© Springer International Publishing Switzerland 2015 H. Liu et al. (Eds.): ICIRA 2015, Part II, LNAI 9245, pp. 472–482, 2015. DOI: 10.1007/978-3-319-22876-1_40

Tension Sensing for a Linear Actuated Catheter Robot

Junghwan Back1, Rashed Karim2, Yohan Noh1, Kawal Rhode2, Kaspar Althoefer1, and Hongbin Liu1()

1 Department of Informatics, King’s College London, London, UK WC2R 2LS {JunghwanBack,KasparAlthoefer,HongbinLiu}@kcl.ac.uk 2 Biomedical Engineering, King’s College London, London, UK WC2R 2LS

{RashedKarim,KawalRhode}@kcl.ac.uk

Abstract. Cardiac ablation therapy is an effective minimally invasive treatment of cardiac arrhythmias. The procedure is delicate and complex in nature requir-ing particular sections of the heart to be ablated. When operated by an expe-rienced electrophysiologist, the procedure normally takes 2 to 3 hours. It is often conducted under fluoroscopic X-ray guidance and longer procedure times can increase the radiation burden of both the operator and patient. Earlier, we had proposed a robot-assisted tendon-guided catheter that can be navigated us-ing radiation-free MRI imaging. In this paper, we propose a tension-feedback mechanism that provides vital control over its guiding tendons. We describe how it can be achieved using tension sensing and demonstrate using experi-ments and finite element simulations that feedback based on accurate tension sensing is plausible.

Keywords: Robot-assisted catheter · Catheter linear actuation · Catheter tension sensing · Tension control

1 Introduction

Cardiac catheterization uses thin steerable tubes, known as catheters, for minimally invasive procedures for treatment of cardiac arrhythmias [1]. During the procedure, the catheter remains in direct contact with the heart tissue delivering ablation at tar-geted sites. Minimally invasive catheter ablation procedure promises advantages, such as minimal incisions, quicker recovery time, less bleeding, and other economic effects [2, 4]. A lack of dexterity, speed, and force capability of catheters causes that skills and functionalities of manual catheterization are still lower level than open heart sur-gery, although promising the advantages [7]. Moreover, the complex and delicate nature of the procedure makes it difficult to steer the catheter manually, causing an extension of surgery time. The extended procedure time increases the radiation bur-den of both surgeon or procedure operator and patient. According to [5-6], 20 years careered cardiologists can have a radiation exposure of about 1000mSv and 100mSv to the head and lower body, respectively. This directly correlates to a 5% chance of developing cancer in later life. Reducing procedure times can thus be important for the well-being of clinicians.

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The procedure for treating cardiac arrhythmias involves a number of stages. The catheter is inserted into the patient’s heart with access through femoral veins. Once in the heart, the real-time electrophysiological state of the heart is mapped allowing the clinician to locate sites of ablation. The catheter is driven under fluoroscopic X-ray guidance to the located site and the ablation is delivered whilst the catheter remains in contact with the heart muscle. There are two possible avenues for reduction of the X-ray radiation dose on the procedure operator. Firstly, the catheter control can be improved by using catheter-tip tissue contact force and position information. Second-ly, the catheter can be remotely controlled with the operator standing away from the radiation source and navigated on MRI images of the patient. The MRI can be advan-tageous as it not only provides anatomical information but also the underlying tissue characteristics.

These suggested solutions can be realized by an MRI-compatible robot-assisted cardiac catheterization with remote navigation. Numerous robot-assisted catheter platforms have been developed by researchers over the last decade such as the master-slave systems presented in [7-10]. Commercial robot catheter systems are also availa-ble, such as Amigo (Catheter Robotics), and CorPath (Corindus Vascualr Robotics) [11-12]. The catheters used in these systems cover the 3D workspace by combining a bi-planar deflection and turning catheter handle such as Biosense Webster’s CS cathe-ter [13]. However, a turning catheter shaft is a non-predictable parameter for auto-mated robotic control due to the flexural catheter shaft material. The procedure opera-tor controls it manually using catheter location and torque feedbacks.

