Patent Application
20080200798
The present invention relates generally to a guide for guiding a medical instrument to a target site within a patient's body, and more particularly to a trajectory guide, visible using magnetic resonance imaging or x-ray based computed tomography (CT), for guiding a medical instrument, e.g. a biopsy instrument, to a target site, the position of which has been determined by means of magnetic resonance imaging or x-ray based computed tomography.
Acquiring and providing diagnostic images of internal body structures and diseased or altered tissue is of great importance in many areas of medicine. Techniques to acquire such images include X-ray computed tomography (CT), ultrasonic imaging, emission tomography and magnetic resonance imaging (MRI). MRI and CT provide two-dimensional cross-sectional images through a patient, using magnetic fields and X-rays, respectively, visualizing internal structures in color or gray scale. These sections can be combined to visualize three-dimensional structures on or within a body. One advantage of using magnetic resonance imaging is that it does not expose the patient to harmful radiation while at the same time producing detailed images of the area of interest. X-ray based computed tomography, on the other hand, is a more economical choice, and therefore more widely available.
When performing interventions such as e.g. biopsies or treatment of diseased or altered tissue, it is of utmost importance to plan the intervention carefully by choosing the entry point and trajectory path in order to reach the location of the desired site with minimum risk for the patient. By using for example an MRI scanner or a CT scanner, the physician is provided with images from which he or she can plan the procedure by choosing the entry point, the direction and the depth for an interventional device so as to reach the target site. Occasionally some kind of guiding device is used; however, in most cases, the puncturing device is manipulated without employing any device for ensuring the accuracy of the puncturing needle reaching the target.
A variety of guides employed to properly position a medical instrument within the body of a patient are known in the state of the art. U.S. Pat. No. 4,608,977 and U.S. Pat. No. 4,733,661 disclose guidance devices for use in conjunction with a CT scanner. U.S. Pat. No. 5,263,956 shows a ball joint for holding a neurosurgery tool in a predetermined orientation relative to a patient's skull. Devices used in stereotactic surgery are disclosed in U.S. Pat. No. 6,110,182 and U.S. Pat. No. 4,805,615. Notably, the abovementioned devices are not optimally adapted for use in conjunction with any simultaneous imaging techniques, and rely mostly on the judgment of the physician in determining the trajectory of the introduced medical instrument.
U.S. Pat. No. 6,206,890 shows a trajectory guide apparatus for use in introducing surgical and observational instruments into a patient's brain. The apparatus comprises a base plate, a movable guide member mounted as a ball joint and, in the case of using computed tomography or magnetic resonance imaging, a separate guide rod with two locator markers, visible using the chosen imaging technique. However, U.S. Pat. No. 6,206,890 fails to indicate any design specifications concerning the two locator markers. Especially when locating small and/or deeply located lesions, accurate means to target the desired site are necessary. On the other hand, for practical reasons in a scanner environment, there is a limited space to maneuver the device.
One puncturing guide is disclosed in WO2004/021898 and US2004/0260312, assigned to the present assignee, and comprises a needle guide mounted on a base plate, where the needle guide is freely movable around a set entrance point on the patient's skin. The guide can therefore first be positioned and secured on the patient's body and thereafter manipulated to achieve the best entrance angle to reach the desired tissue site, without altering the entrance point. Using x-ray based CT, it has been found by the present assignee that by employing a metal needle guide, an artifact will in most cases be produced in the CT image, indicating the extended trajectory path into the tissue, even prior to puncturing the skin. The advantage of using a guide according to abovementioned patent applications in both magnetic resonance imaging and x-ray based computed tomography is its small size, as these techniques may sometimes limit access to the patient during the procedure. However, this puncturing guide is not visible in magnetic resonance imaging and is therefore of minimal use when planning the intervention with regards to optimizing the angle of the needle guide using magnetic resonance imaging. The entire contents of the '898 and '312 documents are incorporated herein by reference for the devices, methods, and techniques described therein.
Considering the practical limitations in both the field of view when performing a scan, and the physical restrictions of the scanner environment, and, on the other hand, simultaneously needing to provide sufficient accuracy for the trajectory, it is further desirable to optimize the distance between the alignment markers, in order to achieve a reliable trajectory path in each individual procedure.
