Question

1. What do you understand from helical CT? Discuss. What are the advantages of helical CT over the step and shoot sequential CT?
2. What is the purpose of antiscatter grid in a CT instrument? Discuss its use in CT.
3. What is filter back projection and how is it used in CT?

Answer 1

1)The Helical CT scans were introduces in the late 1990's. This was a better technique when compared to the prexisting techniques present at that time. With helical CT, the patient is moved through a rotating x-ray beam and detector set. From the perspective of the patient, the x-ray beam from the CT traces a helical path from which the name Helical CT arises . The helical path results in a three-dimensional data set, which can then be reconstructed into sequential images for a stack.

2) Advantages Of Helical CT over Step and Shoot Sequential CT:

a)Shorter examination time which decreases image degradation even in infants and young children in Helical CT

b) The entire imaging is taken in a single slice in Helical CT unlike the step and shoot sequential CT which requires multiple slices.

c) The Capability for retrospective image reconstruction is available only in Helical CT.

d) Improved Multiplanar reconstructions in Helical CT.

3) Purpose of Anti-Scatter Grid and its use in CT:

  The antiscatter grid or the Scatter Removal Grid plays an important role for enhancing image quality in projection radiography by transmitting a majority of primary radiation and selectively rejecting scattered radiation.By selectively allowing primary x-rays to be transmitted and scattered x-rays to be absorbed in the grid, image contrast is significantly enhanced; however, the grid attenuates some of the desired primary x-rays that are incident directly on the lead strips and allows transmission of some scattered radiation photons that have a small scattering angle, or scatter in a direction parallel to the lead strips, or are multiply scattered with an exit angle from the patient that can be transmitted through the grid

4) Filtered back projection is an analytic reconstruction algorithm designed to overcome the limitations of conventional back projection; it applies a convolution filter to remove blurring. This involves "smearing back" the projection across the image at the angle it was acquired. By smearing back all of the projections, you reconstruct an image

Its Uses in CT:

The Real CT contains noise which causes blurring in the imaging system. In order to get around this we use certain filters which are soft and this enables Filtered Back Projection where the noise signals are attenuated.

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