No single modality is all-encompassing for musculoskeletal diagnosis. Rather, each modality is like a tool in a toolbox, used to perform specific functions and solve specific diagnostic problems. For instance, while radiographs (“X-ray films”) are useful as screening tools for appendicular (extremity) fractures, magnetic resonance imaging (MRI) is a more useful tool for diagnosing meniscal tears in the knee. Used in varying combinations, the different modalities can diagnose and characterize a wide range of musculoskeletal pathology. Herein, we describe the various common modalities in clinical application and some examples of their usages.
Radiographs are the predominant modality of musculoskeletal imaging (at the very least in terms of numbers of studies). In their current form, X-ray machines and scanners use electronic devices to produce and detect X-rays. The device used to detect the X-rays may in some sense be said to be similar to the detector in your digital camera, except that these detector plates are designed to detect photons from the X-ray region of the spectrum rather than photons of optical (light) wavelengths. Once formed at the detector plate, X-ray images are stored electronically on computers in a manner similar to how images are stored on your digital camera (albeit with specialized formatting). These X-ray images are then viewed with image storage, display, and editing software libraries called picture archiving and communication systems (PACS).
There is, of course, a more fundamental difference between image formation in digital photography and digital (or computed) radiography. In digital photography, optical photons emanate from the flash element of the camera, are reflected from the object being photographed, and are picked up by the detector in your camera, creating an image of the subject's “surface.” Remember that X-rays have a shorter wavelength and higher energy than visible light, and more easily pass through tissue. X-rays thus pass through the patient to the detector plate, being only partially stopped (generally, scattered or absorbed) in the process. The resultant image is a cumulative superposition of multiple overlapping structures the X-ray photons encountered along their pathway through the patient. How does this occur? The internal anatomic structures of the patient are of varying densities, with structures of higher density (such as bones) preferentially attenuating the beam, and organs of lower density (such as the lung) allowing more photons to pass through. A transmission, or “shadow” image of the internal structure of the patient is so created by the X-ray photons passing through the patient.
Five fundamental tissue densities are defined in the human body, forming a method of scaling the brightness of the resulting images and so identifying anatomic structures. At the lowest end of the density spectrum is air, which appears on X-ray images as black or extreme dark gray regions. Next is fat tissue, also low density, showing itself as dark gray. Fluid is higher in density, and not usually seen in isolation, being ...