Fluoroscopy

Principle

Fluoroscopy is an imaging technique commonly used to obtain real-time images of the internal structures. In its simplest form, a fluoroscope consists of an X-ray source and fluorescent screen between which the object, the patient, is placed. Modern fluoroscopes couple the screen to a CCD video camera allowing still images or images to be played on a monitor or an X-ray image intensifier for digital imaging. (A CCD camera is a charge-coupled device is an image sensor, consisting of an integrated circuit containing a matrix of coupled, light-sensitive capacitors).

Fig. 1 A modern fluoroscope

The x rays are attenuated in dependence on the type of structure of the body. They cast a shadow of the structures on the fluorescent screen. Images on the screen are produced as the unattenuated x rays are absorbed by atoms, which process gives rise to the emission of free electrons with a high kinetic energy (the photoelectric effect). While much of the energy given to the electrons is dissipated as heat, a fraction of it is given off as visible light by exiting atoms in the fluorescent molecules. Then, by ‘’de-excitation’’ light is emitted (see Fluorescence), the fluorescent process and this produce the image.

Application

Common fields of application are the gastrointestinal tract (including administration of barium, and enteroclysis), orthopedic surgery (operation guidance), Angiography of the leg, heart and cerebral vessels, Urological surgery (e.g. retrograde pyelography), implantation of cardiac rhythm devices (pacemakers, implantable cardioverter defibrillators and cardiac resynchronization devices).

Risks

The risk on radiation damage by ionizing should be balanced with the benefits of the procedure to the patient. Although the length of a typical procedure often results in a relatively high absorbed dose, digitization of the images captured and flat-panel detector systems has reduced the radiation dose.

Radiation doses to the patient depends especially on length of the procedure, with typical skin dose rates quoted as 20-50 mGy/min (Gy is Gray, the applied dose. 1 Gy is 1 J/kg tissue). Because of the long length of some procedures, in addition to standard cancer-inducing stochastic radiation effects, deterministic radiation effects have also been observed ranging from mild erythema, equivalent of a sun burn, to more serious burns. While deterministic radiation effects are a possibility, they are not typical of standard fluoroscopic procedures. Most procedures sufficiently long in length to produce radiation burns are part of necessary life-saving operations.

More Info

X-ray Image Intensifiers

At present, the original X-ray image intensifiers are replaced by CCD cameras or modern image intensifiers, which no longer use a separate fluorescent screen. Instead, a cesium iodide phosphor is deposited directly on the photocathode of the intensifier tube. The output image is approximately 10⁵ times brighter than the input image. This brightness gain is comprised of a flux gain (amplification of photon number) and minification gain (concentration of photons from a large input screen onto a small output screen). Each of them approximates a gain of a factor of 100. This gain is such that quantum noise, due to the limited number of X-ray photons, is now a significant factor limiting image quality.

Flat-panel detectors

Also flat-panel detectors replace the image intensifier in fluoroscope design. They have increased sensitivity to X-rays, and therefore reduce patient radiation dose. They have also a better temporal resolution, reducing motion blurring. Contrast ratio is also improved: flat-panel detectors are linear over a very wide latitude, whereas image intensifiers have a maximum contrast ratio of about 35:1. Spatial resolution is approximately equal.

Since flat panel detectors are considerably more expensive they are mainly used in specialties that require high-speed imaging, e.g., vascular imaging and cardiac catheterization.

Imaging concerns

In addition to spatial blurring factors, caused by such things as the Lubberts effect, (non-uniform response of an imaging system at different depths), K-fluorescence reabsorption (reabsorption in the K-orbit of the atom) and electron range, fluoroscopic systems also experience temporal blurring due to system lag. This temporal blurring has the effect of averaging frames together. While this helps reduce noise in images with stationary objects, it creates motion blurring for moving objects. Temporal blurring also complicates measurements of system performance for fluoroscopic systems.