Patent Application
20110077528
The field of the invention is a process and apparatus for optical imaging, and more particularly to intravascular imaging and treatment.
Optical Coherence Tomography (“OCT”) uses light from a broad band light source, which is split by an optical fiber splitter with one fiber directing light to a sample path and the other optical fiber directing light to a reference path mirror or reflective interface. The distal end of the sample path can be interfaced with a scanning device, or the like. The reflected light from the scanning device is recombined with the signal from the reference mirror or reflective interface forming interference fringes, allowing precise depth-resolved imaging or optical measurements to be taken. Conventional OCT thus measures spatially resolved backscattered intensity with a resolution on the order of a few micrometers. Fourier domain OCT (“FD-OCT”), sometimes known as Spectral Domain OCT (“SD-OCT”), can obtain a high sensitivity and imaging speed.
A thrombus, or blood clot, is the final product of the blood coagulation step in hemostasis. It is achieved via the aggregation of platelets that form a platelet plug, and the activation of the humoral coagulation system (i.e. clotting factors). A thrombus is physiologic in cases of injury, but pathologic in case of thrombosis. A “red thrombus” is formed rapidly by the coagulation of stagnating blood, composed mainly of red blood cells rather than platelets and small amounts of fibrin. A “white thrombus” is a thrombus of opaque dull white color composed essentially of blood platelets and only a few red blood cells, cellular debris and fibrin. “Mixed thrombus” can include pink material or discrete regions of white and red thrombus.
Efforts have been made to deduce hemoglobin and thrombi presence from the OCT signal and ablate thrombus. The present invention attempts to solve this problem, as well as others.
A method and apparatus for simultaneous hemoglobin reflectivity measurement and OCT measurement, thrombus detection and treatment, and OCT flushing is described herein. The foregoing and other features and advantages of the invention are apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.
Generally speaking, the method and apparatus for performing simultaneously an OCT scan of a blood vessel and a site-to-site measurement of hemoglobin reflectivity 100 are shown in
The apparatus for simultaneous OCT measurement and hemoglobin reflectivity 100 is derived from any OCT system for imaging coronary arteries comprising a light source, which when incident on hemoglobin provides a source of contrast, wherein the light source may be such that the reflection from hemoglobin is substantially greater or less than light reflected from the luminal surface of a blood vessel. The OCT system may include a Fourier domain OCT (“FD-OCT”), sometimes known as Spectral Domain OCT (“SD-OCT”), or a Time-Domain OCT scanning (“TD-OCT”), where the optical path length of light in the reference arm of the interferometer is rapidly scanned over a distance corresponding to the imaging depth range. The OCT systems may be polarization-sensitive or phase-sensitive and adjusted accordingly.
As shown in
In one embodiment and as shown in
In operation, the apparatus couples the Hemoglobin beam light source (“Hb beam” or “HB light”) into the OCT interferometer 120 so that Hemoglobin beam light 112 enters the sample path 150 of the interferometer and is incident on the specimen 164 being imaged, as shown in
The component for coupling the Hemoglobin beam is an optical element 130 that provides for the simultaneous transmission of the Hemoglobin beam light and the OCT light beam. The optical element 130 may be either a wavelength division multiplexer (“WDM”), a fiber based WDM, or a dichroic filter such as an optical filter element, as shown in
The Hemoglobin beam light reflects from a specimen 164, which includes a hemoglobin sample and returns to the OCT interferometer system 120. The reflected Hemoglobin light may be coupled out of the OCT interferometer in the sample path 150 of the OCT interferometer 120, or in a detection path 170 of the OCT interferometer 120, as shown in
The reflected Hemoglobin beam light is directed into the optical detector 140, which may be a photovoltaic detector 140 that is sensitive to Hemoglobin beam light. Generally speaking, the Hb detector 140 coupled to the Hb light source 112 is not required as long as at least one Hb detector 140 is in the detection path 170. The Hb light 168 that returns from the specimen 164 only couples back to the Hb source path because the circulator is designed to operate at the OCT source wavelength and not necessarily the Hb wavelength. In these cases when the OCT source and Hb wavelengths are different, some Hb light reflected from the specimen (e.g., blood vessel) can couple into the entrance port of the circulator. In most implementations, the Hb detector 140 associated with the Hb source light 112 can removed and one Hb detector 140 in the detection path 170 may be implemented. The reflected Hemoglobin beam may be directed into the photovoltaic detector using either a dichroic filter 142 or a wavelength division multiplexer. The optical element 130 to direct Hemoglobin light into and out of the OCT interferometer 120 could be located before the circulator 132 into port 1 of the circulator, as shown in
In one embodiment, the measurement of the reflected Hemoglobin beam is completed for each OCT A-scan. The reflected Hemoglobin beam that is detected or measured for each A-scan may be integrated over various time periods of the OCT scan. In another embodiment, the reflected Hemoglobin beam is measured over the time period between OCT A-scans, where the time period between OCT A-scans is when the OCT source light is off and the interference fringes are not being recorded. In this embodiment of recording Hemoglobin reflected light between OCT A-scans, the Hemoglobin light does not introduce additional noise into the detection circuitry for the OCT signal. Alternatively, if the Hemoglobin light is recorded over a time period that overlaps with the OCT A-scan, then measures are taken to limit or substantially eliminate any Hemoglobin light from entering the OCT detectors while allowing the OCT light to enter the OCT detectors. In this embodiment, the optical dichroic filters may be used to separate OCT source light and Hemoglobin reflected light into physically distinct circuitry. However, if the wavelength of light used for the Hemoglobin beam and the OCT beam are substantially different, then the detectors' spectral sensitivity may be set to different spectral sensitivities as to not require additional filtering.
