Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-03-30
2002-07-16
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06419632
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a diagnostic ultrasound apparatus and a method of diagnosis with ultrasound images capable of visualizing images of blood flow as an element in motion in an object to be imaged at a higher degree of resolution and high sensitivity, and in particular, to the apparatus and method preferable to an contrast echo technique performed with an ultrasound contrast agent injected into an object for acquiring blood flow images.
2. Description of the Related Art
A diagnostic ultrasound apparatus has a wide variety of advantages, such as, relatively compact in size, lower price, no X-ray exposure, and availability of blood flow imaging on an ultrasound Doppler technique, thereby providing an indispensable today's imaging modality in the field of clinics.
Particularly, the blood flow imaging on the ultrasound Doppler technique, which functions strongly in finding lesions in the cardiac system and others, is called color flow mapping (CFM) or color Doppler tomography and has been standardized in almost all the diagnostic ultrasound apparatus. This color flow mapping two-dimensionally displays blood flow information in almost real time, where a flow toward an ultrasound probe is displayed in red, while a flow away from the probe is displayed in blue.
To perform such display, it is required that the same location in an object be ultrasound-scanned a plurality of times, N, to acquire a time-sequential echo signal and a velocity of blood cells at a desired depth location is detected from the echo signal on the Doppler technique. That is, a Doppler signal is derived from an amount of phase shift per unit time of a reflected signal (blood flow signal) originated from blood cells through scanning of the same location at intervals, and converted into a velocity of blood flow.
In an echo signal associated with ultrasound scanning at each time, there are mixed of a reflected wave from moving substances such as blood cells and another reflected wave from stationary substances, such as the wall of a blood vessel and tissue, which scarcely move. It is characteristic that the latter is dominant in reflected intensity, but there occurs almost no Doppler shift in the reflected wave (clutter signal) from the stationary reflection. members belonging to the latter, while the former has the Doppler shift therein. Thus a Doppler signal is extracted from the echo signal by a quadrature phase detector (comprising a mixer and an LPF), and a blood flow Doppler signal is efficiently extracted by an MTI filter which removes a clutter signal component from the Doppler signal on differences in Doppler shift amounts. The blood flow Doppler signal then experiences frequency analysis carried out using N-piece Doppler data at each depth location, producing an average of the spectrum. (Doppler frequency), dispersion, or reflection intensity from blood cells. The Doppler frequency f
d
is converted into a Doppler velocity v
d
according to this formula:
v
d
=f
d
·c/
(2
f
M
·cos&thgr;) (1),
wherein c denotes a sound velocity, f
M
does the frequency of a reference signal in the mixer, &thgr; does an angle made between an ultrasound beam and blood flow. This information about blood flow thus-obtained is, in general, two-dimensionally displayed on a monitor, with a B-mode image employed as a background.
A CFM mode used in performing this color flow mapping (CFM) will now be compared with a B mode in terms of resolution, S/N, dynamic range for display, aliasing frequency, realtime performance, and others.
The number of burst waves associated with transmitted ultrasound waves differs between the B mode and CFM mode. The burst wave number is defined as the number, per cycle, of ultrasound pulses having the length of a transmission repetition T
0
that is the inverse of an ultrasound transmission frequency f
0
.
The B mode is directed to observing a tomographic image, that is, an image composed of ultrasound signals echoed from tissue. The reflected signal from the tissue is able to have a satisfactorily high S/N, because the reflected signal can be detected at fully large signal intensities within a range of ultrasound pressures which are determined with consideration of safety for an object to be diagnosed. Hence the burst wave number can be set to a lower value, such as one to two waves, and a range resolution can be increased satisfactorily, fulfilling both the S/N and range resolution.
By contrast, the CFM mode is used for observing blood flow, which corresponds therefore to a reflected signal from blood cells (blood flow signal). This blood flow signal is considerably less in signal intensity, approximately −40 to −80 dB, than that acquired from the tissue. Under the same transmission pulse condition as that in the B mode, the CFM mode provides an inferior S/N, with blood flow information substantially unavailable.
The S/N can therefore be improved by increasing the power of an ultrasound pulse to be transmitted. However, in general, since the transmitted ultrasound pressure has a limitation that has been determined with consideration of safety for an object in the B mode, it is difficult to raise the pressure any more. As a result, the number of burst waves is determined at a larger value, such as three waves or more, to enhance power of the ultrasound pulse to be transmitted. An excessively large burst wave number cause, however, range resolution to be deteriorated, thus an upper limit of the burst wave number being determined dependently on an allowed value of the range resolution.
Although the S/N of the blood flow signal can be improved in this way, the power of the blood flow signal still remain smaller by approx. a few dozes of dB than the power of a single reflected from tissue, even when the burst wave number would be raised up to the upper limit within the tolerance thereof. This causes differences in dynamic ranges for display. The dynamic range for B-mode display is large; for example, 100 dB in maximum, although the dynamic rage for the power mode displaying power under the CFM mode is small; for example, 40 dB in maximum.
The ultrasound pulse is repetitively transmitted at a pulse repetition time T
r
. Thus in the velocity mode displaying Doppler velocities under the CFM mode, an aliasing phenomenon will occur, due to the sampling theory, at ±f
r
/2 which is half the pulse repetition frequency f
r
=1/Tr inverted from the pulse repetition time. The values of ±f
r
/2 are referred to as aliasing frequencies. The sign ± means that the direction is separated. From the foregoing equation (1), an aliasing velocity v
r
/2 corresponding to the aliasing frequencies is obtained as follows, by setting &thgr;=0:
v
r
/2=(
f
r
/2)·(
c
/2
f
M
) (2).
Because c and f
M
are constant, the aliasing velocity v
r
/2 becomes constant as well. This aliasing velocity is normally displayed, for diagnosis, on a TV monitor together with a two-dimensional image indicative of blood flow information.
In the B mode, a tomographic image is obtained by performing one time transmission and reception of an ultrasound pulse along the same raster (beam) direction, while in the CFM mode, imaging is based on Doppler signals obtained by performing the transmission and reception of an ultrasound pulse a plurality of times along the same raster direction. Thus, the CFM mode is largely lowered in frame rate than the B mode. For instance, where the transmission and reception is desired to be repeated sixteen times in the same direction, the transmission and reception is required to be repeated seventeen times in total, including scanning of the B mode. If the number of frames for the B mode is ten frames per sec., the number for the CFM mode is six frames per sec., thereby reducing realtime performance.
As a countermeasure to improve the realtime performance in the CFM mode, a technique called “parallel simultaneous reception” has now been in practical use, where the transmissi
Mine Yoshitaka
Shiki Eiichi
Imam Ali M.
Jaworski Francis J.
Kabushiki Kaisha Toshiba
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