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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3835_Библиотеки_им_академика_М_И_Перельмана
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Chapter 11
https://t.me/medicina_free
High-Resolution Ultrasound Imaging
System
Weibao Qiu and Hairong Zheng
Introduction
Intravascular ultrasound (IVUS) usually uses ultrasound with center frequency of
20–60 MHz, which is noted down as high frequency ultrasound (Sun et al. 2008a;
Silverman et al. 2007), ultrasound biomicroscopy (Vogt and Ermert 2007;Foster
et al. 2000) or micro-ultrasound (Stuart Foster et al. 2011; Foster et al. 2009)in
ultrasound society. Since the center frequency of ultrasound is higher than traditional
ultrasound (2–15 MHz) for high-resolution imaging, the system design is different
from the traditional scheme. Separate commercial modules with high performance
are usually chosen for research application (Sun et al. 2008b) regardless of their
high cost and large size. It would provide a platform for concept verification due to
their fast implementations. But for industry, the system would usually consider the
cost and integration. Therefore, the design of the electric system should be easy to
manufacture and mass produce.
The most widely available systems are using a single element transducer based
catheter (Römer et al. 2000; Qiu et al. 2012); therefore, this chapter would only cover
the system aspect for rotational IVUS. Array-based system requires multiplexer IC
in the catheter which is not included in this chapter. This chapter introduces some
design considerations of a high frequency ultrasound system specifically for IVUS
applications. Four parts are included in this chapter: pulse generation, echo receiver,
basic imaging algorithm, and some advanced IVUS technology developed in recent
years.
W. Qiu (B) · H. Zheng
Paul C. Lauterbur Research Center for Biomedical Imaging, Institute of Biomedical and Health
Engineering, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences,
Shenzhen 518055, China
e-mail: wb.qiu@siat.ac.cn
H. Zheng
e-mail: hr.zheng@siat.ac.cn
© Springer Nature Singapore Pte Ltd. 2020
Q. Zhou and Z. Chen (eds.), Multimodality Imaging,
https://doi.org/10.1007/978-981- 10-6307-7_11
257
258 W. Qiu and H. Zheng
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Fig. 11.1 The block diagram of an imaging system for intravascular ultrasound
A typical high frequency ultrasound imaging system for IVUS is shown in
Fig. 11.1 (Qiu et al. 2012). A miniaturized ultrasound transducer with a surface
2
size of about 0.6 * 0.6 mm
is placed at distal end of the catheter to go inside the
vessel (Li et al. 2014;Maetal.2015). It is able to transmit and receive an ultrasound
signal to acquire a cross-sectional view of the coronary artery. A motor control unit
is employed to rotate and pull back the catheter for the imaging (Ma et al. 2015). The
catheter, motor control unit, and the electronic system are usually separate parts to
compose a whole IVUS system. In terms of an electronic system, a pulse generation
and echo receiver are needed for ultrasound imaging. A high-voltage pulse is used
to excite the transducer at desired frequency and spectrum to optimize the performance of the transducer (Qiu et al. 2012). There are several pulse types for different
imaging strategy, which will be present in the second section of this chapter. As for
the echo receiver, signal amplifier, band-specific filter, analog to digital converter
(ADC), and processing unit are required (Qiu et al. 2013). Field programmable gate
array (FPGA) based digital processor is frequently employed to process the ultrasound echo signal for flexibility and programmability (Qiu et al. 2012). After being
processed by the local field processor, the data are transferred to the computer for
further investigations and displayed. The scheme of high-speed data transfers could
be USB 3.0 and PCIE interface in current transmission technology (Qiu et al. 2012).
Graphical user interface (GUI) software is employed to receive and display the IVUS
images for real-time visualization. A graphic processing unit (GPU) could be used
for the data processing, depending on the algorithms employed. The details of the
echo receiver will be presented in the third section. There are several ways to deal
with the IVUS imaging, and some basic imaging processes will be introduced in the
fourth section. In the end, some advanced technologies from the technical point of
view will be presented.
