Image Guided Surgery Flashcards
Explain what Computer Assisted Surgery is used for?
assist and visualize.
Explain how instrument are used in CAS?
The surgical instruments are tracked, and patient anatomy and tool position are captured. Instruments are used in conjunction with preoperative or intraoperative images.
Indirect guidance of tools:
- Intraoperative display of operative field with location of probe or instrument.
- Location overlaid onto any medical image.
Explain the procedure of CAS?
-Tracking unit receives and integrates signals from position sensor and trackers.
-Computer integrates signals from tracking unit with images (registration).
-Display visualizes images with tracker position and orientation.
-Navigated images are updated in real time as the instruments or patient move.
-Surgical tools are calibrated with tracker to obtain a geometrical model.
What are the equipment used? Hardware and Software?
Hardware:
- Computer workstation.
- Display system.
- Tracking system.
- Surgical instruments.
Software:
- Data management.
- Image review.
- Surgical navigation.
What are the 2 fundamental process?
Registration: process relates patient in operating theatre to preoperative image data.
Tracking: mechanism of following position of patient and instruments within operative field.
What is Registration? What are the coordinate systems? What are the two methods for localising reference points?
-Matches virtual world of images to the real world of the patient and operating room.
Refers to the correlation between preoperative images and fixed point on the patient anatomy.
-Coordinate Systems:
- Each component of the tracking system has its own coordinate system.
- Each coordinate system has to be related to one reference system.
-Anatomical landmarks.
- Fiducial markers: artificial landmark.
How do you locate anatomical landmarks?
Landmarks have to be visible both on patient and image data.
Probe which is visible to the tracking device is used to locate the landmarks. Sufficient to result in registration accuracy of 3-4 mm.
Locating fiducial markers?
Artificial landmark.
Placed on patient before scanning on day of surgery.
Markers are visible in both MRI and CT as well as on patient.
Accuracy can be increased using this method.
What is the goal of tracking? What are the devices and their functionalities?
The goal is:
- To determine position and orientation of tools and anatomical structures in image guided procedures.
- Visualize tool position and path in patient.
Tracking devices: sensors provide dynamic positional information.
The requirements for tracking?
- Refresh rate: ~100Hz with a latency of less than 1ms, regardless of the number of tracked objects
- Concurrency: tracks up to n sensors concurrently.
- Working volume: meets the needs of the procedure.
- Obtrusiveness:
o sensors are wireless and can function for several hours.
o All hardware components can be positioned so that they do not restrict the physical access to the patient.
o The system does not have any effect on other devices used during the procedure. - Completeness: sensors are small enough to embed in any tool and provide all 6DOF.
- Accuracy: resolution less than 0.1 mm and 0.1 degree.
- Robustness: not affected by the environment (light, sound, ferromagnetic materials, etc.).
- Cheap: cost less than $5000.
General Overview of the procedure of IGS?
Acquisition of preoperative images. Identification of patient landmarks in images:
- anatomical structures
- fiducials which are placed before image acquisition
Registration of patient.
Tracking of patient and instruments to confirm their position at all times.
How do optical trackers work?
Infrared light tracking surgical instruments and patient.
Camera rigs used to track fiducial markers that are attached to instrument or anatomical structure of interest. The position of the marker with respect to the cameras base line can be computed by triangulation.
Camera measures distance of markers from camera. Base distance between optical cameras is known. It can be either active or passive:
- Active: infrared-emitting markers are activated by an electrical signal.
- Passive: spherical markers reflect infrared light, emitted by illuminators on the position sensor.
What are the advantages and disadvantages of optical tracking?
Advantages: can be used with any traditional instruments. Disadvantages: line of sight problem.
- Cameras and markers have to be always in visual contact.
- Objects or persons between cameras and markers interrupt the visual contact.
Explain Electromagnetic Tracking?
Summary:
- Sensing changes in magnetic field.
- Field generator close to surgical field.
- Field sensor mounted on instrument and patient.
An active transmitter (magnetic field generator) is used to induce a current in sensor coils than can be embedded into the tracked objects.
Current induced in sensors is directly related to field strength and therefore position of sensor coils.
Each sensor consists of three coils, each perpendicular to the other two which allows to measure 6 degrees of freedom.
Field strength is a function of position and orientation between sensor and electromagnetic field.
What are the advantages and disadvantages of electromagnetic tracking?
Advantages: no line-of-sight issues and better operating room ergonomics.
Disadvantages:
- Magnetic field distorted by ferro-magnetic materials.
- Traditional surgical tools cannot be used since they are mostly ferro-magnetic.
