Radiology Studies - X-ray

Background

Conventional radiography is the most frequently used modality for evaluating bone and joint disorders, particularly those involving trauma. In general, the orthopedic radiologist should obtain at least two views of the bone(s) involved, at 90° angles to each other and each view showing two adjacent joints.¹ 

Historical Overview 

The first radiograph was produced in 1895, when Wilhelm Roentgen took an x-ray image of his wife’s hand. To date, conventional radiography plays a critical role in diagnostic imaging. Until recently, all x-rays were produced using a film cassette on which to project the image. The film is then processed and printed. Computed radiography (CR) is a filmless system that substitutes a phosphor imaging plate for the film cassette. The resulting digital images are transferred to a computerized picture archiving and communication system (PACS), which uses computer networks to store and transmit the images to physicians for immediate access and in diverse locations. Digital radiography (DR) substitutes a fixed electronic detector, or charge-coupled device for the film cassette and phosphor plate.² 

Description 

X-rays are a form of radiant energy with a wavelength that is significantly shorter than that of visible light. As the wavelength decreases, energy increases giving X-rays the ability to not only penetrate additional densities compared to visible light but also remove electrons from the orbiting shells of atoms in the object being imaged and thus producing indirect ionization radiation.³ 

Broadly, the components required to form an image include an X-ray generator, an object, and an X-ray detector.4The X-ray generator is responsible for creating the X-ray beam. The beam is passed through the object to be imaged while simultaneously being directed at the detector. 

The generator primarily consists of a filament and an angled target. This structure is encased in a vacuum. The filament if charged with electric current when the machine is turned on forming into the cathode, and thus converting the target into the anode. Electrons which are negatively charged are ejected from the cathode and directed towards the anode target. The target is usually composed of tungsten as it posses’ properties suitable in imaging the human body. These charged electrons hit the angled target, ejecting electron from the target. The angle helps in guiding these newly ejected electrons towards the object under investigation and towards the X-ray detector outside of the X-ray generator. 

As the x-rays pass through the object (human body), they are attenuated via absorption and scatter by different bodily tissues. There are five radiographic densities ranked by the ability to attenuate the x-ray beam: air, fat, soft tissue, bone, and metal. Substances that have little attenuating effects (e.g., air, fat) appear darker than those that have large attenuating effects (eg, bone). Some x-rays are complete blocked when they encounter very dense objects, thus appearing completely white on the image. Thick structures attenuate more radiation compared to thinner structures of the same composition.² 

The type of ionization radiation produced by X-rays is indirect ionizing radiation, meaning it can interact with water particles creating free radicals, which in turn can result in free radical cellular injury. Majority of this cellular damage is repairable. The non-reparable damage can result in DNA damage, introducing mutations which can result in cancer formation. Fortunately, the radiation although not insignificant, is low, thus majority of the cellular damage is repaired. However, multiple X-rays in a short time span, or multiple x-rays in an individual’s life may pose a higher risk. However, the radiation form a single chest X-ray is less than a transatlantic flight. 

Most radiographic views are named based on the direction the beam passes through the patient. The most common views are anteroposterior (AP; front to back), posteroanterior (PA; back to front), lateral (from the side) and oblique (at an angle). Radiographs are also named according to the position of the patient, including erect, supine and prone.^2,3&4 

The table below provides a summary of the conditions for which conventional radiography may help in diagnosis and includes a list of the routine and special views typically obtained.

Table. Conventional radiography: summary of diagnoses by recommended routine and special views

AP, anteroposterior; CMC, carpometacarpal; SLAC, scapholunate advanced collapse.

*Note: according to OrthoBullets, radiography is not needed for these diagnoses.

• X-rays are a form of ionization radiation and can alter the cellular division and other intercellular processes. Radiation effects can range from skin damage to the development of cancer.
• Some organs are more susceptible, specific the organ systems undergo frequent cellular division i.e. reproductive organs, stomach, fetus etc.⁵
• It is important to always important to consider these radiation effects which become a stronger consideration when a younger patient requires frequent imaging.

1. Greenspan A, Beltran J. Orthopedic Imaging: A Practical Approach. Sixth ed. Philadelphia: Wolters Kluwer; 2015.
2. Brant WE. Diagnostic Imaging Methods. In: Brant WE, Helms CA, eds. Fundamentals of Diagnostic Radiology. Fourth ed. Philadelphia: Lippincott Williams & Wilkins; 2012:15-25.
3. Riesz PB. The life of Wilhelm Conrad Roentgen. AJR Am J Roentgenol. 1995;165 (6): 1533-7. doi:10.2214/ajr.165.6.7484601
4. Jay R. Parikh, Richard A. Geise, Edward I. Bluth, Claire E. Bender, Gordon Sze, A. Kyle Jones, and for the Human Resources Commission of the American College of Radiology. American Journal of Roentgenology 2017 208:3, 595-602