Retrospective Cardiac Gating with A Prototype Small-Animal X-ray Computed Tomograph

We developed imaging technologies that enable researchers to answer biological questions. Now, there's a need for gating techniques to freeze motion in small-animal CT.We wanted to show that it's possible to do gated scans with standard CT components. It's all accomplished in software, no need for special hardware.

Understanding the limitations and capabilities of commonly available pre-clinical detectors for X-ray CT imaging leads to enabling technology with meaningful experimental results. Physiological cardiac motion in rodent studies is often considered an insurmountable challenge which had led to a lack of scientific reports. We hope that with the work described here, we'll convince the field that it is possible to overcome this difficulty with commonly available hardware.

Our approach does not require physical connection of electrodes and can be achieved without surgery intervention. X-ray CT with retrospective cardiac gating allows accurate quantification of small structures in and around the heart and rolling measurements of the heart chambers at different time points during a cardiac cycle. This technology substantially enhances our ability to study cardiovascular diseases and develop effective therapies.

To begin, place an anesthetized mouse on the workspace. Pinch the webbing between the toes to test the depth of anesthesia. Intravenously inject 100 microliters of CT contrast agent through the tail vein just before imaging.

Transfer the anesthetized mouse inside the CT imaging chamber. Launch the CrumpCAT software. To perform non-gated CT imaging with high resolution, first enter a study ID on the user interface.

Select Mouse Hi-Res in the Protocol drop-down menu. Then, click the Scan button on the user interface to acquire 720 projections with an exposure time of 80 milliseconds per projection. To perform non-gated CT imaging with medium resolution, enter a study ID on the user interface.

Select Mouse Standard in the Protocol drop-down menu. Click the Scan button on the user interface to acquire 720 projections with an exposure time of 100 milliseconds per projection. To perform gated CT imaging, enter a study ID on the user interface.

Select Cardiac Gating in the Protocol drop-down menu. Click the Scan button on the user interface and acquire 21, 600 projections with an exposure time of 20 milliseconds per projection. For respiratory signal extraction, designate a rectangular region of interest, or ROI1, to represent the diaphragm.

Sum up the pixel intensities inside ROI1 for each projection to generate the respiratory signal. For cardiac signal extraction, designate a second rectangular region of interest, or ROI2, in the template near the heart. Sum up the pixel intensities inside ROI2 for each projection to generate the cardiac signal.

Next, launch the CT image in a DICOM viewer, such as AMIDE. Set the CT value as minus 500, 500 to enhance the visible contrast. For quantification of the left ventricle volume, draw a 3D freehand region of interest to identify the left ventricle in each phase.

Calculate voxels with a threshold CT value of 730 Hounsfield units. Non-gated CT imaging revealed clearer identification of cardiac calcifications on high-resolution images compared to medium-resolution images, with contrast-to-noise ratio values of approximately four and 3.2, respectively. Gated CT imaging demonstrated a contrast-to-noise ratio improvement from approximately 4.2 in respiratory-gated only to about 5.2 in respiratory-and cardiac-gated.

The left ventricle thresholded at a CT value of 730 Hounsfield units. A loop as a short video better reveals the change in the left ventricle volume during a cardiac cycle.