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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5814_Библиотеки_им_академика_М_И_Перельмана
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220 Diagnostic ultrasound: imaging and blood ow measurements
A
+
–
tion
Polariza
Direction
L
Figure 9.6 Multilayer piezoelectric materials.
where K is the dielectric constant and C = KA/L is the original capaci-
tance. Goldberg and Smith (1994) showed that for a 1.5D array element of
0.37 × 3.5 × 0.66 mm consisting of three piezoelectric layers at 2.25 MHz
of a prototype array, with a 2.5 pF cable shunt capacitance, a light epoxy
backing and a λ/4 matching layer, a 100 Ω input electrical impedance, and
a 197 pF clamped capacitance, were measured in comparison to an impedance of 800 Ω and clamped capacitance of 24 pF of a 0.37 × 3.5 × 0.77 mm
element consisting of a single piezoelectric layer of a control array of
similar construction. An increase of 10 dB in round-trip sensitivity was
observed from pulse-echo measurements. Multilayered piezocomposites
have also been incorporated into array transducer design and shown to
enhance their performances as well (Zipparo et al., 1999).
Multilayer ex circuits may be used to overcome the interconnection
problem, but the cable size would still be unmanageable. A multilayer ex
conguration for a 6 × 39 mm 1.5D array is shown in Figure9.7, where (a) is
a photo of the array assembly and (b) is an enlarged view of a section. The
number of channel counts conceivably may be reduced by multiplexing or
by adopting a sparse array approach (Smith et al., 2002; Lockwood et al.,
1996). A fully sampled 2D array reported consisting of 9212 elements is
now commercially available for 3D real-time volumetric imaging. It is not
clear in this 2D array design whether there are equal numbers of elements
in the azimuth and elevation. It is, however, known that for this 2D array,
a subarray beamforming architecture is implemented. The elements rst
are grouped into 128 groups. Delays are applied to the elements in the
group, summed, and cabled to the mainframe, where intergroup delays
are applied and summed again. Presumably in this approach electronic
channel count is reduced, but still allows a fast frame rate of 20–30 per second.
221Chapter nine: Multidimensional imaging and recent developments
dx
(a)
200 µm
40 mm
(b)
Figure 9.7 (a) A photo of a 1.5D array with multilayer ex. (b) Enlarged view of a
section of the ex. The line width and via hole diameter are 25 and 50 μm, respectively. (Courtesy of D. Wildes, GE.)
Sparse arrays have been pursued for many years as an alternative to partially circumvent this problem.
A more recent advance in transducer technology that has a great
potential in making the fabrication of a 2D array more cost-effective and
in solving the interconnection problem is the capacitive micromachined
ultrasonic transducer (cMUT) (Ladabaum et al., 1998; Oralkan et al., 2002;
Mills, 2004). This approach differs completely from the traditional transducer design strategy and possesses the advantage in that semiconductor
technology, which allows integration of transducers and electronics and
miniaturization, is used. A cMUT cell is illustrated in Figure9.8. The silicon nitride membrane and the silicon substrate form a capacitor, which
makes the membrane vibrate upon the application of an AC voltage, VAC,
when biased by a DC voltage, VDC. The gold layer is an electrode. The
approximate dimensions of these structures are silicon nitride membrane 0.35–10 μm thick, the gap between the membrane and polysilicon
d0 ≈ 0.05–12 μm, and silicon substrate ≈ 500 μm. Cell radius, a, goes from
10 to 200 μm. The force, Fc, acting on the membrane can be obtained from
the following equation:
F
c
dx
2
CV V
1
d
=− =−
1
22
ddxKA
2()
dx
=
−
0
KAV
2( )
−
0
2
(9.1)
2
222 Diagnostic ultrasound: imaging and blood ow measurements
+=
Silicon Oxide
Silicon Nitride
Ga
Gold
p Width
Figure 9.8 Construction of one capacitive micromachine’s ultrasonic transducer
(cMUT) cell.
Vacuum
P-doped Silicon
where A is the area of the cMUT cell = πa2, V is the voltage across the
cMUT (a capacitor), x is the membrane displacement, and K is the dielectric constant. The static force when V = VDC and x = 0 is given by
2
KAV
DC
=
F
ST
(9.2)
2
2
d
0
It is clear from this equation that the sensitivity of the device is
inversely proportional to the gap width. It increases as the gap width
decreases. It is also proportional to the bias DC voltage VDC. The resonant
frequency of the device is given by (Hietanen et al., 1992)
2.405
=
n
πτρ
2fa
m
where τ and ρm are the tension and density of the membrane = 120 n/m
and 3 × 10–3 kg/m3 for silicon nitride. For a = 5 μm, fn is about 12 MHz.
