Single-shot phase cancellation ultrasound contrast imaging

Details Single-shot phase cancellation ultrasound contrast imaging Single-shot phase cancellation ultrasound contrast imaging

2000-04-13

2002-04-16

Lateef, Marvin M. (Department: 3737)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

C600S458000

Reexamination Certificate

active

06371914

ABSTRACT:

The invention relates to a method of ultrasound imaging of organs and tissues by detection of ultrasound backscatter from a region which may contain nonlinear scatterers such as microbubbles used as a contrast agent, the method comprising projecting an ultrasound beam to a zone of tissue to be imaged, receiving the echo reflected from the tissue as a radiofrequency response signal, processing the radiofrequency response into a demodulated video output signal, storing the output in a video scan converter, and scanning the tissue to produce a video image of the region under investigation. The invention also comprises a system for ultrasonic imaging of organs or tissues which may contain nonlinear scatterers such as microbubbles used as a contrast agent, the system comprising an ultrasonic probe for transmitting and receiving ultrasonic signals, signal processing means, means for storing the processed signals and a display element. Use of the system for imaging of organs, tissues and blood vessels is also disclosed.
BACKGROUND ART
Wide acceptance of ultrasound as an inexpensive non-invasive diagnostic technique coupled with rapid development of electronics and related technology has brought about numerous improvements to ultrasound equipment and ultrasound signal processing circuitry. Ultrasound scanners designed for medical or other uses have become cheaper, easier to use, more compact, more sophisticated and more powerful instruments. However, the changes of acoustic impedance occurring within the living tissue are small and the absorption of ultrasound energy by different types of tissue (blood vessels, organs, etc.) are such that some diagnostic applications remain unmet challenges, despite these technical developments. This situation changed considerably with the development and introduction of administrable ultrasound contrast agents. Introduction of contrast agents made from stabilized suspensions of gas microbubbles or microballoons into the bloodstream and organs to be investigated have demonstrated that better and more useful ultrasound images of organs and surrounding tissue may be obtained with ultrasound equipment. Thus, pathologies in organs like the liver, spleen, kidneys, heart or other soft tissues are becoming more readily recognizable, opening up new diagnostic areas for both B-mode and Doppler ultrasound and broadening the use of ultrasound as a diagnostic tool.
Recently, ultrasound techniques, i.e. scanners, electronic circuitry, transducers and other hardware and software components are showing great progresses in their abilities to exploit, to a fuller extent, the specific properties of ultrasound contrast agents. This is made possible by the vision by ultrasound instrument manufacturers of the vast potential offered by these contrast agents towards more accurate diagnosis, thanks to enhanced imaging capabilities and quantification of blood flow and perfusion. Thus, what was almost independent developments of these related segments of the field are now providing opportunity to draw on synergies offered by studies in which the electronic/ultrasound characteristics of the apparatus and the physical properties of the contrast agent are combined. A few examples of such studies reported improvements from specific agents/equipment combinations, such as harmonic contrast imaging. These synergies are thus opening new areas of experimentation, innovation, and search of more universal methods for producing greater tissue resolution, better image and greater versatility of ultrasound as a diagnostic technique. There is no doubt that, provided their implementation is kept relatively simple, these will be widely accepted.
An attempt towards improved ultrasound imaging is described in WO-A-93/12720 (Monaghan) which discloses a method of imaging of a region of the body based on subtracting ultrasound images obtained prior to injection of a contrast agent from the images of the same region obtained following administration of the contrast agent. Based on this response subtraction principle, the method performs superposition of images obtained from the same region prior to and after administration of the contrast agent, providing an image of the region perfused by the contrast agent freed from background image, noise or parasites. In theory, the method described is capable of providing a good quality images with enhanced contrast. However, in practice, it requires maintenance of the same reference position of the region imaged for a long period of time, i.e. long enough to allow injection and perfusion of the contrast agent and maintenance of an enormous amount of data. Therefore the practical implementation of the method is very difficult if not impossible. The difficulty is partly due to inevitable internal body movements related to breathing, digestion and heart beat, and partly due to movements of the imaging probe by the ultrasound operator. Most realtime imaging probes are commonly handheld for best perception, feedback and diagnosis.
Interesting proposals for improved imaging of tissue containing microbubble suspensions as contrast agent have been made by Burns, P., Radiology 185 P (1992) 142 and Schrope, B. et al., Ultrasound in Med. & Biol. 19 (1993) 567. There, it is suggested that the second harmonic frequencies generated by non-linear oscillation of microbubbles be used as imaging parameters. The method proposed is based on the fact that normal tissue does not display nonlinear responses to the same extent as microbubbles, and therefore the second harmonics method allows for contrast enhancement between the tissues with and without contrast agent. Although attractive, the method has its shortcomings, as its application imposes several strict requirements. Firstly, excitation of the fundamental “bubble-resonance” frequency must be achieved by fairly narrow-band pulses, i.e. relatively long tone bursts of several radio-frequency cycles. While this requirement is compatible with the circuits and conditions required by Doppler processing, it becomes hardly applicable in the case of B-mode imaging, where the ultrasound pulses must be of very short duration, typically one-half or one-cycle excitation. In this case, insufficient energy is converted from the fundamental frequency to its “second-harmonic”, and thus the B-mode imaging mode cannot greatly benefit from this echo-enhancing method. Secondly, the second harmonic generated is attenuated, as the ultrasound echo propagates in tissue on its way back to the transducer, at a rate as determined by its frequency, i.e. at a rate significantly higher than the attenuation rate of the fundamental frequency. This constraint is a drawback of the “harmonic-imaging” method, which is thus limited to propagation depths compatible with ultrasound attenuation at the high “second-harmonic” frequency. Furthermore, in order to generate echo-signal components at twice the fundamental frequency, “harmonic imaging” requires non-linear oscillation of the contrast agent. Such behavior imposes the ultrasound excitation level to exceed a certain acoustic threshold at the point of imaging (i.e. at a certain depth in tissue). During nonlinear oscillation, a frequency conversion takes place, causing in particular part of the acoustic energy to be converted from the fundamental excitation frequency up to its second harmonic. On the other hand, that level should not exceed the microbubble burst level at which the microbubbles are destroyed, and hence harmonic imaging will fail due to the destruction of the contrast agent in the imaging volume. The above constraints require that the imaging-instrument be set-up in such a way as to ensure the transmit-acoustic level to fall within a certain energy band: high enough to generate second harmonic components, but low enough to avoid microbubble destruction within a few cycles.
Thus, methods which treat electronic echo signals during normal realtime (“on the fly”) investigations are those most desirable, allowing better imaging and wider use of ultrasound diagnostic imaging. Such methods are based on

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