It can be useful to occasionally readdress things that are common place in CT scanning, like slice thickness and interval. Although we use it everyday in our clinical practice, it can be easy to forget the basic principles behind it.
Slice Thickness
First off, I want to distinguish between two "types" of slice thickness that are used interchangeably:
1) Detector "Slice Thickness" - this can be used to describe the size of the individual components of the detector array, and correlates to thickness of the thin slice series. That is, if a scan was acquired using a detector slice thickness of 1.5 mm, then you can not produce an image with voxel size less than 1.5 mm along the z-axis. Most modern multi-detector scanners these days automatically acquire data on the thinnest slice possible, which is often < 1 mm.
2) Reconstruction Slice Thickness - this determines the voxel depth of your multiplanar reconstructions (MPRs). That is, how much data is included in a single slice. This can be changed to virtually any number in post processing.
Slice thickness ultimately determines the trade-off in image quality between spatial resolution (how clearly you can differentiate small changes in the image) and image noise (the standard deviation of the image):
Increase Slice Thickness = Decrease Spatial Resolution and Image Noise
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The slice thickness you choose for your MPRs depends on what you are trying to show. For a scan of the abdomen we are looking at relatively large soft tissue objects, so we prefer less image noise and can usually accept a drop in spatial resolution as the trade-off. On the other hand, when looking at the PTBs we need to visualise tiny structures so need higher spatial resolution, and because we are mostly looking at bone and air, the image noise can increase without degrading the image quality too much.
Have a look at the video below for my analogy on slice thickness:
Slice Interval
This is the distance between the centre of two adjacent slices, and it ultimately determines the number of images in a series. You can select one of three slice interval settings:
1) Contiguous: Interval = Thickness. So where one slice ends, the next one starts.
2) Non-Contiguous: Interval > Thickness. So some areas of anatomy will be missed between two adjacent slices. This creates less images in the series.
3) Overlapped: Interval < Thickness. So some areas of anatomy will be shown in two adjacent slices. This creates more images in the series.
This video shows what an axial lung MPR would look like using each interval setting:
The majority of your MPRs will (or should) be contiguous, because we are not missing areas of anatomy like non-contiguous, and we are not creating more images than necessary like overlapped.
Overlapped images do have their place when you need to track small or tortuous objects, such as bronchi or bowel loops.
Non-contiguous images should not really be used these days because unless the actual scan has been acquired using Axial non-contiguous slices to save dose (such as an HRCT Chest Expiration series).
So there you have it, slice thickness and interval explained in 5 minutes!
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