The cMUT element behavior has been analyzed with a simple
rst-order electromechanical model (Ladabaum et al., 1998), shown in
Figure9.9, where the medium in the cMUT is assumed to be a vacuum,
and as a result, there is no mass loading effect. The force equation at the
cell membrane can found by applying Newton’s second law:
2
() ()
dxt
m
dt
−
2
KAVt
2( )
dxt
2
() 0
kx t
2
−
()
0
(9.3)
where k and m are the mass and the spring constant of the membrane. This
is a nonlinear second-order differential equation, which is very difcult to
223Chapter nine: Multidimensional imaging and recent developments
2
3
d
2
T cell)
x
Spring
Mass
Capacitor (cMU
Figure 9.9 A rst-order model of cMUT operation.
solve. To calculate the collapse voltage, set V(t) = VDC and assume no time
variation; Equation (9.3) becomes
KAV
DC
kx
=
2
dx
−
2( )
0
This third-order polynomial equation can be found to have a real root
for VDC >> kx at
collapse
=
x
0
and
3
8
kd
collapse
=
V
27
0
KA
A reduction in spring constant k or a spring softening effect due to the
electrostatic force acting on the membrane is often observed. The softened
spring constant was found to be
KAV
=−
kk
soft
DC
3
d
0
As the resonant frequency of a spring-mass system is related to the square
root of the spring constant, a drop in the resonant frequency of the cMUT
is expected if the biased voltage is increased.
224 Diagnostic ultrasound: imaging and blood ow measurements
++
22
CA
2FV
cDCAC
ide etch
Small holes allow the nitride to be deposited
Another observation that can be made from the simple equation that
relates the force acting on the capacitor and voltage, Equation (9.1), neglecting x, is that Fc ≈ V2. If an AC voltage is applied, V = VDC + VAC, it can be
shown that
~2
FV VV V
cDCDCA
C
Assuming VDC >> VAC, the time-varying part of ⊕
V
. The larger
the biased voltage, the greater the force or pressure that is produced.
Capacitive micromachined ultrasonic transducers (cMUTs), as the
name implies, are fabricated utilizing MEMS methods. Two different
approaches (Oralkan et al., 2002; Mills, 2004), surface machined and bulk
machined, have been developed. The surface-machined approach developed by Ladabaum et al. (1998) is described in Figure9.10. A clean P-type
silicon wafer is prepared. A thin (1 μm) silicon oxide is grown on top of
the wafer with a wet oxidation process as a sacricial layer. In the next
step, a layer of silicon nitride (0.35 μm thick) is deposited on top of the
oxide layer via low-pressure chemical vapor deposition (LPCVD). A resist
layer is then spin-coated. Electron beam lithography is subsequently performed, followed by plasma nitride and wet oxide etch. A second silicon
nitride layer of 0.25 μm thickness is deposited on the released membranes.
As the nal step, a chrome and gold layer of 0.05 μm thickness is evaporated onto the nitride layer. A 3 MHz 128-element linear array was fabricated with cMUT (Oralkan et al., 2002). Each element of the array had a
dimension of 200 × 600 μm consisting of 750 circular cMUT cells of 36 μm
radius. Each had a silicon nitride membrane thickness of 0.9 μm and gap
P-type silicon wafer
Nitride and ox
Figure 9.10 The MEMS process for cMUT fabrication.
Oxide growth
Nitride deposition
Mask
Lithography
Nitride deposition,
metalization, and
electrode patterning
225Chapter nine: Multidimensional imaging and recent developments
(a)
MEMS structure on the surface; each cell has the width of a human hair
(b)
Figure 9.11 (a) Photo showing several cMUT cells. (b) Atomic force microscopic
image of one cMUT cell. (Courtesy of Sensant Corp.)
width of 0.11 μm. The silicon substrate thickness was 500 μm. The device
was found to have a bandwidth of 80% and a penetration depth of 21 cm
in water. The level of cross talk was observed to be higher than that of
comparable conventional linear arrays.
Figure9.11(a) and (b) shows, respectively, a photo of several cMUT cells
in which the lighter-colored structures are the cell membrane and conductive paths and an atomic force microscopic image of one cell. A 192-element
226 Diagnostic ultrasound: imaging and blood ow measurements
cMUT Linear Array
ct
(a) (b)
Interconne
Array
Figure 9.12 A photo of a cMUT 192-element linear array. (Courtesy of Sensant
Corp.)
linear array fabricated from this technology is shown in Figure 9.12, and
a corresponding image acquired by this array is shown in Figure 9.13,
along with an image obtained by a conventional PZT array. An improved
axial resolution is clearly seen due to the large bandwidth of cMUT. An
additional advantage of cMUT is that the need for matching the acoustic
impedance between the transducer and the loading medium is no longer
necessary. There are, however, a few shortcomings with this technology: a
slight decrease in sensitivity, much higher input electrical impedance, and
the need for a bias voltage in the order of 100 V.
In another development, piezoelectric micromachined ultrasonic transducers (pMUTs) that use the bending mode of a piezoelectric layer spin-coated
onto a silicon membrane have been studied for medical imaging as well
(Dausch et al., 2008). This type of device makes use of the electromechanical
Figure 9.13 Image comparison of breast broadenoma at 9 MHz acquired by (a)
a 192-element cMUT array and (b) a conventional PZT array. (Courtesy of Sensant
Corp.)
