the following description refers to the accompanying drawings that illustrate certain embodiments of the present invention . other embodiments are possible and modifications may be made to the embodiments without departing from the spirit and scope of the invention . therefore , the following detailed description is not meant to limit the present invention . rather , the scope of the present invention is defined by the appended claims . it should be understood that the order of the steps of the methods of the invention is immaterial so long as the invention remains operable . moreover , two or more steps may be conducted simultaneously or in a different order than recited herein unless otherwise specified . fig1 a illustrates a portion of an imaging probe 10 a , using a conventional ivus ultrasonic transducer 12 , an optical transducer 14 which includes an angled - tip optical lens assembly 16 attached to a single mode fiber 18 , a standard miniature rf cable 20 delivering power to the ivus ultrasonic transducer , and a torque cable 22 providing a stable revolution rate to the assembly . torque cables are generally preferred in this dual probe catheter as the optical fiber is known to have a very low torsional ( rotational ) stiffness . for example , a 1 cm length of standard telecomm fiber 125 μm in diameter with approximately 1 millionth of a n - m of applied torque will twist one degree . therefore , it is unrealistic to expect the fiber to be sufficiently torsionally rigid to drive the complete assembly . in fig1 a , both the optical transducer 12 and the ivus ultrasonic transducer 14 are angled to minimize unwanted parasitic reflections from reaching the respective transducers , and to create an aligned cross - sectional “ cut ” through the tissue . as shown , the acoustic beam ( ab ) emanating from the transducer is parallel to optical beam ( ob ) emanating from the fiber . the direction of these two parallel beams is rotated by an angle α relative to the longitudinal axis of the probe . as shown in the figure , a small amount of longitudinal displacement is acceptable . as a first order approximation , this allowable displacement is the approximate maximum beam width of the combined probe 10 a . in most cases , this will be the width of the ultrasound beam , which typically has a width of ˜ 100 to 300 um ( the oct beam width is typically 25 um ). keeping the longitudinal displacement below this longitudinal displacement limit ensures the beams remain overlapped . furthermore , having the two beams at 180 degrees opposite to each other ensures easier real - time or post - processing alignment of the two images for an overlay display . fig1 b depicts a probe 10 b for imaging whereby the overall diameter is reduced . here , a metal coated fiber 24 is shown inside an insulated tube 26 . these two cylindrical surfaces ( tube and coating ), the dielectric constant of the insulation , and the insulation thickness can be configured to form a simple coaxial transmission line for the rf signals . such rf signals may vary from 10 to 60 mhz depending on the ivus ultrasonic transducer design . fig1 c illustrates another probe embodiment 10 c with a different conduction mechanism . specifically , in the probe 10 c shown , the inner 28 and outer 30 coils of a torque cable 22 form a coaxial transmission line 32 . an insulated spacer 34 is inserted between the inner and outer coils to prevent a short circuit condition . the embodiment shown in fig1 c allows rf power to be transmitted using an integral torque wire . in one embodiment , the transducer is coated with epoxy . in one embodiment , both the ultrasound transducer and the optical fiber rotate together , being driven by the same torque wire . the distal tip epoxy encases the optical fiber , the ultrasound transducer and its associated supply wires . hence , the epoxy is selected for suitable optical and acoustic properties , as well as the required electrical insulation . various epoxies and silicon compounds can be purchased and / or specifically tailored that meet these requirements . fig1 d illustrates a cross - section of the embodiment of fig1 c . the two wires connected to the transducer shown in fig1 c and 1d are rigid and rotate with the transducer . fig1 e illustrates another optical probe embodiment wherein two ivus ultrasonic transducers t 1 , t 2 operating at different frequencies are integrated in the device . the lower frequency transducer t 1 allows for ultrasound with s deeper scanning range , but lower resolution . conversely , the higher frequency t 2 transducer allows for ultrasound with increased resolution but less depth penetration . in one embodiment , one transducer operates at about 5 mhz and the other transducer operates at about 60 mhz . by using transducers with differing frequency ranges , an optical probe gains the advantages of both transducers , and mitigates disadvantages of each transducer , respectively . this dual transducer probe achieves the same overall goals as the combined oct / ivus catheter in the case where very high resolution (˜ 10 um , oct ) is not needed in favor of very high penetration (˜ 3 - 5 cm ) offered by a lower frequency ultrasound transducer . fig2 depicts a probe embodiment 40 that incorporates a mechanism for transmitting both rf energy and optical energy to the rotating assembly . specifically , a transformer scheme is used wherein a first coil 42 is attached to the rotating assembly 44 , and a second coil 46 is integrated with the connector 48 of the optical probe . this configuration has the advantage that both coils move with the assembly during a ‘ pull - back ’ ( longitudinal ) scan operation . such pullbacks are used in both oct and ivus scans . when coupled with a rotation , a spiral scan pattern