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The next part of understanding cardiac CT physics is to

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examine the advanced CT technologies and

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procedures.

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Especially, we're going to talk about perfusion CT

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and spectral CT. Spectral CT is a broad term

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that also includes dual energy CT and photon counting CT.

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All these advanced CT methodology became

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possible only with multi-detector CT.

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The physics of CT perfusion can be looked at under the following

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four questions:

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What is CT perfusion? How is CT perfusion

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done? Applications for CT perfusion,

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and how to do CT perfusion safely,

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especially from the radiation point of view.

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So CT perfusion is an imaging procedure that involves

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functional evaluation of tissue vascularity.

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This actually is an advancement of the CT

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technology, how we can use to

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evaluate functional aspect of tissue vascularity.

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And the perfusion CT is based on temporal

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changes through tissue attenuation after

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intravenous administration of iodinated contrast material.

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In fact, this provides us a way to see the vascularity

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changes, and one of the most commonly done application

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is brain perfusion CT.

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So how is it done? The way it's done is after

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injecting iodine contrast material,

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a series of CT scans are done over the same region,

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which means you repeat the acquisition of volume of interest

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during first pass of contrast material is obtained,

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which usually can last one to two minutes.

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Then you follow the same acquisition at the same location of

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delayed phase. That can last from anywhere from two to ten

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minutes.

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Here is an example of a typical brain

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perfusion CT acquisition.

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As you can see here on the graph, on the

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x-axis is the time after contrast administration,

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and the y-axis is the tube current utilized for these type of studies.

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The very first one is called the masking acquisition.

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We want prior to the administration of the contrast.

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So that is used to subtract every other image to

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get the difference in the vascularity of the contrast agent

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moving through the area.

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So for example, here, the white is considered as a masking

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acquisition. After 11 second, you administer the

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contrast material, and you have a series of acquisition,

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one after the other, shown here in a red boxes.

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We call it as early arterial phase.

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And then you have the yellow boxes done at the peak arterial phase,

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followed by a late arterial phase, and then after

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40 second, you can acquire couple more to acquire the venous phase.

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So one might wonder, how many scans are we doing?

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There is a series of scans done because only by doing this,

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you can actually get a temporal part of the

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contrast agent moving through a particular area.

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But that also brings the consideration of what is the radiation

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dose, what it can do for any of these deterministic

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effect. I'm going to talk about that next.