Modifications for P551 Fall 2014

Transcription

1 LAB DEMONSTRATION COMPUTED TOMOGRAPHY USING DESKCAT 1 Modifications for P551 Fall 2014

2 Introduction This lab demonstration explores the physics and technology of Computed Tomography (CT) and guides the students through the process of using DeskCAT to scan, reconstruct and display CT images of a test object. This lab demonstration is different from subsequent student lab exercises in that it is non-quantitative. In contrast, Lab 0 focuses on instilling an understanding of the physics and technology of CT and provides an opportunity to learn about the operation of DeskCAT. Educational Objectives To learn how 2D projection images are created To learn how 3D CT images are created from 2D projections To gain experience with common display modes for 2D and 3D images To understand the operation of the DeskCAT Optical CT scanner, including both hardware and software features Why Learn This? You are likely to come in contact with CT scanners as part of a medical diagnostic procedure at some time in your career. Professionally, you may also make use of the rich 3D information produced by CT scanners (physicians, scientists or engineers) or become scanner operators (imaging technologists, medical physicists). In each case, an understanding of the imaging process and results of CT scanning will be highly beneficial. Overview Medical Imaging Medical imaging is routinely used in diagnosis, treatment planning and surgical guidance. One of the most common imaging techniques, radiography, uses X-rays to create a two-dimensional (2D) projection image of a patient. A 3D CT image can then be created through the mathematical reconstruction of a series of 2D projection images. There are also many non-medical uses of computed tomography and X-ray imaging in industry (inspection for impurities), security (airport scanners), and research (animal scanners). DeskCAT DeskCAT is an optical CT scanner. It operates on the same physical principles as a medical CT scanner except that it uses light instead of X-rays. Optical CT is ideal for education because it is safe and allows the scanner to be intuitive, accessible and interactive. DCT-LM Modus Medical Devices Inc. All rights reserved. 2

3 Method and Discussion In this lab you will: 1. Acquire 2D projection images of a Mouse phantom 2. Acquire and reconstruct 3D CT images of a Mouse phantom 3. Use different display modes to explore 3D CT images of a Mouse phantom Note: In the following procedures, discussion points and questions are included to enhance understanding and to provide guidance for preparing a laboratory report. Please answer all the questions asked in your report. Lab Materials: Mouse phantom 2L Water (preferably distilled, already loaded into scanner) DeskCAT Multi-slice Optical CT Scanner DeskCAT Quick Start Guide DeskCAT User s Guide Optional phantoms and Lab manuals for subsequent labs Optional laser pointer Optional digital camera, phone camera or web cam Optional clear plastic ruler 6 (15 cm) Preparation Start & Setup DeskCAT Software 1. Click on the DeskCAT icon to start program. The Open Project dialog box appears. 2. Create a new project, or select an existing one. Click Open to continue. 3. Inspect the Camera Video window (upper left), to see if there are any air bubbles in the field of view. Air bubbles may interfere with the accuracy of your results. They can be removed by directing a stream of water from a syringe through either of the access ports. Alternatively, the provided small sponge or paint brushes can be used to remove bubbles, or a short length of wire can be used as a poker to remove the bubbles. 4. Adjust the camera setting to achieve maximum brightness without saturating the image. Select Scanner Camera Settings. Adjust Frame Rate/Shutter Speed until no or only a few red pixels are visible in the Camera Video. Red pixels indicate saturation. Calibrate Scanner DCT-LM Modus Medical Devices Inc. All rights reserved. 3

