The Ultimate Sensor

In the original Star Trek series, DeForest Kelley played ship's surgeon Dr. Leonard McCoy. This character was especially memorable for his frequent medical pronouncement, "He's dead, Jim," and for his handheld medical scanner, a device that looked like a pepper shaker with a spinning cap. This magical sensor could instantly and noninvasively diagnose any medical condition.

   Ed Ramsden
Ed Ramsden

Although present-day medical technology hasn't quite caught up with that world, some of what we do have in the opening years of the twenty-first century would likely astound many of the science fiction writers responsible for that television show.

Nuclear Magnetic Resonance

Shortly after the close of World War II, Felix Bloch and Edward Purcell discovered the phenomenon of nuclear magnetic resonance (NMR). The nuclei of atoms containing odd numbers of nucleons (protons and neutrons) have a measurable magnetic dipole moment, and will tend to line up with an externally applied magnetic field like tiny bar magnets. If this alignment is perturbed by the application of a secondary field, the nuclei will try to realign when the secondary field is removed. In the process of realigning, the nuclei will precess like toy tops (Figure 1), and radiate an electromagnetic signal whose frequency is proportional to the strength of the externally applied magnetic field. For the case of hydrogen atoms, where the nucleus consists of a single proton, the resulting nuclear precession, or resonance frequency, is about 42.5 MHz/tesla of applied field.

How to Detect NMR Effects

Nuclear magnetism is an extremely subtle phenomenon, at least compared with the more commonly observed magnetic effects such as ferromagnetism and diamagnetism, which have their origins in the behavior of an atom's electron shells. Consequently, sensitive instrumentation is required to discern this effect. Figure 2 shows a conceptual apparatus for detecting NMR effects. A strong magnet is used to establish the main field to which the nuclei will align. Other coils will be aligned orthogonally to perturb the nuclei, and also to detect the precession signal. An RF signal source is required to stimulate the nuclei, as is a sensitive RF receiver to detect the precession signal.

Figure 1. In the process of realigning with an external magnetic field after perturbation by a secondary field, nuclei will precess like toy tops
Figure 1. In the process of realigning with an external magnetic field after perturbation by a secondary field, nuclei will precess like toy tops

A Sensor Platform

Despite requiring sophisticated measurement techniques for detection, NMR effects can be used as the basis for several different types of sensors. For example, if you place a sample of an unknown material into a magnetic field of known strength, you can use the resonant frequencies you observe to identify the material's chemical composition. Because the electrons surrounding a nucleus have an effect on its resonant response, in many cases you can also use NMR techniques to determine a compound's molecular structure. This is the basis for what is known as NMR spectroscopy, a widely used analytic technique.

Alternatively, if you have a sample of known composition, you can measure the strength of an unknown magnetic field. This is the basis of an instrument known as a proton magnetometer. In this device, a pulsed stimulus field is applied to a hydrogen-rich material such as paraffin. When the stimulus field is released, the hydrogen nuclei (protons) will re-align with the ambient field, and in the process emit an electromagnetic signal with a frequency proportional to the ambient field strength (~4.25 kHz/gauss). Because it is possible to measure time, and consequently frequency, very accurately, proton magnetometers are among the most accurate and stable methods of measuring magnetic fields.

Figure 2. Sensitive instrumentation is required to discern NMR effects; here is a conceptual apparatus that could accomplish this task
Figure 2. Sensitive instrumentation is required to discern NMR effects; here is a conceptual apparatus that could accomplish this task

Magnetic Resonance Imaging

So far we have seen how the NMR effect can be used to measure both composition and magnetic field strength. How does this tie in with Dr. McCoy's medical scanner? Well, a third possibility for a sensor occurs when you know the applied field and you know the material you are looking for. In this case, the strength of response can indicate the amount of the material present. This is the basis for magnetic resonance imaging (MRI) scanners. These devices are "tuned" to measure concentrations of hydrogen, a major constituent of biological building blocks such as water, lipids, and proteins. Because different types of tissue contain varying amounts of hydrogen, it is possible to differentiate among them based on their NMR responses. A technique for scanning the human body to find cancerous tumors, based on this insight, was first patented by Dr. Raymond Damadian in 1974.

A modern MRI machine uses a powerful, typically superconducting, magnet to establish a large uniform field around the patient, typically on the order of 1–3 tesla. A high-power RF source and antenna provide the excitation for the hydrogen nuclei being probed. An MRI machine's ability to resolve spatial detail results from the addition of a number of secondary magnet coils that are used to establish controlled variations (gradients) in the primary magnetic field. Because the MRI scanner can control the strength of the magnetic field at any given point in its scanning volume, it can determine the spatial location from which an induced NMR signal is coming. By taking numerous measurements, the MRI scanner can build up a 3D database of NMR responses from the patient being scanned. From this database, various images of tissue can be computed and displayed, revealing the patient's internal organs and structures in unprecedented detail.

One aspect of the twenty-third century has definitely arrived a little earlier than expected. Now all they have to do is make it fit in a pepper shaker!

Ed Ramsden, BSEE, a member of the Sensors Editorial Advisory Board, designs sensors for the heavy-truck industry in Portland, OR.