Ultrasound imaging is routinely used in medicine for non-invasive imaging of tissues and organs, both for diagnosis and for image guided surgery. The resolution of ultrasound imaging is inherently limited by the frequency of the ultrasound used, with higher frequency providing higher resolution but at the expense of reduced tissue penetration. The most common forms of medical ultrasound imaging, cardiac and fetal imaging, utilize ultrasound frequencies in the range of approximately 2 to 10 Megahertz (MHz) in order to image deep tissues. The ultrasound transducers used in general imaging are typically phased array designs, with a plurality of fixed transducer elements used to form a scan head of a probe. The individual transducer elements are switched electronically to sweep the ultrasound beam across an area to be imaged at high frame rates.
High frequency ultrasound imaging, generally in the range of 50 to 100 MHz, is used to obtain images of smaller structures requiring higher resolution than standard imaging. However with high frequencies, it is technically difficult to fabricate a phased array of transducer elements due to the small wavelengths and resultant precision and small size required of the array. Typically, a high frequency ultrasound system scan head consists of a single element transducer, which is moved mechanically with high precision across the area to be imaged. Such methods of mechanical scanning were commonly used in low frequency ultrasound imaging prior to the development of phased array methods.
In ultrasound imaging, the transducer must transmit and receive the ultrasound signals through an appropriate medium, acoustically coupled to the tissue surface of the body area of interest. Ultrasound is highly reflective from a gas interface, therefore the coupling medium must be a solid, liquid or gel substance with the appropriate acoustic properties. The coupling medium must create a continuous acoustic path between the transducer and the body of interest without any entrapped gas, with minimal attenuation and minimal reflectivity of the signal. High frequency single element transducers usually incorporate one or more acoustic matching layers on the face. The matching layer allows the signal to be transmitted from the acoustic element, a material with high acoustic impedance, into a media of much lower impedance, such as water or tissue. The coupling materials between the transducer's matching layers and the tissues are desired to have acoustic impedance characteristics similar to that of tissues, with low attenuation and scattering at the ultrasound frequencies utilized to obtain the best possible signals for imaging. A wide range of plastics, rubbers, and gels are effective at the low frequencies used in general imaging. Common low frequency ultrasound scanners usually employ a urethane polymer covering. A flowable, viscous coupling gel is applied to the covering and to the patient to complete the acoustic pathway. However, ultrasound attenuation and scattering properties increase with frequency. The requirements for materials to interface between the transducer and a tissue surface become more stringent with high frequency ultrasound imaging. In ranges at and above 50 MHz, most commonly used coupling materials are no longer acceptable.
High frequency ultrasound is particularly useful in imaging the eye due to the small dimensions of the associated tissue structures and the relative lack of imaging depth required. Currently available ophthalmic ultrasound scanners use frequencies of approximately 50 MHz to image the eye, with a single element transducer mechanically scanned over the area to be imaged. Typical scanners require that the eye have an eyecup placed around its perimeter and filled with water to acoustically couple the ultrasound signals from the transducer directly to the tissue surface.
However, the use of an eyecup is cumbersome, uncomfortable to the patient, and limits the use of ultrasound imaging during surgical procedures due to the difficulty in maintaining a sterile field and the inconvenience of having to apply the cup when images are required. Imaging systems configured in this manner also represent a serious risk to the patient as there is no barrier between the moving transducer and the surface of the eye.
U.S. Pat. No. 4,483,343 to Beyer et al., entitled “Ultrasonic Applicator” Nov. 20, 1984, describes an ultrasonic applicator system using a liquid containing sack, which is coupled to the transducer element or elements. The liquid containing sack may be adjusted in height and applied to a body surface under examination.
U.S. Pat. No. 4,649,925 to Dow et al., entitled “Ultrasonic Transducer Probe Drive Mechanism with Position Sensor” Mar. 17, 1987, describes a mechanical scan drive mechanism for an ultrasound probe with a cap over the imaging end, described to be made of polyethylene or other material, which is highly transmissive to ultrasound.
U.S. Pat. No. 4,722,346 to Chen, entitled “Stand-Off Device with Special Fluid” describes a standoff device for use with acoustic transducers in which a chamber having a diaphragm portion containing a liquid acts as the coupling medium.
U.S. Pat. No. 4,796,632 to Boyd et al., entitled “Standoff Adapter for Ultrasound Probe” Jan. 10, 1989, describes an adapter and standoff for an ultrasound probe comprising a molded elastic coupler that is filled with fluid to act as the acoustic coupling medium.
U.S. Pat. No. 5,078,149 to Katsumata et al., entitled “Ultrasonic Coupler and Method for Production Thereof” Jan. 7, 1992, describes an ultrasonic coupler provided with an acoustic coupling medium formed of a water containing polymeric gel produced by integrally cross-linking an aqueous solution of water-soluble polymeric compound, and a holder for accommodating the coupling medium.
U.S. Pat. No. 5,626,554 to Ryaby et al., entitled “Gel Containment Structure” May 6, 1997, describes a gel containment structure, a bladder that is used to conform to the body surface for the application of ultrasound.
U.S. Pat. No. 5,770,801 to Wang et al., entitled “Ultrasound Transmissive Pad” Jun. 23, 1998, describes a pad for transmitting acoustical waves between an ultrasound probe and a target surface. The pad includes a first porous layer and a second porous layer with an ultrasound couplant such as water, glycerin, or silicone oil disposed between the layers.
U.S. Pat. No. 5,997,481 describes a probe cover for an ultrasound imaging probe, the cover including a reservoir for containing a quantity of ultrasonically transmissive gel. U.S. Pat. No. 6,039,694 to Larson et al., entitled “Coupling Sheath for Ultrasound Transducers” Mar. 21, 2000, describes a homogenous, solid, elastic, biocompatible sheath that is conformal to the ultrasound transducer. The sheath is described to be comprised of about 20 to 95% biocompatible liquid, such as water or saline, resulting in a desirable level of acoustic coupling, with acceptable low levels of acoustic artifacts, distortion and attenuation, and provides a microbial barrier between the transducer and surgical field or skin.
U.S. Pat. No. 6,132,378 to Marino, entitled “Cover for Ultrasound Probe” Oct. 17, 2000, describes a cover for an ultrasound probe that is a cup-like device that fits over the probe. A membrane comprising two polyurethane sheets with gel disposed between them is attached to the base of the cup to act as the acoustic coupling medium between the probe and the tissue surface against which the membrane is placed.
U.S. Pat. No. 6,139,502 to Fredriksen, entitled “Ultrasonic Transducer Probe and Handle Housing and Stand-Off Pad” Oct. 31, 2000, describes an ultrasonic transducer probe and handle housing to which a stand-off pad may be affixed to the base and may be adapted to contain a fluid.
U.S. Pat. No. 6,302,848 to Larson et al., entitled “In Vivo Biocompatible Acoustic Coupling Media” Oct. 16, 2001 describes a medical ultrasound coupling medium and lubricant in gel or liquid form, comprised of polyethylene oxide and aqueous solvent solutions.