Robert Adolph Ackermann

Making MRI quieter

We have mitigated acoustic noise in a 1.5 T cylindrical MRI scanner equipped with epoxy-potted, shielded gradients. It has been widely assumed that MRI acoustic noise comes overwhelmingly from vibrations of the gradient assembly. However, with vibration-isolated gradients contained in an airtight enclosure, we found the primary sources of acoustic noise to be eddy-current-induced vibrations of metal structures such as the cryostat inner bore and the rf body coil. We have elucidated the relative strengths of source-pathways of acoustic noise and assembled a reduced-acoustic-noise demonstration MRI system. This scanner employed a number of acoustic noise reduction measures including a vacuum enclosure of a vibrationally isolated gradient assembly, a low-eddy-current rf coil and a non-conducting inner bore cryostat. The demonstration scanner reduced, by about 20 dBA, the acoustic noise levels in the patient bore to 85 dBA and below for several typical noisy pulse sequences. The noise level standing near the patient bore is 71 dBA and below. We have applied Statistical Energy Analysis to develop a vibroacoustic model of the MR system. Our model includes vibrational sources and acoustic pathways to predict acoustic noise and provides a good spectral match above 400 Hz to experimentally measured sound levels. This tool enables us to factor acoustics into the design parameters of new MRI systems.

Advanced Cryocooler Cooling for MRI Systems

Advances in cryocooler technology during the past several years have enabled the design of new cooling methods for magnetic resonant imaging (MRI) systems. Open cycle operation of MRI systems using a Gifford-McMahon (GM) cycle cryocooler to cool two thermal shields, one at 20 K and the second at 80 K, has been the standard approach used to minimize helium usage in these systems. The concept has worked very well and enabled the development of an important medical imaging modality. However, the 12 K temperature limit of these cryocoolers has limited the design flexibility of the MRI magnet system by requiring a cylindrical design with two thermal shields and a large helium container to extend the operating time. The development of Gifford-McMahon cycle cryocoolers capable of cooling below liquid helium temperature, or providing larger cooling capacities between 4.2 and 10 K, has removed this design barrier and provided greater overall system design flexibility. The paper describes the impact that new GM cryocooler developments, based on rare earth intermetallic compounds in the second-stage regenerator, has had on MRI designs. By extending the cooling capacity of these units to below 4.2 K with rare earth materials, new MRI products have been developed that operate as closed cycle systems without the need for replenishing liquid helium to maintain the magnet at temperature for long periods of time. The paper describes the evolution of MRI systems at the General Electric Company from open cycle systems to two new developments using conduction cooling and helium recondensing to eliminate the need for refilling with helium. The paper reviews the design of a conductively cooled system developed for an open MRI magnet used for interventional therapy and a helium recondensing system that was incorporated into GE’s product line. In addition to a description of these systems, the operational reliability of cryocooled systems will also be reviewed.

The Thermal Performance of a 1.5 MVA HTS Generator

A 1.5‐MVA high temperature superconducting ( HTS ) generator of novel design has been designed, built and successfully tested by the General Electric Company. The 1.5‐ MVA generator has served as the engineering prototype for a much larger 100‐MVA beta unit now under design.The HTS coil in the 1.5 ‐ MVA demonstrator is designed to operate in the range of 20–40 K and is cooled with a closed‐cycle helium refrigeration system employing GM type cryocoolers. This paper will discuss the calculation of the thermal loads to the rotor from all anticipated sources. These sources include conduction losses through the coil suspension system, radiative heat loads to the cold‐system components, residual gas conduction losses, helium‐transfer coupling losses and lead losses. These predicted losses were compared to those measured during actual electrical testing of the rotor at 3600 RPM in order to validate the predictive calculations employed for the 100 MVA machine.

A two-stage pulse tube cryo-cooled MRI magnet

A compact, cryogen-free 0.5 Tesla superconducting Magnetic Resonance Imager (MRI) magnet has been conductively-cooled using a two-stage Pulse Tube cryo-refrigerator. With the absence of cold moving parts and seals, Pulse Tube cryo-refrigerators offer lower transmitted vibrations to a MRI magnet and potentially longer MTBF rates. A 7 kW input power compressor package was connected to the Pulse Tube. The Pulse Tube system provided an estimated 1.3 Watts of cooling power at 4.2 K. Vibration measurements made on the vacuum enclosure could not detect vibration or noise due to cryo-refrigerator operation even in the presence of very low levels of background noise. The cryo-refrigerator has been in continuous operation for >4000 hours without degradation of thermal performance.

