Machine Vision News
Vol. 4, 1999

"Crystal clear!"

Real-time image processing and visualisation brings medicine into the new age

With the increasing demand for minimally invasive surgery, MRI scanners have a major part to play not only as a diagnostic tool but also as means of guiding the surgeon´s hand. However, before MRI scanners can be used in an interventional role, certain technological problems must be overcome. A multidisciplinary consortium that brings together Oulu Hospital, the Electrical Engineering Faculty at Oulu University, Daum GmbH, an innovative German engineering company, and Picker Nordstar Oy, a global leader in the medical imaging industry, have been working together on the IRVIT project to solve exactly those problems.  
Radiology is the field of medicine that uses various forms of acoustic and electromagnetic energy for producing diagnostic images of the human body. Since the discovery of X-rays by Rontgen in 1895, X-ray imaging has dominated the history of radiology and it has only been during the last three decades that alternative imaging methods have begun to emerge. The first of these was ultrasound, which used acoustic waves in forming an image. Shortly afterwards computed tomography revolutionised the world of medicine when it made it possible to acquire cross-section images from a living human body. One drawback, however, was that computed tomography still used X-rays, which are harmful to living organs.  
In the late 1970s magnetic resonance imaging (MRI) was introduced. MRI is able to produce two- or three-dimensional images of great quality, showing cross-sections through body parts at regular intervals. The images are so precise that radiologists are often able to get as much information from a scan as from looking at the tissue directly. The MRI scanner uses a modulated magnetic field and radio frequency (RF) energy to ´excite´ the hydrogen protons of organic tissue. When the protons ´relax´ again, they send out an RF signal which is measurable. Using special measurement techniques, the proton relaxation signal can be interpreted into a cross-section image of a human body.  
MRI is the state-of-the-art radiological imaging method that has far superior soft tissue contrast to any other radiological  imaging methods. What is more, in contrast to X-ray based methods, it has the major advantage of providing images without the use of harmful ionising radiation. Not surprisingly, its applications have grown rapidly in recent years.  
Interventional radiology 
Interventional radiology uses imaging to guide surgical operations. It is part of the recent trend of so-called keyhole surgery, which aims at minimal invasiveness. Obviously when operations are performed through such small openings, image guidance is needed. For example, by using ultrasound or X-ray fluoroscopy, a needle can be guided to the chosen part of a tissue and inject medication into or take a tissue sample from a tumour. As medical technology has improved, the distinction between interventional radiology and surgery has become fuzzier.
Currently 99.5% of MRI scanners are sold for diagnostic imaging. However the recent development of the new open magnet systems has made ´access´ to the patient during imaging possible. This development means that there is now the potential to combine state-of-the-art imaging with surgical or radiological interventions.  
Performing surgical operations however, is not straightforward: the room or environment where the MRI scanner is located has extremely strict instrumentation demands. For example, any devices and instruments brought into the MRI room must not interfere with the image acquisition of the scanner. In other words, they should neither emit RF energy nor should they distort the magnetic field inside the imaging volume. At the same time, though, the devices and instruments should themselves be able to withstand strong magnetic field and RF emission. 
If interventional MRI is to become a surgical reality, a great deal of development work is required. New instruments need to be designed, operating room instrumentation will have to be altered to meet MRI requirements, and the scanners themselves will need to be modified to raise the image acquisition to a more interactive level. Many scientific fields and techniques will be required in the realisation of this goal, and naturally modern image processing technology will have a significant role to play. 
IRVIT demonstration 
Intraoperative Real-time Visualisation and Instrument Tracking (IRVIT) is an EU-funded research project under the HPCN-TTN initiative.  
The IRVIT project target is to demonstrate how innovative technology can be used in interventional MRI. The two key features to be demonstrated are:
  1. accurate instrument tracking 
  2. real-time image processing and visualisation.  