Therefore, the Hemoglobin reflection measurement may be performed either concomitantly with the OCT A-scan or between successive OCT A-scans when OCT detection circuitry is not recording signals. In one embodiment, the Hemoglobin reflection measurement is between OCT A-scans and does not interfere with OCT measurement in any way. In another embodiment, hemoglobin and OCT measurements partially overlap. And in one embodiment, Hemoglobin measurement is performed during the OCT A-scan. In another embodiment, the Hemoglobin reflection measurement is performed at any time and the OCT measurement may be disregarded.
The reflected Hemoglobin light beam provides a relative measure of Hemoglobin light at each lateral imaging position. When the Hemoglobin beam is strongly absorbed by Hemoglobin, a relatively weak reflected Hemoglobin signal indicates the presence of Hemoglobin. Alternatively, if the Hemoglobin beam is strongly backscattered by Hemoglobin, a relatively weak signal represents the absence or low levels of Hemoglobin. In one embodiment, if the hemoglobin beam has an optical wavelength of 532 nm, hemoglobin absorbs this 532 nm wavelength strongly and a low signal amplitude represents presence of hemoglobin while a large signal amplitude may represent the presence of white thrombus.
Other causes might be responsible for the strong or weak Hemoglobin signal. For example, the Hemoglobin signal may be weak because flowing blood is absorbing the Hemoglobin beam rather than red thrombus being present. Thus, the OCT image may first identify presence of a thrombus, and subsequently the Hemoglobin beam determines the Hemoglobin content. Likewise, the Hemoglobin signal may be weak because the Hemoglobin beam is not focused well on the thrombus, which could make white thrombus appear as red thrombus due to the lower Hemoglobin signal, even though it's lower due to other effects and not lower due to Hemoglobin absorption. In this case, the OCT signal/image is first used to gauge how those other effects will change the relative values of the Hemoglobin signal before making a determination of the Hemoglobin content. Measuring the Hemoglobin content based on the Hemoglobin signal reflectivity is refined with prior knowledge from the OCT image.
In another embodiment, two or more Hemoglobin signals with different wavelengths could provide additional (differential) discrimination of the Hemoglobin content. The algorithm to decide Hemoglobin content should include both the two or more Hemoglobin signal beams and also the exponential nature of the OCT intensity decrease vs. depth, which are complimentary because the OCT signal provides a measure of scattering and the Hb signal provides a measure of absorption. Using both should provide better sensitivity than one measurement alone.
Thrombus Detection and Treatment
Ambiguity may arise in detected hemoglobin reflected signal intensity due to effects such as blood in the image field, imperfect focusing distance from tissue to catheter, or other. However, these effects will also be apparent in the OCT image. Thus, the OCT image can be used to determine the structural characteristics in the image and to detect whether thrombus is present, and then the Hemoglobin measurement can be employed to detect the Hemoglobin reflectivity at the location of interest. A matrix may help in differentiating white from red thrombus. For each type of thrombus, there is reflectivity of OCT light and the Hb light. For white thrombus, the OCT light scatters less strongly in white thrombus so that the vessel wall behind the thrombus is brighter for white thrombus compared to red thrombus. The OCT signal distinguishes between red and white thrombus based on the relative attenuation of the OCT light through the white thrombus. The Hb light will be more strongly backreflected from white thrombus than red thrombus. The white thrombus will not absorb the HB light and thus give rise to a larger backreflected signal of the Hb light. For red thrombus, the OCT light is more strongly attenuated by red thrombus than white thrombus. By examining the attenuation of OCT light through the thrombus, an estimate can be made whether the thrombus is red or white. The Hb light will be strongly absorbed by red thrombus and thus the magnitude of Hb light backscattered from the red thrombus will be less. By combining the OCT attenuation and HB reflectivity, a specific estimate if the thrombus is red or white may be obtained, where white corresponds to relatively low OCT attenuation and relatively high Hb reflectivity and red corresponds to relatively high OCT attenuation and low Hb reflectivity.
