In order to insonify the body tissues, a beam formed either by a phased array or a shaped transducer is scanned over the tissues to be examined. Traditionally, the same transducer or array is used to detect the returning echoes. This design configuration lies at the heart of one of the most significant limitations in the use of ultrasonic imaging for medical purposes; namely, poor lateral resolution. Theoretically, the lateral resolution could be improved by increasing the aperture of the ultrasonic probe, but the practical problems involved with aperture size increase have kept apertures small and lateral resolution poor. Unquestionably, ultrasonic imaging has been very useful even with this limitation, but it could be more effective with better resolution.
In the practice of cardiology, for example, the limitation on single aperture size is dictated by the space between the ribs (the intercostal spaces). For scanners intended for abdominal and other use, the limitation on aperture size is not so obvious, but it is a serious limitation nevertheless. The problem is that it is difficult to know the exact position of the elements of a large apparatus with multiple and separate physical points of contact (“footprints”) on the patient. For optimum performance, all of the separated transmit and receive elements should be in the same scan plane. In addition, each element position must be known to within 1/10 wavelength (for example, 0.03 mm at 3 MHz). With conventional ultrasound probes, regardless of array vertical displacement or integration (e.g., 1.5D or 2D), there has never been a need to solve alignment and position issues between multiple arrays or multiple individual elements. The methods and apparatus included here teach how to solve these problems for Universal Multiple Aperture ultrasound probes.
In constructing and maintaining a Universal Multiple Aperture Probe using a combination of two or more individual arrays, attention must be paid to each array's ultrasound beam displacement relative to a central array Z axis. The displacement or rotational axes referred to are X, Y and Z. X varies about the longitudinal array axis, Y varies about the central array axes, also termed twist, and Z varies about the transverse or lateral array axis. A fixture and method for measuring the variation of each array was developed and implemented.
Element position is equally important as displacement from the central array Z axis. The positional relationship of each array element to every other element needs to be established within an individual array and from array to array.
The type of crystal used in each array is irrelevant. That is, any one, one and a half, or two dimensional crystal arrays (1D, 1.5D, 2D, such as a piezoelectric array) and all types of Capacitive Micromachined Ultrasonic Transducers (CMUT) can be utilized in multi-aperture configurations.