Hand held mechanical compression device for inducing tissue strain

An apparatus for ultrasonic elastography includes a housing, a moveable surface, and a transducer in ultrasonic communication with the moveable surface. A driver is arranged to mechanically move the moveable surface with respect to the housing. The housing can also be formed such that the moveable surface is a moveable exterior surface of the housing, such as a compliant membrane, which is moved in response to the driver to induce tissue compression. The housing may be arranged to be held in an operator's hand. Alternatively, the housing may be arranged for an insertion into an opening in a patient's body. The driver may move the transducer or moveable exterior surface in a cyclical manner at a low frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus 10 shown in FIG. 2 includes a housing 12 which is provided with an optional handle 14 to be held by the person performing the examination. Housing 12 encloses at least a portion of a transducer 16 for applying ultrasonic signals to a patient. A flat plate 18 may also be provided to provide a larger surface area of compression thereby providing a more uniform stress distribution. The flat plate 18 may be a solid plate formed from an ultrasonically transmissive material and interposed between the surface of the transducer and the patient. Alternatively, the plate 18 may have an opening through which the transducer 16 extends such that the surface of the transducer 16 is substantially coplanar with the surface of the plate 18 which will be placed against the patient. A driver 20 is provided within housing 12 for mechanically moving transducer 16 and plate 18 (if provided) to induce compression in the tissue being examined. An examination may be performed by holding the plate 18 against a patient's skin and activating a button 22 which causes driver 20 to move the transducer 16 and plate 18 to provide compressive force to the tissue being examined. Preferably, the direction of compression is substantially orthogonal to the surface of the transducer 16 and plate 18 . Transducer 16 is arranged to provide transmission and reception of ultrasonic signals through plate 18 into the tissue of a patient so that the tissue may be examined during the compressive movement of transducer 16 and plate 18 . In one arrangement driver 20 may be for example, a mechanical motor or a stepper motor which drives the transducer 16 and plate 18 in a sinusoidal or linear cyclical motion, such as a sine wave or a triangular wave. The driver can also be arranged to provide a single compression of varying force. In this case, multiple sets of ultrasound data can be acquired at varying degrees of compression. While driver 20 is moving transducer 16 , ultrasonic signals are recorded prior to and during compression of the tissue to derive elastographic information concerning the tissue being examined. The apparatus 10 of FIG. 2 can be arranged to be held by the optional handle 14 during examination of tissue. Alternatively, the housing 12 can be sized and shaped to fit in the hand of a user. FIG. 3 shows an alternate arrangement for the apparatus according to the invention. The FIG. 3 apparatus is cut away such that the optional handle 14 is not visible. The FIG. 3 apparatus 30 includes a housing 32 within which is mounted an ultrasonic transducer 34 . Housing 32 is generally hollow and contains fluid. The lower end of housing 32 is closed by a membrane 36 , which may be stretched across a planar opening. A driver unit 40 is arranged to change the amount of fluid in housing 32 . In one embodiment, the driver unit 40 can take the form of a syringe which is manually compressed to alter the fluid volume in housing 32 . Alternatively, the driver unit 40 can include a fluid reservoir 42 , a pump 44 and may include a microprocessor 46 for controlling the operation of pump 44 . Pump 44 is connected to the interior of housing 32 through a conduit 38 . In the apparatus of FIG. 3 , the membrane 36 is applied against the tissue of a patient and the fluid is provided to housing 32 by pump 44 (or a syringe) to move membrane 36 away from transducer 34 to compress the tissue while transducer 34 sends and receives ultrasonic signals. FIGS. 4 and 5 illustrate an apparatus according to the invention which is particularly adapted for examining tissue which is adjoining a cavity in a patient's body. For example, the apparatus 60 and FIGS. 4 and 5 may be used to provide ultrasonic elastographic examination of the prostate gland by inserting the apparatus into the rectum and causing the apparatus to compress the tissue as will be described. Apparatus 60 includes a base 66 on which there is mounted a cylindrical transducer 62 of the type currently used for transrectal ultrasound, such as in prostate examination. A membrane 64 , such as a condom, surrounds transducer 62 and is secured at the base of transducer 62 so as to establish a fluid