DATA ACQUISITION FROM IN VITRO TESTING OF AN OCCLUDING MEDICAL DEVICE

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1 DATA ACQUISITION FROM IN VITRO TESTING OF AN OCCLUDING MEDICAL DEVICE Florentina ENE 1, Carine GACHON 2, Nicolae IONESCU 3 ABSTRACT: This paper presents a technique for in vitro testing of an occluding medical device. The testing apparatus is equipped with a data and image acquisition system. The aim of the integrated components of the system is to measure the pressure of the fluid inside the testing tube and the displacement of the device inserted in the tube. National Instruments equipments and software were used. LabVIEW and Vision Assistant provided the tool for data and image acquisition and output the values for pressure and displacement. The entire system gives the necessary information to reach conclusions about the performances of the medical device and can be used in combination with the testing rig. KEYWORDS: In vitro, Pressure, Displacement, LabVIEW. 1. INTRODUCTION In vitro tests are experiments made in a nonliving model for verifying and proving the intended functions of a medical device. This paper presents a technique of in vitro testing for a human sterilisation medical device. In vitro testing has been developed for different applications and it is also very popular in studying biomedical applications. The term "In vitro" refers to test procedures performed "outside the body," as in a laboratory, as opposed to "In vivo" procedures performed in or on the body directly. The advantages of in vitro tests are that they are quick, relatively inexpensive and specific mechanisms of action can be tested. The disadvantage of these tests is that the homeostatic mechanisms and pathways found in animals are not present (Gallo, 2011). There is a demand for in vitro testing of every medical device in order to prove its efficiency by the results of the tests. The medical device, for which the in vitro testing system has been designed, has the role of blocking the male reproductive system organ, vas deferens, resulting in a reversible sterilisation. A parallel project deals with its design. The testing rig (figure 1) setup is used to obtain conditions similar to the human body and to test the behaviour of the device. The rig is made of a silicon tube with 10 mm outer diameter and 7 mm inner diameter. Inside this tube, the medical device has been placed and a fluid will be introduced in the tube by one of its extremities. 1,2 Galway-Mayo Institute of Technology (GMIT), Dublin Road, Ireland 3 POLITEHNICA University of Bucharest, Romania, Splaiul Independenţei 313, sector 6, Bucureşti flori.ene@gmail.com, carine.gachon@gmit.ie, ionescu_upb@yahoo.com Then, this free extremity will be blocked creating a closed circuit for the test. On the exterior of the tube a contracting device is placed, consisting in a pair of circular devices with inner diameter smaller than the diameter of the tube, and it will move along the whole length of the tube. This mechanism will propel the fluid forward as the muscles of the vas deferens do in the human body. SILICONE TUBE FLUID MEDICAL DEVICE CONTRACTING DEVICE Figure 1. Testing rig The overall in vitro testing consists of two steps: 1. To test and to optimize the properties of the fluid inside the tube in order to ensure that similar conditions with human body fluid are achieved; 2. To test the device with the preset conditions. Each of these steps is important for a precise and accurate experiment, but also to formulate conclusions about the success of the medical device. Developing in vitro testing rig involves setting some requirements for the components of in vitro testing system. Following the problem description, a research was conducted to specify the instrumentation devices necessary for both steps of the in vitro testing (Ene, 2006). Taking into account the relationship between flow-speed-pressure of a fluid described by mathematical equations, it was considered that the best alternative to measure the properties of fluid will be the use of a Particle Image Velocimetry (PIV). This proved to be an

