CT scan generation 

CT Scan was first introduced in 1971 by Godfrey Hounsfield. It was a single detector machine used to study the brain.

First generation ct scanner-

There was only one X-ray source in this generation and there was only one X-ray detector to collect X-ray data. The course and the detector were closely connected to the other. The X-ray beam used in this generation was a pencil beam. In this, the tube detector movement was of translate-rotate type. The X-ray tubes move from one side of the patient to the other side to achieve a set of x-ray parallel projection in translation. After this, the tube rotates on a small angle and translates to the other side.

This process is for one projection and is repeated until 180 projection is done. Due to this translator rotate geometry, this generation is called translite rotate scanner. In the first generation CT scanner, only 2 detectors were used, which were on the other side in front of the x-ray tube. Both these detectors used to detect the x-ray coming on the patient and 2 slices of the image were made at a time.

The main feature of ct scan of this generation was the pencil beam Xray geometry to be used in it, because only 2 detectors were used, so the X-ray beam was not kept very short due to which scatter radiation was also very useful.

The biggest drawback of this generation of ct scan was the time taken during the scan and image reconstruction.

Second generation ct scanner-

In hopes of reducing the time it takes to achieve head CT, the first growth for the CT scanner was the use of a narrow fan beam with a beam of 30 detectors with an angle of about 10 degrees. The linear array was used. This was a major change from the first generation of seen pencil beams and two detectors. This generation ct scan has reduced the time of image acquisition significantly. Although the angle of the fan beam x-ray was still not large enough, there was still a need for a linear movement of the x-ray tube and detector, but the amount of this linear displacement was dramatically reduced.

The initial version consisted of 3 detectors that rotated 3 ° after translate, thus there were a total of 60 translations instead of 180 translations for a section. In later versions, up to 53 detectors were used.

The biggest drawback of the ct scan of this generation was its fan beam, which increased the image resolution due to the increase in the amount of scatter radiation reaching the detector. In this generation, the x-ray tube was still translate-rotate for the image, so the scan time was still too much.

For this reason, the first and second generation ct scan was used for imaging the head and brain, because the thorax and abdomen have continuous moving organs.

Third generation CT scanner-

Translational motion, which was used in the first and second generation scanners, was quite time consuming. The main goal in CT development was to work the scan time from 20 sec to perform brain imaging as well as abdomen and chest imaging. For this, it was proposed that after every rotation movement the translating stage should be abolished.This promoted the introduction of a wide aperture x-ray beam (fan beam), which could reach the entire patient (piece) at a time: this meant that X-ray tubes and detectors were now at every angle without rotational movement. Could rotate freely through and gather multiple slices in every rotation. This ended the translational movement from ct. In this, the aperture of X-ray beam is 40 to 60 °. A large array of detectors is used in it.This new array consists of 400 to 1000 detector elements. X-ray tubes and detector arrays are connected to each other and move around the patient simultaneously.

Advantage-

The biggest feature of the ct scan of this generation is to reduce the scan time by one DM, which was just a few seconds. Some systems could complete the projection in less than 5 sec. Third generation ct scan system is still available in the market which scans in less than one second.

Disadvantage-

1. It is more expensive to use two to four detectors in place of 400 to 1000. But it is argued that such a short time of scan time gives extra benefit to the patient.

2. Third generation ct scan system produces a special image artifact called ring trtifacts. These artifacts lack such lanes and their calibration in the use of such a large number of detectors.

Fourth generation CT scanner

Fourth generation ct scan was developed to reduce ring artifacts generated by third generation. Calibration of the detector is difficult to rotate around the patient. So in this generation, detector arrays are placed around the patient and in a stationary ring that does not rotate, only the x-ray tube rotates around the patient. Every detector processes the fan shaped x-ray beam.

This generation consists of rotate stationary. The scan time is reduced due to rotation of only X-ray tubes and generators. There were 600 detectors in its early versions which increased to 4500 in later versions.

Advantage-

Only x-ray tubes and generators rotate around the patient, so the scan time is reduced drastically.

Disadvantage-

Because the detectors are located in the ring around the patient and the x-ray tube itself rotates around the patient. This detector is less efficient to use because only 1/4 of the detectors are used in data acquitation at any given time, the rest of the detector is of no use.

Fifth generation ct scanner-

This generation was developed primarily for cardiac tomographic imaging. These scanners are called Cine-CT scanners or Electron beam scanners (EBCT). By the first generation of this generation, CT scanners had made considerable progress in imaging most of the body. However, due to high speed and continuous heartbeat, short scan time was required to get the best image of the heart. But the moving components of the ct scan machine were the biggest obstacle in reducing this scan time. Hence there is no moving part in the fifth generation CT scanner.

Instead of X-ray tube rotation, these electron beam scanners have a large X-ray tube inside which the patient resides during the scan. Behind the patient, there is an electron gun, which ejects electrons. This electron beam electronically tilts down, deflects, and falls onto the large, half circle tungsten target that surrounds the patient. The collision of electrons with the target ring produces an X-ray beam, which crosses the patient and falls on the detector facing it.

Advantage-

Its biggest advantage is very short scan time, so that the CT-movie of beating heart can be made. In this, scan time works by 50 msec, which can capture the contraction and relaxation of the heart.

Disadvantage-

The scanners of this generation were primarily made for heart scans, so were developed only for cardiologists. These scanners are very expensive. And they are also not versatile, ultimately not very popular in the field of medical imaging.