An Investigation of the Effect of Window and Level Controls on the accuracy of the ExacTrac Patient Repositioning System Dan Goldbaum and Russell Hamilton

Attix, Introduction to Radiological Physics and Radiation Dosimetry, p Attix, Introduction to Radiological Physics and Radiation Dosimetry, p.4: “Biological Systems (e.g., humans) are particularly susceptible to damage by ionizing radiation, so that the expenditure of of a relatively trivial amount of energy (~4 J/kg) throughout the body is likely to cause death, even though that amount of energy can only raise the gross temperature of the body by about 0.001 degrees Celsius.”

Agenda We propose an investigation of the accuracy of the ExacTrac patient repositioning system as a function of window and level settings for x-ray and digitally reconstructed radiograph images. Outline I) The Novalis linear accelerator with ExacTrac patient repositioning package A) Repositioning the patient II) Visual Display of medical images: Manipulation of visual contrast A) Overview B) An example of the effect of different window and level settings III) Proposed Research Plan A) Geometrical and Anthropomorphic Phantoms for “known” results B) Retrospective Patient Study

Treatment Device: Novalis linear accelerator Above: Novalis LINAC Right: Position adjustment examples using the table and gantry.

Treatment planning using initial CT images Screenshot from treatment Planning procedure using CT images. CT scan of head phantom

What’s the Plan? BrainScan Treatment Planning system.

X-Ray 6D x-ray imaging system for position verification and readjustment [Screen shot from ExacTrac 5.0.2. Courtesy of Russell Hamilton.] Above: Floor mounted x-ray tubes with flat panel detectors. After initial positioning of the patient, X-ray images for the patient are recorded for comparison with Digitally reconstructed radiographs (DRRs) calculated from the initial CT simulation.

ExacTrac X-Ray 6D System The ExacTrac system finds the displacement from the patient position specified in the treatment plan by generating Digitally Reconstructed Radiographs (DRR’s) for several different reference displacements from the initial CT images. The DRR that most closely matches the fresh x-ray images gives the displacement from the position in the treatment plan. [J-Y Jin et al., Med. Dos. 33, 124 (2008). ]

ExacTrac generates directions for patient placement ExacTrac generates displacements for each of the 6 degrees of freedom (3 translational + 3 rotational) based upon comparison of X-ray images and DRR’s Repositioning is carried out robotically, and the resulting patient position is Verified using fresh X-ray images.

How does the accuracy of the ExacTrac system depend upon the window and level settings for image contrast control? Image contrast settings are not explained. ExacTrac generates a default Setting that is frequently used We set out to investigate the effect that varying the image contrast controls has on the fusion algorithm which will be manifest by the repositioning accuracy. But, first, what do I mean by “image contrast settings” and “window” and “level” Above: ExacTrac “fusion” of DRR and fresh x-ray image.

Visual display of a medical image, pt Visual display of a medical image, pt. 1: From detection to visualization, a rough overview

Visual display of a medical image, pt. 2: Window and Level Controls Visual display of a medical image, pt. 2: Window and Level Controls. A “toy” example with 8 digit input pixels. (a) (b) (a) (b) (c) (d) (e) (f) (c) (d) (e) (f)

Visual display of a medical image, pt Visual display of a medical image, pt. 3: several visualizations of the same medical image

THANK YOU FOR LISTENING! Final Slide! Take home message We propose an investigation of the accuracy of the ExacTrac patient repositioning system as a function of window and level settings for x-ray and digitally reconstructed radiograph images. We will use both phantoms and real patient data to determine the ExacTrac system performance as a function of window and level settings. Further questions to be considered How do the visual contrast controls used by the ExacTrac system relate to the more traditional window and level controls? How does the ExacTrac system take into account the visual contrast of the x-ray and DRR images to be fused? Can we generate simple MatLab code that demonstrates the same principles? THANK YOU FOR LISTENING!

Research Plan I. Prepare a treatment plan for a geometric phantom with embedded markers A. On the treatment table, displace (rotate and/or translate) phantom by known amount B. Manually adjust visual image contrast settings C. Use the ExacTrac system to reposition the phantom D. Evaluate the accuracy of the repositioning E. Repeat A-D for multiple displacements and visual image contrast settings II. Repeat I, using anthropomorphic phantoms with isocenters in the head, thorax and pelvis. III. Perform a retrospective study using real patient data.