The thrombosis may be present in the coronary arteries, cerebral arteries, peripheral arteries, pulmonary arteries, venous vessels, or any other vessels subjected to thrombosis. The mechanical properties of the thrombus change with age, so fresh thrombus is mechanically softer and more pliable than older thrombus, which becomes harder and less pliable. Because older thrombus is less pliable and more rigid, risk of an infarction is greater with old thrombus that is mechanically dislodged from the vessel lumen. For these reasons, thrombolysis of older thrombus is especially important and targeting of this older thrombus can be accomplished by the method and apparatus allows simultaneous visualization of the intravascular thrombus while conducting laser thrombolysis. The more rigid thrombus may be detected by birefringence, polarization-sensitive OCT, which more readily understood by nonprovisional application entitled “Fiber-Based Single Channel Polarization-Sensitive Spectral Interferometry”, U.S. Ser. No. 12/131,825, filed Jun. 2, 2008, incorporated by reference herein. Additionally, differences in the thrombus, such as the amount of platelets or concentration of hemoglobin may be determined by the OCT imaging and hemoglobin beam. The intracellular components may also be discerned depending signal intensity, polarization state, birefringence, and phase-sensitive information obtained and described herein.
An OCT image of white thrombus 200 is shown in
The method and apparatus allows simultaneous visualization of the intravascular thrombus while conducting laser thrombolysis 300, as shown in
Alternatively, the thrombus laser source 314 is coupled to the OCT interferometer 320 in the sample arm 350 before the Rotary/Pullback Motors 360, as shown in
Alternatively, a dichroic beamsplitter 328 separates the reflected Hemoglobin beam 368 before entering the circulator 332 and directs it to the Hemoglobin reflection detector 340 through a PBS 322, as shown in
The thrombus laser 314 can emit optical energy over a multiplicity of optical wavelengths, frequencies, and pulse durations to achieve the controlled heating of the red and white thrombi. The thrombus laser source 314 is further explained below. In one example, the heating of the thrombi with green light near the green spectrum can be used to cause ablation of the thrombus by light absorption by red blood cells and/or platelets located in the thrombi. In order to achieve heating of the thrombi and lyse the thrombi, the pulse duration is selected by the optical absorption length (δ) of light in the thrombus or the lateral spotsize of light incident on the thrombus (d). The mechanism of breaking the thrombus into smaller sized fragments is one by which light is absorbed by the thrombus; the absorbed light generates thermal injury in the thrombus that results in thermal elastic expansion of the thrombus material; the thrombus material that is mechanically damaged is fragmented into a piece of material. By continuing with this process the thrombus is fragmented into smaller parts or micron-sized pieces. The principle of selective photothermolysis can be used to specify the appropriate pulse duration for targeted particles or clusters of particles of a given size, which is further explained below.
Selective Photothermolysis
Selective pulsed laser photothermolysis can be used to heat the thrombi and selectively injure and/or kill these cells. By absorbing light energy, the thrombi or clusters of thrombi temperature increases and can induce explosive vaporization of a thin layer of fluid in contact with the thrombi, as to cause a microexplosion within the cell. A conventional vapor bubble can be created that expands on the nano-second timescale as the initial high vapor pressure overcomes the surface tension of the fluid. The expansion and collapse of bubbles can also cause a second shock wave that travels outward and interacts with the cell to disrupt the cellular membrane. Thrombi that have hemoglobin can be killed, while adjacent cells can remain viable. Additionally, the heating energy, for example, a pulsed laser light can be used to selectively heat the thrombi to induce apoptosis, protein inactivation through denaturation or coagulation of protein form increased temperature of the thrombi by the pulsed laser, or damage to specific cellular structures by the interaction of the heated thrombi and cellular structures.
