WO2014097086A1

WO2014097086A1 - Interventional x-ray system with automatic centration of roi in centre of image

Info

Publication number: WO2014097086A1
Application number: PCT/IB2013/060903
Authority: WO
Inventors: Erik Martinus Hubertus Petrus Van Dijk; Sander Slegt
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-12-21
Filing date: 2013-12-13
Publication date: 2014-06-26

Abstract

A system including an imager (100) having a radiation source (XR) for emitting a radiation beam (PR). The imager is operable to acquire a current image of an object (PAT) at a current field-of-view (FOV_i) whilst a device (GW) resides in or around said object (PAT). The system includes an apparatus (A) for adapting a field-of-view of the imager, so that a follow-up image is acquirable by the imager at an updated field-of-view (FOV_i+1) that is centered around a new device position or is centered around an expected device position.

Description

Interventional X-ray system with automatic centration of ROI in center of image

FIELD OF THE INVENTION

The present invention relates to an imaging system, to a method of adapting a field of view of an imager, to a computer program element and to a computer readable medium.

BACKGROUND OF THE INVENTION

Minimal invasive interventions such as PCI (Percutaneous Coronary

Intervention) or similar interventions are almost always image-guided. In other words, an x- ray imager or similar imaging equipment is used to acquire a series of images whilst an intervention device such as a catheter or guidewire resides in the patient's body, and an intervention radiologist is navigating said device to a lesioned site such as a stenosis.

However, the image guided nature of such an intervention means that the patient and/or medical personnel is exposed to radiation dosage, which, in the case of the radiologist, can add up considerably over time when the radiologist is carrying out a large number of such interventions. Minimizing both, staff and patient dosage, in a given clinical procedure is therefore desirable in interventional radiology. One way of addressing the dosage issue is the concept of "tight" collimation as disclosed in US2006/0566665. In tight collimation, a relevant region of interest is automatically detected and the x-ray beam of the x-ray imager is limited by operation of a collimator, so that it is only the detected ROI that is irradiated by the x-ray beam. This allows reducing the dosage area product because the irradiated area is minimized.

It has been found however, that radiologists tend to disable the collimation functionality in certain situations to operate with an essentially un-collimated x-ray beam. In other words, the envisaged reduction in x-ray dosage is at times not achieved even though the imager has the (tight) collimator or similar automatic collimator control functionality.

SUMMARY OF THE INVENTION

There may therefore be a need for an alternative imaging system that would invite physicians to make more ample use of an imager's collimation functionality. The object of the present invention is solved by the subject matter of the independent claims where further embodiments are incorporated in the dependent claims. It should be noted that the following described aspect of the invention equally apply to the method, to the computer program element and to the computer readable medium.

According to a first aspect of the invention there is provided an imaging system comprising:

an imager having a radiation source for emitting a radiation beam, the imager operable to acquire a current image of an object (such as a human or animal patient) at a current field-of-view whilst a device resides in or around said object;

an apparatus for adapting a field-of-view of the imager, the apparatus comprising:

an input port for receiving positional information in relation to the device; a tracker configured to register, based on the received positional information, a new device position;

a re-adjuster configured to use the registered new device position to effect a change of a relative position between the radiation source and the object, so that a follow-up image is acquirable by the imager at an updated field-of-view that is centered around the new device position or is centered around an expected device position. Effecting a change of the relative position includes interfacing with corresponding actuators (such as stepper motors or similar) and issuing suitable control commands and or signals.

It has been found that some physicians believe there is a conflict between an environment where the ROI region of interest is changing dynamically and the use of symmetric collimation. The system proposed herein allows addressing of those concerns. In the context of minimal invasive procedures where for example a catheter is navigated through the vasculature of a patient. The region of interest is essentially constantly changing. The region of interest is mainly the area around the catheter' s tip and the area in front of the catheter taking the direction of movement because the physician is most interested in where he is going rather than where he has been. In this case the use of a symmetric collimator can be awkward if the ROI happens to be located at the outer fringes of the currently irradiated volume. However, this situation cannot happen with the presently proposed apparatus because the acquired images are essentially always centered round the catheter's tip either by moving the table on which the patient is deposited or by moving the imager's frame holding at least the x-ray source and, in some embodiment, also the detector. In other words, the system operates to automatically keep the ROI in the center of the imaging frame or field of view of the imager. ROI centered images allow efficient use of even a symmetric collimator to effectively shutter out the radiation around the ROI. According to one embodiment, the system comprises a collimator that automatically limits the irradiated area to the instant ROI ("tight collimation"). Although use of a symmetric collimator is envisaged herein, asymmetric collimation can be used also.

Once the instant ROI has been determined by operation of the tracker, the system's x-ray source and/or table can be moved/shifted to keep the ROI in the center of the FOV. The shifting of imager's frame is a preferred embodiment since a shift of the table (and with it the patient) might interfere with tubes/catheters or other medical equipment running to and from the patient. However in some embodiment it is only the table that is shifted and/or tilted.

