MOST interventional device guidance during minimally invasive procedures is performed using X-ray fluoroscopic imaging. X-ray fluoroscopy does not provide the physician good tissue contrast, lacks depth information, and delivers ionizing radiation to the patient. One alternative is ultrasonic image guidance during minimally invasive surgical procedures because it provides the clinician with excellent soft tissue contrast without ionizing radiation. However, there is often a problem identifying interventional devices using ultrasound because many devices are specular reflectors that do not return the ultrasonic echoes back toward the transducer. Various techniques are used to improve the visualization of the devices, such as adding an echogenic coating or scoring, etching, or dimpling. Each of these methods improves the backscatter from the device. Studies have shown that making such modifications can increase visualization of the treated device when compared with a similar untreated device [1
]. Despite the improved visualization of part of the device, the localization of the device tip is not necessarily improved in all cases.
Other researchers have attempted to track catheters by attaching sensors that interact with the ultrasound beam. McDicken et al.
described ultrasonic sensitive needle stylet combinations that included a piezoelectric crystal at the end of the stylet. The signals received by this crystal were superimposed on the B-mode image to aid in needle identification [5
]. The EchoMark system (EchoCath, Princeton, NJ) used an omnidirectional receiver that was positioned at a particular point of interest on the catheter that would receive ultrasonic energy transmitted by a standard transducer [7
]. Researchers at Duke expanded on this idea by attaching a receiver to the tip of a catheter and imaged the device with a real-time 3-D (RT3-D) ultrasound system [9
]. Accurate results were produced in these studies, but limitations with this approach are the additional costs and possible safety issues that result from attaching a sensor to the interventional device tip.
Other ultrasonic guidance techniques use color Doppler techniques to track moving interventional devices. The “pump technique” is performed by repeatedly advancing and retracting either the device or a guide wire within the device to create motion that is detected by the ultrasound system. This technique is effective but limited because tip identification is not improved and the color Doppler signal is only present while the device is being manipulated [10
]. The ColorMark (Echocath, Princeton, NJ) is an FDA-approved device that couples low-frequency vibrations (1 to 3 kHz) into an aspiration needle [11
]. These vibrations were detected and displayed with the color Doppler feature of a 2-D ultrasound system during a clinical trial to treat pericardiocentesis. Researchers testing this technology reported positive results in both in vitro and in vivo experiments, but noted that the planar nature of the 2-D ultrasound beam limited the overall effectiveness. In an attempt to overcome this planar problem, we implemented this method for real-time detection and guidance of minimally invasive devices using the Model 1 RT3-D ultrasound (Volumetrics Medical Imaging, Durham, NC) [12
The Model 1 RT3-D ultrasound system is a commercial version of the real-time volumetric scanner originally developed at Duke University in the early 1990s [14
]. The 3-D system uses a matrix array transducer () and up to 512 transmit channels to steer and focus the ultrasound beam through a pyramidal volume. The receive data are acquired by the 2-D matrix array using up to 256 receive channels and 16:1 receive-mode parallel-receive processing to generate 4096 B-mode image lines at up to 30 volumes per second. From the pyramidal volume of received echo data, the scanner displays two simultaneous orthogonal B-scan image planes and up to three C-scan image planes parallel to the array face (). The B-scan image planes can be swept through the pyramid while the C-scan image planes can be inclined at any desired angle and positioned at any depth. The system can also display real-time 3-D rendered images of echo data collected between two user-selected parallel C-scan image planes. Finally, the system offers both 3-D pulsed wave (PW) spectral Doppler and 3-D color Doppler features over the complete pyramid. This system has been used to show the usefulness of RT3-D echocardiography for the examination of left ventricular function [16
], detection of perfusion defects [17
], guidance of right ventricle (RV) endomyocar-dial biopsy [18
], measurement of peak left ventricle (LV) flow velocities [19
], and evaluation of congenital cardiac abnormalities [20
Schematic of pyramidal scan showing the scanned volume of echo data generated by the matrix array and the orthogonal B-scans and C-scans slices. Also shown within the echo data is the tip of a vibrating interventional device.
In our previous work [12
], we coupled 1 to 3 kHz vibrations from a unimorph piezoelectric buzzer into four different interventional devices. As illustrated in , these vibrations were coupled into the device, causing deflections along the length of the device. When the vibrating tip was inserted into the 3-D pyramidal scan created by the matrix array, signals from the vibrating device were acquired and sent to the 3-D scanner for Doppler processing before being displayed. The devices and sample experimental color Doppler image slices from volumetric scans are presented in the previous work [12
]. Despite positive results, our experiments showed limitations with reliability and consistent device tip detection during in vivo experiments. In an attempt to overcome some of these reliability issues, we developed a 1-D analytical model to maximize tip vibrations by placing the piezoelectric buzzer near a natural antinode [21
]. Our desire to maximize the vibrations arose from experimental [22
] and theoretical [23
] works that show the Doppler signal strength is directly related to the vibration amplitude. Our model results showed an order-of-magnitude displacement increase when placing the buzzer at a natural antinode compared with a natural node. This model can be used for any device as long as the geometry and material properties are known.
In this paper, we describe improved radiofrequency (RF) and color Doppler filters and use the analytical model [21
] to aid in the placement of the piezoelectric buzzer on two clinically relevant and commonly used interventional devices: an atrial septal puncture needle and an endomyocardial biopsy forceps. Some conditions, such as mitral stenosis with enlargement of the left atrium, cause distortion of the septal anatomy that make septal puncture difficult or even dangerous with the possibility of inadvertent puncture of the aorta, coronary sinus or inferior vena cava, or perforation of the right atrial (RA) or left atrial (LA) wall when using X-ray fluoroscopic guidance. Such problems can be avoided if the procedure is performed under real-time ultrasonic guidance to ensure tip identification [24