A spatially localized temperature increase can be generated within individual macrophages or other cells when incident photons are absorbed by the thrombi. Spatially selective confinement can be accomplished by using laser dosimetry with a wavelength that is absorbed by the thrombi and pulse duration for spatial confinement within the macrophage or other cells. Selection of appropriate pulse duration can be used to allow application of the principle of selective photothermolysis so that temperature increase can be confined more to thrombi or been targeted by the thrombi. Neighboring cells not comprising the hemoglobin can be spared.
The principles of selective photothermolysis can be used to determine the proper lysing protocol or parameters. Using selective photothermolysis, four exemplary parameters that can be determined in selecting a killing protocol include wavelength of the energy source, dose (energy/area), pulse duration, and spot size. To select an appropriate wavelength, the absorption properties of the particle or cluster of particles and the cell and/or surrounding tissues can be determined. A wavelength of killing light energy can be selected to be more strongly absorbed by the particle than the cell or any surrounding tissue or any tissue or composition between the source and the particle. For example, the absorbance spectrum of fat, normal aortic tissue and oxygenated hemoglobin are known and nadir at about 700 nm. Although water has a nadir at about 500 nm, its absorbance is negligible at about 700 nm, and non-existent at 532, as shown in
Thus, wavelength can be determined based on the absorption of the targeted particle and the absorption of other compositions in the subject, such as tissues, endogenous chromophores, protein composition, or any other absorptive characteristic of the subject imposed between the energy source and the target particle. The pulse duration can be determined by estimating the thermal relaxation time of the target particle. Thermal relaxation time can be based on the geometry of the particle and the diffusion of heat into media or tissue surrounding the target particle. An appropriate dose can also be determined. The dosage used can be related to the pulse duration. As pulse duration is lessened, the temperature used to kill a cell can be elevated. The change in temperature used for a given pulse duration for killing a cell can be determined by using the Aharenius damage integral, which is known to those skilled in the art. The spot size used can also be related to fluence. Thus, a desired spot size can be selected based on the desired fluence. Spot size can be selected to be approximately equal to the depth of the targeted cells.
A detectable internal strain field can be generated in the thrombi when a metallic composition, i.e. hemoglobin, is under the action of an external force or energy. The internal strain field can be detected using phase sensitive OCT using block correlation signal processing techniques that have been applied in elasticity imaging in ultrasound imaging. The external force may be provided by the application of the thrombus laser, i.e. a pulsed light source can be applied to the thrombi and a thermoelastic strain field can be detected with phase-sensitive OCT system, which can be readily understood by U.S. patent application Ser. No. 11/784,477, filed Nov. 8, 2007, and herein incorporated by reference. Action of the external force on each hemoglobin can produce movement of the hemoglobin (znp(t)) that produces a change in the cellular membrane tension level or an internal strain field within a cell. Action of a force on each hemoglobin in a thrombi or tissues produces a movement of the hemoglobin (znp(t)). Movement of the hemoglobin can be along the z-direction. The hemoglobin can also have movement in any direction that can be written as vector displacement, unp(ro) for a hemoglobin positioned at ro. Hemoglobin displacement unp(ro) can produce a displacement field (u(r,ro)) in the proteins in the thrombi containing the hemoglobin and surrounding cells. In the case of a homogeneous elastic media, the displacement field (u(r,ro)) can be computed for a semi-infinite half-space following, for example, the method of Mindlin (R. D. Mindlin, “A force at a point of a semi-infinite solid”, Physics 1936, 7:195-202, which is incorporated by reference for the methods taught therein). In the case of an inhomogeneous viscoelastic media, a finite element method numerical approach can be applied to compute the displacement field in the cell. The displacement field (u(r,ro)) produced by a hemoglobin positioned at ro can induce an internal stain field that is determined by change in the displacement field along a particular direction. The strain field (εij(r,ro)) is a tensor quantity and is given by Equation (1),
where ui(r,ro) is the i'th component of the displacement field and xj is the jth coordinate direction. For example, when j=3, x3 is the z-direction. The internal strain field in thrombi due to all hemoglobins in the thrombi and surrounding thrombi is a superposition of the strain fields due to each hemoglobin. A detectable change in thrombi can also be caused with light energy. For example, pulsed laser light can be applied to contact the hemoglobin comprised by a thrombi. The application of light energy can cause a detectable change in optical path due to a change in optical refractive and thermal elastic expansion. The light energy can also cause motion of the cell, particle, or tissues proximate to the thrombi for detection by optical coherence tomography. Such movement can be caused by thermal elastic expansion. Alternatively, sound energy can motion of the cell, particle, or tissues proximate to the thrombi for detection by optical coherence tomography.
