2 CT Operation Theory
The CT Scanner consists of the following subcomponents:
•
X-ray source
•
CT Detector
•
Gantry
•
Power Distribution Unit
•
Table
•
System Cabinet
•
Scanner Desktop
2.1 X-ray Source
The source and detector components are housed on a gantry with a cylindrical patient bore. An
X- ray tube that is part of the source subcomponent is housed on the rotating gantry,
diametrically opposite to the detector. The high voltage generator is the second part of the X-ray
source. It provides high voltage to the X-ray tube across the anode and the cathode, along with
current to the filament, which is part of the cathode
The X-ray tube contains filaments, a cathode and an anode. The filament provides the electrons
that create X-rays. The X-ray system generates a current that heats the filament until electrons
start to “boil off” and break away from the filament. We refer to the filament current as “mA.”
Increasing the mA increases the number of electrons that become available to make X-ray.
Higher concentrations of electrons improve image resolution.
The X-ray system creates a high voltage, or kV, potential between the cathode and anode. The
negative charge on the cathode repels the electrons that boil off the filament. The positive
charge on the anode attracts the negatively charged electrons. The electrons strike the rotating
anode target and displace electrons in the target material. This interaction creates heat and X-
ray photons. The target rotates to help spread the heat over a larger area. Increasing the kV
increases the electron strike speed, which in turn increases the intensity or “hardness” of the X-
ray photon beam.
The X-ray tube's heat capacity and dissipation can determine the frequency and length of CT
Exposures, or limit the mA. A Helical or Cine exposure can last up to 60 seconds and axial
exposures last from 0.28 to 1.0 seconds.
2.2 CT Detector
The CT Detector is a wide coverage cone beam detector with multiple detector rows along the
longitudinal plane. The Detector channels are arranged as an arc diametrically opposite to the
X- ray tube on the rotating CT gantry. The detector consists of a scintillator that converts X-rays
into light, diodes for light conversion into current and analog to digital converter that converts the
current into digital signal. The Data Acquisition System (DAS) samples each detector cell up to
about 2496 times per gantry rotation, amplifies and quantifies the current from the cells and
transmits the resultant data to the scanner desktop.
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
636
2 CT Operation Theory
