Development of a Positioning System for 3D Ultrasound

Poulsen, Carsten

Etd

Development of a Positioning System for 3D Ultrasound

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Ultrasound has developed from 2D into 3D ultrasound in recent years. 3D ultrasound gives enhanced diagnostic capabilities and can make it easier for less trained people to interpret ultrasound images. In general there are two ways of getting a 3D ultrasound image : By using a 2D array scanner (giving 3D images directly) or by using a series of 2D scans and combine these scans to build a 3D volume. The only practical scanning technique that can be used for portable systems is freehand scanning that combines a series of 2D images. 3D images acquired by using a conventional ultrasound transducer and the freehand scanning technique are, however, often misaligned laterally and have unevenly spacing. These errors can be corrected if the position associated with each 2D image is known. Commercially available positioning systems use magnetic or optical tracking, but these systems are very bulky and not portable. We have proposed another way to get the position by tracking on the skin surface. This is done by obtaining digital images of the surface at a very high rate and then cross correlating each image to reveal the change in position. Accumulating these changes will then give the correct location (in two dimensions) relative to a starting point. Correct volume and surface rendering can therefore be achieved when a scan is done. A custom-made housing was made to mount an optical sensor to the ultrasound transducer. The optical sensor was placed in the housing and the hardware circuit from an optical mouse was used to interface to a USB interface. An implementation with an optical fiber was also made since this could fit easily to the transducer handle. In Windows a custom-made mouse driver was used to extract the position information from the sensor. This driver allowed multiple mouse devices in the system and removed the acceleration of the mouse, giving a correct transfer of the position. A DLL (Dynamic Link Library) was used to interface to a 3D ultrasound software called Sonocubic. Using the DLL and a custom modified version of Sonocubic 3D construction software has allowed a correct compensation of the acquired ultrasound images. To validate the accuracy of the optical sensor an optical mouse was placed in an XY-recorder to compare the acquired position with the actual position. The test revealed that the accuracy of the optical sensor is very high. A 55 mm movement of the sensor gave a deviation of 0.56 mm which is well within the expected result. A computer generated phantom was made to see if the compensation algorithm was working. The test revealed that the compensation algorithm and the software is working perfectly. Next a vessel phantom was scanned to see that the compensation algorithm (lateral compensation) was working in real life. The test showed that a correct lateral compensation was made. Finally 3D phantoms were custom made to test the accuracy of the system by estimation of a known volume. The system was able to estimate the volume in a phantom within an accuracy of 6 %. Performance of the system with direct imaging, using the optical sensor and a lens, was compared to an implementation with an optical fiber, two lenses and the optical sensor. The optical fiber was difficult to implement since the image contrast was degraded severely through the optical fiber and the lenses. This made it difficult for the correlation algorithm to function correctly and tracking could therefore not be done on a skin surface. Code for an FPGA was made in VHDL to extract the actual images from the optical sensor and display them directly on a computer screen. This was necessary to see how well the sensor was in focus. This proved to be a really useful tool for adjusting the optical system for maximal contrast. The optical tracking on a skin surface is a good way to assist a user doing a freehand scanning to get images without geometric distortion. Furthermore, it is the only real positioning system for a portable system. One requirement for this system is, however, that the object being scanned is flat and does not curve or vary vertically. For most applications this is not the case, and we are therefore proposing an implementation with microgyros that is able to give angle information as well. This would give the system a total of up to 5 instead of just 2 degrees of freedom. The status of this is currently that it can be easily implemented in the DLL, but it is not implemented in the 3D reconstruction software, Sonocubic.

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