Any critique of radio graphic image quality must include an evaluation of quantum mottle, a fundamental limitation of the imaging process. Using an illustrative analogy, this article explains the concept of quantum mottle and defines the quantum sink in an imaging chain. The article also describes the interdependence between quantum mottle and other system parameters, including spatial resolution, contrast resolution and total system speed.
Radiographic image quality characterizes the ability of an imaging system to accurately depict structures in a radiographed object. Six components affect image quality -- spatial resolution (or sharpness), contrast, density, radiographic mottle (or noise), distortion and artifacts. Of these six parameters, radiographic mottle is the most difficult to understand and relate to the imaging process. Radiographic mottle creates a grainy, blotchy, textured or snowy appearance in a radiographic image.
If an image were obtained of a phantom of uniform composition and thickness, the resulting film would have an irregular appearance. Optical density is not consistent throughout the image of the phantom, but rather small differences are present among nearby regions. One reason for this variation is the manifestation of radiographic mottle. Consequently, the imaging process does not render a completely faithful reproduction of the object. High levels of noise produce more variation in film optical density and inhibit the depiction of low-contrast structures.
Film graininess, nonuniformity of screen phosphor and quantum mottle contribute to overall radiographic mottle. Of these three factors, quantum mottle is the dominant component and is the most important consideration regarding noise in the image. Quantum mottle is defined as the statistical fluctuation in the number of photons per unit area that contribute to image formation.
Subject contrast -- in the form of varying x-ray intensity exiting the patient -- must be detected and then converted to an observable form by the imaging system. X-ray photons absorbed in the intensifying screen become the initial information carriers for image formation. However, the detected photos exhibit statistical variations because x-ray emission from the target and interactions with matter, including the patient and the screen, are random events. Quantum mottle becomes more pronounced when the number of x-ray photos absorbed in the screen is decreased, such as by reducing the radiation dose.
The following analogy illustrates the concept of quantum mottle. Imagine that two flat, metal objects are placed on the sand at the beach. They could be of any shape or size, but suppose they are a simple circle and triangle. These two items are confined to a specific region with delineated boundaries -- a sandbox. A passing radiography student is blindfolded and then asked to place his or her finger at various points within the sandbox. The finger acts as a detector, sensing the nature of the items it touches. In this example, the metal objects feel hard to the student's touch and the sand feels soft. Metal or sand could be present at any location within the sandbox, and the exact locations and shapes of the metal objects are unknown to the student until he or she can sample all regions throughout the sandbox.
Suppose a second radiography student is asked to record, on a sheet of paper, the location and attributes (hard or soft) of every point that the first student touches in the sandbox. After the first student has sampled a few points, the blindfold is removed and he or she is asked to describe the shapes of the objects by studying the recorded data alone.
The student fails to recognize the shapes of the metal objects because the number of points he or she has sampled is insufficient to visualize their pattern. The total area probed by the student is very small compared with the total area of the sandbox.
As more points are sampled, the metal design begins to become discernible; additional sampling clarifies the structure even further.
Radiographic imaging is similar in many ways to the discrete sampling process employed by the two radiography students exploring the sandbox. In radiography, x-rays detected at the screen denote relative transmission through objects along the photons' paths. X-rays readily pass through radiolucent objects and are absorbed in the screen (represented in the analogy by the sand). Alternatively, x-rays completely removed by the object are not incident on the screen (represented in the analogy by the metal). The final image on film is a composite of these sampling events. Increased photon absorption by the screen (a higher number of information carriers in the imaging chain) reduces quantum mottle and improves visualization of the structure.