1. Effects
of
kVp
and
mAs
on
image
spa4al
resolu4on,
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
filtra4on:
Eave
=
(1/2-­‐1/3)Emax
• KVp
and
filtra,on
=
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
1015
• S
=
exposure
4me,
dura4on
of
x-­‐ray
produc4on
• Indicates
quan4ty:
Number
of
photons
is
propor4onal
to
mAs

4. Rules
of
Thumb
•
•
•
•
•
kVp
and
exposure:
Exposure
α
(kVp)2
For
a
fixed
exposure
technique:
(kVp1/kVp2)5
=
mAs2/mAs1
kVp
determines
quan0ty,
quality,
and
transmission
through
the
object
whereas
mAs
determines
quan0ty

5. Image
Contrast
• Medical
Imaging
is
the
Process
of
Conver4ng
Tissue
Characteris4cs
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
Sensi4vity
Increases
Image
Contrast
and
the
Visibility
of
Objects
in
the
Body

6. Different
defini4ons
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
resolu4on
(contrast
to
noise
ra4o)
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
Reconstruc,on:
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. Hounsfield
Unit
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.

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
filtra@on
=>
minimizes
the
photoelectric
interac@ons

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. Spa4al
Resolu4on
• Ability
of
an
image
system
to
dis4nctly
depict
two
objects
as
they
become
smaller
and
closer
together
• Directly
related
to
mAs
=
quan,ty
of
photons
making
the
image
• kVp
set
for
a
technique
• LOTS
of
other
factors
that
affect
spa4al
resolu4on
• Quan4fied
using
MTF
(cycles/mm)

14. Spa4al
resolu4on
of
different
imaging
systems
-­‐
MTF
Ref: Bushberg

15. Noise
• Local
varia4ons
in
contrast
due
to
a
background
texture
called
noise
that
does
not
represent
the
ahenua4on
in
pa4ent
• Random:
e.g.,
caused
by
random
varia4ons
in
x-­‐
ray
photons
interac4ng
in
the
4ssue
• Screen-­‐film
radiography:
Visual
percep,on
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
pa,ent
dose.

16. Signal-­‐to-­‐Noise
Ra4o
•
•
•
•
If
N
=
photons/pixel
then
SNR
=
√N
Noise
and
Dose:
to
increase
the
SNR
by
2
the
dose
to
the
pa4ent
(N)
has
to
be
increased
by
4

17. Thank
You!