4 5. Under Calibration Geometry Calibration select Auto-Cal and accept the values. *Note: calibration must be done with NO phantom loaded. The calibration may come back that it has "failed" and will not accept the calibration constants. Until you can calibrate the device, you will not be able to continue. Pay attention to the positions of the two red and two blue vertical lines. Look in the Users Manual to diagnose which of two problems is causing this: (a) If due to bubbles in the image, follow the instructions above to remove any bubbles that you can see in the image. They may be trapped on either of the two glass windows, either in the main field of view or along the edges. Also make sure that the water level is at least a mm or two above the top edge of the window. Bubbles up there will be flagged by the software. As a last resort, simply using your fingers (while watching the camera) to wipe away the bubbles works. (b) If due to "camera pan", in the menu click on Camera Settings, and then select Enable Camera Panning. In the past, this has been fixed by dragging the slider from the seeming default value of 56 out to ~65. Try calibrating again, and if successful, accept the calibration constants. Obtain New Reference Image 6. Click on New Reference Image button on Side Panel to capture a single reference image. The captured reference image will be saved by the system for use in image reconstruction. Note: Reference images are used to compensate for light source inhomogeneities. The DeskCAT scanner and software are now ready to scan. DCT-LM Modus Medical Devices Inc. All rights reserved. 4

5 Phantoms Prior to acquiring images with DeskCAT it is helpful to observe and understand some physical properties of the test objects which are about to be scanned. These test objects are commonly known as phantoms. The phantoms used with DeskCAT are made of clear silicone with the addition of colored dyes. They are translucent to light in the same way that patients are translucent to X-rays. Alternative materials for phantoms are water (used in Lab Exercise # 2) and clear gelatin. DeskCAT is provided with multiple phantoms that have been designed to enable the different experiments described in the subsequent lab exercises. In general, phantoms are test objects with known properties which, when imaged, provide information about how an imaging system performs. In medical imaging, different kinds of phantoms are used for quality assurance to ensure that an imaging system is performing as expected or within tolerances. 7. Observe the Mouse phantom hold it up in room light hold it up in a dark room in front of a bright white screen shine the light from a laser pointer through it what optical effects do you observe? Hint some optical effects to consider are reflection, refraction, scattering, absorption, and color. X-rays are higher energy photons than light. Do X-rays interact with matter the same way that light does? What are the differences and similarities? 8. Acquire a few close-up digital photos of the Mouse phantom from different directions using a camera phone if you have one (if not, ask the instructor). Make sure that you have one image from the front of the mouse and at least one image from an oblique angle. Save these photos for use later in this demonstration and in your lab report. DCT-LM Modus Medical Devices Inc. All rights reserved. 5

6 9. Optional observe other DeskCAT phantoms, how are they different from the Mouse phantom? Can you relate the differences in the phantoms to the educational goals of the subsequent labs? DCT-LM Modus Medical Devices Inc. All rights reserved. 6

7 DeskCAT 0 Projection Image 10. Load the Mouse 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. As the phantom is lowered into the scanner you will see the image of the Mouse phantom appear in the Camera Video window. Maximize the Camera Video window 11. Select Scanner Motor Control. Drag the Motor Control window to a location that does not obstruct your view of the Mouse phantom. Use the Motor Control interface to rotate the Mouse Phantom. 12. Right click in the Camera Video window to gain experience with the Zoom and Window/Level controls. Window and Level are related to image brightness and contrast adjustment. Display contrast can be increased or decreased by making the window narrower or wider. Likewise, display brightness can be increased or decreased by adjusting the level (Figure 3).

8 Figure 3: A graphical representation of how window and level are used to adjust display contrast and brightness. DCT-LM Modus Medical Devices Inc. All rights reserved. 6

9 The image that is displayed in the Camera Video window is an optical 2D projection image. It is the equivalent of an X-ray 2D projection image in medical imaging. A projection image is formed when a photon beam traveling from source to detector is attenuated by an object. The numerical value of each pixel in the image is the brightness of the source dimmed by attenuation values on the line through the phantom between the source and the detector. In the image of the phantom, the brain and intestines of the mouse are darker than the rest of the mouse. This is because these objects have higher optical densities and therefore attenuate more light. In a medical radiograph bones attenuate more X-rays than surrounding tissues because bones are more dense. However, when you look at bones in a skeleton they appear white. What is the difference between the density of the Mouse s brain in an optical projection image and bones in an X-ray projection image? What is the difference between the interaction of light with the phantom and X-rays with tissues in a patient? Figure 4a: Shows the clinical setup for acquiring a chest radiograph. The flat panel acquires the projection image. DCT-LM Modus Medical Devices Inc. All rights reserved. 7

