Image Contrast, Noise, Resolution

1. Effects of kVp and mAs on image
spatial resolution, contrast, dose,
and noise
Vibha Chaswal, Ph.D.

2. Peak Voltage (KVp) of an X-ray
tube
•Highest X-ray energy
is determined by peak
voltage applied
across the x-ray tube.
•With filtration:
Eave = (1/2-1/3)Emax
•KVp and filtration =
Quality of the X-ray
beam
Ref: Bushberg

3. Milli Ampere Second (mAs) tube
current
•Tube current is the rate of electron flow from
the cathode to anode, measured in
milliamperes (mA)
•1 mA = 6.24 x 10
15
•S = exposure time, duration of x-ray
production
•Indicates quantity: Number of photons is
proportional to mAs

4. Rules of Thumb
•kVp and exposure:
•Exposure α (kVp)
2
•For a fixed exposure technique:
•(kVp
1
/kVp
2
)
5
= mAs
2
/mAs
1
•kVp determines quantity, quality, and
transmission through the object whereas mAs
determines quantity

5. Image Contrast
•Medical Imaging is the Process of
Converting Tissue Characteristics
into a Visual Image
•Contrast: Difference in the image
gray-scale between closely adjacent
regions on the image.
Contrast sensitivity: imaging
system's ability to translate
physical object contrast into image
contrast
Increasing Contrast Sensitivity
Increases Image Contrast and the
Visibility of Objects in the Body

6. Different definitions of contrast
•Subject contrast: difference in some aspect of
the signal prior to its being recorded
(x-ray operators use different kVp and mAs to
control subject contrast)
•Displayed contrast: digital imaging
(CT (x-ray tomography) imaging uses mAs for
increasing contrast resolution (contrast to
noise ratio) kVp dependence:Once kVp is set,
out of sight out of mind

7. Dose and contrast versus kVp
For screen film radiography
Ref: Bushberg

8. Computed Tomography: 3 steps
Scan: produces image data
Reconstruction: produces digital image => a matrix of pixels with CT numbers
Digital to analog conversion: produces visible analog image represented by
different shades of gray

9. X-ray attenuation depends on both the density and atomic number (Z)
of materials and the energy of the x-ray photons. For CT imaging a
high KV (like 120-140) and heavy beam filtration is used. This
minimizes the photoelectric interactions that are influenced by the Z of
a material. Therefore, CT numbers are determined by the density of the
tissues or materials.
Hounsfield Unit

10. Displayed Contrast: CT
•Defined by difference in gray scale values of
closely lying adjacent structures
•Gray scale values assigned to pixels during DI
to analog conversion of DI
•Visible contrast can be controlled by window,
level and zoom or post-processing techniques
•CT imaging uses a high KV (like 120-140 kVp)
and heavy beam filtration => minimizes the
photoelectric interactions

11. Digital Radiography
•Enhance contrast
digitally using
Window and Level
and
Ref: Bushberg

12. Digital Radiography
•Post-
processing
using the
Edge
enhancement
filter
Ref: Bushberg

13. Spatial Resolution
•Ability of an image system to distinctly depict
two objects as they become smaller and
closer together
•Directly related to mAs = quantity of photons
making the image
•kVp set for a technique
•LOTS of other factors that affect spatial
resolution
•Quantified using MTF (cycles/mm)

14. Spatial resolution of different
imaging systems - MTF
Ref: Bushberg

15. Noise
•Local variations in contrast due to a background
texture called noise that does not represent the
attenuation in patient
•Random: e.g., caused by random variations in x-
ray photons interacting in the tissue
•Screen-film radiography: Visual perception of
noise is reduced when the detected x-ray
photons increase.
•Increasing mAs and kVp decrease noise as the #
of detected photons increase, so does patient
dose.

16. Signal-to-Noise Ratio
•If N = photons/pixel then
•SNR = √N
•Noise and Dose:
• to increase the SNR by 2 the dose to the
patient (N) has to be increased by 4

17. Thank You!