INTRODUCTION TO MEDICAL IMAGING- 3D LOCALIZATION LAB MANUAL 1. Modifications for P551 Fall 2013 Medical Physics Laboratory

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1 INTRODUCTION TO MEDICAL IMAGING- 3D LOCALIZATION LAB MANUAL 1 Modifications for P551 Fall 2013 Medical Physics Laboratory

2 Introduction Following the introductory lab 0, this lab exercise the student through experiments that demonstrate the geometry of 2D and 3D imaging by quantitatively locating objects in a phantom. Educational Objectives To understand the geometry of radiographic projections and CT images To navigate and make measurements in 2D and 3D images Why Learn This The DeskCAT scanner operates on the same geometric principles as a diagnostic CT scanner. By learning the operation of the DeskCAT scanner, you will gain a better understanding of CT imaging. Overview Medical imaging is routinely used in medical diagnosis and treatment planning. Radiography is a 2- dimensional (2D) technique that dates back to 1895, while computer- aided 3D digital techniques have evolved rapidly since the 1970s. From 2D to 3D In radiographic imaging, the internal geometry of the 3D object being imaged is flattened onto a single 2D projection plane. The result is spatial information that is incomplete, as depicted in Figure 1. Fiducial markers (small spheres) in 3D space being imaged Projection image retains information in only 2 of 3 dimensions (projection of a sphere is a circle) Figure 1: Graphical depiction of a projected image Modus Medical Devices Inc. All rights reserved. 2

3 By acquiring projections at different angles, it is possible to determine the 3D position of objects within a body. In CT imaging, projections are acquired at many different angles, and the internal 3D structure of the body is reconstructed using a computer program. For simple small fiducial markers, as depicted in Figure 1, a single projection will determine the positions of each marker in 2 dimensions, and only one more projection is needed to determine their position in the 3rd dimension. However, for more complex anatomical shapes, CT reconstruction is necessary to construct accurate, unambiguous images. Method In this lab you will: 1. Determine the positions of fiducial markers by acquiring two projection images. 2. Test the geometric accuracy of a 3D CT reconstruction. 3. Solve for distances Δu, Δv, and Δw, shown in Figure 2. Figure 2: Geometry of Fiducial Marker phantom from side view (left) and axial views (right) The fiducial markers have the following properties: The two upper and two lower markers both lay in planes parallel to the u- v plane, and the vertical distance between these two planes is Δw. In the axial views shown in Figure 2, the lines joining the co- planar fiducial markers have lengths Δu and Δv and are approximately orthogonal. Lab Materials: Fiducial Marker phantom (shown at right) Blank Silicone phantom 2L Water (preferably distilled) DeskCAT Multi- slice Optical CT Scanner 2013 Modus Medical Devices Inc. All rights reserved. 3

4 Project Set up and Scanner Calibration 1. Start the DeskCAT software and create a new project. 2. If you have not already done so, follow the instructions in Lab 0 for calibrating and setting up to take a data scan. 3. Load the Blank Silicone phantom into the scanner by attaching the phantom to the Rotary Stage using the Jar Clamp and mounting the Rotary Stage onto the scanner. Ensure that the Rotary Stage is properly aligned using the alignment tab. 4. Acquire a reference image with the New Reference Image button on the Side Panel. Determine Marker Positions with Projection Image 5. Load the Fiducial Marker phantom into the scanner by attaching the phantom to the Rotary Stage using the Jar Clamp and mounting the Rotary Stage onto the scanner. Ensure that the Rotary Stage is properly aligned using the alignment tab. 6. Select Scanner à Motor Control tab from the menu bar. This tool lets you rotate the phantom. 7. Observe the phantom in the Camera Video window (top left). 8. Rotate the phantom until the top two markers appear to overlap and the bottom 2 markers appear separated (see Figure 3). Select Set Current Position As Home. 9. Select the Move To 90 button to rotate the top two markers to their furthest distance apart. 10. Open the Projection Viewer window by clicking the Projection Viewer button on the Side Panel. 11. Select Enable Snapshot. Acquire a snapshot by selecting Take Snapshot. Figure 3: Projection image of fiducial markers. The top markers are overlapping, while the bottom 2 markers are separated. 12. Measure the positions of the top two fiducial markers on screen by placing the cursor over the center of each marker and recording the coordinates. The position of the cursor is displayed 2013 Modus Medical Devices Inc. All rights reserved. 4