10 Figure 4b: Shows the equivalent setup for the DeskCAT Optical CT Scanner. DCT-LM Modus Medical Devices Inc. All rights reserved. 8

11 13. While viewing the phantom in the Camera Video window, do you see evidence of the optical effects that you observed in step 11 (before placing the phantom in the scanner)? Figure 5 is a schematic showing a top view of the aquarium with a phantom and refracted light paths through the aquarium. The water in the aquarium is used to reduce the effect of refraction. Figure 6 shows a projection image where some of the water has been removed from the aquarium. What evidence of refraction do you see in this image? Is refractive index matching necessary in X-ray imaging? Figure 5: Illustrates the effect of refraction along a light path. The refractive index of the matching liquid will determine which path the light follows. For example, DCT-LM Modus Medical Devices Inc. All rights reserved. 9

12 an accurate refractive index matching liquid will result in light following the straight path through the phantom. DCT-LM Modus Medical Devices Inc. All rights reserved. 1

13 DeskCAT 0 Water Level Figure 6: A projection image of a Mouse phantom in a half filled aquarium. Water is a better refractive index match to the phantom than air. Therefore, the effects of refraction are more evident above the water level. 14. The source light in the scanner can be switched between red and green. What color are you using? Why is the image in the Camera Video window shown in grey scale? In X- ray imaging, what is the equivalent of changing color? Switch back to the original light color in the scanner before continuing. Perform Data Scan 15. In the Side Panel select 200 projection images. 16. Click Data Scan on the Side Panel to perform scan. 200 projection images are acquired and saved (in the project folder) as the phantom is rotated through 360 degrees. An image of the center slice sinogram appears as the scan progresses. Note: During a scan with DeskCAT the phantom is rotated. Mechanically this is simpler and easier than rotating the source and detector. In contrast, a medical CT scanner rotates the source and detector because it is not practical to rotate patients.

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15 Review Scan 17. Select the High (0.5mm) Voxel Resolution radio button in the Side Panel. 18. Open the Projection Viewer by clicking on the Projection Viewer button in the Side Panel. In the Main Tab at the bottom of the Projection Viewer window you can use the radio buttons to show the Reference (I 0 ), Data (I), I 0 /I (ratio of the reference image to data image for each pixel), and ln(i 0 /I) (natural logarithm of the ratio) images. Note: The importance of these calculated images is beyond the scope of this introductory lab. However, it should be noted that a CT image is reconstructed using the ln(i 0 /I) images at the selected resolution. It is possible to change the selected resolution from Low (2mm) to Very High (0.25 mm) by closing the Projection Viewer, selecting a different resolution and then reopening the Projection Viewer. It is possible to show the full image resolution by selecting the Full Image checkbox in the Projection Viewer window. The projection images can be stepped through manually, or viewed through playback using the Play and Pause controls. Zoom and Window/Level values of the projection images can be manipulated by right clicking on the image and selecting the appropriate tool. 19. Once you have gained experience using these controls please select a projection image that shows a front view of the mouse. Compare this image to the front view of the mouse acquired with your digital camera earlier in the lab. DeskCAT uses a greyscale digital camera. Why is the DeskCAT projection image different from your digital camera image? Hint: consider the path of light from source to detector. Is it possible to find a projection image in DeskCAT that is from the same direction as the oblique image taken earlier with your digital camera? Note: In a CT scanner the object being scanned is between the source and the detector. In DeskCAT, the light photons converge from a large area diffuse source through the object being scanned and into the small aperture of the camera. In a cone beam X- ray CT scanner, the X-ray photons diverge from a small source through the object being scanned and onto a large area detector. In each case the geometry is a rectangular cone, allowing the same reconstruction algorithm to be used despite having reverse photon directions. DCT-LM Modus Medical Devices Inc. All rights reserved.

16 Figure 7: (a) DeskCAT scanner schematic showing light photons direction of travel (b) Schematic of a cone beam X-ray CT scanner, showing the X-ray photons direction of travel. DCT-LM Modus Medical Devices Inc. All rights reserved.