Thermal Conductivity of 34-700 Carbon Fiber Composites at Cryogenic Temperatures

A steady-state test rig for measuring the thermal conductivity of thin carbon fiber composites at cryogenic temperatures has been constructed and calibrated using a 304 stainless steel sample. Thermal conductivity measurements were carried out on both unidirectional and multi-directional 34-700 carbon fiber-NCT 301 epoxy composite samples from 8 K up to 150 K using the test rig. Thermal diffusivity measurements were made at 295 K and were then used to calculate the thermal conductivity. Unidirectional samples with fibers oriented at 0, 90, and 45 degrees to the direction of heat flow have been evaluated. The thermal conductivity of a sample with laminate plys in the configuration ±45/90/0(3)/90/±45 has also been measured. At room temperature, the thermal conductivity in the 0° direction is 7 times that in the 90° direction. For the multidirectional sample, the thermal conductivity ranges from 0.22 W/(mK) at 5.7 K to 2.98 W/(mK). The general trend of the data is similar to that of the T-300 carbon fiber samples.

Performance of a Dynamically Balanced Gifford Mcmahon Cryogenic Refrigerator Cold Head

A quiet, vibration-free, Gifford McMahon (GM) lO-K refrigerator cold head has been developed for use in vibration-sensitive cryogenic systems. A unique, pneumatically driven displacer eliminates vibratory forces and enables several inherently long-life features to be incorporated into the design: Opposed dynamically balanced and pneumatically controlled displacer motions balance the inertia forces and eliminate the end-of-stroke impact loading (striking the housing) common in pneumatically driven GM refrigerators. In addition, the cold head employs clearance seals in a novel design approach to eliminate wear and extend life, and stationary regenerators to prevent compaction of the matrix material. This paper discusses the cold head and its principles of operation. The dynamic model developed to describe the cold head displacer motions is presented, as well as the methods used to control the motions for minimum vibration and optimum performance. Test data provide a measure of the cold head’s thermal performance at 10 K, and demonstrate that a 2.5 to 1 reduction in noise and a 5 to 1 reduction in vibration levels (g) over comparable commercially available GM refrigerators are possible with this design.

A Gaseous-Helium Cooling System for a High-Tc Superconducting Coil

A gaseous-helium cooling system for a high-Tc coil operating at 20K is described. The system uses a two-stage Gifford-McMahon (GM) coldhead and counterflow heat exchangers to cool a pressurized flow of helium gas from room temperature down to approximately 19K. The cold helium gas circulates through a vacuum-insulated transfer line to a single-pass heat exchanger in contact with an epoxy-impregnated high-Tc coil The coil was wound using 2000 meters of Bi-2223 tape produced by Intermagnetics General Corporation (IGC). A water-cooled rotary compressor supplies pressurized helium gas to both the GM coldhead and the helium gas cooling circuit, in parallel. Descriptions of the coldbox configuration, the flow control and temperature monitoring systems, the high-Tc coil, and the test dewar are given. Test results, including the cooldown and energization of the Bi-2223 coil, are included.

Regenerative Cryogenic Refrigerators

The development of satellite communications in the 1950s led to the first commercial need for small cryogenic refrigerators to provide 4 K cooling for ground-based parametric amplifiers. The use of these refrigerators has increased significantly since that time with the evolution of several additional cryogenic applications. The most successful of these new applications are: Military infrared surveillance and target acquisition systems Vacuum cryopumps Superconducting magnets for medical and electrical power systems Superconducting electronic devices

Regenerative Heat Exchanger Theory

Regenerator theory deals with the physical equations defining the thermal and fluid flow fields that exist in a regenerator. These equations describe the temperature distributions in the matrix material and fluid as functions of both space and time and lead to a complex set of differential equations for which no closed-form solutions exist. In the following sections we explore the different types of regenerator designs commonly used in cryogenic devices and develop both the thermal and fluid dynamic equations that define their performance.

Regenerator Performance Data

In this chapter we review generator performance data that have been published over the past 40 years. Beginning in the 1950s, researchers at Stanford University, Stanford, California, and the U.S. Naval Postgraduate School, Monterey, California, published results of experimental programs conducted to develop basic regenerator performance data. Their papers describe the considerable interest shown at that time in the application of rotary regenerators to the gas-turbine cycle for high-temperature exhaust-gas thermal regeneration.