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There are two types of acquisition in cardiac CT.

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One is called the retrospective ECG gating, and this is

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how we started when we started cardiac CT in the

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beginning. The principle behind here is like,

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the scanner will continuously acquire data around the heart

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when the patient is transported through the heart while acquiring the data.

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Imagine you are using a 64-slice scanner, which

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means the maximum X-ray beam width is about 40

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millimeters.

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One rotation around the heart does not suffice to cover the

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entire heart, so it has to rotate about three to five times

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around the heart to cover the entire heart volume.

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By doing so, in the retrospective ECG gating,

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the scanner will automatically match with the ECG of the

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patient, and when the scanner is started, the

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patient is transported through the gantry while you're

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continuously acquiring the data.

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This yellow portion is called the patient radiation dose exposure

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into the entire area.

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Afterwards, we only reconstruct the image in the diastolic

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area, not in the systolic area, because in the systolic

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area, the perturbation is so fast you won't be able to get

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any useful information. Therefore, you reconstruct

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part of the image, which is required to reconstruct the image in this

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portion, and throw everything else.

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That's where this impact of retrospective ECG gating

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resulting in a higher patient dose.

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So the radiation dose is typically higher than prospective triggering.

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When do we do that? For example, this is a 64-slice

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scanner,

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which is about 40-millimeter beam width.

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You need to cover the entire heart through multiple rotations,

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means multiple heartbeats at every time you are doing

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it, and you're moving to the next position of the bed to acquire the next

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image information as you continuously acquire.

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You want to make sure the data available for reconstruction,

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there's no gap between one adjacent to the next heartbeat.

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That's why you tend to overlap the anatomy to make

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sure there is redundant data for image reconstruction.

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Because of the overlapping, that is what is called the very low

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pitch, the radiation dose in the retrospective ECG gating can

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be high.

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So the way it works is cardiac CT, typically

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with retrospective gating, uses a pitch value of 0.2 to

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0.4,

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which means from one heartbeat to another heartbeat, as you are

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acquiring the data, the data you want is acquired in such a way

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there is no gap between the data of the rotation.

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So we always tend to overlap it. Because of that, the

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radiation dose is inversely proportional to pitch.

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Therefore, the lower the pitch value means the higher the

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retrospective ECG-gated radiation dose to the patient.

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Immediately after the development of retrospective ECG

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gating, we started using what is called a prospective ECG triggering

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method,

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in which instead of acquiring throughout the heart cycle and through the

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multiple heart cycles, the scanner was advised to

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turn on at a predetermined position between

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the R to R peak.

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And then the table is moved to the next position, wait for a present

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heartbeat, again acquire, and you keep doing it

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until you cover the entire heart,

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which is what's shown here. You can see here a couple of heartbeats were skipped,

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but you are acquiring data such that the volume of the

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data acquired does not have any gap from the past in the

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previous position. You slightly tend to overlap, but not

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as much as retrospective. Plus, you don't expose the heart

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throughout the heart cycle, you only expose the heart only at a certain

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point of the heart cycle.

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This particular study basically demonstrates the difference between the

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helical method, which is called the retrospective gated

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technique,

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versus prospective triggering method.

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You can see here this is a prospective method where you

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have the radiation dose to the patient almost

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one-fourth of that of the retrospective gated.

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But the scientist authors have shown that there is no difference in the image

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quality between the two acquisitions, but the patient dose was

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remarkably lower in the sequential method compared to the helical method,

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and that's the prospective method.