Chapter 7 Radiographic Image Formation and Exposure Factors

Radiographic Image Formation l In this section we will see how the image is formed on the x-ray and what are the effects of the various factors that we control before making the exposure.

Radiographic Image Formation l X-rays can pass through solid objects - the degree of penetration depending upon the wavelength of the rays and the composition of the object. Some rays are completely absorbed; the others are absorbed in varying degrees. l The degree of absorption depends upon the type of material, the thickness, and the atomic number of the material that the x-rays must pass through.

Radiographic Image Formation l Since the body is made up of many materials of many various thicknesses, x- ray is absorbed in varying degrees and therefore reaches the film in varying degrees. This is how the x-ray image (or shadow) is formed. X-ray is a shadow picture of differential absorption. l This process is described in more detail in the chapter on “darkroom procedures.”

Exposure Factors l There are several factors involved in radiography, all of which combined determine the quality of the radiograph produced.

Exposure Factors They are: l Kilovoltage l Milliamperage l Exposure Time l Milliampere-Second l Film-Anode Distance l Part-Film Distance l Size of Focal Spot

Kilovoltage (Quality) l Kilovoltage is the electromotive force or electrical pressure that pushes the electrons from the cathode to the anode during exposure, therefore controlling the speed that the electrons travel, the force of the impact at the anode target and the quality of penetrating power of the x-rays produced.

Kilovoltage l The kilovoltage determines the wavelength of the x-rays produced. High kV. produces short wavelength x-rays (hard rays), the type with great penetrability; lower kV results in longer wavelengths (softer rays) and less penetrability.

Kilovoltage l We can then see that selecting the kV predetermines the quality of the x-rays produced. (the penetrating power of the beam). l The kV selected for the exposure depends upon the thickness of the part to be x-rayed. A general rule that has sometimes been used to determine the kV is twice the thickness of the part, in cm, plus 25.

Milliamperage (Quantity) l Milliamperage determines the exact number of electrons which will be forced from the cathode to the anode in a specific amount of time and under a specific kilovoltage. l This is explained by the fact that the MA applied to the cathode filament determines its temperature (its degree of incandescence) and therefore the number of electrons that will be boiled off.

Milliamperage l It is the MA together with the time that determines the total quantity of x-rays produced; for each electron that strikes the anode target, there will be an equivalent amount of x-ray energy produced.

Exposure Time (S) (Quantity) l Exposure time is the length of time (in seconds) that the anode target will be bombarded with electrons and therefore during which x-rays are produced. The exposure time and the amount of x-ray produced per second (as determined by the MA setting) determine the total amount of radiation produced.

Milliampere-Second (MAS) l Milliampere-second refers to the product of the milliampere and the exposure time. It indicates the total quantity of x-ray produced. The same results will be had with high MA and low time or low MA and high time. Low MA at longer time will risk patient movement and result in a poor quality film, therefore, high MA and low exposure time is preferred.

Milliampere-Second (MAS) l The MAS represent a combination of the MA and the exposure time in seconds. For example, the MAS used for a lumbosacral (A-P) is 10 and any of the following combinations or any other combination making 10 MAS could be used: 10 MA for one second, 30 MA for 1/3 second, 50 MA for 1/5 second, 100 MA for 1/10 second.

Film-Anode Distance (FAD) l AKA Tube-Film Distance (TFD) AKA Focal film Distance (FFD) l refers to the distance from the anode target to the film. It governs radiographic distortion. The shorter the FAD, the greater the distortion; the longer the FAD, the more nearly parallel the rays and the less distortion or magnification.

Film-Anode Distance (FAD) l The previous statement would seem to indicate that we should always use an FAD as long as the size of the room permits, but this is not so. An increase in FAD reduces the overall illumination so that the MAS must be increased.

Film-Anode Distance (FAD) l This increases the possibility of the patient moving if you only increase the time. l With modern day equipment you can increase the MA and therefore less chance of patient movement and better diagnostic yield. l It decreases the overall illumination due to the inverse square law (so your technique factors must be adjusted accordingly).

Part-Film Distance (PFD) l AKA Object-Film Distance (OFD) l The part-film distance should be as close to zero as possible. An increase in the PFD/OFD will cause distortion.

Size of the Focal Spot l The size of the focal spot is determined by the size of the filament (the surface area of the wire filament capable of emitting electrons when heated to incandescence). This is the size of the electron stream. l The larger the filament, the larger the focal spot. The larger the focal spot, the poorer the film detail.

Range of Exposure Factors