GENERATIONS OF CT SCANNERS

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:

   

This picture shows the very 1^st 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 2^nd generation scanners.

SECOND GENERATION CT SCANNER

The 2^nd generation CT scanners were developed to overcome some of the challenges of the 1^st 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 3^rd 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 4^th 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.

next we will discus CT scanning protocols