted to be non-catheter embodiments. The catheter optically images the lumen at a rate high enough to allow high quality imaging of large regions of body tissue in a short period of time. These regions can be dynamically imaged in a safe manner, i.e., without the introduction of large amounts of blood-displacing fluids that can have serious adverse effects on the recipient.

FIG. 1depicts medical system100, which is a preferred embodiment of the systems and methods described herein. This embodiment includes medical device102, which is preferably a catheter, elongated tubular member104and inner core106. Catheter102includes distal end103and is insertable into a living body and can be advanced through a body lumen such as a blood vessel, artery, or a body canal, while at the same time imaging that body lumen or canal. Optical imager108is located at or near the distal end of inner core106. Inner core106is slideably received within elongated member104and can be rotated radially around center axis130as well as translated axially along center axis130. In order to image the lumen, any fluids that do not allow sufficient propagation of light, e.g., blood, are preferably displaced or sequestered prior to imaging. However, as will be seen, the improved systems require blood displacement for a shorter duration. In this embodiment, elongated member104includes openings110for introducing a substantially transparent fluid to displace any blood present in the lumen. This substantially transparent fluid is preferably saline, however any fluid can be used that allows the propagation of a sufficient amount of light to properly image the lumen according to the needs of the individual application. This method of displacing fluid while imaging a body lumen both axially and radially allows the generation of a three-dimensional image of the circumference and length of the body lumen.

Inner core106includes flexible drive shaft112that is configured to rotate optical imager108in a manner which is well known in the art. Inner core106also includes an optical signal line (not shown) that is preferably housed within the drive shaft112and optically couples imager108with imaging system200.FIG. 2depicts another embodiment of medical system100, including imaging system200, control system202and proximal end204of catheter102. Imaging system200is preferably an optical coherence tomography (OCT) imaging system but can also be an optical coherence domain reflectometry (OCDR) system or any optical imaging system that allows for high speed imaging. Imaging system200processes the optical information contained in an optical signal reflected by the body lumen and received by imager108. Control system202includes hardware and software for controlling the movement of inner core106. More specifically, control system202preferably includes drive system206for mechanically driving the rotation and axial translation, or pull back, of inner core106within elongated member104. The operation and functionality of these various systems is discussed in more detail below. Also depicted in each figure is guiding catheter140, which is optionally used to facilitate the introduction of catheter102into the body lumen. Guiding catheter140can also be used to introduce saline into the body lumen.

A detailed description of medical system100is facilitated by a discussion of several example applications in which system100can be implemented.FIG. 3depicts system100in one such example application. More specifically,FIG. 3illustrates distal end103of catheter102inserted within a cutaway of body lumen302. In this embodiment, system100scans the circumference of lumen302over pre-determined length304. Imager108directs tissue pulse306towards body lumen302and impacts a first location at the surface of body lumen302forming circular cross-section308. In order to provide better illustration of this embodiment, the size of cross-section308has been exaggerated inFIG. 3. Tissue pulse306penetrates body tissue302and reflects back to imager108and is communicated to imaging system200. The information in the reflected signal is used to generate a two-dimensional image of the depth, in the z-direction, and the length, in the x-direction, of lumen302.

Inner core106then rotates radially in direction310, the y-direction, and translates, or moves, axially along axis130in direction312to a second location on lumen302. A second tissue pulse306is directed to the second location and the information in the reflected signal is communicated to imaging system200and the process repeats until the circumference of lumen302is imaged along length304. The images taken at the consecutive locations can be combined together to form a three-dimensional image of lumen302. Throughout the imaging process any blood present in lumen302is either sequestered or displaced by the infusion of saline from opening110.

In one example embodiment, length304is 50 millimeters (mm) and saline is infused at a rate of 4 cubic centimeters (cc) per second (sec). As in any optical imaging application, care must be taken to avoid harm to the recipient from excessive amounts of saline infusion. For a typical recipient, a safe amount of saline that does not result in significant harm is approximately 30 cubic centimeters. As one of skill in the art will readily realize this safe level varies with the recipient's body characteristics. To maintain patient safety by limiting the level of saline infusion to an acceptable level, length304is preferably imaged in 8 seconds. This is a pull back rate of 6.25 mm/sec and results in a tolerable total saline infusion of 32 cubic centimeters.