17 Sinogram During each scan a sinogram appears in the Center Slice Sinogram window. This image is created by stacking the central line of each projection image (identified by the red arrows in the Camera View window). The vertical axis of the sinogram represents the angle of rotation (projection number). Objects that do not rotate during a scan appear as vertical stripes in the sinogram display. 20. Save a screen shot of the sinogram image for your report. Observe the sinogram can you see any obvious or faint vertical lines? Where could the objects that cause such vertical lines be located in the scanner? 21. Place an object such as a clear plastic ruler into the aquarium so that it can be seen next to the Mouse phantom. This object must be tall enough to intersect the central line in the projection image (Figure 8). Please note that this object will not rotate during the scan. Repeat the data scan, observe the sinogram and save a screen shot for your lab report. Identify the non-rotating object in the resulting sinogram. Remove the nonrotating object and repeat the data scan before continuing. Figure 8: Shows a ruler placed inside the aquarium and the resultant center slice sinogram. DCT-LM Modus Medical Devices Inc. All rights reserved.

18 Reconstruct and Display 2D and 3D CT Images Note: For the purpose of discussion and questions, please know that you can repeat the scan as many times as you would like because there is no X-ray radiation dose to the phantom (or to the operator) and there is no X-ray tube to wear out. 22. On the Side Panel select the following: High (0.5 mm) Voxel Resolution Note: only if a computer with substantial memory and CPU capability is connected should you select Start Reconstruction With Scan or Update 3D Display. Otherwise, enabling these operations can result in a scan taking as long as an hour. Doing these steps separately is much faster and far less computationally demanding. 23. From the menu bar, select Reconstruction Reconstruction Options and select the Hamming Filter. Filters are used to reduce high frequency noise in the reconstructed image. The selection of which filter to use during reconstruction is beyond the scope of this introductory lab. 24. Press Start Data Scan button on the Side Panel. During the scan: the Camera View window is updated as projection images are acquired the sinogram is created during the scan 25. When the scan is complete click Ok to continue. 26. Now, Start Reconstruction, watch the 2D CT image in the Slice Reconstruction window update as each projection/slice is reconstructed. When complete, then click Update 3D Display. 27. Once reconstruction is complete, reconstructed images can be viewed and manipulated in the Slice Reconstruction window or the 3D Viewer window. Note: The reference image, calibration parameters, projection images and reconstructed CT images are saved in the project folder as they are generated. Select Project Status on the Side Panel at any time to review the project data which has been generated and saved. 2D CT Image 28. By observing the Slice Reconstruction window during a scan you will see how projection DCT-LM Modus Medical Devices Inc. All rights reserved.

19 image data is back projected into a single slice. Repeat the scan until you understand qualitatively how the projection data is back projected and displayed. Following reconstruction, you can observe different 2D slices by clicking and dragging the arrow in the Slice Reconstruction window. 29. Explore the following commands in the Slice Reconstruction window: Right click in the window and manipulate the Zoom and Window/Level controls DCT-LM Modus Medical Devices Inc. All rights reserved.

20 Right click and select the Line Profile Tool. Click and drag a line within the image to see a line profile Right click within the Slice Reconstruction window and select the Region Profile Tool. Click and drag in the image to see a Region of Interest (ROI) Histogram. Right click again to change the ROI from a rectangle to a circle Right click in the Line Profile or ROI Histogram window to display a menu of functions allowing you to save the image, save the data, or export the data in the window 30. Once you have become comfortable with the display and manipulation of 2D CT images you should save some screen shots for your laboratory report. 3D CT image By observing the 3D Viewer window during a reconstruction, you will see how projection image data is back projected into a 3D image (Update 3D Display must be selected). It may be helpful to maximize the 3D Viewer window and repeat the scan/reconstruction as you explore the 3D image using the different display modes. Navigating the 3D Image using Multiplanar Reformatting Multiplanar Reformatting (MPR) is one of the most important display modes in medical imaging. MPR allows you to visualize 2D planes within a 3D image. Manipulating the 3D image within DeskCAT software is easy to do with a bit of practice. The image will not be damaged by experimenting with the display. You may use the Reset View button to return the image to its starting position. The description below is a brief summary of the tools for manipulating the 3D image. Practice with each tool until you become comfortable with navigation. To rotate cube, 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 move 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 axis 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 DCT-LM Modus Medical Devices Inc. All rights reserved.

