INTRODUCTION

The production of a quality radiograph depends on many factors. Chapters 1 through 7 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 provide a detailed explanation of these factors, yet there is one element that remains to be discussed. This crucial factor involves the evaluation of a finished radiograph.

The ability of the technologist to evaluate a radiograph properly is imperative. Without this ability, the attempt to attain quality is futile. The need for a second radiograph at one time or another is unavoidable, no matter what the skill level of the radiographer. Assessing what is wrong with the radiograph and making the proper corrections are the skills that we seek. Radiographic quality depends on the technologist’s understanding of the concepts and variables that produce a good radiograph.

Radiography can be an extremely difficult subject to grasp. The concepts of x-rays and how they are formed is complex. Mastering the physics of radiography is a challenge for all students. Quality radiographs are not attained by “luck” but by a conscious understanding of the variables. This understanding can change a radiographic image into a piece of artwork.

PHYSICS OF RADIOGRAPHY: A REVIEW

X-rays are generated in an x-ray tube, which consists of a cathode side (with a negative electrical charge) and an anode side (with a positive electrical charge). In the tube a stream of fast-moving electrons is attracted and directed from the cathode to the anode. As the electrons collide and interact with the atoms of the target on the anode, a great amount of energy is produced; 1% of this energy is in the form of x-rays.

The cathode consists of a wire filament that emits electrons when it is heated. The temperature of the filament is controlled by the milliamperage (mA) setting on the console of the machine. As the mA is increased, the temperature of the filament increases and the filament produces more electrons. The period during which the electrons (x-rays) are permitted to leave the x-ray tube is in fractions of seconds. The number of electrons and the period set for their release determine how many x-rays are available. Therefore the milliamperage-seconds (mAs) controls the total number of x-rays produced.

The anode, which attracts negatively charged electrons, is made of a metal (tungsten) that can withstand high temperatures. This tolerance is necessary because of the great amount of heat produced during the collision of electrons. Ninety-nine percent of the energy produced during the impact of electrons is in the form of heat; only 1% is x-rays. The anode is constructed at an angle so that the electrons are directed downward (toward the cassette) through a window in the metal housing of the x-ray tube.

The electron speed necessary to create a high-energy impact is achieved by applying thousands of volts (kilovolts) across the anode and cathode. The available electrons travel at a tremendous speed toward the positive charge of the anode. High voltage produces x-rays with greater penetrating power and intensity. Therefore the kilovoltage peak (kVp) controls the penetrating power of the x-rays.

DENSITY AND CONTRAST: A REVIEW

Chapter 5 gives a detailed explanation of density and contrast. To apply this knowledge in a practical manner, a review of the salient points is necessary.

Radiographic density is defined as the degree of blackness on the radiograph. Density is primarily affected by mAs. The higher the mAs, the greater the density and the more blackness on the radiograph. The mAs controls the total number of x-rays available. If x-rays make film black, more x-rays emitted by the machine will cause more blackness on the film. The kVp may also influence density and increase blackness on the radiograph; mAs and kVp can be differentiated because the latter also changes the contrast.

Radiographic contrast is defined as the density difference between two areas of a finished radiograph. If the difference between two areas is great, the contrast is described as high. If there is a slight difference in density (an overall gray appearance), the contrast is low.

Radiographic contrast is affected primarily by the kVp. The higher the kVp, the lower the contrast. The kVp governs the penetrating power of the x-ray beam. If a high kVp setting is used, more x-rays reach the film because of the increased penetration (pushing power). The kVp also governs the energy spectrum of the x-ray beam. High-kVp techniques have not only higher peak-energy photons in the beam, which enhance patient penetration, but also have a wider variation of energies among all the photons in the beam, allowing for more variation in the degree of penetration among the photons. This broad photon energy spectrum contributes to the greater gray spectrum (long scale or low contrast), even with high-versus low-kVp techniques. Scatter radiation, which is more prevalent with high-kVp techniques, can influence image contrast as well, but the use of grids and the use of fast screens (i.e., rare-earth screens) minimize this effect.

VIEWING A RADIOGRAPH

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