In light of the above, a 4-way tendon driven catheter is more suitable for automated robotic control. There are 4-way tendon commercial systems such as the Sensei (Hansen Medical) catheter system [14]. These existing systems provide tele-operative steering control and permit clinicians to be away from radiation during the procedure. The procedure time is notably reduced, one recent study reports 41% reduction in time [15] and another study reports improved patient outcomes based on long-term follow-up. [16]. These recent studies show promise of further reduction of procedure times with robot-assisted remote navigation. MRI integration can significantly im-prove such procedures as soft tissue visualization is currently non-existent in existing systems. Research from our group has shown that soft tissue-tool interaction is even possible where direct access of tissue is not available [17].

Earlier, we developed a new catheter steering system with an MR-compatible ca-theter-tip [3]. In this work, we increase the controllability and dexterity of this 4-way tendon driven catheter using tension sensing. Tension sensing is made possible using cantilever beam structure and strain gauge.

2 System Overview

The entire catheter system is as shown in Fig. 1 where our present work differs from [3] with the introduction of a linear actuated platform. The four main components of the robot are the actuation, the steerable MRI-compatible multi-segment catheter tip, the image feedback and the tension sensing. In the linear-actuated catheter robot, four

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linear guides are installed to control four tendons of catheter tip. Linear guides reduce backlash and slack. The liner actuations are controlled by a DSP controller (Arduino Mega) which interfaces with custom-made controller software, and the five actuators are controlled parallel using PWM duty cycle control. The tension sensor was devel-oped using strain gauge and special cantilever beam structure for tension feedback. The tension sensor communicates with the main control system via the DSP control-ler. Thus, an embedded source in DSP controller receives target tension, and can con-trol the linear actuation to achieve a constant tension. Also, the developed system can be controlled by a joystick handle to provide for remote image navigation. The catheter is remotely navigated with custom-made image guidance software using the techniques in [18,19]. In this work, a phantom glass-heart model was pre-scanned with CT, allowing images of resolution 0.68 mm × 0.68 × 1.00 mm to be acquired. The software displays the real-time position of the catheter on the CT heart model. Position information is obtained with magnetic tracking (NDI Aurora® EM) of the catheter’s tip.

Fig. 1. Schematic diagram showing configuration of the linear actuated catheter robot.

3 Catheter Tip

The installed catheter tip is made of 12 segments connected together as shown in Fig. 2 following the design proposed in [3]. Each segment is a helical structure to be bended 4-way through four tendons driven, and the catheter tip has dimensions which conform to the required standards for such procedures. A single segment is 3 mm with a length of 9 mm and the inside lumen is 1.2 mm to allow insertion of optional ele-ments. Each tendon guiding channel is 250 μm in diameter, and the total length of the catheter tip is 117 mm. The catheter tip is shielded by flexible outer tube to prevent

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any breakage and thus disconnection of the catheter segments. It also prevents elonga-tion of catheter tip. In addition to this, a linear stage actuation is employed to provide axial movement of the catheter tip, and also a variable deflection curvature radius is achieved through sliding of a nitinol tube in the catheter tip. The catheter tip provides for a full 3D workspace.

Fig. 2. (a) Catheter steerable segments configurations. The Lumen tube (black rod) is showed as an example, (b) generated various bending radius using sliding of the stiffener in the catheter tip. The all cases of (b) are generated by 3N tension.

4 Linear Actuated Catheter Robot

In the robot-assisted catheter control, actuator variables such as velocity and angular displacement are important control feedback parameters that can provide for safety limitations of the system. In this work, the catheter shape is estimated by tension. To enable this, suppression of the mechanical slack in the tendons is needed to provide reliable feedback from actuators. To reduce the mechanical slack and the backlash associated with rotary motors, each tendon is driven by a linear actuator (Newmark®, ET-200-21, resolution: 7.5 μm, max velocity: 200 mm/sec). The 4-Way tendon driven catheter tip is used, so that four commercial linear actuators actuate the four tendons such as x-axis and y-axis. A custom made linear guide moves these four linear actua-tors for z-axis translation.

