Field of the invention

The present invention relates to image-guided placement of tools in medical procedures. In particular, this invention relates to tracking tools used for measurement or research of biological tissue, and for therapy thereof, by stimulating the tissue electromagnetically. Another particular field of interest for the invention is image-guided surgery, where a surgical or biopsy tool or is guided to the tissue based on tomographic images. To be precise, the present invention relates to what is stated in the preamble portion of the independent claims.

Prior art

Biological tissue can be stimulated using electromagnetic pulses. This is beneficial for measuring the condition of the patient or for therapeutic purposes. One application is transcranial magnetic stimulation (TMS), where an electromagnetic pulse is generated by a current-carrying coil placed over the head. The pulse has the capacity to stimulate neu- rons in the brain. The resulting neuronal stimulation depends on the characteristics of the applied electromagnetic field; important characteristics include the field strength, its vector direction with respect to the stimulated neurons, and the pulse shape and length. Therefore, in TMS it is important to be fully aware of the location and orientation of the stimulating coil with respect to the head. Image-guidance tools are used for this purpose.

Image-guidance refers to the application of tools with respect to anatomical structures in the human body on the one hand, and to tomographic images of the body on the other. The tomographic images are acquired by using magnetic resonance imaging, computed tomography, ultrasound or other techniques. A tool is then placed into the tissue with the help of 3D stereotactic tracking. For doing this, the tomographic images are first co- registered to the living body, thereby creating a virtual 3D world on a computer screen with the tool visible as on overlay on the anatomical images. By providing fast feedback to the operator, the operator is capable of placing and orienting the tool in the desired location within the body. A particular field of interest related to image-guidance in the scope of this invention is stereotactic guidance of non- invasive nerve stimulation to the human body. This can be done by a changing magnetic field that induces a stimulating electric field in the tissue. When targeted in the brain, for instance, magnetic nerve stimulation is capable of trigger- ing activity in neurons in selected parts of the brain. Image-guided method of magnetic stimulation is particularly useful for mapping cortical functions and for detecting changes in the neuronal excitability due to disorders, trauma etc. Methods and apparatuses related to the described art are disclosed in publications EP 1366782 Al and US 2008058582 Al.

Another particular application of the described art is image-guided surgery, where a surgical tool is guided to a location within the tissue based on tomographic images of the body or a part of it. The surgical intervention can be a biopsy, where a biopsy needle is placed, e.g., into a location of the body where MRI shows contrast-enhancement possibly related to a cancerous tissue. The intervention can also be a resection of tissue. Image- guided surgeries are performed everywhere in the body.

A frequently used tool for stereotactic localization is an optical tracking unit. Optical tracking units send infrared light and measure, using CCD cameras, the light that is reflected back by a plurality of reflectors. The reflectors are placed on the tools that are to be tracked. Traditionally, these reflectors are plastic spheres with reflective surface mate- rial. The reflectors are conventionally placed accurately in predefined geometry and at least 3 reflectors are needed to determine the location and orientation of the tool with respect to the tracking unit.

In magnetic stimulation, a reference tracker equipped with three to four reflectors is placed on the forehead of a subject, and a set of three to 12 or more reflectors are placed on the stimulating coil. The tracking unit then sends the coordinates and orientations of the reference tracker and the coil to a computer. For example, a commercially available NDI Polaris Spectra (also Vicra) unit locates tools by measuring the location of infrared- reflecting spheres. At least three of these spheres are attached to a tool, such as transcranial magnetic stimulation coil, in a known geometry. The system then can measure the 3D location with 6 degrees of freedom with respect to the localization unit. Disadvantages of the prior art

However, conventional tracking apparatuses feature considerable disadvantages. An essential problem is that they are not particularly robust but rather high-maintenance. The reflectors of conventional tracking systems are coated with reflecting material that is vul- nerable to scratching due to everyday use. They also gain dirt, which is difficult to clean off the traditionally porous surfaces. Also, current reflector materials do not allow multiple sterilizations and are rather fragile and tend to break if accidentally dropped to the ground. The tracker bodies to which the reflectors are attached have similar disadvantages as the reflectors.

Object of the invention

It is the objective of this invention to mitigate at least some disadvantages of the prior art and to provide a tracker that provides better visibility to optical tracking systems for locating medical tools more accurately and reliably.