Typical optical imaging systems radially rotate at a low rate of 26 revolutions every second, with either 256 or 512 separate imaging locations314, referred to as vectors, in one complete circumference of lumen302. The set of vectors imaged in one circumference of lumen302is referred to as an imaging plane. Because of this low rotational speed, the typical imaging system cannot image any significant length of lumen302without infusing dangerously high amounts of saline and, therefore, these systems operate at a low translational speed or do not translate at all. As will be demonstrated in the following example embodiment, optical imaging system100is capable of operating at both a relatively higher rotational speed and a relatively higher translational speed than the typical imaging system, allowing optical imaging system100to image while maintaining patient safety.

In this example embodiment, the distance between imager108and lumen302is 1 mm and the angle between vectors314is 0.7 degrees, resulting in a distance between vectors314of 0.012 mm, with 512 vectors in one imaging plane. Preferably, the distance between two consecutive imaging planes, or pitch, is approximately equal to the distance between two consecutive vectors314, resulting in a pitch of 0.012 mm/rev. In addition, the distances can be adjusted to allow overlap between adjacent cross-sections308. Overlap between vectors314allows higher degrees of resolution and accuracy. To achieve a rate of pull back of 6.25 mm and a pitch of 0.012 mm, the rate of rotation is preferably 520 revolutions per second (rev/sec), or 31200 revolutions per minute (rpm). This can be determined by taking rate of pull back and dividing it by the pitch between imaging planes. A rate of 520 rev/sec with 512 vectors per revolution results in a processing speed of 266240 vectors/sec (520 rev/sec*512 vectors/rev).

A second example can be given to illustrate operation of system100at an even higher speed. In this example embodiment, in order to further limit the total amount of saline infusion, scanning of length304takes place in 4 seconds, at a pull back rate of 12.5 nun/sec. With the same pitch as the previous example, the rotational rate of inner core106is 1040 rev/sec, or 12.5 mm/sec divided by 0.012 mm/rev. With 512 vectors314per revolution, system100processes at a rate of 532480 vectors/sec, or twice as fast as the example embodiment depicted above.

These two examples serve to illustrate the relation between length304, scanning time, lumen size, pitch and the number of vectors314in an individual rotation. These values will vary depending on the size of lumen302and the needs of each particular application. For example, a shorter length304than 50 mm would in turn allow more vectors314per revolution, a lower pitch between revolutions, or a lower revolution rate in order to maintain the same processing speed. One of skill in the art will readily recognize the interrelation between these variables and the effects a modification of one would have on the others. Accordingly, the systems and methods described herein are not limited to any one example embodiment and can be adjusted and configured according to the needs of each individual application.

In a preferred embodiment, each revolution of inner core106produces one frame resulting in a frame rate of 520 per second. Each frame is preferably stored in a memory to allow viewing at any rate. Since a typical human perceives motion at a viewing rate of approximately 30-35 frames/sec, imaging system200can be configured to display these frames from memory at a rate equal or near this basic viewing rate. However the frames can be viewed in real-time (520 frames/sec) or any other desired rate according to the needs of the individual application. At a frame rate of 32 frames/sec, the viewing of the scanned frames from length304would take approximately 130 seconds.

Referring back toFIG. 2, imaging system200is optically coupled with proximal end204of catheter102through drive system206. Control system202controls the operation of drive system206including the rotation and translation rates of inner core106. Drive system206includes housing212and rotary joint214. In order to allow the dynamic high speed operation of system100, drive system206preferably includes one or more high speed ball bearings208and one or more high speed seals210. Bearings208are preferably retractably coupled between rotary joint214and housing212and are configured to rotate at the rate required by the individual application. For instance, in an embodiment capable of performing in the range of speeds demonstrated in the above two examples, bearings capable of rotating in the approximate range of 30,000 to 65,000 rpm are preferably used. In addition, high speed seals210can also used to prevent saline or other infusion fluids from escaping from within member104into drive system206.

In another embodiment of system100, a lubricant, such as a silicone-based lubricant, is placed in the space between elongated member104and inner core106to combat the friction effects resulting from the high rotational rate within catheter102. In an example embodiment, the diameter of inner core106is approximately 0.6 mm for a 3 french catheter102, which translates into 1.9 mm of circumference and a relative speed of approximately 1 meter/sec at a rotational rate of 520 rev/sec and 2 meters/sec at a rotational rate of 1040 rev/sec. These rates can result in significant friction, especially as the rotational rate increases or the diameter of inner core106increases. In addition, to combat friction, the space between member104and inner core106, i.e., the difference between the respective diameters, can be enlarged to reduce the amount of contact between member104and inner core106. In this embodiment, a separate lumen is contained within member104and connects flush port218with opening110in order to allow saline to be infused without mixing with lubricant.

