Magnetic resonance imaging (MRI) is a diagnostic and research tool used in the neurosciences. In 2012, the NEUROSPIN centre will take delivery of an 11.75 teslas magnetic resonance imager (MRI) with a 90 cm diameter opening capable of scanning the patient’s entire body. The use of this magnet in molecular imaging, together with new pharmaceutical contrast agents, will enable a deeper understanding of the brain by improving the images by a factor of ten. The development of ISEULT is part of a larger Franco-German project being carried out in collaboration with major industrial companies active in the field, including Guerbet, Siemens Medical Solutions and Alstom.
Cross-section of the ISEULT 11.7 T magnet. The superconducting windings are shown in orange, the structure held at 1.8 K in blue, and the cryostat in grey.
The challenges for the ISEULT magnet
The design of the ISEULT magnet includes a number of characteristics that set it apart from conventional MRI magnets. There are five main technological problems to be overcome before such a magnet can be manufactured:
? A high intensity 11.75 T magnetic field.
? A large usable volume of several liters compared with other imaging systems already installed at the NEUROSPIN site which generate a field of 17 T but in a volume that is one hundred times smaller.
? Temporal stability, with variations in the magnetic field of less than 10-9 T over a period of ten minutes.
? A magnetic field that is homogeneous to within 5.10-7 T throughout the volume of interest corresponding to the brain of the patient.
? Containment of the magnetic field inside the experiment room.
Superconducting cable (9.2 mm x 4.9 mm) for the main coil in a trough. Ten strands of composite NbTi-Cu wire inserted in a copper trough and coated with solder.
The coil consists of several thousand kilometers of niobium-titanium superconducting wire wound in double pancake coils and carrying a current of 1483 A with a stability of 0.05 ppm / hour. This superconductor is maintained at a very low temperature of 1.8 K by means of 5000 liters of superfluid helium isolated from the exterior by series of insulating enclosures. A development program including a number of prototypes and special test stations will be required in order to understand and overcome these problems.
The R1 prototype: Stack of six prototype pancake coils, with an inner radius of 185 mm and an outer radius of 240 mm, mounted in a mechanical support structure in order to study the mechanical characteristics of the ISEULT conductor when subjected to a high magnetic field (12 T) and current (5000 A).
Completion of the development phase
A series of small specialized prototypes were manufactured between the end of 2009 and the start of 2012, and tests were carried out on model coils in order to demonstrate the principles of assembly and to verify the capability of the conductor to operate under the nominal magnetic field and associated forces. One of the prototypes, identified as R1, consisted of an assembly of six double pancake coils. Each double pancake coils consisted of eleven turns of the final ISEULT conductor and used the same components and assembly procedures as the final magnet. After winding, the six double pancake coils were assembled into a mechanical support structure consisting of stainless steel flanges, and an axial force of 240 tonnes was applied via twelve aluminum tie rods. The R1 prototype was then inserted into the 8 T magnet in the SEHT test station in order to apply a maximum field of 12 T to the conductor.
The initial cooling and testing of the R1 prototype was successfully completed in late December 2011 and early January 2012. By ramping up the current in both SEHT and R1, the conductor was subjected to an azimuthal stress reaching 225 MPa, well above the maximum nominal stress of 170 MPa expected in ISEULT. Additional tests at 4.2 K showed that the conductor could reach 96 % of the theoretical critical current, equivalent to a variation of 0.1 K from the theoretical critical temperature. This result compares well with the temperature margin of 1 K assumed during the dimensioning of the ISEULT magnet. In conjunction with the application of an axial stress of almost 110 MPa, these tests have demonstrated the ability of the conductor to withstand the mechanical stresses that they will be subjected to in ISEULT with a margin of almost 20 % while under the magnetic field conditions representative of those of ISEULT.
These excellent results completed the final qualification phase of the components and manufacturing procedures for the manufacture of the ISEULT coils. The way was now clear for Alstom to begin manufacture of the 170 production double pancake coils on February 1, 2012.
