acquisition 2 Flashcards
what is the linear coefficient?
Describes the fraction of attenuated incident photons in a mono-energetic beam (per thickness of a material)
increasing atomic no. and increasing the physical density of the absorbing material increases the photon energy except at k-edges
what are the possible interactions included in the linear coefficient?
coherent scatter
Compton scatter
photoelectric effect
What is the image reconstruction process?
pre-processing and reformatting raw data
convolution = mathematical distortion of raw data
back projection - basic
FBP- filtered back projection
iterative reconstruction - much clearer - good resolution
what is convolution?
fills in gaps of missing data
1 scan view (body section) - contains 2 objects
essentially digital filter applied to introduce negative terms to suppress extra voxel values (make the image look blurry/indistinct) that make up image cage.
data provided is not a complete image so back project onto image surface and shadows in missing data.
image beam has to be rotated around the body to obtain another view
project this view onto other images
see the beginning of an image
Two views don’t give high enough quality/resolution - so several hundred are used to produce a clinical image
Convolution is a mathematical concept that implies the product of two functions. In practical terms for radiology, convolution implies the application of a mathematical operation to a signal such that a different signal is produced. Convolutions are applied in image processing for CTs and MRIs.
what is convolution?
fills in gaps of missing data
1 scan view (body section) - contains 2 objects
essentially digital filter applied to introduce negative terms to suppress extra voxel values (make the image look blurry/indistinct) that make up image cage.
data provided is not a complete image so back project onto image surface and shadows in missing data.
image beam has to be rotated around the body to obtain another view
project this view onto other images
see the beginning of an image
Two views don’t give high enough quality/resolution - so several hundred are used to produce a clinical image
(Convolution is a mathematical concept that implies the product of two functions. In practical terms for radiology, convolution implies the application of a mathematical operation to a signal such that a different signal is produced.)
what is Fourier trandformation?
it is a primary mathematical model in the creation of computerised medical images
conversion of data into more useful/recognisable forms - using mathematical algorithms
CT specific:
created using different image densities and enhancing them to create recognisable ray scale - hounsfield units
What is the matrix?
no. of voxels we can have - 512 (rows) x 512 (pixels) per voxel
edges ill-defined goes through filter and then edges are clear (can see curves)
what is kernel?
what does it do?
Kernel is a convolution algorithm:
the process used to modify the frequency contents of projection data prior to back projection during image reconstruction in a CT scanner
corrects the image - blurring and sharpens image
What is incident intensity?
intensity of the x-ray beam striking the patient
and it is proportional to the tube output (mAs)
Beam attenuation?
beam is attenuated by the patient - depends on the tissues density
same as conventional X-RAY
How is attenuation measured?
Attenuated radiation is measured by detectors situated opposite the X-ray tube
What are essential properties of CT detectors?
high detection efficiency - absorbing all the emergent x-ray beams to which they’re exposed
high conversion energy - enabling all the detected energy to contribute to image formation
wide dynamic range - so that all x-ray intensities - weakest to strongest - are converted into proportional output signals
What is attenuation coefficient?
measure of how easily a material can be penetrated by an incident energy beam
quantifies how much the beam is weakened by material
what is acquisition?
spiral
process in which a single continuous set of spiral scan data is acquired without an intervening pause
what is acquisition?
spiral
process in which a single continuous set of spiral scan data is acquired without an intervening pause
what is interpolation ?
modification of scan data before reconstruction in order to synthesise a set of data that yields a clinically acceptable representation of cross section of anatomy
what is slip ring technology?
allows high speed scanning with good temporal resolution and increased gantry speeds - 0.33 secs
desirable to achieve cardiac imaging without using beta blockers to slow patients heart rate
Collimation in CT:
Double collimated
at tube exit - before patient - controls patients dose
post patient at detector entrance - ensures beam matches width
controls slice thickness - detector eliminates scatter which would degrade the image
1.26mm - 80cm variable potential
controlled by software programme
Describe the mathematical calculation:
Done by computer
the resultant data is worked out by subtracting the attenuated signal (from the detector) from the incident beam intensity signal
The CT collects hundreds of readings from the detectors and this effectively divides the body into a criss-cross matrix of separate very small tissue volumes (voxels)
What is the basic image construction?
Pre-processing - raw data is produced by processing scan data
convolution - provides sharp/smooth images + enhances appearance of tissue of interest
back projection - complicated process in which the convolved scan data is projected back into the image matrix (clearer + sharper)
Pre-processing :
raw data - values of all measured detector signals during a scan
the attenuation and correlated x-ray position - from tis ct images are reconstructed
A preprocessing step is implemented in order to reduce the noise in CT images. The goal is to increase the signal to noise ratio (SNR) as well as to increase the contrast of the structures of interest,
Mathematical procedures:
convolution
back projection
iterative projection
1st stage - undertake a series of dose measurements
for each measurement the computer has to know the position of the tube and detector
hence the voxels the measured the beam passed through
what is the process of back projection?
attenuation value is averaged over each pixel (voxel)
beams are projected in two directions (voxel)
as each beam passes through the object + a summed linear coefficient is obtained
assumed linear coefficient is spread evenly across each beam path
each table of calculated linear attenuation coefficients are added
resulting table provides the estimates of each pixels original linear attenuation coefficient
computer starts to work out and give us information
what is the pipeline pr
pre-processing - convolution - back projection
too few projections causes artefacts
typically need about 2000 projections
blurring where back projection lacks - filtered back projection improves
what is simple back projection?
reconstruction of an image by taking each view + smearing/guessing it along the path it was originally acquired on
sum of all projected views
leading to blurred images
what is filtrated back projection?
each view is filtered before the back projection to counteract the blurring
each 1 dimensional view is convolved with a 1 dimensional filter kernel - set of filtered views
much clearer views
what is iterative reconstruction?
Iterative reconstruction refers to an image reconstruction algorithm used in CT that begins with an image assumption, and compares it to real time measured values while making constant adjustments until the two are in agreement.
algothrimic method which uses statistical info and geometric models with the raw data to process + reduce noise
what is display window width and window level?
The window width is the range of the grayscale that can be displayed.
The center of grayscale range is referred to as the window level. - should be roughly the same as hounsfield units
what is windowing?
windowing is the process in which the image grayscale components of an image is manipulated via the numbers - hounsfield units
to change the appearance of the picture to highlight particular structures
adjustment of window level?
window level - brightness
range of CT no. hounsfield units that image contains:
wider - the more shades of grey
- contrast will be poor
narrow (better) - 50-30 hounsfield
wide - 2000 hounsfield
adjustment of window?
window width - contrast
lower level - image is brighter
higher level - image is darker
hounsfield unit - tissues:
bone - +400 -+1000
soft tissue - +40 - +80
water - 0
fat - -60 - -100
lung - -400 - -600
air - -1000