According to one embodiment, the shift of the frame is restricted to a planar motion parallel to the horizontal plane of the examination platform/table. In one embodiment, the line between x-ray source and a detector (that receives the attenuated radiation after passage through the patient) remains essentially perpendicular to the examination platform plane during the shift.

However in one embodiment more complicated shifts parallel to a slanted detector plane/platform plane is envisaged. According to one embodiment, the frame and table are moved in combination. Specifically, a shift of the frame is combined with tilting the examination platform. According to one embodiment the frame holding the x-ray source is an articulated imager robot arm that afford shifting x-ray source (and with it the detector) across planes at any angulation in an angular range.

The table or imager frame movement can either be fully automatic or semiautomatic. In the semi-automatic embodiment, the shift occurs only upon a user interaction. The proposed shift may be indicated to the user first on a screen and the frame and/or bed sets into motion only upon the user hitting an "ok" button or upon the user actuating other input means such as a pedal or similar.

According to one embodiment, the imager is an x-ray imager of the C-arm type and the frame holding the x-ray source and detector is a C-arm. However other imagers based on other radiation types and/or of different construction and frame types may also be used in the system proposed herein.

According to one embodiment, the device's current and/or new position is taken relative to a tip portion of the device but other prominent parts of the device can be used as well. The medical device is, in one embodiment, a guidewire but the system can also be used with guide catheters or PCI catheters or indeed with any other devices for introduction into the patient's body given the medical devices have sufficient radiation opacity.

According to one embodiment, the positional information is represented by in- image information in the current image. The tracker operative to image process said image to extract said positional information and to use the extracted positional information when registering the new device position. The in-image information, such as the "footprint" (projection view) of a characteristic part of the device such as tip portion thereof is detected by segmentation (for example grey-value thresholding) in the current image or across series of images. Tracking the position of the device across a series of images allows extracting velocity information to define the device's motion which can used to establish, eg by kinematic extrapolation, the expected new device position for a future time instant. This information can then be combined with the current imager geometry to pinpoint the position of the device in the imager's frame of reference.

According to one embodiment, the system comprises a robot. The imager is communicatively coupled to the robot. The robot is operative to advance the device through the object, the device thereby assuming the new device position and the robot generating a stream of spatial coordinate information, the tracker configured to use said stream of spatial coordinate information when registering the new device position.

Using a robot to navigate the guidewire or other device helps ensuring that movement of the catheter is not too sudden or abrupt for the system to properly follow the movement of the catheter. When using a robot, the positional and or velocity information generated by the robot is harnessed instead of or in combination with the in-image information to compute the device's position in the frame of reference of the imager. In other words there is backflow of positional/kinematic information in relation to the device's position from the robot to the x-ray system.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described with reference to the following drawings wherein:

Fig. 1 shows an imaging arrangement for supporting an interventional procedure;

Fig. 2 shows a detailed view of the arrangement of Fig. 1; Fig. 3 shows the action on the imager's geometry of a control apparatus that is used in the arrangement of Fig. 1;

Fig. 4 shows an operation of the control apparatus according to one

embodiment;

Fig. 5 is a flow chart for a method of adapting a field of view of an imager.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to Figure 1 there is shown an arrangement for image-guided support of an intervention. Broadly, the arrangement includes an x-ray imager 100, an examination table TB, and, in one embodiment, a robot ROB for steering a catheter guidewire GW or a similar medical device.

During the intervention a patient PAT is lying on examination table TB such that the x-ray beam PR emitted by an x-ray source XR passes through the region of interest, that is, the x-ray beam generated by x-ray source XR irradiates one or more clinically relevant organs of anatomic interest. Examples for such an intervention includes PCI

(percutaneous coronary interventions) carried out by an interventional radiologist hereinafter referred to simply as the "operator" or "user".

In coronary and (other) vascular applications, the interventional radiologist typically first gains access to the treatment area (target ROI) by navigating the guidewire GW to the target area. During this navigation phase, the instantaneous ROIs at any one time during the intervention may be defined and identified by the tip portion TP of guidewire GW. In subsequent phases of the procedure, other devices are moved along the path of the guidewire to the target treatment area ROI. Depending on the type of intervention, various types of devices can be used. Examples are catheters for deployment of stents, ballooning catheters, catheters for coiling of cerebral aneurysms or for gluing of arterio-venous malformations (AVMs) in patent's brain.

In the following the term ROI is a context dependent term and is meant to include the relevant tip position of the guidewire GW at any given instant. In other words the ROI changes as guidewire GW progresses from entry point into the patient's body until arrival at the target ROI (the lesioned site). The human vasculature is a complex network of interconnected vessels. The challenge in PCI or similar minimal invasive interventions is for the operator to safely navigate guidewire GW by successfully negotiate a number of bifurcations or shunts in the patient PAT's cardiac vasculature to eventually arrive at the target ROI. In other contexts and for other devices the ROI is defined by reference to other suitable, prominent device parts.

The exploratory phase of the intervention is visually supported by the imager. Imager 100 operates to acquire a sequence of fluoroscopy frames FL (referred to as "fluoro" hereinafter) as the respective devices GW progress through the patient's passage ways.