is created inside the arterial lumen . however , this approach results in an increased cost for a one - time - use catheter . fig3 illustrates an alternative coupling scheme wherein the fixed coil 42 is part of the drive unit 50 ( motorized assembly providing rotational and longitudinal motions ). in this embodiment , the fixed coil is permanent , and must be long enough to efficiently couple the rf energy into the rotating catheter coil over the entire pullback length . although incorporating the fixed coil to the drive unit imposes additional requirements to the drive electronics , the decrease in catheter usage provides an overall cost savings . currently , conventional slip - ring technology is widely used in the field of optical imaging . alternatively to fig2 and 3 , slip - ring technology can be used in ivus probes described herein . however , for a probe with a centered optics configuration , the slip - ring is more difficult to manufacture than in the ivus - only case . fig4 illustrates an embodiment that includes capacitive micro - machined ultrasonic transducers ( cmut ) 52 integrated in a coronary imaging probe 54 . the advantage of the cmut is the small size of the transducer , which is fabricated via conventional electronic cmos processes . the small size and photolithographic fabrication allows customized arrays of transducers to be built with the drive electronics on the same substrate . in this example , an array is formed in an annular region around the optical transducer . as a result , a co - focused , aligned and combined beam can be formed , which eliminates the need for software registration and removes a potential source of error . however , this probe tip may be larger than the embodiment shown in fig1 . fig5 a illustrates a fused oct - ivus image 56 , wherein the demarcation line 58 is chosen near the oct penetration limit . as shown , by registering the relative images of the ultrasound 60 and the oct scans 62 , it is possible for a clinician to view a composite image that shows additional physiological data . this approach can be used to image subsurface lipid pools . fig5 b illustrates a fused oct / ivus image wherein the oct portion appears in the image center and the ivus portion appears in the periphery . the outer boundary indicates approximately the boundary where the two regions intersect . not shown in the embodiments depicted in the figures is a guide catheter . typically , the guide catheter is a larger bore catheter used to introduce the smaller imaging catheter into the main arterial trunk . from the guide catheter , a flush solution can be expelled to create a clear , blood - free imaging region when oct imaging is performed . alternative embodiments may include a flush lumen within the imaging catheter whereby the flush solution is ejected at the imaging tip rather than from the guide catheter . the aspects and embodiments of the invention can incorporate various components of varying dimension and materials as is known to one of ordinary skill . various specific dimensions and materials are described herein , however , these exemplary materials are not meant to be limiting , but only to evidence additional more specific embodiments . for all of the measurements discussed below , the dimension given also includes a range of greater than about 10 - 20 % of the dimension given and less than about 10 %- 20 % of the dimension given . in addition , for all of the measurements discussed below , the dimension given also includes a range of greater than about 20 - 50 % of the dimension given and less than about 20 %- 50 % of the dimension given . further , in addition , for all of the measurements discussed below , the dimension given also includes a range of greater than about 50 - 100 % of the dimension given and less than about 50 %- 100 % of the dimension given . in one probe embodiment , the viewing window used is a transparent epoxy - based window . further , in another embodiment , the transducers used have a first dimension of about 0 . 1 mm and a second dimension of about 0 . 5 mm . the forward viewing angle is about 10 degrees in one embodiment of the probe . the end - cap used in one probe embodiment includes a metal . the probe can include a hollow core that is substantially filled with an epoxy material in some embodiments . in one embodiment , the width of the shield rf cable is about 0 . 18 mm . it should be appreciated that various aspects of the claimed invention are directed to subsets and substeps of the techniques disclosed herein . further , the terms and expressions employed herein are used as terms of description and not of limitation , and there is no intention , in the use of such terms and expressions , of excluding any equivalents of the features shown and described or portions thereof , but it is recognized that various modifications are possible within the scope of the invention claimed . accordingly , what is desired to be secured by letters patent is the invention as defined and differentiated in the following claims , including all equivalents .
in one aspect , the invention relates to a probe . the probe includes a sheath , a flexible , bi - directionally rotatable , optical subsystem positioned within the sheath , the optical subsystem comprising a transmission fiber , the optical subsystem capable of transmitting and collecting light of a predetermined range of wavelengths along a first beam having a predetermined beam size . the probe also includes an ultrasound subsystem , the ultrasound subsystem positioned within the sheath and adapted to propagate energy of a predetermined range of frequencies along a second beam having a second predetermined beam size , wherein a portion of the first and second beams overlap a region during a scan .