Abstract:
Alcohol is used as a coupling medium between an ultrasonic transducer and patient skin providing rapid settling of quantitative measurements.

Description:
BACKGROUND OF INVENTION 
     The present invention relates to medical ultrasound imaging equipment and, in particular, to a biocompatible material for coupling ultrasonic energy between a patient&#39;s skin and an ultrasonic transducer used in such equipment. 
     Ultrasound may be used to measure the physical characteristics of living tissue. In echo ultrasound, an ultrasonic transducer is placed against the skin to transmit sound into the patient, and then to receive echo signals caused by the reflections of the ultrasonic energy across interfaces between materials of different acoustical properties. In transmission ultrasound, an ultrasonic transmitting transducer is placed against the skin on one side of the patient to transmit sound through the patient to be received by a second ultrasonic transducer placed against the skin on the other side of the patient. 
     Transmission ultrasound finds considerable use in quantitative measurements of tissue characteristics and, in particular, in the field of bone densitometry where measurements of speed of sound and broadband ultrasonic attenuation (BUA) may be used to characterize bone health. Such systems are described in U.S. Pat. Nos. 4,930,511, 5,042,489, 5,054,490, 5,099,849, 5,119,820, 5,218,963, 5,343,863, 5,349,959, 5,483,965, 5,603,325, 5,840,029, 6,027,449, 6,277,076, and 6,364,837, all assigned to the assignee of the present invention and hereby incorporated by reference. 
     The delivery of ultrasound into the patient requires an efficient coupling path between the transducer and the patient&#39;s skin usually facilitated by a coupling material. Such coupling materials are typically selected to be hypoallergenic, slow to dry, and of comparable acoustic qualities to water (the principal constituent of tissue). Some commonly used coupling materials are glycerol, water, and oils. 
     In many applications, a gel material is used because of its lessened tendency to flow from the region in which it is applied and its ability to fill gaps between the ultrasonic transducer and the skin. U.S. Pat. No. 4,002,221 describes a gel coupling agent having a viscosity similar to mayonnaise and formed from copolymers of methylvinyl-ether and maleic acid and carboxy-polymethylene polymer with alkali metal salts as thickeners. U.S. Pat. No. 4,459,854 describes a coupling material composed of a hydrogel being a copolymer of vinyl pyrrolidone and phenolethyl methacrylate. Numerous of ultrasound coupling gels are commercially available. 
     Precise quantitative measurements in bone densitometry studies often require a stabilization period after the coupling material has been applied to the patient and the ultrasonic transducers have been positioned. This stabilization period is about one minute with maximum stabilization occurring as much as fifteen to twenty minutes later. The mechanism underlying this need for a stabilizing period is not well understood. 
     SUMMARY OF INVENTION 
     The present inventor has determined that the stabilization period needed for quantitative bone density measurements is essentially eliminated (approximately fifteen seconds) when a thin coating of alcohol is used as the coupling material between the ultrasonic transducer and skin. 
     While the inventor does not wish to be bound by a particular theory, it is believed that the reduction of stabilization time may result from alcohol&#39;s ability to penetrate or wet the skin very quickly, which in turn may be a function of its extremely low surface tension. In contrast, gels, for example, may tend to entrap air and impede the wetting of the skin&#39;s surface. 
     Unlike gels and other coupling materials, the high volatility of the alcohol makes for easy cleanup and does not leave a residue on the patient&#39;s skin. Alcohol has a sterilizing property, presents a low safety hazard, is easily dispensed from a spray bottle without danger of contamination, and is readily available in the hospital or clinic environment. 
     The invention may be particularly suitable for ultrasound systems that use an inflated bladder having a convex outer surface where little gap filling is required and where the contact area of the bladder during inflation spreads outward clearing bubbles from the region. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a cross-sectional view through a transmission ultrasound machine intended for measurement of the os calcis bone of the heel, showing opposed transducers with deflated coupling bladders spaced apart to receive the heel therebetween, and showing application of an alcohol spray to skin surfaces; 
     FIG. 2 is a figure similar to that of FIG. 1 showing the heel in place between the bladders with the bladders inflated to provide coupling of ultrasonic energy from the transducers to the skin through an alcohol layer; and 
     FIG. 3 is a chart showing change in measured bone density values over time obtained with the alcohol spray versus a conventional gel coupling material. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIGS. 1 and 2, a bone densitometer  10  providing for ultrasonic measurement of bone health includes a housing  12  having an opening  14  in the upper surface sized to receive a patient&#39;s heel  16 . 
