Towards a procedure-optimised steerable catheter for deep-seated neurosurgery

In recent years, steerable needles have attracted significant interest in relation to minimally invasive surgery (MIS). Specifically, the flexible, programmable bevel-tip needle (PBN) concept was successfully demonstrated in vivo in an evaluation of the feasibility of convection-enhanced delivery (CED) for chemotherapeutics within the ovine model with a 2.5 mm PBN prototype. However, further size reductions are necessary for other diagnostic and therapeutic procedures and drug delivery operations involving deep-seated tissue structures. Since PBNs have a complex cross-section geometry, standard production methods, such as extrusion, fail, as the outer diameter is reduced further. This paper presents our first attempt to demonstrate a new manufacturing method for PBNs that employs thermal drawing technology. Experimental characterisation tests were performed for the 2.5 mm PBN and the new 1.3 mm thermally drawn (TD) PBN prototype described here. The results show that thermal drawing presents a significant advantage in miniaturising complex needle structures. However, the steering behaviour was affected due to the choice of material in this first attempt, a limitation which will be addressed in future work

Full 3D motion control for programmable bevel-tip steerable needles

Minimally invasive surgery has been in the focus of many researchers due to its reduced intra- and post-operative risks when compared to an equivalent open surgery approach. In the context of minimally invasive surgery, percutaneous intervention, and particularly, needle insertions, are of great importance in tumour-related therapy and diagnosis. However, needle and tissue deformation occurring during needle insertion often results in misplacement of the needles, which leads to complications, such as unsuccessful treatment and misdiagnosis. To this end, steerable needles have been proposed to compensate for placement errors by allowing curvilinear navigation. A particular type of steerable needle is the programmable bevel-tip steerable needle (PBN), which is a bio-inspired needle that consists of relatively soft and slender segments. Due to its flexibility and bevel-tip segments, it can navigate through 3D curvilinear paths. In PBNs, steering in a desired direction is performed by actuating particular PBN segments. Therefore, the pose of each segment is needed to ensure that the correct segment is actuated. To this end, in this thesis, proprioceptive sensing methods for PBNs were investigated. Two novel methods, an electromagnetic (EM)-based tip pose estimation method and a fibre Bragg grating (FBG)-based full shape sensing method, were presented and evaluated. The error in position was observed to be less than 1.08 mm and 5.76 mm, with the proposed EM-based tip tracking and FBG-based shape reconstruction methods, respectively. Moreover, autonomous path-following controllers for PBNs were also investigated. A closed-loop, 3D path-following controller, which was closed via feedback from FBG-inscribed multi-core fibres embedded within the needle, was presented. The nonlinear guidance law, which is a well-known approach for path-following control of aerial vehicles, and active disturbance rejection control (ADRC), which is known for its robustness within hard-to-model environments, were chosen as the control methods. Both linear and nonlinear ADRC were investigated, and the approaches were validated in both ex vivo brain and phantom tissue, with some of the experiments involving moving targets. The tracking error in position was observed to be less than 6.56 mm.Open Acces

SMART IMAGE-GUIDED NEEDLE INSERTION FOR TISSUE BIOPSY

M.S

A Novel Flexible and Steerable Probe for Minimally Invasive Soft Tissue Intervention

