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Magnetic Resonance Imaging, a Success Story for Superconductivity

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Magnetic Resonance Imaging, a Success Story for Superconductivity FEATURES Magnetic Resonance Imaging a success story for superconductivity l René Aarnink1 and Johan Overweg2 - DOI: 10.1051/epn/2012404 l 1 Minimally Invasive Healthcare Department Head, Philips Research Eindhoven l 2 Principal Scientist, Tomographic Imaging Systems Department, Philips Research Hamburg Superconductivity is at the heart of MRI scanners, helping to give doctors an unprecedented view of structures deep within the human body. Nevertheless, challenges still exist in realizing the full potential of this important imaging modality, some of which relate to physics and some to economics. ithout superconductivity, one of to- The field strength generated by a current-carrying con- day’s most important medical imag- ductor can be calculated using Ampere’s Law. If the MRI ing modalities would not be possible. magnet were a simple long solenoid with uniform wind- Magnetic Resonance Imaging (MRI), ing density, this would reduce to: Wwhich depends on being able to align the weak magnetic moment of the protons in a patient’s body with a mag- B = µ0 I N/L netic field, requires the use of magnetic field strengths in m Working screen for Magnetic excess of 1T. This can only be achieved practically through where B = field strength in Tesla; I = current per turn; N = Resonance Imaging the use of superconducting electromagnets. number of turns; L = coil length and µ0 is the permeability 26 EPN 43/4 Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn/2012404 MAGNETIC RESONANCE IMAGING FEATURES of free space (to a close approximation, equal to that of shorter scan times, increasing patient throughput and the patient/air within an MRI scanner). reducing patient discomfort. There is therefore a trend This simple formula works out at approximately 1 × 106 toward higher field strength scanners, with 3T MRI scan- ampere-turns (around 10,000 turns of wire driven with ners already being used in routine clinical care and 7T 100 amps) for a typical MRI magnet length of 1.2 metres machines being used in (pre-)clinical research. and a field of 1 tesla. For several reasons, practical MRI However, 7T MRI scanners are extremely expensive and magnets actually have around 5 times as many ampere- will probably remain so, in particular because the current- turns. Firstly because of their small length-to-diameter carrying capacity of the superconductors employed de- ratio; secondly because of the increasing turns density creases rapidly above 5T, leading to a disproportionate needed towards the ends of the coil to make the field increase in the amount of con- homogeneous; and lastly because of the need for active ductor needed. In addition, the stray-field compensation (the magnet consists of two electro-magnetic (Lorentz) forces concentric coil sets with opposing polarity, shaped in that have to be contained become such a way that the outer coil to a large extent cancels a real problem, increasing with Zero boil-off magnets are the external magnetic field of the inner coil). the square of the field. The main now state of the art Without the use of superconducting materials such as problem with Lorentz forces on “ Niobium Titanium, cooled with liquid helium to below superconducting conductors is ” their critical temperature, the energy loss in such coils that they can lead to motion of the conductors, which would make modern MRI scanners impractical. Thanks causes friction heat. It only takes a few microjoules of to these materials, however, MRI scanners are already heat energy to raise a small piece of the superconductor commonplace in hospitals, providing superior soft-tissue above its critical temperature (the heat capacity at 4K is imaging to assist doctors in the diagnosis of a wide range extremely small) and if this non-superconducting zone of diseases, including cancer, Alzheimer’s, stroke, trauma starts to propagate through the coil, the entire stored en- injury and Parkinson’s disease. ergy of the magnet (many megajoules) is dumped into the liquid helium cooling the magnet (this is known as Field strength and uniformity a quench). MRI magnets are designed to survive such The characteristics of the magnetic field generated by an event without permanent damage, but re-cooling and an MRI scanner have a significant impact on its per- bringing the magnet back to the required field strength formance. The field strength has to be very uniform in is time consuming and costly. the imaging volume (typically the cross-section of the human body, with a spherical volume of around 45 to Cryostat design 50 cm diameter in the center of the magnet) and it has In most of today’s MRI magnets, the superconducting to remain extremely stable during the scan. In addition, coil