An Acoustic Tracking Approach for Medical Ultrasound Image Simulator
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Ultrasound examinations are a standard procedure in the clinical diagnosis of many diseases. However, the efficacy of an ultrasound examination is highly dependent on the skill and experience of the operator, which has prompted proposals for ultrasound simulation systems to facilitate training and education in hospitals and medical schools. The key technology of the medical ultrasound simulation system is the probe tracking method that is used to determine the position and inclination angle of the sham probe, since this information is used to display the ultrasound images in real time. This study investigated a novel acoustic tracking approach for an ultrasound simulation system that exhibits high sensitivity and is cost-effective. Five air-coupled ultrasound elements are arranged as a 1D array in front of a sham probe for transmitting the acoustic signals, and a 5 × 5 2D array of receiving elements is used to receive the acoustic signals from the moving transmitting elements. Since the patterns of the received signals can differ for different positions and angles of the moving probe, the probe can be tracked precisely by the acoustic tracking approach. After the probe position has been determined by the system, the corresponding ultrasound image is immediately displayed on the screen. The system performance was verified by scanning three different subjects as image databases: a simple commercial phantom, a complicated self-made phantom, and a porcine heart. The experimental results indicated that the tracking and angle accuracies of the presented acoustic tracking approach were 0.7 mm and 0.5°, respectively. The performance of the acoustic tracking approach is compared with those of other tracking technologies.
KeywordsUltrasound simulation system Ultrasound examination Acoustic tracking Air-coupled ultrasound transducer
Medical ultrasound examinations have become essential for diagnostic, therapeutic, and surgical procedures in hospitals and other clinical environments. An ultrasound system can provide high-resolution cross-section images of the abdomen and its internal organs that are useful in many clinical applications, such as cardiology, obstetrics, and gynecology, as well as of the breast, thyroid, and vascular and musculoskeletal systems. However, the efficacy of an ultrasound examination is highly dependent on the skill of the operator. Misjudgment can occur if the operator lacks sufficient experience, particularly when detecting rare disorders. It is therefore very important that junior sonographers receive a training course in ultrasound examinations . According to the recommendations from the American College of Cardiology and the American Heart Association, trainees who intend performing transthoracic echocardiography in adult patients need 12 months of training to achieve proficiency, which includes a minimum of 300 examinations and the interpretation of 450 Doppler examinations [2, 3]. The requirement for such a large number of examinations represents a large burden on both hospitals and trainees, and moreover it may still be insufficient for ensuring that trainees are able to detect cases of rare disorders. This situation has prompted proposals for the development of an ultrasound simulation system for assisting ultrasound examination training and education.
An ultrasound simulation system can provide very effective training without impacting patient safety because it can simulate highly accurate scenarios, uses very realistic tools, and affords opportunities to manage patient complications . The high dependence of the outcome of an ultrasound examination on the probe-handling skill of the operator means that the system for tracking the probe position is a very important component of an ultrasound simulation system . Several tracking approaches have been developed in the last decade, including optical tracking (OT), electromagnetic tracking (EMT), and inertial tracking (IT). Any tracking system can be characterized by its sampling rate, sensitivity, precision, working range, and degrees of freedom (DoF), and each tracking approach has its own advantages and disadvantages.
The OT approach for ultrasound simulation systems provides high accuracy, high update rates, and a relatively large workspace [6, 7, 8]. For instance, an ultrasound education ultrasound simulation system for the abdominal region that integrates Web cameras and several planar optical markers printed on paper sheets has been reported . However, OT requires the line of sight to be maintained between the tracking device and the probe, and so the presence of any obstacle, ambient light, or infrared radiation between the detector and markers degrades the performance when using the OT approach. In EMT, a magnetic field sensor placed on a probe measures electrical currents induced as the probe moves in a magnetic field generated by an electrical field generator [10, 11, 12, 13, 14, 15, 16, 17]. However, EMT has the disadvantage that signals from sources such as power cables and surrounding instruments can interfere with the tracking system signals and thereby impair the tracking accuracy. This makes it challenging to use EMT in an environment where various metallic objects are moving around in the magnetic field generated by the electrical field generator. However, EMT is still the most popular approach for tracking the probe position in ultrasound simulation systems [18, 19, 20, 21, 22]. The IT approach is a navigation technique that employs accelerometers and gyroscopes to track the position and orientation of an object relative to a known starting point. While IT is cheaper than other approaches, the large measurement error that accumulates over time is a major disadvantage for an ultrasound simulation system . The acoustic method has been also used to track the probe position in a 3D space [24, 25, 26, 27, 28, 29, 30, 31, 32]. In this method, sound-emitting devices were mounted on the ultrasound probe, and the fixed microphones were mounted above the patient. The microphone was used to receive the sound signals from the emitting devices as the probe is moving. The position and orientation of the probe then was determined by measuring the speed of sound in air between each emitter and microphone. However, the microphone must be placed over the patient and must close enough to the emitter in order to obtain a good signal-to-noise ratio. In addition, many studies used several tracking approaches for 3D ultrasound imaging [33, 34, 35, 36].