containing vessel. Conventional transrectal ultrasound transducers are commonly provided with a fluid inlet in the base and a fluid outlet at the opposite end. This allows fluid to be provided to the space between membrane 64 and transducer 62 , such as by a syringe. In conventional transrectal ultrasound, the introduction of the fluid is used to establish an ultrasonic interface between the rectal cavity and the transducer. Fluid can also be introduced by a pump 70 which is in fluid communication with the interior of membrane 64 by a conduit 68 . As shown in FIG. 5 the addition of fluid to the space between the transducer 62 and membrane 64 causes the expansion of the membrane. The expansion of the membrane 64 as shown causes compression of the tissue of interest. The membrane can be formed such that expansion is primarily directed in a direction orthogonal to the axis of transducer 62 , such as by forming the end with substantially less compliance than the sidewalls. This can be achieved by forming the end with a material with different elastic properties or with a different material thickness to limit expansion along the axis of the transducer. In the embodiments of the invention described herein, the driver can be selected and arranged to induce low frequency cyclical motion or vibration. The frequency of this cyclical motion, and at which compression is applied, is ordinarily below 10 Hz to provide a quasi-static measurement condition. For example, the handheld mechanical compression device, such as exemplified in FIG. 2 , can be provided with a driver 20 for displacing the transducer 16 and/or plate 18 in a low-frequency-cycled manner. At the beginning of a tissue strain estimation process, the handheld mechanical compression device 10 is placed against the tissue to be analyzed. When the handheld mechanical compression device is activated, the transducer 16 initially collects a pre-compression frame of RF signal data in response to a signal from a controller (not shown). The handheld mechanical compression device 10 may be activated by a switch 22 or by other means known in the art. The driver 20 then automatically displaces the transducer 16 and/or plate 18 in a low frequency cyclical manner, such as with a sinusoidal motion, thus inducing a cyclical tissue strain. The transducer 16 then collects post-compression RF echo signal data at predetermined points of the compression cycle. This can be achieved by synchronizing the operation of the transducer 16 and the driver 20 using a controller (not shown), such as a microprocessor. At times near the zero-crossings of the sinusoidal cyclical compression, displacement from applied low frequency vibration is substantially linear and the applied displacement within a fixed time interval is at a maximum. If a data frame sequence is acquired in this region, strain can readily be computed using elastographic signal processing methods. Data acquisition can be synchronized to the vibration driver to ensure that data is acquired in the right time interval. While not preferred, data can be acquired at less linear portions of the sinusoidal compression cycle so long as the data are acquired at known points of the sinusoidal motion. Alternatively, in place of a sinusoidal displacement from the driver 20 to induce tissue compression, a low frequency triangular waveform or reverse saw-tooth waveform, can also be used to induce tissue strain. An advantage of such waveforms is that the rate of change is substantially constant within a linear portion of the curve. However, such waveforms require the use of a driver 20 which can accommodate such triangular motion (e.g., stepper-motor based). In one embodiment using low frequency vibration to induce tissue compression, the driver 20 can take the form of a conventional audio speaker driven by an appropriately shaped driving signal, such as from a function generator. Alternatively, precise vibration devices (e.g., mini-shaker type 4810 by Bruel & Kjaer, Denmark) or stepper-motor controlled systems can also be used. According to the method of the present invention, an apparatus as described herein is applied against the tissue of the patient and held in place by hand or otherwise. The apparatus is used to apply strain to the tissue and ultrasonic data is recorded prior to and following the application of strain such that the tissue may be characterized in terms of its elastic properties. In addition, the driver can be arranged to Japply a variable degree of force during a single compression and multiple sets of ultrasonic data can be acquired during such a compression. While there have been described what are believed to be the preferred embodiments of the present invention, those skilled in the art will recognize that other and further changes may be made thereto without departing from the spirit of the invention and it is intended to claim all such changes and modifications as fall within the scope of the invention.