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![X-ray Intensity
The intensity of the x-ray beam of an x-ray imaging system is measured
in milligray in air (mGya) [formerly milliroentgen (mR)] and is called
the x-ray quantity.
Another term, radiation exposure, is also often used. All have the same
meaning, and all are measured in mGya (mR).
The mGya (mR) is a measure of the number of ion pairs produced in air
by a quantity of x-rays.](https://image.slidesharecdn.com/factoraffectingthequantityofxray-230810183303-45a7b4b3/85/Factor-affecting-the-quantity-of-Xray-pptx-4-320.jpg)
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Factor affecting the quantity of Xray.pptx
- 1. X-ray Emission By :Sohail Ahmad Khan NWIHS Radiology
- 2. OUTLINE X-ray Quantity X-ray Intensity Factors That Affect X-ray Quantity Milliampere Seconds (mAs) Kilovolt Peak (kVp) Distance Filtration
- 3. X-rays areemitted through a window in the glass or metal enclosure of the x-ray tube in the form of a spectrum of energies. The x-ray beam is characterized by quantity (the number of x- rays in the beam) and quality (the penetrability of the beam).
- 4. X-ray Intensity Theintensity of the x-ray beam of an x-ray imaging system is measured in milligray in air (mGya) [formerly milliroentgen (mR)] and is called the x-ray quantity. Another term, radiation exposure, is also often used. All have the same meaning, and all are measured in mGya (mR). The mGya (mR) is a measure of the number of ion pairs produced in air by a quantity of x-rays.
- 5. Ionization ofair increases as the number of x-rays in the beam increases. The relationship between the x-ray quantity as measured in mGya (mR) and the number of x-rays in the beam is not always one to one. Some small variations are related to the effective x-ray energy.
- 6. Radiation exposurerate expressed as mGya/s, mGya/ min, mGya/mAs (mR/s, mR/min, or mR/mAs) can also be used to express x-ray intensity. X-ray quantity is the number of x-rays in the useful beam. Most general-purpose radiographic tubes, when operated at approximately 70 kVp, produce x-ray intensities of approximately 50 μGya/mAs (5 mR/mAs) at a 100-cm source-to-image receptor distance (SID). Figure 8-1 is a nomogram for estimating x-ray intensity for a wide range of techniques. These curves apply only for single-phase, full-wave–rectified apparatus.
- 8. Factors That AffectX-ray Quantity Milliampere Seconds (mAs) Kilovolt Peak (kVp) Distance Filtration
- 10. Milliampere Seconds (mAs) X-ray quantity is directly proportional to the milliampere seconds (mAs). When mAs is doubled, the number of electrons striking the tube target is doubled, and therefore the number of x-rays emitted is doubled.
- 11. X-ray quantityis proportional to mAs.
- 12. Kilovolt Peak (kVp) X-ray quantity varies rapidly with changes in kilovolt peak (kVp). The change in x-ray quantity is proportional to the square of the ratio of the kVp; in other words, if kVp were doubled, the x-ray intensity would increase by a factor of 4.
- 13. Mathematically, thisis expressed as follows: X-ray quantity is proportional to the kVp2.
- 14. Theoretically, doublingthe x-ray intensity by kVp manipulation alone requires an increase of 40% in kVp. This relationship is not adopted clinically because as kVp is increased, the penetrability of the x-ray beam is increased, and relatively fewer x-rays are absorbed in the patient. More x-rays go through the patient and interact with the image receptor. Consequently, to maintain a constant exposure of the image receptor, an increase of 15% in kVp should be accompanied by a reduction of one half in mAs.
- 15. Note thatby increasing kVp and reducing mAs so that image receptor exposure remains constant, the patient dose is reduced significantly. The disadvantage of such a technique adjustment is reduced image contrast when screen-film is the image receptor. There is little change in contrast when using digital image receptors.
- 16. Distance X-ray intensityvaries inversely with the square of the distance from the x-ray tube target. This relationship is known as the inverse square law X-ray quantity is inversely proportional to the square of the distance from the source.
- 17. When SIDis increased, mAs must be increased by SID2 to maintain constant exposure to the image receptor. Compensating for a change in SID by changing mAs by the factor SID2 is known as the square law, a corollary to the inverse square law.
- 18. Filtration Adding filtrationto the useful x-ray beam reduces patient dose. X-ray imaging systems have metal filters, usually 1 to 5 mm of aluminum (Al), positioned in the useful beam. The purpose of filtration is to reduce the number of low-energy x-rays. Low-energy x-rays contribute nothing useful to the image. They only increase the patient radiation dose unnecessarily because they are absorbed in superficial tissues and do not penetrate to reach the image receptor.
- 19. When filtrationis added to the x-ray beam, patient dose is reduced because fewer low-energy x-rays are in the useful beam. An estimate of exposure reduction can be made from the nomogram in Figure 8-1, where it is shown that the reduction is not proportional to the thickness of the added filter but is related in a complex way.
- 21. The disadvantageof x-ray beam filtration can be reduced image contrast when using screen-film caused by x-ray beam hardening. X-ray beam hardening increases the number of high energy x- rays in the beam by removing the lower-energy non penetrating x-rays.
- 22.