Traditional ultrasound images are formed by first transmitting ultrasound to a medium of interest and then receiving the ultrasound signals resulting from the interaction of the transmitted signals with the medium. This kind of an image is usually a representation of the mechanical properties of the medium and provides structural or anatomical information. The interaction of the medium with other forms of energy can provide additional information about the functional differences even in a structurally indifferent, uniform medium. For instance, when a short laser pulse is transmitted into a tissue, the introduced light energy is absorbed and scattered in a different manner by different parts of the tissue. The optical absorption depends on the wavelength of the light and the properties of the medium at the molecular or even atomic level. Regions with stronger absorption characteristics in a tissue generate stronger acoustic signals via the thermoelastic effect, which is simply the thermal expansion of the imaging regions resulting in a mechanical disturbance and hence an acoustic signal. By collecting these light-induced acoustic signals using a transducer or array of transducers, one can construct an image that is a representation of the light absorption characteristics of the sample. One example of this approach is to image the microvasculature in tissue by detecting blood oxygenation, which is usually a sign of angiogenesis indicating a cancerous lesion. In this example, the increased light absorption of the oxygenated blood is used to create a high-contrast image.
Micromachined Transducers Source
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Existing functional ultrasound imaging methods are based on mechanically scanned single transducers, or the combination of a laser source with a one-dimensional commercial imaging probe. These approaches do not provide real-time three-dimensional images. In addition, current devices are bulky and not suitable for intracavital applications.
The present invention provides an apparatus for functional imaging of an object that is compact, sensitive, and provides real-time three-dimensional images. The apparatus includes a source of non-ultrasonic energy, where the source induces generation of ultrasonic waves within the object. The source can provide any type of non-ultrasonic energy, including but not limited to light, heat, microwaves, and other electromagnetic fields. Preferably, the source is a laser. The apparatus also includes a single capacitive micromachined ultrasonic transducer (CMUT) device or an array of CMUTs. In the case of a single CMUT element, it can be mechanically scanned to simulate an array of any geometry. Among the advantages of CMUTs are tremendous fabrication flexibility and a typically wider bandwidth. Transducer arrays with high operating frequencies and with nearly arbitrary geometries can be fabricated. The wider bandwidth of CMUTs provides better image resolution and potential for novel imaging methods.
The present invention also provides a method of functionally imaging an object. The method includes the steps of exposing the object to a source of non-ultrasonic energy, where the source induces generation of ultrasonic waves in the object, and detecting the generated ultrasonic waves with a CMUT device.
The present invention provides an apparatus for functional ultrasound imaging of an object, including a source of non-ultrasonic excitation energy and a single CMUT or an array of CMUTs. The source may be any type of source, including but not limited to light (with different wavelengths depending on the absorption characteristics of the imaging target), rapid thermal heating, microwaves, radio-frequency (RF) electromagnetic waves and other electromagnetic fields, electron beams, etc., but is preferably a laser. The CMUT arrays may be in any type of configuration. FIG. 1 shows examples of array configurations according to the present invention, including an annular ring array (FIG. 1(a)), an annular array (FIG. 1(b)), a one-dimensional linear array (FIG. 1(c)), a two-dimensional rectangular array (FIG. 1(d)) and a cylindrical array (FIG. 1(e)). CMUT arrays may also be formed on a curved surface. In addition, arrays may be formed around the target object to allow tomographic image reconstruction methods. A single CMUT or multiple CMUTs can be mechanically scanned to simulate an array with more elements.
Several apparatus designs are possible according to the present invention, based on different types of non-ultrasonic radiation sources and CMUT arrays with different geometries. For medical applications, these apparatuses can be used externally or from within the body. Some sample designs for functional ultrasonic imaging apparatuses employing a laser excitation and a CMUT array are shown in FIG. 2. FIG. 2 (a) shows an apparatus with a linear CMUT array 110 in conjunction with an optical fiber 120 to provide a short laser pulse in the form of laser beam 122. This apparatus has an imaging field indicated by dashed lines 112. This type of apparatus provides a two-dimensional cross-sectional image. To obtain a volume image with this kind of apparatus requires mechanical scanning. A real-time three-dimensional functional image can be acquired by using a two-dimensional aperture that can be electronically scanned. One example of such an apparatus is shown in FIG. 2 (b). This apparatus again has an optical fiber 120 to provide a short laser pulse 122. This apparatus employs a two-dimensional rectangular array 130, which provides an imaging field, indicated by dashed lines 132, which is perpendicular to the laser beam 122. The array can also be used in parallel with the laser beam 122. Such an approach is shown in FIG. 2 (c) where an annular ring array 140, with imaging field indicated by dashed lines 142, is used to form a real-time three-dimensional functional image. The internal cavity of the array 140 is occupied by the optical fiber 120 to provide the laser pulse 122. Another advantage of the ring array is that the working channel can contain not only the optical fiber that brings in the light beam, but also may bring in a therapeutic device to burn an occlusion, scissors to extract a piece of tissue, or any other needed working tool. The arrays depicted in these sample designs can be integrated with supporting integrated circuits to improve the overall image quality. These examples are provided to help visualize the general approach according to the invention and are not meant to describe all possibilities.
