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Technical Developments |
1 From the Department of Radiology (S.B.S., M.E.B.) and James Buchanan Brady Urological Institute (S.B.S., A.P., L.R.K., D.S.), Johns Hopkins Medical Institutions, Jefferson Building, Rm 173, 600 N Wolfe St, Baltimore, MD 21287; and Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Md (A.P., D.S.). Received June 29, 2001; revision requested August 20; revision received December 3; accepted January 7, 2002. Supported in part by National Science Foundation Cooperative Agreement EEC 9731478, Engineering Research Center for Computer-Integrated Surgical Systems and Technology; and National Cancer Institute grant 1R21CA088232-01A1. Address correspondence to S.B.S. (e-mail: ssolomo@jhmi.edu).
| ABSTRACT |
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© RSNA, 2002
Index terms: Computed tomography (CT), guidance, 70.12119 Computed tomography (CT), radiation exposure, 70.12119 Fluoroscopy, technology Phantoms Radiations, exposure to patients and personnel, 70.12119 Radiations, measurement, 70.12119
| INTRODUCTION |
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The major limitation of CT fluoroscopy is the relatively high radiation exposure to patient and physician. To make the real-time adjustments in needle trajectory, the physicians hand is in proximity to the scanning plane. Physician hand exposure has been theoretically and empirically determined to be approximately 2 mGy per procedure (3). Kato et al (4) calculated that, on the basis of an annual dose limit of 500 mSv for the hands, a physician with continuous hand exposure would be limited to performing only four CT fluoroscopic procedures each year. A number of procedural techniques and shields have been suggested to minimize radiation exposure (5). Kato et al (4) and Daly et al (6) used needle holders and Nawfel et al (3) used lead drapes to minimize hand exposure. Others have noted that experience and training may lead to a reduction in exposure (7). Paulson et al (8) recently reported reducing radiation exposure by lowering the milliampere setting and acquiring intermittent spot images during the procedure. Intermittent spot-check imaging has gained greater acceptance, as it generally can allow successful completion of the intervention with a substantial reduction of radiation exposure.
Robots have been introduced into the operating room to hold and move instruments precisely. Robots allow greater precision and accuracy and lack tremor when compared with humans (9,10). Neurosurgeons have used robots to perform stereotactic biopsies on the basis of previously acquired CT images (1113). Cardiac surgeons use robots to translate gross movements on a magnified image into fine robotic movements in the body (14). One of the advantages of robots that is capitalized in telesurgical applications is the fact that the surgeon does not need to be in the same location as the patient (15). Multiple reports of remote surgery exist (1618). This advantage of operating remotely may be used to limit radiation exposure in CT fluoroscopyguided interventional procedures.
The purpose of our study was to evaluate the feasibility of using a robotic system to precisely orient and advance a needle with CT fluoroscopic guidance for various interventional procedures.
| Materials and Methods |
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The remote center of motion is a compact robot for clinical applications, in which a fulcrum point is located distal to the mechanism (21,22). Typically, this fulcrum point is at the skin entry site. The robot can precisely orient a surgical instrument or biopsy needle while maintaining the needle tip at the fixed skin entry point. This capability allows the needle to be aimed at any desired trajectory from the skin insertion point. The robot assembly is supported by a passive arm with a frame mounted to the CT table. This arm allows supporting and positioning of the robot in proximity to the target organ. The estimated time for mounting the robot to the CT bed and for subsequent image registration was 15 minutes.
Needle advancement can be computer controlled, in which the robot is registered with the CT image, or manual joystick controlled, in which the physician uses the live CT fluoroscopic images and "drives" the needle to the target with the joystick. There are two joystick controls. One controls the forward-reverse advancement and one controls the trajectory orientation. Separation of these two functions allows increased safety.
The needle holder can accommodate a needle of any size. Eighteen-gauge core biopsy needles and 15-gauge radio-frequency (RF) devices were used in the interventions in this study.
Robot Accuracy Testing
A model was constructed with 22 1-mm-diameter nipple markers dispersed throughout the three-dimensional phantom. The phantom was placed within the CT gantry, and the robotic arm, which was holding an 18-gauge needle, was positioned 510 cm from the nipple markers. After robot registration, images were acquired that included the needle tip and each of the target markers. With use of a computer mouse, the needle tip and the target point were defined for the robot computer. Then the robot drove an 18-gauge needle to each marker. Two accuracy measurements were made for each test (Fig 2). First, the targeting accuracy, which was defined as the angle between the ideal trajectory and the actual trajectory of the needle after robotic alignment, was measured from the target to the direction of the needle. Second, the overall error, which was defined as the distance from the target to the tip of the needle after the robot had advanced the needle to the target, was measured.
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Entry Site Selection
The patients were placed on the CT fluoroscopy table (Somatom Plus 4 with CARE Vision; Siemens Medical Systems, Iselin, NJ). Patients were placed supine or prone, depending on the expected skin entry site. The patients legs were slid into a frame that was attached to the table and from which the robot arm extended. A nonenhanced breath-hold spiral CT scan was obtained to help localize the lesion and plan the procedure. The appropriate section for needle entry was selected, and the table was moved to that position. Metallic nipple markers were placed on the patients skin at the selected section, with use of the CT scanner laser light for guidance. Another single-breath-hold image was acquired at the selected section. The nipple markers that were visible on the CT image allowed selection of the appropriate skin entry site. Each patients skin was cleaned with povidone iodine, and lidocaine was administered locally over the planned entry site. A small dermotomy was made in the skin at the appropriate entry site.