Although it is envisaged in some embodiments that it is the operator him or himself manually guiding the guide wire GW to effect its progression, in other embodiments a suitably configured robot ROB is used instead. The robot ROB includes an undercarriage on which there is rotatably arranged an articulated arm carrying at one of its end means to guide guidewire GW. According to one embodiment the undercarriage is attached to the examination table TB. In other embodiments the robot is arranged spatially separate and resides at a defined location in the examination theatre away from the imager and

examination table assembly.

Robot ROB's arm and hence the guidewire GW is positionable and movable along any of the three dimensions x, y, z. Robot ROB is switchable into a feed mode to effect the progression or retraction of the guide wire GW. Movement of robot's ROB arm and guidewire GW forward-feed or backward-feed is effected by a robot driver RD. Robot driver RD operates to intercept control commands from a user input means such as a joystick RA communicatively coupled to said driver RD. Motions imparted by the operator's hand on said joystick RA are translated into control commands which are interpreted by the driver RD. In response thereto, lower level electric impulses or signals are then forwarded to corresponding actuators (stepper motors or similar means) in the robot ROB to mechanically effect the user- desired motion that correspond to the joystick motion.

In other words, robot ROB affords to effectively remote-control the guidewire' s GW progression. Using the robots ROB as described herein not only enhances the precision with which the guidewire GW (and eventually the catheter) is navigable but also helps reduce radiation dosage incurred by the operator. This is because the operator does not have to be physically close to the patient but can remain at a radiation safe location away from the patient during the intervention. To this effect an "intervention cockpit" arrangement is used in one embodiment situated remote from the patient and x-ray imager 100. The cockpit arrangement is surrounded by radiation opaque glass and includes a robot control console including the joystick RA. The acquired sequences of fluoroscopic images FL are then displayed on one or more screens M likewise positioned in the cockpit. An example of a robot ROB for remote-controlling interventions is described in US2012/0179167. Imager 100 includes a rigid C-arm CA having affixed thereto at one of its ends a detector D and to the other end the x-ray tube XR and a collimator COL. X-ray tube XR operates to generate and emit a primary radiation x-ray beam PR whose main direction is schematically indicted by p. As will be explained in more detail below with reference to Fig. 2, said vector p indicates the central ray of said x-ray beam. The position of arm CA is adjustable so that the projection images can be acquired along different projection directions p. The arm CA is rotatably mounted around the examination table TB. The arm CA and with it the x-ray tube XR-detector D assembly is driven by a stepper motor or other suitable actuator. Overall operation of imager 100 is controlled by operator from a computer console CON. Console CON is coupled to screen M. Operator can control via said console OC any one image acquisition by releasing individual x-ray exposures for example by actuating a joystick or pedal or other suitable input means coupled to said console CON.

During the intervention and imaging, examination table TB (and on it patient PAT) is positioned between detector D and x-ray tube XR such that the lesioned site or any other related region of interest ROA is irradiated by primary radiation beam PR.

Broadly, during an image acquisition the collimated x-ray beam PR emanates from x-ray tube XR, passes through patient PAT at said region ROI, experiences attenuation by interaction with matter therein, and the so attenuated beam PR then strikes detector D's surface at a plurality of the detector cells. Each cell that is struck by an individual ray (of said primary beam PR) responds by issuing a corresponding electric signal. The collection of said signals is then translated by a data acquisition system ("DAS" - not shown) into a respective digital value representative of said attenuation. The density of the organic material making up the ROI, for example rib cage and cardiac tissue in case of a PCI, determines the level of attenuation. High density material (such as bone) causes higher attenuation than less dense materials (such as the cardiac tissue). The collection of the so registered digital values for each (x-)ray are then consolidated into an array of digital values forming an X-ray projection image for a given acquisition time and projection direction.

Unfortunately, image structures representative of anatomic structures such as blood itself and vessel tissue, are not normally discernible from an ordinary X-ray image without further preparation. This is because said anatomic structures lack the requisite radiation opacity. To address this, an imaging technique has been devised called

angiography. In an angiography, a sequence of X-ray images ("angiograms" hereinafter referred to as "angios" also) are acquired whilst a previously administered contract agent is required to reside at the region of interest ROI. The contrast agent is essentially a "dye" or a radiation opaque fluid, which is administered to the patient manually or preferably by a power injector to so at least temporarily confer, for imaging purposes, the much needed radiation opacity.

In order for the fluoros FL to be capable of showing the relevant anatomic structure at any given instance during the intervention, the x-ray imager's geometry can be changed in manifold ways to so effect the above mentioned change of the projection direction p. Changing the imager's geometry amounts to changing the spatial configuration of the imager's parts relative to each other and/or the patient PAT and can be effected by energizing suitable ones of a plurality of actuators such as stepper motors M1-M9. Other actuators can also be used like ball screw arrangements where appropriate. Changing the imager's geometry includes in particular changing the position of C-arm CA and or rotating same around the patient PAT or, more specifically, around the ROI. The imager's geometry can be defined by a common spatial frame of reference x,y,z that can be used also for defining the position of the robot's arm ROB and hence the position of the guidewire GW measured, for example, by the position of guidewire GW's tip TP in said frame of reference. The common frame of reference is indicted in Fig 1 as directions x,y along the table TB's edges

(longitudinal and across, respectively) spanning the table TB's plane. The third spatial dimension may be taken vertically along direction z perpendicular to said x,y-plane of table TB.