The change in strain field surrounding the thrombi can be detected using phase-sensitive optical coherence tomographic imaging modalities. In this approach phase sensitive interference fringes can be detected before and immediately after the application of a force on the hemoglobin. Utilizing block correlation algorithms for ultrasound elasticity imaging of spatially-resolved interference fringes recorded before and after application of a force on the hemoglobin particle can be used for determination of the spatially resolved strain field surrounding the cell. Thus, the thrombi can be detected by detecting the change in the thrombi caused by the interaction of the pulsed light energy causing a change in the thrombi with the hemoglobin using such a modality. The spatially resolved strain field due to application of the external force can be detected using a phase sensitive optical coherence tomographic imaging modality. Phase sensitive OCT imaging modalities can comprise a probe for transmitting and receiving light energy to and from the cell. The light energy used for OCT imaging modalities can be distinct from the light energy used to cause a change in the thrombi as would be clear to one skilled in the art. Thus, the OCT modality can use light energy for detection of the thrombi that is typical of OCT imaging systems. The systems described herein can also be used with a light source for causing a change in the cell. OCT imaging light energy can therefore be distinguished from light energy or energy that causes a change in the thrombi or thrombi changing energy. The probe can be sized, shaped and otherwise configured for intravascular operation. The probe can further comprise a magnetic source for applying the magnetic field to the cell. The magnetic field can be applied to the thrombi from a magnetic source located external to the subject or internal to the subject. The external source can be located in a probe or can be distinct from a probe. The external force can also be the application of pulsed laser light that is selectively absorbed by the hemoglobin of the thrombi and that generates a thermoelastic strain field surrounding the composition or particle. By recording images before or after pulsed laser exposure, the thermoelastic strain field in the tissue may be determined using block correlation algorithms applied for ultrasound elasticity and thermal imaging.
Atherosclerotic rabbit thoracic aorta injected with Iron Oxide Nanoparticles and saline 48 hours prior to imaging with optical coherence tomography and after injection.
OCT temperature measurement may also be employed by recording an A-Scan or B-scan before and a second after pulsed laser excitation, where the OCT system is a phase sensitive OCT system. After recording scans before and after pulsed laser excitation, the interference fringe signals are correlated using a block correlation algorithm and then differentiated with respect to tissue depth to obtain a measure of the relative phase change due to the pulsed laser excitation. With a calibration of the combined thermo-refractive change and the thermo-elastic displacement, a depth resolved estimate of temperature resulting from the absorption of pulsed laser light is obtained Lipids in an atherosclerotic lesion might be more easily identified by an anomalous thermo-refractive and thermoelastic change.
Ablation Threshold
The ablation threshold of thrombus material is given by the partial vaporization theory. In this theory, light absorption leads to the generation of heat and rapid expansion of water to a vapor bubble. The rapid expansion of the vapor bubble leads to mechanical failure of the thrombi membrane and lysing of the thrombus in the region of light absorption. Ablation threshold may be predicted by the partial vaporization theory, which states that vaporization of water occurs when the temperature is raised to 100° C. The full energy of vaporization is not required before certain nucleation sites begin to form vapor bubbles. Thus the onset of ablation can be predicted by the following Equation (2):
where Eth is the energy required to reach ablation threshold, ρ is the density, c is the specific heat, ΔT100 is the number of degrees needed to reach 100° C., and μa is the absorption coefficient. This theory applies when the laser pulse is thermally confined, i.e., when the laser energy is deposited in the target absorber before the resultant heat has time to diffuse. Thermal confinement is achieved when the following Equation (3) is satisfied:
τp<τth; (3)
where τp is the laser pulse duration, τth is the time of thermal confinement. Thermal confinement time may be limited by the absorption depth (δ) or the lateral spotsize (d) of light incident on the thrombus. The laser pulse duration (τp) is selected to be less that the thermal relaxation time. The thermal relaxation time (τth) relevant to selecting the laser pulse duration is the lesser of the longitudinal (δ2/4χ) or lateral thermal relaxation time (d2/16χ) where χ is the thermal diffusivity of the thrombus material (˜0.14 mm2/s).
