Last updated: 4/24/2012 10:30am

• Students: Alexis Cheng
• Mentor(s): Emad Boctor, Russell Taylor, Jin Kang

Photoacoustic (PA) registration has been shown to be able to at least replicate the functionality of common surgical tracking systems such as electromagnetic or optical trackers. This technology involves shining points of laser onto a phantom or tissue that can be seen by a stereovision camera system. The laser energy is absorbed by the phantom or tissue and generates an acoustic wave that can be detected by an ultrasound probe.

The goal of this project is to develop a PA registration and visualization system that can perform a direct registration from 3D stereocamera (SC) space to 3D ultrasound space in an ex-vivo tissue environment and achieve sub-millimeter error results based on target registration error (TRE).

The PA effect was first observed in 1880 by Alexander Graham Bell. The general principle of this phenomenon is that light waves will generate an acoustic wave when absorbed by matter due to thermal absorption and excitation. It was initially developed for use in the communication field, but has since been used in the medical field as well in the form of PA spectroscopy. This biomedical application is mainly used for functional and structural imaging thanks to the large contrast in absorption coefficients between blood and tissue [1].

When matter is exposed to pulses of light, the light energy becomes absorbed by the molecules within the matter. The absorbed energy thermally excites the molecules and generates heat. This causes the matter to expand and generate acoustic waves. These waves can be detected by an assortment of acoustic devices such as ultrasound probes.

• To demonstrate accurate registration of stereovision camera to 3D US coordinate system

Surgical tracking systems are widely used in computer integrated surgery. Some common ones include electromagnetic and optical tracking systems. These systems both require markers to be placed on the object of interest. In actuality, the markers are the objects that are being tracked. To determine the object of interest with respect to the markers, a calibration process must be performed. For example, the calibration of an optical tracking system can have an error of about 3.08mm and 3.26degree [2]. The PA registration that this project proposes does not require a calibration process and aims to achieve a sub-millimeter error.

Another factor of error for electromagnetic tracking systems is the presence of interference. This can come from large metal sources or electrical sources. The PA registration system will not have these limitations. However, it does have an issue that optical tracking systems share. This issue is that the visibility of site of interest is critical and necessary. One advantage that our system has in this issue is that the SC will be extremely close to the site of interest, which may not always be the case with standard optical tracking systems.

• Minimum: (March 26)
  1. Phantom and ex-vivo tissue experimental results with 3D Ultrasound
  2. Fiber delivery system successfully projects multiple laser points concurrently
  3. Visualization is able to show the Ultrasound points overlaid with the stereovision camera points
  4. Automatic segmentation is working on individual 2D Ultrasound images or individual slices of the 3D volume

• Expected: (April 16)
  1. Minimum Deliverables
  2. Visualization is able to display a representation of the Ultrasound volume in the stereovision camera image
  3. Automatic segmentation is working on an entire 3D Ultrasound volume

• Maximum: (May 7)
  1. Expected Deliverables
  2. Ability to collect 3D RF data automatically without manually actuating motor
  3. Complete integration of system

• Fiber Delivery System
  • There are two main reasons to develop a fiber delivery system. First of all, we can use it to shine multiple laser points onto the phantom or tissue at the same time. This allows us to eventually move towards a real-time system. Secondly, a fiber delivery system will allow the PA registration system to move to a laparoscopic environment. This is necessary because the end goal of this system is to be used in laparoscopic procedures.
  • Connect a fiber bundle to the laser source and split off the fibers at the other end. Lenses are then attached to each fiber to collimate the light.

• 3D Ultrasound
  • The main reason to use 3D Ultrasound as opposed to 2D Ultrasound is because we want to register 3D points between the SC space and the Ultrasound space. It also allows for easier placement of the 3D ultrasound probe because it was difficult to place the 2D ultrasound probe such that its image plane could see the PA signal.
  • Collect 2D ultrasound radio-frequency data when the 3D probe is in a static location. Actuate the motor by a certain step size. Repeat this procedure until the desired volume is collected.

• Automatic Segmentation
  • The reason for this is to allow results to be reliably reproduced from a set of data. The manual method required the user to select an intensity threshold with trial and error.
  • Determine intensity thresholds based on histogram of intensities. Make image or volume binary. Determine signal points based on the intensity weighted centroid of each high intensity region.

• Validation
  • The reason for validation is to show that this PA registration system is successful in meeting the project goals.
  • A visualization of ultrasound points in the camera space overlaid on the camera image. A representation of the ultrasound volume should also be visualized in the same space.
  • A TRE will be calculated based on other fiducials placed within the phantom or tissue. The fiducials will be tested using the computed transformation from the PA signal points.