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Pulse Generation
In this section, basic requirements of the pulse generation for the excitation of IVUS
transducer are introduced. Optimized excitation would increase the efficacy of the
excitation. The transducer is a piezoelectric material which requires a short electrical
pulse to generate acoustic waveform. The spectrum of the electrical pulse should
cover the inherent resonant frequency of the transducer. Several waveform schemes
which can be used for IVUS have been listed in Table 11.1. The properties of the
electrical pulses are listed to elaborate the performance of each one.
The unipolar pulse is the easiest one to be generated. It is a high-voltage ultra-short
unipolar spike, usually a negative unipolar pulse. It has been extensively used due to
relatively easy accomplishment and the wide bandwidth for a variety of frequencies
(Qiu et al. 2012). It is able to cover from sub-MHz to 100 MHz frequency range with
very high peak amplitude (more than 200 V), which has been applied for B-mode
medical imaging and non-destructive testing (NDT). The shorter the pulse, the wider
the frequency range that can be covered. 10 ns negative pulse could cover to as high
as 100 MHz (−3 dB). There are several systems available for the unipolar pulse
generation (e.g., Panametrics 5900 pulser/receiver, Olympus NDT Inc., Waltham,
MA, USA; UT340, Utex scientific instruments Inc., Mississauga, Ontario, Canada).
However, the drawback of this kind of pulse is also obvious that some of the peak
energy is concentrated outside of the bandwidth of the transducer, especially at
relatively low frequency range, which would slow down the center frequency of the
pulse. Unwanted signals and heat may be generated. The efficacy is relatively low in
for a unipolar pulse, especially for high-resolution ultrasound imaging.
Bipolar high-voltage pulse is the most widely used waveform in medical ultrasound equipment including the IVUS imaging system (Brown and Lockwood 2002;
Xu et al. 2007). The bandwidth of a bipolar pulse can be adjusted to match the
bandwidth of the transducer for high energy transmission. A high signal-to-noise
ratio (SNR) can then be achieved. For this reason, the center frequency of the bipolar pulse should be tunable for the transducer in the catheter with different center
frequencies. There are several implemented methods for the bipolar pulse. Metaloxide-semiconductor field effect transistor (MOSFET) is usually used for ultra-short
high-voltage pulse (Brown and Lockwood 2002;Xuetal.2007). Paired MOSFET
(e.g., TC6320, Supertex Inc., Sunnyvale, CA) with both N- and P-channel is able
to provide bipolar pulse. Some parameters should be aware in IVUS system is the
timing related value and current related value. Since the center frequency is usually
in the range of 20–50 MHz, which is high frequency range. The spectrum of the pulse
should be also in this range, which would require the pulse to have a fast transaction
time. Rise time, turn-on delay time, fall time, and turn-off delay time are the important
parameters for the good waveform. The on-state resistance should be as low as possible to low down the power cost when the pulse is excited. Larger resistance would
decrease the current of the pulse and consequently increase the power consumption
in the device and low down the driving performance of the pulse. The number of
the pulse cycle is also a parameter could be used to adjust the image. Longer pulse
260 W. Qiu and H. Zheng
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Table 11.1 Electrical pulses for the excitation of the transducer
Pulse
type
Negative
pulse
(Sun
et al.
2008)
Waveform Typical
cycle
1 B-mode >100 MHz
Imaging
mode
Bandwidth
range
(pulse
length < 10 ns)
Typical implementation
N-MOSFET
Quasisine
wave
pulse
(Qiu
et al.
2012;
Xu et al.
2007)
Sine
wave
pulse
(Qiu
et al.
2012)
Arbitrary
wave
(Qiu
et al.
2012)
1–2 B-mode ~30 MHz Paired
3–7 Flow-
mode
1–2 B-mode ~30 MHz Power
3–7 Flow-
mode
~5 µs Coded
excitation
mode
~15 MHz Paired
~15 MHz Power
~50 MHz Power
MOSFET and
transformer
MOSFET and
transformer
amplifier
amplifier
amplifier
could generate more power to penetrate deeper into the tissue. However, the spatial
resolution will be sacrificed. It could be used for Doppler measurement (Xu et al.
