Tuesday, July 14, 2020

Generations of CT Scanners

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In our previous blog we have discussed about Personal  Protective Equipment.

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In Today’s blog we are going to see about different generations of CT scanners.

Invention of Computed Tomography is a revolution in the healthcare sector. The contribution of Computed Tomography in diagnostic and therapeutic fields in immense. Therefore it is important for every healthcare professional to know in detail about the evolution of CT scanners.

Let’s get into the topic.

INTRODUCTION

Computed Tomography was introduced by G.N. Hounsfield at Central Research Laboratories. The idea of computed tomography raised when G.N. Hounsfield was trying to identify what is inside a box by taking X rays at various angles around a box. Computed Tomography was commercialized in 1972.

WHY COMPUTED 

TOMOGRAPHY?

Computed Tomography provides 3 Dimensional cross sectional images of internal organs and structures. Computed Tomography gives three dimensional informational on a single plane. The ct scanners are used to view images of internal organs, bones, soft tissues and blood vessels. Gives information on the size and location of organs.

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Before getting into the ct technology, we will discuss about the generations of ct scanners as following.

FIRST GENERATION

1st generation ct scanner has a single X ray source and single X ray detector. It takes 180 profiles in 180 seconds. Translate/rotate process. In order to acquire every slice across the part of the body, the X ray tube and detectors has to be moved linearly before rotating the position of X ray tube to acquire images at different projection angles. The X ray tube and detector translate across one slice (left), then rotates a very small amount and then translate back towards right side. First generation CT Scanners used a pencil beam of one degree.

One of the disadvantages of 1st generation ct scanner is that scanning time is between 25-30 mins. Resolution was very poor in 1st generation ct scanner.


SECOND GENERATION

2nd generation ct scanner is faster that previous generation. It used a narrow fan beam of 10 degrees which results in a linear array of 30 detectors. It covers 10 profiles per degree. It takes 180 profiles in 18 seconds. Since it is a narrow fan beam, 2nd generation ct scanner still requires a Translate/Rotate process.

Scanning time was improved than the first generation CT scanners. More number of detectors contributed to a more scattered radiation. It decreased the resolution of the image.


THIRD GENERATION

3rd generation ct scanner used a larger fan beam of 40 – 60 degrees which covers the entire patient (one slice) at a time. It is a Rotate/Rotate process. Larger fan beam eliminated the need for translate motion across the single slice. Now X ray tube and detector can rotate together across the desired projection angles, instead of taking multiple multiple slices per projection angle.

Larger array of detectors (more than 800) were used. Scanning time was reduced to 2 seconds.

Because of large array of detectors, there is problem in calibration of each detectors which resulted in a ring artifact of the image produced.


FOURTH GENERATION

4th generation ct scanner has stationary detector ring and rotating X ray tube to overcome detector drift (ring artifact in third generation).  Circular array of fixed detectors is used. Thus it is a Rotate/Stationary process.

Because of reduced movement of detectors, scanning time is even reduced.

One of the disadvantage of 4th generation ct scanner is the lesser usage of detectors, only 1/4th of detectors are in use at a single time. Stationary detector requires a larger acceptance angle for radiation, therefore it is more sensitive to scattered radiation than third generation CT scanners.


FIFTH GENERATION

5th generation ct scanner is also called as Electron Beam scanners and Cine CT scanners.

Since heart is one of the fastest moving organ in human body, it requires even a quicker scan time to take cardiac imaging. In order to achieve that all the movements in a CT scanner was restricted. Thus 5th generation ct scanner used a Stationary/Stationary process. It consists of a large X ray tube, inside of which patient lies during the scan. It is a large tungsten ring that circles around the patient, opposite to the detector ring. When electrons emitted from electron beam falls on the tungsten ring, it produces the X ray beam which passes through the patient and collected in the opposite detector ring.

It takes 17 CT slices per second with a sweep time of 50 micro seconds. Image is acquired in less than 100 micro seconds.

5th generation ct scanner is very useful in cardiac imaging. It produces the images of contraction and relaxation of heart.

However 5th generation ct scanner is very costly and popular only in the field of cardiology.


SIXTH GENERATION

6th generation ct scanner is also called as Helical CT scanners.

In all previous generations, gantry needs to be stopped to change angle projections. So image acquisition is not a continuous process. In order to overcome this problem Helical CT scanners was introduced with a slip ring technology.

Here patient is moved through a rotating X ray beam and detector set. X ray beam from CT traces a helical path. Helical path results in a three dimensional data, which can be reconstructed into sequence of images. Entire helical CT scan can be performed in a single breath hold.

One of the disadvantage of helical CT scanners is that image is produced in a helical form, thus no full slices of data are available. However this problem can be compensated through a reconstruction process.


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SEVENTH GENERATION

7th generation ct scanner used a cone shaped X ray beam. Unlike pencil beam and fan beam in earlier generation, cone beam does not pass through the narrow collimeter. Therefore intensity of the X ray beam is not strongly reduced and can interact more efficiently with a detector array.

And also linear array of detectors in previous generations are modified as flat panel detectors or multiple detector array.

Thus 7th generation ct scanner can acquire more information in a very short span of time.

 

Now I hope you have acquired some knowledge about generations of ct scanners. We will discuss about Imaging Technology of Computed Tomography and artifacts of Computed Tomography in our next blog.

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COMPUTED TOMOGRAPHY | TECHNOLOGY | ARTIFACTS

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2 comments:

  1. The explanation is great but it would be great if you could add more about the process

    ReplyDelete