According to one embodiment, a stiff overhead suspension arm SA is arranged as an upside down "L" shape carrying at its lower extremity a cradle CR. The other extremity of L-shaped suspension arm SA is attached to an overhead carriage OHC that is mounted preferably to the ceiling of the examination theatre. Suspension arm SA is attached at a pivot point to a crossbar. Said pivot of overhead carriage OHC is energizable via stepper motor M9 to slide along the crossbar. According to one embodiment, crossbar itself is energizable via stepper motor M8 to slide along the pair of rails.

In other words, C-arm CA, and with it the detector D-X-ray source XR assembly, can be shifted across the x,y plane for any given projection direction p. Direction p can be defined by an angle a as explained in more detail below.

In yet other words, C-arm CA may be positioned to "embrace" table TB either from the bed's long edge x or from it shorter (cross) edge y.

C-arm CA defines an "incomplete" circle or ellipse. The geometric center of said circle or the appropriate focal point of the ellipse is called the iso-center. The C-arm CA is so arranged that it can be rotated around a horizontal axis passing through the iso-center. Said horizontal axis may be defined either by the x or y direction depending on whether c- arm CA has been positioned (via operation of overhead carriage OHC as explained earlier) along the long or short edge of the table TB. To afford this horizontal rotation, C-arm CA is coupled, in one embodiment, to suspension arm SA by way of cradle CR. Cradle CR includes a suitable ball bearing and/or rail arrangement (see dashed circles in Fig 1) and so allows C- arm CA to selectably slide in said cradle XR in clockwise or counterclockwise orientation (see curved double-head arrow in Fig 1) when so energized by a respective one Ml of stepper motors M1-M9.

According to one embodiment it is also cradle CR itself that is rotatably mounted to suspension arm CA so hat C-arm CA can be rotated around a further horizontal axis parallel to x-y-table plane around angle y

According to one embodiment a stepper motor, such as stepper motor M9, is addressable to effect a rotation of c-arm CA around vertical z axis at overhead carriage pivot point as shown in Figure 1 by angle β.

Adjustment of the projection angle alpha a, (and, if applicable, γ) is mostly guided by anatomic considerations and is pre-set by an acquisition protocol or ad-hoc selected by operator for the specific intervention to be carried out.

The projection angle a defines the angle at which the projection view on the organ of interest is acquired as mentioned earlier. In other words the projection angle a defines the angle at which primary radiation cone PR passes through the relevant part of the patient's body. Using the common reference frame x,y,z as defined herein, projection angle measures the deviation relative the vertical z-axis.

In one embodiment, it is also table's TB geometry that can be changed.

Specifically, table's TB height is adjustable and the table TB's plane is tiltable.

According to one embodiment table TB is also likewise slideable in the x-y plane by a suitably undercarriage. Similar to the overhead carriage OHC, a rigid table support is slidably mounted on a crossbar. Stepper motor M3 is energizable so that said support (and with it table TB) slides along the crossbar along the x-axis. Table crossbar itself slides along y axis on suitably arranged rails when energized by motor M4.

Those schooled in the art will appreciate that the c-arm imager 100 as described above and as shown in Fig 1 is but one embodiment. Imagers other than of the c- arm type (such U-arm) may also be used herein. Other means of moving C-arm CA in the x,y plane are also contemplated. For example, rather than being ceiling mounted, the imager's arm CA may be floor mounted on a similar rail system as explained for table TB. According to one embodiment, c-arm CA is arranged on an articulated robotic arm.

Reference is now made to Figure 2 where a part of the imager's geometry is shown in more detail in relation to radiation cone CBC. Specifically, the detector D-x-ray tube XR assembly is shown with the c-arm CA itself cut out for clarity. The view affords a view from above on the assembly so is opposite to the direction of travel of the x-ray beam CBC.

Detector cells (shown as squares on the detector image plane) are arranged on detector D plane and are addressable by a local detector frame of reference u,v. If the projection angle a=0, the coordinate axis of the said detector frame of reference and the x,y axis of the common frame of reference x,y,z are parallel. In general (a≠ 0), one frame is an orthogonal transform of the other.

Primary radiation beam PR emanates from x-ray radiation source XR as an un- collimated cone beam BC which is then collimated by collimator action COL. Collimation motor M2 is used to energize various collimator blades CB that together define a collimation window or aperture. Blades CB or sheets ("shutters") are formed from lead or tungsten or other highly radiation-opaque material.