• Access to laboratories

1. Dr. Boctor's lab
   • I already have access
2. Dr. Kang's lab
   • I already have access
3. Robotorium
   • I already have access

• Access to equipment

1. Laser
   • This is located in Dr. Kang's lab
   • I won't be able to run any experiments without the laser, but can develop and validate each of my algorithms on data that I have already collected
2. Optics
   • We already have some parts, but also have permission from Dr. Boctor to purchase new parts within reason
   • If we don't have optics parts or damage the ones that we have, we can fire the laser in free space
3. Ultrasound machine
   • This is located in Dr. Boctor's lab
   • There is another ultrasound machine in Dr. Boctor's lab that we can feasibly use if one of them is in use
4. 3D Ultrasound probe
   • This is located in Dr. Boctor's lab
   • There is another 3D Ultrasound probe if one of them is being used
5. Sonix DAQ device
   • This is located in Dr. Boctor's lab
   • We won't be able to collect any data if this is unavailable
6. Phantom Materials
   • This is located in Dr. Boctor's lab
   • We can buy more gelatin or plastisol if we run out
7. Ex-vivo tissue
   • We can buy this from the hospital market

1. Milestone name: Phantom Construction
   • Planned Date: February 27, 2012
   • Expected Date: March 6, 2012
   • Criteria: Create a phantom suitable for 3D PA imaging
   • Status: First attempted failed
2. Milestone name: Ex-vivo Tissue Phantom Construction
   • Planned Date: February 27, 2012
   • Expected Date: February 21, 2012
   • Criteria: Create a phantom with ex-vivo tissue suitable for 3D PA imaging
   • Status: Completed, will make another one at some point after this one perishes
3. Milestone name: 3D Ultrasound
   • Planned Date: February 27
   • Expected Date: February 27
   • Criteria: Able to collect collect data with a 3D US probe and segment the PA signal from the 3D volume
   • Status: Have a successful workflow with Ultrasonix 3D US probe. Will try to do the same with NDK 3D US probe
4. Milestone name: Fiber Delivery System
   • Planned Date: March 5, 2012
   • Expected Date: May 7, 2012
   • Criteria: Able to project multiple laser points
   • Status: Initial prototype can project multiple points. Still looking for method of collimating fiber output.
5. Milestone name: Visualization
   • Planned Date: March 26, 2012
   • Expected Date: April 2, 2012
   • Criteria: Ability to display SC and US points and representation of US volume in the SC space
   • Status: Complete. Able to generate movie of ultrasound representation in SC image
6. Milestone name: Automatic Segmentation
   • Planned Date: April 16, 2012
   • Expected Date: May 7, 2012
   • Criteria: Ability to segment desired PA signal from a set of images or 3D volume
   • Status: framework working on US side. SC side needs a bit more work.
7. Milestone name: System Integration
   • Planned Date: May 7, 2012
   • Expected Date: May 7, 2012
   • Criteria: All pieces fit together
   • Status: Pending

• Project Plan
  • Project plan presentation
  • Project plan proposal
• Project Background Reading
  • See Bibliography below for links.
• Project Checkpoint
  • Project checkpoint presentation
• Paper Seminar Presentations
  • PDF of Critical Summary
  • PDF of Paper
  • PDF of Seminar Presentation
• Project Final Presentation
  • PDF of Poster
• Project Final Report
  • Final Report
  • links to any appendices or other material

• Reading List
  1. Hoelen C. et al. “Three-dimensional photoacoustic imaging of blood vessels in tissue”. Optics Letters 1998. Vol. 23-8:648-650
  2. Kuo N. et al. “Photoacoustic imaging of prostate brachytherapy seeds in ex vivo prostate”. SPIE 2011
  3. Oberhammer P. et al. “Optimization and Quantification for Rigid Point Based Registration for Computer Aided Surgery”. Advances in Medical Engineering 2007. Vol. 114-3:230-235
  4. Pham D. et al., “Current Methods in Medical Image Segmentation”. Annual Review of Biomedical Engineering 2000. Vol. 2:315-337
  5. Spike, B. “The Photoacoustic Effect”. Physics 325 Lecture Notes 2006
  6. Vyas S. et al., “Intraoperative Ultrasound to Stereocamera Registration using Interventional Photoacoustic Imaging”. SPIE 2012
  7. Xu M. et al. “Photoacoustic Imaging in Biomedicine. Review of Scientific Instruments”. Review of Scientific Instruments 2006, 77
• References
  1. Spike, B. “The Photoacoustic Effect”. Physics 325 Lecture Notes 2006
  2. Boctor E. et al., “A Novel Closed Form Solution for Ultrasound Calibration”. ISBI 2004

PA Project Source Code