2 expensive choice for this application, but the advantages of this measurement instrument were acknowledged. To analyse the behaviour of the device, its eventual displacement inside the tube is important and it can be accurately measured by a vision system. This paper describes the development of the system for data and image acquisition and analysis. 2. COMPONENTS The system has been created to integrate the results of the two tests in a user friendly environment DAQ System Data acquisition (DAQ) involves gathering signals from measurement sources and digitising the signal for storage, analysis and presentation on a personal computer (PC). A data acquisition system is composed of elements that transform the signal received in a data that can be analysed. The elements of a DAQ system (figure 2) are: sensors, signal conditioning, DAQ hardware, driver and application software and the PC (***, ) DAQ DEVICE PCI 6221 Lab VIE PIEZOELECTRIC PRESSURE SENSOR SC 2075 CONNECTOR BOARD Figure 2. DAQ System The sensor indicates the type of measurement that will be made by tests. Different alternatives were considered for measuring the pressure, the flow or the velocity of the fluid inside the tube. The most appropriate method for tests was selected taking into consideration its integration with the other components of the testing rig. For financial reasons the final method that has been chosen was a piezoelectric pressure sensor. This is a gauge sensor with the pressure range between 0 and 30psi. DAQ hardware is NI PCI 6221 from National Instruments (***, ). The connector block NI SC-2075 provides BNC and spring terminal connectivity for 68-pin M Series DAQ devices. The built-in ±15 V or adjustable 0 to 5 V power supply and LEDs for digital lines make the SC-2075 a cost-effective device, ideal for academic laboratories. The software that is used for this project is NI LabVIEW 7.1. NI LabVIEW is an environment for signal acquisition, measurement analysis, and data presentation, giving the flexibility of a programming language without the complexity of traditional development tools (Jamal and Pichlik, 1999). To create an application with LabVIEW the following are necessary: The creation of a user interface by putting controls and data indicators from the Controls Palette onto the Front Panel; The design of the graphical code by defining functionality with the VIs in the Diagram Panel and wiring them together. The DAQ Assistant is a graphical interface used to configure measurement tasks and channels, and to customize timing, triggering, and scales without programming IMAQ System An image acquisition (IMAQ) system involves processing an image or a sequence of images recorded by a camera in order to determine certain parameters or to make an inspection for the object in the image. The main components of an image acquisition are: camera, camera connectivity, IMAQ hardware, IMAQ driver and application software and PC (*** ). The camera used is Basler A601f. This camera features a CMOS sensor size 656x491, pixel size of 9.9µmx9.9µm and maxim 60fps. With a compact size it is easy to integrate with their IEEE 1394 interface. The IMAQ hardware used for this application is NI CVS-1454 compact vision systems (*** ). This gives flexibility, integration and ruggedness for all of the inspection, alignment, gauging, and identification applications. Vision Assistant (*** ). is a tool for prototyping and testing image processing applications. To prototype an image processing application, build custom algorithms with the Vision Assistant scripting feature. The scripting feature records every step of the processing algorithm. After completing the algorithm, it can be tested on other images to make sure it works. NI Vision Assistant can automatically generate a LabVIEW block code. It is possible to run the diagram generated or to integrate it into an automation or production test application, which may include motion, instrument control, or DAQ Equipment set-up The elements of both DAQ system and IMAQ system have been coupled together in order to create the steps of in vitro testing (figure 3).

3 SCIENTIFIC PAPERS camera has been placed at 15 cm distance from the testing rig. MULTIMETER POWER SUPPLY Figure 3. The testing equipment The DAQ device, NI PCI 6221 has been inserted in the PC and the software LabVIEW 7.1 has been installed. The connector block SC 2075 has been attached to the port of the DAQ device. The piezoelectric sensor has been placed on the SC 2075 board by inserting the four pins into the board terminals. Conforming to the sensor electrical connection diagram the pins were connected to the positive and negative channels of the power supply and the connector board (figure 4). PINS1&3-PS CH6-PS SENSOR PINS2&4-PS PRESSURE Figure 4. The sensor connections The measurement of the pressure implies calculating the pressure gradient between two sensors. To simulate the second sensor (due to the impossibility of purchasing it) a second voltage with the value 0.06V has been set on the second display of the power supply (PS) as shown in figure 5, with the scale factor 0.5MPa/V. The camera Basler 601f has been connected to the NI CVS-1454 compact vision systems and this connected to the PC and to the power supply. The software Vision Assistant has been installed in the PC. The artefact testing rig used is the silicon tube with 10mm external diameter and a cylindrical, soft material, plug inserted into the tube. The artefact testing rig has been positioned on a coloured background so that the object in the image can be clear and for avoiding elements of the background to interfere on the image analysis process. The CONNECTOR BOARD SC2075 PRESSURE SENSOR TUBE PRESSURE REGULATOR Figure 5. The DAQ equipment used 3. METHODS FOR SOFTWARE DESIGN A LabVIEW program was created that will analyse and present the results of the measurements from the testing rig. The application involves the use of two modules from NI software package: LabVIEW and Vision Assistant. LabVIEW programs are called virtual instruments, or VIs. LabVIEW is used, firstly, to create a VI for data acquisition and analysis of the results, meaning measurement of fluid pressure. Secondly, it is used to create a connection between the results of the two steps of the in vitro testing. Vision Assistant programs are called scripts. This is used to acquire and analyse images of the testing rig in order to determine the displacement of the medical device. The analysis of the images will give the value of the displacement of the medical device in the tube. The program created by using both LabVIEW and Vision Assistant from NI is the tool to complete the both steps of the in vitro testing of the medical device: Testing and measuring the parameters of the fluid - measurement of the fluid pressure Testing the medical device- measurement of the displacement of the medical device The two steps were created independently and after completion the programs were integrated in a final program used to monitor the results. LabVIEW has two interfaces: Front Panel and Block Diagram. The two of them are linked. The Front panel is the operator interface and the Block Diagram is the programmer interface. The Block Diagram connections are created between the

