A CT-scan compatible robotic device for needle placement in medical applications

Several medical applications require devices capable of placing different substances inside the human body. Due to the nature of the task it is desirable to perform these actions with visual feedback, whereas the most viable solution, especially for deep target points, is computer tomography (CT). The paper presents an innovative device, which can be fitted inside the CT gantry, and has decoupled motions to ensure maximum accuracy during the needle placement. It will be shown that for needle placement tasks 5 degrees of freedom (DOF) are sufficient to achieve the task. The geometric and kinematic model of the robot will be presented. The workspace and precision mapping are computed. Some simulation results will show the robot capabilities as well as its placement in the CT scan environment. Introduction “A wise man should consider that health is the greatest of human blessings, and learn how by his own thought to derive benefit from his illnesses.” (Hippocrates) Needle placement technique covers a wide area of medical applications, where the task can be described, in a very simple way, as the insertion of a needle on a linear trajectory, from outside up to a target point situated in the patient body. Needle placement covers applications like biopsies, fluid extraction from internal organs (cardiac tamponade, lungs) or placement of radioactive seeds in different body organs in minimally invasive cancer therapy [1-3]. In [1] it is shown that a robotic device enhances the needle placement precision beyond the natural human capabilities. Most robotic devices developed for needle placement applications are dedicated for a single, very specific task, thus narrowing its range of applications to a singular one. Application definition. The robotic structure should introduce, based on radiologic data, rigid needles of diameter varying from 0.6 mm up to 2 mm and a length from 50 mm up to 250 mm inside the patient body following a linear trajectory. Due to the long distances, the variable tissue density one can define the first requirement for the robot: the needle is introduced, in a linear trajectory by an independent mechanism, to ensure constant precision for the entire displacement and to avoid any deviations from the predefined trajectory. The areas of interest for needle placement in different medical applications where the use of a robotic device is justified cover the human thoracic and abdominal area where the target point can be situated in close vicinity to different structures (blood vessels, ganglions, other organs) whereas an error can cause a cataclysmic event leading up the death of the patient. Several applications require the placement of multiple needles in a circular or rectangular matrix display thus defining the second requirement for the robot: it should be capable of placing multiple needles on parallel trajectories. Thus an advantage would be to be able to fix the robot orientation after its initial setup and modify only the insertion point from one needle to the other. Besides emergency situations where the intervention must be performed very fast, needle placement is carried out following a set of imagistic investigations ranging from simple radiologic exams to echography, computer tomography (CT) and magnetic resonance imaging (MRI). These techniques provide a set of images enabling the preplanning of the procedure where the physician Advanced Engineering Forum Online: 2013-06-27 ISSN: 2234-991X, Vols. 8-9, pp 574-583 doi:10.4028/www.scientific.net/AEF.8-9.574 © 2013 Trans Tech Publications, Switzerland This is an open access article under the CC-BY 4.0 license (https://creativecommons.org/licenses/by/4.0/) defines safe linear trajectories to reach the targeted point(s). Nevertheless, in order to increase the safety of the procedure it is desired to have a real-time visual feedback of the needle placement during the procedure to reduce the risk of damaging any internal body structures [2]. During the first steps of introducing robotic systems in medical arena, the imagistic methods that can be used during the needle placement were defined [3,4]: High Intensity Focal Ultrasound (HIFU), CT, MRI and Positron Emitted Tomography (PET Scan). Each of these techniques has advantages and limitations. The best option as image quality is the use of MRI but due to the high magnetic fields generated by the machine there are a lot of restrictions for both patient and robot. The ultrasound machines have a low image quality and a limited use especially for deep targets. CT can be considered at the moment as the most versatile option even though the image quality is lower compared to MRI machines but it is still good enough for real-time needle placement procedures. Figure 1 illustrates an example of a CT machine equipped with supplementary laser sensors (the lasers were enhanced in the figure to provide better contrast) for position calibration. This type of CT scanners is designed especially for fast real-time image feedback but they cannot be used in diagnostics [5]. Figure 2 shows a CT image with a needle inserted inside the human body [6]. Figure 1. CT-Sim scan with laser positioning system Figure 2. CT scan with needle placement (arrow) for transpulmonary biopsy [6] The need to provide real-time visual feedback for the needle position during its insertion defines the next technical characteristic of the robotic system: it must be small enough to fit inside the CT gantry in the patient proximity. Definition of an innovative robotic structure In [1] the needle placement precision of manual template guidance was calculated to be around 3 mm (2.7 ± 0.7 mm). Many applications require a higher precision (1 mm) making the manual task impossible to achieve. Considering the fact that the target point inside the body is known and the trajectory is defined, it means that also the entrance point is known, and being visible it is very easily and precisely reached. This means that the error is generated mainly by the orientation angle of the needle. A simple estimation of the acceptable angular deviation for a maximum error of 1 mm is given by Eq.1 where d represents the trajectory length inside the body and αmax is the angular deviation: π α 180 d 1 2 1 asin 2 max ⋅       ⋅ ⋅ = (1) Advanced Engineering Forum Vols. 8-9 575