Implantable ferromagnetic markers

1 . An implantable marker for use in surgical guidance comprising

one or more ferromagnetic elements formed of at least one ferromagnetic material; and

at least one diamagnetic element formed of at least one diamagnetic material; wherein the at least one diamagnetic material comprises graphite having a substantially isotropic grain structure and the one or more ferromagnetic elements are juxtaposed the at least one diamagnetic element, wherein the graphite is isostatically pressed graphite.

2 . The implantable marker as claimed in claim 1 , wherein the graphite has a bulk susceptibility of more than −0.16×10 −4 .

3 . The implantable marker as claimed in claim 1 , wherein the graphite has an apparent bulk susceptibility in the marker of more than −0.9×10 −4 .

4 . The implantable marker as claimed in claim 1 , wherein the isostatically pressed graphite has an apparent bulk susceptibility in the marker of at least about −1.2×10 −4 .

5 . The implantable marker as claimed in claim 4 , wherein the isostatically pressed graphite contains fewer than 300 ppm of impurities.

6 . The implantable marker as claimed in claim 4 , wherein the isostatically pressed graphite contains more than 99.9% carbon.

7 . The implantable marker as claimed in claim 1 , wherein the isostatically pressed graphite has a density of at least about 1.75 g/cm 3 and optionally has a density of about 1.85 g/cm 3 .

8 . The implantable marker as claimed in claim 1 , wherein the graphite is heat treated graphite.

9 . The implantable marker as claimed in claim 1 , wherein the one or more ferromagnetic elements comprise one or more wires or strips of ferromagnetic material having a length to diameter (or length to square root of cross-sectional area) ratio of at least 50.

10 . The implantable marker of claim 1 , wherein the ferromagnetic material comprises a ferromagnetic metal, amorphous metal or ceramic ferrite.

11 . An implantable marker for use in surgical guidance comprising:

one or more ferromagnetic elements formed of at least one ferromagnetic material; and

at least one diamagnetic element formed of at least one diamagnetic material;

wherein the at least one diamagnetic material comprises graphite having a substantially isotropic grain structure and the one or more ferromagnetic elements are juxtaposed the at least one diamagnetic element, and

wherein the at least one diamagnetic element has a total volume that is about 100-10,000 times greater than a volume of the one or more ferromagnetic elements.

12 . A method of manufacturing an implantable magnetic marker for use in surgery, the method comprising steps of:

forming one or more ferromagnetic elements from at least one ferromagnetic material;

forming at least one diamagnetic element, wherein the at least one diamagnetic element comprises isostatically pressed graphite having a substantially isotropic grain structure; and

thereafter assembling the one or more ferromagnetic elements and the at least one diamagnetic element such that the one or more ferromagnetic elements are juxtaposed the at least one diamagnetic element, wherein the one or more ferromagnetic elements and the at least one diamagnetic element are configured and arranged to produce mutually opposing magnetic moments in a presence of an applied magnetic field.

13 . The method of manufacturing the implantable magnetic marker as claimed in claim 12 , wherein the method includes step of performing an isostatic pressing process to provide the isostatically pressed graphite having the substantially isotropic grain structure.

14 . The method of manufacturing the implantable magnetic marker as claimed in claim 12 , further comprising step of heat treating the graphite at a temperature in excess of about 2,200 degrees Celsius.

15 . A detection system for locating an implantable marker, the system comprising:

the implantable marker for use in surgical guidance comprising one or more ferromagnetic elements formed of at least one ferromagnetic material and at least one diamagnetic element formed of at least one diamagnetic material; wherein the at least one diamagnetic material comprises graphite having a substantially isotropic grain structure and the one or more ferromagnetic elements are juxtaposed the at least one diamagnetic element, wherein the implantable marker is configured to be detected by a susceptibility probe;

at least one drive coil arranged to excite the implantable marker with an alternating magnetic field and at least one sense coil arranged to detect a signal received from the excited implantable marker;

a magnetic field generator arranged to drive an alternating magnetic field through the at least one drive coil; and

at least one detector arranged to receive the signal from the sense coil and detect one or more harmonics of the drive frequency in the received signal, wherein the susceptibility probe comprises the at least one detector; and wherein the graphite is isostatically pressed graphite.

16 . The detection system of claim 15 , wherein the at least one diamagnetic element has a total volume that is about 100-10,000 times greater than a volume of the one or more ferromagnetic elements.

17 . The detection system of claim 15 , wherein the strength of the sensing field gives rise to a field strength that ranges from about 40 μT and about 400 μT within about 5 mm of the susceptibility probe.

18 . The detection system of claim 17 , wherein the strength of the sensing field gives rise to a field strength that ranges from about 40 μT to about 400 μT within about 5 mm of the susceptibility probe.

19 . The implantable marker of claim 15 , wherein the alternating magnetic field is a sensing field, wherein the sensing field has a strength that ranges from about 0.1 mT to about 2.0 mT.

20 . The implantable marker of claim 19 , wherein the strength of the sensing field gives rise to a field strength that ranges from about 40 μT to about 400 μT within about 5 mm of the susceptibility probe.