Each tendon in the linear guide is evenly arranged in the catheter shaft, so that a tendon in catheter shaft has less friction. However, each tendon is inserted into the shaft after a change in direction. This was the primary contributor of friction in the tendon. The component is designed using a bearing to reduce friction as shown in Fig. 3. The friction was reduced by friction coefficient between silk and metal, and bearing rotation without friction.

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Fig. 3. Mechanical design of the linear actuated catheter robot platform includes tension sensor, reducing friction and linear actuation.

5 Tension Sensor

To reduce the backlash associated with rotary motors, each tendon is driven by a li-near actuator (Newmark®, ET-200-21, resolution: 7.5 μm, max velocity: 200 mm/sec). To provide tension feedback for the tendon driving, a tension sensor using cantilever beam structure and strain gauge was developed. In this work, the sensor beam structure was modified to increase the sensitivity by increasing the bending moment range on the beam. Each tendon is tied to the tension sensor, which is fixed on the linear actuator. In practice, the elongation of the tendon under tension, the los-ing and over tension by the external forces are of frequent occurrence and thus the tension PD controller is developed. Two 6D magnetic trackers (NDI Aurora® EM) are seeded into bottom and top of catheter tip to provide catheter position feedback. The magnetic tracker in the bottom is to compensate orientation of catheter, and the mag-netic tracker in the top is to provide catheter tip position feedback. In this paper, the tension sensor and control will be explained mainly, and the overview of the catheter robot design including the 4-way tendon driven catheter tip will be introduced.

The shape of the catheter-tip is dictated by the tendon tensions. The tension is thus an essential parameter for automated robotic control. Moreover, during remote navi-gation, the mechanical safety condition can be detected using the tension measure-ment alone. Fibre optic based sensing methods developed in our previous works have been proved suitable for miniaturized setting, but they often need recalibration after a number of uses. [20-25]. To provide a solution fit for purpose, we developed the ten-sion sensor using strain gauges. Strain gauges are robust and provide with a larger measureable range.

To enable measurement of tension in the tendons, a cantilever beam structure based tension sensor using strain gauge is developed. The dimension of the sensor body is

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4mm × 8mm × 11mm. In order to take linear stage of the structure and material, the tension sensor has an initial bending radius, and then a strain gauge is attached on middle surface of the sensor body to measure a voltage difference depending on the applied tension, as shown in Fig. 4.

Fig. 4. Tension sensor structure and arrangement in the platform

A high resolution sensing feedback allows precise control in a closed loop system. It is understood that higher resolution tension sensing is correlated with more precise catheter motion control. However, suitable maximum tension on the catheter is 3N, and also the DSP controller (Arduino) analog input guarantees a 10-bit resolution on between 0V to 5V. This requires increasing the resolution of the tension sensor to achieve less than 0.05 N. This can be achieved by amplifying the generated output voltage range of the tension sensor. A possible way to amplify the output voltage is increasing the gain of differential amplified in a signal processing circuit. However, an unreasonable increasing gain could incur a slower frequency response.

An alternative solution is amplification using sensor design modification. The vol-tage difference is generated in a Wheatstone bridge circuit, and it is correlated with bending of the sensing structure. To increase the bending, a part of the structure is extruded cut circularly as shown in Fig. 5, and it was simulated in Solidworks Finite elements Method (FEM) analysis. During the simulation, a 3D printer ABS-P400 material property was assumed, and the applied maximum tension was 3N. As a simu-lation result, the stress of the tension sensor was amplified about 2.5 times after the circular extrude cut as shown in Fig. 5. In practice, the average output voltage differ-ence is amplified from 0.16V to 0.4V. The design modification is applied to the tension sensor, and it was realized in the robotized catheter platform as shown in Fig. 5.

