Digital Image Characteristics
During radiographic imaging, the radiation through the patient undergoes a lot of change in x-ray intensities due to the transmission and absorption of x-rays from anatomic tissue. The image receptor receives this exit radiation and creates a latent image or invisible image.
In digital imaging , the latent image is stored as a digital data and processed by the computer for viewing on the display monitor. It has a specialized imgae receptor that produces a computerized radiographic image. Currently two types of digital radiographic system are available which are Computed Radiography (CR) and Direct Radiography (DR) system . In both imaging systems, radiographic images can be manupulated by the computer in many ways.
Digital image is made up of numerical data which can be manupulated by computer. In this, the contrast and brightness can be changed very much by the computer.
Pixel
Digital image is recorded as Matrix . It is made up of a combination of matrix rows and columns, from which picture elements of square shape are called, which are called Pixel . The size of the Pixel is micron.
100 micron = 0.1mm
Here each pixel has a numerical value which is displayed as a single brightness level on the display monitor. The location of the pixel in the image matrix corresponds to the area in the patient or volume tissue.
A given anatomic area or FOV (Field of View) consists of the following pixels in a different matrix.
| Matrix Size | No of Pixel |
| 1024X1024 | 1048576 |
| 2048X2048 | 4194304 |
Large matrix size and very high number of small pixels improve image quality, but in addition to this computer processing time, network transmission time and digital storage space also increase.
Pixel Size = FOV / Matrix Size
Here the FOV increases for a fixed matrix size, so the pixel size also increases. (Direct Relation)
If the Matrix size increases for a fixed FOV, the pixel size decreases. (Inverse relation)
Pxel Bit Depth
The numerical value of each pixel depends on the relative attenuatuin of the x-ray radiaiton passing through the tissue. Bone tissue where more xray attenuate is less for the pixel's numerical value, and for soft tissue, the numerical value for the pixel is higher.
Each pixel has a Pixel Bit Depth or number of bits , it does the amount of precision in digitizing the analog signal into a digital signal. By which gray shade is obtained for display in digital image.
Bit depth is determined by an anlog to digital converter . It is an essential part of any digital imaging system.
The bit depth is represented by the power of 2 n. Here 2 binary numbers are used. Hence the bit depth is represented by the following formula -
No of bits = 2 n
Here n bit is the depth.
| Bit depth | No of bits | No of Shades |
| 12 | 2 12 | 4096 |
| 14 | 2 14 | 16384 |
| 16 | 2 16 | 65536 |
The more gray shades a system digitizes and displays, the higher the contrast resolution. Therefore, the image exhibits more anatomic detail and can produce differentiation in small anatomic area.
Pixel Density
The number of pixels present in the per unit area of an image is called pixel density. Pixel density increases as spatial resolution increases.
Pixel Pitch
The second pixel between the center of two pixels is called pitch. Pixel Pitch is less in small size pixels which increases the spatial resolution.
