Intro to X-ray Imaging

Question 1

What makes a high quality image?

Answer 1

Good spatial resolution

Good signal to noise ratio

Question 2

What is geometric magnification?

Answer 2

m = image size / object size
m = (S1 + S2) / S1

S1 = distance from source to object
S2 = distance from object to detector

Question 3

What is the typical beam of source?

Answer 3

Cone beam

Question 4

What does focal spot size cause?

Answer 4

Penumbra: blurring (spatial resolution) due to finite focal spot

The image is no longer localised in umbra due to the magnification

Larger the focal spot, greater the penumbra

p = S2/S1 F

F = real focal spot

Question 5

What effects the size of penumbra?

Answer 5

Distances (geometric magnification)
Focal spot size

Question 6

How is the focal spot measured?

Answer 6

By using a pinhole (in a slab of metal) which is places as close to source as possible

An image is taken and the focal spot is measured in the detector plane

This requires extra calculations to find real focal spot:

real focal spot = measured focal spot times S1/S2

Question 7

Why is filtering used?

Answer 7

As the lower energy photons get absorbed and deposit dose but aren’t able to be detected so need to get rid of them

High energy photons don’t attenuate as easily due to their small wavelength

Question 8

Which beam requires more filteration?

Answer 8

more energetic beam

Question 9

What metrics are compared in detectors?

Answer 9

Efficiency
Spatial resolution
Noise
Dynamic range

Question 10

Which factors determine unsharpness in an image?

Answer 10

Source focal spot size (F)
Intrinsic detector resolution (D)

They form a total sharpness, determined by system magnification:

M = (S1 + S2) /S1

Question 11

How is the intrinsic detector resolution approximated?

Answer 11

D = FMWH

Question 12

What is the best case scenario and more realistic scenario for detector resolution?

Answer 12

Best case scenario: signal confined within pixels (FWHM = pixel size)

More realistic scenario: signal spread across pixels due to light scattering (FWHM > pixel size)

(detector resolution is not same as pixel size)

Question 13

What happens when an object is moved towards source?

Answer 13

magnification increases, the detector pixel are projected onto the object plane i.e. demagnifying it

This leads to a higher detector resolution (D’ = D/M)

Question 14

What is negative effect of moving the object towards source?

Answer 14

This leads to a larger source blur at the detector

(F’ = (S1/S2) F)

Question 15

What are the two counteracting effects of unsharpness?

Answer 15

When the object is placed in contact with detector, the blur due to the source is zero (S2 = 0) but that due to the detector is maximum
U_d = D/M

When object is moved away from detector, the blur due to the detector decreases but that due to the source increases
U_s = F’ / M

Need to find the optimal distance i.e. optimal magnification

Question 16

What should be done if the FWHM of the detector blur is much smaller than the source focal spot?

Answer 16

Place your object close to the detector

Question 17

What should be done if the source focal spot is negligibly small compared to the detector resolution

Answer 17

Use a large magnification

Question 18

What type of distribution are both effects?

Answer 18

Gaussian blurring
(focal spot spreads in Gaussian manner)

Question 19

What is the total unsharpness equation?

Answer 19

U = √(U_s)^2 + (U_d)^2

which turns into:

U = √ ((M - 1 / M) F)^2 + (D/M)^2

U needs to be maximised