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Here is a description or difference between the detector types.

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On the left-hand side is the energy-integrating detector

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utilized in all the CT scanners from all the vendors.

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So the way it works is the X-ray photons, when it

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hits the detector, it first interacts with a

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scintillating material,

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which converts the X-rays into light photons, and the

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light photons then interact with the electronics of the

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detector, creating electrical pulses or electrical signals.

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On the other hand, with the photon-counting detector, the

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material, semiconductor material, directly can

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convert the X-ray photon into electrical signal,

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and these signals are proportional to the energy of the X-ray

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photon. Therefore, the number of X-ray pulses are produced,

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electrical signals are produced, and that's counted, and

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that's what the distinction between the photon-counting detector and

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energy-integrating detector.

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So in the energy-integrating detector, it utilizes a

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scintillating material to convert the X-rays to light

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photon,

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which are then absorbed by photodiodes to create

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electrical electronic signals. A photon-counting detector

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converts X-rays directly to electrical signal.

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That is the important difference.

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The detector is a single thick layer of a semiconductor

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diode. The incident X-rays directly absorbed in the

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diode creates positive and negative charges that are

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then pulled out by the thin film transistor, creating an image.

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So now,

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what is the characteristic of a photon-counting detector?

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As you can see here,

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each pulse created by each X-ray interaction creates

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will result in a detector signal proportional to the

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strength of the X-ray photon energy.

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Thereby, you can see here different pulses heights are produced.

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By having buckets or bins to only

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count signals of a certain energy range, you

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cannot count everything else. Thereby, you can have uniquely

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create these photon-counting buckets.

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So each photon generates electrical pulses with

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height proportional to the energy deposited.

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The detector counts numbers of pulses with heights that

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exceeds a threshold dose. For example, it only counts this pulse.

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Now, it does not count about other one.

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You can actually set a threshold

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level higher than the electronic noise.

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Therefore, this electronic noise can be completely avoided.

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That's one of the major advantage because this will allow to

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acquire images at a lower radiation dose, because

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with the energy-integrating detector, if the radiation dose is too low,

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the signal produced almost merges with the electronic noise,

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thereby degrades the image quality.

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Here is now that you can subtract the electronic noise, you can acquire the

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image at the low signal, and still you can salvage the image quality.

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That is the main advantage of a photon-counting detector.

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Here's an example of an image panel showing on the left-hand side is

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the photon-counting detector. On the right-hand side is the

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obtained with energy-integrating detector.

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One can maintain the same radiation level.

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The noise can be reduced further down with the photon count detector,

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or one can accept the noise increase.

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The radiation dose can be decreased from 23 milligray to

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14.6 milligray in these cases without

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impacting the overall diagnosis, and that is

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supposed to be one of the major advantage of the photon-counting detector.

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The other advantage is the spatial resolution.

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Because now you can design a detector with a very

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small size, you can actually improve the spatial

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resolution quite dramatically.

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Since photon-counting detector do not have separate scintillating material

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elements and septa, hence manufactured can be very small

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element, can go almost from 0.28 millimeter to

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0.07 millimeter. There are some limitations which I'm not going

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to talk about. There are limitations such as charge shedding and crosstalk.

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But I want to show you here is the spatial resolution in the upper part of

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the lung lobe obtained with the energy-integrating detector

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versus the photon-counting detector clearly shows

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dramatic improvement in the spatial resolution.

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Let's examine the clinical benefit of the photon-counting

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detector in these four panels. Here are four different

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panels, and the top images are obtained with the

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photon-counting detector,

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and the bottom one is obtained by the energy-integrating detector.

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This is a look to see how the spatial resolution is

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improved with the photon-counting detector.

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This is an image of the front arm. You can see the trabecular

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structure very clearly compared to the images obtained with the

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energy detector, which is now improved, introducing

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new application to be done with the photon-counting detector.

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One of the other major advantage of the photon-counting detector is the

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reduction of the electronic noise and metal artifact.

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For example, in this particular energy detector, even though the

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metal artifact is quite dramatically lower, but you can still see

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streaks coming out of the pedicle screw in the lumbar spine.

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That is all eliminated with the photon-counting detector because you

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can individually bin these energies and reconstruct.

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Here is an example of how the iodine contrast image is

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improved with the photon-counting detector compared to the

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energy-integrating detector. More so, the radiation

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dose efficiency is clearly shown with the

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photon-counting detector.