What Are The Generation Of CT scan ( Computed tomography) ?
In generation Of CT Scan, The CT scan have been developed to acquire the x-ray transmission data for image reconstruction. The CT scan have gone through a number of design change since the technology. The aim of the CT scan generation is to provide faster acquisition times, better spatial resolution and shorter computer reconstruction times and simplification mechanical motion.
A) First Generation Of CT scan :-. The original EMI unit was the first generation of CT scan. The first generation of ct scan is rotate/ translate pencil beam and Only one detector per tomography section. Only two x-ray detectors were used, and they measured the transmission of x-rays through the patient for two different slices. A thin x-ray beam passed linearly over the patient, and a single detector followed on the opposite side of the patient. The tube and detector were then rotated slightly, and the process was repeated until a 180° arc was covered. Scan times were very long. This design is no longer in use. The original EMI scanner was designed specifically for evaluation of the brain. In this unit the head was enclosed in water bath between the x-ray tube above and a pair of detector. A took about 4.5 Minutes per scan with 1.5 minutes to reconstruction a slice.
Advantage
— Pencil beam allow efficient scatter rejection.
Disadvantage.
–It take 4.5 min to 5 min for one slice and it take approx 22 min to 25 min total Scan of head.
— Water bath acted as a bolus.
— The NaI detector had a significant amount of after glow.
— NaI hygroscopic.
— Complex mechanical motion of translate – rotate.
— Poor spatial resolution.
B) Second Generation Of CT scan:- The second generation CT scan also rotate / translate system. The 30 detector are used in second generation. This increased the utilization of the x-ray beam by 30 times, compared with the single detector used per slice in first-generation systems. A relatively narrow fan angle of 10 degrees was used. In principle, a reduction in scan time of about 30-fold could be expected. It takes 18 sec per slice scan. It takes 15 times faster than the first generation CT scan scan time.
Advantage:-
— The second generation of ct scan is 15 times faster than the first generation CT scan.
Disadvantage:-
— Water bath acted as a bolus.
— The NaI detector had a significant amount of after glow.
— NaI hygroscopic.
— The second generation CT scan increase more scattered radiation as compared to First generation of ct scan.
— Complex mechanical motion of translate – rotate.
C) Third Generation of CT scan:- The third generation CT scanner is also rotate / rotate system with wide fan beam and the angle of fan beam was 50° . Both the x-ray tube and detectors rotate around the patient. The original rotate – rotate scanner used 288 detector but newer version of some units use 700 detectors. In third generation the fan beam and detector array formed into arc view entire patient at all time. The curvilinear detector array results in constant source to detector path length which Is advantage of good image reconstruction. Xenon and scientillation crystal were used because the detector and x-ray tube are in precise alignment through entire scan motion. It take at 1 sec or slightly less scan time of per slice. The third generation scanners lead to a situation in which each detector is responsible for the data corresponding to a ring in the image. Any drift in the signal levels of the detector over time affects the µt values that are back projected to produce the CT image, Causing ring artifacts.
Advantage:-
1) In third generation CT scanner produce an image in 1 sec.
2) Xenon detector was used which is highly directinal and efficient reject scattered radiation.
3) The patient dose was less.
Disadvantage:-
1) More expensive as compared to First and second generation of CT scan.
2) More scatter radiation recorded by a fan beam unit than first and second generation scanners.
3) It produce ring artifacts
D) Fourth generation of CT scan:- Fourth-generation CT scanners were designed to overcome the problem of ring artifacts. The fourth generation of CT scan is rotate/ fixed system. The detector completely surrounded the patient and the detectors are do not move because detectors are fixed. Only the x-ray tube rotates in 360° circle inside the detector ring and the X-ray beam is collimated to form a fan beam. Modern fourth-generation CT systems use about 4,800 individual detectors. During acquisition with a fourth-generation scanner, the divergent x-ray beam emerging from the x-ray tube forms a fan-shaped x-ray beam.
Advantage:-
1) fourth generation of ct scan design to eliminate ring artifacts.
2) As detector are fixed so signal wire connected to detector remain fixed and therefore no fear of cable wear and tear as in case of 3rd generation CT scan.
Disadvantage:-
1) It gives High patient radiation dose because the distance from tube to patient is less.
2) It is more expensive as compared to 1st, 2nd and 3rd generation of CT scan.
3) It produce more scattered radiation.
E) Fifth generation of CT scan:- The fifth generation of CT scan is a stationary / stationary system. Fifth generation of CT scanner has been developed specifically for cardiac tomographic imaging. This “cine-CT” scanner does not use a conventional x-ray tube; instead, a large arc ( 210°) of tungsten encircles the patient and lies directly opposite to the detector ring. It uses an electronic gun that deflects and focuses a fast moving electron beam along tungsten target ring in the gantry. Since the detector is also in the form of ring, it permits simultaneous acquisition of multiple image sections. The image are obtained in 50 ms times and can produce fast frame rate CT movies of the beating heart. The hole heart can be acquired in 0.2 sec.
F) sixth generation of CT scan:- The third generation + Fourth generation + slip ring technology + Helical motion = sixth generation of CT scan. The sixth generation of ct scan design 1990. A slip ring is a circular contact with sliding brushes that allows the gantry to rotate continually. The use of slip-ring technology eliminated the inertial limitations at the end of each slice acquisition, and the rotating gantry was free to rotate continuously throughout the entire patient examination. This design made it possible to achieve greater rotational velocities than with systems not using a slip ring, allowing shorter scan times. The helical CT scan acquired data while the table is moving. The total Scan time required to image the patient can be much shorter, excluding time required to translate the patient table. It allows the less contrast agent and increases patient through. In some cases the entire scan can be done within a single breath hold of the patient.
G) Seventh generation of CT scan:- The seventh generation of ct scan uses multidetector array ( MDA). When multiple detector arrays are used the collimator spacing is wider and therefore more of the x-rays that are produced by the x-ray tube are used in producing image data. With conventional, single detector array scanners, opening up the collimator increases the slice thickness, which is good for improving the utilization of the x-ray beam but reduces spatial resolution in the slice thickness dimension. With the introduction of multiple detector arrays, the slice thickness is determined by the detector size and not by the collimator. This represents a major shift in CT technology. 4 continuous 5 mm detector array gives 20 mm collimator spacing. The number of x-ray detected is 4 times higher then that of single array of 5 mm. Further 10 mm, 15 mm, 20 mm, slices may be obtained from the same acquisition.
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