21 31. Manipulate the image of the Mouse phantom until the brain and intestines are visible. Save a screen shot of this image for your report. Navigating the 3D Image using ISO Surface Render ISO Surface Render displays the surface of objects within a 3D image that have the same density (pixel value). The cube can be rotated and moved the same way as in MPR. The radio buttons in the upper left of the 3D Image Window allows you to select different surface rendering. Note: In this and the following display modes it may be helpful to turn on the Select Radius of Valid Data button on the Main Tab at the bottom of the 3D Viewer window. Decreasing the value from 1.00, crops the image perimeter. Use this feature to remove the jar walls in the displayed image. DCT-LM Modus Medical Devices Inc. All rights reserved.

22 32. By adjusting the ISO Value (slider just below the image) you can display surfaces of different densities. Adjust the slider to show the surface of the mouse, the brain and/or the intestines. Save a screen shot of these images for your lab report. Navigating the 3D Image using Maximum Intensity Projection Maximum Intensity Projection (MIP) is a reprojection display mode where lines are projected through the image one line for each display pixel and the brightest pixel on each line is shown on the display. This mode is commonly used because it is a fast algorithm that is easy to implement. The cube can be rotated and moved the same way as in MPR. 33. By observing and manipulating the MIP display during a scan/reconstruction (Update 3D Display must be selected), you will see how projection image data is back projected into a 3D CT image. Repeat this process until you understand qualitatively how the projection data is back projected and displayed. 34. By adjusting the Intensity Slider you can alter the MIP display. Save a screen shot for your lab report. Navigating the 3D Image using Density Sum Density Sum is a more computationally intensive reprojection display mode than MIP. It produces an image that is similar to an acquired projection image from the scanner; however the 3D image can be viewed from any angle (including projection angles that are not possible in the scanner). In this technique, lines are projected through the image one line for each display pixel and the average pixel value along on each line is shown on the display. The cube can be rotated and moved the same way as in MPR. 35. By adjusting the Level Slider, you can alter the Density Sum display. Save a screen shot for your report. Culminating Activities 36. Line Profile and Region of Interest Histogram tabs are similar to the tools in the Slice Reconstruction window. Explore each tool, referring to the appropriate section of the User s Guide if necessary. Save screen shots for your report. 37. In your laboratory report describe, in a few paragraphs, how a 2D projection image is Further Study formed. Similarly, describe how a 3D CT image is formed. DCT-LM Modus Medical Devices Inc. All rights reserved.

23 DeskCAT is a cone beam CT scanner. What other geometries of scanners are there (e.g., multislice, helical)? Hint - sometimes these are called different generations of CT scanners. How long does it take to scan and reconstruct an image using the DeskCAT scanner? How do the number of projections and the resolution of the 3D image affect the speed? What is the speed of medical CT scanners? In what medical applications would high speed be an advantage? DCT-LM Modus Medical Devices Inc. All rights reserved.

24 INTRODUCTION TO MEDICAL IMAGING-3D LOCALIZATION LAB MANUAL 1 Modifications for P551 Fall 2014

25 DeskCAT Student Exercise: Introduction to Medical Imaging 1 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. DCT-LM Modus Medical Devices Inc. All rights reserved. 2

26 DeskCAT Student Exercise: Introduction to Medical Imaging 1 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 DCT-LM Modus Medical Devices Inc. All rights reserved. 3

27 DeskCAT Student Exercise: Introduction to Medical Imaging 1 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 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 DCT-LM Modus Medical Devices Inc. All rights reserved. 4

28 DeskCAT Student Exercise: Introduction to Medical Imaging 1 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 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 DCT-LM Modus Medical Devices Inc. All rights reserved. 5

29 DeskCAT Student Exercise: Introduction to Medical Imaging 1 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? DCT-LM Modus Medical Devices Inc. All rights reserved. 6