In the example shown in Figure 2, a symmetric collimator is shown capable of defining square shaped apertures. In other embodiments, the pairs of blades are energizable independently from the other pair (asymmetric collimator) so that non-square rectangular apertures can be formed also if so required. Initial cone beam BC is thereby restricted to collimated beam CBC which in general has a frusto-conical shape. For present purposes, the term "cone" is be construed broadly and includes in particular the "classical" circular cone but also cones with rectangular bases ("pyramids"). Said collimated beam CBC then irradiates a portion of patient PAT's anatomy. The imager's field of view FOV for any given projection angle (in Figure 2 alpha equals zero is assumed however the following

consideration applies to any angle) is defined by the location where a central ray p of collimated cone CBC ingresses patient's PAT body. This point of ingress also defines the points on the ray's in-tissue path and also the point where said central ray p eventually impinges detector plane D. Figure 2 represents the desirable situation where said center ray p strikes the center cell of detector's D image plane u,v.

Figure 2 further shows the desirable situation where collimated radiation cone CBC is centered around the tip portion TP of guidewire because guidewire GW's tip TP happens to lie on center ray p's in-tissue path. Figure 2 also serves to distinguish the two concepts of centration and collimation. Collimation is merely the restriction of the radiation beam whereas centration is defined by the direction that results in center ray P striking substantially the central detector cell of detector D or at least striking a group of central detector cells as the notion of the "central ray p" is a rather a conceptual one as central ray p is more realistically construed as a bundle of central rays together forming a "core" or "central beam" of collimated cone CBC. However the situation in Figure 2 is merely a snapshot at a certain instant at which guide wire GW assumes a certain position during the course of its progression through patient PAT. In other words, without extra effort, guidewire footprint will be "off-center" at a later instance.

In order to better cope with this dynamics of the moving guide wire or indeed with that of any other movable medical device throughout the imaging session, the arrangement according to Figure 1 includes an apparatus A that acts to maintain centration of the x-ray source-detector D assembly throughout the intervention, that is, throughout the progression of the guide wire through the patient. The effect of control apparatus A can be seen in Figure 3 to which reference is now made. At time instant tl we have a situation which is similar to Figure 2 where x-ray source XR-detector D assembly is centered round a current tip position PI. At a later instant t2, tip TP has progressed to a different position P2 but now x-ray source-detector D-assembly is still centered round the new position P2 because control apparatus A has timely effected a corresponding shift of x-ray source XR-detector D assembly. Said shift is indicated in Fig 3 by the arrow between the two dashed lines. Fig 2 shows how the field of view in one instant FOVi is updated for another field of view FOVi₊i to so maintain centration around tip TP.

In other words, apparatus A interfaces with motor controller MC to

dynamically update the imager's (imaging) geometry so that the C-arm CA, at least in a lateral motion parallel to the x,y plane, follows the catheter's tip point throughout its progression through the patient. To this effect and as shown in Figure 1, apparatus A instructs motor controller MC accordingly and said controller MC then issues corresponding command signals via a bus system BUS to respective ones of the various stepper motors Ml- M9. Operation of control apparatus A will now be explained in more detail.

Operation

Control apparatus A operates to change imager's geometry, in particular to reposition C-arm CA, so as to maintain x-ray tube XR's centration around the catheter point TP. To this effect, control apparatus A comprises input port IN, a tracker TR and C-arm position re-adjuster RAD.

It is contemplated herein that control apparatus operates in two basic embodiments: in one embodiment control apparatus CA receives a stream of coordinate positions as produced by the robot's driver RD. In other words, in this embodiment, the control operator CA operates to control motor controller MC based on positional coordinate information as received from the robot ROB. The received stream of positional information are interpreted as clues on the whereabouts of the guidewire GW's footprint in the common frame of reference x,y,z.

According to the other basic embodiment, control apparatus A is capable of operating without a robot by reading in streams of fluoroscopic or angiographic images obtained by the imager and the positional clues on the whereabouts of the guidewire GW's footprint is derived from in-image information and the imager's geometry. In one

embodiment control apparatus A receives and operates on both streams, that is, on the coordinate stream from the robot and the fluoroscopic stream as produced by the imager.

When operation is based to on the fluoroscopic frames FL, tracker TR includes a segmentor that image-processes each of the received sequence of frames to extract therefrom the position of the catheter tip in each x-ray frame. The so extracted "footprints" (that is, projection views of the guidewire) can then be used to define the instantaneous regions of interests ROIi around which the C-arm is to be centered during the course of the intervention.

In the embodiment, where the robot ROB is used also, additional information is transferred from the robot driver RD to control apparatus A. This additional information could include not only the stream of the instant spatial coordinates of the catheter tip (as derivable from the position of the robot arm) but also includes the type of selected catheter or guidewire, orientation of the catheter/guidewire and the velocity of the motion imparted by the movement of the robotic arm on the catheter or guidewire. Velocity of said guidewire GW motion can then be analyzed in terms of its spatial vector components v_x, v_y, v_z in the common reference frame because the position of the robot ROB relative to the imager is known.

According to one embodiment it is only the guidewire GW's translation in the x,y plane that is derived from the positional and velocity information. Because there is always a certain latency between the positional/velocity information as received from the robot at apparatus A and between the sequence of projection images FL, a synchronization mechanism is envisaged that helps match the respective ones of the set of positional/velocity robot information with the corresponding frames in the stream of fluoros to which the robotic information pertains. In other words, the respective tip positions as extracted by tracker TR's segmenter can always be matched to a respective set of robot supplied guidewire positions/ velocities that was relevant at the acquisition time of the respective fluoro frames. This synchronization is achieved in one embodiment by evaluating time stamps that are generated by the imager and the robot when the images FL or the positional/velocity information are produced, respectively.