2007), differentiating the flow from the tissue (Zhou et al. 2007). Although it has been
widely applied for a general imaging system, it really depends on the system scheme
for flow measurement in the IVUS application. It is a trade-off for the whole system.
The bipolar pulse scheme provides higher efficiency than the unipolar pulse (Qiu
et al. 2012). However, the amplitude of the bipolar pulse is limited to 150 V pp, since
11 High-Resolution Ultrasound Imaging System 261
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From
transducer
Front-end
electronics
Limiter
LNA
Imaging receiver
TGC
amplifier
Clock
Filter
PLL
ADC
LVDS
CLK
FPGA
RAM
Digital
circuit
PCIE/USB
To
computer
Fig. 11.2 Schematic of the receiver for IVUS imaging
high power performance P-channel MOSFET is usually not as good as N-channel
MOSFET.
Although the MOSFET is still popular in an ultrasound system, the shape is not
a sinusoidal wave due to its inherent features. We called quasi-sine wave for this
method. The spectrum would be better with pure sinusoidal wave for the excitation,
and special power amplifier could be designed for this purpose. Single cycle and
multiple cycles of sinusoidal waveform could be implemented in this scheme (Qiu
et al. 2012, 2013, 2017). The power amplifier for this application should be a linear
power amplifier to maintain wide spectrum. There are two things to be aware of when
designing this type of power amplifier. Firstly, this power amplifier should have a fast
start feature, which makes sure the first pulse could be generated quickly. In addition,
the power amplifier should be switched off after the pulse generation. Since the linear
power amplifier does have static current to maintain the working state, consequently,
there is some noise also generated by the power amplifier. The output of the amplifier
is usually connected to the receiver part directly, which would definitely transfer some
noises to t he receiver. So that, the power amplifier should have a switch-off circuitry
inside for a high-resolution imaging. For the more advanced imaging technique, a
linear power amplifier can also show its beauty of nature. Arbitrary waveform can be
generated for coded excitation to improve the penetration depth of the imaging, and
the imaging resolution can be maintained in a good effort (Qiu et al. 2017). Chirp
waveform is the typical one for this application.
Receiver Circuitry
The ultrasound echo signal can be produced when ultrasound propagates in the
targeted tissue. The echo signal would be collected by the transducer to be converted
to electrical signal by piezoelectric effect. A receiver circuit should be designed to
receive and process the echo signal, in which Fig. 11.2 shows a typical schematic of
such a receiver circuit (Qiu et al. 2012).
262 W. Qiu and H. Zheng
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Fig. 11.3 Photograph of the integrated PCBs for IVUS imaging
The system could be implemented by separate modules or an integrated PCB. The
echo signal received from the catheter is sent to the electronics for data acquisition
and image processing. After passing the protection circuitry, signal amplification,
and band-pass filtering, the echo signal is converted to a digital signal by an ADC.
An FPGA is usually employed as a core processor of the imaging platform, which
processes the digitized signal for real-time images. The algorithms implemented
in the FPGA can be programmed for different setting. An external random access
memory (RAM) device with large capacity and high-speed interface is employed
for FPGA to support sophisticated algorithm such as digital scan converter (DSC).
High-speed PCIE or USB interface could be used for real-time imaging and fast radio
frequency (RF) data transfer. The following sections explain the detailed design for
IVUS imaging. Table 11.2 shows the typical electronics requirement for an IVUS
system. Figure 11.3 shows some PCBs specifically designed for IVUS data acquisi-
tion and processing.
