Magnetic resonance imaging (MRI) has become a mainstay of medical imaging facilities. Superior soft-tissue contrast versus CT scans and the use of non-ionizing radio waves to visualize a rich matrix of functional information – including blood volume/oxygenation and localized metabolic activity within tumour sites – represent a winning combination for clinicians in the diagnosis and treatment of all manner of diseases.

Underpinning that clinical capability is an array of enabling technologies, the largest and most expensive of which is the cryogenically cooled superconducting magnet that sits at the heart of today’s cutting-edge MRI scanners. Clinical MRI machines typically have a magnetic-field strength in the range 0.1 to 3.0 T – though research systems for human and small-animal applications are available at much higher fields (up to 25 T). In every case, these multimillion-dollar systems require a magnetic field that combines extreme stability with extreme uniformity (to within a few ppm) to ensure optimal imaging performance.

To service that need, Swiss manufacturer Metrolab Technology SA, a market-leader in precision magnetometers, has developed a portfolio of measurement tools and accessories to enable MRI equipment manufacturers to quantify and map the magnetic subsystems at the heart of their clinical MRI scanners. Metrolab’s products are used by MRI equipment vendors throughout the technology and innovation cycle: to support R&D on next-generation systems; in the production and assembly plant; and during the installation of new MRI machines in clinical facilities.

“The biggest application – in terms of the volume of test units we ship – is for the installation and commissioning of new MRI scanners in hospitals and clinics all over the world,” explains Philip Keller, marketing and product manager at Metrolab.

Magnetic iterations

The core product in Metrolab’s portfolio is the NMR magnetic-field camera, the latest iteration of which – the MFC2046 – provides magnetic-field measurements with a resolution of 10 ppb; overall positioning tolerances well under 1 mm; and a measurement range from 0.2 to more than 25 T (versus a 7 T limit with the previous-generation system). The camera comes with a MFC9046 probe array, a unit with up to 255 measurement points that generates detailed field maps inside the MRI magnet bore in roughly five minutes.

“With our new-generation MFC2046 camera system, we’re also optionally combining the functionalities of our single-point, wide-range magnetic probe into the probe array,” says Keller. “The former is used for magnet ‘ramping’, while the probe array offers detailed field mapping – a combination that saves MRI manufacturers time and money in the assembly plant and during MRI system installation in the clinic. It’s a win-win because technicians no longer need to swap out two test instruments to perform these different sets of measurements.”

Consider a typical installation scenario in which a new MRI machine is shipped to a clinical customer. For safety reasons, the magnet is usually dispatched from the factory without any magnetic field, ahead of installation in a magnetically and electrically shielded room at the customer site. At this point, the next task for the manufacturer’s technician team is to bring the MRI scanner’s magnetic field up to the specified operating level by injecting current into the magnet’s superconducting coils.

“The wide-range probe [in the MFC9046 probe array] is used to track this ramping process from zero to say 1.5 or 3.0 T,” explains Keller. “Then, as the magnet nears the desired field level, this high-precision single-point probe enables the technicians to get the field setting just right.”

Installation of the MRI magnet continues with an iterative, fine-tuning process known as shimming. The aim here is to make the magnetic field inside the bore of the MRI scanner more homogeneous by placing pieces of iron (shims) in the appropriate place or by adjusting the current in special shim coils. “The technicians must first ramp the magnet to its nominal field, followed by detailed field mapping and the shim adjustments,” says Keller. “They then have to remap the magnetic field to make sure the shimming has had the desired effect in terms of field homogeneity.”

Think small

As well as applications in whole-body MRI, Metrolab’s magnetic-field camera and probe arrays are also suitable for use with smaller-scale MRI systems with magnet bores as small as 40 mm. Small-bore MRI scanners are used by drug companies to image a range of animal subjects  – mice, rats and guinea pigs, for example – in drug evaluation trials. These test animals serve as “human models” that allow scientists, through repeat MRI scans, to evaluate the therapeutic efficacy (as well as secondary side-effects) of new drug regimes over time. Elsewhere, small-bore MRI systems find niche applications in sports medicine – and specifically the imaging of extremity injuries to knees, elbows and ankles.

In the past, Metrolab served this specialist instrumentation market with its single-point NMR probe. To generate a map of the magnetic field, a technician had to position the probe at hundred of points within the magnet bore – a process that took several hours. Now, with the MFC9046 multiprobe array, it’s possible to generate the same field map in around five minutes. “The compressed data acquisition time represents a significant gain from a production and installation perspective,” claims Keller. It also provides better positional accuracy and minimizes inconsistencies due to magnet drift.

Measurement speed aside, one of the main engineering challenges when mapping a small-scale MRI system is the size of the magnet bore. “With our new MFC9046 system we use a pulse-waved NMR measurement technique instead of continuous-wave,” notes Keller. “That means we can have the electronics remote from the probe head and in turn make the probe array a lot smaller versus our previous-generation unit.”

Another key requirement – given that the output is a map of magnetic field inside the small MRI magnet bore – is the positioning accuracy of the field measurements. “You need to have accurate magnetic-field measurements, but you also need to have accurate positions in geometric space,” explains Keller. “As you rotate the probe array inside the magnet bore, the mechanical accuracy of the positioner has to be spot on, with sub-mm tolerances.”

To extend its coverage of the small-scale MR market, Metrolab has also developed a new miniature probe array (MFC9146) for field-mapping of NMR spectroscopy systems used in materials science and applied chemistry laboratories.