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Full Assembly of the Ultrasound Transducer Guidance System
Transducer Guidance System Motion Components
Objectives
Objectives
The primary objective of this project is to create a high precision guidance system for an ultrasound transducer that electronically measures its position for each 2D image slice. The position data will be used to reconstruct 3D images of the oral cavity. This guidance system should allow for smooth and continuous imaging while giving reproducable results. In order to achieve such objectives, the team designed, analysed, fabricated, and tested the following high precision transducer guidance system pictured in Figure 1.
One crucial functional requirement of the system is to restrict the movement of the transducer to a semicircular arc as demonstrated by the animation in Figure 2. It's required that the transducer's tip be in constant contact with the patient's gumline during the entire scanning process. To achieve this, the guidance system was designed to operate about a central axis placed below the patients chin. The transducer is to be mounted the arm rotating about this axis. A linear rail and carriage component was designed to offer adjustability in the radial direction, allowing for the transducer to traverse either towards or away from the axis of rotation to accomodate for unique oral topography of the different patients using the device.
Another critical functional requirement of the guidance system is to record the transducer tip's position with an accuracy of at least 100 microns. This requirement was met using two separate absolute magnetic encoders working in tandem to collect the transducer's positional data. One linear encoder and one axial encoder, each with a system accuracy of 10μm and 0.05° respectfully, were studied and sourced from position encoder company RLS. To maintain the encoders' high precision and high reliability, each encoder is housed within a cusom designed and machined mount that allow the encoder's full funcionality while ensuring their security.
The secondary requirements of this high precision guidance device is to restric patient head movement and to allow for the transducer to be easily removed or replaced. Reducing patient head movement is important as any movement during the ultrasound scanning may result in image distortion. To combat this issue, the team purchased an adjustable chin rest that eliminates most pitch, roll, and yaw head movement. As for the transducer mount, a custom clamp with a locking mechanism was designed and molded to fit the organic contours of the transducer body
Figure 1. CAD rendering of completed assembly
Figure 2. Transducer's path of travel along a human jaw
Project Background
Project Background
Ultrasound technology is currently used in many medical applications, however it is not fully utilized in dentistry despite its broad scope of applications such as observing dental fractures or measuring gingival thickness. A UCSD research group from the Jokerst Bioimaging Lab is working to use ultrasound in measuring dental pocket depths and assessing gingival health.
The current approach to measuring dental pocket depths involves a probe that is placed between the gums and each tooth to see how far it reaches into the gum line. This process is outlined in the top image of Figure 3, where various different stages of gum disease is illustrated. Not only is this process painful for the patient, but it also provides an imprecise measurement with variation as high as 40%. Ultrasound imaging, however, is a less painful and more accurate diagnostic tool that could improve dental health. Currently, this technology is not being used in the field of dentistry due to the fact that ultrasonography for dental imaging is in the eaarly stages of research and development. The continuation of research into ultrasound imaging for dental applications is important because it offers modern soluutions to the issue of lengthy and uncomfortable dentist trips. The all encompassing general goal of the project is to ultimately improve the public's overall quality of life.
To detail the process of 3D ultrasound imaging, a handheld transducer is pressed against the teeth and scanned across the mouth by its operator. However, this process poses the issue of image distortion due to irregular mouth topology, patient movement, and shaking during the handheld scanning.
The team is working closely with the Jokerst Bioimaging Lab to create a guidance system for an ultrasound transducer that electronically measures its position for each 2D image slice. The position data will be used to reconstruct 3D images of the oral cavity.
Product Description
Product Description
The transducer guidance system mainly consists of 6 integral components operating together in order to accurately acquire the transducer’s positional information. Precise positional data is recorded using digital encoders and the software and communication interface. The transducer is traversed along the guidance system via the rotary joint and linear motion components. A transducer mount is used to fix the transducer onto the guidance system carriage. A head restraint/positioner is required to maximize patient comfort while minimizing head movement.
The data aquisition components within the transducer guidance system include one rotary encoder, one linear encoder, and the software and communication interface. Both encoders use an absolute scale as to eliminate the need for a predetermined home position for the device. They are both magnetic to reduce physical wear and to decrease the likelihood of dirt and debris affecting the positional readout. As mentioned earlier, the linear encoder and the axial encoder each have a system accuracy of 10 μm and 0.05° respectfully.
The rotary joint consists of a machined stainless steel axis with tapered roller bearings used to smoothly rotate the arm connected to the axial encoder housing. The rotational arm, shown in red in Figure 4, consists of machined 6061 aluminum and is the main component that houses the rotary encoder and allows it to spin about the axis over the magnetic scale. Tapered roller bearings were chosen for their ability to withstand large axial and radial loads. To reduce backlash from the bearings, a spring washer is used to apply constant pressure.
In addition to the guidance system's rotating element, there is also a linear component. A 80mm linear rail along the arm's length allows for the transducer to move in the radial direction. The chosen linear motion carriage is preloaded to minimize deflection as this component is what the transducer mount sits upon. Fixed atop the linear rail carriage is a custom clamp molded to fit the organic shape of the transducer handle's design. This transducer mount is a 3D printed piece designed specifially for a Verasonics brand transducer model. Two set screws attached to a clamping surface is used to sandwiching the transducer, securely holding it in place.
Finally, to reduce the patient's mead motion, a head positioning chin rest typically used for opthalmy was purchased and acts as the base of the system. The head positioner is a purchased optometry heavy duty chin rest from the company Good-lite. Some key features of this product include table clamps to fix the entire system to the a flat surface and an adjustable chin height from 10.75 in to 14.25 in.
Figure 4. Ultrasound transducer guidance system mannequin test
Figure 5. Ultrasound transducer guidance system human test
File from iOS (3).MOVFigure 6. Video of encoder operation with positional readout
Summary of Performance Results
Summary of Performance Results
The device’s operation satisfies most of the functional requirements as listed.
Going through the requirements one by one, the transducer mount satisfies the basic functional requirement of tightly clamping the transducer. With the help of a machined alignment surface, a transducer is able to be properly mounted in the same position every time.
In order to zero the device’s linear encoder for proper data acquisition in the radial direction, a removable calibration cylinder with a known radius is used. Upon testing the system’s motion using the calibration cylinder, the act of pressing the transducer up against the surface and traversing it along the curved cylinder felt smooth with no jamming issues between the cylinder surface and the transducer tip.
The purchased head restraint component allows for the patient to rest their chin on a surface while being scanned. This significantly reduces head motion during the procedure. However, during testing it was found that the forehead strap may not be viable in restricting the pitch and yaw motion of the head.
After successfully attaching and calibrating the transducer within the mount, the team performed preliminary tests on the scanning procedure. One test was done on a mannequin from the Jokerst Bioimaging Lab and a second test was done on one of the team members. The results of these tests showed that the transducer is able to successfully traverse about the front 6 to 10 teeth on both the upper and lower gum line in one smooth and continuous motion. Very little force was required to manipulate the device, allowing for the procedure to be easily accomplished with one operator. However, after conducting these tests, a remaining concern the team still has regards the positioning of the transducer against the mouth. Although possible, it is difficult to perfectly flush the transducer against the gum, so some training may be required to successfully operate the device.
As for the position readout of the device, the team is able to output and track the position data. The mechanical range of the device was measured to be 110°.
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