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Unsharpness Calculations and Resolution

Unsharpness Calculations and Resolution. By Professor Stelmark. Source-to-object distance (SOD) refers to the distance from the x-ray source (focal spot) to the object being radiographed. SOD can be expressed mathematically as follows:.

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Unsharpness Calculations and Resolution

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  1. Unsharpness Calculations and Resolution By Professor Stelmark

  2. Source-to-object distance (SOD) refers to the distance from the x-ray source (focal spot) to the object being radiographed. SOD can be expressed mathematically as follows:

  3. Focal spot size FSS = Dimensions of the nominal focal spot in millimeters (mm)

  4. Calculating Geometric Unsharpness The amount of geometric unsharpness can be calculated for each of the following images to determine which image has more geometric unsharpness.

  5. TEMPORAL RESOLUTION SPATIAL RESOLUTION CONTRAST RESOLUTION RADIATION DOSE NOISE IMAGE ARTIFACTS

  6. SPATIAL RESOLUTION The degree of sharpness in the image The degree of blurring in the image Spatial resolution (also known as high-contrast resolution) is the ability of an imaging system to create separate images of closely spaced high-contrast (black and white) objects (as with contrast resolution, spatial resolution also applies to all imaging modalities). In other words, do the two objects appear sharp and clear, or do they blur together?

  7. The spatial resolution is specified in terms of line pairs per centimeter (lp/cm) or line pairs per millimeter (lp/mm). A line pair is a pair of equal-sized black-white bars. The number of line pairs in digital imaging is specified as spatial frequency. 11 lp/mm 7 lp/mm 17 lp/mm 11 lp/mm

  8. The most common method of measuring spatial resolution is to use a value known as spatial frequency. The unit of spatial frequency is the line pairs per millimeter (lp/mm) and is obtained with a resolution chart (Fig. 3-13). A line pair includes an opaque line and a radiolucent space. In the resolution chart, one lp/mm would have a 0.5 mm lead bar separated by 0.5 mm of radiolucent material. Two lp/mm would have 0.25 mm lead bars separated by 0.25 mm of radiolucent material and so the greater the lp/mm value, the smaller the object that can be imaged and the better the spatial resolution. The limiting spatial resolution (also known as the Nyquist frequency) is the maximum number of lp/mm that can be recorded by the imaging system. The resolving power of the unaided human eye is approximately 30 lp/mm when inspecting an image up close, and at normal reading distance (about 25 cm), it is about 5 lp/mm.

  9. SPATIAL FREQUENCY- 15 lp/mm. WHAT IS THE SMALLEST SIZE OF AN OBJECT THE IMAGING SYSTEM CAN RESOLVE 1/15 = 0.06 mm/lp 0.06 /2 = 0.03 mm object

  10. Spatial Resolution Influencing Factors in Digital Imaging • FOV • Matrix size • Type of Algorithm

  11. Contrast Resolution The ability of an imaging system to distinguish structures with similar x-ray transmission as separate entities (the term not only applies to intensifying screens, but all imaging systems including computed radiography (CR), digital radiography (DR), conventional and digital fluoroscopy, computed tomography, magnetic resonance imaging, and sonography).

  12. Contrast Resolution Influencing Factors in Digital Imaging • SNR • Dynamic Range • Pixel Depth • Type of Algorithm

  13. TEMPORAL RESOLUTION Temporal resolution is an indication of a X-ray system's ability to freeze motions of the exposed object. An oversimplified analogy is the “shutter” speed of a camera. When a photo is taken at a sports event, a higher shutter speed should be used to reduce the blurring effects caused by the moving athletes.

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