A general object of the present invention is to provide a trajectory guide which can be used in a way that maximizes safety and minimizes patient discomfort during the intervention. Another object of the present invention is to provide a small and economical trajectory guide visible in magnetic resonance imaging and computed tomography for use in medical devices such as puncturing guides for biopsies and other surgical guides. A further object is to provide a method of using the trajectory guide to improve the accuracy when verifying the angle of entrance of a needle guide using magnetic resonance imaging or computed tomography prior to perforating the patient's skin and tissue.
Thus, an object is achieved by providing a specifically tailored trajectory guide as an aligning or guiding member of a medical device intended for introduction of instruments into the body of a patient. The trajectory guide is provided with at least one distal and one proximal marker visible in magnetic resonance imaging or computed tomography. These markers are of such a size and distance from each other that they can be used to determine that the trajectory guide is dependably aligned with the target tissue. This is achieved by providing a function which can be used to determine the accuracy of the trajectory guide.
In a first embodiment the trajectory guide is provided as a hollow tube or instrument guide attached to a base which is attached or placed directly over the desired target tissue. The trajectory guide is freely angularly movable relative to the base and can therefore easily be aligned with the desired tissue site. In another embodiment, the trajectory guide is supplied as a separate member which can be attached to a trajectory guide having an instrument guide for a puncturing device, e.g. a biopsy needle. In a further embodiment, the trajectory guide is movable around a point that coincides with the defined puncturing entrance point. With such a puncturing guide, the tip of the puncturing device may be positioned at the puncturing point in a first operation and, in a subsequent operation; the entrance angle can be set without moving the position of the needle tip. In such an arrangement, the guide rod's distal marker is placed at the distal end of the rod, such that the distal tip of the guide rod is visible using magnetic resonance imaging and indicates the exact entrance point of the puncturing device.
a and 6b illustrate the use of an extended rod, e.g. a biopsy instrument, in an image of a body section.
The following description will describe the embodiments mainly in connection to using magnetic resonance imaging for determining the trajectory path using the present invention. It should however be noted, that it is within the scope of the present invention to apply a corresponding approach to using x-ray computed tomography or any other cross-sectional imaging technique, with the appropriate changes in materials used, as is known in the state of the art.
One embodiment of the trajectory guide 1 of the present invention (shown in
In another embodiment, illustrated in
In a further embodiment, shown in
The segments have the shapes of two semi-circular equally sized bows pivotally attached to the base plate 16 at positions separated by 90°, and a slit is provided in each bow, wherein the movable guide member 17 is adapted to be arranged in the slits at the intersection point of the bows and wherein the guide member 17 is movable around a point that coincides with the defined puncturing entrance point.
The distal end of the trajectory guide 17 is introduced in a bore 18 in the center of the base plate 16. The inside of the ring-shaped retainer 21 is threaded and fits on a corresponding thread on the upper part of the guide member 17. Before the retainer 21 is tightened, the guide member 17 can slide inside the slits in the first and second segments 19, 20, with the distal tip of the guide member 17 being in contact, or almost in contact, with an object, such as the skin of a patient, beneath the base plate 16. The center of the first semi-sphere constitutes the rotational center of the guide member 17, which means that e.g. a puncturing needle introduced into the guide member 17 can be positioned in different angular orientations without changing the entrance point for the puncturing needle through a patient's skin. The retainer 21 is used to lock the guide in place after determining the optimal entrance angle.
Also in this embodiment the movable guide member 17 is provided with markers 22, 23 that are visible in the cross-sectional imaging technique used. Again, use and placement of these markers will be further described below. Similarly to the embodiment illustrated in
In a third embodiment a combination of the previous embodiments is provided, such that at least one marker is located on the trajectory guide and at least one marker is located on the guide rod. The two markers should be spaced apart along the trajectory path such that they can be used in optimization of the trajectory angle, as will be described below.
