Biomedical applications of superconductivity broadly revolve around two different technologies. The most familiar application of superconductor technology is that of nuclear Magnetic Resonance Imaging (MRI). The less familiar application is in Biomagnetics. Biomagnetism (now called Magnetic Source Imaging or MSI) is the measurement of magnetic fields produced by biological systems such as the human body. It is different from magnetobiology, which is the study of magnetic field effects on biological systems. We will discuss both MRI and MSI applications and indicate the role of High Temperature Superconductors (HTS) in both applications. Since these applications of HTS materials are in their infancy, we discuss them as future developments. The Los Alamos role in this development is that of developing better HTS tapes. However, we are more directly involved with a technique related to MSI, namely biosusceptometry (see Biomedical Applications).
Nuclear Magnetic Resonance Imaging (the word nuclear was dropped to reduce the concerns of the general public, hence MRI) is a noninvasive method for seeing inside the body without using ionizing radiation. The technique now has widespread use in hospitals in diagnosing injuries to joints and bones, detecting tumors, and in general detecting diseases that change human anatomy.
Briefly, MRI technology is based upon the detection of the positions in the body of hydrogen nuclei (protons). In a constant applied magnetic field the magnetic moments (due to their spins) of the protons almost align with the applied field. This causes them to precess about the direction of the applied field with a characteristic frequency, called the Larmor frequency. This frequency is proportional to the strength of the applied field. Applying a second small magnetic field at radio frequencies (rf) flips the spins of the protons from a parallel to antiparallel orientation relative to the initial applied field direction. After the rf field is removed the proton spins relax to their ground state energy (parallel field orientation) and emit an rf signal that can be detected. By computer processing that signal, we can obtain information about the distribution of the protons (hence the hydrogen atoms) and their chemical environment.
Modern MRI technology is more complex in that it applies a DC field with a small gradient (non-constant). This is done to obtain good spatial resolution. This also in turn requires sophisticated computer software and lots of computing power. The field strengths of modern MRI machines are typically 0.5 to 5.0 T (tesla; 1 tesla = 10,000 Gauss; the earth’s magnetic field is ~ 0.5 Gauss). If field strengths were the only requirement for high resolution MRI, then conventional electromagnet (non-superconducting) MRI technology would have led to a much more rapid development of this field than has been the case. However, conventional magnets are not stable enough to generate the extremely stable (in both space and time) magnetic fields needed. It is the persistent currents present in superconducting magnets that provide the needed stability. The spatial variation of a modern superconducting MRI magnet is about 1 part in 105, and the time variation is about 1 part in 109. Without this stability, modern MRI pictures would be diffuse, unfocused, and of course, of limited use as a diagnostic tool. Conventional electromagnets don’t come close to having this kind of stability. Even with superconducting magnets it is difficult to control spatial variations. An ideal magnet in the form of a solenoid would have a uniform field inside, but real magnets have fringe fields. To obtain uniform fields, MRI manufacturers must use tricks such as adding extra windings or small steel shims to the magnets.
Besides these scientific issues, there are also economic and operating concerns for hospital use of MRI machines. First, there is the price: a modern MRI unit costs from one to two million dollars, so it is a major investment for hospitals. The superconducting magnets are usually 20% of that cost. However, the maintenance contract can be $100,000 to $150,000/year. Also for low temperature superconducting (LTS) magnets the liquid helium usage can run $8,000 to $12,000/year. Cryocooler systems can reduce helium losses and hold refrigeration costs to this low level; otherwise they may be double that cost. Also there is the extra cost of the load bearing structures for these machines, which can weigh from 3 to 5 tons. If the hospital is lucky enough to have high usage of the MRI machine the cost per patient per session may be as low as $1,000- $1,500; with low usage the price goes up.
Despite these costs and concerns, MRI technology has become a valuable diagnostic tool worldwide. It first became available in large hospitals in large metropolitan areas of industrialized countries. In rural areas transportable units built on a trailer bed may be used among smaller hospitals in the region. In underdeveloped countries where liquid helium technology is unavailable, more expensive closed-cycle refrigeration units must be used and this increases costs. One way around this problem is to use permanent (non-superconducting) magnets. These can be made to produce a uniform field, but at lower field strengths (0.1 to 0.2 T). Because the signal-to-noise ratio decreases with lower magnetic field strength, these units require more sophisticated computer technology. However, these low-field MRI units may have a role as an initial diagnostic in rural or underdeveloped areas.
Do HTS magnets have a role in future MRI developments? Of course, the answer to this question depends on advances in HTS technology. With the development of coated conductor wire for magnets we believe the price of HTS magnets will become more competitive with LTS magnets. Because HTS magnets will have an advantage in operating costs, need fewer infrastructures, and may be lighter in weight they most likely will initially have a niche in markets in rural and underdeveloped regions.