Summary of the invention The present invention is based on a tracking apparatus that has a plurality of reflectors attached to the subject of tracking, i.e. the tool, such as a stimulator coil. The tracking apparatus further has at least one tracking unit having means for emitting and receiving radiation and also being able to locate the plurality of reflectors with aid of the tracking radiation. The plurality of reflectors is enclosed in an enclosure that is made of shielding material essentially transparent to the tracking radiation. To be precise, the invention is characterized by what is stated in the characterizing portion of the independent claim 1.

Advantages

Considerable advantages are gained with the aid of the invention. Because the reflector spheres are protected by an enclosure, they are not exposed to everyday wearing making them more robust and their maintenance easier while not compromising tracking accuracy. Since the enclosure is made of protective material, it receives all the wearing leaving the reflectors undamaged. According to one embodiment of the invention, further advantages can be gained by making the enclosure of a material such as acryl or plexiglass that is easy to clean and maintain, which improves already enhanced maintenance.

According to another embodiment of the invention, the reflector spheres are fitted within a tube-like enclosure, which improves the visibility of the reflectors while there is reduced need for complex supporting structures impeding visibility. Therefore a tracker can be constructed with at least 3 reflectors always visible in each orientation and location of the tool. In addition, the described arrangement requires less reflector spheres compared to conventional apparatuses.

The invention enables the above-mentioned solutions and advantages to be obtained also in other medical applications utilizing 3D optical localization.

Brief description of the drawings

In the following the present invention is described in greater detail with references to the accompanying drawings, in which:

Fig. 1 shows a traditional stimulator coil with exposed reflectors.

Fig. 2 shows a stimulator coil according to the invention with enclosed reflectors.

Fig. 3 shows the trajectory of the enclosure according to one embodiment of the invention.

Fig. 4 shows an overview of a tracking system according to prior art.

Detailed description of the invention

As illustrated in Figs. 1 and 4, a plurality of reflectors 2 has traditionally been attached to tools, such as a stimulator coil 1, without shielding structures. When tracking a stimulator coil 1, at least three reflectors are usually required. In addition, the three reflectors must naturally not be aligned on the same imaginary straight line. However, the more trackers are coupled to the object of tracking 1, the more accurate information can be gathered about the location and position of the object. In the example, according to the present invention, depicted in Fig. 1, a stimulator coil 1 has been equipped with 12 reflectors 2 to provide a sufficient amount of vital tracking information assisting with firing stimulating pulses to the correct part of the patient's brain in the right angle. A large number of reflectors 2 is needed for ensuring that at least one reflector 2 is always visible to the detecting device regardless of the position and orientation of the tool 1 under observation.

The reflectors 2 are usually spherical plastic protuberances equipped with a reflecting surface, similar to retrorefiecting material used in reflective stripes found on high- visibility jackets, for example. The reflectors 2 are therefore adapted to reflect tracking radiation emitted by at least one tracking unit 4. These tracking units 4 emit radiation, such as infrared radiation, which is reflected back to the units. An often used tracking unit 4 is the Polaris Passive, or Spectra or Vicra Optical Tracking Unit manufactured by Northern Digital Inc. (Canada). Based on the emitted and received radiation pattern, the tracking units 4 (Fig. 4) are able to conclude the location and position of the object of tracking. This naturally requires establishing a fixed pattern of reflectors 2 so that the reflected radiation pattern is unequivocal. The tracking unit 4 then sends the coordinates and orientations of the tracking object to a computer 7, or a suchlike data processor. The system can then measure the 3D location with 6 degrees of freedom with respect to the tracking unit.