Current trends in surgical intervention favour a minimally invasive (MI) approach, in which complex procedures are performed through increasingly small incisions. Specifically, in neurosurgery, there is a need for minimally invasive keyhole access, which conflicts with the lack of maneuverability of conventional rigid instruments. In an attempt to address this fundamental shortcoming, this thesis describes the concept design, implementation and experimental validation of a novel flexible and steerable probe, named “STING” (Soft Tissue Intervention and Neurosurgical Guide), which is able to steer along curvilinear trajectories within a compliant medium. The underlying mechanism of motion of the flexible probe, based on the reciprocal movement of interlocked probe segments, is biologically inspired and was designed around the unique features of the ovipositor of certain parasitic wasps. Such insects are able to lay eggs by penetrating different kinds of “host” (e.g. wood, larva) with a very thin and flexible multi-part channel, thanks to a micro-toothed surface topography, coupled with a reciprocating “push and pull” motion of each segment. This thesis starts by exploring these foundations, where the “microtexturing” of the surface of a rigid probe prototype is shown to facilitate probe insertion into soft tissue (porcine brain), while gaining tissue purchase when the probe is tensioned outwards. Based on these findings, forward motion into soft tissue via a reciprocating mechanism is then demonstrated through a focused set of experimental trials in gelatine and agar gel. A flexible probe prototype (10 mm diameter), composed of four interconnected segments, is then presented and shown to be able to steer in a brain-like material along multiple curvilinear trajectories on a plane. The geometry and certain key features of the probe are optimised through finite element models, and a suitable actuation strategy is proposed, where the approach vector of the tip is found to be a function of the offset between interlocked segments. This concept of a “programmable bevel”, which enables the steering angle to be chosen with virtually infinite resolution, represents a world-first in percutaneous soft tissue surgery. The thesis concludes with a description of the integration and validation of a fully functional prototype within a larger neurosurgical robotic suite (EU FP7 ROBOCAST), which is followed by a summary of the corresponding implications for future work

DEVELOPMENT OF A KINETIC MODEL FOR STEERABLE CATHETERS FOR MINIMALLY INVASIVE SURGERY

The steerable catheters have demonstrated many advantages to overcome the limitations of the conventional catheters in the minimally invasive surgery. The motion and force transmission from the proximal end to distal tip of the catheter have significant effects to the efficiency and safety of surgery. While the force information between the catheter and the body (e.g., vessel) can be obtained by mounting sensors on the distal tip of the catheter, this would be more intrusive and less reliable than the one without the sensors, which is described in this disseration. In addition, the small diameters of the catheters may also restrict the idea of mounting sensors on the distal tip. The other approach to obtain the force information is to infer it from the information outside the body. This will demand an accurate mathematical model that describes the force and motion relation called kinetic model, and unfortunately, such a kinetic model is not available in the literature. In this dissertation, a kinetic model for steerable catheters is presented wich captures the following characteristics of the steerable catheter, namely (1) the geometrical non-linear behavior of the catheter in motion, (2) the deformable pathway, (3) the friction between the catheter and the pathyway, and (4) the contact between the catheter and pathway. A non-linear finite element system (SPACAR) was employed to capture these characteristics. A test-bed was built and an experiment was carried out to verify the developed kinetic model. The following conclusions can be drawn from this dissertation: (1) the developed kinetic model is accurte in comparison with those in literature; (2) the Dahl friction model, the LuGre friction model and the simplified LuGre friction model are able to capture the friction behavior between the catheter and the pathway but the Coulomb friction model fails (as it cannot capture the hysteresis property which has a significant influence on the behavior of the catheter); (3) the developed kinetic model has the potential of being used to optimize the design and operation of steerable catheters with several salient findings that (3a) the maximal contact force between the catheter and the pathway occurs on the tip of the distal part or the connecting part between the distal part and catheter body of the catheter and (3b) the rigidity and length of the distal part are crucial structural parameters that affect the motion and force transmission significantly. There are several contributions made by this dissertation. In the field of the steerable catheter, biomechanics and bio-instrumentation, the contributions are summarized in the following: (1) the approach to develop the kinetic model of the steerable catheter in a complex work environment is useful to model other similar compliant medical devices, such as endoscope; (2) the kinetic model of the steerable catheter can provide the force information to improve the efficiency and safety of MIS (minimally invastive surgery) and to realize the “doctor-assisted” catheter-based MIS procedure; (3) the kinetic model can provide accurate data for developing other simplified models for the steerable catheters in their corresponding work environments for realizing the robotic-based fully automated MIS procedure. (4) The kinetic model of the steerable catheter and the test-bed with the corresponding instruments and methods for the kinetic and kinematic measurements are a useful design validation in the steerable catheter technology as well as for the training of physicians to perform the catheter-based interventional procedure by adding more complex anatomic phantoms. In the field of continuum manipulator and continuum robots, the approach to develop the kinetic model is useful to model other manipulators and robots, such as snake-like robots