The 11.7 T magnet is currently being manufactured
The 190 km of conductor needed for the windings was manufactured in the USA by Luvata Waterbury Inc. between 2010 and 2012. The final double pancake coils are currently being manufactured by Alstom in Belfort. At the end of December 2012, more than 125 units had been completed out of a total of 170. The active shielding coils were also ready to be wound at that time. The remaining components of the magnet had been ordered and were either in manufacture or had been delivered.
The cryogenic system has been designed to cool the magnet continuously, 24 hours a day and 365 days a year. It has already been installed in the NEUROSPIN basement. The heart of the system is a helium refrigerator supplied by Air Liquide at the end of 2010. The acceptance tests showed that its performance was better than planned.
The DC power system consists of two power converters. The first of these brings the magnet up to the nominal current of 1483 A at 40 V, and the second then provides continuous power to the magnet at the nominal current in stabilized mode. These two power units will be delivered to NEUROSPIN in the middle of 2013.
Radio frequency antenna array
A magnetic resonance image is formed by processing the relaxation signal from a previously excited atomic nucleus, usually hydrogen. An antenna is used to excite the nuclei with electromagnetic radiation, and to receive the relaxation signal. The operating frequency increases in proportion to the static magnetic field, from 128 MHz at 3 T to 500 MHz at 11.7 T. At frequencies up to 128 MHz, a single antenna is used to provide a sufficiently uniform excitation. At higher frequencies, the interaction between the transmitted wave and the material of the body results in strongly heterogeneous excitation, and an antenna array is needed.
A preliminary eight channel 7 T antenna array has been built by SACM. The parallel transmission methodology used to achieve a uniform excitation will be finalized at the NEUROSPIN center using a phantom, an object with dielectric properties similar to those of a human head. Parallel transmission seeks to achieve a uniform excitation in two ways. The amplitude and phase of the signal fed to each element in the antenna array may be adjusted while maintaining identical pulse timings to all elements in order to generate a uniform radio frequency electromagnetic field across the region of interest. Unfortunately, this method is not effective across the entire brain region. Another degree of freedom in the time domain is required with individually shaped pulses being fed to each channel and transmitted simultaneously with the gradients of the magnetic field. During excitation, these gradients allow movement within the Fourier space, a dual of the image space, in order to cover a range of spatial frequencies and avoid the destructive interference that leads to artifacts. The flip angle become homogeneous when using this second method.
Examples of images, free from artifacts and contrast losses, obtained at 7 T using parallel transmission and the eight channel antenna.
In order to obtain the first images of the human brain at 7 T in 2010, the eight channel antenna was certified by the Bureau Veritas to be in compliance with the IEC 60601-1 standard relating to medical electrical equipment. The experiment was conducted with authorization from the French Agency for the Safety of Health Products (AFSSAPS), now renamed the National Agency for the Safety of Medicines and Health Products (ANSM), under a protocol entitled, ‘Assessment of the value of magnetic resonance imaging and spectroscopy at 7 teslas in the study of brain structure and function’. This work was also approved by the Committee for Personal Protection (CPP). A twelve channel antenna array is has been built and is currently under test with the aim of doubling the receive sensitivity. A patented signal distribution system is used to drive the array with just eight transmitters. The measured increase in sensitivity should lead to a marked improvement in the image resolution compared with a 3 T scanner. The 11.7 T antenna for the ISEULT project has already been built to the same design. It will be commissioned and tested on site as soon as the magnet is delivered.
The magnet will be assembled at Alstom following delivery of the cryostat components. Delivery of the magnet to Saclay is due at the end of 2013. Once delivered to NEUROSPIN, the magnet will be connected to the cryogenic and electrical systems prior to a full set of qualification tests at 1.8 K and the ramp-up to the nominal magnetic field. The first images obtained at 11.7 T using the radio frequency antenna developed at SACM are expected to be produced during the course of 2014.
Last update : 01/23 2019 (3377)