In one embodiment, tracker TR operates to convert the positional and velocity information as supplied by the robot ROB into image plane u,v coordinates. According to one embodiment this conversion is achieved by using certain auxiliary information items obtained in relation to the catheter or guidewire GW and/or the imager alongside the frame of fluoros or a single one of the fluoros. A set of possible "candidate" three-dimensional orientations and positions of the guidewire in the common frame of reference x,y,z is determined as a function of the fluoro and said auxiliary information item or items. The set of possible candidate orientations and positions is reduced to one definite orientation and position of the catheter by performing a series of defined guidewire motions or

"catheter/guidewire wriggling". "Catheter wriggling" is an exploratory step wherein, at first, small movements of the tip are performed along a predetermined path. Preferably, the catheter tip TP is at first moved in a first plane for a predetermined number of times, before it is moved in a second plane for a predetermined number of times. In other words the orientation of the plane is varied and a sequence of fluoros is acquired. The extent of wriggle motion in the images is then compared for each of the different planes. The plane that is essentially parallel to the detector plane will show the largest extent of tip wriggling as compared to the other planes thus the plane of the tip can be pinpointed. Said plane together with the respective robot arm coordinates then define the tip TP position in the common frame of reference. In other words, a positional/velocity information flow from the robot back to the imager can be harnessed to pinpoint the guidewire tip TP at any given time after observation of a few cycles of tip wriggling in various planes. Of course the amount of said exploratory tip wriggling is adapted to the confines of the current anatomic locale.

In other embodiments the conversion is achieved by a calibration either pre- procedural or during the intervention by automatically associating each new set of robot information and the respective change of tip position as registered in between two

consecutive frames in the fluoro sequence. Tracker TR then operates to process the robot and/or image information to define an instantaneous region of interest ROI. In a basic embodiment it is the respective instantaneous tip position that is used as the ROI. In other embodiments it is not only the sequence of instantaneous positions of the tip TP that is used to the define the ROI at any one time but it is also the instantaneous (that is, averaged over one or more pairs of consecutive tip positions) velocity information (that is, direction and speed) as supplied by the robot ROB whilst moving the guidewire is also used. Using the velocity information allows to account for the imager's latency. The imager's latency or lag or "response time" is the time it takes for the imager's C-arm position to change. Response times for other imager geometry changes can also be taken into consideration.

Reference is now made to Figure 4 to explain the operation of tracker TR when using the velocity component in the definition of the instantaneous ROI. Figure 4 shows a fluoro from the stream of fluoros acquired at a certain instant t. The fluoro includes a footprint of the guide wire GW 401 with the footprint of its tip 405. A velocity component V as supplied by the latest robot information and/or velocity as analyzed from the previous recordings in the fluoro sequence is then applied to the instant tip point TP 405 (X) to so obtain a kinematic estimate for the expected position of tip at position 410 which then defines the location of the center (C) of the next ROI. In other words, at any given time the next ROI position is determined via the following formula:

C = X -t a * V

where "a" is a factor that is set depending on the response time of the system or may be set according to user preference to fine-tune the operation of apparatus A with a given imager when said apparatus is used an add-on for the imager. The ROI positions so computed ensure that the detector's D image plane is not blocked out by collimator COL action in the direction in which the catheter or guidewire is to proceed. It also ensured that the subsequently acquired frame is centered around the tip TP and that the footprint position of tip TP in the fluoro stays clear of the fluoro' s edge portion. The operator therefore has a clear sight during the intervention on where the guidewire is to go next and does not feel the collimator is cluttering his or her vision.

The computed position of the guidewire tip position 410 or, in the simpler embodiment, the instant tip position without lag adjustment, can then be used in the following step by re-adjuster RAD to instruct motor controller MC to effect a change in the imager's geometry. Re-adjuster RAD operates as a closed loop feedback controller. In other words re-adjuster RAD registers at pre-defined time intervals the current position of the image's current geometry and compares same with the instantaneous ROI position as supplied in the manner described above by tracker TR. Image re-adjuster RAD compares the difference between the current imager geometry and the desired ROI position as supplied by the tracker to obtain a position correction value. This correction value is then forwarded to motor controller MC and interpreted by same. Motor controller MC then translates the correction information into corresponding lower level hardware control impulses or signals which are then transmitted via the bus system BUS to the respective stepper motors M1-M9.

According to one embodiment, accounting for the imager's lag when changing the geometry can also be achieved without robot information but by analyzing the trajectory of the catheter tip as obtained from previous locations as recorded across a sequence of images. This historical path of the catheter's tip can be obtained for example by using optical flow algorithms.

