the use of acquisition and video interfaces via serial or parallel buses. The
keyboard and the mouse are the most common input devices used in com-
puters, although joysticks, light pens, and trackballs are occasionally used
as input devices. While the keyboard is essential for the input of alpha-
numeric data such as patient identification, date, time, and operator’s name,
the mouse and trackballs are used to select items from the menu. Light
pens, a mouse, and touch screens are often used for the selection of regions
of interest (ROI) in an image.
Common output devices include display screens (video monitor) for
texts, images or graphics, and printers for printing. Display screens normally
have a capability of a gray scale or a color scale for comparison between
the intensities or amplitudes of different regions of the image.
Operation of a Computer
A computer operates according to instructions provided by an operator.
These instructions are given in the form of one or more programs. A col-
lection of programs is called the software, which is developed by specialists
according to the specific need for a project. The most essential program for
the operation of a computer is the so-called operating system such as DOS
(disk operating system), Windows 98, Unix, and Linux. The utility of this
program is to facilitate communication between the computer and opera-
tor’s instructions. The operating system transfers the program instructions
from the input device to the memory, commands the CPU to carry out the
specific instruction and returns the data to the output devices. Other utili-
ties of this system include file transfer from one location to another, storing
data in the external storage device, and display of the data.
Data must be provided as input to the computer for processing, and in
nuclear medicine they are available in the form of counts or voltage pulses
obtained from scintigraphic studies. Data are processed according to
instructions in the software program, and the processed data are then stored
in computer memory or external storage spaces or displayed on video mon-
itors. The time to complete a task by the computer depends on a number
of factors such as the speed of the CPU, the size of the main memory, serial
or parallel processing of data, and the data transfer rates of the I/O devices.
The faster CPU, the larger size of memory and the parallel buses provide
speedy computation.
Digitization of Analog Data
In nuclear medicine, signals from a gamma camera are acquired in analog
form, which are digitized before storing and further processing by the com-
puter. Conversion of analog signals to discrete digital values is performed
by the so-called analog-to-digital converters(ADCs) and the process is
called digitization.
142 11. Digital Computers in Nuclear Medicine