Image Quality

Question 1

What factors affect image quality?

Answer 1

(1) Noise
(2) Artifacts
(3) Distortion
(4) Spatial Resolution / Resolution
(5) Contrast

Question 2

Physical characterization of a digital detector

Answer 2

(1) Response curve
(2) Contrast to Noise ratio (CNR)
(3) Spatial resolution (MTF)
(4) Noise (NPS)
(5) Efficiency (DQE)

Question 3

What is meant by the dynamic range of an X-ray detector?

Answer 3

It is the measure of a response of a detector exposed to X-rays.
It is the range of exposure for which a meaningful image can be obtained.

Question 4

With respect to dynamic range, compare a screen-film detector system with a digital detector.
How does the response curve appear?

Answer 4

Larger dynamic range with digital detector.

The response curve for the screen-film system is steeper than the digital detector.

Question 5

Define contrast.

Answer 5

It is the difference in brightness (grey scales)
in two areas of an image.
• Contrast is what makes an image useful, what
provides the information.

Question 6

Types of contrast

Answer 6

(1) Object contrast
(2) Subject contrast
(3) Image contrast (detector etc)
(4) Displayed contrast

Question 7

Explain what is meant by object contrast.

Answer 7

The actual physical difference
in the materials that cause a
difference in how they absorb
x-rays
• What will introduce a
difference in the uniform xray field:
– Tissue thickness difference in
part of the body
– Density difference in part of the
body
– Composition difference in part
of the body

Question 8

How may we measure contrast?

Answer 8

(1) Contrast to Noise ratio (CNR)

(2) Contrast threshold

Question 9

What is contrast to noise ratio?

Answer 9

-An indicator of detectability.
- Contrast= (Intensity_1 - Intensity_2)/ Intensity_1
- Noise= sigma/Intensity_1
CNR = Contrast/noise

CNR= (Intensity_1- Intensity_2)/sigma

Rose criterion: detectable if CNR= 5

Question 10

Define spatial resolution.

Answer 10

It is the ability to see small things in the
image. The ability to resolve two very close
points as distinct points.
• When the points come too close (beyond the
resolution capabilities of the system) they are
seen as one larger point.

Question 11

How can spatial resolution be measured?

Answer 11

• Bar pattern on phantom : determine the limiting resolution.

Question 12

What factors affect spatial resolution?

Answer 12

The spatial resolution depends on several technical and physical factors, such as
the spread in the scintillator, the sampling of the pixels and the bandwidth of the
readout electronics.

Question 13

What is MTF?

Answer 13

MTF stands for the modulation transfer function.
MTF is a measure of the ability of an imaging
detector to reproduce image contrast from
subject contrast at various spatial frequencies
• MTF progressively decreases with increasing
spatial frequency
• MTF is normalized to unity at zero spatial
frequency by convention
• The most common way to measure the MTF of a
system digital radiography is based on the use of
a sharp-edge phantom

Question 14

Describe the shape of a MTF vs spatial frequency graph.

Answer 14

Describes the loss of information in the image for increasing frequencies.

Question 15

Define noise.

Answer 15

Defined as uncertainty or imprecision of the
recording of a signal
• X Ray imaging: when recorded with small
number of X- photons has high degree of
uncertainty, more photons give less noise
• Other sources of noise:
– Grains in radiographic film
– Large grains in intensifying screens
– Electronic noise of detector or amplifier

Quantum or electronic noise is responsible for
the fluctuations that can be observed on the
images and can hide the useful info
• Image quality decreases suddenly with
increasing noise
• Several (and diverse) noise sources contribute
to the total noise