Ultrasound imaging system using multi-stage pulse compression

Details Ultrasound imaging system using multi-stage pulse compression Ultrasound imaging system using multi-stage pulse compression

2002-09-05

2003-12-30

Jaworski, Francis J. (Department: 3737)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

C600S443000

Reexamination Certificate

active

06669640

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an ultrasound imaging system, and more particularly, to an ultrasound imaging system having an efficient hardware structure and capable of providing a high-resolution ultrasound image by adopting a multi-stage pulse compression scheme.
BACKGROUND OF THE INVENTION
The ultrasound imaging system is widely used in the medical field for the purpose of displaying a sliced image (ultrasound image) of a “target object” such as an internal organ of a human body. In such ultrasound imaging systems, an ultrasound image is formed by transmitting ultrasound signals towards the target object, receiving the signals reflected from the target object, more specifically, from a surface of the target object (e.g., an interface between skin and subcutaneous fat, between subcutaneous fat and abdominal muscles, etc., where the acoustic impedance is discontinuous), and converting the received ultrasound signals into electrical signals. For ultrasound signal transmission purposes, the ultrasound imaging system uses a transducer and a pulser for driving the transducer. The transducer generates ultrasound signals in response to a pulse applied from the pulser.
Most of the conventional state of the art ultrasound imaging systems employ a short pulse as ultrasound transmission signals. In such systems, the power of the signals received at the transducer is remarkably lowered since the transmitted ultrasound signal undergoes severe attenuation when passing through a highly dense medium, such as the human body. As a result, obtaining the desired information on the target object, e.g., in cases where the target object is located deep inside a body, becomes difficult. Increasing the peak voltage of the pulse being transmitted may solve the problems associated with the attenuation of the ultrasound signals. However, there is a certain limitation to increasing the peak voltage of the pulse, since this may affect the internal organs of the human body.
Instead of increasing the peak voltage of the pulse, the average power of the pulses can be raised. As a result, the Signal to Noise Ratio (SNR) can be improved remarkably. This method is called “pulse compression,” and is used, for example, in radar equipment. An ultrasound imaging system of the type that employs pulse compression normally uses a coded long pulse having a long duration instead of the conventional short pulse. In this type of ultrasound imaging systems, the resolution in the direction of ultrasound wave propagation, the “axial resolution,” is determined by the convolution taken between the characteristic function of the transducer and the coded long pulse, in contrast to a conventional system employing short pulses of high peak voltage where the axial resolution is determined by the impulse response characteristics of the transducer. Therefore, in order to avoid the degradation of the axial resolution that may be caused by the use of the coded long pulse, such ultrasound imaging apparatuses use a correlator-based pulse compressor that takes a cross-correlation between the received ultrasound signal and the coded long pulse as transmitted. Using the correlation at the pulse compressor can prevent degradation of the axial resolution, allowing the same level of resolution to be maintained as if a short pulse were transmitted. Accordingly, a relatively low voltage of the long duration can be advantageously used without sacrificing the SNR.
Additionally known in the art, ultrasound imaging systems may also be based on a phased array. Such an ultrasound imaging system includes a plurality of channels, each channel including a transducer, a transmitter (i.e., pulser) and a receiver coupled to the transducer. The transmitter functions to transmit ultrasound signals (or pulses) towards the target object such as a human body. Note that the transmitters at the plurality of channels do not transmit ultrasound signals at the same time. Instead, they transmit the ultrasound signals with a different timing so that the ultrasound signals as transmitted from the transmitters reach a desired position within the target object at the same time, thereby being transmit-focused at a predetermined location within the target object. The transmitted ultrasound pulses pass through various internal organs of the human body and are reflected from a certain portion of the internal organs and directed to the transducer array.
The ultrasound signals reflected from the target object are received by the transducer array and are converted into electric signals. The time when the reflected signals reach each of the transducers varies depending on the location of each transducer in the array relative to the target object. That is, the farther away from the center position of the array the transducer is located, the more time period is required for the ultrasound signals to reach the transducer. In order to compensate for the differences in arrival time among the transducers, a beamformer is used to receive focus the converted electrical signals. The beamformer incorporates appropriate time delays into the electrical signals, which correspond to the received ultrasound signals, giving rise to the same effects as if all the transducers receive the reflected signals at the same time. The time delays as applied by the beamformer vary depending on the depth of the reflecting surface of the target object and the locations of the transducers.
The beamformer is further explained below with reference to
FIG. 1
, which illustrates the structure of a beamformer in a conventional ultrasound imaging system. As shown, beamformer
100
comprises transducer array
10
including a plurality of transducers, delay stage
11
comprised of a corresponding number of delay elements DLY
1
-DLY
64
to the transducers, adder
12
, and pulse compressor
13
connected to the output terminal of adder
12
. The reflected ultrasound signals are converted to electric signals at the transducers and are transmitted to delay stage
11
. Each delay element at delay stage
11
compensates the input signals by a predetermined time delay depending on the location of the corresponding transducer relative to the center of transducer array
10
. Therefore, the differences in arrival time among the transducers can be compensated by the use of delay elements, which are connected to the output terminals of the transducers. The delay-processed signals from delay elements DLY
1
-DLY
64
are added together in adder
12
. Pulse compressor
13
pulse-compresses the output signal from adder
12
. According to the beamformer of
FIG. 1
, the system configuration can be simplified, but problems arise where the beamformer of
FIG. 1
adopts dynamic receive-focusing. If receive-focusing is performed dynamically in the beamformer of
FIG. 1
where pulse compression occurs after receive-focusing, delay times necessary for receive-focusing may be inaccurately computed, as explained below.
Preferably, the beamformer of
FIG. 1
may adopt dynamic receive focusing, according to which a focusing point is dynamically changed while the ultrasound signals are propagating through the human body. According to the dynamic receive-focusing, the time delay value for the center transducer is fixed to a predetermined value. For some transducers adjacent to the center transducer, the time delay is controlled to be shorter than the fixed time delay for the center transducer. For the remaining transducers far from the center transducer, the time delay is controlled to be close to the fixed time delay of the center transducer. With the dynamic receive-focusing, the time delays for the transducers are continuously controlled to ensure that the signals reflected from the same focusing point can be summed. As a result, the time delays for the outside transducers transition from a low to high value, as the receive-focusing operation proceeds. Therefore, the ultrasound signals from the outside transducers are distorted as if their frequencies were lowered.
Turning again to the problems encountered with the

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