Optimization of Phase Contrast Imaging Luke Powers 1, Alfred Luk 1, Christopher Weaver 1, Jonathan Fermo 1 Advisors: Paul King, Ph.D. 1 ; Frank E. Carroll, M.D. 2 ; Edwin Donnelly, M.D., Ph.D. 2 ; Robert Traeger; Gary Shearer 3 1 Department of Biomedical Engineering, Vanderbilt University, Nashville, TN 2 Vanderbilt University Medical Center Department of Radiology and Radiological Sciences 3 W.M. Keck Foundation Free-Electron Laser Center, Vanderbilt University DesignProject Overview Phase Contrast Radiography (PC-R) Overview Traditional radiography utilizes differences in x-ray absorption to develop an image. Phase Contrast Radiography (PC-R) utilizes differences in the refraction and diffraction of the x-ray beam at interfaces as it transverses the object, resulting in an edge effect enhancement seen on the image produced. The basis of phase contrast imaging effects originates from differences in the x-ray index of refraction, n, which is given by n = 1 – δ – i β where the imaginary component, β, is utilized in absorption imaging and the real component, δ, is utilized in PC-R. PC-R Considerations Spatial Coherence Unlike x-ray absorption, phase contrast effects depend on the spatial coherence (d) of the x-ray wave front. It is proportional to the x-ray wavelength (λ) and source-to-object distance (R 1 ), and inversely proportional to the source focal spot size (f). A larger “spatial coherence length” yields more pronounced phase contrast effects. Therefore this technique is dependent on different physical properties than those of traditional absorption imaging techniques. This difference gives PC- R the potential to detect edges that would appear invisible on conventional radiography. PC-R provides a new method for soft-tissue imaging contrast and has significant potential for early cancer detection in mammography and specimen radiography. However there is much future work to be done in terms of characterizing the parameters required for various tissues to successfully move PC-R into a clinical setting. A system to accurately position a specimen for PC-R testing would assist in the optimization of such parameters. The goal is to divide out the absorption component and be left with a “phase only” image. This is accomplished by taking two images: one being the absorption component only and the other containing both components. The absorption only image is taken when the object is touching the detector (z=0). When z is increased to get the second image, the new image is larger than the first. Phase Extraction I(x) z=0 = Absorption component 1+λz/2 π Φ”(x) = Phase component λ = Wavelength z = Object to detector distance The image with both absorption and phase components must be scaled down to the size of the absorption only image. Poor accuracy when scaling will create false edges in the PC-R image. Therefore, phase extraction requires the positioning of the two images to be accurate and precise. Main objective: To construct a device for computer-controlled automatic movement of an object and detector to be used in phase contrast imaging research Future Research: Optimizing parameters for PC imaging of specific objects/tissues Constructing reproducible images without false edges Imaging using scattering, defines pixel resolution < 150 microns Computed Tomography Device Description Social Impact Breast cancer is the most common type of cancer among women in the United States today and is the leading cause of cancer death among women age forty to forty-nine. The National Cancer Institute estimates that, based on current rates, 13.2 % of women born today will be diagnosed with breast cancer at some time in their lives. PC-R has significant potential for the early detection of cancers in mammography, due to its ability to detect objects invisible on conventional radiography. Development of this procedure may reduce the death rate of breast cancer patients significantly. Cost/Market Analysis Cost of Items Purchased$1, Estimated Cost of Items Available from FEL $6, Estimated Total Cost of Parts$7, Estimated Market Value of Device $30,000 The PC-R images of the cuvettes tested for the presence of enhanced edge effects. The image taken at a R2 distance of 90 cm contained the characteristic edge effect profile in the intensity plot. The next step would be to extract the phase component from the image and make adjustments for magnification effects. This data confirms that our device can be used to successfully generate images with identifiable phase components. An analysis of the errors in each stage revealed that the error for the horizontal and vertical stages is 10 μm. The error in the detector stage is inches (about 25 μm). The error in the rotational stage is gradians. The optical interrupter has a differential distance of 25 μm. This would only provide error for the MAR 345 stage, since it has a resolution of 12.7 μm/step. The optical interrupter only affects absolute distance and not relative distance. Based on the collected results, the error in each stage is negligible. The resolution achievable for each stage was high enough to be used in parameter optimization for phase contrast imaging. 1. MAR 345 stage: Range = 100 cm Resolution = 12.7 µm/step Horizontal stage: Range = 6 cm Resolution = 25 µm/step Vertical stage: Range = 5 cm Resolution = 25 µm/step Rotational stage: Range = 400 grad Resolution = grad/step Power Supply Optical interrupters/cables (4) Microcontroller chip Microstepper drivers (4) Interface boards (4) iPocket ethernet controller Control System Overview Each stage’s motion is controlled through a PBASIC program, which is stored on the BS2sx microcontroller chip. The stages are given 3 commands: home, forward, or reverse. The other input needed is how far the stage must move. The optical interrupters are used to detect the home position. PBASIC always makes sure the stage is moving to the desired location from only one direction in order to prevent inaccuracies due to hysteresis buildup in the stage’s ball screw. The stages are then individually controlled through a LabVIEW Virtual Instrument (VI), which communicates with the BS2sx microcontroller chip via Ethernet connection. The user inputs the distance desired and the direction, while the VI keeps track of the current location of the stages. The VI also acts as a filter for various error-causing commands and prevents them from being sent to the BS2sx microcontroller chip. The monochromatic x-ray source was undergoing maintenance and could not be used for the testing of the device. Imaging was conducted in the x-ray lab at the Free-Electron Laser (FEL) Center using: Molly (Molybdenum) X-Ray Tube Produces a polychromatic beam with a focal spot of 100 μm and energy peaks at mostly 17.5 & 19.5 keV. Can serve as a monochromatic source by isolating a particular energy with a mosaic crystal MAR 345 Digital Detector 100 micron resolution mode. Computer controlled, raw scan data was transmitted from detector to computer. Raw image data converted into.TIFF files and analyzed using Image J Plastic Cuvette Transparent plastic, 1.3 X 1.3 cm, filled with water. Chosen over other objects to image due to sharp edges present within the object in order to better detect edge effects. Placed in middle of rotational stage The experiment was composed of 7 scanning sessions. The R1 distance = 70 cm and the R2 distance ranging from 90 – 30 cm, with 10 cm movement increments per session. Testing & Results Acknowledgements Conclusion This project was made possible with the resources available at the W.M. Keck Foundation FEL Center, the Vanderbilt BME Department, and help from the following people: Gary Shearer, Dr. Frank Carroll, Robert Traeger, Scott Degenhardt, Dr. Edwin Donnelly, Dr. Paul King Magnification due to similar triangles Illustration of edge effect enhancement Figure A: PC-R image of a breast cancer specimen. The tumor (white arrow heads) is invading the chest wall (white arrows). Figure B: The "phase-only" image further reveals strands of tumor invasion (black arrows) not apparent in Figure A. Plastic cuvette on rotational stage. X-ray image at R 2 = 90 cm. Contains both absorption & phase components. Magnification of cuvette edge. Contrast adjusted to show edges. Yellow line indicates ROI. (Right) Plot of x-ray intensity data on ROI drawn on magnified cuvette edge. Edge enhancement apparent in plot. (Above) Edge effect of phase contrast image compared to that of an absorption only image. LabVIEW VI Front Control Panel