In X-ray CT systems, X-rays are used to image the internal structure and features of a region of a subject or an object. The imaging is performed by an X-ray CT system which images internal structure and features of a plurality of thin planar slices or a 3D volume of a region of an object using X-rays. For medical applications, the imaging objects include human bodies. An X-ray CT system generally comprises an X-ray source that provides a cone-shaped X-ray beam and an array of closely spaced X-ray detectors that face the X-ray source.
The X-ray source and array of detectors are typically mounted in a gantry so that a patient/object being imaged with the CT system, generally lying on an appropriate support couch, can be positioned within the gantry between the X-ray source and the array of detectors. The gantry and couch are moveable relative to each other so that the X-ray source and detector array can be positioned axially at desired locations along the patient's body. The gantry comprises a stationary structure referred to as a stator and a rotary element referred to as a rotor which is mounted to the stator so that the rotor is rotatable about the axial direction. In third generation CT systems, the X-ray source and detectors are mounted on the rotor. Angular positions of the rotor about the axial direction are controllable so that the X-ray source can be positioned at desired angles, referred to as view angles, around a patient's body. To image a slice in a region of a patient's body, the X-ray source is positioned at the axial position of the slice and the X-ray source is rotated around the slice to illuminate the slice with X-rays from a plurality of different view angles. At each view angle, detectors in the array of detectors generate signals responsive to the intensity of X-rays from the source that pass through the slice. The signals are processed to determine the amounts by which X-rays from the X-ray source are attenuated over various path lengths through the slice that the X-rays traverse in passing though the slice from the X-ray source to the detectors. The amounts by which the X-rays are attenuated are used to determine an X-ray absorption coefficient of materials in the slice as a function of position in the slice. The absorption coefficient is used to generate an image of the slice and identify compositions and densities of tissues in the slice.
The X-ray detectors comprised in a detector array of CT system are generally packaged in a plurality of modules, known as detector-modules, each of which comprises a plurality of X-ray detectors. Most modern CT systems are multi-slice CT systems designed to simultaneously image a plurality of slices of a patient. The X-ray detectors in each CT detector-module of a multi-slice scanner are arranged in a rectangular matrix of rows and columns. The X-ray detector matrices of any two CT detector-modules in a CT system are substantially identical and comprise a same number of rows of detectors and a same number of columns of detectors. The modules are positioned one adjacent to and contiguous with the other in a closely packed array with their rows of detectors aligned end to end so that the X-ray detectors form a plurality of long parallel rows of X-ray detectors.
A multi-slice X-ray CT system is usually named or featured by the maximum number of slices that it can simultaneously image, for example, an 8-slice CT system means that it can simultaneously image at most 8 slices; a 16-slice CT system can simultaneously image at most 16 slices.
The X-ray detectors in each long row of detectors lie on an arc of a circle having its center located at a focal point of the CT system's X-ray source and the design of these detectors is specifically determined by the radius of the circle which is hereinafter referred to as the focal distance. The design of X-ray detectors placed on the arc of one focusing distance of one CT system cannot therefore be used on another CT system of a different focusing distance.
X-ray detectors typically include a plurality of anti-scatter grids for collimating X-ray beams received at the detector. The grids are comprised of thin septa of a high Z material to absorb off axis X-rays not intended to the scintillator pixel below the grid opening. The grid openings converge at the focal spot of the X-ray source. Below the grid there is a scintillator for converting X-rays to light energy, and attached to the back of the scintillator, photodiodes for receiving the light energy and producing electric charges there from. The anti-scatter grids are aligned and bounded with the elements of the scintillator arrays to very tight and exact locational tolerances. Each of the X-ray detector modules are accurately aligned on a backbone to focus on the focal spot of the X-ray source.
The X-rays typically are generated from the focal spot in the X-ray tube and collimated to project a conical beam of the required size onto an array of the multi-pixel detectors on the opposite side of the object being imaged. The ideal image is generated if the X-ray photons coming from the focal spot, through the object, and onto the detector travel in a straight line. Because the imaged objects can contain disperse dense materials, bone, metal, etc., that reflect or scatter the X-ray photons, the scattered photons travel at some arbitrary angle and can enter the detector array in some other location than their initial trajectory would have had them enter. The result is X-ray photons with no useful information of the imaged object creating background noise by diluting the photons that contain image information decreasing signal to noise. As the noise increases the image quality degrades.
To help absorb the unwanted scattered photons it is typical to add some type of thin guiding plate, tunnels or shielding material made of X-ray absorbent shielding material made of lead, tungsten, etc. between each row of pixels, aligned on edge to the detector face. The shielding plates are critically aligned with the taper of the X-ray beam and only allow the X-rays that are aligned with the local angle of the X-ray beam and if functioning correctly only allow the X-rays that are aligned with the focal spot to enter the detector. In some cases the signal to noise (SNR) can be reduced by adding additional absorbing plates orthogonal to the initial plates creating a 2D plate (anti-scatter grid) that encloses each pixel of the detector to further exclude off-axis photons.
To increase resolution of imaging the detector pixels need to be made smaller, resulting in higher density with less space between each pixel requiring the shielding be more closely spaced. If the anti-scatter grid is increased in height, towards the X-ray source to increase effectiveness and if it is precisely aligned it will absorb more off-axis X-ray photons increasing SNR.
As the anti-scatter grid height increases it also makes alignment of the detector with regard to the focal spot more critical. If the system and its related tolerances can't hold or predict if one detector in the array is reliably aimed at the focal spot then the result can be shadowing of the X-ray photons, and degradation of the image data.
Another factor is that as the X-ray tube heats the focal spot shifts some unpredictable amount creating more shadowing and further degrading images.
The following U.S. patents and published applications are incorporated herein by this reference: U.S. Pat. Nos. 4,872,191; 5,991,357; 6,693,291; 6,744,053; 7,177,387; 7,236,560; 8,287,187; 8,483,353; 8,314,412; 8,525,119; 2012/0049074; 2014/0050296; 2014/0219415; and 2014/0355734.