Towards a less claustrophobic scanning

It is unlikely anyone undergoing a magnetic resonance imaging (MRI) scan would describe the experience as pleasant. Having to lie very still in a narrow tube for anything up to an hour, while the machine loudly bangs its way through the scanning procedure, is hardly fun. In fact, some patients find it so traumatic they have to be sedated - at times, put under general anaesthetic - before being scanned. That is if they can fit inside it at all.

However, claustrophobia and a tight squeeze could soon become problems of the past as more of the latest generation of 'open' scanners, with very wide bores or completely open gaps for the patient, appear in clinics around the world. These systems also offer the potential for MRI-guided surgery.

Wider bore systems

According to the World Health Organization, there are at least 300 million clinically obese adults worldwide, so not surprisingly this has been a driving factor for developments in scanner technology. Siemens, for instance, have been producing scanners with wide bores - 70cm as opposed to the standard 60cm - since 2004.

"We see a trend towards more patient comfort. Growing obesity will push this trend, and patients are better informed than they used to be - they look on the Internet and pick an institution. So with an ever higher density of MR systems in the world, our customers are in a competitive situation," says Mathias Blasche, principal key expert in the MR Business Unit of Siemens Healthcare. Children, intensive care patients, or anyone dependent on medical equipment can also benefit from wider bores, explains Blasche. "We see better image quality with anxious patients who are prone to move during the scan if they do not feel comfortable," he adds.

All patients can gain from these developments. Siemens's new wide-bore Magnetom Espree-Pink breast scanner, for example, allows spectroscopy and biopsies to be carried out in situ, aided by dedicated software and a specially designed coil (see 'How MRI works' p22) that can be adjusted to optimise scanning of any breast size.

Don't take it lying down

Some conditions, like certain back injuries, can actually be made worse by patients lying flat on their backs in a conventional scanner. In open scanners they don't have to.

At the Nuffield Orthopaedic Centre (NOC) in Oxford, where they installed a FONAR 360° open scanner in a purpose-built building in 2006, they can now scan patients lying on their sides. Their scanner also accommodates larger patients and has seen the number of people requiring pre-scan sedation fall from roughly one a week (when using a conventional scanner) to one every two months, says Ruth McDonnell, MRI lead radiographer at the NOC.

"Patients are often relieved to find the scanner is not a tube like they've seen on TV. Being able to see out of the scanner throughout the examination makes them feel much less enclosed," explains McDonnell. Philips's patient studies echo this experience: their Panorama HFO open scanner reduced the rejection rate by claustrophobic patients by a half.

Systems like the FONAR UPRIGHT multi-position scanner take the idea a stage further. "It can scan patients upright in weight-bearing positions, in positions of pain, and in positions of flexion, extension, lateral bending and rotation," says a spokesperson for US-based FONAR. Patients can also be scanned sitting upright or lying horizontally. This provides a new route to diagnoses for doctors and physiotherapists, and is particularly valuable for spinal problems that can look significantly different when lying down.

 Italian manufacturers Esaote, who produce dedicated musculoskeletal scanners, allow study of joints and the spine in weight-bearing positions via their G-scan. As the whole unit can be rotated from horizontal to vertical, the scan can be performed at any angle. In their most open system, the C-scan, the patient inserts the joint to be studied while the rest of their body is outside the machine.

The technical challenge

Open and wider-bore MRI machines may seem like an obvious step that should have been taken years ago. However, the technical challenges involved in obtaining a high-resolution image from these systems has been considerable. For a start, in order to get a clear MR image the magnetic field provided by the scanner magnet needs to be uniform - and the shorter the magnet, the more difficult this is to achieve.

"We addressed this by developing a [superconducting] magnet with seven field-generating coils instead of six," says Blasche from Siemens, who specialise in wide bore scanners. This gave a field uniform enough to image any organ in a single step, despite a short 125cm system length. Siemens also worked on improving the perfor-mance of the radio-frequency (RF) system and RF coils that create the pulse required to obtain an MRI signal, as the more RF channels available, the higher the signal-to-noise ratio and the faster the imaging.

