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Interactive medical phantom for practicing functional stereotactic interventions
Implementation from idea to first prototype

Interactive medical phantom for practicing functional stereotactic interventions

introduction: What is stereotactic method and stereotactic neurosurgery?

Stereotactic technique of high precision positioning of surgical instruments for therapeutic exposure is widely used for neurosurgical operations in patients suffering from different range of movement disorders, such as Parkinson's disease, essential tremor, dystonia and other functional diseases. The technique of stereotactic surgery consists in plunging a stereotactic instrument along a preliminarily planned trajectory into a required anatomical region of the brain for therapeutic action - in stereotactic neurosurgery this region is a "target" for action. The targeting error should be minimal because deviation from the required target by more than 1 mm in functional stereotactic can significantly reduce the efficiency of the surgery. Exposure factors can be chronic electrostimulation, thermal destruction, cryodestruction, laser ablation, and others.

The surgeon plans the trajectory of stereotactic instrumentation in advance in order to avoid damaging important brain structures (blood vessels, functionally significant brain areas) using neuroimaging data of different modalities (magnetic resonance imaging (MRI), computed tomography (CT), functional MRI, MRI-tractography, etc.). Modern ideas about the organization of brain functions and neuroanatomy allow to assume both the therapeutic effect and the side effect when the influence is extended to known anatomical areas, such as corticospinal tract, spinothalamic, dentatorubrotalamic tract. Thus, the effect of exposure during electrostimulation of deep brain structures affects the manifestation of the disease, for example, reduces the severity of tremor in a patient with Parkinson's disease, stiffness in the limbs, symptoms of hypokinesia; besides the positive effect, stimulation of unwanted brain structures can also cause side effects (general malaise, difficulty in pronouncing words, tightness of muscle groups and so on). Most often the patient is awake during the operation, so that the neurological status can be assessed directly at the moment of the therapeutic intervention. The efficiency of stereotactic intervention depends primarily on the correct choice of a stereotactic target for exposure for a particular clinical case, as well as on the accuracy of hitting the planned target.

Often an additional decision (changing the position of the stereotactic instrument) is required during the surgery depending on the effect of the intervention. This entire process requires a thorough understanding of neuroanatomy and neurophysiology, techniques of operating stereotactic equipment, the ability to plan an intervention at the workstation, and knowledge of the sequence of certain surgical manipulations.

How to perform stereotactic surgery?

The process of training a neurosurgeon to perform such interventions is a complex task. Master classes and assisting an experienced surgeon during surgery and reading specialized literature are the main ways to learn. The task is also complicated by the fact that there are many different types of stereotactic equipment, stereotactic frames, robots and frameless stereotactic systems whose use in such surgeries has its own peculiarities. One of the ways to learn in medicine is to practice on medical phantoms. A stereotactic phantom is a device that simulates a patient's head and allows stereotactic guidance of a surgical instrument into a target using stereotactic techniques.

Such phantom has two functions. Firstly, it is getting (practicing) skills of working with stereotactic equipment. The second important function is to evaluate the accuracy of hitting a stereotactic instrument into a planned target inside the phantom. The main characteristics of stereotactic phantoms are size and shape, material, filling, input and target target design, compatibility with standard stereotactic frames and head-holders, compatibility with standard spatial registration methods, and compatibility with pointing error devices [, Marko, et al. "Stereotactic Neuro-Navigation Phantom Designs: A Systematic Review." Frontiers in Neurorobotics, vol. 14, October 2020, pp. 549603. DOI.org (Crossref), https://doi.org/10.3389/fnbot.2020.549603].

Basic definitions in Stereotactic

For the purpose of further exposition, let us introduce a number of definitions and review the basic elements of stereotactic operation according to the state of the art.

