Digital image is a continuous variation in two dimentional i.e in x coordinates and Y coordinates being converted to  variation in the form of a finite  numbers or values  called pixels. These number or values are repesented in the form of digital numbers. It refers light intensity function, denoted by f(x,y)  where the value of f  at spatial co-ordinate (x,y)  gives the intensity  (brightness) of the image at that point (Figure 1).  Since the light is a form of energy, f(x,y)  must be non zero  and finite,  that is  0 < f(x,y) < infinity. In medical images, whether it is from Nuclear Magnetic Resonance Imaging, CT Imaging or Ultrasound, all images are digitized both spatially and in amplitude.  Digitization of the spatial co ordinates (x,y) is referred to as image sampling  while amplitude digitization is called gray-level quantization. The matrix size (N) as well as number of discrete gray levels (G) allowed for each pixel are taken as integer value & power of two .

N    =   2 ⁿ

G    =   2 ^m

where n, m and G denote the number of samples,  the number of bits and number of grey  level respectively. Number of bits required to store digital image is given by

b  =  N * N * m

In medical image modalities image data can be acquired in different matrix size, say 64 X 64, 128 X 128,  256 X 256 or higher matrix. The matrix can be square or rectangular. Each pixels in a matrix may have 8, 10, 12, 16 bit grey level. In medical data strored in Dicom or properietry software comprises  12 bits  grey levels (i.e., in case of 12 bit grey levels varies from 0 to 4095  integer  value).

Medical Images can be stored in different format. The length of file varies in different format. The data file of image is in two parts; header information and and image data. Dicom format/standards are commonly  used in medical imaging data. The header information in Dicom stores data regarding patient details, hospital details, sequence/parameters details etc. This information helps in identifying the patient details, processing and analyzing image data to improve interpretation.

Medical image file data length in different format (for matrix size of 256×256)

Data length Orignal file                      BMP format                                           JPEG format

Siemens (format)

134 KB                                                193 KB                                                          14 KB

Dicom (format)

134 KB                                                193 KB                                                          14 KB

Gamma Camera Image

129 KB                                                193 KB                                                          14 KB