The detachable guide rod described above can preferably comprise a fastening means such as that illustrated in
One unique feature of the present invention is the presence of alignment markers in all embodiments. The alignment markers 7, 8, 22, 23 provide reference points in the images obtained by e.g. magnetic resonance imaging, which is further described below. When performing a biopsy or other surgical procedure requiring guiding an instrument to a specific site within a body, e.g. a tumor or other aberrant tissue, accurately determining the exact trajectory path prior to commencing the invasive procedure improves reliability and performance of the procedure, and can in many cases also reduce the time needed for the invasive part of the surgery, thereby reducing discomfort for the patient. In magnetic resonance imaging two-dimensional images are obtained, which represent virtual slices of a body section. The represented slices can be chosen to be at any angle in relation to the scanner or the body being depicted. These slices can commonly be between 1 mm and 2 cm thick. Consequently, all tissue visualized within this thickness will be depicted in one plane in the produced image. To determine whether a trajectory guide such as described herein is placed so as to allow access to the desired site within the body tissue, it is important to determine the precise angle of entrance into the body. In the obtained image, an in-plane angle of entry is easily calculated. However, as the two-dimensional image represents matter within a three-dimensional space, it can be difficult to visualize the out-of-plane offset angle. This is especially true using a trajectory guide consisting of a continuously visible elongated segment, as is illustrated in
The present inventors have realized that the uncertainty in determining the out-of-plane trajectory angle in relation to the target tissue can vary depending on the distance chosen between the two markers. Specifically, if a too small distance is chosen between the markers, and depending on the thickness of the body section chosen for imaging, it can be difficult to determine whether the entire rod is oriented entirely parallel to the surface plane of a particular depicted cross-section. On the other hand, a very large distance between the two markers is not practical, due to the limited actual space within the scanner environment and the need to limit the field of view for computational reasons. In addition, as the patient is not absolutely still during a scanning procedure due to breathing etc, it would be very difficult to keep two distantly spaced markers within the same slice for any longer period of time. Especially when performing procedures where the targeted tissue is of a smaller size, it is crucial to be able to accurately determine the trajectory path in three-dimensional space.
The high accuracy is achieved in the present invention by constructing markers that are sufficiently separated from each other on the trajectory guide to be able to determine a deviation from the image plane.
To be useful as a trajectory guide, both the two markers need to be visible in the produced image. This is illustrated in
L
min
=t/(sin αmax)=t/(sin(arctan (R/D))) (1)
wherein D is the depth of the lesion measured from the entrance point to the center of the lesion, and t is the thickness of the slice depicted in the image. R is the radius of the targeted tissue mass, e.g. a lesion, or, in practice, approximately half the extension of the lesion as estimated or measured in a plane perpendicular to the image plane, and αmax is the maximum allowed deviation angle from the image plane in order to still target a lesion of size 2 R. There are some practical aspects of the imaging technique that need to be considered. As a person skilled in the art will know, the thickness, t, of the slice depicted is generally chosen such that the target radius, i.e. R, is larger than or equal to t. The thickness is typically 3-5 mm. The image plane is preferably located approximately over the center of the lesion. It is also assumed that the signal from a single marker will weaken substantially as more than half of the marker is outside the image plane.
Referring to
tan αmax=(sin αmax)/(cos αmax)=R/D (2)
αmax=arctan (R/D) (3)
L
min
=t/(sin αmax) (4)
Notably, the two parameters D and R are available to the user in the MRI image, who can thereby calculate αmax.
Furthermore, the projected distance between the markers, Lproj, can also be obtained from the image. Referring to
L
proj
=L
min*cos αmax (5)
t/L
proj=tan αmax (6)
For very small angles, as is common in MRI, Lproj will approximately equal Lmin, and is such cases equation (1) and (5) can be approximated to
L
min
=t*D/R (7)
Therefore, for deeply located small lesions, i.e. where R is approximately equal to t and D is very much larger than t and R, the distance between the visible markers will have to be approximately the same as the depth of the lesion in order to obtain good accuracy of the intervention. For larger more shallow located lesions, equation (1) should be considered.
In practice, when the distance Lmin is known on the guide, the visible distance Lproj in the image at a known t gives a reasonable estimate of whether the target of size 2 R at depth D (in the image plane) will be accurately targeted using the trajectory guide at hand. This is illustrated in Example 1. By supplying tabular data there is also room for adjustment of the image plane thickness to reach a desired accuracy.
Using the abovementioned equation and the empirical limitations observed by physicians in using e.g. magnetic resonance imaging when taking a biopsy, the markers are preferably placed at a distance of between 1 and 30 cm from each other, and more preferably between 5 and 15 cm. The markers should also be small enough to each define a specific reference point in the image.
