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Filtered Back Projection (FBP)
FBP uses 250,000 + simultaneous equations of ray sums taken at differing angles of a sine wave to compute the values of attenuation coefficients within a cross section
As the name suggests, it is composed of two main steps
filtering the data (along the row direction in the detector)
performing the back projection operation where the data is painted back in the image along the direction which it was measured (9)
Iterative Reconstruction (IR)
The scanner will use a portion of measued projection data and then uses mathematics to fill in the blank data sets with accuracy.
The level of IR used is determined by the ADMIRE setting on the force scanner. Our protocols have been optimised with appropiate ADMIRE levels for the particular scan
Increased IR= less dose and smoother image.
Lower IR= higher dose with noisier image. (1)
Our scanner uses a combination of FBP and IR to make our images
Hounsfield units (HU)
Greyscale seen on CT images is described in HU.
Air= - 1000HU
Fat= -60 to -120HU
Water = 0HU
Bone = +1000HU (6)
Slice thickness and intervals
Slice thickness: Width of z-axis data displayed in slice
Thin slices: Noisier, higher spatial resolution, more images to review (above image)
Thick slices: Less noise, lower spatial resolution, less images to review (below image)
Slice intervals: Distance of z-axis between slices
eg 3mm_3mm axial reconstructions. 3mm of data displayed in slice and 3mm gap between slices.
Slice thickness can only be made as thin as scan was acquired.
Windows and Kernals
The windows and kernals determine how you want your data set to be displayed. The achieve qualty recons you need both these vaules to be working together.
Kernels: Filters your data prior to performing back projection. The kernel is a numerical value. Higher the number the sharper the image
Window: Describes the grey scale mapping we are aiming to achieve in our image. We have premade windows in CT designed for particular regions. Each window has a designated range of HU included (window width (WW)) and centre HU value (window centre (WC)).
Wider WW displays a wider range of HU. It is useful when you are looking at regions with large differences in HU such as chests (air in lungs, soft tissue and bones)
Smaller WW is better to to see more detail within areas that have similar HU structures such as the abdomen
Top Image: Kernel Br40, Window is Abdomen (WW, WC)
Bottom Image: Kernel is Br64, Window is Bone (WW, WC)
(2, 3)
MIPs
Maximum Intensity Projection
Used for angiograms and oncology chest imaging
MinIPs
Minimum Intensity Projection
We mainly use these in Chest CT to better evaluate air trapping and the airway
VRT
Volume Rendered Techniques or 3D
Made off soft tissue thin slices
References
Bushberg, Jerrold T., et al. The Essential Physics of Medical Imaging. 3rd ed., Lippincott Williams and Wilkins, 2011.
https://radiopaedia.org/articles/kernel-image-reconstruction-for-ct-1
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