CT and dual energy CT - Reconstruction

12 important questions on CT and dual energy CT - Reconstruction

Relation between attenuation and intensity and image brightness?

High attenuation means low intensity.
High attenuation is bright image.

What is the difference between a monochromatic and polychromatic x-ray beam?

The gradual decrease in photons that are left because of different tissue is less visible.

How is an image reconstruction made?

You get a signal (you have an attenuation value) and you spread that value (evenly) over the boxes of your picture. You project them back along the same path. So each projection of each path gives information about the source.
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Why is a picture blurred after back projection?

Because you spread out the signal evenly, you will get a blurry image because of the oversampling at the center and less sampling at the edge.

What does a sharpening filter give you?  

- High spatial resolution. How accurately the measured activity is localized in the body.
- Sharp edges
- More noise

What does a smoothing filter do?

- Less noise; afwijkingen weghalen (better contrast resolution)
- Lower spatial resolution
- Low edge details.

What is iterative reconstruction?

1. Estimate the image, bases on raw data
2. Forward projection; based on the estimate picture
3. Comparison of back projection and the from the forward projection
4. Reconstruct estimate. And repeat the process.

What is the reference of Hounsfield Units?

Water.
attenuation coefficient of intest - u of H20 / u H2O X 1000

When does filtering happen in the reconstruction process?

  • It happens with the raw data.

When do you use which window? Narrow or wide?

When a tissue has a lot of different densities. You can use a wide window with all kinds from white to black. A little change in HU => little change in brightness.

When a tissue has not that much of differences. You can use a narrow window. You will have increase contrast. A little change in HU => high change in brightness.

So what kind of window do you chose for bone or for abdomen?

Bone => high difference in density => wide window
Abdomen => small differences => small window

What happens to the pixels with a value outside the window?

Black or white. So the tissue of contrast will be more visible.

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