Each tendon is installed on each linear actuator directly, thus keeping the magnetic field and circuit noise effect to a very small output voltage of the Wheastone bridge circuit. This is accomplished using a signal processing circuit for amplifying and fil-tering the output signal as shown in Fig. 6.

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Fig. 5. FEM simulation results in Solidworks, (A) after the circular extrude cut, and (B) is initial tension sensor design which has only initial bending.

Fig. 6. Tension sensor signal processing circuit including Wheatstone bridge, differential am-plifier with gain 178, RC passive band-pass filter, and inverting amplifier.

When the sensor body is bent, the changing output voltage generated by the strain gauge resistor value is captured. This is amplified by a differential amplifier stage by about 178 times. Also, a passive band-pass filter was used, with a band in the range 57Hz to 1.5 kHz, selected after an FFT measurement. Finally, the output signal is inverted to be connected to an Arduino analog input pin. As mentioned previously, when a 3N tension is applied, the average output voltage range can be about 0.4V. Depending on the strain gauge attaching condition, it has a slightly different sensing linear stage scale. Thus, these four tension sensors are calibrated individually using step weights. To validate the tension sensor, weights of 0.5 N were increasingly added to the tension sensor until they reached 3N. The calibrated parameter for each tension is as below in Table 1. It can be seen from Fig. 7 that the tension sensor measures the weights with good accuracy. When the load is released, the tension sensor reading immediately drops to zero.

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Table 1. The table shows each tension sensor’s specifications.

Axis Output Voltage Difference Calibration Value Resolution

X+ 0.48 V 6.25 0.030N X- 0.40 V 7.50 0.036N Y+ 0.38 V 7.89 0.038N Y- 0.42 V 7.14 0.034N

These calibrated tension sensors have similar calibration value and resolution as

shown in Table 1. Also, these tension sensors are tested to observe hysteresis and repeatability as shown in Fig. 7. The final resolution, average accuracy and hysteresis of the tension sensor are 0.03N, 97.5% and 6% respectively. Especially, the increas-ing resolution of the tension sensor about 0.03N allows 0.5mm resolution during ca-theter motion control.

Fig. 7. Linarites (upper) and hysteresis (lower) of the four tension sensors

6 Tension Control Based on PD Controller

For close loop control in automated catheter robot assistance system, tension control is an essential requirement for tendon driven catheter. However, the material of the tension sensor body and elongation of the tendon are damper components, and it causes the tension signal to be slightly changed after the application of constant ten-sion. Moreover, when an external force is applied to the catheter, loss of tension can occur. Therefore, tension sensing requires a continuous control. To maintain constant tension, a simple PD controller was designed to provide constant tension. To evaluate the PD controller, the tension sensor is connected with a spring to generate tension as shown in Fig. 8.

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Fig. 8. Tension control experiment set-up using a spring

Fig. 9. the tension control results with applied target tensions were 1N to 3N.

P and D gains control velocity of stepper motor of linear guide depending on de-sired and measured tensions. During this PD controller evaluation, it showed 0.4 sec rising time, 0.12N tolerance error, and fast settling time. The rising time can be re-duced by increasing the P gain, but it will cause an overshoot and slow settling time. Moreover, an overshoot during tension control can generate a large undesirable con-tact force of the catheter-tip. So the tension should be generated without an overshoot in the target time. However, when the tension is generated by an unexpected contact force by collisions, fast response times are needed from the tension control. Finally, although the tension sensor and PD controller is promising applicable accuracy and resolution, the response of PD controller can be improved with real-time gain adjust-ment, for example with a fuzzy based PD-PI controller in [26-29].