Registration
Use of a number of image registration methods is possible (23). With the registration method used in this study, we took advantage of the laser light that is incorporated in the section-selection component of all CT scanners.
The mechanical arm with the sterilized needle holder, which contained the appropriate needle for the given procedure, was manually placed so that the tip of the needle was in the skin dermotomy site (Fig 1). The needle shaft was moved to reflect the CT laser light (Fig 3, A). This point was noted to the computer. Then a second position, with the needle tip still in the entry site, was selected. This second position was selected to also reflect the laser light along the needle shaft. The second position was noted to the computer (Fig 3, B). Thus, the two needle positions that reflected the laser light were used to identify a plane for image registration. The third coordinate came from the position of the CT scanner table.
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Procedure Assessment
Procedures were assessed with CT fluoroscopy of the needle tip in the target tissue. In addition, for the cases of neobladder access and nephrostomy tube placement, urine return was evidence of successful targeting. Pathologic findings were correlated with imaging findings for the 10 biopsies performed with the robot.
| Results |
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Patient Results
All interventional procedures were performed successfully without complication. These procedures included 10 percutaneous core biopsies (kidney, n = 7; lung, n = 2; liver, n = 1), 11 RF ablations (kidney, n = 9; spine, n = 2), nephrostomy tube placement (n = 1), and neobladder access (n = 1) (Fig 5, Table).
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In four cases, the target was not met adequately, and fine-tuning adjustment with joystick control was required to ultimately reach the target. The four cases included patients 1, 3, 8, and 14, with target diameters of 2.5, 1.0, 2.0, and 2.0 cm, respectively. The remaining cases were targeted satisfactorily, on the basis of image interpretation.
| Discussion |
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In this study, the robotic arm held and advanced the needle. Since the physicians hands did not need to be in the scanning plane at all, physician radiation exposure was dramatically reduced. In fact, radiation exposure to the physician can be completely eliminated if the joystick and computer are placed in the control room, where the physician can view the technologists monitor to perform the procedure.
Radiation exposure to the patient can also be reduced with use of the robot. Since the computer can reliably advance the robots needle to the target, continuous imaging may not be necessary as long as respiration is relatively controlled. This will significantly reduce patient radiation exposure. This method of allowing the computer to advance the needle without continuous CT fluoroscopy may also be useful in cases where CT fluoroscopy is not available. Once the needle is placed by the robot, a subsequent image can be acquired to assess position. Any inaccuracies in registration or movement due to respiration can be updated with the joystick manual control.
One of the challenges in standard CT fluoroscopyguided procedures is that the entry site may not be in the same plane as that of the lesion. Multiple manual manipulations are often necessary to determine the correct needle trajectory. In the system used for this study, this challenge can be avoided. The needle tip or entry site for registration and the lesion can be in different scanning planes. The ability to have the computer advance the needle tip to the plane of the lesion can avoid having to "hunt" for the needle tip.
The limitation to use of the robot is the extra preparation time. The robot needs to be attached to the CT table, and registration must be performed. In the future, if the robot can be mounted on the CT table, registration could be performed during installation and not have to be repeated with each additional patient. In the current situation, however, approximately 15 minutes of extra preparation time are needed for frame mounting and robot registration.
Another limitation relates to the mechanism of needle advancement by the robot. Since the robot has a friction rolling mechanism to advance the needle, it will strip off the insulation of RF ablation probes. In these cases, we advanced the needle manually for the known distance when the angle of trajectory was selected by the robot. We are developing a new mechanism of advancement to avoid this issue.
Use of the robot will limit the tactile feel that an interventionalist often relies on when performing procedures. Many researchers have applied haptic interfaces to simulate the force feedback that a physician may sense when performing a procedure such as angioplasty or laparoscopy (24,25). These interfaces have been focused on virtual reality educational simulations. Some of these techniques may be incorporated into the joystick control of the robot, which remains an area for further investigation.
The robot may also be useful with standard CT scanners without CT fluoroscopy. In these cases, the robot can advance the needle to the target, which would obviate the repeated scanning and hunting for the needle tip that occur in conventional procedures. Use of the robot can potentially save procedure time and hopefully improve accuracy.
The robot itself is relatively easy to use without special training. It is driven by a personal computer, and touch screen functions are included. While we used a research prototype system, a commercial unit might cost in the range of $20,000. The steps simply include the technologist transferring the registration images to the computer and the radiologist using the mouse to select the needle tip and target.
Last, several interventional tools, such as the RF devices, do not fit well into the CT gantry. It is conceivable that a shorter needle might be used to "register" the robot for the correct trajectory; then, out of the CT gantry, the appropriate device could be placed into the robot and advanced to the target.
In summary, CT fluoroscopy is a helpful tool for performing interventional procedures with image guidance. The limitation of added radiation exposure to the physician can be dramatically reduced by using a robotically controlled needle driver, as described in this study.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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Abbreviation: RF = radio frequency
Author contributions: Guarantor of integrity of entire study, S.B.S.; study concepts and design, S.B.S., M.E.B., L.R.K., D.S.; literature research, S.B.S., A.P.; clinical studies, S.B.S., M.E.B., L.R.K.; experimental studies, S.B.S., A.P., D.S.; data acquisition, S.B.S., A.P., D.S.; data analysis/interpretation, S.B.S., M.E.B., A.P., D.S.; manuscript preparation, S.B.S., A.P., D.S.; manuscript definition of intellectual content, S.B.S.; manuscript editing, revision/review, and final version approval, all authors.
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