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I'm going to talk about CT dosimetry under the CT

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physics. When it says CT dosimetry, that includes how we

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measure CT dose and how it is used for

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estimating patient dose. And this is very important

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because CT scanning has always been very critiqued

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for the patient dose, and I want to explain how we measure patient dose

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and what things can be done to effectively

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strategize to optimize the patient dose.

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So in this particular section,

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I want to talk about how CT doses are measured

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and what are the key CT dose descriptors.

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And I want to explain how we can understand some of the displays

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available with every patient image and talk

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about effective dose estimation and the uncertainty

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associated with the risk estimations.

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So how are CT dose measured?

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First and foremost, CT dose is not measured directly on

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patient.

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It is measured using standard phantoms, shown on this

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particular image.

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And we call first terminology called computed

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tomography dose index, short form

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CTDI. It is expressed in milligray.

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So this is just an index, and that is measured

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using these phantom at the center of these phantoms

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on rotation, and biophysicist does this using this phantom

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to measure what is called a CTDI.

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And that is expressed in milligray.

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That is the unit of absorbed dose.

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From the time CT were developed, fortunately, it has been very

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standardized, these phantoms.

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The 16-centimeter diameter phantom is called adult

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body phantom.

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Even though it is different from actual reality, but that's the

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circular phantom we have been using to measure the scanner

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output. From there, we can estimate the patient dose

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and so forth.

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A smaller 16-centimeter phantom is called the

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head phantom,

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and there is a even further called 10-centimeter diameter.

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Sometimes we can use it for pediatric head and so forth.

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But these are circular plastic phantom.

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It is called some type of a

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slightly denser than water PMMA phantom,

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and they have holes. The holes allows the physicist to

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insert the ion chamber and scanning the phantom at the

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center will collect information on this

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chamber to measure radiation dose absorbed in the phantom.

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I just also want to differentiate something unique with CT.

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In radiography and mammography or in fluoroscopy, the

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radiation dose gradient in a patient is always like

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this, which means you're imaging a three-dimensional object, and

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that representation is a 2D, so the surface is only one

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direction. You take an X-ray, you take a mammogram, you see

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the maximum dose on the surface of the patient, and it goes

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down because the radiation dose either is transmitted, or

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scattered, or absorbed. By the time it comes out,

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this is the signal we use for image reconstruction.

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So the surface dose is maximum here on this one.

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In the CT, what we see is, since we are acquiring the

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data around the patient, we see the surface

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dose as the maximum and the central dose is

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much lesser. That's why when we are seeing some very obese patient

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images, you see lot of graininess in the center,

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and that's because of the less photon are contributing to the

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image, and we have a term called photon starvation

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artifacts.

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So

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typically, the radiation dose distribution is as

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follows. In a head phantom, or the object

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is small, the surface dose measured is same

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as the central dose measured

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because the object is small.

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On the other end, when the object is large, such as in body phantom,

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we see the surface dose is almost

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twice that of the center phantom.

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And this is important for us because for how we estimate the dose.

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Because we want to make sure how the dose is distributed in the phantom or the

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object or a patient, and thereby we can estimate the patient

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risk and so forth.