Meanwhile, new types of receiver coil have had to be developed for completely open systems to obtain good image quality. "The rule in MRI is that the axis of symmetry of the receiver coil needs to be perpendicular to the orientation of the main magnetic field. Conventional high-field superconducting MRI systems have a horizontal magnetic field that is parallel to the long axis of the body, so they can use planar (flat) coils to image the spine. The open MRIs operate with a vertical magnetic field and use solenoidal ('wrap-around') receiver coils," explain FONAR. 

As truly open scanners have a gap between two magnet pole-pieces, a new 'iron-frame' technology was also needed to provide a suitable magnetic field. This forms the basis for all open MRI scanners. Iron-frame technology uses iron to provide a conduit for the magnetic flux to complete its circuit from the north pole of the magnet to the south pole. In the FONAR 360° scanner, it is partly built into the scanner room walls.

The surgery of the future?

Open scanners lend themselves to MRI-guided surgical procedures, known as 'interventional MRI' - something currently in its infancy. The hope is that complex surgery, including the removal of cancerous tumours, could be performed under MR guidance making it more accurate. But there is a catch.

Many metals are strongly attracted by the large magnetic field generated by MRI scanners, so surgeons cannot use conventional surgical instruments. "A scalpel must be sharp, but if it cannot be made from a ferromagnetic or conductive material, what material do you use? Interventional MRI is not just an engineering challenge, but a challenge for materials scientists too," explains Steve Keevil, senior lecturer in Imaging Sciences at St Thomas' Hospital in London.

Guy's and St Thomas' Hospital has carried out MRI guiding of cardiac catheters since 2001 using a conventional scanner. "We have been restricted to using devices that are MR safe by chance," says Keevil. Meanwhile, the NOC have conducted preliminary research into interventional procedures using their open scanner. "However, there have been limitations including the supply of MR-safe needles suitable for interventional musculoskeletal work," says McDonnell.

While some specialist devices, such as biopsy kits and needles produced by DAUM, do exist there are few general instruments available. Significant investment will be required to develop new products and get them through all the regulatory tests, then, once approved, the market for them will be very small. "Interventional MR has not grown a lot within the last ten years and, given the practical problems, you do wonder whether it is ever really going to take off," Keevil warns.

Spanner in the works

Even if interventional MRI never reaches its true potential, open systems still greatly improve the patient experience and offer examinations impossible by any other means. Not surprisingly then, the number of open systems in clinical use is steadily increasing. But what can we expect in the future?

Developments in the MRI field are driven both by the needs of the end-users and by advances in science and materials, says Blasche from Siemens. "We take care to understand the needs of our customers in different institutions, different countries, and with different healthcare systems," he says. "At the same time, we closely watch new developments and learn from experience. The best solutions are created when all these factors match perfectly."

"We see a trend towards higher magnetic field strength," continues Blasche. "Twenty years ago, most routine scanners were sold at 1 tesla (T). Ten years ago, 1.5T became the clinical standard and is still approximately two-thirds of the world market, but the share of 3T systems has increased to approximately 20 per cent over the last few years."

High magnetic field strengths are generally desirable in MRI because the higher the field, the higher the image resolution. Superconducting magnets produce the highest fields, yet lower cost electromagnets (as used by FONAR) and permanent magnets (the base of Esaote's scanners) can produce enough resolution for many studies. According to Esaote's Strategic Marketing Department, optimising all their scanners' components allows good image quality for bone, muscles and tendons in joints at the relatively low fields they use (0.25T for their G-scan for example). They see musculoskeletal scans being increasingly performed in dedicated systems.

"We believe it is a waste of healthcare resources to use a 3T system for a routine knee examination," they say. FONAR also cite cost-effectiveness, along with high performance and a desire to "provide a set of unique imaging applications" as main R&D drivers.

The future development - and use - of MRI scanners could, however, be thrown into jeopardy by the 2004 European Commission (EC) directive restricting occupational exposure to electromagnetic fields (Directive 2004/40/EC). This was due to come into effect in April 2008 and would, in its present form, limit the use of all MRI systems, including the open scanners. But as a result of further investigations showing workers routinely exceed the recommended limits for exposure with no evidence of side-effects and lobbying - in part by the UK's Institute of Physics - the implementation has been stalled until April 2012.

Keevil, who was part of the lobbying process, says a working party is currently trying to re-word the directive in such a way that MRI will not be affected. If they don't succeed, we could be forced back to poorer quality imaging of soft tissues using harmful ionising radiation, which surely cannot be in anyone's interest.