When minimally invasive surgery, also known as keyhole surgery, is performed, usually the target is not directly visible. For this reason, surgeons need a means of "seeing" inside the patient so that they can place the surgical instrument precisely. X-ray fluoroscopy, ultrasound and optical endoscopy are the most commonly used imaging technologies, but MRI is the state-of-the-art technology for medical diagnostic imaging.
A problem with MRI is that, under certain conditions, the images produced can suffer from geometric distortions due to lack of uniformity of the activating magnetic field. Under clinical conditions, it is vital that the instrument tracking technology is able to compensate for these distortions.
The AMIT system uses a technology called electron spin resonance (ESR), which is a close relative of MRI, to measure the position of instruments. The advantage of ESR is that it uses the same physical principle as the MRI technology and therefore suffers the same geometric distortions. In this way, distortions are automatically eliminated and the relative accuracy of the calculated position is not impaired.
ESR enables the precise location of surgical instruments relative to the patient's organs to be displayed in real time. Dr. Osmo Tervonen of Oulu University Hospital, Finland, co-ordinator of the project stated: "It is very important for surgeons and radiologists to know the precise position of the tip of the surgical instrument. If we need to take a biopsy of a tumour, we need to know exactly where it's being taken."
A key benefit of the AMIT system, developed with funding from the IST programme, is that the ESR technology enables the very tip of the instrument to be positioned, without the need for wires. Rival systems calculate the co-ordinates of a reference point located somewhere on the shaft of the instrument, and then use extrapolation to work out where the tip is positioned. "We developed ESR to be used in wireless mode, and this makes it much more acceptable for use in a clinical environment", commented Dr. Tervonen.
"Since we are calculating the actual position of the tip of the instrument, it doesn't matter if the shaft of the instrument bends", stated Dr. Tervonen. "In fact, we can use catheters very effectively, because the system automatically calculates the catheter's profile and enables it to be tracked within the body volume."
Conventional MRI is not suitable for real-time clinical imaging. Although it is possible to obtain an MRI image in only one second, in practice much longer scan times are required to calculate 3D images of acceptable quality. AMIT explored a method of reducing the image acquisition latency by combining high-resolution images produced prior to the operation with low-resolution images produced in real time during the course of the operation.
One of the goals of AMIT was to develop an operation-specific user interface and visualisation technology. Although it is important that the surgeon has direct control of the ESR system, it has to be used within an integrated framework of the whole imaging and guidance system. Minimally invasive surgery assisted by MRI guidance is a relatively new area. The AMIT project has developed sample applications using needles and catheters, with the aim of advancing the state-of-the-art. This has enabled new procedures to be developed and helped to make current operations even safer.
"This whole area is developing", stated Dr. Tervonen. "Minimally invasive surgery assisted by MRI guidance is used for cancer treatment to take tissue samples from tumours, but it can also be used for other procedures such as surgical interventions around the nerves in the lumbar region. An area where it will find application is in stereotactic procedures which target very specific areas of the brain. The treatment of Parkinson's disease requires certain nuclei in the lower brain to be targeted, and ESR would be useful because it is so precise and accurate."
Source for this article is the European IST Results news service. More information on the AMIT project is available in the VMW March 2001 article Finnish imaging project to promote MRI guidance for minimally invasive procedures.