For every 4 cm increase in patient tissue thickness requires a doubling of exposure (time) in order to achieve an image of equal density.
They also say 3cm more mass would increase double or 1/2 image.
Notes below in bold states, that a step in kVp corresponds to a 50% change in exposure (see attached). This guideline allows the kVp to be adjusted instead of mAs when the mAs calculated goes beyond the tube limits or when a change in image contrast is required.
The HVL refers to the thickness of a material required to produce an exit beam that is half the intensity of the primary beam. In the literature there is a discrepancy between the HVL of human tissue. In some papers, the HVL is 4 cm,33 while others state that it is 3 cm.15, 30 McDaniel's water bath study found that the HVL lies between 3.3 and 3.8 cm so it is difficult to determine which of these is correct. A HVL of 3 cm provides the basis of the 25%
Step-by-step explanation:
Milliamperage
Exposure time changes in mA affect the rate of exposure, that is, the number of photons produced per second during an exposure. For this reason, a change in mA will alter the quantity of exposure to the IR. An increase in mA will increase the quantity of exposure; decreased mA will reduce the quantity of exposure. Exposure is directly proportional to mA; that is, if the mA doubles, the quantity of exposure also doubles. Technically, when the mA is doubled, the number of electrons at the filament doubles. During the exposure, the number of photons emitted from the tube doubles as well. The opposite is true if the mA decreases by 50%. The electrons at the filament and the photons emitted will be halved. Dose to the patient is also directly proportional. For example, if the mA is doubled, the dose to the patient is doubled.
Exposure Time
Exposure time also controls the exposure to the IR. This factor affects the exposure by determining how long the exposure will last. Obviously a longer exposure time will increase the exposure to the IR, and a decrease in exposure time will reduce the IR exposure. Like the mA described earlier, the quantity of exposure is also directly proportional to the exposure time. Dose to the patient is also directly proportional. For example, if the exposure time is doubled, the dose to the patient is doubled.
The quantity of exposure and the patient dose are directly proportional to the mAs. For each of the four exposure techniques listed earlier, the volume of photons emitted and the dose to the patient will be equal. The density or blackening effect on the image will also be equal. The unit mAs is the primary controller of radiographic density.
It is important to understand that the mA, exposure time, or the equivalent, mAs, all follow the same directly proportional rule in terms of exposure and dose. If any of these three factors are doubled, both the exposure and the dose are doubled. If any of the three are cut in half, the exposure and dose are cut in half.
The pegged kilovoltage technique
This was developed as an improvement to the optimal kilovoltage technique. The pegged kilovoltage technique follows the same principle as the optimal kilovoltage technique, however there are specific kVp values that must be used attached. The mAs is adjusted based on a half value layer (HVL) of 3 cm, where a change in thickness of 3 cm requires the mAs to be doubled or halved. This allows adaptation of exposure without a change in image contrast. This approach to mAs adjustment is more precise than the optimal exposure technique. Furthermore, a step in kVp corresponds to a 50% change in exposure attached. This guideline allows the kVp to be adjusted instead of mAs when the mAs calculated goes beyond the tube limits or when a change in image contrast is required.
