By Prof. Stelmark

Digital Imaging In digital imaging, the latent image is stored as digital data and must be processed by the computer for viewing on a display monitor. Digital imaging can be accomplished by using a specialized image receptor that can produce a computerized radiographic image. Two types of digital radiographic systems are in use today: computed radiography (CR) and direct digital radiography (DR). Regardless of whether the imaging system is CR or DR, the computer can manipulate the radiographic image in various ways after the image has been created digitally.

A unique characteristic of digital image receptors is their wide dynamic range. Dynamic range refers to the range of exposure intensities an image receptor can accurately detect; this means that moderately underexposed or overexposed images may still be of acceptable diagnostic quality. Because the image is constructed of digital data and is viewed on a display monitor, it can exhibit a wider range of brightness or densities. As a result, anatomic areas of widely different x-ray attenuation, such as soft tissues and bony structures, can be more easily visualized on a digital image.

Digital images are composed of numeric data that can be easily manipulated by a computer. When displayed on a computer monitor, there is tremendous flexibility in terms of altering the brightness (density) and contrast of a digital image. The practical advantage of such capability is that, regardless of the original exposure technique factors (within reason), any anatomic structure can be independently and well visualized. Computers can also perform various postprocessing image manipulations to improve visibility of the anatomic region further.

A digital image is recorded as a matrix or combination of rows and columns (array) of small, usually square, “picture elements” called pixels. The size of the pixel is measured in microns (0.001 mm). Each pixel is recorded as a single numeric value, which is represented as a single brightness level on a display monitor. The location of the pixel within the image matrix corresponds to an area within the patient or volume of tissue

For a given anatomic area, or field of view (FOV), a matrix size of 1024 × 1024 has 1,048,576 individual pixels; a matrix size of 2048 × 2048 has 4,194,304 pixels. Digital image quality is improved with a larger matrix size that includes a greater number of smaller pixels Although image quality is improved for a larger matrix size and smaller pixels, computer processing time, network transmission time, and digital storage space increase as the matrix size increases.

The numeric value assigned to each pixel is determined by the relative attenuation of x-rays passing through the corresponding volume of tissue. Pixels representing highly attenuating tissues, such as bone, are usually assigned a low value for higher brightness than pixels representing tissues of low x-ray attenuation.

High brightness pixel Low brightness pixel

Each pixel also has a bit depth, or number of bits that determines the amount of precision in digitizing the analog signal and therefore the number of shades of gray that can be displayed in the image. Bit depth is determined by the analog-to- digital converter that is an integral component of every digital imaging system. Because the binary system is used, bit depth is expressed as 2 to the power of n, or the number of bits (2 n ). A larger bit depth allows a greater number of shades of gray to be displayed on a computer monitor. Binary digits are used to display the brightness level (shades of gray) of the digital image. The greater the number of bits, the greater the number of shades of gray, and the quality of the image is improved.

A system that can digitize and display a greater number of shades of gray has better contrast resolution. An image with increased contrast resolution increases the visibility of recorded detail and the ability to distinguish among small anatomic areas of interest.

A system that can digitize and display a greater number of pixels has better spatial resolution. An image with increased spatial resolution increases the visibility of recorded detail and the ability to resolve small structures. Pixel density The number of pixels per unit area

The distance measured from the center of a pixel to an adjacent pixel determines the pixel pitch or pixel spacing

Increasing the pixel density and decreasing the pixel pitch increases spatial resolution. Decreasing pixel density and increasing pixel pitch decreases spatial resolution.

Image Manipulation

R L

WW 40 WW 60 WW 80WW 100 WW 40 WW 30 WW 20WW 10

WL 10WL 20 WL 40 WL 60

SMOOTHER SHARPER

Matrix 5 x

FOV 20 cm

40 mm