Then according to the following table, the fluence rates (Power Density or Intensity) for Combined Thrombus Laser/OCT Thrombolysis can be calculated as shown in Table 1:
The last column represents the power density in a silica optical fiber with a 10 micron core diameter—similar to the Single Mode Fiber-28 (“SMF-28 fiber”) used for the OCT system for the pulse energy in the fourth column. A safe threshold for staying clear of damage in silica fibers is: 5×108 W/cm2. Based on the calculation in the Table 1 above, the pulse duration is at least about 100 μs or longer. When this pulse duration is longer than one OCT A-scan, a complete A-scan cannot be recorded between the pulsed laser irradiation because the ablating pulse is longer than one A-scan. In these cases the OCT imaging and laser thrombolysis may have to be performed at alternating times
If the spot diameter of focused OCT light on the luminal wall is assumed to be 50 μm or is 1.96×10−3 mm2, then the energies required at the ablation threshold can be computed, as shown in Table 2:
For each pulse duration, the method and apparatus of laser thrombolysis can achieve sufficient ablation. The time to lyse a thrombus is given by the ablation efficiency is about 2 mg/mJ more or less independent of the pulse duration. Ablation efficiency is the mass in grams of tissue removed by the laser per energy of laser pulse used.
The user can then select the thrombus laser source 314 to emit a pulsed laser light energy. In accordance with one exemplary protocol, the thrombus laser source 314 can be in the green spectrum of optical energy, approximately 532 nanometers and a pulse duration of about 200 microseconds. The pulsed laser green light is incident on the thrombus, absorbed and causes a temperature increase leading to vapor formation and lysing of the thrombolytic material. Higher temperature increased can also be achieved. For example, temperatures increases up to and greater than 65 degrees C. can be achieved. Different wavelengths of light can be used to identify and heat the red, white and mixed thrombi. Wavelength sensitivity of different types of thrombus can also enhance the specificity of lysing red, white and mixed thrombi. Additionally, the user may select the power, pulse, and wavelength of the laser depending on the stage of the thrombosis. Higher power or an increase in frequency may be needed for late stage thrombosis, while lower power and frequency may be appropriate for early stage thrombosis.
The thrombus laser beam may be derived from various pulsed laser light sources include q-switched, free-running, intracavity frequency doubled lasers, femtosecond lasers, diode pumped fiber lasers, UV excimer lasers and the like. In one embodiment, apparatus combines novel diode pumped fiber lasers that can produce diffraction limited (M2<1.5) high energy pulses (mJ) with a 100 μsec to 10 msec pulse duration that are absorbed by the thrombus material, such as hemoglobin and platelets. The diode pumped fiber laser sources are unique and ideally suited for OCT guided laser thrombolysis as they can provide diffraction limited ablative laser pulses of the appropriate pulse duration (100 μsec-100 msec), energy (5-20 mJ), and wavelength (400-1000 nm). A wavelength in the 350-600 nm regions is selected for laser thrombolysis to selectively ablate red and pink thrombus without incurring injury to the vessel wall. An exemplary diode pumped fiber laser 400 is shown in
Exemplary Yb-Doped Fiber Laser
The Yb-doped fiber laser 410 as shown in
An exemplary FOPA or masteroscillator power amplifier (“MOPA”) 500 is shown in
In order to couple efficiently light into a single-mode optical fiber such as those utilized in OCT intravascular imaging systems, a high beam quality is desired. The diode pumped fiber laser source provides a near diffraction limited beam quality at the fiber output. If only the fiber core diameter and the numerical aperture are known, and a step-index multimode fiber is assumed. There is no formula to exactly calculate the beam quality in that case, because it depends on the distribution of optical power over the fiber modes, and this distribution itself depends on the launching conditions. However, the beam quality M2 factor can be roughly estimated, assuming that the power is well distributed over the modes, so that the numerical aperture represents a reasonable (perhaps slightly too high) estimate for the actual beam divergence. This leads to the Equation (4):
where a is the fiber core radius (i.e., half the core diameter). Such power dimensions should be accounted for to couple efficiently light from the thrombus laser into the optical fiber used in the OCT system. M2 factors near unity correspond to diffraction limited beams. Use of large mode area fiber lasers and amplifiers (Yb fiber lasers and amplifiers) allows producing near diffraction limited beam quality of the thrombus laser light that can then be coupled efficiently into a single mode optical fiber such as that used in an intravascular OCT imaging system.
The method of simultaneous visualization of the intravascular space while conducting laser thrombolysis 600 comprises performing an intravascular OCT pullback image of a vessel being interrogated, generally shown as a flowchart in
Alternatively, the laser thrombus method and apparatus can be combined with a distal embolic protection device 390, generally shown in
The method and apparatus comprise a combined action of an intravascular OCT system with a thrombus laser with nearly diffraction limited (M2<1.5), high average power (hundreds of watts), high repetition rate, long pulse duration, high pulse energy laser sources for thrombolysis. At the back-end of the system, there could be a software modification or imaging processes to overlay the image of the OCT thrombus image before thrombolysis and after thrombolysis.
