These are the dominant type of scanners on the market because they have been manufactured longer and offer a lower cost of production and purchase. Spinning tube, commonly called spiral CT, or helical CT, is an imaging technique in which an entire X-ray tube is spun around the central axis of the area being scanned. The axial slice (right) is the image that corresponds to number 33 (left). Spiral CT Drawing of CT fan beam and patient in a CT imaging system CT scan of the thorax. This results in an increased time of scanning. The table has to make a stop in taking slices. Then the table increments again and another slice is taken. The table increments to a particular location and then stops which is followed by the X-ray tube rotation and acquisition of a slice. Sequential CT also known as step-and-shoot CT is a type of scanning method in which the CT table moves stepwise. On the basis of image acquisition and procedures various type of scanners are available in the market. The 1979 Nobel Prize in Physiology or Medicine was awarded jointly to South African-American physicist Allan MacLeod Cormack and British electrical engineer Godfrey Hounsfield "for the development of computer-assisted tomography". While CT is most prominently used in medical diagnosis, it can also be used to form images of non-living objects. Since its development in the 1970s, CT scanning has proven to be a versatile imaging technique. CT scan can be used in patients with metallic implants or pacemakers, for whom magnetic resonance imaging (MRI) is contraindicated. The multiple X-ray measurements taken from different angles are then processed on a computer using tomographic reconstruction algorithms to produce tomographic (cross-sectional) images (virtual "slices") of a body. ĬT scanners use a rotating X-ray tube and a row of detectors placed in a gantry to measure X-ray attenuations by different tissues inside the body. The personnel that perform CT scans are called radiographers or radiology technologists. Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.A computed tomography scan (usually abbreviated to CT scan formerly called computed axial tomography scan or CAT scan) is a medical imaging technique used to obtain detailed internal images of the body.
Hail is a good reflector of energy and will return very high dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rainrates which are associated to the dBZ values. The higher the dBZ, the stronger the rainrate.
Typically, light rain is occurring when the dBZ value reaches 20. The scale of dBZ values is also related to the intensity of rainfall. The value of the dBZ depends upon the mode the radar is in at the time the image was created. Notice the color on each scale remains the same in both operational modes, only the values change. The other scale (near left) represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75). One scale (far left) represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). Each reflectivity image you see includes one of two color scales. The dBZ values increase as the strength of the signal returned to the radar increases.
So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used. Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). "Reflectivity" is the amount of transmitted power returned to the radar receiver. The colors are the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan.
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