Regarding CT scan all are true except:
Correct Answer: 50% reduction in kVp reduces 50% radiation dose
Description: 50% reduction in KVP reduces 50% radiation dose Quantity of exposure is directly propoional to (kVp)2. Thus reducing kVp by 1/2 would reduce the radiation dose to 1/4 . X rays are produced by energy conversion when a fast-moving stream of electrons is suddently decelerated in the target of an x-ray tube. The target of an x-ray tube is usually tungsten or an alloy of tungsten. The exposure dose to the patient depends on Quantity and Quality of the x ray beam. X ray Quantity, also referred to as radiation exposure, refers to the number of photons in the x ray beam. More photons increase the dose to the patient. X ray Quality refers to the energy or penetrating power of the photons in the beam. Several factors affect the quality and quantity of the x ray beam. kVp: The kVp affects both the quantity and the quality of the radiation beam. The kVp across the X ray tube influences the force of attraction experienced by an electron released by the filament as it moves towards the anode. Thus if the kVp is increased, then the kinetic energy (E) of the electron increases. As X ray production by Bremsstrahlung is propoional to E 2,quantity or Intensity will be directly propoional to kVp`. If the kVp is doubled, then the intensity increases by a factor of four. The quality or the penetrating power of the beam increases as the kVp increases. Quality of radiation depends almost entirely on the kVp. mA/mAs: mA/mAs affects the quantity of radiation. It has no effect on quality of beam. It has no effect on quality of beam. If the mA is doubled, then the quantity is doubled, and the dose is increased by a factor of two. When the current through the X-ray tube (mA) is doubled the number of electrons flowing across the tube in unit time is doubled. If all other factors remain unchanged. each electron will have the same chance of creating X ray photons and so the number of photons will be doubled. The same applies to mAs. The quantity of the X ray beam is directly propoional to the mA through the tube. Filtration: filtration affects both the quality and quantity of the beam. A filter is always inseed in the x ray beam to remove low energy photons. This removal reduces the quantity, and as a result, the mean energy to the beam increases. The beam becomes more penetrating or harder. Thicker filters reduce the quantity of the beam, but increases beam quality. A filter is intended to protect the patient by removing these low-energy photons. Target material: Target material with higher atomic numbers increase both the quantity of photons slightly and quality (energy) of the beam. Tungsten produces a significantly more efficient spectrum than molybdenum. Source to image receptor distance (SID): The SID affects the quantity of photons but has no effect on the quality. The quantity is affected by the inverse square law, which states that the intensity (quantity) is inversely propoional to the square of the distance. If the distance is increased, then the quantity decreases by 1/d2. Summary: Quantity of X-rays generated is directly propoional to atomic number (z), current (mA), length of time of exposure (s) and square of the voltage applied (kVp). Quantity of x-ray = (kVp)2 xmAxsxZ d2 Quality of radiation depends almost entirely on the voltage (kVp). Effects of Various Parameters on Quality and Quantity of X Rays Variable Quantity Quality Anode material (Z) a Z Affects position (energy) of characteristic x rays Tube potential (kVp) a kVp2 Determines presence or absence of characteristic x rays; determines maximum x-ray energy Tube current (mA) a mA None Time a time None Distance a 1/distance,- None Filtration (HVL) Decreases with increasing filtration (higher HVL) Increased percentage of high-energy x rays with increased filtration (higher HVL)
Category:
Radiology
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