FIG. 4depicts an example embodiment of imaging system200and catheter102configured to image lumen302at high speed. In this embodiment, imaging system200is an interferometric OCT imaging system; however, system200is not limited to this form of OCT imaging and can be used with any imaging technique that allows for high speed imaging. Interferometric imaging system200includes wide band light source402, beam splitter404, diffraction grating406, mixer408, filter410and detection array412. System200allows the generation of a three-dimensional image of lumen302with only one-dimensional scanning, through the use of parallel processing and time gating. The use of time gating has been discussed in detail in Yasuno, et al.,Spectral Interferometric Optical Coherence Tomography with Nonlinearβ-Barium Borate Time Gating, Optics Letters, Vol. 27, No. 6, Mar. 15, 2002, and the use of parallel processing has been discussed in Zeylikovich et al., System and Method for Performing Selected Optical Measurements on a Sample Using a Diffraction Grating, U.S. Pat. No. 5,943,133, both of which are expressly incorporated by reference herein.

Wide band light source402generates wide band light beam pulse420and directs it towards beam splitter404, which, in this embodiment, is a 50/50 beam splitter. Beam splitter404splits beam420in two, generating tissue pulse422and reference pulse424. One of skill in the art will readily recognize that interferometric OCT systems can be implemented with various beam splitting ratios and accordingly, system100is not limited to any one ratio of beam splitting. Light source402preferably generates light beam420in a pulsed manner to allow imaging with high intensity light without significant risk of tissue damage. The reduction in duty cycle of the beam reduces the total energy delivered to the body yet still allows high intensity which is preferably timed to coincide with the acquisition of the reflected pulse. Powering pulsed superluminescent diodes (SLD's) or lasers preferably requires processing system416to generate a timing signal to trigger a discharge type power supply (not shown) and is well known to one of skill in the art. However, light source402is not limited to a pulsed source and can be delivered in a continuous wave format if the characteristics of system100, including the signal-to-noise ratio, are such as to allow the use of a lower intensity continuous source.

Light beam pulse420has a wide bandwidth which is indirectly proportional to the coherence length of the beam and accordingly, allows for higher imaging resolution. Light source402can be any wide band pulsed light source, including a short pulse laser or an SLD. Also, source402can be implemented as an array of multiple SLD's to allow for a wider bandwidth. These embodiments will be discussed in more detail below.

For ease of illustration, light beams are depicted as being directed through free space, however, in a preferred embodiment these light beams are directed with the aid of an optical channel such as a fiber optic. The use of fiber optics can also eliminate the use of mirrors for directing the light pathway. Tissue pulse422is directed through lens L1and into inner core106to imager108, which in turn directs pulse422onto lumen302. Lens L1focuses pulse422in one spatial direction into body lumen or other body tissue302. Tissue pulse422penetrates lumen302and is modulated, or reflected and backscattered, from multiple points within lumen302. Imager108receives this reflected tissue pulse422and directs it back to beam splitter404through inner core106. Reflected tissue pulse422contains the spatial and temporal profiles of lumen302, where the depth information is contained within the temporal profile. Beam splitter404then directs reflected pulse422towards spectrometer414, including diffraction grating406and lens L2.

One of skill in the art will readily recognize the existence of numerous different spectrometer configurations that can be used in interferometric OCT imaging. In this embodiment, diffraction grating406and lens L2spatially decompose reflected tissue pulse422into temporal spectral components which are directed onto mixer408. Mixer408can be any mixer or device capable of mixing, or combining, multiple light pulses and outputting the mixed light pulse. In a preferred embodiment, mixer408is a time gate configured to accept reflected pulse422and reference pulse424and output a light wave426corresponding to the temporal duration of reference pulse424and having substantially the same spatial profile as reflected pulse422.

Time gate408is used to cancel any phase skew in reflected pulse422. This phase skew occurs as a result of the modulation and diffraction of pulse422, as well as the temporal duration of pulse422, which, for instance, can be in the range of several picoseconds and can cause spatial signal shifting on detection array412. A delay line (not shown) is used to adjust the flight time of reference pulse424which is also directed onto time gate408by multiple mirrors M1-M5. In this embodiment, time gate408is a nonlinear β-barium borate time gate crystal and is used to mix the incident tissue pulse422and reference pulse424to generate light wave426. Light wave426is preferably a harmonic wave that corresponds to the temporal duration of the reference pulse424, which, in this embodiment, is approximately 150 femtoseconds (fs). Harmonic wave426has the same spatial profile as tissue pulse422with a shorter duration corresponding to reference pulse424and significantly reduces the phase skew of the incident tissue pulse422. In one embodiment, time gate408is triggered by reference pulse424and mixes tissue pulse422and reference pulse424upon incidence by reference pulse424on time gate408.