The problems posed by interventional MRI are very similar to those of the better known field of telemanipulation. The MRI intervention system is a closed feedback loop where the surgeon moves an instrument according to the visual feedback he gets. Moving the instrument triggers an image update, which shows the instrumentÕs new position with respect to the anatomy. In order to be able to control the image acquisition and thereby provide visual feedback, it is necessary to be able to assess the location of the instrument accurately.
This is where the IRVIT project comes in. A new technological device, the ESR marker, has been developed and this can be used to measure the location of the surgical instrument. The marker is a very small crystal that can be integrated into the tip of an instrument 1.2mm in diameter. By measuring the electron spin resonance (ESR) signal from the crystal, the location of the instrument can be etermined. As the MRI scanner is used to measure both the ESR marker location and the natomical images, most of the sources of error will be the same, and thus their net effect is cancelled out. As a result excellent relative localisation accuracy can be achieved. 
Figure 1:  
ESR technology provides an accurate and reliable method of instrument location, even when the instrument bends.

During an intervention the images are almost always acquired relative to the tip of the instrument. There exist methods that can measure the location of the non-invasive part of the instrument body. With these systems the tip of the instrument is extrapolated and thus bending of the instrument can dramatically impair accuracy. Because the ESR marker is small, it can be located directly into the tip of the instrument. Therefore instrument bending does not impair accuracy. An example of a case where ESR marker tracking is used is shown in Figure 1.
The images obtained by the scanner itself are insufficient for the efficient performance of a surgical intervention and various image processing techniques are necessary in order to make the most important features more easily visible. In addition, real-time volume rendering techniques can provide essential added value to the visual feedback. The applications of these technologies are currently being studied at Oulu University Hospital. 
Another important image processing task in IRVIT is the registration of real-time images with earlier preoperative images. The current image acquisition latency of the MRI scanners is a result of the limits of imaging physics and of the MRI scanner design. In diagnostic use, the image acquisition latencies do not play as critical a role as they do in interactive interventional MRI. MRI scanner manufacturers are currently working on eliminating both sources of latency but for the present and near future, real-time imaging is not yet a possibility. Fortunately this problem can be alleviated if the preoperative images can be used parallel to the intraoperative ones. The research into image registration methods now being done at the University of Oulu aims to solve this problem. A demonstration system of the IRVIT project will be built into the Picker Outlook Proview scanner at Oulu University Hospital in Finland. This demonstrator will be available for evaluation at the end of June 1999, when the project is due for completion. Figure 2 shows a preview of the forthcoming system.
Figure 2:  Caption: A radiologist testing the IRVIT demonstrator system.The Consortium

Any project in the field of interventional MRI requires a multidisciplinary consortium and skills, and thus the consortium of the IRVIT project consists of four complementary partners, each leaders in their respective fields. 
Daum GmbH is an innovative German engineering company which specialises in developing instruments for minimally invasive surgery and interventional radiology. In a very short time, its high quality MRI-compatible instruments have become well known all over the world. Within the IRVIT project, Daum has used its substantial expertise in MRI-compatible materials and instruments to integrate the ESR marker into an instrument. 
Picker Nordstar Oy is a subsidiary of Picker International, a global leader in the medical imaging industry. Its products include Outlook Proview, one of the MRI systems with the most open shape available on the market today. With a large patient gap (i.e. there is sufficient space for the surgeon to work around the patient) and flexible patient handling, Outlook Proview offers the ideal platform for developing interventional procedures. The Outlook Proview scanners are designed and manufactured in Finland. 
The Department of Radiology at Oulu University Hospital is one the leading clinics involved in MRI research and interventional radiology in Northern Europe. Not only does the clinic have over ten engineers and physicists researching MRI imaging, image processing and real-time volume endering, but the modern interventional MRI facilities at the hospital are ideal for performing and developing both radiological and neurosurgical procedures. The Department of Radiology therefore brings significant accumulated expertise in state-of-the-art image-guided procedures to the IRVIT project. 
The Department of Electrical Engineering in the University of Oulu has a Machine Vision and Media Processing Group with over 60 engineers and mathematicians specialising in signal, image and media processing. This group has applied its expertise in image processing to the specific applications of the IRVIT project. 
Technical co-ordinator of IRVIT 
Lasse Jyrkinen 
Picker Nordstar Oy 
Tel: +358-8-553 2787 
Fax: +358-8-553 2612 
HPCN-TTN network (URL)