Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-10-25
2002-12-24
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06497666
ABSTRACT:
BACKGROUND
This invention relates to medical ultrasonic imaging, and in particular to systems for improved imaging of contrast agent included in imaged tissue.
The nonlinear response of contrast agents such as microbubbles improves the detectability of microbubbles in tissue. Contrast agents effectively generate unique ultrasound signals such as second harmonic, third harmonic, subharmonic, or other harmonics that are not transmitted in significant quantities by a transducer. Good second harmonic contrast agents, in particular, have been shown to generate strong signals with minimal destruction of a population of contrast microbubbles.
Another unique property of contrast agents is that they are disrupted by impinging sound waves. Ultrasound pulses in particular can move, crack, rearrange, split, and destroy microbubbles and their gas-encapsulating shells. When this type of disruption occurs, at least two received pulses will be decorrelated when at least two identical pulses are transmitted during at least two separate transmit events. A loss-of-correlation (LOC) between received signals may be used to detect contrast agents, since areas that contain little or no contrast agent will return similar signals among the multiple received pulses and produce strong signal correlations.
There is a need for an improved LOC imaging technique that offers excellent specificity for contrast agents, high bandwidth resolution, and high frame rates. Further, there is a need to increase the spatial uniformity of detected contrast agents within an image frame when LOC techniques are implemented. Still further, there is a need to offer a contrast-only image that may or may not be displayed with an image substantially free of detected contrast agent.
Among the prior-art techniques for detecting contrast agents at least five disadvantages exist, as follows.
1.) Poor Axial Resolution Due to Narrowband Receive Filtering
A technique often labeled as “harmonic imaging” specifically filters each received pulse/signal with the goal of suppressing fundamental energy and retaining second harmonic energy. This technique limits the bandwidth of returned signals and therefore limits axial resolution. “Power Harmonics” by ATL or harmonic “Power Contrast Imaging” (PCI) by Acuson are two examples of this technique. Since the ratio of returned contrast agent energy to tissue energy for second harmonic signals is typically larger than the ratio of returned contrast agent energy to tissue energy for fundamental signals, second harmonic signals have been preferred for imaging contrast agents. By adequately suppressing fundamental energy by filtering each received pulse separately within a set of multiple received pulses, less tissue flash is introduced into the images if a clutter filter is applied across the two or more received pulses at each range. The purpose of the clutter filter is to remove signals from stationary or slowly moving targets. Since returned fundamental signals are the largest amplitude signals returned from stationary or slowly moving tissue as compared to harmonic signals, separately pre-filtering each returned pulse with a narrowband filter that suppresses fundamental energy reduces the amount of stopband rejection and/or stopband width required of the clutter filter. Fewer taps may be used with these types of clutter filters, and therefore fewer pulses need to be transmitted, thereby improving frame rates and minimizing unnecessary agent destruction.
Other examples of prior art that selectively filters second harmonic signals include “Harmonic Power Mode Doppler . . . ” by Burns et. al [1] and “Ultrasonic Diagnostic Imaging with Contrast Agents” by Averkiou [2]. The former reference specifically describes a narrow-band digital filter in the Instrumentation subsection of the Methods section. The latter specifically describes the desire for narrowband filtering in Col. 5, lines 23 through 35, with reference to
FIG. 6
, and Col. 6, lines 13-19. Full citations for these and the other references cited in this section are provided in Table 1.
In other prior-art systems that selectively image the fundamental signal and suppress the second harmonic signal, e.g., the fundamental PCI imaging mode offered by Acuson, axial resolution is similar or inferior to those techniques that specifically suppress fundamental frequency components and retain second harmonic frequency components.
2.) Inability to Separate an Anatomical Reference Image from a Contrast-Only Image
The two-pulse, alternate polarity techniques (U.S. Pat. Nos. 5,706,819 [3], 5,951,478 [4], and 5,632,277 [5]) and multiple-pulse alternate polarity techniques (“Pulse Inversion Doppler” U.S. Pat. No. 6,095,980 [6]) and some Contrast Pulse Sequences (Reference [7]) are unable to show an accurate second harmonic contrast-only image, since tissue second harmonic signals cannot be accurately separated from contrast agent second harmonic signals. Techniques that alternate the transmit envelope polarity between different transmit events modulate the fundamental frequency signal components into the stopband of a clutter filter, which is applied across multiple received pulses at a single point in space, while second harmonic frequency signal components remain unmodulated and minimally suppressed by the passband of the clutter filter. Since the second harmonic signals generated from nonlinear propagation through tissue are not adequately suppressed, a stationary or slowly moving tissue image is inherently integrated with detected contrast agent signals. This is most significant for high mechanical index (MI) imaging, where tissue harmonic signals are strong. This is a significant clinical limitation, since contrast-only images can look much different than images that integrate contrast agent information with anatomical information.
Techniques that can generate contrast-only images can offer additional diagnostic information not available from techniques that alternate the transmit envelope polarity between different transmit events. For example, a 1 cm diameter lesion as seen in a B-mode image may appear as a 0.5 cm diameter lesion in a contrast-only image due to strongly anechoic areas near the perimeter of the lesion absorbing contrast agents. Without the ability to separate these two types of images after the acquisition and detection of a single frame, diagnostic confidence is reduced. Contrast-only image information can facilitate more accurate diagnosis and provide functional information.
3.) Inability to Pre-Scan a Region Before Imaging with a Contrast-Specific Imaging Technique
Current techniques used for contrast agent imaging such as the two-pulse techniques mentioned above lack the ability to pre-scan a region at low power before generating an image showing detected contrast agent with high specificity. For some contrast agents, such as Levovist, high MI scanning is preferred for achieving high agent specificity. However, high MI scanning disrupts the contrast agent, and can quickly deplete a region of contrast agent. Imaging techniques that inherently integrate anatomical tissue signals with contrast-specific signals do not facilitate efficient acquisition and user-selectable display of high-specificity contrast agent images. Once the transducer is placed on the subject, contrast agent is immediately disrupted and depleted. Pre-scanning with an alternating polarity pulsing technique at low transmit power to locate an area of interest and then manually turning up the transmit amplitude/power can be used, but contrast agent is destroyed before the optimal maximum transmit power is reached. Thus, unnecessary agent destruction occurs before the high specificity contrast agent image is generated. Further, optimal specificity may not be possible due to the lesser contrast agent concentration. A manually-activated, instantaneous, substantial increase in transmit power would be useful, but the inability to separate the background image from the contrast-only image still makes
Guracar Ismayil M.
Phillips Patrick J.
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