In one embodiment of the invention, a silicon substrate is used to allow the described non-ultrasonic energy sources to be integrated on the same substrate with the CMUT array. Vertical cavity surface emitting lasers, microfabricated electron beam sources, and nanokylstrons for microwave generation are examples of sources that may be integrated with the CMUT array.
The present invention also provides a method of functionally imaging an object, including the steps of exposing the object to a source of non-ultrasonic energy, generating ultrasonic waves in the object, and detecting the ultrasonic waves in the object. This method is shown schematically in FIG. 4. Object 410, with high absorption region 412, is exposed to non-ultrasonic excitation energy, indicated by arrows 422, from source 420. The non-ultrasonic energy then generates ultrasound waves in the object 410. These waves are in turn detected by CMUT array 430. The received signal 440 is an indication of a strong absorber of the non-ultrasonic excitation energy.
The excitation energy can also be used for therapeutic applications. For example, the design described in FIG. 2(c) could be used for both photoacoustic imaging and tissue ablation by increasing the power level of the laser source. Similarly, microwaves and RF fields can be used for ablation of tissue. The method of the present invention may also be used to monitor the therapy, such as the extent and the nature of the lesion resulting from the ablation procedure. Other uses of the present invention are applications such as non-destructive testing and acoustic microscopy.
The transducer array has 256 elements (1616 elements). Each element is 250 μm250 μm. Thus, the entire array size is 4 mm4 mm. The transducers have a center frequency of 5 MHz. The CMUT array was fabricated using surface micromachining with membranes made of silicon nitride. A few of the key CMUT device parameters are shown in Table 1. A more thorough description of the design and fabrication of the CMUT array has been reported elsewhere. A description of the CMUT array and integrated electronics has also been previously reported. The transducer array is flip-chip bonded to a custom-designed integrated circuit (IC) that comprises the front-end circuitry. The result is that each element is connected to its own amplifier via a 400-μm long through-wafer via. Integrating the electronics in this manner mitigates the effect of parasitic cable capacitance and simplifies connecting the transducer array to an external system. The IC allows for the selection of a single element at a time. Thus, 256 pulses are required to acquire a single image with no averaging. For a propagation limited system, this allows a maximum achievable frame rate of 100 frames/sec for imaging a 3-cm volume in oil. TABLE 1CMUT Device ParametersCell diameter, μm36Element pitch, μm250Number of cells per element24Membrane thickness, μm0.6Cavity thickness, μm0.1Insulating layer thickness, μm0.15Silicon substrate thickness, μm400Flip-chip bond pad diameter, μm50Through-wafer interconnect diameter, μm20 Results
In this chapter the basic principles, the fabrication process, and some modeling approaches of the novel micromachined ultrasonic transducers (MUTs) are described. These transducers utilize the flextensional vibration of an array of micromembranes. They are usually called cMUT (capacitive MUT) or pMUT (piezoelectric MUT) depending on the actuation principle, electrostatic or piezoelectric. For water coupling applications, both these kinds of transducers offer a better matching to the load compared with the typical piezoelectric transducers and therefore they have a larger intrinsic bandwidth. Here emphasis is given to the cMUTs because they have shown good electroacoustic characteristics, which parallel, or even exceed, those of conventional piezoelectric transducers. Good echographic images of internal organs of the human body have been obtained demonstrating the possibilities of this technology to be utilized in commercial 1D and 2D probes for medical applications. At present pMUTs are in a very early stage of development and the potential advantages over the cMUTs are still to be demonstrated. 2ff7e9595c
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