CT Protocols - reasons for choice of slice-thickness and slice-spacing in PE (CTPA) protocols

Background:

The variability of chosen spacing between slices and slice thicknesses in PE (CTPA dedicated scans) is very large.

Most of the scans have slice spacing=slice thickness. However, a very large portion of the scans, have overlapping slices (spacing < thickness).

The most common I've seen, in several different institutions, are [Written below as (spacing, thickness) in mm]: (0.6 mm spacing, 1.2 mm thickness), (0.8, 1), (0.7, 1), (0.9, 1), (1.5, 3), (0.5, 1), (0.6, 0.8), (1,2), (2,2)

And there is a long tail of additional combinations: such as (0.3, 0.5), (0.3, 0.6), (0.8, 1.2), (1, 1), (1,1.5), and up to (2.5, 3), (3,3)

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Questions-

There are a few things that are bothering me the most about this:

1. For small findings such as PEs, why not always choose the thinnest and tightly spaced slices? Why ever go above (0.5,0.5)? Surprisingly (for me) all the common protocols include thicknesses that are >= 1 mm.

Actually some of the most common protocols I've seen (marked in bold above) use a thickness of 3mm, or even (2mm spacing, 2 mm thickness).

1. Many common protocols are using overlapping slices [when the spacing is smaller than the thickness, e.g. (0.6, 1.2)].

What is the reason for using overlapping slices?

What is the logic behind choosing the amount of overlap, for example - in (0.6,1.2) the overlap is very large, and in (0.8,1) the overlap is very small.

submitted by /u/RichMaxwell
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source https://www.reddit.com/r/Radiology/comments/hex6wz/ct_protocols_reasons_for_choice_of_slicethickness/