The depth structure of lumen302is then Fourier transformed by lens L3and spatially projected through filter410onto detection array412. Filter410is preferably a bandpass filter and is used to eliminate noise and interference components outside harmonic wave426. Detection array412is an array of light detectors such as a charge-coupled device or semiconductor-based imager that is capable of detecting the light intensity of harmonic wave426during the short duration of incidence on array412. Array412preferably includes a sufficient number of light detectors to detect all diffracted portions of reflected tissue pulse422with a degree of resolution suitable for the needs of the application. Array412is communicatively coupled with processing system416, which is configured to process the projected depth structure and assemble the two and three-dimensional images in a viewable format. Processing system416is preferably a computer, but can be any customized or standard data processing system with sufficient capability to process the images at a rate determined by the needs of the individual application.

In some embodiments, the high imaging and acquisition speed of system100can result in more noise throughout the system, and can result in a decreased signal-to-noise ratio (SNR). In one preferred embodiment, the signal strength or intensity, of light source402is increased in order to compensate for higher noise levels, such as by implementing light source402as an SLD array including two or more superluminescent diodes. In one embodiment, source402is two arrayed SLD's with offset center bandwidths. After imaging, processing system416algorithmically fits the spectral density of arrayed source402into a gaussian distribution, effectively creating a gaussian, high intensity, wide beam source. This spectral shaping technique allows system100to raise the signal strength of source402and widen the bandwidth of source402and in turn increase the SNR without losing resolution and accuracy resulting from a non-gaussian spectral density. Spectral shaping is discussed in detail in Tripathi et al.,Spectral Shaping for non-Gaussian Source Spectra in Optical Coherence Tomography, Optics Letters, Vol. 27, No. 6, Mar. 15, 2002, which is expressly incorporated by reference herein. The implementation of this spectral shaping technique in combination with the embodiment of imaging system200depicted inFIG. 4can allow high speed imaging with an increased SNR.

In this embodiment, the combined spectral densities of the two SLD's is calculated by Fourier transforming the interferometric responses of a tissue pulse to a single surface, such as a mirror or glass slide. This combined spectral density can be stored in memory within processing system416. Then, the spectral density is determined from an image created by the incident harmonic wave426by calculating the average square root of the power spectrum. An ideal gaussian source spectrum can then be determined by preferably using the zeroth moment of the spectral density obtained from the lumen response and the first and second moments of the spectral density obtained from the single surface response. The ratio of the ideal gaussian source spectrum and the measured source spectrum defines a spectral correction curve. Multiplying the Fourier transform of each individual image by the spectral correction curve gives the spectrally filtered gaussian response of each image. The coherence function envelope can then be obtained through digital quadrature demodulation.

In this embodiment, the gaussian coherence function of non-gaussian source402can be obtained by Fourier transforming harmonic wave426, applying a correction to each Fourier component and inverse transforming the corrected signal. The signal processing algorithm for this spectral shaping technique is preferably stored and performed in processing system416. Preferably, the SLD's implemented in source402are orthogonally polarized and can be combined using a polarizing beam splitter. Therefore, the improved systems reduce the duration of blood displacement and can create 3D images from an interior body scan.

In addition to the embodiments described herein, system100can be used in conjunction with other features known in the art, such as additional guiding catheters, guidewires and prostheses, including inflatable balloons and stents. Furthermore, the length and composition of catheter102and the constituent components varies on the needs of the individual application. Elongated member104can be made from any suitable material or combination of materials including Pebax 70A, Tecoflex, polyethylene, nylon, hypo-tube, natural rubber, silicone rubber, polyvinylchloride, polyurethanes, polyesters, polytetrafluorothylene (PTFE), and thermoplastic polymers. It can also be formed as a composite having a reinforcement material incorporated within catheter102in order to enhance strength, flexibility, and toughness. Suitable enforcement layers include wire mesh layers and the like.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. For example, the reader is to understand that the specific ordering and combination of process actions described herein is merely illustrative, and the invention can be performed using different or additional process actions, or a different combination or ordering of process actions. For example, this invention is particularly suited for applications involving high speed optical imaging in a catheter, but can be used in any design involving optical imaging. As a further example, each feature of one embodiment can be mixed and matched with other features shown in other embodiments. Features and processes known to those of ordinary skill in the art of optical imaging may similarly be incorporated as desired. Additionally and obviously, features may be added or subtracted as desired. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.