7 Future Work and Conclusion

The tele-catheterization with robot assistance catheter control proposed in this work is an ideal solution to significantly reduce radiation exposure on the procedure operator during catheterization. To realize the delicate catheter control, reliable feedbacks from

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catheter localization, force and actuator are needed. To satisfy the robot-assisted ca-theter control requirement, linear actuation, tension sensing, and robotic image guid-ance was integrated into a robotized tele-steerable catheter system. Resolution, average accuracy and hysteresis of the tension sensor are 0.03N, 97.5% and 6% re-spectively. The increasing resolution of the tension sensor of about 0.03N allows a 0.5mm resolution to achieve catheter motion controlling. Moreover, the tension PD controller is promising with a good accuracy and resolution of 0.4 sec rising time, 0.12N tolerance error, and a fast settling time. With the introduction of a linear ac-tuated catheter robot to our existing system in [3] and integration of image and ten-sion feedback, we envisage that it will be possible to arrive quickly at the heart tissue target for delivering ablation.

References

1. Tunay, I.: Position control of cathters using magnetic fields. In: Proceedings of the IEEE International Conference on Mechatronics (2004)

2. Baim, D.S.: Grossman’s Cardica Catheterization, Angiography, and Intervention, p. 992. Williams & Wilkins, Baltimore (2005)

3. Ataollahi, A., Karim, R., Soleiman Fallah, A., Rhode, K., Razavi, R., Seneviratne, L., Schaeffter, T., Althoefer, K.: 3-DOF MR-Compatible Multi-Segment Cardiac Catheter Steering Mechanism. IEEE Transactions on Biomedical Engineering (2013) (preprint on-line)

4. Natale, A.: Atrial fibrillation in 2012: Advances in catheter-ablation treatment of AF. Na-ture Reviews Cardiology 10(2), 63–64 (2013)

5. Picano, E., et al.: Cancer and non-cancer brain and eye effects of chronic low-dose ioniz-ing radiation exposure. BMC Cancer 2(1), 157 (2012)

6. National Council on Radiation Protection and Measurements Limitation of Exposure to Ionizing Radiation. National Council on Radiation Protection and Measurements: No 116, Bethesda (1993)

7. Khoshnam, M., Patel, R.V.: Pseudo-Rigid-Body 3R model for a steerable ablation catheter. In: IEEE International Conference on Robotics and Automation (ICRA) (May 2013)

8. Kesner, S.B., Howe, R.: Position Control of Motion Compensation Cardiac Catheters. IEEE Transactions on Robotics 27(6), December 2011

9. Kettler, D.T., Plowes, R.D., Novotny, P.M., Vasilyev, N.V., del Nido, P.J., Howe, R.D.: An active motion compensation instrument for beating heart mitral valve surgery. In: Proc. IEEE/RSJ Int. Conf. Intell. Robots Syst., pp. 1290–1295 (October/November 2007)

10. Guo, S., Wang, P., Guo, J., Wei, W., Ji, Y., Wang, Y.: A novel master-slave robotic cathe-ter system for vascular interventional surgery. In: IEEE International Conference on Me-chatronics and Automation (August 2013)

11. Amigo rmote catheter systme. http://www.catheterrobotics.com/product-main.htm 12. How CorPath Robotic Angioplasty Works. http://www.corindus.com/about-corpath/how-

corpath-works 13. Biosense Webster CS uni-Directional Catheter. https://www.biosensewebster.com/

products/non-navigational/uni-directional-catheter.aspx 14. Sensei Robotic System. http://www.hansenmedical.com/us/en/cardiac-arrhythmia/artisan-

extend-catheter/product-overview

482 J. Back et al.

15. Arujuna, A., Karim, R., Zarinabad, N., Gill, J., Rhode, K, Schaeffter, T., Wright, M., Rinaldi, C.A., Cooklin, M., Razavi, R., O’Neill, M.D., Gill, J.S.: A randomized prospec-tive mechanistic cardiac magnetic resonance study correlating catheter stability, late gado-linium enhancement and 3 year clinical outcomes in robotically assisted vs. standard cathe-ter ablation. Europace (February, 2015)

