Biomedical Imaging
Motion Compensated Unrestrained Awake Animal SPECT Imaging
Oak Ridge National Laboratory (ORNL), Thomas Jefferson National Accelerator Facility (JLab), University of Maryland (UM), and Johns Hopkins University (JHU) have collaborated to develop a SPECT-CT system for imaging unanesthetized, unrestrained mice. The full operational system is composed of a commercial precision x-ray CT/SPECT gantry, two high-resolution gamma cameras and an infrared (IR)-based motion tracking system. Thee Dell workstations are used to control the three sub-systems (gantry, IR tracking and gamma cameras) and record three separate data sets that are later used to correct for motion in image reconstruction. The three data sets that are time tagged via a common clock are: gantry location, mouse pose and gamma camera list mode data. A fully functional system including motion corrected SPECT reconstruction has been demonstrated. The completed system is installed and being tested on awake mice at JHU (Fig. 1).
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| Fig. 1. Photographs of the system installed at JHU. Shown on the left is the Siemens MicroCAT II with the x-ray
shield in place. The internal view on the right shows the gantry with the JLab SPECT detectors and mounted ORNL optical tracking system. Three CMOS IR cameras are seen in the enlargement. |
The awake mouse is confined within a transparent burrow at the center of rotation of the gantry. The mouse head position is tracked by three infrared reflecting hemispheres attached to its head that are imaged by three CMOS based infrared cameras. Figure 2 gives the overall functional block diagram for motion measurement.
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Fig. 2. Functional block diagram of optical tracking system. |
Tracking of the mouse’s head is achieved by imaging the three retro-reflectors glued in a triangular arrangement on the head of the mouse. The tracking apparatus measures the pose of the mouse’s head with 6 degrees of freedom at a rate of 15 frames per second with sub-millimeter accuracy. The awake mouse is confined within a transparent burrow at the center of rotation of the gantry. The optical tracking system is composed of three CMOS optical cameras and each camera is encircled with a circular array of infrared emitting LEDs such that the light output direction is aligned near coaxially to spherical retro-reflective markers. The setup provides simultaneous (stereo) blur-free images of the markers. The three camera system provides improved accuracy, range of motion, and robustness over an earlier implemented two camera system. The three CMOS camera image acquisition and processing and algorithm development is handled by a Dell workstation.
Image reconstruction is accomplished using an iterative list-mode maximum likelihood expectation maximization algorithm (OSEM) that incorporates time-stamped event data, pose information, and gantry rotation position.
The system has been tested at Johns Hopkins University, first with moving phantoms and then with several awake mice. See Figure 3 for an example result of the tracking of an awake mouse injected with Tc99m-MDP. In this example a white female BALB-C 32.1g mouse was injected through a tail vain with 3.3 mC of Tc99m- methylenediphosphonate (MDP). Tc99m-MDP is used for bone scintigraphy where it is typically taken up by healthy bone. The resulting SPECT images are, therefore, similar to a low resolution x-ray CT of the skeleton. After about three hours after injection the mouse was anesthetized for ~10 minutes with the inhalant isoflurane to allow placement of IR reflectors with instant adhesive. The mouse was imaged ~3hr post injection using a 1 mm diameter pinhole collimator with 1.5x magnification. List-mode data were acquired at 120 gamma stops over 360 degrees, at 20 sec /position. The list mode projection data with mouse head tracking was reconstructed using OSEM with tracking data incorporated. |
Fig. 3. Three-dimensional views from a SPECT reconstruction of a Tc99m-MDP injected mouse without motion correction (lower right); with motion correction (lower left); static (upper left); and a static CT reconstruction of the same mouse (upper right). |
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Fact Sheet available here in PDF format
Point
of Contact:
James S. Goddard, Jr., Ph.D.
Image Science and Machine Vision
Measurement Science and Systems Division
Oak Ridge National Laboratory
P.O. Box 2008, MS-6075
Oak Ridge, Tennessee 37831-6075
Office: (865) 574-9034
E-mail: goddardjsjr@ornl.gov
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