Table 11.2 Electronics
requirement for an IVUS
system
Items Performance
Frequency range 10–80 MHz
High-voltage tunable pulse >100 V pp
Adjustable gain >45 dB
Gain fluctuation <±1.5 dB
ADC >11 bits, 150 MSPS
Dynamic range >45 dB
Data transferring speed 100 MByte/s
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Termination and Protection
The transmit path and receive path are connected together for the imaging application
in the circuit since there is only one transducer employed in the IVUS. There are two
points that need to be paid attention to for this part: electrical matching and highvoltage protection (Choi et al. 2011, 2014a, b). The impedance of the transducer is
usually designed to 50 , and therefore, the receiver should be also 50 for the
matched transmission. It is better to tune the circuit to 50 to avoid the reflection over
ultrasound cable. The reflection may occur if the circuit is mismatched electrically,
which will cause a ring down signal. The imaging resolution will be degraded by the
elongated ringing signal. Once the impedance of the receiver is matched to the cable
impedance, the ringing effect will be minimized.
The protection circuits, also named as limiter, are used to block the high-voltage
waveform from the pulse generator (Choi et al. 2014a, b). The high-voltage excitation
pulse will get into the receive path which may cause the saturation of ADC. It needs
time to be recovered to the normal state from the saturation state. The imaging performance is decreased during this time period. Moreover, the high-voltage waveform
may damage the receiver since the amplitude of the waveform will hit the root of the
maximal signal of the circuit. There are several methods for the protection circuit
including passive circuit and active circuit, which are shown in Fig. 11.4. There is
no need for external power for passive circuit, whereas external power or control is
needed for active circuit. Resistor diode circuit is the easiest way for the protection;
however, the bandwidth is poor and some energy is absorbed during the transmission
period. Figure 11.4b shows another passive protection circuit with the use of depletion MOSFETs (Choi et al. 2014b). Figure 11.4c shows an active protection circuit
with biased diode bridge.
Low Noise Amplifier and TGC
The quality of IVUS images is significantly influenced by the front-end analog electronics, including the low noise amplifier and time gain compensation (TGC) amplifier. Poor electrical components and circuitry design can increase the noise level and
corrupt the weak echo signals. This is particularly important for the IVUS imaging,
where the echo signal is extremely weak. The lowest amplitude of the signal is lower
than 50 µV, which is not able to be acquired directly by the digital converter. About
45 dB gain (about 180 times amplitude increase) could be applied to the small signal
for the data acquisition (Qiu et al. 2017). The noise figure, which is one important
parameter for the low noise amplifier, should be taken care for the amplification strategy. The dynamic range of the IVUS image is influenced by the performance of the
amplifier. Another rule is to use smaller number of amplifiers because each amplifier
can definitely introduce noise. It should be noted that, once the echo signal is distorted
in the front-end section, it is very difficult to be recovered by post-processing even
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Q1
R2
R
GND
Q2
RX
GND
R1
4
C2
3
2
1
+V
TX RX
TX
C1
TX RX
-V
GND
Fig. 11.4 Different protection schemes for IVUS imaging
Table 11.3 Selection of the amplifier for IVUS imaging
Vendor Brand Gain (dB) Bandwidth (MHz) Noise
figure
Miteq 1114 32 500 1.1 >200
Tyco electronics SMA231 26 250 1.7 183.18
ADI AD8331 43.5 120 4.15 11.45
Texas instruments THS4509 20 1900 17.1 10.46
a
Digikey price (acquired from www.digikey.com),bMouser price (acquired from www.mouser.com)
Cost
(USD)
a
b
a
a
with sophisticated algorithms. Therefore, more attention should be paid to diminish
the noise level and to achieve a high sensitivity (Table 11.3).
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Analog to Digital Converter
Digital echo signal is needed for the imaging of intravascular vessels. After the
amplification of the echo signal, the signal will be digitized by the ADC. The sampling
rate of the ADC must be at least double than the maximum frequency of the ultrasonic
echo signal to avoid signal distortion based on the Nyquist sampling theorem. The
center frequency of ultrasound for single element IVUS imaging is usually around
40 MHz. Assuming the −6 dB bandwidth is about 60%, where the upper frequency
range is higher than 52 MHz. Therefore, the sampling frequency should be higher
than 104 MHz. Usually, the sampling rate should be higher than 150 MHz for high
frequency ultrasound to acquire wider spectrum data. In the early stage, quadrature
sampling techniques were employed to minimize the requirement of sampling rate by
using two separate ADCs as half of the above requirements (Foster et al. 2002). But
the circuitry control is complicated, which may affect the performance of the digitized
process. An excitation delay method was proposed recently. By delaying half or a
quarter of full cycle of excitation pulse would enable a low sampling frequency
(Qiu et al. 2018). However, it may not be suitable for IVUS (mechanical rotation
mode) application since the excitation is needed multiple times. Table 11.4 shows
the high-speed ADCs which could be used for IVUS imaging.
