[Physics] Difference between conventional spiral/helix CT and multi-slice spiral CT

medical-physicsx-rays

I'm currently learning about CT scanners, and come across conventional spiral/helix CT, and then multi-slice spiral CT.

What I can get out of it is, that the multi-slice scanner have a lot more detectors than the other, which, if I'm not mistaken make it faster to do scans.

So basically, I'm not sure what the benefits are, if any, in conventional spiral CT ?
I guess, that due to collimators, the X-rays in conventional spiral CTs are not as many for every turn, since the width of the detectors are not as thick as in multi-slice, meaning, that extra X-rays would just go through a patient, but not detected. But all in all you have to do more turns, if you want to measure the same volume, so the X-ray dose should be around the same, or…?

So am I missing something, or is there no advantages at all for conventional spiral CTs compared to multi-slice spiral CTs, or than maybe the price ?

Best Answer

There is not a lot of "straight up physics" here, but it's an interesting question nonetheless.

There is a lot of clinical (not just economic) value in fast CT scans. Three in particular that are worth noting:

  • if you want to do a contrast-enhanced (flow) scan, you need to see the contrast "everywhere" before it becomes too diluted. The faster you can scan the whole body, the better; otherwise you may need to give the patient multiple doses of contrast, which is undesirable (not without risk)
  • Suppress respiratory motion: for many scans the patient is asked to hold their breath; if you have a sick patient, they cannot hold their breath for long and a fast scan is preferred. The more axial FOV you cover in one rotation, the shorter the time to scan the entire chest.
  • Stop cardiac motion: if you can image the entire heart with the detector, and your gantry rotates fast enough, you can "stop the heart" and produce stunning 3D images of the heart, including detail of the coronary arteries.

Of course there is a cost to having larger detectors, and fast rotation speeds. Some of the complexities include:

  • You need collimation on the detector side to reject scatter: this requires very precise alignment as the pixels on the detector become smaller
  • As the axial extent of the FOV increases, you run into a problem called the "heel effect": since the anode is tilted at an angle of about 8° (so the apparent focal spot size is much less than the actual size of the spot made by the electron beam on the anode), as you change the angle at which you look at the anode, the size of the spot (and the apparent intensity) changes. At one end, this makes the spot look small and you are count starved; at the other end, the spot is large and the intensity of Xrays is greater. This is circumvented by making the anode angle larger - this makes the flux smaller, and the thermal problems bigger... just when faster rotation requires MORE flux.
  • As the gantry rotates faster, the acceleration of components increases as the square of the velocity. For a gantry rotating at 0.25 seconds (so it covers 180° + fan angle of detector in under 0.2 second, enough to "freeze" the heart), an object at a radius of rotation of 50 cm experiences a centripetal acceleration of about 32 g... and one of those components is the CT tube that contains an anode with a mass of several kg, at a temperature of 1200 °C (white hot), rotating at about 5000 rpm on vacuum (i.e. unlubricated) bearings. That's an engineering challenge...

Yes - multi detector CT scanners are more expensive; but their rapid adoption after initial introduction demonstrates they have value. But they are very challenging to design.

See also this article for some cardiologist thoughts on multi slice scanners.

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