If field strength alone determined image quality, open MR simply would not exist. Most physicians would refer patients to high-field systems, and large and claustrophobic patients would constitute the majority of people imaged on open systems.

Instead, the difference between high-field and low-field systems is like playing golf on a wide or narrow fairway, said radiologic technologist William Faulkner. The end result may be the same, but the strategy changes.

To many observers, the “game” is played with field strength, and high-field systems offer inherently better signal to noise. Unless, that is, the variables affecting field strength are tweaked.

“Field strength basically buys you signal to noise,” said Faulkner, an MRI consultant and director of education for Chattanooga Imaging in Chattanooga. “If you have high field strength, you have signal to noise in the bank. You can use that excess signal to noise to buy faster scan times.”

Good imaging depends on many factors: T1 relaxation is important with open systems, as T1 is field-strength-dependent, and reducing field strength shortens T1. As field strength is reduced, TR also must be reduced to maintain tissue contrast. As TR drops, a noticeable loss in signal to noise results, Faulkner said. There is, however, a noticeable improvement in gray and white matter contrast.

“At low field, you need to lower TR if you’re going to do T1-weighted spin-echo; additionally, GRE imaging can be used to get good contrast in gray and white matter,” he said. “This is extremely important in contrast-enhanced studies. If you try to cram your high-field protocol down a low-field approach, that dog just won’t hunt.”

For example, because lower field strengths produce chemical shift artifacts, lower receiver bandwidths can be selected to increase signal to noise on open systems.

Another option is the use of three-dimensional spoiled gradient-echo, which most open systems are capable of performing. This is an excellent source of signal to noise and offers good tissue contrast. Use of smaller coils also helps to maintain signal to noise, Faulkner said.