is enclosed in a helium tank containing approxi- the proton polarization, which determines the strength mately 1000 litres of liquid when full. Magnets are usu- of the NMR signal, increases linearly with field strength. ally cooled down and filled only once, in the factory High field strengths therefore enable increased signal- where they are made, and remain cold for their entire to-noise ratios and better image quality. They also allow life (typically 10 years). How MRI worKS When a person is in an MRI scanner, the Gradient fields in the three dimensions powerful magnetic field and powerful hydrogen nuclei (i.e., protons, found in allow the scanner to work only with pro- radio waves. abundance in the human body as water) tons from a ‘slice’ at a time, allowing the MRI is used to image every part of the align with the strong magnetic field. A ra- creation of a whole volume that can be body, but is particularly useful in neu- dio wave at just the right frequency for the looked at in three dimensions. rological conditions, disorders of the protons to absorb energy pushes some of Contrast agents may be injected intrave- muscles and joints, for evaluating tu- the protons out of alignment. The protons nously to show enhancement of blood mors and showing abnormalities in the then snap back to alignment, producing a vessels, tumors or inflammation. Unlike heart and blood vessels. n detectable rotating magnetic field as they CT scanning, MRI uses no ionizing radia- do so. Since protons in different areas of tion and is generally a very safe proce- source: the body (e.g., fat versus muscle) realign at dure. Patients with some metal implants www.websters-online-dictionary.org/ different speeds, the different structures and cardiac pacemakers are prevented definitions/Magnetic+resonance+ of the body can be revealed. from having an MRI due to effects of the imaging EPN 43/4 27 FEATURES MAGNETIC RESONANCE IMAGING Any heat leak into this cold mass causes helium to high-vacuum space and the magnet’s suspension sys- evaporate (at a rate of around 1.4 litres per hour per tem has a low thermal conductivity, the single biggest watt of thermal load). If the cold mass is enclosed in a contributor to this heat load is thermal radiation. Older MRI magnets relied on two refrigerator-cooled shields in the vacuum space to reduce the heat load to less than 50 mW (the radiated heat decreases with the fourth power INCREASED Patient comfort AND acceSS of the surface temperature of the shield seen by the cold mass), resulting in a loss of less than 1 litre of helium per Some patients feel very claustrophobic in the bore of a conven- day. More modern refrigerators have about 1W of cooling tional cylindrical magnet MRI scanner, and the sudden loud noises power at 4K, allowing boil-off from the helium tank to be emitted by the scanner when the gradient coils are activated often stopped completely. These zero boil-off magnets are now worsens this feeling and can cause the patient to move suddenly. the state of the art. The rapidly increasing cost of helium Such movements can degrade image quality, so sedatives and/or more than justifies the extra cost of such a refrigerator. ear plugs are often used to calm the patient and minimize the effect. However, it takes about 7 kW of electrical power to drive the refrigeration system. One of the challenges in magnet design is therefore to make these refrigeration systems more efficient and reliable so that the loss of helium due to planned and un-planned downtime of the refrigeration system is mini- mized. When the magnet is being shipped or temporar- ily stored before installation, the refrigerator is also kept running whenever possible. With the escalating cost of helium, which is a by-product of natural-gas extraction, another important challenge for the future is minimizing the helium content of the mag- net. A few hundred litres of liquid helium or less would actually be sufficient to keep the magnet superconduct- ing, so the main challenge in developing magnets without a large helium buffer volume is to make the refrigeration system so reliable that it never fails, or so efficient that it can re-cool the magnet without using much external m The Panorama HFO, Philips’ high-field, wide open system, offers outstanding clinical helium after a failure. versatility with leading solutions for all clinical applications. And it is preferred by patients because of its comfortable space and wide open bore. High-temperature superconductors One way of eliminating the need for helium in MRI In an attempt to increase patient comfort and provide clinicians with scanners completely would be to use high-temperature better access to the patient, for example to perform MRI-guided clin- superconducting materials such as Yttrium Barium Cop- ical interventions, open-magnet MRI scanners have been developed. per Oxide (YBCO) tape.
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