16. Ullah, W., Hunter, R.J., Haldar, S., Ailsa Mclean, R.N., Dhinoja, M., Sporton, S., Earley, M.J., Lorgat, F., Wong, T., Schiling, R.J.: Comparison of Robotic and Manual Persistent AF Ablation Using Catheter Contact Force Sensing: An International Multicen-ter Registry Study. Pacing and Clinical Electrophysiology 37(11), 1427–1435 (2014)

17. Liu, H., Noonan, D.P., Althoefer, K., Seneviratne, L.D.: Rolling mechanical imaging: a novel approach for soft tissue modelling and identification during minimally invasive sur-gery. In: 2008 IEEE International Conference on Robotics and Automation (ICRA), pp. 846–850, Pasadena, CA (2008)

18. Karim, R., Mohiaddin, R., Rueckert, D.: Left atrium pulmonary veins: segmentation and quantification for planning atrial fibrillation ablation. In: SPIE Medical Imaging 2009 (2009)

19. Karim, R.: Surface flattening of the human left atrium and proof-of-concept clinical appli-cations. Computerized Medical Imaging and Graphics 38(4) (2014)

20. Polygerinos, P., Ataollahi, A., Schaeffter, T., Razavi, R., Seneviratne, L.D., Althoefer, K.: MRI-compatible intensity-modulated force sensor for cardiac catheterization procedures. IEEE Transactions on Biomedical Engineering 58(3), 721–726 (2011)

21. Liu, H., Puangmali, P., Zbyszewski, D., Elhage, O., Dasgupta, P., Dai, J.S., Seneviratne, L.D., Althoefer, K.: An Indentation Depth-force Sensing Wheeled Probe for Abnormality Identi-fication during Minimally Invasive Surgery. Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, H 224(6), 751–763 (2010)

22. Althoefer, K., Zbyszewski, D., Liu, H., Puangmali, P., Seneviratne, L.D., Challacombe, B., Dasgupta, P., Murphy, D.: Air-Cushion force sensitive probe for soft tissue investigation during minimally invasive surgery. In: IEEE Conference on Sensors, pp. 41–46, Lecce, Italy (2008)

23. Liu, H., Li, J., Poon, Q., Seneviratne, L.D., Althoefer, K.: Miniaturized force indentation-depth sensor for tissue abnormality identification. In: 2010 IEEE International Conference on Robotics and Automation (ICRA), pp. 3654–3659 (May 2010)

24. Searle, T.C., Althoefer, K., Seneviratne, L.D., Liu, H.: An optical curvature sensor for flexible manipulators. In: 2013 IEEE International Conference on Robotics and Automa-tion (ICRA), pp. 4400–4405 (2013)

25. Noh, Y., Sareh, S., Back, J., Würdemann, H., Ranzani, T., Secco, E., Faragasso, A., Liu, H., Althoefer, K.: A three-axial body force sensor for flexible manipulators. In: 2014 IEEE International Conference on Robotics and Automation (ICRA), pp. 6388–6393 (2014)

26. Gao, X., Zheng-Jin F.: Design study of an adaptive Fuzzy-PD controller for pneumatic servo system. Control Engineering Practice, January 2004

27. Fuzzy PID Control of a Flexible-Joint Robot Arm with Uncertainties from Time-Varying Loads. IEEE Transactions of Control Systems Technology 6(3), May 1997

28. Back, J., et al.: Control a contact sensing finger for surface haptic exploration. In: 2014 IEEE International Conference on Robotics and Automation, ICRA 2014 (2014)

29. Liu, H., Nguyen, K.C., Perdereau, V., Back, J., Bimbo, J., Godden, M., Seneviratne, L.D., Althoefer, K.: Finger Contact Sensing and the Application in Dexterous Hand Manipula-tion. Autonomous Robots 39(1), 25–41 (2015)