4 controls defined in the Front Panel and any other functions necessary to obtain the desired solutions Measurement of two pressures sub VI The VI called Measurement of Two Pressures is the tool which analyses and presents the results of the pressure measurements from the two sensors connected to the testing rig. Measurement of Two Pressures has been made by completing the following tasks (figure 6): Figure 6. Measurement of Two Pressures subvi- Block Diagram a. Configuration of DAQ Assistant. The configuration of DAQ Assistant means the selection of the type of signal to be acquired from the DAQ card, the channels to which the sensors are connected, the scale factor of the sensors and the type of acquisition of the sample. b. Saving results to file. The function Write LabVIEW measurement File allows the data to be saved in a different file which path will be decided by the user at the time of the test. c. Creating the graph Distance=F (time) for the contracting device. With the input controls Total Time and Velocity of the Contracting Device defined on the Control Panel and with the incrementing alternative that the structure For Loop offers the graph Distance=F (time) has been created. d. Determination of the pressure gradient graph. The pressure gradient is the difference between the two measured pressure divided by the distance between the sensors. From the signal output of the DAQ Assistant the two signals were selected using the function Select Signal, the pressure gradient was calculated and the results displayed in a graph. e. Presenting the results of the pressure in a table. To display the recorded data a table was created using the function Build Table Displacement measurement subvi The Displacement Measurement VI has been created with the help of the software Vision Assistant 7.1. Because this is as well a part of the NI software package the script created will be transformed in LabVIEW code. Firstly, the Vision Assistant script has been developed (figure 7), by completing the necessary steps of IMAQ and analysis process. Figure 7. Vision Assistant Script The following steps have been carried out: a. Image acquisition. The acquisition of the images by the camera has been made with the function Sequence, which acquire a certain number of frames. Ten frames were acquired for

5 comparing the differences between the first and the last image. After the acquisition the pictures were saved to be processed ulteriorly for the analysis as the LabVIEW didn t allow a real time acquisition in the used version. Also, it has been acquired one image with the function Snap of the artefact testing rig and a ruler in the background, which will be used for the calibration of the image. b. Image analysis. The Vision Assistant provides a series of function for the image analysis. The process is obtained by creating a script. The script is the tool of the program containing the selected functions in the order that they have been placed. o Image calibration In order to make accurate measurements the images has to be calibrated. The calibration step is made with the function Calibrate image by selecting two points and identifying the distance in both x and y direction with value in real world measurements units. o Pattern Matching The function Pattern Matching locates regions of a grayscale image that match a predetermined template. Using three times Pattern Matching it was identified the ends of the testing tube and the medical device. o Edge detection Edge Detection function locates the edges along a line drawn on the image. A horizontal straight line has been drawn along the centre of the testing rig. The function has found four edges along that line and has displayed the x and y position of the four points of the intersection between the line and the edges. o Measurement of the distances Caliper computes measurements such as distances, areas, and angles based on results returned from other machine vision and image processing functions. To attain the distance measurement (figure 8) has been selected the points found in the previous step, Edge Detection. c. Presenting results. Vision Assistant provides a tool to present the results of all the steps created in the script. The option View Measurements has been selected from the menu Tools. All the results are presented in two measurement units: pixel and cm. Figure 8. The Caliper results d. Analysis of all the images. From the Tools menu has been chosen the option Batch Processing. This provides the solution for the analysis of multiple images with the same script. The all-ten images have been selected and the option of saving results just for the Caliper function Tests VI The integration of the LabVIEW codes for the measurement of the data was made with a Case structure in the Block Diagram that has been controlled by the boolean button Chooses the tests which permits the selection of the test to be run. Figures 9 and 10 show the Block Diagrams of the final Tests VI which will be used for developing the tests of this application. Figure 9. Case structure for pressure measurement Figure 10. Case structure for displacement measurement