There are actually several choices for the high-speed ADCs. Attention should
be given to the sampling rate, effective number of bits (ENOB), and signal-to-noise
ratio (SNR). Main vendors for ADC are Texas Instruments Inc. from Dallas, TX, and
Analog Devices, Canton, MA. In addition, a low-jitter clock generator is important
for accurate clock source of high precision data acquisition.
A low pass filter or band-pass filter should be employed before the ADC to remove
noise before the digitized procedure. Some comments (RLP83+, Mini-Circuits,
Brooklyn, NY) can be used for anti-aliasing filter. The insertion loss of the low
pass filter should be minimized as low as possible, e.g., less than 1 dB.
Data Process Unit
When the digitization process is complete, the digital signal is transmitted to the
FPGA by high-speed interfaces such as low-voltage differential signaling (LVDS)
Table 11.4 High-speed ADCs for IVUS imaging
Vendor Brand Speed (MSPS) Bits SNR (dBFS) Cost (USD)
TI ADS4129 250 12 70.4 56.21
ADI AD9230 250 12 65.5 98.53
Linear LTC2152 250 12 68.5 60.17
a
Digikey price
a
266 W. Qiu and H. Zheng
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bus or CMOS bus. There are several data process units in local field such as FPGA,
digital signal processor (DSP), ARM, etc. FPGA is a field process unit, which is the
most important digital processor unit in medical ultrasound equipment (Qiu et al.
2012). There are plenty of I/O interfaces in a FPGA chip, which enables the chip
can be connected to multiple ADCs with a same timing sequence. FPGA is an integrated component which contains a hierarchy of reconfigurable interconnections
designed to be programmed with a different code. Reconfigurable properties and
high-speed implementation make FPGA very suitable for the design of flexible and
programmable system for IVUS application. In ultrasound imaging system, FPGA is
employed to support high-speed ADC interface, signal processing, and data transfer
to the computer. Table 11.5 shows the commercially available FPGA for IVUS application. There are two vendors named Altera and Xilinx, who are major suppliers for
the FPGA components. There are plenty of choices for the selection of FPGA. Several criterions can be used for the selection of an FPGA: IO number, logic element
resources, speed level, and special requirement such as LVDS interface, PLL, or DSP
block, etc. The designer should consider carefully and simulate the algorithms for
the image processing. Complex image processing algorithms definitely require large
logic elements and DSP blocks (including multipliers).
Computer Connection
After image processing in the FPGA, the data need to be transferred to a computer for
post-processing and display. Different connection methods could be used for IVUS
imaging. Table 11.6 shows the features of different connection methods. USB 3.0
now becomes a convenient method for the data transfer.It is able to support more than
300 MB/s data throughput for the imaging applications. PCIE is a stronger transfer
strategy for the imaging application. The highest speed is about 15.8 GB/s, which
enable a super-speed data transfer. However, the driver firmware is more difficult for
both the PC side and FPGA side for the PCIE interface.
Table 11.5 FPGA components for high-resolution ultrasound imaging
Brand Components LE (K) Memory (Mb) Pin Cost
Altera 5CGXFC7D7F31C8N 149.5 7.5 480 249.95
10AX057H4F34E3SG 570 40.1 492 1795
Xilinx XC7S50-2FGGA484C 52.2 2.6 250 63.77
XC7K410T-1FFG900C 406.7 27.9 500 1496
a
Digikey price
(USD)
a
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