CT- section 2 Flashcards
what are the three distinct phases of a CT formation
1) data acquisition
2) image reconstruction
3) image display
what is data acquisition
‘scanning phase’- produces raw data but not the image
what is image reconstruction
processes the raw data converting it to image data
what is image display
the image data is displayed on a CRT monitor. image manipulation may take place at this stage
what happens during the scanning phase
a fan-shaped x-ray beam scans around the body, is attenuated by the patient anatomy and is detected by the detectors
what 3 things does attenuation depend on
1) density
2) atomic number
3) thickness of a structure
what is the linear attenuation coefficient
is the degree to which the x-ray intensity is reduced per thickness by the material it passes through
how often does the DAS sample the detected x-ray beam
at regular time intervals
DAS produces numerical measurements called
views
what does each view produce
a line of data in the form of a graph called an attenuation profile.
This means that the amount the x-ray beam has been attenuated along its path is now represented numerically in the form of a graph. known as RAW DATA
what happens during image reconstruction
the raw data is used to reconstruct an image in a process known as back projection
what are the attenuation profiles for a particular cross sectional slice of anatomy projected onto
a matrix
what is the one drawback of back projection
it causes streaks or artefacts on the image
how is streaks/artefacts made by back projection minimised
the data is filtered before back projection takes place
what is filtering
involves a number of complex mathematical steps, where algorithms are applied to raw data to remove the blurring seen in unfiltered back projection
the more views used…
the more accurate representation of the original
what is a matrix made up of
squares called pixels
the pixels are actually a cube of data known as
a voxel
each pixel/voxel will have a numerical value due to…
the back projection of the numerical data from the attenuation profiles produced during data acquisition
the numerical value of each voxel is expressed in
Hounsfield Units (HU)
what does the HU represent
the average density of the anatomical tissues within the voxel
what are the HU units for water, bone and air respectively
water = 0 HU bone = +1000 HU air = -1000 Hu
what does each HU value have
a corresponding shade of grey
once the HU’s are displayed on the matrix on the monitor what does it create
an image
what is this data then referred to as
image data
what is window width (WW)
determines the range of HU that will be represented on the image.
what does the window level select
the centre value of the window width
what is the benefit of windowing
gives CT a very high contrast resolution. this is because a window can be set to display and make visible very small differences in tissue densities
