Implantable array with reference structure and imaging method thereof

Abstract

An implantable array adapted for placement in anatomical tissue of a human or animal body comprises a substrate and a reference structure formed by a plurality of notches disposed in at least one outer edge of the substrate, the reference structure defining a spatial relationship to a predefined point of the array.

Description

Technical Field

[0001] The present invention relates to an implantable array having a reference structure that is visible in magnetic resonance images (MRI) when the array is implanted in, for example, a human or animal body, and to a method of imaging an electrode array. Background Art

[0002] Such an array could be an array of electrodes implanted on the surface of the brain, for example, to assess cortical electrical activity in epilepsy patients before surgery.

[0003] Therefore, MRI is used to locate the electrode contacts of an implanted electrode array relative to the patient's anatomy. However, metallic components or other well-conducting materials such as carbon and conductive polymers can cause MRI artifacts that can affect the results, especially near these components and materials.

[0004] Thin-film implants, which may have metal thickness 100 times less, could mitigate these artifacts, but they produce subtle MRI signal voids in many clinical MRI sequences and are therefore not suitable for electrode localization by MRI. Very small metal contacts or very thin metal layers may not be visible at all in clinical MRI.

[0005] But the most urgent thing is that doctors need to know the location of the implant, and the intracranial EEG value increases with the improvement of electrode positioning accuracy. Summary of the Invention

[0006] The present invention is based on the problem of providing an implantable array for MRI-based positioning of its standard or microplanar electrode contacts, as well as a method for MR imaging of such an array.

[0007] This object is solved by an implantable array and a method of imaging an implantable array according to the respective independent claims. Further aspects of the invention are defined in the dependent claims.

[0008] Thus, there is provided an implantable array adapted for placement in anatomical tissue of a human or animal body, comprising

[0009] substrate, and

[0010] a reference structure formed by a plurality of notches arranged in at least one outer edge of the substrate,

[0011] A reference structure that defines a spatial relationship to predefined points of an array.

[0012] Advantageous embodiments of the present invention may include the following features.

[0013] The notches may be positioned in at least two adjacent outer edges of the array.

[0014] Each notch may be spaced apart from the other notches.

[0015] The spatial relationship may be on the surface of the array.

[0016] The grid may be a regular grid of rectangles.

[0017] The location can be at least one of the following

[0018] - electrode contacts,

[0019] - Fluid interfaces,

[0020] -Points with geometric significance.

[0021] The implantable array may comprise at least one layer of a polymer, in particular selected from silicone rubber, polyurethane, polyimide, epoxy resin, liquid crystal polymer and parylene.

[0022] The array may be an electrode array.

[0023] Arrays can be planar or flexible.

[0024] There is also provided a method of magnetic resonance imaging (MRI) of an implantable array according to any one of the preceding claims, when implanted in a human or animal body, comprising:

[0025] acquiring at least one first T2-weighted image of the body region including the implantable array and at least one second T2-weighted image of the body region including the implantable array,

[0026] The first image is acquired at the first echo time TE_1, which is less than the transverse relaxation time T2* of the silicone resin.

[0027] a second T2-weighted image is 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 less than the transverse relaxation time of body tissue,

[0028] generating a difference image based on at least one second image and at least one first image,

[0029] The first sequence and the second sequence include the same relaxation time TR, inversion time TI and flip angle (α) settings.

[0030] Thus, the at least one and the second image may be acquired based on the first predetermined gradient echo sequence and the second predetermined gradient echo sequence.

[0031] The present invention and its embodiments are described in further detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Schematically shows an implantable electrode array according to an embodiment of the present invention,

[0033] Figure 2 shows an implantable electrode array according to an embodiment of the present invention,

[0034] Figure 3 A cross-sectional view of an implantable electrode array according to an embodiment of the present invention is shown.

[0035] Figure 1 and Figure 2 An embodiment of an implantable array is shown, namely a planar implantable electrode array. The implantable electrode array comprises at least one substrate 1 made of a biocompatible polymer. The polymer may be selected from silicone rubber, polyurethane, polyimide, epoxy resin, liquid crystal polymer and parylene. Specific embodiments

[0037] The implantable electrode array is preferably flexible so that its form can be adapted to the form of the location in the cortex of the person or animal in which it is placed. The electrode array comprises sensor electrode contacts 2 of diameter c on its surface 8. The sensor contacts are equidistant from one another and are arranged in columns and rows, i.e., they define a regular rectangular grid on the surface of the electrode array.

[0038] See also Figure 3 , the electrode contacts 2 can be located in pores 6 in the polymer 1 having a diameter smaller than the diameter c. A typical diameter c of the electrode contacts 2 is 4 mm. Figure 2 A, the hole 6 may have a diameter of 2.3 mm. A typical electrode array may comprise, for example, 3x3 electrode contacts or 3x6 electrode contacts, depending on the intended application. A typical spacing between two electrode contacts 2 (inter-electrode spacing) may be 10 mm. See Figure 2 B. According to another embodiment of the present invention, the microelectrode array may have contacts 4 with a diameter of about 0.3 mm.

