University of Iowa Hospitals and Clinics: Department of Radiology: Teleradiology
Teleradiology systems allow rural primary care physicians to consult with radiologists at a distant location while viewing the same image
Sending Station Specifications
The three most important specifications for a teleradiology
sending station are resolution, compression, and transmission speed.
1. Image Resolution
Resolution is the ability of an imaging system to differentiate
among objects. When a sending station digitizes a x-ray film it
breaks it into a two dimensional matrix of small elements called
pixels.
As the image is read by the digitizer, the information contained in each pixel is assigned a number which represents the amount of density (information) it contains. This number is called the gray scale (or density) number. A pixel that has a lot of in formation (black) would be assigned a higher number than a pixel with little information (light). The more pixels in an image and the greater the range of density numbers per pixel, the better the image resolution.
Typical resolution matrix sizes offered today by vendors are 512 X 512 (512 pixels wide by 512 pixels high), 1024 X 1024, and 2048 X 2048. Typical gray scale ranges offered are 256 (8 computer bits deep) to 4096 (12 computer bits deep) shades of gray.
While increasing the matrix and gray scale range improves the image resolution, it also requires more information that has to be sent via the transmission network. For example: an image that is digitized at 512 X 512 X 8 requires 2,097,152 bits of info rmation to be transmitted, while a image that is digitized at 1024 X 1024 X 12 has 12,582,912 bits of information to be transmitted. The latter is 6 times larger than the former and would take 6 times longer to transmit to the receiving station.
2. Compression
Compression is a software technique by which certain pixels in
the digitized image are dropped to decrease transmission time.
Compression is expressed as a ratio. A compression ratio of 10:1
means that for each pixel of information retained from the original
digitized matrix, 10 have been dropped before transmission. There are
numerous compression algorithms in use today ranging from 2:1 to 15:1
or higher. Compression algorithms below about 3:1 are usually
considered lossless; that is, no information contained in the
original digitized image is lost. Compression ratios above this are
considered lossy (destructive) and can result in image degradation.
3. Transmission (modem) speed
A modem is the interface unit between the image digitizer and the
transmission network. It converts digital image data to electrical
impulses which can be sent along the transmission media. The rate at
which a modem can perform this conversion is given in bits per second
(bps).
The ideal teleradiology sending station would have very high resolution, little or no compression, and very high transmission speeds. This is not possible in the real world because optimizing one parameter negatively affects another (e.g. increasing re solution matrix size increases transmission time). How does one select a teleradiology sending unit to balance resolution, compression, and transmission speed parameters? If economically feasible, one selects a sending station that has a reasonably fast modem (about 19,200 bps), operator-selectable resolution of 512 to 2048 bits and several selectable compression levels. A station with this flexibility will allow the sender (and receiver) to decide on a case-by-case basis which is more important; quality of the received image or the speed at which it arrives. If selectable resolution and compression are not an option, the sending station should have a reasonably high fixed resolution (1024 X 1024 X 12) and lossless compression (3:1).
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