Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-11-24
2003-02-18
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S455000, C600S443000
Reexamination Certificate
active
06520915
ABSTRACT:
FIELD
This patent specification relates to the field of ultrasound information processing. In particular, it relates to a method and system for detecting and displaying fluid flow in medical ultrasound applications.
BACKGROUND
In recent decades ultrasonic imaging technology has played an increasing role in examining the internal structure of living organisms. The technology has applications in diagnosis of various medical ailments where it is useful to examine structural details in soft tissues within the body. Ultrasound imaging systems are advantageous for use in medical diagnosis as they are non-invasive, easy to use, and do not subject patients to the dangers of electromagnetic radiation. Instead of electromagnetic radiation, an ultrasound imaging system transmits sound waves of very high frequency (e.g., 2 MHz to 10 MHz) into the patient and processes echoes reflected from structures in the patient's body to derive and display information relating to these structures.
As described in Zagzebski,
Essentials of Ultrasound Physics
(Mosby 1996), which is incorporated by reference herein, principal pulse-echo ultrasound display modes includes A-mode (amplitude mode), B-mode (brightness mode), and M-mode (motion mode). An A-mode (amplitude mode) display is a simple plot of instantaneous echo amplitude versus time, measured after the transmission of an acoustic pulse along a single line of a target region. A B-mode (brightness mode) image is a two-dimensional intensity image of echo amplitude for all points in a target region, measured and continually refreshed as acoustic pulses are transmitted along different lines in the target region. An M-mode (motion mode) display is a one-dimensional intensity image of echo amplitude along a single line in the target region that is slowly swept across the screen as time moves forward. Of these principal echo display modes, only the B-mode display provides an actual 2-D “visual” representation of the acoustic reflectivity of tissues in the target region.
More recently, ultrasonic imaging systems have additionally been able to detect fluid flow (e.g., blood flow) in a target region. The detection and measurement of fluid flow is based on the Doppler effect, whereby returned acoustic signals reflected from the flowing fluid are shifted in frequency with respect to the incident interrogating signals. In color Doppler imaging, also referred to as color flow imaging, a sequence of pulses is transmitted down each line in the target region, and phase changes in the echo signals are detected and processed to determine the direction and velocity of fluid flow for each location in the target region. As known in the art, the measured flow direction is only a binary metric—either “toward” or “away from” the transducer—because fluid flow can only be detected in terms of its projection along the path of the incident interrogating pulse. Thus, the true fluid velocity can only be measured to within a factor of cos(&thgr;
d
), where &thgr;
d
is the angle between the actual fluid flow direction and the path of the incident interrogating pulse. The term Doppler frame is used herein to denote a timewise-adjacent sequence of pulses for determining fluid velocity at each location in a target region.
In conventional color Doppler imaging, color Doppler frames are transmitted during time intervals lying between conventional B-mode frames. In a typical conventional ultrasound system having color Doppler capability, a flow image containing the flow information is superimposed on a conventional B-mode image output. In the resulting display, stationary tissue is depicted by a standard B-mode intensity value, while flowing fluid is depicted by red (for fluid flowing toward the probe) or blue (for fluid flowing away from the probe). The measured velocity at each location is typically indicated by a color saturation or luminance value, whereby low velocities are depicted by “dim” color and high velocities are depicted by “bright” color.
An alternative to color Doppler imaging, referred to in the art as power Doppler, power flow, or energy flow imaging, does not measure the direction or velocity of fluid flow. Rather, only the amount of Doppler energy in the echo signal is measured for each location in the target region. Thus, for a given location in the target plane receiving the sequence of “m” Doppler pulses, where the magnitude and phase of the reflections are given by a complex sequence R(k)={R
1
, R
2
, R
3
, . . . , R
m
}, the complex sequence R(k) is high-pass filtered to remove effects of stationary tissues. The remaining signal represents the Doppler energy in the sequence R(k). Conventional power Doppler systems display a flow image that is a monochromatic or color intensity image of this Doppler energy, the flow image being superimposed on a conventional B-mode display. As described in Zagzebski, supra, power Doppler systems are usually credited with being more sensitive to the presence of fluid flow as compared to color Doppler systems, while being generally insensitive to the actual velocity of the fluid flow. Other tradeoffs and comparisons between color Doppler and power Doppler modes are described in Zagzebski, supra.
One disadvantage of conventional Doppler systems (color Doppler or power Doppler) lies in their lack of frame rate and spatial resolution as compared to B-mode systems. While a typical B-mode system may have a frame rate of 30 frames/sec at a spatial resolution of 1024 lines at 1024 samples/line, the invocation of a Doppler feature can drop the frame rate to as low as 8 frames/sec, with the flow image being a mere 64 lines at 64 samples/line. It is to be appreciated that these parameters are presented by way of example only in order to illustrate certain aspects of the prior art, and are not intended to limit the scope of the preferred embodiments disclosed infra. A primary reason for the low frame rate and resolution for Doppler systems lies in the substantial number of data samples needed per location to get acceptable velocity measurement accuracy. The number of data samples per location corresponds to the number of pulses (vectors) that need to be sent down a given line during Doppler frame acquisition. According to the uncertainty principle, the frequency shift cannot be detected accurately when the observation period is short. Although interleaving schemes known in the art (e.g., fire vector 1 for line 1, vector 1 for line 9, vector 2 for line 1, etc.) can increase the time spacing between vectors without decreasing the frame rate, there is still a substantial number of vectors needed per line (e.g., 24) to get acceptable frequency shift (i.e., velocity) measurements. Each vector needs a dedicated round-trip time from the probe, a 10-cm trip in a typical scenario. Using the speed of sound at 13 &mgr;s/cm, then the Doppler frame acquisition time is equal to (13 &mgr;s/cm)(10 cm/vector)(64 lines/frame)(24 vectors/line)=199.7 ms/frame. And, as stated supra, this all needs to take place between B-frames, resulting in very low overall frame rates. Furthermore, although the frame rate may be increased by decreasing the number of samples per location or by decreasing the number of locations considered, lower resolution and/or velocity accuracy will result.
Another problem found in conventional Doppler systems relates to clutter. Clutter signals, sometimes referred to as flash artifacts, are undesirable Doppler signals that arise from structures and targets in the body that do not represent fluid flow but which nevertheless may have Doppler shifts. Clutter signals may be caused by slow tissue or vessel wall motion arising from heart beats, arterial pulsations, or respiration. Clutter signals can also arise due to movement of the transducer by the operator. These unwanted signals are typically filtered out, so that the flow image only represents true fluid flow and suppresses clutter. Clutter signals that have not been adequately suppressed are subsequently confused with flow signals, and are typically seen in
Lin Shengtz
Phung Hue
Cooper & Dunham LLP
Jaworski Francis J.
Patel Maulin
U-Systems Inc.
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