  • Intracranial space is a region of space that contains all anatomical structures under the skull bones (brain, vessels, liquor spaces, etc.).
  • Cranial space - area of space corresponding to the skull bones.
  • Extracranial space - all area of space except for cranial and intracranial space.
  • Brain coordinate system - ordered intracerebral space where each point of the brain is assigned coordinates in a given coordinate system.
  • Medical image modality is a term in radiology to refer to a form of imaging, e.g., CT brain imaging, various sequences of magnetic resonance imaging of the brain, CT brain angiography, etc.
  • Localizers (localizers, adapters) - various types of auxiliary stereotactic devices with the presence of landmarks (reference points), using which the coordinate system of neuroimaging data is transformed into the coordinate system of stereotactic manipulator.
  • Stereotactic tomography (localization) - neuroimaging examination (MRI, CT and others) performed to a patient before the surgery and necessary for stereotactic calculations.
  • Stereotactic calculations - a series of affine geometric transformations required to "snap" between coordinate systems. By "snapping" we mean the possibility to determine an object's coordinates in one of the coordinate systems by the coordinates of the same object in another coordinate system using the known transformation function.
  • Localization neuroimaging data - tomograms of patient's head with stereotactic localizer.
  • Alignment (merging) of medical images - a method of geometric transformations (linear and non-linear) of medical images, used to align several images to the main (reference) image, providing spatial matching of anatomy on images of different modalities of the same patient.
  • Reference image - neuroimaging modality, coordinate system of which is used as the main one, into which coordinate systems of all other modalities will be transformed for this patient.
  • Registration - the process of linking neuroimaging data space and physical space of the head, brain, stereotactic localizer and manipulator.
  • Spatial matching (linking, binding, geometric transformation) - calculation of function or transformation matrix, using which it is possible to restore object position from one coordinate system to another.
  • Stereotactic markup (stereotactic tomography or localization tomography) - performing computed tomography or magnetic resonance imaging of a patient's head with a localizer fixed to it.
  • Stereotactic target or target target - required point in the brain structure that has specified coordinates in space, where the tip of stereotactic instrument will be directed for therapeutic action.
  • Stereotactic manipulator - positioning device for aiming stereotactic instrument at a target.
  • Contrast mark - object clearly visualized on tomograms.
  • Segmentation of neuroimaging data - process used to classify tissues in a neuroimaging dataset.

    Framebased stereotactic neurosurgery: basic stages

    Let us consider a classical stereotactic operation using frame stereotactic. Stereotactic target is located in the intracranial space and is physically inaccessible for direct measurement of its coordinates, so some extracranial landmark - stereotactic localizer - is used to determine its coordinates.

    Fig. 1 shows a pictorial scheme for stereotactic operation using frame stereotactic.

    Stereotactic procedure has several stages: stereotactic marking, surgical planning, registration, operative stage, intraoperative and postoperative assessment of procedure efficiency. The stages may vary depending on the type of stereotactic equipment used, but the basic principles of stereotactic guidance remain the same.

    The main task of stereotactic guidance is to "link" the space of neuroimaging data to the physical space of a patient's head and stereotactic manipulator by performing a series of affine geometric transformations where the stereotactic localizer acts as a key element.

    The stereotactic frame (1) under local anesthesia is rigidly fixed to the patient's head (2) by screwing pointed screws (3) through the skin into the skull bones. Then a stereotactic localizer (4) is attached to the frame - stereotactic localizer is the basis for setting the origin of the coordinate system (z, x, y) for the stereotactic frame, manipulator and the brain. There are reference points (5) on the localizer. The patient is stereotactically tomographed with the localizer in place. Thus, the obtained tomograms (6) in voxel space (k, i, j) visualize intracranial structures with intracranial target and extracranial reference points (points, elements) of the localizer.

    Surgical planning stage. Obtained stereotactic tomograms are loaded into the planning station (computer), also all neuroimaging data of different modalities (MRI tractography, PET brain and other, made to the patient at preoperative stage) necessary for surgical planning are loaded, which are combined with localization tomography and receive a single neuroimaging (virtual) space. At this stage the surgeon works in the neuroimaging space (virtual space of the patient's head), sets a target target depending on the clinical task, chooses an entry point on the skull (place of a trepanation opening), thereby setting the trajectory of a stereotactic instrument plunge. The planning station has internal algorithms that search for localization reference patterns and calculate the space transformation function (affine transformation) of localization tomography through a series of steps - first into the physical space of the stereotactic localizer, then into the space of the stereotactic manipulator. In fact, the surgeon obtains the necessary parameters that need to be set on the scales of the stereotactic manipulator (7) to ensure that the stereotactic instrument (8) (cannula) is brought to the target point (target) of the brain (9).

    Surgical step. In the operating room, the manipulator (7) is fixed to the stereotactic frame (1). The surgeon, using the data calculated in the previous stage, sets the required parameters on the manipulator scales and performs surgical intervention. At first the surgeon makes trepanation opening in the skull in the point corresponding to access point, then using manipulator (7) he plunges stereotactic instrument (8) along the calculated trajectory into the required stereotactic target (9). The therapeutic action is performed. Intraoperatively the clinical effect is evaluated. If necessary depending on the effect the surgeon could change the position of stereotactic instrument, for example, move the active end of the instrument more laterally or stop the action at all (stop the operation) if it has a pronounced side effect.

