Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2001-06-15
2003-09-23
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S602000, C073S626000, C073S659000, C073S861250, C600S447000
Reexamination Certificate
active
06622560
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ultrasound imaging system. In particular, the invention relates to an ultrasound imaging system based on a pulse compression technique using a spread spectrum signal and a FIR filter having an efficient hardware structure.
2. Description of the Related Art
Conventionally, a medical ultrasound imaging system obtains information about a human body by transmitting short ultrasound pulses into the body and receiving a signal reflected from inside the body.
FIG. 1
shows a block diagram of a conventional short-pulse ultrasound imaging system
100
, which comprises a transducer array
1
having a plurality of transducers, a pulser
11
, a TX(transmission) focus delay memory
14
, a TX/RX(receiving) switch
21
, a receiver
31
, a beamformer
37
, an RX focus delay adjuster
36
, a signal processor
41
and a scan converter
42
.
Specifically, a delay pattern of ultrasound pulses to be transmitted into an object, e.g., a human body, from the transducer array
1
is first stored in the TX focus delay memory
14
. Thereafter, a binary sequence corresponding to the delay pattern stored in the TX focus delay memory
14
is generated and provided to the pulser
11
.
As a method of determining the delay pattern for each of the transducers, a fixed-focusing technique is commonly used, which focuses the energies of the ultrasound pulses on a predetermined point inside the human body. Recently, as one of efforts to resolve the problem of limited resolution due to the fixed-focusing transmission compared to dynamic focusing receiving, a synthetic aperture technique has been studied. With the synthetic aperture technique, one or more transducers can be used for transmitting ultrasound pulses and bi-directional dynamic focusing is possible for both the transmitting and receiving pulses. By using the synthetic aperture technique, the resolution can be improved while SNR (signal-to-noise ratio) is decreased.
The pulser
11
is a bipolar pulser, which supplies an amplified signal (e.g., +80 or −80 volt) to the transducer array
1
in response to the binary sequence from the TX focus delay memory
14
. The voltage output of the pulser
11
, having a predetermined amplitude, is applied to each transducer of the transducer array
1
at a time determined by the delay pattern.
The transducer array
1
transmits the ultrasound pulses, in response to the output voltage of the pulser
11
, into the object. A portion of the transducers in the transducer array
1
may selectively be used for one time transmission even if the transducer array
1
includes N, e.g., 128, transducers. For example, only 64 transducers within an aperture may be utilized for transmitting the ultrasound pulse at one time.
After transmitting the ultrasound pulses into the body, the transducer array
1
receives a pulse signal that is reflected from the body.
The TX/RX switch
21
acts as a duplexer for isolating the receiver
31
from the pulser
11
to protect the high voltage output from being applied to the receiver
31
. The switch
21
connects the transducer array
1
to the pulser
11
during the transmission mode and to the receiver
31
during the reception mode.
The receiver
31
includes a pre-amplifier for amplifying the received signal, a TGC (time gain compensator) for compensating the attenuation during propagation of the ultrasound pulses and an analog-to-digital converter for converting the amplified received signal to a corresponding digital signal.
The beamformer
37
performs RX focusing for the corresponding digital signal that is provided from the receiver
31
in accordance with the delay pattern from the RX focus delay adjuster
36
.
The signal processor
41
performs the signal processing such as envelope detection, log compensation to produce a B-mode image signal.
The scan converter
42
converts the B-mode image signal to a signal, which can be represented on a display device (not shown).
Due to the decrease in power of the ultrasound pulse during the propagation into highly attenuating medium such as rubber, soft tissue and the like, the short-pulse imaging system may not obtain correct information for a target object deep inside the body.
Since the medical ultrasound imaging system
100
may cause damage to the body if it increases the peak voltage of the transmitted short pulses, the power of the received signal cannot be increased by increasing the power of transmission pulse.
On the other hand, a pulse compression technique that is used in a radar apparatus is capable of improving the SNR of the ultrasound imaging system by increasing the average power of the transmitted pulse instead of increasing the peak voltage thereof. In an imaging system using such a pulse compression technique, generally, a long-duration waveform signal (“long pulse”) instead of the short pulse is transmitted to the body to increase the SNR.
In the medical imaging system
100
using the conventional short pulse, the image resolution in the ultrasound propagation direction depends on the impulse response of the ultrasound transducer which is selected and used due to the use of short pulses with a high voltage. However, in the imaging system using the pulse compression technique, the image resolution is determined by the convolution of the ultrasound transducer and the long pulse.
In an imaging system using the pulse compression technique, by using a pulse compressor having a FIR (Finite Impulse Response) filter at the ultrasound receiver, it is capable of effectively increasing the SNR by transmitting the long pulse signal having a lower voltage than the peak voltage in the short pulse technique.
In the ultrasound imaging system using the long pulse signal, the system performance is known to depend on characteristics of the long pulse signal used therein. In particular, the image quality is based on the relationship between the frequency characteristics of the long pulse signal and the ultrasound transducer. The system performance also depends on how the pulse compressor or the FIR filter is implemented.
Further, since the pulse compressor should be used per each channel for dynamic RX focusing, hardware complexity of the receiving part of the system depends on a structure of the pulse compressor.
In the ultrasound imaging system using the long pulse signal, a spread spectrum signal, e.g., a chirp signal (a linear frequency modulation signal), can be used as the long pulse signal. Particularly, The chirp signal has frequency characteristic that matches with the spectrum of the transducer of the ultrasound imaging system having a limited bandwidth. The chirp signal after passing the conventional FIR filter also has its peak side lobes that are −13 dB below its main lobe. However, the conventional spread spectrum signal is not suitable for use in the medical ultrasound imaging system because side lobes of the spectrum of the output signal from the pulse compressor should be −50 dB or more below the main lobe, to be used in the medical imaging system.
SUMMARY OF THE INVENTION
It is, therefore, a primary objective of the present invention to provide an ultrasound imaging method and system based on a pulse compression technique that uses a spread spectrum signal having tolerable side lobes.
Another objective of the present invention is to provide a FIR filter having an efficient structure and a method of determining the coefficients of the FIR filter for use in the ultrasound imaging system using the spread spectrum signal.
In accordance with one aspect of the present invention, there is provided an ultrasound imaging method for forming an image of an object using signals reflected from the object after transmitting an ultrasound pulse to the object, comprising the steps of (a) converting a predetermined first spread spectrum signal to the ultrasound signal at one or more transducers and transmitting the ultrasound signal to the object, (b) performing pulse compression on a reflected signal of the ultrasound signal refl
Song Tai Kyong
Yoo Yang Mo
Carlson Dale L.
Kinney Michael K.
Medison Co. Ltd.
Saint-Surin Jacques
Wiggin & Dana LLP
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