Læknablaðið - 15.03.1983, Side 16

72
LÆKNABLADID
subject (KupfmCiller 1924, Nyquist 1924, Hart-
ley 1928, Gabor 1946). The theory was soon
accepted, Elias (1953) and Hopkins (1953)
showed its possibilities in optics and Röhler
(1957) published the first applications for
quantitation of the roentgenologic informa-
tion transmission. Later on information theory
has been explained in many roentgenologic
publications (Schober 1961, Holm 1964, Soila
1966).
The basis of the theory.
The Model of Shannon’s communication sy-
stem consists of the following links:
a) the source of information, which chooses a
certain message from a group af messages,
b) the transmitter, which changes the message
for the channel,
c) the transmission channel,
d) the receiver, which changes the message
into a form which can be understood in the
next phase,
e) the destination.
All the parts of the model have their own
information transmission capacity, and the
most favourable system can be combined only
by balancing the links with each other. The
information transmission capacity of the who-
le chain can be calculated from the informa-
tion transmission capacities of the links.
When applying this model to the roentgen
diagnostic chain, the object can be thought as
the source of information, and the effect of the
object on the energy of roentgen rays as the
transmitter.
The fluoroscopic screen or the roentgen
image intensifier are the main part of the
channel and the receiver, and the eye or a
recording device are the destination. Thus it
will be seen that a modern roentgen diagno-
stic chain often has to be divided into several
information transmission channels the one
after the other«.
Það er athyglisvert, að bæði Lassonen og
ýmsir aðrir höfundar, sem unnið hafa að
rannsóknum á röntgengreinarkerfinu, hafa lát-
ið staðar numið, þegar kerfisferillinn er kom-
inn að þeim þætti, er ég hefi merkt d) á
ferilmyndinni 4.5.1.
Samkvæmt þeim skilgreiningum, sem gefn-
ar eru hér að framan, má fræðilega gera ráð
fyrir því í upplýsingakerfi, að inntak = úttak,
eða að bezta yfirfærslueigind (transfer func-
tion) sé=l. Mjög fá kerfi innihalda þó svo
fullkomna »transducers« og í raun verður
ávallt að gera ráð fyrir því, að yfirfærslueigind
sé < 1. Um þetta segir Lassonen ((40), tilvitnun
til (56)):
»In Shannon’s theory the dimension of the
information transmission capacity is binary
digit, »bit«. The amount of information in bits
is the two-based logarithm of the group of
a. Myndataka b. Myndagerð
4.5.1. Kerfisferill röntgengreiningar. (Sbr. lesmál 4.5. og skipurit 4.2.1., 4.2.2.). Nokkuð breytt eftir Rossman
(42).