Whereas MRI technology is based upon the generation of strong (10 T) stable magnetic fields, MSI (Magnetic Source Imaging ) technology relies on the ability of superconductors to detect very small magnetic fields (10-14 T). This sensitivity is achieved by what are called Superconducting QUantum Interference Devices or SQUIDs. So before describing MSI applications, we will briefly describe the basics of SQUIDs.
SQUIDs are based upon Josephson junctions. (See our page on Josephson junctions for other applications of SQUIDS.). These junctions consist of two superconductors separated by a weak link of either non-superconducting material or a constriction in the superconducting material. The main criterion of the weak link is that superconducting electrons have to tunnel through the junction. A SQUID consists of a superconducting loop (or loops) with one or more tunnel junctions. The dcSQUID (two Josephson junctions connected in parallel) are the most widely used detector in medical applications discussed here. Without going into details, a SQUID is used to measure magnetic flux through a pick-up loop. The tunnel junctions can be operated so that a small magnetic flux change is converted into a large voltage signal across the loop. For LTS the SQUID must, of course, operate at or below liquid helium temperature. HTS SQUIDS are operated at or below liquid nitrogen temperature.
As mentioned above SQUIDS are used to detect small magnetic fields. In the human body, currents are generated that produce these small magnetic fields. It is the neurons in the brain and excitations in muscle fibers that generate these currents when they are activated. For example, when a single neuron "fires," a pulse of charge flows along the neuron. The magnetic field from the current of a single neuron cannot be detected. However, neurons in the brain are aligned and clustered, so a cluster of thousands of neurons firing simultaneously generates a detectable magnetic field. A SQUID placed outside the skull can measure these fields. Also, the neuron cluster does not have to be near the surface to be detected. SQUIDS can detect neuron clusters firing deep within the brain. These magnetic biological signals from excitable cells (neurons, muscle fibers or nerve cells) are usually at low frequencies, i. e., below 50 Hz. In this frequency range only SQUIDS have the sensitivity needed for meaningful detection. Detection of magnetic fields generated in the brain is known as magnetoencephalography or MEG. Remember that like MRI, MSI is a non-invasive technology. Because these magnetic fields generated by brain activity are so much weaker than many external fields that may be present such as urban noise or the earth’s magnetic field, extensive precautions must be taken to eliminate the effects of external fields in MEG applications. This is also true of all the applications discussed below.
An MSI unit is made up of many components. First, these units must be in a magnetically shielded room. These shielded rooms are typically built with two or three high magnetic permeability layers and one layer of aluminum and so they expensive. The sensors (SQUIDS) are coupled to the source by a flux transformer. This flux transformer consists of a detection coil connected to the source and an input coil connected to the SQUID. Also, of course, the SQUID must housed in a low-temperature dewar. Finally, the multi-channel (many SQUIDS) MSI units used for medical diagnosis need to be easily operated, but have fairly complex electronics.
There is also now a long list of applications of SQUIDS for detecting magnetic signals from muscle or nerve activity. The detection of signals from the stomach is called magnetogastrogram (MGG), signals from the small intestine is called magnetoenterogram (MENG), signals from skeletal muscle is labeled magnetomyogram (MMG), signals from the heart leads to the technology of magnetocardiogram (MCG or MKG) to list a few. Finally, we mention that originally it was thought that HTS SQUIDS would play no role in these applications. Because thermal noise can degrade the signal it was realized that operating a SQUID at liquid nitrogen temperatures would increase the thermal noise by a factor of 20 over operating at liquid helium temperatures. Fortunately, with a lot of technical development it has been demonstrated that HTS SQUIDS may achieve about the same sensitivity as LTS SQUIDS.
MSI has advanced rapidly in the last ten years and has become a unique diagnostic tool. Of the many applications of MSI we will just give examples from the most active areaMCG. The full power of the technique is seen in studies and clinical applications of the functioning of the human heart (MCG) and brain (MEG). The superior spatial resolution of MCG or MEG as compared to ECG or EEG yields a more accurate picture of the heart or brain functionality.
MCG is applied to several different aspects of the functioning of the heart. It is used to study cardiac arrthythmias, to evaluate the risk of sudden cardiac arrest, or in evaluating heart transplant rejection, to mention a few. The traditional method of determining heart arrthythmias is by invasive catheter mapping. Now it is possible to perform non-invasive MCG measurements to obtain the same information. The risk of sudden cardiac arrest is associated with the malfunctioning of the left ventricle which is seen as a strong peak in either a MCG or ECG recording. Also, the ability to non-invasively detect with MCG acute rejection events in heart transplant patients is becoming an important clinical tool.
Finally, we mention a recent study using an HTS MSI unit. A patient who had undergone a heart attack was tested with both ECG and MCG measurements immediately after the attack and then again one hour later. The ECG recordings were the same, whereas the MCG recording one hour later was very different from the original recording. The implication from this study is that prominent changes in the heart following the attack were picked up in the MCG measurement, but not in the standard ECG measurement. It also demonstrates the potential benefit of HTS SQUIDS in medical applications. See also Biomedical Applications for another application.
NEWS & AWARDS