    Assessment of the procedure efficiency. After the surgery the patient undergoes control brain tomography in order to exclude complications as well as to assess the accuracy of hitting the planned target point of the brain.

    Not only guides fixed to stereotactic frame but also robotic complexes allowing automatic positioning of stereotactic instruments can act as manipulator (7). In such cases patient's head registration is performed directly in the operating room using an additional series of transformations in order to "tie" the coordinate system of the manipulator and the patient's brain.

    Thus, any modern stereotactic surgery, regardless of the type of equipment used, is reduced to the principle of "binding" of neuroimaging data space to the physical space of a patient's head (brain structures) by means of an explicit or implicitly specified extracranial landmark (localizer).

    Stereotactic training phantoms

    Stereotactic phantoms simulate a human head with an intracranial target and allow to use the same methodology of stereotactic surgery that is performed on the human brain.

    Methods of modeling of stereotactic target and surgery in general proposed in the known state of the art are insufficient for complex training process. Thus, non-anthropomorphic phantoms absolutely do not reflect anatomical and spatial configuration of a patient's head and are actually used only in preclinical and laboratory studies. The surgeon has to abstract his surgical vision quite heavily when working with these types of phantoms.

    The design of anthropomorphic phantoms is tried to be close to the human head anatomy, and working with them is the closest to the real conditions of stereotactic intervention for a surgeon. However, at the stage of stereotactic imaging of known anthropomorphic phantoms the surgeon receives a phantom tomography, which does not correspond to real neuroimaging data of the human brain. Thus, the surgeon is not practicing those principles of surgery planning that are used in brain surgery - selection of the required stereotactic target depending on the diagnosis and manifestation of the disease, planning the trajectory of instrument immersion bypassing functionally significant brain regions and vessels.

    Besides, anthropomorphic phantoms first of all model human skull, however, taking into account conditions of stereotactic localizer fixation stage, it is necessary to model other anatomical elements and landmarks - soft tissues, skin. Often at certain stages of some surgical options, e.g. neurostimulator implantation, it is required to perform subcutaneous extension in the area of anterior chest wall.

    Functional stereotactic surgery is not only a high-precision tool hitting the required anatomical target, but also a surgeon's ability to correctly select brain structures (stereotactic targets) for exposure depending on the assigned clinical task, as well as evaluation of exposure efficiency during surgery.

    Taking into account the above-described limitation of existing stereotactic phantoms according to the state of the art, they have not received proper attention in the practice of training surgeons for stereotactic interventions. The existence of so many variants of the presented phantoms reflects the need to create a universal phantom.

    We have developed a new type of stereotactic phantom with feedback, which is compatible with stereotactic equipment, allows you to interactively assess the effectiveness of surgery depending on the clinical task and allows the surgeon to work with training data of human brain neuroimaging when planning the surgery.

    According to the present principles, the phantom is a set of hardware and software including a non-contact positioning evaluation system, a stereotactic tomography data modification module, and a decision-making unit.

    The phantom intracranially contains registration reference marks with training neuroimaging data of human brain and the system of non-contact evaluation of stereotactic tool positioning. The stereotactic tomography data modification module allows virtual replacement of the intracranial space of the stereotactic tomography phantom with training neuroimaging data of the human brain. Decision-making unit processes data coming from intracranial system of non-contact evaluation of stereotactic tool positioning during the surgery and in real-time assesses the expected clinical effect of the stereotactic intervention performed.

    The concept of a new generation of educational anthropomorphic interactive phantoms providing virtual, physical and clinical simulation in functional neurosurgery, supporting various types of stereotactic equipment.

    Fig.2. Interactive medical phantom for practicing functional stereotactic interventions

    A - Anthropomorphic phantom (1) with installed stereotactic frame, the presence of a platform (2) for applying of a trepanation hole, intracranial cavity (3) which contains a system for non-contact assessment of positioning (4) with a working area (5), intracranial registration markers (6) with neuroimaging data of human brain and the tip of a guided stereotactic instrument with tags (7) in the stereotactic target; B -  An algorithm for the stereotactic tomography data modification module that virtually replaces the intracranial space of a stereotactic tomography phantom with training neuroimaging data of the human brain; С - A way to simulate the phantom's intracranial space with neuroimaging data from the human brain during training stereotactic surgery.

    The technology is explained in more detail on the Description page.

    You can find information about the development of the prototype and the first tests on the Roadmap page.