Differential Detection, Staging and Treatment of Intravascular Thrombus
The present system and method is well suited to detect and intravascular thrombus in the arterial or the venous system, including within both high pressure pulsatile flow vessels such as the coronary arteries and within low pressure non-pulsatile flow vessels such in leg veins or venous grafts. A primary trigger for arterial thrombosis is the rupture of an atherosclerotic plaque, which develops through the accumulation of lipid deposits and lipid-laden macrophages (foam cells) in the artery wall. The thrombi that form at ruptured plaques are rich in platelets, which are small (about 1 μm in diameter) anucleate cells produced by megakaryocytes in the bone marrow. These disc-shaped cells circulate in the blood as sentinels of vascular integrity and rapidly form a primary haemostatic plug at sites of vascular injury. When an atherosclerotic plaque ruptures, platelets are rapidly recruited to the site, through the interaction of specific platelet cell-surface receptors with collagen and von Willebrand factor. After this adhesion to the vessel wall, the receptor-mediated binding of additional platelets (termed platelet aggregation) then results in rapid growth of the thrombus. Platelets also become activated at this stage. A major pathway of activation involves the cleavage and, consequently, the activation of the platelet receptor PAR1 (protease-activated receptor 1; also known as the thrombin receptor) by the protease thrombin (also known as factor II), which is activated by the blood coagulation cascade. Activated platelets then release the contents of granules, which further promote platelet recruitment, adhesion, aggregation and activation.
The coagulation cascade is the sequential process by which coagulation factors of the blood interact and are activated, ultimately generating fibrin, the main protein component of the thrombus, and this cascade operates in both arterial and venous thrombosis. The cascade is initiated by exposure of the blood to tissue factor (also known as factor III), a protein that is present at high concentrations in atherosclerotic plaques. Circulating tissue factor is also present at increased concentrations in patients with cardiovascular disease and might contribute to thrombosis after plaque rupture.
Venous thrombosis can be triggered by several factors, including abnormal blood flow, e.g., the absence of blood flow; altered properties of the blood, e.g., thrombophilia, or alterations in the endothelium. In venous thrombosis, the endothelium typically remains intact, but is converted from a surface having anticoagulant properties to one with procoagulant properties.
Thrombus staging may be accomplished by the OCT system and method of the present invention in which the differential cellular characteristics of disorganized early stage thrombus are employed to discriminate from the more highly organized later stage thrombus. It is generally understood that earlier stage thrombus is formed proximally on a lesion relative to the later stage thrombus. Additionally, disorganized early stage thrombus is more susceptible to thrombolysis using lower energy or pulsed laser energy than that required to lyse later stage thrombolysis.
Thus, an initial OCT imaging pass may be made to identify and stage the thrombus lesion as being early stage, later stage or a combination thereof. Once the staging information is obtained and the position of the respective staged thrombus determined, either the power of or the pulse rate of the Thrombus Laser is adjusted to correlate to the type of thrombus and applied to the thrombus. In one embodiment of the inventive method, the power to the Thrombus Laser is reduced to lyse the earlier stage thrombus and clear it from the lesion. In another embodiment of the inventive method, the Thrombus Laser is pulsed at a rate to differentially lyse the early stage thrombus and clear it from the lesion. In yet another embodiment of the inventive method, the power and/or pulse rate is then increased to lyse the later stage, more highly organized thrombus.
IVUS/OCT Catheter
Additionally, a combined IVUS/OCT catheter may first identify suspicious thrombi regions with Intravascular Ultrasound (“IVUS”). An exemplary IVUS/OCT catheter is readily understood by nonprovisional application entitled “OCT-IVUS Catheter for Concurrent Luminal Imaging”, U.S. Ser. No. 12/173,004, filed Jul. 14, 2008. In a first step, the thrombus is identified and imaged with IVUS. The elastic properties of the thrombus can also be estimated to determine if the thrombus is soft or rigid. In a second step, a limited volume contrast injection is used and the thrombus can be imaged with OCT. After imaging the thrombus with OCT and making a determination of the thrombus color (white or red), the thrombus laser may be applied to break the thrombus into small fragments for removal. The procedure is: (1) first image with IVUS; (2) image with OCT; (3) identify thrombus type; and (4) perform laser thrombolysis.
