Differential Absorption Lecture PDF

Differential absorption Dr. Nahla Atallah X-RAYS INTERACT with matter in the following five ways: (1) coherent scattering, (2) Compton scattering, (3) photoelectric effect, (4) pair production, and (5) photodisintegration.  DIFFERENTIAL ABSORPTION  One of the five ways an x-r...

Differential absorption Dr. Nahla Atallah X-RAYS INTERACT with matter in the following five ways: (1) coherent scattering, (2) Compton scattering, (3) photoelectric effect, (4) pair production, and (5) photodisintegration.  DIFFERENTIAL ABSORPTION  One of the five ways an x-ray can interact with tissue, only two are important to radiology,  Compton scattering and the photoelectric effect. Similarly, only two methods of x-ray production, bremsstrahlung x-rays and characteristic x-rays—are important.  More important than interaction of the x-ray by Compton scattering or photoelectric effect, however, is the x-ray transmitted through the body without interacting.  Figure 9-10 shows schematically how each of these types of x-ray contributes to an image.  The Compton-scattered x-ray contributes no useful information to the image. These scattered x-rays result in image noise, a generalized dulling of the image by x-rays not representing anatomy. To reduce this type of noise, we use techniques and apparatus (device) to reduce the number of scattered x-rays that reach the image receptor. X-rays that undergo photoelectric interaction provide diagnostic information to the image receptor.  Because they do not reach the image receptor, these x-rays are representative of anatomical structures with high x-ray absorption characteristics; such structures are radiopaque. The photoelectric absorption of x-rays produces the light areas in a radiograph, such as those corresponding to bone. Other x-rays penetrate the body and are transmitted to the image receptor with no interaction whatsoever. They produce the dark areas of a radiograph. The anatomical structures through which these x-rays pass are radiolucent.  Basically, an x-ray image results from the difference between those x-rays absorbed photoelectrically in the patient and those transmitted to the image receptor, This difference in x-ray interaction is called differential absorption.  Approximately 1% of the x-rays incident on a patient reach the image receptor.  Thus the radiographic image results from approximately 0.5% of the x-rays emitted by the x-ray tube.  Producing a high-quality radiograph requires the proper selection of kVp, so that the effective x-ray energy results in maximum differential absorption. Un fortunately, reducing the kVp to increase differential absorption and therefore image contrast results in increased patient radiation dose. Dependence on Atomic Number Consider the image of an extremity (Figure 9-12). An image of the bone is produced because many more x-rays are absorbed photoelectrically in bone than in soft tissue.  Recall that the probability of an x-ray undergoing photoelectric effect is proportional to the third power of the atomic number of the tissue.  Note that the relative probability of interaction between bone and soft tissue (differential absorption) remains constant, but the absolute probability of each decreases with increasing energy. With higher x-ray energy, fewer interactions occur, so more x- rays are transmitted without interaction. Compton scattering is independent of the atomic number of tissue. The probability of Compton scattering for bone atoms and for soft tissue atoms is approximately equal and decreases with increasing x-ray energy. The probability of Compton scattering is inversely proportional to x-ray energy (1/E).  The probability of the photoelectric effect is inversely proportional to the third power of the x-ray energy (1/E3). Dependence on Mass Density Mass density is the quantity of matter per unit volume, specified in units of kilograms per cubic meter (kg/m3). Sometimes mass density is reported in grams per cubic centimetre (g/cm3).