5 along the bottom left of the Projection Viewer screen. For greater accuracy, make all measurements as close to the center of the marker as possible. 13. After measurements of the two top markers are complete, exit the Projection Viewer and return to the Scanner à Motor Control window. 14. Using the motor control function, rotate the phantom until the bottom two markers appear to overlap in the Camera Video window. Select Set Current Position As Home. 15. Select the Move to 90 button to rotate the bottom two markers to their furthest distance apart. 16. Obtain another snapshot, from within the Projection Viewer and make position measurements on the bottom two markers. 17. Using the two sets of measurements, calculate the distances of Δu, Δv, and Δw and record your results. Determine Marker Positions with CT image 18. Remove the phantom from the aquarium. 19. Load the Blank Silicone phantom into the scanner 20. Select 320 projections and acquire a reference scan using the New Reference Scan button on the Side Panel. 21. Load the Fiducial Marker phantom into the scanner. 22. Acquire a data scan using the Start Data Scan button on the Side Panel. 23. Once the scan is complete, select the High (0.5 mm) Voxel Resolution option and press the Start Reconstruction button to perform a reconstruction. 24. Once the reconstruction is complete, maximize the 3D Viewer window (bottom right window). 25. Select the Multiplanar Reformatting viewing option. Note that there are three planes that intersect the 3D image and that the perimeter of the 3D image is shown as a wireframe cube. Navigating the 3D Image Multiplanar reformatting (MPR) is one of the most important display techniques in medical imaging. It shows 2D planes within a 3D image. Manipulating the image is easy to do with a bit of practice. The image will not be damaged by experimenting with the display. Use the Reset button to return the image to its starting position. The description below is a brief summary of the tools for manipulating the image. Practice with each tool until you become comfortable navigating the 3D image. To rotate the cube containing the 3D image, left click outside of cube and drag. To pan, middle click outside of cube and drag. To zoom, right click outside of cube and drag up/down OR scroll mouse wheel. For point cursor (xyz coordinates and attenuation value): left click on plane. To push/pull plane: middle click on plane and drag. To tilt plane: middle click on border of plane and drag. To adjust window: right click on plane and drag left/right. To adjust level: right click on plane and drag up/down. To turn on/off planes: select x, y or z plane check box in Main Tab at bottom of window To turn on/off wireframe: select View Outline checkbox in Main Tab at bottom of window To turn on/off axis labels: select View Axes checkbox in Main Tab at bottom of window 2013 Modus Medical Devices Inc. All rights reserved. 5

6 Determine Marker Positions with CT image, continued 26. Move the z plane through the 3D image until you intersect two of the markers. 27. Move the plane carefully through the markers and get it to rest as close to the centers of the markers as possible. You should be able to clearly see the two markers, and also a bright outer ring. The outer ring is caused by light reflection off the outer surface of the phantom. 28. Right- click in the window to access options. Adjust the Window and Level so the markers are clearly visible and the outer ring is invisible. 29. Capture a screenshot of the 3D image at this window and level setting and include it in your lab report. 30. Click on the center of each marker. The coordinates are shown at the bottom left edge of the window. Record these coordinates for each of the markers. Carefully determine the location of the center of the markers for best accuracy. 31. Move the plane through the image until you intersect the second pair of markers, and record the position of each marker. 32. Using the above coordinates, calculate Δu, Δv, and Δw. Compare these results to those calculated previously with 2D projection data. Discussion / Additional Questions 1. Do your coordinate values agree between the two methods? Give possible reasons for any discrepancy between the two sets of results. 2. Why can t the coordinates of the fiducials be measured in display modes other than MPR? 3. Why is the Blank Silicone phantom used to generate reference images? 4. Why are the phantoms scanned in water? What effect would be seen if they were scanned in air? Are there artifacts of a similar nature in x- ray CT? Further Study 5. How are fiducial markers used in radiography and radiation therapy? What type of marker is used in clinical practice using x- ray beams? 6. How can fiducial markers be used for quality assurance purposes in radiography? 2013 Modus Medical Devices Inc. All rights reserved. 6