GENERATIONS OF CT
SCANNERS
The different generations of CT scanner was developed over
the years to increase scan time and efficiency of scanning. The changes were
majorly on the X-ray tube and detector arrangements.
Major adjustment in the technology of CT scanners were manifested in
- Tube orientation and shape of beam (from pencil beam through narrow beam to fan beam)
- Number of detectors (from single detectors to multiple detectors).
- Detector arrangement.
- Slip ring technology.
This primarily led to decrease in scan time and increase in efficiency of the scanners.
FIRST GENERATION CT
SCANNERS:
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This picture shows the very 1st
generation of CT scanners. It was used to scan only the head.
The scans were done using a water filled box. A narrow
pencil width X-ray beam and a single detector mechanism were used for data
acquisition. Scan was performed in a rotate-translate motion.
Let me explain a rotate-translate motion.
For scan 1, the X-ray tube and the single detector scan from
one end of the patient to the other end.
E.g. when scanning the head, the x-ray tube and the detector moves from
the right side of the head to the left. This is translation (a horizontal
movement across the patient)
For scan 2, the X-ray tube and the single detector rotate
into a new position (circular movement around the patient) and the
translational scans are taken in this new position.
For scan 3 to the last scan, the procedures are repeated.
Note: scans are only recorded during the translational
movement and for one complete slice; the rotation has to complete 3600.
This scanners where limited because
1.
Only head scans could be performed.
2.
Generates a lot of heat requiring water and an
elaborate cooling system.
3.
Scan time was very slow. About 1 minute per
slice
This led to the development of the 2nd generation
scanners.
SECOND GENERATION CT
SCANNER
The 2nd generation
CT scanners were developed to overcome some of the challenges of the 1st
generation scanners. The scanners have narrow fan beam (100),
multiple detectors, multiple angles of acquisitions but still perform
rotate-translate scans. Scan time was greatly reduced to about 20seconds per
slice.
THIRD GENERATION CT
SCANNER
This generation of scanners was developed to primarily
reduce scan time
It has the following features
- Fan beam
- Multiple detector array (500 -1000 detectors)
- Only rotational movement. Rotate-rotate (no more rotate-translate)
- Scan time of as fast as 0.5sec per rotation or per slice.
Most modern day scanners have this design. However, the 3rd
generation scanners has a limitation which is the presence of a rotate-rotate
ring artifact.
FOURTH GENERATION CT SCANNERS
This generation of CT scanners was designed to eliminate the
ring artifact. It has the following
modifications
1.
A fan beam.
2.
Fixed ring of detectors. The detectors are
arranged round the gantry.
3.
Only the tube rotates.
Electron Beam CT:
This was designed for ultrafast scans to freeze cardiac
motion in Cardiac CT scans. An electron beam is emitted from a cathode and
focused on a track of tungsten anode target from which X-rays are produced and
collimated into a fan beam for use in CT scans.
Slip Ring Technology
and Helical Scans:
All generations of CT scanners (except 4th gen.) required
winding and unwinding of connection cables causing inter-scan delays. Slip ring
was designed to eliminate this. A slip ring is a drum with grooves along which
electrical contactor brushes slide. Data are transmitted from detectors via
various high capacity wireless technologies thus allowing continuous rotation.
This enables the helical scan where data are collected continuously as the
patient moves through the rotating gantry.
Most modern day 3rd generation CT scanners are incorporated with slip rings. This has eliminated the need for 4th generation scanners in eliminating ring artifacts.
