In the design and implementation of Picture Archiving and Communication Systems (PACS), two types of problems have to be solved. The first is in the aspect of image data formatting and the second is in the image transmission.
Radiography, X-ray Computed Tomography (CT), radionuclide emission tomography, magnetic resonance imaging (MRI), and ultrasonic imaging are the widely used medical imaging. Microscopic images are the examples of bioimaging. Some of these biomedical imaging modalities, such as X-ray CT, MRI, etc, directly create digitized images, which are stored in computer disks. Some of them produce non-digitized images, which are recorded on films or video-tapes. Using scanners (flatbed, overhead, slide) or video camera, the non-digitized images can be digitized. Due to the diversity of physical principles and image reconstruction procedures of these imaging modalities, the image data formats of the digitized biomedical images are extremely different. In addition, the data size of the digitized images are usually very large, especially when the higher resolution is required. For instance, a transaxial thoracic image, created by the 3-rd generation of X-ray scanner, has about 150 X 103 bytes, a digitized radiographic film (14” X 17”), in the moderate resolution (300 pixel-per-inch), occupies about 20 X 106 bytes. Therefore, seeking an unified image data format and compressing the image data for digitized biomedical images are the first issue in developing PACS .
Computer networks provide the effective means for exchanging images. Currently, mainframe computers are linked by networks such as Bitnet, ARPAnet, etc. Personal (micro) computers (including workstations) are linked by LAN etc. Micro-mainframe connectivity is also available. In order to establish the communications among these computers, the same protocol is required. Once the compressed images are captured, opening, z.e., redisplaying the captured images requires the same or compatible applications (softwares). For the further processing and analysis, these compressed images may be required to change to the standard text data format (e.g. ASC) to meet a variety of needs of different computer systems and processing techniques. It is evident that speed in image transmission is another key factor, especially when image data size is very large. Thus, speeding up image transmission, decoding the compressed images, and selecting protocol and application are the second issue in developing a PACS .
The problems addressed above have been considered and solved. An Image Transfer System, ITS, has been developed in the University of Maryland School of Medicine (Baltimore) and tested between micro- to micro-computers, micro- to mainframe computers, and mainframe to mainframe computers.
1. The Tag Image File Format, TIFF, is utilized as an unified data format for image data compression and transmission. TIFF was originated in 1986 and some ideas behind it came from the ACR/NEMA communication protocol for radiological images. We extend it to more general medical images .
Comparing with traditional data file formats which use a fixed-position organization, TIFF employs a different structure based on tagged information. This structure is commonly used in data base design. Each tag of TIFF consists of several fields and a pointer. It describes not only the height and width of image, but also contain resolution information, support grayscale a d color data, and allow for private and public data compression schemes. Although the tag structure of TIFF may seem to add an unnecessary layer of overhead, it provides a key advantage over a fixed- position format. It is very easy to add new tags to the format without requiring all supporting software to be rewritten .
Standard data compression schemes are defined within TIFF. For bilevel image, a simple run- length encoding is used. For grayscale and color images, a form of LZW (Lempel-Ziv and Welch) encoding is used. According to our results, compression ratio is up to 3.65 .
2. A special firmware and a selected protocol (University of Maryland School of Medicine is going to file patent protection) are utilized to transfer images (in TIFF format) between micro- to micro-computers, and micro- to mainframe computers. This firmware provides a speed (in image transfer) which is four (4) times faster than that in normal data transfer .
Once the transferred images are captured, they can be directly displayed and printed. Using phototypesetter printer such as Linotronic, a photo-quality prints can be produced. Using several public softwares, these captured images can be directly manipulated. A decoding technique which can translate the coded, compressed TIFF image data into standard text image data is also developed. Thus, almost every image processing and analysis approach, e.g. edge-detection and region-segmentation in the lower-level analysis and object recognition and labeling in the higher- level analysis, either in mainframe or in micro-computer, can use these image data for further processing .
Our research and development showed that ITS has the following advantages:
A. it can accept both digitized and non-digitized images, as inputs, from different biomedicalimaging modalities;
B. various image data formats can be converted to an unified format, TIFF, which is extensible,portable, and revisable;
C. it can achieve pretty high data compression ratio, up to 3.65;
D. it can achieve quite fast transmission speed, 4 times faster than normal data transfer;
E. its communication protocol and applications are simple and public;
F. its decompression (i.e. decoding) procedure makes the captured image be processed in almostevery computer system and by existing processing/analysis techniques;
G. it is very user-friendly: it does not require special computer training;
H. it is also extremely cost-effective;
ITS demonstrates the great promise and is a practical Picture Archiving and Communication System.