Imaging through scattering media

During surgery, there is often the need for obtaining structural information of regions deep below the surface of the tissue. This information is needed to protect blood vessels and other structures which should not be damaged during surgery.

However, many methods which are available during surgery are not able to resolve structures deep into tissue or are too cumbersome to use during surgery. Optical imaging for example cannot image deeper than a couple of mms. This is due to the fact, that shorter wavelengths are more effected by scattering whereas longer wavelengths suffer from stronger absorption in water. Hence, there is no wavelength which can be used to image deep into tissue as light always suffers from either absorption or scattering.

It remains a challenge to visualize structures deep below the surface of tissue. A method which provides this kind of information during surgery would add great value. It would enable the surgeon to better see where he wants to go while enabling him to avoid damaging structures such as blood vessels or nerves. Allowing him to quickly find the structures he wants to work on, while allowing for less removal of tissue. Greatly reducing the risk to the patient during surgery.

To date, there are mainly four techniques which are used to visualize structures below the surface of the tissue during surgery:

  • Optical Imaging techniques (Widefield Imaging, Optical Coherence Tomography, …)
  • Magnetic Resonance Imaging (MRI)
  • Medical Ultrasound
  • X-Ray Imaging

However, each of these techniques has significant limitations. Hence, a new method is needed which overcomes the limitations of the established techniques.

Attenuation length over wavelength
  Optical Imaging
 Magnetic Resonance Imaging (MRI)
 Medical Ultrasound
 X-Ray Imaging

Advantages
  • Is directly integrated in one of the main working tools of the surgeon
  • Is already the main tool for visualization
  • Provides 3D visualization in the entire body
  • Provides various contrasts (different MRI sequences) which can be used to differentiate various kinds of tissues
  • Standard clinical ultrasound can provide 3D visualization several tenth of cms into tissue
  • X-Ray Imaging can provide visualization through tissue (2D as well as 3D)

Disadvantages
  • Imaging is limited to a couple of mms below the surface due to scattering in tissue and absorption (mainly due to water)
  • MRI is mainly used to produce preoperative data. This information becomes unreliable during surgery as tissue is shifted around (body is opened, tissue is removed, …)
  • Reliable intraoperative data is not readily available. There are only very few hospitals with intraoperative MRI machines as these systems are very expensive and using intraoperative MRI interrupts the workflow during surgery (metallic instruments need to be removed, patient has the be brought to MRI machine, …)
  • Requires physical contact, as acoustic impedance needs to be matched. If for example the soundwaves have to travel through air to reach the tissue, large parts of the soundwave are reflected at the boundary between tissue and air. Impedance matching is often difficult to achieve during surgery as either the ultrasound device needs to be brought into contact with the tissue or vacancies which have been created during surgery have to be filled with water. Hence, it often interrupts the workflow.
  • Very limited resolution
  • Radiation dose is a major concern not only for the patient but also for the surgeon who has to do the surgical procedure on a regular basis for various patients.
  • Taking X-Ray images interrupts the workflow during surgery. X-Ray shielding and the X-Ray machine needs to be put in place and needs to be removed before and after the images are taken
  • Intraoperative X-Ray is expensive which limits its use (However, not as expensive as MRI)

Further reading

 

MRI

Medical ultrasound

X-Ray Imaging

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