Modern MRI systems have converged on a common framework. A strong homogeneous B0 field provides polarization, and the readout field. An RF field B1 excites the spins. Gradient fields encode the signal, which is acquired by an array of receive coils. This results in a high performance system. However, all of the components of the system are large, and the B1 and gradient systems require significant amounts of power. Such systems are far from portable.

However, there are many other ways to perform MRI. The key ideas for portable systems have been around since the beginning of NMR. There are several different elements. One is imaging at low fields, and even the earth's field. Another is the idea of obtaining polarization separately from the acquisition field. The final key idea is using spin echoes to acquire signals in the presence of strong, but inhomogeneous fields. This presentation will trace these different threads from their origins through to current systems.

The idea of using the Earth's field goes back to the beginning of MRI, and the Packard-Varian experiment. This was published as an abstract [1], and has been the basis for many other systems. The imaging volume is surrounded by an electromagnet oriented perpendicular to the earth's field. The electromagnet is energized to produce polarization in the sample, and then rapidly switched off. The magnetization then precesses about the earth's field.

This approach was widely studied for characterizing oil wells in the 1950's [2], a field called Nuclear Magnetic Logging, or NML. Some of the investigators are very familiar to the field of MRI, including Felix Bloch, Erwin Hahn, Russel Varian, and Henry Torrey. Technologies and approaches originally designed for NML have continued to appear in MRI.

The same idea can be used for an imaging system, as shown by Stepisnik [3], with the addition of gradient coils for spatial encoding. Vegetables were imaged in situ growing in a garden 20 m outside of the lab. These studies would be difficult to do with a more conventional imaging instrument. Adding pulsed gradients allowed measurement of diffusion in sea ice by Paul Callaghan [4]. This was a very portable system, since it had to be transported to antarctic sea ice packs!

Any of these approaches can be used to build MRI systems that are light enough to be very portable.

The main contributor to the weight and bulk of current MRI systems is the need to provide a strong, uniform polarization field. Another approach has been to use a low, homogeneous field for readout, and rely on another mechanism for polarization. This can be a strong switched pulsed field [5,6], as is used in earth's field image imaging. Effectively this separates the two functions of a conventional MRI system's B0 field. A strong inhomogeneous field provides polarization, while a weaker homogeneous field provides the acquisition resonant frequency. The two independent magnets can be less expensive that a single strong homogeneous magnet.

There are many other sources of polarization that have been used. One is hyperpolarization, such as helium or xenon [7,8]. Another is polarization with electron spin resonance, and the Overhauler effect [2], which was explored with NML. Since the polarization doesn't come from the thermal equilibrium from the B0 field, the B0 field only has to be strong enough to ensure that body noise dominates the acquired signal.

The other approach for well logging, and portable systems was the use of strong inhomogeneous magnets for polarization and imaging. The inhomogeneous field results in a very short T2*, but this can be refocused using RF pulses. This can be used used to recover T2, and measure diffusion using pulsed field gradients [2]. In well logging, two opposed magnetic produce a disk-like region of sufficient homogeneity to make measurements. The same approach has been miniaturized to make an intravascular system for characterizing atherosclerotic plaques [9], using exactly the same system geometry. A similar approach can be used to make a handheld probe (NMR Mouse) that can study the NMR properties of external objects [10].

The use of RF refocusing can also be used for portable imaging systems, such as described in [11] and commercialized by Magnevu for extremity imaging. In this case, a long sequence of RF refocusing pulses are used to collect spatial frequency samples.

Current MRI scanners have converged on a common configuration, that determines the power, weight, and complexity of these systems. However, there is a long history of alternative geometries that can make systems that are lighter and more portable. With the advent of many alternate sources of polarization, these alternate system geometries will have a renascence.