Implantable electrode array with a reference structure

Description

[0001] The present invention relates to an implantable electrode array with a reference structure which is visible in magnetic resonance images (MRI) when the array is implanted for example in a human or animal body,.

[0002] Such an array may be an electrode array which is implanted on the surface of the brain e.g., for pre-surgical assessment of cortical electrical activity in patients with epilepsy.

[0003] Hereby, MRI is used to localize the electrode contacts of the implanted electrode array with respect to the anatomy of the patient. However, metal components or other good electric current conductive materials like carbon and electric conductive polymers may cause MRI artifacts that compromise the results especially in the direct vicinity of these components and materials.

[0004] Thin-film implants which may feature 100x less metal thickness may mitigate these artifacts, but thin-film implants produce inconspicuous MRI signal voids in many clinical MRI sequences, and are therefore not suitable for localizing the electrode with MRI. Very small metal contacts or very thin metal layers may not become visible in clinical MRI at all.

[0005] But imperatively, physicians need to know the implant position, and the value of intracranial EEG increases with the precision of electrode localization.

[0006] US 2018 / 085186 A1 relates to an implant device, such as a screw or a rod, which is identifiable after implantation and comprises a main portion of the implant device and a coded portion of the implant device. The coded portion comprises a radiopaque element and indicia on the radiopaque element or indicia as a plurality of radiopaque elements disposed within the coded portion around an entire circumference of the implant device. The indicia are discernible by x-ray, fluoroscopy, computed tomography, electromagnetic radiation, ultrasound, or magnetic resonance imaging.

[0007] US 2018 / 078386 A1 relates to an implant containing a structurally encoded region, the implant comprising an implant body defining adjacent first and second encoded regions, the first encoded region comprising a first series of shaped inclusions in a first pattern of relatively differing opacity figures, and the second encoded region comprising a second series of shaped inclusions in a second pattern of relatively differing opacity figures, the first and second encoded regions being disposed such that, when the first encoded region and second encoded regions are viewed by reading illumination from a position wherein the first pattern and second pattern overlap, a third pattern is revealed by the reading illumination, the third pattern being different than the first and second patterns, and comprising shape or surface characteristics representing structurally encoded data.

[0008] US 5 123 422 A relates to an intra-cochlea electrode carrier for implantation into a helically shaped tympanic canal has a plurality of segments. Each segment has opposite first and second surfaces, the first surface defining a length between opposite ends of the segment and a width between opposite sides of the segment, a height in cross section to the width being defined between the opposite first and second surfaces of the segment. Respective electrodes are on the first surfaces of the segments. Hinges on the opposite ends of the segments connect one end of each segment to one end of another segment, whereby to connect the segments into an elongated electrode carrier, the hinges having sufficient heights relative to the widths of the first surfaces of the segments and providing sufficient spacing between the ends of the connected segments for the electrode carrier to follow a helix of a helically shaped tympanic canal when implanted therein with the electrodes on the first surfaces of the connected segments oriented toward a nerve-receptor surface of the tympanic canal. A connector connects the electrodes of the connected segments.

[0009] Salam, M. et al. "Subdural porous and notched mini-grid electrodes for wireless intracranial electroencephalographic recordings", Journal of Multidisciplinary Healthcare, vol. 7, 9 December 2014, pages 573-586, ISSN: 1178-2390 relates to a device according to the pre-characterizing portion of claim 1.

[0010] The present invention is based on the problem to provide an implantable electrode array which allows for MRI-based localization of its standard or micro-sized planar electrode contacts.

[0011] This problem is solved by the implantable electrode array according to claim 1.

[0012] Further aspects of the invention are defined in the dependent claims.

[0013] Accordingly, the invention comprises an implantable electrode array suitable for being placed in anatomic tissue of a human or animal body, comprising a substrate, and a reference structure, the reference structure being formed by a number of notches arranged in at least one outer edge of the substrate, the reference structure defining a spatial relationship with predefined points of the array, wherein the predefined points are electrode contacts, the notches defining a grid over the surface of the electrode array, wherein the notches are arranged in at least two adjacent outer edges of the array, wherein the electrode contacts being in the crossings of the grid lines.

[0014] Advantageous embodiments of the invention may comprise the following features. Each notch may be spaced apart from each other notch.