According to one embodiment it is only the x,y position of the imager's arm CA that is changed. In other words in this embodiment motor controller MC would only instruct the motors M8 and M9 in the overhead carriage to effect the respective shift of C- arm whilst maintaining the instant projection direction, that is, whilst maintaining the direction a of the instant center ray p of beam PR. The operation as explained above is then repeated for any newly detected position of catheter tip TP. In other words, from the point of view of the operator, as he or she changes the catheter position (either by direct hand action or by using robot ROB) the C-arm's position would change in a manner that would appear to be continuous to the operator and directly responsive to the progression of the guidewire GW.

As an alternative to the above, rather than moving the C-arm CA it is envisaged instead to move the examination table TB by energizing respective step motors M3 and M4 as to effect a shift or tilt of said table to so off-set for the guide wire's motion.

For ease of presentation in the previous Figures 1-3 a projection angle alpha equals zero has been assumed. However other projection angles, for example alpha equals 30° can of course also be used and the motion of the catheter wire can still be compensated for so as to retain the centration of each image frame around the instantaneous position of the catheter's tip TP.

According to one embodiment the C-arm CA can be shifted parallel to a detector image plane angled at a≠ 0. In other words a non- zero projection direction alpha can be maintained whilst the C-arm is shifted in a direction perpendicular to said projection direction. However in simpler embodiments the C-arm's position CA can be shifted only in the x,y plane or in even simpler embodiments only in either x or in y direction, that is, along the longitudinal axis of the image table TB or across.

The C-arm shift parallel to the u,v image plane for non-zero projection directions a can is implemented by a suitable combination of the C-arm shift in the x,y plane of the common frame of reference and a table tilt movement.

According to one embodiment the collimator's stepper motor M2 operates to collimate the blades tightly around the instantaneous ROI. In other words in this embodiment rather than to have the collimator in an "open" position at all times, it is only a user definable area around the center ROI that the beam is allowed to radiate. This can reduce x-ray dose on the patient and the operator.

Once the re-adjuster RAD has determined that the newly computed imager geometry (or c-arm position in particular) has been indeed assumed, a corresponding "OK" signal is issued for example on the screen M or by acoustical or otherwise visual indication (control lamp flashing) to the operator. The operator can then decide to accept the computed adjusted imager position and the image acquisition with the now centered x-ray-detector assembly can commence upon the operator actuating a release mechanism. A fully automatic embodiment is also envisaged so no previous release signal by the operator is required but images are acquired as soon as the newly computed imaging position is assumed.

The communication between controller A and imager 100 and/or robot ROB may be wired but wireless protocols or similar are also envisaged.

In short, the arrangement proposed herein including control apparatus A allows the operator to obtain a sequence of centered projection images FL at the

instantaneous region of interest ROI. For example, the catheter or guidewire tip position or any other prominent part of this or another medical device is shown substantially at the center of each imager frame. The acquired sequence of centered images can then be displayed on screen M by a suitable rendering graphic visualization processing. For example known road mapping techniques can be used to a graphically display otherwise indiscernible outlines of the relevant vessel's structure by using one or more angiographic images. Operation of the apparatus A as explained above is analogous during use in an angiographic imaging run.

According to one embodiment the system includes visualization means such as graphics display generator (not shown) to fuse and render for display on the screen M the collimated images with other pre-recorded images to provide the physicians with visual clues about the surrounding anatomy. Examples include cardiac- or neuro-roadmap information where a graphical element is generated to outline the catheter's footprint as extracted from the x-ray images. Said graphic is blended in and/or overlaid on each of a pre-recorded run of images acquired with un-collimated exposure or on each frame of other fluoro runs.

According to one embodiment, the components of control apparatus A are separate modules arranged in a distributed architecture and connected in a suitable communication network. However, this is an exemplary embodiment only. For example, the components may be arranged in control apparatus A as a dedicated FPGA or as hardwired standalone chips. In an alternate embodiment, the components of may be resident in work station CON running as software routines. The components may be programmed in a suitable scientific computing platform such as Matlab® or Simulink® and then translated into C++ or C routines maintained in a library and linked when called on by work station CON.

Reference is now made to Figure 5 showing a flow chart of a method for adapting the field of view of an imager.

At step S501 positional or velocity information of the guide wire or other medical device residing and movable in the patient is received. According to one

embodiment the positional information includes coordinate information and/or velocity information as produced by a robot or similar device that automatically or upon user actuation steers or guides the guidewire through the patient.

At step S505 a new position of the guide wire is registered based on the received positional and/or velocity information.

At step S510 a current relative position between radiation source and object to be imaged is changed. The change is based on the registered new device position.

Once the change in imaging geometry has been so affected, an image is acquired at step S515 by the imager at an updated field of view that is now centered around the registered new device position.

At step S520 it is then determined whether new positional position is being received, and, if yes, the previous steps S505 through S515 are then repeated to so obtain a sequence of centered images each image centered around the respective new device positions.

According to one embodiment, in step S510 an expected new device position is computed based on the determined new device position and on a velocity information obtained from previous images and/or received from a robot or similar remote controllable device used to steer or control the motion of the device. In said computation a time period is used to compute the expected position. Said time period corresponds to the imager's time lag, that is, the time it takes for the imager's geometry to change. In this embodiment the follow up images are then centered round this predicted or expected device position. Changing the imager's geometry includes a change, in a horizontal plane, of the x-ray source's position relative to the object. The plane is defined by a plane parallel to a platform on which the object resides during the imaging. In one embodiment, the change in restricted to said horizontal plane but shifts across slanted planes are also envisaged in some embodiments.