As discussed earlier, the reflectors 2 are vulnerable to wear and tear because they are exposed to superficial damages caused by everyday use. Wearing of the reflecting surface of the reflectors 2 impede their ability to reflect tracking radiation, which compromises the whole tracking process. To address this problem, an enclosure is provided according to the present invention. As illustrated in Fig. 2, the reflectors 2 of the stimulator coil 1 are enclosed in a shielding enclosure 3. According to one embodiment, a similar enclosed reflector arrangement is also provided for the patient. The enclosure 3 is made of shield- ing material, such as acryl glass. The material material is preferably suitable for cleaning, disinfection or sterilization through application of heat, chemicals, irradiation, high pressure or filtration, or their combination. The material shall also be essentially transparent to the tracking radiation 5 emitted by the tracking unit 4. In this context, by essentially transparent is meant material is adapted to allow at least 50 % of the radiation in ques- tion to permeate. This way the enclosure 3 protects reflectors 2 from wearing while letting tracking radiation 5 through. Preferably, the enclosure 3 lets at least 85 per cent of the tracking radiation 5 through to the reflectors 2. In addition to being protective and essentially transparent to tracking radiation 5, the wall thickness of the enclosure 3 is preferably as thin as possible to avoid excess weight making the structure light and easy to operate. The suitable thickness varies with the geometrical shape of the tracker. The thickness can vary along the length of the tracker. The farther off the enclosure 3 is adapted from the reflectors 2, the thicker its surface can be. As a rule of thumb, the wall thickness near the reflectors 2 can vary between 0.5 and 5 mm.

According to the invention, the material and its thickness should be such that the radiated light does not significantly refract in the material. Refraction bends the light and can in- troduce errors in the localization. Snell's law of refraction can be used to choose the material and the wall thickness. The deflection y of the light rays in a material of thickness x, and refractory index of n, can be estimated as:

y = x tan(ΔΘ),

where Δθ = θi - Θ ₂ , with θi as the incidence angle. The refraction angle θ ₂ is obtained from the Snell's law Θ2 = asin ( sin(θi) / ή), when the incident light is in air. For example, if the reflector should be seen at an angle of ±30 degrees with respect to the surface of the protective enclosure 3, and the protective enclosure 3 is planar acrylic (refractory index = 1.49), the wall thickness of 3 mm would result in 0.55 mm error in localization due to the enclosure. However, it interestingly has transpired that 0.5 to 1 mm errors are typi- cally allowable in image-guidance applications, the reflection error caused by the enclosure wall does not compromise the operation. Due to this revelation, the trackers may be equipped with an enclosure 3 conforming to allowable error tolerances described above.

It is advantageous that the surface shape of the protective enclosure is circular in shape when the reflectors are spherical in shape. A tube-like structure is one solution, which limits refraction in one direction and thereby allows use of thicker wall of the protective enclosure.

According to one embodiment of the invention, at least three, preferably four, spherical reflectors 2 are attached to a stimulator coil 1. The reflectors 2 are fitted within a tube- like enclosure 3 having a circular cross-section. The plurality of reflectors 2 is required for the tracking unit 4 to determine the locations of the stimulator coil 1 for all rotation angles. According to another embodiment, the reflectors 2 may also be fitted into a large essentially transparent enclosure of another shape. For example, instead of a tube-like enclosure 3, the casing can be a prism accommodating the reflectors 2 in such a manner that the sphere locations appear the same from all directions, i.e., light refraction is minimized.

The coating can be used on the surface of the enclosure to reduce reflection of the infrared light of the tracking unit. The coating naturally needs to be essentially transparent to the IR radiation. The enclosure can be made of composite materials, or even of compos- ite materials where one part has positive refractory index and the other part has negative refractory index, whereby the refractoriness of the enclosure is further limited, preferably cancelled out.

According to another embodiment of the invention, the outer surface of the enclosure 3 is coated with an antireflective material, whereby the tracking radiation is further prevented from reflecting away from the tool 1.

In a method according to the present invention tracking radiation 5 is emitted and received with a tracking unit 4 as described above and based on the received radiation 5 pattern, the position of the tool 1 is computed. This naturally requires first establishing, how the reflectors 2 are positioned relative to the tool 1. According to one embodiment, reflectors are attached to a TMS coil and a patient under treatment. In the method, the tracking radiation 5, such as infrared light, is reflected by the reflectors through an enclosure 3 that is essentially permeable to the radiation 5. In this context, by essentially permeable is meant that the material in question is adapted to allow at least 50 % of the radiation in question to permeate. Methods for emitting and receiving tracking radiation as well as computing the position of the reflectors and tool is known.

The above description of the invention is only of illustrative nature. Various alternatives and modifications can be devised by those skilled in the art without departing from the spirit of the invention. Accordingly, the present invention embraces all such alternatives, modifications, and variances that fall within the scope of the following claims.