Here we provide a description of CT geometries from parallel, fan and cone-beam CT. The CT geometries have evolved from Parallel Beam at the inception of CT to Fan-Beam CT in 3rd generation CT and to Cone-Beam CT as the vendors has evolved to large angle multi-detector CT (MDCT).

chapters:
00:00 Intro
01:12 Paralell Beam CT
02:18 Fan Beam
03:15 Cone Beam


After going through the different generations of CT scanners we also wanted to focus on a few terms related to ways that x-rays cover the body during scanning. These go from parallel-beam, to fan-beam to cone-beam geometry in terms of speed of acquisition (i.e. the time required to get all the data needed to make an image).

Parallel beams of x-rays were used in the first generation of CT. The x-ray source was collimated and turned into single ray, a.k.a. a pencil beam. In this case the source and detector had to move to get another single ray.

In first generation CT we need to do all that just in order to get just one view of data, then the whole gantry rotates and to get the second view of the data we have to repeat again and again until we go at least half way around the patient.

After all of that in parallel beam CT we still have acquired only one slice of the patient. This then needs to be repeated for each slice after translating the table. That is why parallel beam CT was replaced with fan-beam CT.

Fan-Beam CT was introduced in 3rd generation CT. The reason it is called Fan-Beam, is because the beam coming out of the x-ray tube makes the shape of a fan.

Instead of translating you just turn on the x-rays and then the whole slice in this direction and here is covered in x-rays so we can get one rotation relatively quickly and don’t have to wait for those translations anymore.

Each rotation in fan-beam CT provides just one slice. Therefore, many rotations are needed to cover the full anatomy. This is where cone-beam CT comes in for even more acceleration in the speed of acquisition.

Then what came after fan-beam CT is called Cone-Beam CT and the geometry is the same as the third generation CT (i.e. an x-ray source and x-ray detector mounted straight across one another). However, in this case the detector has many more rows so the shape of the x-ray beam coming out looks more like a cone than a fan.

Each rotation is still fast (just like in fan-beam CT) but fewer rotations are needed in order to scan the entire anatomy of interest. As you can see in the figure more of the volume is covered at one time and thus we are need fewer rotations in cone-beam CT.

On a modern (state-of-the-art) CT scanner one can scan the whole heart or the whole head in just one single rotation (more discussion on this below).

Now Cone-Beam CT is the basis of modern CT scanners and it is, also called multi-detector CT (MDCT), as there was a gradual progression of rows of CT detectors on systems from 1,2,4,16, 64, until today where state-of-the art systems can have 256 or more rows of CT detectors. That is a huge leap in coverage from the early days of 3rd generation CT.

In this summary table we highlight the differences from parallel-beam to fan-beam to cone-beam. With a sum-what arbitrary distinction drawn between fan-beam and cone-beam when the number of detector rows is larger than 16. There were more major differences required in the CT reconstruction when the number of rows became greater than 16, so that is where we will draw the distinction between fan-beam and cone-beam systems.


Parallel-Beam Translate Rotate 1 slice
Fan Beam —- Rotate 1-16 detector rows
Cone Beam —- Rotate More than 16 detector rows

We also note that sometimes in the literature when people use the phrase cone-beam CT they are referring to systems which use a flat-panel detectors (that were originally designed for radiography and fluoroscopy procedures) in order to perform the CT. These systems were always cone-beam in nature as they evolved from interventional radiography systems rather than from early 3rd generation CT scanners. In future posts we will discuss these other scanners that also have very large cone-angles as cone-beam CT has been applied to a number of disciplines including: interventional radiography, cardiography, radiation therapy guidance and dental imaging.

At a high-level we went from first-generation CT being Parallel-beam CT to then Fan-Beam CT much faster to acquire a single slice. Now the state-of-the art is Cone-Beam (multi detector) CT (MDCT) where the a whole organ can be scanned in a fraction of a second.

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