By: 1 December 2009

The advent of digital imaging has enhanced convenience and lowered costs of radiography, and the technology is rapidly becoming universal.

Two options currently exist. Computed radiography (CR) uses an imaging plate (phosphor-based) in place of film, which is scanned by laser to digitize the image. It has the advantages of high sensitivity and resolution, with exposure compensation to maximize contrast. Digital radiography (DR) employs a variety of imaging plates with similar advantages to CR, but the image is instantaneously captured digitally and displayed on a computer monitor. A central feature of both CR and DR is that they are readily integrated into Picture Archiving Computer Systems (PACS) which manage image storage and retrieval.

Exhibit 1: SKI frame with turntable to position patient. Registration grid is mounted to the frame. Turntable is rotated for frontal and orthogonal views.

Despite its advantages (logistics, image sharpness, contrast, magnification, computerised metrics, etc.), digital image capture does not automatically provide a more reliable radiograph than traditional methods. Imprecision may arise from other sources such as poor positioning of the patient and image distortion due to parallax.1 Standardized Imaging is a concept designed to limit, or correct for, these sources of error. It is applicable to CR, DR or film-based methods. An example of such a method is Standardized Knee Imaging (SKI)2,3 in which each patient is set up in a standardized position within a frame (See Exhibit 1), which also inserts one or more registration grids into the x-ray field.

The grid image(s) are superimposed on to the target image, and the combined image is processed digitally to correct for any parallax error. Using SKI we can expect reproducibility of angular measurements to be within