Biomechanics of a parasitic wasp ovipositor : Probing for answers

Insects such as mosquitoes, true bugs, and parasitic wasps, probe for resources hidden in various substrates. The resources are often, located deep within the substrate and can only be reached with long and thin (slender) probes. Such probes can, however, easily bend or break (buckle) when pushed inside the substrate, which makes probing a challenging task. Nevertheless, the mentioned insects use their probes repeatedly throughout their lifetime without apparent damage. Furthermore, the probes are also used for sensing the targets, can be steered during insertion, and can transport both fluids (e.g. blood, phloem sap) and eggs. Insect probes seem highly versatile structures that satisfy many functional requirements, including buckling avoidance, steering, sensing, and transport. Similar requirements also hold for minimally invasive medical procedures, where slender tools are used to minimize damage to the patient. Understanding the probing process in insects can bring insights in the insect ecology and evolution and it may also help in the development of novel surgical tools. In this thesis, I focus on the mechanical and motor adaptations of insect probing, while other aspects are only briefly discussed. In chapter 2, we review the literature on the probing structures and their operating principles across mosquitoes, parasitic wasps, and hemipterans. Probes are either modified mouthparts (mosquitoes, true bugs) or special tubular outgrowths of the abdomen (parasitic wasps). Despite having different developmental origins, the probes share three major morphological characteristics, which may reflect the shared functional requirements of buckling avoidance and steering: (i) the probes consist of multiple, interconnected elements that can slide along each other, (ii) the probe diameters are very small, which leaves no space for internal musculature, and (iii) the distal ends (tips) of the probe elements are asymmetric and often bear various serrations, hooks, bulges, or notches. How such slender multi-element probes avoid buckling during insertion has been hypothesized in the so-called push–pull mechanism. According to this mechanism, the probe is inserted into the substrate by reciprocal movements of the elements. The insects therefore simultaneously push on some of the probe elements, while pulling on the others. The tip serrations are directed such, that they primarily increase the friction upon pulling of the elements. This puts the pulled elements under tension and makes them effectively stiffer in bending (like when pulling a rope). The elements under tension can serve as guides along which the other elements are pushed inside the substrate without the risk of buckling. The insect alternates the pushing and pulling between the elements to incrementally insert the probe in the substrate. This mechanism has, however, never been quantified in insects and it was hitherto unknown whether the animals rely on it during probing. The probe tip asymmetry presumably facilitates steering. The asymmetric tip geometry leads to asymmetric reaction forces from the substrate on the tip during insertion, which push the probe tip sideways into a curved path. Controlling the tip geometry therefore allows for control of probing direction. Although offsetting the elements by sliding already changes the shape of the probe tip, these changes might be too small to induce the necessary change of probing direction. A number of mechanisms that enhance the tip asymmetry during the sliding of the elements have been suggested. However, few mechanisms have been observed or studied in vivo, so it is not completely clear how insects steer with their probes. Additionally, the effect of the substrate on both the steering and insertion mechanisms is unknown. To understand the biomechanics of insect probing, we investigated the probing behaviour of the braconid parasitic wasp Diachasmimorpha longicaudata. This is an ideal species for studying the buckling avoidance and steering, because it: (i) possess a slender ovipositor several millimetres in length, (ii) probes into solid material (e.g. citrus fruits), and (iii) attack fruit-fly larvae that are freely moving within the substrate (i.e. steering can be expected). The ovipositor of D. longicaudata is similar to other hymenopterans and consists of three interconnected elements (valves), one dorsal and two ventral ones. The interconnection is a tongue-and-groove mechanism, which allows for sliding of the valves, but prevents their separation. The ovipositor has an asymmetric tip—the distal end of the dorsal valve is enlarged (bulge), while the ventral valve tips have harpoon-like serrations. Additionally, just proximal to the bulge of the dorsal valve, the ovipositor is characteristically bent in an S-shape. This seems to be a feature present only in D. longicaudata and closely related species. The wasps also possess a pair of sheaths that envelop the ovipositor at rest and throughout most of the probing process, but do not penetrate into the substrate. In chapter 3, we studied the kinematics of ovipositor insertion into translucent, artificial substrates of various stiffnesses. Ovipositor insertion was filmed in a three camera setup, which allowed us to reconstruct the ovipositor insertion in 3D, while also monitoring the orientation of the insect’s body. We discovered that the wasps can explore a wide range of the substrate by probing in any direction with respect to their body orientation from a single puncture point. Probing range and speed decreased with increasing substrate stiffness. Wasps used two strategies of ovipositor insertion. In soft substrates, all ovipositor valves were pushed inside the substrate at the same time. In stiff substrates, wasps always moved the valves alternatively, presumably employing the hypothesized push–pull mechanism. We observed that ovipositors can follow curved trajectories inside the substrate. Detailed kinematic analysis revealed that the ovipositors followed a curved path during probing with protracted ventral valve(s). In contrast, probing with protracted dorsal valve resulted in straight trajectories. We linked the changes in the probing direction to the shape changes in the ovipositor tip. When the ventral valves were protracted, they curved towards the dorsal valve, resulting in an enhanced bevel which presumably