FIELD OF THE INVENTION

The invention relates to a system for navigating a surgical instrument.

BACKGROUND OF THE INVENTION

Surgical interventions performed in interventional radiology consists of introducing a surgical instrument, such as a needle or equivalent, in the body of the patient.

The interventional radiologist uses an imaging system, most likely a Computed Tomography Scan (CT-Scan), or Cone Beam Computer Tomography (CBCT), or a Magnetic Resonance Imaging system (MRI), to see the organs of the patient and choose the target for the tip and the trajectory to be followed by the needle to reach this target. A target can be any structure of the body, such as a region of a tumour, a region of a cyst, a region of a bone, or the like.

In order to help the interventional radiologist to reach the target, a navigation system is necessary. Such navigation systems use a tracking system based on optical, electromagnetic, radiofrequency, inertial, ultrasound or mechanical technology.

The objective of the tracking system is to give the spatial position and orientation in real time of one or more trackers, one tracker having a known spatial relationship with the needle.

It is also possible to use a robot, that can be active, haptic or tele-operated and that contains a needle guide at its extremity or carry directly the needle. It can be considered that a robot is a particular case of a navigation system.

Document WO 2010/086374 describes a method for navigating a surgical instrument such as a needle in a 3D medical image of a patient. To that end, the needle is slidingly arranged in a surgical guide to which a tracker is rigidly attached, and a reference marker is attached to the patient's body and localized by the tracking system. Since the reference marker can be detected in the 3D medical image, it is possible to determine the position and orientation of the surgical guide with respect to the 3D medical image. The needle being a linear instrument, its axis is supposed to coincide with the axis of the guide. Hence, even if the needle is not itself tracked, the system allows determining the position and orientation of the needle axis in the 3D medical image.

During interventional radiology, a first 3D medical image of a region of the patient's body is acquired while the reference marker is attached to the patient's body so as to be visible on the first 3D image. However, in some cases, the reference marker is at a certain distance of the actual region of interest (e.g. a volume containing a target to be treated by the surgical instrument). This means that, in view of navigating the surgical instrument during the surgical intervention, the reference marker has to be displaced to a position closer to the region of interest, and a new 3D medical image has to be acquired. This is time consuming.

Besides, a goal of interventional radiology is to reduce the dose of X-ray absorbed by the patient when the 3D image is CT or CBCT. Said dose depends on the volume of the patient's body that is scanned in order to obtain the 3D medical image. In order to reduce the dose, one could thus consider reducing the irradiated volume.

In the case of MRI, the acquisition of the images is time-consuming. Since the acquisition time depends on the volume size, one could also consider reducing the imaged volume.

However, in order to enable navigation, this reduced volume should always comprise the reference marker in order to allow registration of the 3D medical image with data provided by the tracking system. This constraint limits the possibility to reduce the imaged volume.

In addition, if only a small volume of the patient's body is imaged, it is also more difficult for the radiologist to interpret.

BRIEF DESCRIPTION OF THE INVENTION

A goal of the invention is to allow navigating a surgical instrument with respect to a 3D medical image even if this image corresponds to a volume of the patient's body that does not comprise a reference marker in its entirety.

A further goal of the invention is to reduce the dose of X-ray applied to a patient in interventional radiology and/or to reduce the acquisition time of an image, while still allowing navigation of the surgical instrument in a 3D medical image of the patient.

To this end, an object of the invention is a system for navigating a surgical instrument, comprising a processor configured for:obtaining a first 3D medical image of a first volume of a patient's body, said first volume comprising a reference marker,registering the first 3D image with said reference marker,obtaining a second 3D medical image of a second volume of the patient's body, said second volume being different from the first volume and not containing the reference marker in its entirety, said first and second 3D images being obtained by a single imaging device,registering the second 3D medical image with the first 3D medical image,obtaining a virtual position of the surgical instrument with respect to the reference marker from a tracking system,determining a virtual position of the surgical instrument with respect to the second 3D medical image.

The system described above allows performing a method for determining a virtual position of a surgical instrument with respect to a 3D medical image, in view of navigating said surgical instrument, that comprises:obtaining a first 3D medical image of a first volume of a patient's body, said first volume comprising a reference marker,registering the first 3D image with said reference marker,obtaining a second 3D medical image of a second volume of the patient's body, said second volume being different from the first volume and not containing the reference marker in its entirety, said first and second 3D images being obtained by a single imaging device,registering the second 3D medical image with the first 3D medical image,obtaining a virtual position of the surgical instrument with respect to the reference marker using a tracking system,determining a virtual position of the surgical instrument with respect to the second 3D medical image.

According to an embodiment, the second volume is smaller than the first volume.

According to an embodiment, the first 3D image is a large volume of CT images and the second 3D medical image is a small set of CT slices of a volume of the patient's body comprising a target.

According to another embodiment, the first and second 3D images are MR images.

According to an embodiment, the surgical instrument is a needle guide.

Advantageously, a tracker is rigidly attached to the needle guide for navigation of a needle coupled to said needle guide.

According to an embodiment, the needle is detected as a trace in the second 3D medical image and the registration comprises registering said trace of the needle with the virtual position of said needle.

During said registration of the trace of the needle with the virtual position of said needle, the translation along and the rotation about the needle axis may be determined by an image to image registration technique. Preferably, said image to image registration technique is carried out in a region of interest having a known geometric relationship with the needle guide or the trace of the needle.

According to an embodiment, the second volume is included in the first volume.

Alternatively, at least a part of the second volume extends outside the first volume.

According to an embodiment, the method further comprises updating the first 3D medical image by replacing, in said first 3D medical image, the second volume by the second 3D medical image.

According to an embodiment, the registration of the first and second 3D images comprises a rigid registration based on DICOM tags of said images.

According to an embodiment, the registration of the first and second 3D images uses an image to image registration technique.

Both of said registrations may be combined such that results of the rigid registration based on DICOM tags of the images are used for initializing an iterative computation of the image to image registration technique.

Advantageously, the image to image registration technique is carried out in a region of interest having a known geometric relationship with the needle guide.

According to an embodiment, the reference marker comprises fiducials that are at least partly visible in the second 3D medical image and said image to image registration technique is carried out using said parts of the fiducials.

