Intravascular Ultrasound (IVUS) has become an important interventional diagnostic procedure for imaging atherosclerosis and other vessel diseases and defects. In the procedure, an IVUS catheter is threaded over a guidewire into a blood vessel of interest, and images are acquired of the atherosclerotic plaque and surrounding area using ultrasonic echoes. This information is much more descriptive than the traditional standard of angiography, which only shows an image of the blood flowing through the vessel lumen. Some of the key applications of IVUS include: determining a correct diameter and length of a stent to choose for dilating an arterial stenosis, verifying that a post-stenting diameter and luminal cross-section area are adequate, verifying that a stent is well apposed against a vessel wall to minimize thrombosis and optimize drug delivery (in the case of a drug eluting stent) and identifying an exact location of side-branch vessels. In addition, new techniques such as virtual histology (RF signal-based tissue characterization) show promise of aiding identification of vulnerable plaque (i.e., plaque which is prone to rupture and lead to onset of a heart attack).
There are generally two standard types of IVUS catheters, rotational IVUS catheters and phased array catheters. In a rotational IVUS catheter, a single transducer consisting of a piezoelectric crystal is rotated at approximately 1800 revolutions per minute while the element is excited by a signal. This causes the element to vibrate at a frequency from around 9 to 45 MHz, and above depending on the dimensions and characteristics of the transducer. The single element transducer of the rotational IVUS catheter can be made very thin and therefore able to vibrate at relatively high frequencies, thus achieving a relatively high resolution, especially in the near field (close to the outside diameter of the catheter sheath). In addition, this type of transducer can be excited by a relatively high Voltage, increasing the signal to noise ratio. Because the transducer rotationally sweeps past the guidewire during each rotation, a guidewire shadow is seen in the image that obscures some of the image of tissue in back of the guidewire. In addition, rotation of the transducer, usually achieved by a reinforced coil drive shaft, can be uneven and cause distortion of the image. This effect is known as NURD (non-uniform rotational distortion).
Another type of IVUS catheter is a phased array (or solid state) catheter. This catheter has no rotating parts, but instead has a multi-element transducer (for example 64 elements), in which each element is fired in a specific order by means of several small integrated circuits in the tip of the catheter. The multiplexing and demultiplexing performed by the integrated circuits allows for a minimal number of wires inside the catheter. Due to its structure, this type of catheter requires little or no prepping (e.g. flushing with saline to remove air bubbles from within the catheter) and is very flexible and trackable over a guidewire. It is a difficult, multi-step process to make this multi element transducer. One of the challenges of this method is that it is difficult to make the elements thin enough to achieve frequencies as high as those utilized in rotational IVUS catheters.
In the last decade, a new technology has shown promise in ultrasound transducers. This technology is known as cMUT (capacitive Microfabricated Ultrasonic Transducer). The cMUT transducers typically consist of an array of tiny drums fabricated on silicon or other semi-conductor materials. In the cMUT manufacturing process, a thin sacrificial layer is first deposited in a desired pattern. A thin nitride layer is then deposited over the sacrificial layer. This will form both the “drum shell” (bottom and cylinder) and the “drum head” (membrane). Tiny holes are etched through the nitride layer, allowing the sacrificial layer to be removed. The nitride layer is then sealed and an electrical connection is made, so that the membrane can be excited, causing it to vibrate. Typically, a bias DC Voltage is applied to keep the drum from collapsing. New techniques, however, apply the DC bias Voltage to maintain the membrane in a controlled, imploded (or collapsed) state. An AC Voltage is also applied to create the ultrasound energy by inducing vibration within the drum head (membrane). In addition, signal processing circuitry can be included in the silicon base of the cMUT structure. The cMUT transducer shows promise for lower cost fabrication because of the consistency of semiconductor processing technology.
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