227Chapter nine: Multidimensional imaging and recent developments
∫
λλ
ed
coupling coefcient k31 of a piezoelectric material, i.e., displacement produced
in the 1 direction upon the application of an electric eld in the 3 polarization
direction. It possesses the same advantage of cMUT in ease of fabrication and
integration, with imaging electronics allowing the manufacturing of high-density arrays. An additional advantage is its larger capacitance than cMUT. The
shortcoming is that k31 of a piezoelectric material is typically lower than k33. A
prototype 7.1 MHz 2D fabricated showed a –6 dB bandwidth of 57%. It should
be noted that for pMUT, each pMUT cell represents an array element, unlike
cMUT, where each array element is populated by many cMUT cells.
9.2.2 Sparse arrays
To reduce the number of elements and channel count, sparse arrays may
be used. On a predetermined aperture, piezoelectric elements are randomly placed as shown in Figure 9.14, where all the elements may be
used to transmit and receive, or some of the elements for transmission
and some for reception. The advantage of a sparse array is countered by
the decrease in sensitivity due to the reduction in aperture size and an
increase in the side lobe pedestal or the noise oor outside of the main
lobe. It has been shown to be proportional to 1/(number of elements)
(Lockwood et al., 1996). To overcome this problem, a periodic sparse array
has been suggested by Lockwood et al. (1996). The grating lobes caused
by the large pitch in a sparse periodic array may be alleviated by selecting different pitches for the transmit and receive arrays. At a direction of
φx shown in Figure3.47 at a distance r >> La, where La is the width of the
array, the transmitted radiation pattern, as previously shown in Chapter
3, normalized with respect to the wavelength λ is given by
x
δ
jk
δ=
()Ha
TT
x
x
λ
(9.2)
Figure 9.14 A random sparse array where the solid squares represent piezoelectric elements.
228 Diagnostic ultrasound: imaging and blood ow measurements
∫
λλ
ed
==() () () [][] [*]HHHFTa FT aFTa a
TR
x
a
T
b
u
0
g
Figure 9.15 Transmit aperture function for a linear array of pitch g and element
width b.
where δ = sin φx, λ is wavelength, k is the wave number, and aT is the
transmit aperture function. Again, this equation basically states that
the radiation pattern of an aperture is the Fourier transform of the
aperture function (Steinberg, 1976). For a linear array the aperture
function can be represented by Figure9.15, where b is the element width.
If the acoustic independent variable chosen is medium velocity, at(x/λ) is
represented by a series of pulses with a medium velocity amplitude u0.
Here the distance x is normalized with respect to λ. If the receive radiation pattern is given by
x
δ
jk
δ=
()Ha
RR
x
x
λ
(9.3)
where aR is the receive aperture function, the two-way pulse-echo radiation pattern would be
δ= δδ
TR TR TT
where * denotes convolution and E(x/λ) = aT(x/λ)
aR(x/λ) is frequently
*
(9.4)
called the effective aperture function or co-array function (Steinberg,
1976). In the far eld of the array, i.e., r >> L, the rectangular pulses may
be represented by impulses, as shown in Figure9.16. Assuming that there
are NT and NR elements in the transmit and receive apertures, respectively, following the convolution, the number of elements in the effective
aperture should be NTR = NT + NR – 1 with a width of 2L and a pitch of
½λ. There are a variety of ways to reconstruct the effective aperture from
the transmit and receive aperture functions. Figure9.17 illustrates two of
229Chapter nine: Multidimensional imaging and recent developments
x
λ/2
x
x 2Lx
λ/2
*
x
NT = 8 NR = 8
xLL
2L
N
= 15
TR
x
Figure 9.16 Desirable effective aperture function can be achieved by convoluting
a transmit aperture function with a receive aperture function.
them (Lockwood et al., 1996). This approach is only valid for the far eld
of a nonfocused array and at the focus of a phased array. In addition, a
combination of apodization and addition of extra elements is necessary to
make it work successfully.
9.3 3D imaging
Three-dimensional ultrasound is an important new area of development.
It exploits the tomographical capability of ultrasound by acquiring multiple slices of the images. 3D reconstruction can be accomplished ofine
or in real time using 2D arrays and parallel processing (Nelson, 2000). If
a scanner is capable of displaying the volumetric images in real time, it
is often called a 4D scanner. Ofine 3D imaging is achieved by freehand
scanning with electromagnetic position sensing or by mounting the probe
on a mechanical translator whose position is encoded. The data acquired
are displayed following image processing by optimized algorithms on a
high-resolution monitor. Currently 3D ultrasound images are displayed
λ/2
λ
*
x
λ/2
2λ
*
Figure 9.17 Two approaches that may be used to obtain a desired effective aperture function.
=
x 2L
λ/2
=
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