[0015] The grid may be a rectangular, regular grid.

[0016] The implantable electrode array may comprise at least one layer of a polymer, in particular selected from the group comprising silicone rubber, polyurethane, polyimide, epoxy, liquid crystal polymer, and parylene.

[0017] The array may be a planar and flexible array.

[0018] Provided is also a method of magnetic resonance imaging, MRI, of the implantable electrode array according to the invention as defined above, when implanted in a human or animal body. While the method is not claimed as such, it is useful to understand the invention. The method comprising acquiring at least one first T2-weighted image of a body region comprising the implantable array, and at least one second T2-weighted image of the body region comprising the implantable array, the first image being acquired at a first echo time, TE_1, the first echo time being smaller than the transverse relaxation time, T2*, of silicone, the second T2-weighted image being acquired at a second echo time, TE_2, the second echo time being selected to be equal to or greater than the transverse relaxation time, T2*, of the substrate, but smaller than the transverse relaxation time of body tissue, generating a difference image on the basis of the at least one second image and the at least one first image, wherein the first sequence and the second sequence comprise identical relaxation time, TR, inversion time, TI, and flip angle (α) set-ting.

[0019] Hereby, the at least one and second images may be acquired on the basis of first and second predetermined gradient echo sequences.

[0020] Various embodiments will be described below in further detail in connection with the drawing. It is understood that these embodiments are not limiting the scope of the invention, which is solely defined by the appended claims. Fig. 1illustrates schematically the implantable electrode array according to an embodiment of the invention, Fig. 2illustrates the implantable electrode array according to embodiments of the invention, Fig. 3illustrates sectional view of the implantable electrode array according to an embodiment of the invention.

[0021] Fig. 1 and Fig. 2 illustrate embodiments of the implantable array, that is a planar implantable electrode array. The implantable electrode array comprises at least one substrate 1 made from a biocompatible polymer. The polymer can be selected from the group comprising silicone rubber, polyurethane, polyimide, epoxy, liquid crystal polymer, and parylene.

[0022] The implantable electrode array is preferably flexible such that it can adapt its form to the form of the location in the cortex of a human or animal where it is placed. The electrode array comprises sensor electrode contacts 2 of diameter c on its surface 8. The sensor contacts are equidistant to each other, and are arranged in columns and rows, i.e., they define a regular rectangular grid over the surface of the electrode array.

[0023] The electrode contacts 2 may be located in apertures 6 in the polymer 1 of a diameter smaller than diameter c, refer to Fig. 3. Typical diameters c of the electrode contacts 2 are 4 mm. The apertures 6 may have a diameter of 2.3 mm, refer to Fig. 2A. Typical electrode arrays may comprise e.g. 3x3 electrode contacts or 3x6 electrode contacts, depending on the intended application. A typical spacing between two electrode contacts 2 (interelectrode spacing) may be 10 mm. Microelectrode arrays, according to a further embodiment of the invention, may have contacts 4 with diameters of about 0.3 mm, refer to Fig. 2B.

[0024] Additionally to, or instead of the contacts 2, other predefined points of interest on the implant array may be referenced via the reference structure. The predefined points 2 which can be referenced by the patterns 51, 52, 53, 54, 55 are electrical contacts 2, i.e., sensor contacts, whose material may be selected from metal, conductive polymer, carbon on the implantable array, as described above.

[0025] The implantable electrode array further comprises a reference structure 5. The reference structure 5 is formed by notches 51, 52, 53, 54 along the four outer edges of the substrate 8 of the electrode array. The notches 51, 52, 53, 54 are distinct from each other and regularly spaced apart from each other. Each of these notches is concavely formed, i.e., forms a recess in the respective outer edge towards the inner of the array. All the notches 51, 52, 53, 54 have identical shape.

[0026] The notches 51, 52, 53, 54 are located at positions on straight elongated lines 7 through the contacts 2. The dimension of the notches may be the same as the diameter of the contacts.

[0027] In other words, the reference structure 5, i.e., the notches 51, 52, 53, 54 define a spatial relationship with the electrode contacts 2 of the implant array. The spatial relationship is a regular, rectangular grid 7 over a surface 8 of the array, wherein the electrode contacts are in the crossings of the grid lines.