caused a change in insertion direction. In chapter 4, we investigated the above described steering mechanism by quantifying the bending stiffness (three point bend test) and the geometry (high-resolution computer tomography) of the ovipositor in D. longicaudata. Additionally, we qualitatively assessed the material composition of the valves using fluorescence imaging. The thick dorsal valve bulge might be stiff and could straighten the S-shaped region of the ovipositor during the valve offset, causing bending of the tip. We discovered that the S-shaped region of the ovipositor is significantly softer than its neighbouring regions, which is mostly due to the presence of resilin in the S-shaped region of the ventral valve. Resilin is a rubber-like protein and reduces the stiffness of the otherwise heavily sclerotized valves. Additionally, we showed that the ventral valves have a higher bending stiffness than the dorsal valve along most of their length. The exception is presumably the bulge on the dorsal valve—although we could not directly measure its bending stiffness, its geometrical properties show that it is the thickest (and therefore stiffest) region in the distal end of the ovipositor. Outside the substrate, offsetting of the valves in any direction (i.e. pro- or retraction of the ventral valves) caused a straightening of the S-shaped region of the ovipositor and a curving towards the dorsal side. However, during probing in a substrate, such curving was only observed upon protraction of the ventral valves. We hypothesize this is due to the interaction of the ovipositor with the substrate. Namely, the bevelled ventral valve tips generate substrate reaction forces that promote dorsal curving, while the bevelled tip of the dorsal valve generates substrate forces that promote ventral bending. The interaction between the ventral and dorsal valves straightens the S-shaped region of the ovipositor and enhances dorsal curving. This therefore facilitates strong shape changes of the tip only upon protraction of the ventral valves, while counteracting the ventral curving of the dorsal valve. These opposing mechanisms presumably result in an approximately straight protraction of the dorsal valve. In chapters 2 and 3 we describe how the wasps use the reciprocal valve movements when probing in stiff substrates. As such substrates presumably require strong forces during insertion, the reciprocal valve movements may indeed serve to avoid buckling. However, how the valves are actuated or the forces generated during probing have never been quantified. In chapter 5, we therefore investigated the ovipositor base and the muscles driving the movements of the valves. At the base, the valves attach to plate-like structures that are interconnected with a series of linkages. The muscles attach to these plates and can move them with respect to each other. Such movements also result in the movements of the valves. To analyse the mechanics of this linked system, we performed high-resolution computer tomography scans of wasps in different stages of the probing cycle. This allowed us to compare the configurational changes of the basal plates to the valve offset, and measure the muscle cross-sections and attachment sites. We also calculated the muscle moment arms and estimated the forces and moments of the most relevant musculature actuating the ovipositor movements, by assuming a tensile muscle stress previously reported for insect muscles. For the ventral valves only, we also calculated the forces the valves can exert onto the substrate. The dorsal valve can only be moved by moving the base that is linked inside the abdomen, and therefore force estimation could not be made. The displacement magnitude of the basal plates corresponded to the valve offset, indicating that the valves are indeed moved due to the changes in the arrangement of the basal plates. We also showed that the ventral valve plates move most during the probing cycle, while the magnitude of the dorsal valve plate movements is much smaller. This suggests that the ventral valves move along the dorsal valve, while the dorsal valve moves together with the abdomen during probing. Additionally, in the situation where the animal keeps its abdomen stationary, we estimated the maximal forces actuating the ventral valves. The estimated maximal pushing forces can be higher than the estimated buckling load of the unsupported ovipositor outside the substrate. Assuming the maximal pushing forces are required during probing, antibuckling mechanisms are needed to avoid damaging the ovipositor. Buckling can be limited (prevented) by either supporting the ovipositor outside the substrate with additional sheaths, employing the push–pull mechanism, or both. Subtracting the maximal estimated pushing and pulling forces on the ventral valves, results in a net pushing force that is very close to the buckling threshold of the ovipositor, albeit still slightly higher. The sheaths, although being flexible, might provide the additional support if needed. In this thesis, I show that multi-element probes are inserted into the substrate using reciprocal movements of the individual elements. These movements appear to be necessary in stiff substrates, which presumably require high pushing forces on a single element during probing. This is in accordance with the hypothesis that reciprocal valve movements serve as an anti-buckling mechanism. Additionally, such valve movements are also important for steering of the probe during insertion. The valve offset controls the shape of the probe tip and therefore the net substrate reaction forces that result in bending of the probe. Wasps evolved special structures that enhance the shape changes of their ovipositor tips and facilitate steering. Our findings may be interesting for a broad range of audiences. Entomologists, evolutionary biologists, and ecologists may find them useful when studying the diversification of probing insects, their evolutionary success, or their ecological interactions (e.g. insect–plant, parasite–host). The anti-buckling and steering mechanisms may be helpful when developing novel, man-made probes. These mechanisms allow for minimization of the probe thickness and accurate steering control, which minimizes substrate damage during probing. Our findings may be particularly useful in the development of slender, steerable needles for minimally invasive surgery.</p

