CT scan advantages and disadvantages vs General Radiology

Question

I have been looking at how CT scans work and the disadvantage and advantages they have compare to general radiology techniques, but this has brought up some questions about CT scan dosage.

For general X-rays, the lower the KV the higher the contrast and the higher the dose because more attenuation is happening via photoelectric absorption. For higher KV there are more photons being produced but the dosage is less because higher energy photons attenuate less via photoelectric absorbing.

For CT scans it seems the opposite is true. Higher energy photons increase the patient dose, which bring up two questions:

1. Why if CT uses high energy photons, does it have better contrast imaging that regular radiography? Surely there is less photoelectric absorption and more Compton scattering.

My thoughts are that for each layer that is being scanned there will be a slightly different electron density which is what Compton scattering is proportional to and that there is maybe a collimator to realign the scatter photons to form and image?

2. Why is the dose higher for, higher KV values? My only conclusion is that this is due to the what is called the pitch of the machine, which as I have read is between 1-2, so at 1 you have a higher dosage than 2 as more information is being taken.

But in CT the scan time is not varied as it is pretty much a set thing. In normal radiography you can adjust the dosage but also vary the time depending on what you are looking at, where as in CT the detector spins around in a set timely manor.

Answer 1

For general X-rays, the lower the KV the higher the contrast and the higher the dose because more attenuation is happening via photoelectric absorption. For higher KV there are more photons being produced but the dosage is less because higher energy photons attenuate less via photoelectric absorbing.

This is only true if you keep the x-ray flux passing through the object constant. It also doesn't matter what imaging modality you're talking about.

The output (measured in mGy/mAs) of an x-ray tube used in medical imaging changes with the square of the kV. If you go from 80 kV to 120 kV, and that's the only thing you change, the x-ray tube output changes by roughly a factor of $(120/80)^2$

Why if CT uses high energy photons, does it have better contrast imaging that regular radiography? Surely there is less photoelectric absorption and more Compton scattering.

The contrast detectability with CT imaging doesn't just come from the x-ray energy that's being used. With CT, you're measuring the x-ray transmission through an object at many different angles. Take that transmission data, run it through some reconstruction algorithms, and you get a representation of the original object.

Most CT is also done with narrow beams, so most of the scatter radiation isn't detected.

With general radiography, all you're measuring is the transmission of x-rays through the object. Objects you're interested in are covered up by other objects, which decreases the contrast of the object.

Why is the dose higher for, higher Kv values? My only conclusion is due to the what is called the pitch of the machine, which as I have read is between 1-2, so at 1 you have a higher dosage than 2 as more information is being taken.

There are many factors that affect dose: kV, tube current, pitch, time. You can't really use any of them by itself to talk about dose except in very general terms.