1.  Field of the Invention
This invention relates generally to imaging devices, and more specifically, to systems and methods for determining geometric parameters of imaging devices. 
2.  Background of the Invention
Computed tomography (CT) is an imaging technique that has been widely used in the medical field.  In a procedure for computed tomography, an x-ray source and a detector apparatus are positioned on opposite sides of a portion of a patient under examination.  The x-ray source generates and directs an x-ray beam towards the patient, while the detector apparatus measures the x-ray absorption at a plurality of transmission paths defined by the x-ray beam during the process.  The detector apparatus produces a voltage proportional to the intensity of incident x-rays, and the voltage is read and digitized for subsequent processing in a computer.  By taking hundreds of images from multiple angles around the patient, relatively massive amounts of data are thus accumulated.  The accumulated data are then analyzed and processed for reconstruction of a matrix (visual or otherwise), which constitutes a depiction of a density function of the bodily section being examined.  By considering one or more of such sections, a skilled diagnostician can often diagnose various bodily ailments such as tumors, blood clots, etc. 
Reconstruction of CT images requires an accurate determination of certain geometric parameters of the CT imaging machine, such as the source-to-imager distance (SID), the source-to-axis distance (SAD), the axis of rotation, and/or the piercing point (which is the point of projection of the rotation center onto the imager plane).  These parameters are also useful in procedures for localization of anatomical landmarks. 
Currently, CT imaging machines are constructed to be as rigid as possible, sometimes with active control mechanisms, with the goal of trying to make at least some of the geometric parameters stay constant during operations of the CT imaging machines.  However, despite making the CT imaging machine as rigid as possible, it has been found that certain geometric parameters may still vary as a function of a rotation of the gantry of the CT imaging machine.  For example, due to slippage between machine components and/or strains undergone by the components, the radiation source and the imager may move relative to each other.  The amount and/or direction of movement between components may vary as the gantry of the CT imaging machine is rotated.  Also, due to slippage and strains of machine components, the axis of rotation may be different from what is expected, and/or the piercing point of each image frame may vary as the gantry rotates.  Although some geometric parameters can be determined manually by physically measuring them at the CT imaging machine, it would be very difficult and inconvenient to manually obtain geometric parameters for a plurality of prescribed gantry rotational angles. 
For the foregoing reason, it would be desirable to have systems and methods for automatically determining geometric parameters of imaging devices.