6 4. RESULTS The program created by using both LabVIEW and Vision Assistant from NI is the tool to complete the two steps of the in vitro testing of the medical device. The operator has to choose, firstly which is the test that he wants to do by clicking the Boolean button in the correct position: - Start pressure measurement - Start displacement measurement To run the test for pressure measurement the operator needs to input values for: -Velocity of the contracting device -Total time -Distance between sensors Velocity is the value set on the contracting device placed on the testing rig and the Total time is the number of seconds of the contracting device movement. After pushing the Run button, the operator will be asked to choose the location and name of the file where the results of the test be saved. After a short delay the measurements are recorded and displayed in the Front Panel. The results that will be presented to the operator are: The graph distance vs. time of the contracting device The graph of pressure gradient vs. time The graph of the two pressures values The table with the values of the two pressures, the values of the pressure gradient and the time when they were recorded The name and location of the results file To demonstrate the performance of the VI for pressure measurement a test has been concluded. The values chosen were: velocity 5m/sec., total time 21 sec and distance between sensors 40 mm. Start pressure measurement option has been chosen from the button Chose the test and the Run button has been pushed to begin the acquisition and analysis of the pressure. After the program has been running and recording pressure for 21 seconds the results are displayed as showed in the figure 11. Figure 11. Front Panel for pressure measurement In the first graph, the displacement of the rig the comparison can show how the movement contracting device is presented. This is a linear of the contracting device towards the occluding dependence as the contracting device has a device will influence the pressure of the fluid constant velocity. The second graph shows the inside the tube. The third graph presents the two pressure gradient vs. time. The two graphs can be pressures recorded from the sensors connected to compared for a more accurate understanding of two different channels. the phenomenon in the tube. For the real testing

7 In the table Results table data recorded are presented. The values recorded in this example are for demonstration purpose only. After completion of the first test, it is possible to see the results of the second test. The operator has to choose Start Displacement Measurement from the button Choose the Test and the window Displacement Measurement. When the program is ran the operator has to navigate and choose the first and the last of the acquired images. The program analyses the two images, measures the distances between: The first end of the tube and the first end of the medical device The ends of the medical device The last end of the medical device and the last end of the tube The first and last ends of the tube and by subtracting the values obtained with the first image from the values obtained with the last image the displacement is calculated. In the Front Panel are offered the following results: - The first image acquired - The last image acquired - The values of the displacements A verification of this step has been made. As shown in the figure 12 the two images are displayed in the Front Panel and also the value for the displacement. The first value cm represents the displacement of the medical device from the first end of the tube. Figure 12 Front Panel for displacement measurement In the same way, the displacement for the real medical device during the test can be determined. The values that will result will indicate if the medical device is reliable to maintain its predetermined position in the human body. 5. CONCLUDING REMARKS The objective of this paper was to present a new technique for the data acquisition from an in vitro testing system. The testing rig specifications being given, a data and image acquisition and processing system was created and a detailed description of each of its components was presented here. Using the user friendly environment of LabVIEW and Vision Assistant, the tests for the measurement of the pressure and the displacement were designed. Due to the unavailability of the testing rig and the medical device the system could not be tested in its final configuration. But several benchmarking tests were used to validate the system. The results show that the program can be a useful tool to provide the information needed from the testing rig. Measurement of the pressure will give the possibility to set up the elements of the testing rig at real values that could be similar to human body conditions. Measurement of the displacement will reflect upon the capability of the device to comply with its prescribed functions. The system is ready to be integrated with the testing rig as soon as this one will be available. 6. REFERENCES Ene, F. (2006). Data acquisition from in vitro testing of an occluding medical device, Bach (Eng) in Mech. Eng., GMIT, 2006 Gallo, M. (2011). In Vivo and in Vitro Testing, available at: Accessed: Jamal, R and Pichlik, H. (1999). LabVIEW: applications and solutions, Upper Saddle River, NJ: Prentice Hall PTR, Upper Saddle River, NJ ISBN *** (2011). Introduction to Data Acquisition, available at: p/id/3536, Accessed: *** (2011). Low-Cost M Series Multifunction DAQ, available at: dlr.pdf, Accessed: *** (2010). Machine Vision and Scientific Image Processing, available at: topic/in-vivo-4, Accessed: *** (2010). Rugged Real-Time Compact Vision Systems, available at: us/2005_5732_221_101_lo.pdf, Accessed: *** (2010). Vision Development Module for LabVIEW, LabWindows/CVI, and Measurement Studio, available at: /us/pg596_600_vision_dev_module.pdf, Accessed: