Principles of CT

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Chapter 56 Principles of CT


Hardi Madani, John Curtis and Helen Marmery




Basics




X-rays are used to image thin transverse sections of the body (axial 1–10 mm thick slices).



Slices are stacked on top of one another to produce series.



Each tissue has a Hounsfield unit (HU) value, which represents the density of an object and ranges from –1000 to +1000.



Objects can be described as low (black) or high (white) density:




Air is low density, with an HU value of –1000, and appears black.



Water is intermediate in density, with an HU value of 0, and appears grey.



Bone is high density, with an HU value of +1000, and appears white.



Soft tissues and organs have a range of values and are different shades of grey (see table).



Tissue types: densities and CT numbers (Hounsfield units)







































Tissue Density/colour CT number (HU)
Bone White (high density) +1000
Stones White +100 to +900 or more
Fresh blood Light grey/white +60 to +80
Liver Grey +40 to +60
Water Grey (intermediate density) 0
Fat Murky grey/black –50 to –100
Air Black (low density) –1000

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Feb 18, 2017 | Posted by in GENERAL SURGERY | Comments Off on Principles of CT

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