contrast

Question 1

attenuation factors

Answer 1

Z
density
thickness

Question 2

contrast

Answer 2

difference in attenuation between different materials

variation in detected intensity in image

measured in percentage

depends on detail (thickness and mu) and on energy through mu, but not the amount of xrays you use

Question 3

contrast vs energy

Answer 3

contrast decreases with increasing energy

(low energies, more absorption by patients)

attenuation coefficients decrease with increasing xray energy

Question 4

SNR

Answer 4

Rose criterion: a detail is visible if SNR>5

driven by amount of xrays you use

optimal balance given when optimised figure of merit

most common: signal VS background noise

Question 5

FoM

Answer 5

figure of merit
dose independent
helps determine optimal xray energy to visualise a given detail

Question 6

ways to improve contrast

Answer 6

remove scatter

increase inherent contrast:
contrast agent (highly attenuative substance that locally increases the mu of tissue)

reduce kV

remove effects of overlying tissue:
project onto plane

process images:
software improvements
digital subtraction
dual energy

Question 7

scatter removal methods

Answer 7

field size

air gap

grid

scanning slit

tissue thickness

Question 8

anti scatter grid and important parameters

Answer 8

only allows unscattered photons through
some primary radiation absorbed
increase exposure to compensate

grid ratio
selectivity (if 1, then no improvement)
contrast improvement K
Bucky factor

Question 9

antiscatter grid selection factors

Answer 9

contrast improvement
patient exposure
xray tube loading (voltage/energy of electrons)

Question 10

contrast agents

Answer 10

iodine (blood vessels)
barium (intestine wall)
air

Question 11

digital subtraction angiography (DSA)

Answer 11

take image before and after highly attenuating contrast agent delivered

subtract images

sometimes done in fluoroscopy for live mode

Question 12

dual energy imaging (k edge imaging)

Answer 12

both images taken after the contrast agent injected
but above and below k edge

(for iodine, 33keV)

Question 13

dual energy radiography

Answer 13

take radiographs are 2 very diff energies dominated by photoelectric effect (low) and compton scatter (high)

weighted difference image

(2 equations with 2 unknowns to separate components/distinct structures)