Radiographic imaging

Question 1

radiographic imaging definition

Answer 1

externally produced radiation passing though tissues → used to generate images
- x-ray source = electromagnetic waves → penetrate body

Question 2

X-ray tube construction

Answer 2

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

Question 3

Brehmsstrahlung

Answer 3

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)

Question 4

W.K. Röntgen first X-ray picture

Answer 4

high voltage generated by coil applied across vacuum tube → electrons accelerated inside the tube → emitted photons pass though body → visualized on fluorescent screen

Question 5

photon emission function

Answer 5

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

Question 6

photon emission principle

Answer 6

source → (collimator) → object → (collimator) → detector

Question 7

effect of scattering

Answer 7

to avoid scattered photons one can use a collimator → allows photons only through a hole
- scattering disminishes resolution

Question 8

linear attenuation - Beer Lambert Law

Answer 8

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

Question 9

pQCT

Answer 9

peripheral quantitative computer tomography

- use relatively low energy to penetrate object → high conrast between different tissues

Question 10

QCT

Answer 10

quantitative computer tomography → used for clinical assessment e.g. abdominal imaging → low contrast between muscle and fat

Question 11

x-ray radiography definition

- advantages/disadvantages

Answer 11

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

Question 12

dual energy system

Answer 12

combine low kVp-image with high kVp-image → get a better soft tissue image

Question 13

computed tomography measurement

Answer 13

• CT creates images with depth information
• 2D image is reconstructed from it’s 1D projections
  → slice sensitive, recorded digitaly & reconstructed on computer

Question 14

CT projections and samples

Answer 14

• 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

Question 15

CT scanner types gen. 1-4

Answer 15

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

Question 16

CT scanner type gen. 5 & special systems

Answer 16

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

Question 17

CT reconstruction types

Answer 17

• 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

Question 18

direct reconstruction of CT image

Answer 18

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

Question 19

Image representation in CT

Answer 19

→ 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

Question 20

Radon’s formula / Sinogram

Answer 20

• 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