One solution to accurately targeting all lesions regardless of size and depth is to make a trajectory guide with two markers spaced very far apart, however, as stated above, this is not always practical. Therefore, in other embodiments, the trajectory guide is provided with three or more alignment markers, spaced along the trajectory guide in a straight line, in order to offer additional points of reference for the alignment. The number of markers is preferably between 2 and 20, more preferably 5 to 10. Using more than two markers provides the present invention with the means to determine the level of accuracy of reaching the desired target site, such that a given number of visible markers in an image plane of thickness t, compared to the total number of markers on the guide corresponds to a given accuracy (according to Example 2). This gives the physician the opportunity to assess whether it is advisable to attempt e.g. a biopsy, depending on for instance the size and/or depth of the target tissue. Furthermore, using additional markers could also provide the image with a measuring scale, which would ease the determination of e.g. the precise depth of the target tissue. It should be noted that in the case of providing the trajectory guide with three or more markers, the abovementioned length (L) in determining the accuracy of the trajectory guide represents the distance between the two outermost markers visible in the produced image.
In a further embodiment, at least one marker is mounted on the trajectory guide such that it is movable along the length of the trajectory guide. This adds the advantage of being able to adapt a trajectory guide to specific situations wherein a higher accuracy is desired.
As mentioned above, in some cases, specific constraints arise in individual procedures which limit the physical space available for a medical device. An example of such a constraint is when either the field of view or the actual physical space available in the scanner does not allow the placement of a long guide rod. Therefore, an additional embodiment provides a detachable guide rod comprising at least one proximal section which can be disengaged or broken off, to allow a shorter guide rod to be used. The shortest possible guide rod to be used still comprises at least two markers.
When the present invention is to be used in connection with either magnetic resonance imaging or computed tomography the markers comprise a contrast medium chosen among those common in the state of the art. The contrast medium for computed tomography can comprise, but is not limited to, barium sulfate and iodine-based compounds. The contrast medium for magnetic resonance imaging can comprise, but is not limited to, water, lipids, formulations based on gadolinium, dysprosium, manganese or iron. Considering the chosen imaging technique, the contrast medium can either be present as a liquid, a gel or in solid form. In the first two cases, the substance is enclosed in a capsule or similar container, which is then attached to or encapsulated within the trajectory guide. The markers can be incorporated within the material of the guide member or the guide rod. Additionally, the markers can be produced separately from the trajectory guide and attached to the trajectory guide, either detachably or permanently affixed to the guide member or the guide rod.
All the components of the present invention, other than the alignment markers, should comprise materials which are either not visible in computed tomography and magnetic resonance imaging, or should be visible in such a way that the markers are easily distinguishable in the produced image. The choice of materials in instruments and medical devices is important when using magnetic resonance imaging. All magnetic metals constitute a danger, as they will be pulled towards the magnet with great force. Several accidents have occurred when metal objects have injured the patient during the procedure. Non-magnetic metals do not pose a health hazard, but will quench the signal in an area surrounding the metal object. Therefore, all metals are avoided. With the above in mind, the trajectory guide may comprise components made of any materials known in the state of the art. Non-limiting examples of material which can be used in computed tomography and magnetic resonance imaging are carbon fiber, glass fiber, plastics or ceramics.
Although the present invention has been described with reference to specific embodiments it will be apparent for those skilled in the art that many variations and modifications can be performed within the scope of the invention as described in the specification and defined with reference to the claims below.
The invention is further described in the following non-restrictive examples.
Given that
L
min
=t/(sin αmax)=t/(sin(arctan (R/D))) (1)
a length between two markers on a trajectory guide of 10 cm, a lesion with a radius of 0.25 cm and a slice thickness of 0.2 cm, the lesion will be accurately targeted if it lies at a maximum depth of 12.5 cm.
If five different markers are used, with equal distance between each marker along a straight line, and given that
L
min
=t/(sin αmax)=t/(sin(arctan (R/D))) (1)
a lesion with a radius of 0.25 cm and a slice thickness of 0.2 cm, the indicated markers visible in the image obtained in Table 1 correspond to a maximum depth (Dmax) where a lesion will be accurately targeted. Note that the markers are labeled m1 through m5, where m1 is the most distal marker (i.e. essentially at the entrance point of e.g. the biopsy guide) and m5 is the most proximal marker and the distance between m1 and m5 is equal to L.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0700406-2 | Feb 2007 | SE | national |