Semi-Automated Needle Steering in Biological Tissue Using an Ultrasound-Based Deflection Predictor

The performance of needle-based interventions depends on the accuracy of needle tip positioning. Here, a novel needle steering strategy is proposed that enhances accuracy of needle steering. In our approach the surgeon is in charge of needle insertion to ensure the safety of operation, while the needle tip bevel location is robotically controlled to minimize the targeting error. The system has two main components: (1) a real-time predictor for estimating future needle deflection as it is steered inside soft tissue, and (2) an online motion planner that calculates control decisions and steers the needle toward the target by iterative optimization of the needle deflection predictions. The predictor uses the ultrasound-based curvature information to estimate the needle deflection. Given the specification of anatomical obstacles and a target from preoperative images, the motion planner uses the deflection predictions to estimate control actions, i.e., the depth(s) at which the needle should be rotated to reach the target. Ex-vivo needle insertions are performed with and without obstacle to validate our approach. The results demonstrate the needle steering strategy guides the needle to the targets with a maximum error of 1.22 mm

SHAPE MEMORY ALLOY ACTUATOR CONTROL FOR 3D STEERING OF ACTIVE SURGICAL NEEDLE IN MINIMAL INVASIVE SURGERIES

Ph.D

A mechanics-based model for 3D steering of programmable bevel-tip needles

We present a model for the steering of programmable bevel-tip needles, along with a set of experiments demonstrating the 3D steering performance of a new, clinically viable, 4-segment, pre-production prototype. A multi-beam approach, based on Euler-Bernoulli beam theory, is used to model the novel multi-segment design of these needles. Finite element simulations for known loads are used to validate the multi-beam deflection model. A clinically sized (2.5 mm outer diameter), 4-segment programmable bevel-tip needle, manufactured by extrusion of a medical-grade polymer, is used to conduct an extensive set of experimental trials to evaluate the steering model. For the first time, we demonstrate the ability of the 4-segment needle design to steer in any direction with a maximum achievable curvature of 0.0192±0.0014 mm⁻¹. Finite element simulations confirm that the multi-beam approach produces a good model fit for tip deflections, with a root-mean-square deviation (RMSD) in modeled tip deflection of 0.2636 mm. We perform a parameter optimization to produce a best-fit steering model for the experimental trials, with a RMSD in curvature prediction of 1.12×10⁻³ mm⁻¹