FAQ

All you ever wanted to know about MRI (but were afraid to ask)

(With apologies to Woody Allen)

This section lists some frequently asked questions, and some other things which really don’t fit easily into the rest of the text.

Why all the noise in MRI?

Good question. How can a machine that has no (intentionally) moving parts make noises that hit literally deafening levels of 130dB or more?

The answer is that the sound is due to switching electrical currents through the magnetic field gradient coils. When current is put through a wire that is in a magnetic field, a force is generated, called the Lorentz force. This is the basis of electrical motors, and you may remember physics classes about left- and right-hand rules for what direction the force is in (although perhaps your unconscious has hidden this traumatic memory from your conscious mind?). Anyway, in MRI, to encode spatial information we need to be able to switch currents on and off to each of the three magnetic gradient coils. When the current changes, so does the magnetic force on the coils. These forces are huge, because we have some of the strongest magnetic fields on Earth, and we can switch very large electrical currents (10s of amps) in less than a millisecond. These large, rapidly alternating forces produce vibrations that literally rattle the whole scanner, and these are the noises that you hear.

As you get used to hearing MRI scanners in action, you get to recognize different types of acquisition. The sequence of magnetic field gradient pulses are quite different. For example, rapid imaging sequences such as echo planar imaging (EPI) tend to switch the gradients especially rapidly, resulting in especially loud, high frequency beeps.

Why do we need such a strong magnetic field?

Sadly, the physics works against us here. The reality is that when we put you into an MRI scanner, the magnetization that we generate is not very strong at all. The problem is that the energy associated with aligning your protons to the magnetic field is much, much less than the thermal energy of the same protons at your body temperature. The thermal energy almost overwhelms the magnetic energy, and at typical field strengths your body is only magnetized to about 1/100000 of what it would be if it would be at absolute zero of temperature. The amount of signal that we measure in MRI varies approximately linearly with magnetization, and field strength. So that 3T MRI scanner gives you twice as much signal as the 1.5T scanner down the hall. There are other properties that also vary with field strength, so this isn’t the only story here.

There is an interesting alternative, which has the promise to increase the signal strength in MRI by several orders of magnitude. Hyperpolarization takes materials, for example contrast agents, and magnetizes them at low temperature in a high field. The trick is that the contrast agent has to be warmed to body temperature and introduced into the patient before the magnetization has a chance to return to equilibrium (T1 recovery). This is very difficult, but can be done, for example hyperpolarized xenon gas for lung studies. At the moment, this is something of a niche area of research, but maybe one day it will make a big impact on our field.

Why are MRI scanners so expensive?

The glib answer is because they can be. As MRI is so critical to much of modern medicine, even relatively small improvements in image quality through technical innovation can be extremely valuable. Having said that, modern MRI scanners represent the pinnacle of science and engineering. Here are some of the expensive components.

The main magnetic field is both incredibly strong and incredibly uniform. The field is generated by passing extremely large electrical currents (hundreds of amps) through superconducting wires kept cold in a bath of liquid helium at a temperature of 4 degrees centigrade above absolute zero. Superconductors are materials with zero electrical resistance – not just really, really small, but zero. If the power goes off to your local hospital, the current in the MRI continues to flow, and the magnetic field is maintained. As the sign on the door says, “the magnet is always on”. Superconductors are a whole field of materials science, and generating a wire that can be wound into a magnet, and remains superconducting while being exposed to high magnetic fields is an amazing achievement. In addition to the cost of materials and development, the need to keep the superconducting coils at such low temperatures is substantial. A typical MRI scanner might contain 1000 liters or more of liquid helium, costing tens of thousands of dollars 1. The main magnet is the single most expensive part of the MRI scanner, with the cost rising rapidly with magnetic field strength. While you might not pay much more than $1 million for a 1.5T MRI scanner, a 7T scanner is likely to set you back more than $10 million.

Siting an MRI scanner is also a substantial cost, often comparable with the cost of the scanner itself. The scan room must form a tight RF shield to prevent RF from the scanner getting out (and interfering with electrical devices in the vicinity), and RF from the outside getting in (your local FM radio station introducing noise into your images). Consideration must also be given to the spatial extent of the magnetic field. Regulations require that access to areas with a magnetic field strength of greater than 5 gauss (0.0005T) is restricted. Before the development of actively shielded magnets, this required a large space around, and above and below the magnet to be restricted.

The amplifiers required to drive the magnetic field gradients are the best that money can buy. They can switch very large currents in less than a millisecond with extremely high accuracy, with timing accuracy in the microsecond range. The power and fidelity of such amplifiers is far higher than the audio amplifiers used at rock concerts. On the other hand, each amplifier might cost more than $100,000, so maybe you get what you pay for.

Similarly, the RF amplifiers are extremely high specification, and are capable of generating peak power outputs of about 50kW. The RF coils themselves are also incredibly expensive, and a good head coil can cost more than $100,000. The technology in these coils is disappointingly simple and inexpensive, essentially just copper wires, capacitors and inductors to make resonant circuits. There is quite an art to making a high quality coil though, although it’s still hard to justify the price tag when the cost of the components can be measured in the hundreds of dollars.

Finally, there are the development costs, as well and complying with the rules and regulations for medical devices. Manufacturers employ hundreds of people dedicated to improving their scanners. Those MRI physicists don’t come cheap, although they rarely have to pay for drinks to take to expensive cocktail parties.

Footnotes

1

Helium is a limited resource on Earth. It is collected as a by-product of the extraction of natural gas fuel. Helium is a very small atom, such that if it is released into the atmosphere, it rises to the upper atmosphere where it is travelling fast enough to escape the Earth’s gravity and be lost to space. Modern MRI scanners have very efficient helium recovery systems (the “cold head”, which has a pump that you can often hear) such that the helium may not have to be refilled in the lifetime of the scanner. Manufacturers have very recently developed scanners that require only a few liters of liquid helium.