CT Image Acquisition

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Basic principle:

The mathematical principles of CT were first developed by Radon in 1917. Radon's treatise proved that an image of an unknown object could be produced if one had an infinite number of projections through the object.

Computed tomography (CT) is the science that creates two-dimensional cross sectional images from three-dimensional body structures. The primary goal of any CT system is to accurately reproduce the internal structures of the body as two-dimensional cross-sectional images.

kVp selection:

The selection of kVp sets the energy of the X-rays reaching the patient and should take the scanned object into consideration. Each object will vary in density and atomic number, both of which will impact the X-ray beam's attenuation. Larger, denser objects will require higher or more intense kVp selections to ensure a sufficient number of photons exit the object and are subsequently captured by the detector.

Each substance/tissue has its own attenuation and absorption coefficient for a specific kVp setting. For example, at lower kVp's there is greater opacification of calcifications and iodinated contrast media. Lower kVp settings inherently have more photoelectric affect and lower Compton scatter. Various clinical scenarios require specific X-ray spectra and hence lower kVp settings for optimal image quality. However, these lower settings will decrease the overall signal and will result in an increase in noise. To maintain signal to noise ratios while lowering the kVp there must be an increase in the mAs.

Slice Thickness: Single Detector Array Scanners:

The slice thickness in single detector array CT systems is determined by the physical collimation of the incident x-ray beam with two lead jaws. As the gap between the two lead jaws widens, the slice thickness increases. The width of the detectors in the single detector array places an upper limit on slice thickness. Opening the collimation beyond this point would do nothing to increase slice thickness, but would increase both the dose to the patient and the amount of scattered radiation.

Collimator pitch:

Pitch is a parameter that comes to play when helical scan protocols are used. In a helical CT scanner with one detector array, the pitch is determined by the collimator (collimator pitch) and is defined as:

Collimator pitch = Table movement per 360 degree rotation of the gantry / collimator width at Isocenter

Detector pitch:

The detector pitch is defined as,

Detector pitch = Table movement (mm) per 360 degree rotation of the gantry / detector width at Isocenter (mm)

Algorithm:

Simple backprojection is a mathematical process, based on trigonometry, which is designed to emulate the acquisition process in reverse. Each ray in each view represents an individual measurement of 11. In addition to the value of 11 for each ray, the reconstruction algorithm also "knows" the acquisition angle and position in the detector array corresponding to each ray. Simple back projection starts with an empty image matrix (an image with all pixels set to zero), and the 11 value from each ray in all views is smeared or back projected onto the image matrix. In other words, the value of 11 is added to each pixel in a line through the image corresponding to the ray's path. Simple backprojection comes very close to reconstructing the CT image as desired. However, a characteristic IIr blurring is a by-product of simple back projection. Imagine that a thin wire is imaged by a CT scanner perpendicular to the image plane; this should ideally result in a small point on the image. Rays not running through the wire will contribute little to the image (11 = 0). The back projected rays, which do run through the wire, will converge at the position of the wire in the image plane, but these projections run from one edge of the reconstruction circle to the other. These projections (i.e., lines) will therefore "radiate" geometrically in all directions away from a point input. If the image gray scale is measured as a function of distance away from the center of the wire, it will gradually diminish with a 1/r dependency, where r is the distance away from the point. This phenomenon results in a blurred image of the actual object. When simple back projection is used. A filtering step is therefore added to correct this blurring, in a process known as filtered back projection.

Questions:

1. Lower the KVp will increase the