10/01 Diagnostic Imaging: Open MRI: Play to the = advantage of open MRI

Diagnostic = Imaging
October=20 2001

Open MRI

Play to the advantage = of open=20 MRI

Tweak the variables affecting field strength to = improve=20 signal to noise

By Steven K. Wagner

If field strength alone determined image quality, open MR = simply would=20 not exist. Most physicians would refer patients to high-field = systems, and=20 large and claustrophobic patients would constitute the majority of = people=20 imaged on open systems.

Instead, the difference between high-field and low-field = systems is=20 like playing golf on a wide or narrow fairway, said radiologic=20 technologist William Faulkner. The end result may be the same, but = the=20 strategy changes.

To many observers, the =93game=94 is played with field = strength, and=20 high-field systems offer inherently better signal to noise. = Unless, that=20 is, the variables affecting field strength are tweaked.

=93Field strength basically buys you signal to noise,=94 said = Faulkner, an=20 MRI consultant and director of education for Chattanooga Imaging = in=20 Chattanooga. =93If you have high field strength, you have signal = to noise in=20 the bank. You can use that excess signal to noise to buy faster = scan=20 times.=94

Good imaging depends on many factors: T1 relaxation is = important with=20 open systems, as T1 is field-strength-dependent, and reducing = field=20 strength shortens T1. As field strength is reduced, TR also must = be=20 reduced to maintain tissue contrast. As TR drops, a noticeable = loss in=20 signal to noise results, Faulkner said. There is, however, a = noticeable=20 improvement in gray and white matter contrast.

=93At low field, you need to lower TR if you=92re going to do = T1-weighted=20 spin-echo; additionally, GRE imaging can be used to get good = contrast in=20 gray and white matter,=94 he said. =93This is extremely important = in=20 contrast-enhanced studies. If you try to cram your high-field = protocol=20 down a low-field approach, that dog just won=92t hunt.=94

For example, because lower field strengths produce chemical = shift=20 artifacts, lower receiver bandwidths can be selected to increase = signal to=20 noise on open systems.

Another option is the use of three-dimensional spoiled = gradient-echo,=20 which most open systems are capable of performing. This is an = excellent=20 source of signal to noise and offers good tissue contrast. Use of = smaller=20 coils also helps to maintain signal to noise, Faulkner = said.

Diagnostic = ImagingOctober=20 2001

Open MRI

Play to the advantage = of open=20 MRI

Diagnostic = Imaging
October=20 2001