Combined SPECT and CT Imaging to Visualize Cardiac Functionality

1. In Vivo Imaging Set-up

1. Open up the imaging software.
2. To set-up the CT portion of the scan, allow the X-ray tube to warm up by selecting the option on the software. The system will begin heating up the tube.
3. Anesthetize the mouse. To ensure that the mouse is unconscious, extend one leg and pinch the animal's paw. If the mouse does not produce a withdrawal reflex, the animal is sufficiently anesthetized.
4. Inject the mouse intravenously with the radionuclide. A commonly used SPECT radionuclide is Technetium99m (^99mTc), in part due to its half-life. However, there are also many other available radionuclides that can be used including Iodine-123 (¹²³I) and Indium-111 (¹¹¹In).
5. Wait. The radionuclide will take time to distribute through the bloodstream and begin decaying. The amount of time required is dependent on the radionuclide used and the imaging application. For cardiac applications, scans can begin almost immediately, whereas for tumors, the wait time might be several hours to days. Dependent on the timing of the imaging, the mouse can either remain anesthetized for the entirety of the procedure or can wake up and then be re-anesthetized when ready for imaging.
6. Place the mouse in the mouse bed located on the SPECT/CT stage. The bed should be equipped with a tube for anesthetic gas, a heater, and a means of monitoring ECG and respiration. During imaging, the operator will be unable to observe the mouse directly, so having alternate means to monitor physiological parameters during imaging (i.e., heart rate and respiration) is necessary.

2. SPECT/CT Imaging

1. Move the bed (containing the mouse) inside the collimator.
2. Acquire a single axial image of the mouse. Using this pilot image, set a region of interest for a secondary scan.
3. Define the settings for SPECT including the number of images collected, the time per image, the scan mode (detector rotation path), and the step mode for improved image accuracy or increased imaging speed.
4. To set-up the CT portion of the scan, allow the X-ray tube to warm up by selecting the option on the software. The system will begin heating up the tube.
5. Define the setting for CT, such as the tube current and voltage, the angle of rotation, the speed of the scan, and the number of images taken at each rotation angle.
6. Begin data acquisition. The length of time required to complete the scan will be dependent on the scan parameters selected, but is typically 30-60 min in length.
7. Remove the bed from the collimator.
8. Remove the mouse from the bed and continue to monitor the mouse until it is conscious and able to move around normally.

3. SPECT/CT Reconstruction

1. Reconstruction is typically performed using built-in software. The CT data and the SPECT data can be separately reconstructed and then combined using internal registration.