In another exemplary embodiment of the present invention, a computer program or a computer program element is provided that is characterized by being adapted to execute the method steps of the method according to one of the preceding embodiments, on an appropriate system.

The computer program element might therefore be stored on a computer unit, which might also be part of an embodiment of the present invention. This computing unit may be adapted to perform or induce a performing of the steps of the method described above. Moreover, it may be adapted to operate the components of the above-described apparatus. The computing unit can be adapted to operate automatically and/or to execute the orders of a user. A computer program may be loaded into a working memory of a data processor. The data processor may thus be equipped to carry out the method of the invention.

This exemplary embodiment of the invention covers both, a computer program that right from the beginning uses the invention and a computer program that by means of an up-date turns an existing program into a program that uses the invention.

Further on, the computer program element might be able to provide all necessary steps to fulfill the procedure of an exemplary embodiment of the method as described above.

According to a further exemplary embodiment of the present invention, a computer readable medium, such as a CD-ROM, is presented wherein the computer readable medium has a computer program element stored on it which computer program element is described by the preceding section.

A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

However, the computer program may also be presented over a network like the World Wide Web and can be downloaded into the working memory of a data processor from such a network. According to a further exemplary embodiment of the present invention, a medium for making a computer program element available for downloading is provided, which computer program element is arranged to perform a method according to one of the previously described embodiments of the invention.

It has to be noted that embodiments of the invention are described with reference to different subject matters. In particular, some embodiments are described with reference to method type claims whereas other embodiments are described with reference to the device type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters is considered to be disclosed with this application.

However, all features can be combined providing synergetic effects that are more than the simple summation of the features.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing a claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items re-cited in the claims. The mere fact that certain measures are re-cited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An imaging system comprising:

an imager (100) having a radiation source (XR) for emitting a radiation beam, the imager operable to acquire a current image of an object (PAT) at a current field-of-view (FOVi) whilst a device (GW) resides in or around said object;

an apparatus for adapting the field-of-view of the imager, the apparatus comprising:

an input port (IN) for receiving positional information in relation to the device; a tracker (TR) configured to register, based on the received positional information, a new device position;

a re-adjuster (RAD) configured to use the registered new device position to effect a change of a relative position between the radiation source (XR) and the object (PAT), so that a follow-up image is acquirable by the imager (100) at an updated field-of-view (FOVi₊i) that is centered around the new device position or is centered around an expected device position.

2. System of claim 1, wherein the change of the relative position is effected by moving a frame (CA) of the imager that carries the radiation source (XR).

3. System of claims 1 or 2, wherein the change of the relative position is or is further effected by moving an examination platform (TB) on which the object is disposed on during operation of the imager.

4. System of any one of claims 1-3, wherein the device (GW)'s current and/or new position is taken relative to a tip (TP) portion of the device.

5. System of any one of claims 1-4, wherein the positional information is represented by in-image information in the current image, the tracker (TR) operative to image process said image to extract said positional information and to use the extracted positional information when registering the new device position.

6. System of any one of claims 1-5, wherein the imager is communicatively coupled to a robot (ROB), the robot (ROB) operative to advance the device through the object, the device thereby assuming the new device position and the robot generating a stream of spatial coordinate information, the tracker configured to use said stream of spatial coordinate information when registering the new device position.

7. System of any one of claims 1- 6, wherein the field-of-view (FOVi₊i) of the follow-up image is centered around the expected device position, wherein the tracker is configured to use the new device position to predict the expected device position, wherein a spatial difference between the new device position and the expected device position corresponds to a response time for effecting said change in relative position between the radiation source and the object.

8. System of any one of claims 1- 7, wherein the device (GW) is a PCI catheter and/or the imager is an x-ray imager of the C-arm type and/or the object is a human or animal patient.

9. System of claim 6-8, comprising the robot for advancing the device.

10. Method of adapting a field-of-view of an imager (100) having a radiation source for emitting a radiation beam, the imager operable to acquire a current image of an object at a current field-of-view (FOVi) whilst a device resides in or around said object, the method comprising the steps of:

receiving (S501) positional information in relation to the device; registering (S505), based on the received positional information, a new device position;

based on the registered new device position, effecting (S510)a change of a relative position between the radiation source and the object

acquiring (S515) a follow-up image by the imager at an updated field-of-view (FOVi₊i) that is centered around the new device position or is centered around an expected device position.

11. Method of claim 10, wherein the change of the relative position is effected by moving a frame of the imager that carries the radiation source.

12. Method of claims 10 or 11, wherein the change of the relative position is or is further effected by moving an examination platform on which the object is disposed during operation of the imager.

13. A computer program element for controlling an imaging system according to any one of claims 1-9, which, when being executed by a processing unit is adapted to perform the method steps of claims 10-12.

A computer readable medium having stored thereon the program element of