[0039] In addition to contact 2, or in place of contact 2, other predefined points of interest on the implant array may be referenced by the reference structure. Thus, most generally, a predefined point 2 that may be referenced by notches 51, 52, 53, 54 may be

[0040] a) As mentioned above, the electrode contacts 2, i.e., the sensor contacts, can be made of materials selected from metals, conductive polymers, and carbon on implantable arrays.

[0041] b) fluid interfaces (e.g. inlets or outlets for transferring liquids into or out of the human or animal body),

[0042] c) Other points that have special geometric significance to the physician.

[0043] The implantable electrode array also includes a reference structure 5. The reference structure 5 is formed by notches 51, 52, 53, and 54 along the four outer edges of the substrate 8 of the electrode array. The notches 51, 52, 53, and 54 are different from each other and are regularly spaced apart from each other. Each of these notches is concavely formed, that is, a recess is formed in the respective outer edge toward the interior of the array. All notches 51, 52, 53, and 54 have the same shape.

[0044] The notches 51, 52, 53, 54 are located at positions on the elongated straight line 7 passing through the contact 2. The size of the notches may be the same as the diameter of the contact.

[0045] In other words, the reference structure 5, i.e. the notches 51, 52, 53, 54, defines a spatial relationship to the electrode contacts 2 of the implant array. The spatial relationship is a regular rectangular grid 7 on the surface 8 of the array, with the electrode contacts located at the intersections of the grid lines.

[0046] In an MRI of the electrode array, whenever the electrode array's base 8 is visible, its edge with the notches 51, 52, 53, 54 is visible. Because the notches 51, 52, 53, 54 define a grid on the electrode array's surface 8, including the electrode contacts 2, the location of the sensor contacts 2 can be determined as being located at the (virtual) intersections of the (virtual) grid lines in the MRI. Therefore, the notches 51, 52, 53, 54 can be used as reference structures 5 for locating the electrode contacts 2 (or other points of interest) in the MRI, regardless of whether the electrode contacts 2 themselves are properly imaged in the MRI.

[0047] In the following, a method of magnetic resonance imaging (MRI) of an implantable electrode array as described above implanted in a human or animal body is outlined.

[0048] To date, T2 star (T2* = effective transverse relaxation time) weighted images have been used, in particular images obtained using gradient echo (GRE) imaging sequences.

[0049] At clinical field strengths typically used for MRI (e.g., B = 0.2 Tesla to 3.0 Tesla), substrate materials (e.g., silicone) have very short T2*, on the order of 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 the (virtual) echo time TE = 0 ms. On the other hand, most tissues have a T2* exceeding 2 ms, so their factor EXP(-TE / T2*) does not change much if TE is varied within the range of 0.5-2 ms, while for silicone, the signal at TE_1 of 1-2 ms is much lower compared to TE_2 of 0.5 ms or less. Therefore, the signal difference

[0050] Delta_S=S(TE_1)-S(TE_2)=S_0*(EXP(-TE_1 / T2*)-EXP(-TE_2 / T2*))

[0051] With substrates (e.g., silicone), it does not disappear, but with brain tissue it does disappear.

[0052] Therefore, markers from the underlying material (e.g., silicone) can be selectively visualized in difference images acquired using two different echo times, one of which is smaller than the T2* of the underlying material (e.g., silicone) and the other of which is T2* or slightly larger than the underlying (silicone) T2*, but still lower than normal (brain) tissue.

[0053] Therefore, the method comprises the following steps:

[0054] acquiring at least one first T2-weighted image of the body region including the implantable array, and at least one second T2-weighted image of the body region including the implantable array;

[0055] Acquire a first image at a first echo time TE_1, which is less than the transverse relaxation time T2* of the substrate material (silicone resin);

[0056] acquiring a second T2-weighted image 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 less than the transverse relaxation time T2* of the body tissue;

[0057] A difference image is generated based on the at least one second image and the at least one first image.

[0058] At least one first image and a second image are acquired based on a first predetermined gradient echo (GRE) sequence and a second predetermined gradient echo (GRE) sequence.

[0059] Here, the first sequence and the second sequence include the same relaxation time TR, inversion time TI, and flip angle (α) settings.

Claims

1. An implantable array suitable for placement in anatomical tissue of a human or animal body, said array being an electrode array comprising substrate, and a reference structure (5) formed by a plurality of notches (51, 52, 53, 54) placed in at least one outer edge of the substrate, The reference structure (5) defines a spatial relationship with a predefined point (2) of the array, The predefined point (2) is the electrode contact point, The notches (51, 52, 53, 54) define a grid (7) on the surface (8) of the electrode array including the electrode contacts (2), in, The notches (51, 52, 53, 54) are placed in at least two adjacent outer edges of the array, The invention is characterized in that the electrode contacts are located at the intersections of the grid lines.

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

3. An implantable array according to any one of the preceding claims, wherein The grid (7) is a rectangular regular grid (7).

4. An implantable array according to any one of the preceding claims, wherein The implantable array comprises at least one layer of a polymer selected from the group consisting of silicone rubber, polyurethane, polyimide, epoxy resin, liquid crystal polymer, and parylene.

5. An implantable array according to any one of the preceding claims, wherein The array is planar and flexible.

Images