CT physics 4 Flashcards
what is the modern scanner design
a continuously rotating fan beam - third gen scanner design
what are 3rd gen CT scanners
fixed X-ray source and curved array of detectors which reduces the mechanical complexity and improves image quality
- improves intrinsic collimation so stops x-rays being detected by incorrect detector channel
typical scan time for 3rd gen scanner
scan time (rotation time) reduced to less than one second per 360 degree rotation
what does the hounsfield unit represent
the density of different tissues in the body
- also used to assess contrast in CT scan
what happens when an image in reconstructed in axial slice
made up in matrix of pixels each assigned a HU value which is determined by attenuation of various tissues relative to the attenuation of waterw
what is the HU values given to tissues in the body relative to?
the attenuation of water
what HU value is water given
0 HU
what HU value is air given
-1000 HU
average attenuation value of a bone
average value of 1000 HU
- tissues attenuate more than water
filtered back projection
- images are ‘filtered’ to enhance certain features and remove noise (improve quality)
- back projection - images are ‘back projected’ onto single image involving taking the filtered images and projecting them back into a common space - stacked together to make a 3D image
what is iterative reconstruction
- used to create clearer and more detailed images
- works more slowly
- starts with initial estimate of image then iteratively refines that estimate to match acquired data
- helps reduce noise, errors, and improve image quality of final picture
advantages of IR
- noise reduction without degraded resolution
- improved spatial resolution + artefact reduction
- dose reduction + improved resolution
- lower mA or kV compared to when using filtered back projection
disadvantages of IR
- high computation demands - slower reconstruction technique
- clinicians reported waxy nature to images - normally used to seeing certain amount of noise in CT images
what happens in a helical CT scan
x-ray source and detectors continuously rotate around patient while patient is moved around the scanner creating a path of data acquisition
- no defined slice
what kind of attenuation data does 360 degree interpolation use
uses a combination of attenuation data from multiple angles around the object being scanned
- raw projection data
- sinogram data
- filtered data
- interpolated data
how many rotations is data required from
2 FULL rotations
- makes image width broader causing z-axis blurring
180 degree linear interpolation simplified
- two data points and having to estimate a value at a point between them (point C)
- linear interpolation means finding value of point C
what can interpolation be used for
- smooth transitions between pixels can help reduce jagged edges + improve visual quality of images
- can be used to estimate pixel values at new locations
what does slice profiles in CT refer to
- used to create a cross-sectional image of patients anatomy
when would you use overlapping reconstructed axial image slices in helical CT
- reduces stair step artifact and improves MPRs and 3D image reconstructions
- depending on acquiring of volume of data you can choose
what does z-axis collimation determine
acquired slice thickness during each x-ray tube / detector rotation
what does an increase in pitch lead to
reduction in z-axis resolution
advantages of helical CT
- faster scan times due to acquiring volume of data instead of single slice
- better spatial + contrast resolution
- motion artefacts are reduced
disadvantages of helical CT
- increased radiation dose
- motion artefacts
- complexity and cost
single slice interpolation
2 nearest values used
multislice scanners
- more data points available
- use all measurements within variable filter width for interpolation
