Biomedical Engineering Reference
In-Depth Information
analysis software,* which consists of a file containing only pixel values, usually
stored in binary as short (2-byte) integers, and a separate header file, which
contains a simple data structure of basic information.
In addition, there is a variety of widely used image formats not specifically
designed for medical images. Common examples include tagged image file
format (TIFF), graphics interchange format (GIF), Microsoft window bitmap
(BMP), and a standard known as JPEG, developed by the Joint Photographic
Experts Group. A specialist digital format developed by the Moving Picture
Experts Group, known as MPEG, for storage of movie
video loops, is widely
available and useful for displaying medical images of dynamic processes
such as cardiac pulsations. Each of these image formats stores data in self-
contained files that contain both the image data and header information
detailing color maps, use of data compression, etc. In general, these formats
do not have provision for integrated storage of patient, examination, or
image acquisition details. As with other medical image-specific formats,
information about file structures is widely available in the public domain.
†
Many proprietary software packages allow convenient import and export of
images in the commonly used formats and also, frequently, conversion from
one format to another.
A complicating factor in transferring data from one computer system to
another arises from low level details of data storage. Key factors are the num-
ber of bytes used to store a given type of number format; for example, in most
modern computers floating point numbers are stored as 4, 8, or 16 bytes,
depending on the computer, operating system, and data precision. Even if the
number of bytes per stored number matches or is corrected for, the order in
which multiple bytes are stored has two standard variants. It may therefore
be necessary to pass the data through a byte reversing program prior to use on
another computer (see Figure 4.3). Some image analysis programs will do this
automatically as required. An advantage of the GIF file format is that it only
uses one byte per pixel intensity value and therefore does not suffer from this
problem. A disadvantage is that this limits the dynamic range of the informa-
tion that can be stored. In general, it is always prudent to check that the output
of one system or package can actually be successfully imported to another.
A final issue associated with image transfer, particularly if images from dif-
ferent sources are combined, is the choice of coordinate system. The way that
human images are displayed is a matter of convention; for example, the North
American Radiological convention is to display transaxial images with right
of the patient at the left of the image and the posterior side at the bottom
regardless of how the subject was positioned in the scanner. An on-the-fly
coordinate transformation is frequently performed in retrieving data from the
hard disk, or other archive, and displaying it on a computer screen. That coor-
dinate transformation may be user defined so that the same data may appear
* Analyse is a proprietry format developed at the Mayo Clinic (see
http://www.mayo.edu/ bir
).
†
See footnote on page 80.