     The opening  14  reveals a chamber  22  sized to receive the patient&#39;s heel  16  on a guide plate  24  so that the os calcis  18  is positioned along an axis  26  between a first transmitting transducer  28  and a second receiving transducer  30  flanking the heel  16 . The transducers  28  and  30  each extend through a circular backer plate  34  covered with a flexible silicon membrane  36  attached around its circumference to the circumference of the backer plate so as to define an enclosed volume covering the faces of transducers  28  and  30  exposed to the chamber  22 . A liquid, typically a water and alcohol mixture, is introduced through passageways  38  communicating with the enclosed volume to inflate the membrane  36  expanding it outward to engage the skin of the patient&#39;s heel  16  when the patient&#39;s heel  16  is positioned within the housing  12 . 
     As will be understood from this description, the membranes  36  are outwardly convex to initially contact the foot at a limited area that expands outward with inflation to squeeze out air. 
     During operation of the bone densitometer  10 , a series of ultrasound pulses may be produced by the transducer  28  driven by appropriate circuitry to pass along axis  26  through the os calcis  18  to receiving transducer  30 . The ultrasound pulses received by the transducer  30  are then analyzed to deduce the speed of sound (SOS) through the patient&#39;s heel  16  (being principally that through the os calcis  18 ) and a broadband attenuation (BUA) as is well understood to those of ordinary skill in the art. 
     Referring now to FIG. 1, before insertion of the heel  16 , an alcohol spray  40  may be applied to both the left and right side of the heel  16  in the region where they will contact the membranes  36  upon inflation of the membranes  36 . The alcohol spray  40  is preferably isopropyl alcohol in 70 to 90% concentration of alcohol as may be practically obtained and stored. The alcohol spray  40  is not mixed with other thickeners to maintain a low viscosity. Trace amounts of additional material may be added to the alcohol spray  40  including surfactants, perfumes, colorings, and the like. 
     The viscosity of the alcohol spray  40  is preferably comparable to the viscosity of water at room temperature (approximately one centi-Poise (cP)) and typically no greater than ten centi-Poise. The surface tension of the alcohol spray  40  will be comparable to the surface tension of isopropyl alcohol (twenty-two dynes per centimeter) but in any event will be less than the surface tension of water at room temperature of 72.8 dynes per centimeter. 
     The amount of alcohol spray  40  applied is such as to coat the heel  16  sufficiently so that alcohol just begins to run down the vertical sides of the skin. The alcohol spray  40  may be applied with a conventional spray bottle  42  which need not contact the patient eliminating contamination problems. 
     Alternatively, the alcohol may be applied to the membranes  36  before insertion of the heel  16 . In yet another alternative, the alcohol spray  40  may be applied to either of the heel  16  or membranes  36  with the membranes  36  pre-inflated and the heel  16  is slid between them. The alcohol need not be applied in a spray but may be painted on or the like with other forms of applicators such as a brush. 
     Referring to FIG. 2, the volatile alcohol spray  40  is immediately covered by the membranes  36  (as seen in FIG. 2) that serve to retard additional evaporation prior to the measurement. 
     Referring now to FIG. 3, using the alcohol spray  40  as a coupling material provides measured ultrasonic parameters (e.g., speed of sound, broadband ultrasonic attenuation) that stabilize as generally indicated by plot line  50  approaching an asymptote  52  within approximately fifteen seconds. In contrast, use of a conventional gel provides measured ultrasonic parameters that stabilize as generally indicated by plot line  54  in as much as 120 to 180 seconds. 
     When the heel  16  is removed from the bone densitometer  10 , the alcohol spray  40  which has not been absorbed into the skin, quickly evaporates, avoiding the need to clean gel off of the patient&#39;s foot such as might soil hosiery. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.