The present study investigated a novel acoustic tracking approach for use in a medical ultrasound simulation system. Air-coupled ultrasound elements are embedded in the front of a sham ultrasound probe for transmitting the acoustic signals, and the position and orientation of the sham probe are tracked by receiving the acoustic signals using 2D air-coupled ultrasound elements. After the position of the sham probe is identified by the acoustic tracking approach, its corresponding ultrasound image is displayed according to the position of a real ultrasound examination in the image database as obtained previously via the mechanical scanning of subjects. The validity of this approach was verified in phantom and in vitro porcine heart experiments, and the system performance was compared with those of several commercial ultrasound simulation systems.
2 Materials and Methods
2.1 System Description
2.2 Position Tracking Procedure
After the position and orientation of the sham probe are identified using the acoustic tracking approach, the real ultrasound image from a specific cross section of an organ is displayed on the system screen. The image database was obtained using a clinical ultrasound scanner (t3000, Terason) with a linear array probe (12L5). The probe was fixed on a four-axis motor platform so that it can be moved freely in 3D space by a motor controller. Open-source software is available for the Terason t3000 scanner to allow applications to be developed that run on the Windows operating system, which allows real-time ultrasound images to be acquired on a frame-by-frame basis in the PC as the probe is moved across the area of interest. The position information from the motor and image information from the scanner were integrated together in the image database. When the minimum value of Ri is determined, the corresponding ultrasound image is displayed on the user interface.
2.3 Sample Preparation
3 Results and Discussion
While the experimental results showed that the simulation system performs well, comparison with commercial ultrasound systems revealed that the acoustic tracking approach still has the following limitations. The tracking accuracy is determined by measuring the minimum horizontal distance that causes the array to be indistinguishable between two adjacent positions. The sham probe was fixed on the motor to sweep the receiver unit at a step setting from 0.001 to 1 mm. The experimental results show that 0.7 mm is the minimum horizontal distance between two adjacent positions that can be tracked using the acoustic approach. In other words, the simulation system may display the same image even when the sham probe is moved by up to 0.7 mm. The accuracy of the probe angle was also measured, by inclining the probe from 0.2° to 2°. The experimental results show that a resolution of 0.5° is achievable. In a trial involving sonographers with 10 years of experience in ultrasound examinations, they considered that the moving speed and sensitivity of the sham probe kept up with the images display on the simulation system, and hence provided an accurate simulation of a real examination.
Comparison of the system performances between the acoustic tracking approach, OT, and EMT
OT systems by northern digital inc.
Tracking accuracy: 0.25 mm from 95 to 240 cm
Tracking accuracy: 0.3 mm from 240 to 300 cm
Tracking accuracy: 0.25 mm from 55.7 to 133.6 cm
EMT systems by Northern Digital Inc.
Planar field generator
Tracking accuracy: 0.7 mm
Angle accuracy: 0.2°
Tracking accuracy: 0.48 mm
Angle accuracy: 0.3°
Tabletop field generator
Tracking accuracy: 1.2 mm
Angle accuracy: 0.5°
Tracking accuracy: 0.8 mm
Angle accuracy: 0.7°
Acoustic tracking approach
Five transmitting and 25 receiving elements
Tracking accuracy: 0.7 mm
Angle accuracy: 0.5°
This study investigated a novel acoustic tracking approach for an ultrasound simulation system. Air-coupled ultrasound elements are key components in this approach. Based on the acoustic signals received from a sham ultrasound probe, the position and angle of the moving sham probe can be detected precisely in this system, and the corresponding ultrasound image is displayed simultaneously on the screen. The system performance was verified using three different subjects, with the results showing that the dynamic images from the simulator perfectly match those from an actual clinical ultrasound system. The tracking and angle accuracies of the presented acoustic tracking approach were 0.7 mm and 0.5°, respectively. Future studies should focus on constructing a database of clinical ultrasound images, particularly for rare disorders.
This work was supported by the Ministry of Science and Technology of Taiwan under Grant MOST 104-2314-B-182A-014, and in part, supported by the Ministry of Education, Taiwan. The aim for the Top University Project to the National Cheng Kung University (NCKU).
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