Biomedical Engineering Reference
In-Depth Information
Figure 1.11: We can see that the progressive distance reduction of the linear
scatterers, from (
a
1
,...,
a
4
) (Fig. 1.10) to (
b
1
,...,
b
4
) reduces the time difference
between the maximums of the “train” pulses. The maximums can be separated
reducing the pulse width from
d
1
(Fig. 1.10) to
d
2
, this is equivalent to an increase
in the pulse frequency.
diminishing the pulse width
d
t
, which is equivalent to increasing the frequency
of the emitted pulse. The axial resolution of this technique depends essentially
on two factors:
ultrasound speed c
and
pulse duration d
t
. The functional depen-
dency between the spatial resolution, the frequency, and the ultrasound speed
propagation is given by:
c
f
d
r
=
cd
t
=
cT
=
(1.7)
where
d
r
is the axial resolution,
c
is the ultrasound speed for biological tissues,
d
t
is the pulse width,
T
is the period of ultrasound wave, and
f
is the ultrasound
frequency. For IVUS, the typical values are:
c
=
1540 m/sec and
f
=
30 MHz,
the axial resolution is approximately
d
r
=
1540
/
(30
×
10
6
)
=
0
.
05 mm
≈
50
µ
m,
and the relative error of the axial resolution is given by:
c
c
+
f
f
d
r
d
r
=
The axial resolution dependency versus the ultrasound frequency is shown in
Fig. 1.12.
