Radiographic imaging Flashcards
radiographic imaging definition
externally produced radiation passing though tissues → used to generate images
- x-ray source = electromagnetic waves → penetrate body
X-ray tube construction
electrons are accelerated by high voltage → heat filament (cathode) emits electrons by thermionic emission → x-rays are produced when high speed electrons hit the metal target
Brehmsstrahlung
continous radiation profile
→ electron energy and wavelight dependent
- voltage dependent photon peak at different wavelenghts
- low energy electrons: more contrast → difficult to produce
- high energy electrons: more intensity (more photons are emitted)
W.K. Röntgen first X-ray picture
high voltage generated by coil applied across vacuum tube → electrons accelerated inside the tube → emitted photons pass though body → visualized on fluorescent screen
photon emission function
photon emission: photon shot at the object
→ object absorbs some of the photons & some get scattered (poor spatial resolution) and some get transmitted
→ absorption depends on tissue
photon emission principle
source → (collimator) → object → (collimator) → detector
effect of scattering
to avoid scattered photons one can use a collimator → allows photons only through a hole
- scattering disminishes resolution
linear attenuation - Beer Lambert Law
problem: x-ray is dangerous bc of ionization of the tissue
→ different tissues have stronger attenuation (Bone> muscle > fat → related to density)
- photoelectric effect: interaction with electron & core → local ionization → important up to ~0.1 MeV
- compton scattering: producing ionized photon + lower energy scattered photon
pQCT
peripheral quantitative computer tomography
- use relatively low energy to penetrate object → high conrast between different tissues
QCT
quantitative computer tomography → used for clinical assessment e.g. abdominal imaging → low contrast between muscle and fat
x-ray radiography definition
- advantages/disadvantages
radiography relies upon differential attenuation of x-rays → more absorption = brighter
- high contrast btw soft tissue/air (lungs) & hard tissues (bones) → low radiation dose
(+) high resolution image, rapid aquisition, low dose
(-) no depth information, material wiht similar density difficult to distinguish
dual energy system
combine low kVp-image with high kVp-image → get a better soft tissue image
computed tomography measurement
- CT creates images with depth information
- 2D image is reconstructed from it’s 1D projections
→ slice sensitive, recorded digitaly & reconstructed on computer
CT projections and samples
- sample P(t) of a projection defines the integrated attenuation along the path of the x-ray beam
- set of samples taken under defined angle describes a projection
- all projections recorded for 0°-180° describe the data set for the image reconstruction
CT scanner types gen. 1-4
1st generation: one channel, translation-rotation, 5min per slice
2nd generation: multi channel, translation-rotation, 20sec per slice
3rd generation: rotation of tube and detectors, 1-10s per slice
4th generation: rotation of tube ring detector, 1-10s per slice
CT scanner type gen. 5 & special systems
5th generation: targets on ring detector, rotatin electron beam, no mechanical movement, 200ms per slice
multislice system: based on 3rd generation, 64 detector rows, 0.4s per 64 slice
flatpanel system: cone beam geometry, 2D detector, milliseconds for cardiac applications
CT reconstruction types
- Inverse of Radon’s formula → does not work
- Iterative reconstruction (ART) → Image estimation; compute pseudo-projections; compare to original projection; minimize error
- direct reconstuction → back projection in image plane, summed up over all measured angles; resulting image strongly blurred
direct reconstruction of CT image
Filtered back-projection
- sampling density high near center
- filter applied while data is in frequency domain
Ramp (ram-lak) filter (high pass)
- supresses low spatial frequency components
- enhances boundaries btw areas of high & low density
- amplifies image noise
Shepp- logan filter (high-pass)
- least smoothing & highest spatial resolution
- for image reconstruction of the brain
Hann-Filter (band-pass/window)
- reduces image noise very quickly
- does not preserve edges
Image representation in CT
→ an image represents a matrix of linear attenuation coefficients
- Hounsfields Units (HU)
- image values from -1000 HU (air) to 3000 HU (compact bone)
→ no clear distinction of bone and soft tissue
Radon’s formula / Sinogram
- need to determine number of projection angles
- rule of thumb: number of projection angles should be about the same as the numbers of pixels across the object