[0028] In MRI of the electrode array, the edges thereof with the notches 51, 52, 53, 54 are visible, as long as the substrate 8 of the electrode array is visible. Since the notches 51, 52, 53, 54 define a grid over the surface 8 of the electrode array comprising the electrode contacts 2, the positions of the sensor contacts 2 can be determined as being located at the (imaginary) crossing points of the (imaginary) grid lines in the MRIs. Therefore, the notches 51, 52, 53, 54 can be used as reference structure 5 for localizing the electrode contacts 2 in the MRIs, regardless whether or not the electrode contacts 2 themselves are correctly imaged in the MRIs.

[0029] In the following, the method of magnetic resonance imaging, MRI, of the implantable electrode array as described before, implanted in a human or animal body is outlined. The method is not part of the invention.

[0030] Hereto, T2-star (T2* = effective transverse relaxation time) weighted images are used, especially acquired with Gradient Echo (GRE) imaging sequences.

[0031] The substrate material (e.g., silicone) has a very short T2* at the clinical field strengths that are typically used for MRI (e.g., B = 0.2 Tesla to 3.0 Tesla) of about 1-2 ms or less. The MRI signal S in Gradient Echo imaging (GRE) is given by S(TE; T2*)= S_0 * EXP(-TE / T2*), where TE is the variable echo time, and S_0 is the signal at a (fictitious) echo time TE=0 ms. On the other hand, most tissues have T2* which are longer than 2 ms, so that the factor EXP(-TE / T2*) for them does not vary much, if TE is varied in the range of 0.5-2 ms, whereas for silicone the signal at a TE_1 of 1-2 ms is much lower as compared to a TE_2 of 0.5 ms or less.. Thus, the signal difference Delta_S = S TE_ 1 − S TE_ 2 = S_ 0 * EXP − TE_ 1 / T 2 * − EXP − TE_ 2 / T 2 * is non-vanishing for the substrate (e.g., silicone), but it does vanish for brain tissue.

[0032] A marker from substrate material (e.g. silicone) can thus be selectively visualized in a difference image acquired with two different echo times, where one is less than the T2* of the substrate material (e.g., silicone), and the other is of the order or slightly larger than the substrate (silicone) T2*, but still lower than that of normal (brain) tissue.

[0033] Thus, the method comprises the steps of: Acquiring at least one first T2-weighted image of a body region comprising the implantable array, and at least one second T2-weighted image of the body region comprising the implantable array; the first image being acquired at a first echo time, TE_1, the first echo time being smaller than the transverse relaxation time, T2*, of the substrate material (silicone); the second T2-weighted image being acquired at a second echo time, TE_2, the second echo time being selected to be equal to or greater than the transverse relaxation time, T2*, of the substrate material (silicone), but smaller than the transverse relaxation time, T2*, of body tissue; generating a difference image on the basis of the at least one second image and the at least one first image.

[0034] The at least one first and second images are acquired on the basis of first and second predetermined Gradient Echo (GRE) sequences.

[0035] Herein, the first sequence and the second sequence comprise identical relaxation time TR, inversion time TI, and flip angle (α) setting.

[0036] The invention is defined by the appended claims.

Claims

1. An implantable electrode array suitable for being placed in anatomic tissue of a human or animal body, comprising a substrate, and a reference structure (5), the reference structure being formed by a number of notches (51, 52, 53, 54) arranged in at least one outer edge of the substrate, the reference structure (5) defining a spatial relationship with predefined points (2) of the array, wherein the predefined points (2) are electrode contacts, the notches (51, 52, 53, 54) defining a grid (7) over the surface (8) of the electrode array, wherein the notches (51, 52, 53, 54) are arranged in at least two adjacent outer edges of the array, characterized by the electrode contacts being in the crossings of the grid lines.

2. The implantable electrode array of claim 1, wherein each notch (51, 52, 53, 54) is spaced apart from each other notch (51, 52, 53, 54).

3. The implantable electrode array of the preceding claim, wherein the grid (7) is a rectangular, regular grid (7).

4. The implantable electrode array of one of the preceding claims, wherein the implantable array comprises at least one layer of a polymer, in particular selected from the group comprising silicone rubber, polyurethane, polyimide, epoxy, liquid crystal polymer, and parylene.

5. The implantable electrode array of one of the preceding claims, wherein the array is a planar and flexible array.