Scanheads that were developed in the late 1970's and early 1980's for imaging human tissue are still useful for many ultrasound imaging applications. A transducer located in the scanhead comprises discs of piezoelectric material, which when excited electrically vibrated at a frequency usually chosen to be between 2 and 10 MHz. At these frequencies, the vibrational energy of the transducer was directional and radiated from two faces of a thin circular disc in reasonably well-defined beams. In general, the energy radiating from the back of the transducer is absorbed by a suitable material while that from the front is coupled to the patient by a fluid capable of transmitting ultrasound energy with low loss characteristics. Emerging through a thin, low-loss cap, the energy is further coupled to the patient with a sonolucent gel applied to the patient's skin. Echoes resulting from the interaction of the ultrasound energy with body tissue traverse the same path in reverse, and when they strike the transducer generate an electrical signal whose strength is a function of the echogenicity of a target within the patient and the target's depth below the patient's skin. The location in depth is determined from the time interval between the transmit pulse and the received echo. With this information and the directional information delivered by a position encoder coupled to the transducer, the scanheads generate a gray-scale image of the tissue lying in a scan plane within the patient, which is refreshed and updated with every sweep of the transducer across the image plane. Two sweeps of the transducer comprises one operating cycle, referred to as 1 Hz, and equates to two frames per second.
Two dimensional ultrasound images (also known as B-scans) are made up of a number of adjacent lines of ultrasound data called A-scans, which are acquired from the scanhead through successive sweeps of the transducer. The line of ultrasound data is acquired when a transducer transmits the ultrasound pulse into the tissue being studied and then receives the ultrasound signal reflected by the tissue along a beam axis of the transducer. The lines of ultrasound data are located within the same plane and are usually spaced at constant intervals. Each line of data is acquired with the ultrasound beam axis moved laterally within the plane by a known incremental distance. The ultrasound image may have a linear format, in which the lines are parallel to one another and equally spaced, or a sector format, in which the lines radiate from an apex with equal angles between them. To produce a linear format image, the transducer is moved laterally, without altering the angle between the transducer and the line along which it is moved. To produce a sector format image, the transducer is mounted to a fixture, which rotates about an apex, causing the transducer to move in an arc. As the transducer moves, the position within the scan plane is tracked so that an associated ultrasound system can display the ultrasound line data at the correct locations within the displayed image.
Early clinical diagnostic ultrasound systems used wobbler scanheads to produce the sector format images. These systems used low frequency ultrasound, in the 2 to 5 MHz range. The wobbler scanheads usually consisted of the transducer located within a fluid filled chamber, a motor, a position encoder, and an acoustic window through which the ultrasound passed. The motor drive mechanism usually moved the transducer through an arc, resulting in a sector scan type image format while the position encoder kept track of the transducer position. The wall of the fluid filled chamber, which faced the tissue being imaged, acted as an acoustic window, which was usually made of a hard plastic material. This window allowed ultrasound to pass through with little attenuation. Further, in general, there is a reflected ultrasound wave which does not pass through the window. This wave can reverberate between the transducer and the window several times before dissipating. The reverb components, which strike the transducer, can cause an undesirable artifact in the ultrasound image. The magnitude of the reflected wave is determined by the acoustic impedance mismatch between the material used for the window and the fluid in the transducer chamber. The amount of attenuation is determined by the window material, which occurs as the ultrasound energy passes through the window. Both attenuation and reflections at the window are undesirable.
In the 80's these mechanically scanned transducers began to be replaced by solid state devices which consist of a plurality of narrow piezoelectric elements which, when excited sequentially, can be used to build up an image. These “linear array” scanheads had been developed at the same time as the mechanical ones, but delivered poorer image quality. Further work, throughout the 80's and 90's resulted in the development of “phased array” scanheads, which have the ability to excite groups of elements in ways that allows electronic beam steering and focusing, which in general produce better images than any mechanical scanhead and at frame rates of 60 frames per second. Today, phased arrays are universally used for ultrasound imaging of human tissue. However, a typical phased array system using a transducer operating at five MHz might have a spatial resolution of 0.5 mm.
One disadvantage with higher operating frequencies is as the operating frequency increases, fabrication difficulties make it challenging to build a phased array type imaging system. As a result, current systems operating in the 30-40 MHz range typically use mechanically scanned single element transducers, in scanheads similar in operating principal to the mechanically scanned systems described above. However, high frequencies generally result in higher attenuation and thus the attenuation due to an acoustic window is increased significantly. Accordingly, current high frequency transducers use a non-encapsulated transducer, which is moved back and forth with a linear servo-motor and position encoder system. At higher frequencies (greater than 30 MHz), transducer encapsulation is impractical due to a breakdown of theoretical properties and characteristics of materials with higher frequencies.
For high frequency transducers, since it is not encapsulated, the moving transducer is exposed. Acoustic coupling to the tissue being imaged is accomplished by creating a mound of ultrasound gel on the surface of the tissue, into which the moving transducer is lowered. Satisfactory imaging depends on the existence of a continuous layer of gel between the transducer and the tissue. If the transducer loses contact with the gel, or if an air bubble forms on the surface of the transducer, imaging will be compromised or even impossible. This type of imaging is restricted to relatively low frame rates, because a rapidly moving transducer will disrupt the gel layer and is more likely to lose contact. Further disadvantages of exposed transducers are that they can create a hazard to delicate tissue, and can also expose the transducer to possible damage from impact.
A further disadvantage in mechanical ultrasound scanheads is the use of moving magnet motors. The attraction of the moving magnet type is that there is no need for flexible wires to deliver power to the drive coil because the drive coil is stationary and the permanent magnet is attached to the moving member or rotor. Furthermore, the magnet type motor is inefficient. The usual mechanical scanhead consumes up to three Watts of electrical power, which is converted into heat that must be dissipated through plastic walls of the scanhead housing. As the housing is generally a poor conductor of heat the internal temperature of the scanhead may rise, which in time can degrade materials, alter the acoustic properties of the device, and can even be uncomfortable to the subject. Another reason the magnet motor is inefficient is that in an effort to keep the oscillating mass low, the moving magnets are kept relatively small. To achieve a certain torque, motor currents are correspondingly high, which gives rise to a high I^2R loss. These losses increase roughly as the square of the scanning rate.