The combined IVUS/OCT catheter is able detect the mechanical properties of the thrombus using IVUS elasticity imaging. The mechanical properties of thrombus change with time and the thrombi become more rigid over time. Elasticity imaging of thrombi to detect the mechanical properties of the thrombus examines how the thrombus responds to the flush in the OCT image. Rigid thrombus do not show surface distortion and tend to displace more readily as opposed to soft thrombus, which would indicate more surface distortion and less gross displacement in response to a flush. To differentiate between soft and rigid thrombus the examination of how the thrombus responds to the momentum of the flush material—if the response is greater surface displacement and less gross movement or movement of the center of mass, then that indicates soft thrombus. While if the thrombus surface does not distort and there is gross displacement, which indicates rigid thrombus.
Ultrasound elasticity imaging to noninvasively detect and age thrombus knowing that thrombi harden over time is useful, but the technique relies on whether the age of a thrombus can be predicted from strain estimates, and how accurate these predictions are. Thrombus hardness can be quantified at each scan interval by measures of normalized strains and reconstructed relative Young's moduli. Strain magnitudes exhibit progressive decrease as clots age and the relationship between the normalized strain and the clot age can be developed. Elasticity imaging is a key component of venous compression ultrasound for effective diagnosis and treatment of thrombosis.
OCT Flushing
Hemoglobin reflectivity can be employed to detect start and stop of flush in vessel lumen during OCT imaging. The Hemoglobin reflectivity measurement is expected to be at a maximum during periods before and after the saline flush, and drop substantially when the flush bolus arrives. Detecting this dramatic signal change will be an effective start signal for OCT longitudinal motion or the pullback device of the OCT catheter system.
One embodiment of the method for detecting the start and stop of a flush during OCT imaging 700 comprises reducing the total volume-per-bolus of blood-clearing fluid delivered during intravascular optical imaging, generally shown in
Another embodiment of the method for detecting the start and stop of a flush during OCT imaging comprises coupling the imaging system in communication with an electro-mechanically controlled syringe pump or “power flusher”. The syringe reservoir is pre-loaded with clearing fluid and injects only as requested by the imaging system. The total imaging time and region, the number of intervals, the time between intervals, and other flushing parameters (flow rate, etc.) are controlled by the operator at the physician's discretion. A single command starts the sequence, and control of the flushing and imaging location is automated by the imaging system. A cancel command is available to the operator for interrupting the sequence once it is started. Because a lag may exist between the injection command and clearance of the blood downstream, movement of the imaging transducer may be given a corresponding lag. The lag time can be a function of the transducer position relative to the distal end of the guide catheter (where the fluid is delivered).
Another embodiment of the method for detecting the start and stop of a flush during OCT imaging comprises positioning the imaging transducer in a fixed longitudinal position and continuously acquiring images. A real-time image processing algorithm analyses the incoming images and determines if the vessel is cleared or not cleared of blood. Only during intervals when the algorithm detects a cleared field is longitudinal translation of the imaging transducer initiated and longitudinal pullback begins. Because the imaging system automatically responds to the presence or absence of blood in the imaging field, the physician or operator is free to select the bolus volume/time, pause periods, number of intervals, etc. without directly interacting with the imaging system.
Finally, for any method used for generating the flush intervals, images of non-flushed vessel at the start and end of each interval can be cropped out and all intervals combined into what effectively appears to the user as a single image sequence of the total region of interest. An EKG can be in communication with the imaging system for reduction of image artifacts due to cardiac motion. For example, the start of imaging interval N can be constrained to the same EKG phase as the end of the N-I interval, effectively making the image sequence appear more continuous. The pullback transducer can reverse a small distance between flushing intervals and intentionally overlap the end of the previous interval with the start of the next interval. Overlapped regions can be automatically or manually cropped as necessary to insure optimal continuity in the final sequence.
EKG synchronization to alleviate registration artifacts due to the catheter sliding longitudinally back and forth during heart beat motion may be coupled with the method. If the thrombus is imaged first but then lysed on a second pull-back, the lysing beam should only fire at the same point in the EKG at which the image was acquired (or the pullbacks should be started at the same phase in the EKG).
This application is a continuation from PCT Patent Application No. PCT/US2009/038832, which was filed on Mar. 30, 2009, and which claims priority to U.S. Provisional Application Ser. No. 61/040,630, filed Mar. 28, 2008, all incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 61040630 | Mar 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2009/038832 | Mar 2009 | US |
| Child | 12892229 | US |
