CT History

Important Dates

1917: Johann Radon develops a mathematical theory to reconstruct an image from transmission measurements.

1963: Allan Cormack creates the first CT scanner. This scanner used an X-ray pencil beam and two NaI detectors.

1972: Godfrey Hounsfield (working with EMI, Ltd.) creates the first commercial CT scanner. The system, known as the EMI scanner was only able to image the patient's head. Interestingly, the EMI scanner was developed completely independent of Cormack's prior work.

1979: The Nobel prize in medicine goes to Hounsfield and Cormack for work in computed tomography.

1980’s: Helical CT scan mode was first introduced.

1990’s: Multi-detector array CT was developed, reducing scan time.

2000’s: Cone-beam and dual energy CT were introduced.

Scanner Generations

First Generation

First generation scanners laterally translated both the X-ray tube and imaging detector at discrete gantry angles to create images.


  • Used kilovoltage pencil beam and single detector.
  • Acquisition required laterally translating both X-ray tube and detector.
  • Scans required 5+ minutes per slice.


First Generation CT Scanner
Second Generation CT Scanner

Second Generation

Second generation scanners incorporated broad fan beams and detector arrays but retained the "translate while imaging, rotate, repeat" acquisition.


  • Used broad kilovoltage fan beam and an array of detectors to accelerate acquisition.
    • Detector array had between 3 and 60 elements (discrete detectors).
  • Reduced imaging time to 15-20 seconds per slice (an over 93% reduction in scan time).
  • Increased data acquisition rate strained limited computing power during image reconstruction.


Third Generation

Third generation scanners were the first to acquire projection data while the gantry rotated. This innovation eliminated the need to translate the X-ray tube and detector array, further reducing acquisition time. Early third generation designs were limited in how far they could rotate without rewinding cables by reversing direction. This limitation was overcome in the 1980s with the incorporation of a slip ring.


  • First to collect data during gantry rotation.
    • Later designs incorporated a slip ring.
  • Reduced imaging time to 15-20 seconds per scan.
    • Significantly faster than the 15-20 seconds per slice scans of a second generation scanner.
  • Axial scans only.
    • Helical acquisition is characteristic of generation six scanners.
  • Limitations
    • A single faulty detector element can lead to a ring artifact.
Third Generation CT Scanner

Ring artifact caused by faulty detector element

Slip rings used to facilitate continuous rotation

Fourth Generation

Fourth generation scanners feature a full 360-degree array of detectors meaning only the X-ray tube rotates. This effectively removed the ring artifacts of third generation scanners but was very expensive.


  • 360 degree detector and spinning X-ray tube.
  • Expensive
  • Resolved ring artifact issue of third generation but introduced wobble artifact.
    • Wobble artifact is caused by slight misalignment between center of detector array and center of X-ray tube rotation.
Fourth Generation CT Scanner
Uniform phantom with wobble artifact.
Fifth generation CT scanner

Fifth Generation

Fifth generation removed traditional spinning X-ray tubes entirely. Rather, an electron gun would accelerate  an electron beam through a set of steering "deflection coils." The deflection coils direct the electron beam into a large circumferential target located along the gantry. During acquisition the electron beam is directed around the target allowing for circumferential acquisition without any moving parts. This design is very expensive but allows for acquisition speeds greatly exceeding what is possible with a physically rotating X-ray tube.


  • Replaced X-ray tube with deflected electron tube and circumferential target.
    • Magnetic steering of electron beam allows acquisition.
  • Extremely fast acquisition thanks to no moving parts.
  • Very high cost.

Sixth Generation

Sixth generation CT scanners essentially returned to the hardware of a third generation scanner (rotating fan beam X-ray tube and detector array) but with the addition of a helical acquisition mode. This reduced costs relative to the fourth and fifth generation scanners while increasing acquisition speed above what was possible with a third generation scanner.


  • Introduced helical acquisition mode.
  • Uses third generation hardware
    • Rotating fan beam and detector array mounted on slip ring gantry.
Helical acquisition is characteristic of a sixth generation CT scanner.
Seventh generation scanners incorporated large "cone beam" sources.

Seventh Generation

Seventh generation CT scanners incorporates a larger cone beam X-ray source and detector array. In cone beam acquisition, multiple rows of detectors are arranged together, allowing the system to collect multiple rows of data in a single rotation.


  • Introduced cone beam acquisition.
    • Several slices can be measured in a single gantry rotation.
  • Modern CT scanners are usually seventh generation scanners.


Main Page

Computed Tomography
Table of Contents

Next Page

Magnetic Resonance Imaging
Table of Contents

Not a Premium Member?

Sign up today to get access to hundreds of ABR style practice questions.