Acoustic noise generated during magnetic resonance (MR) imaging is an unwanted side effect that may cause discomfort in patients and healthcare professionals.
The problems associated with acoustic noise include simple annoyance,
heightened anxiety,
verbal communication difficulties (1),
temporary hearing loss and,
in extreme cases,
the potential permanent hearing impairment (2-4). It is been reported temporary shifts in hearing thresholds in 43% of the patients scanned without ear protection and patients with improperly fitted earplugs (2).
Additionally,
acoustic noise may pose a particular hazard to specific patient groups who may be at increased risk (e.g.,
patients with psychiatric disorders,
elderly,
pediatric and sedated patients,
newly born children) (4).
Furthermore,
noise exposure for the fetus could be of concern for both patients and interventional MRI staff (5).
The gradient magnetic field is the primary source of acoustic noise in MR procedure (4,6).
This noise occurs during the rapid alterations of currents within the gradient coils.
These currents,
in the presence of the strong static magnetic field of the MR system,
produce significant (Lorentz) forces that act upon the gradient coils.
Acoustic noise is produced when the forces cause motion or vibration of the gradient coils as they impact against their mountings,
which,
in turn,
flex and vibrate (7,8).
The acoustic noise varies due to the alteration of the gradient output (rise time or amplitude) by modifying MR imaging parameters.
Noise tends to be enhanced by decreases in section thickness,
field of view,
repetition time,
and echo time.
Gradient magnetic field-induced noise levels have been measured during a variety of pulse sequences for MR systems with static magnetic field strengths reporting sound pressure levels that can run as high as 100-120 dB.
(6,
9-11).
The situation is exacerbated in ultra-high speed imaging because of the very high switching rates used in these techniques.
Noise levels can be as high as 140 dB,
which is well above generally accepted safety level permitted in the work- place (7).
Hearing protection is applied routinely to all children and adults undergoing MR imaging.
Earplugs,
when properly used,
can abate noise by 10 to 30 dB,
which is usually an adequate amount of sound attenuation for the MR environment.
Unfortunately,
passive noise control methods suffer from a number of limitations.
They affect the verbal communication with patients during the operation of the MR system,
standard earplugs are often too large for the ear canal of adolescents and infants,
and passive noise control devices offer non-uniform noise attenuation over the hearing range.
While high frequencies may be well attenuated,
attenuation is poor at low frequencies.
This is problematic because,
for certain pulse sequences,
the low frequency range is where the peak MR imaging-related acoustic noise is generated.
Several investigators have described the development of “quiet” pulse sequences,
which substantially decrease acoustic noise and are acceptable for MR imaging and functional MRI examinations.
To date two methods have been used to reduce noise - dampen/isolate the gradient coil from the patient bore,
or reduce switching rate.
Both methods have drawbacks; the first resulting in reduction of bore space and the second reducing performance. New technology has made quieter techniques feasible which range from as low as 80 dB (1/10000 as loud) to nearly silent.
“Quiet” PROPELLER uses a standard 2D PROPELLER sampling scheme.
The k-space trajectory and data sampling scheme can be optimized such that gradient steps are smaller than those in product PROPELLER and result in a scan that produces less noise than product.
This “Quiet” method reduces acoustic noise levels approximately 25-30dB for T2 PROPELLER and T2 FLAIR PROPELLER employing an acoustic noise model to optimize gradient waveforms while minimizing the impact on scan time.
An additional 180º preparation is used to minimize the effect on image quality
The purpose of this study is to evaluate the quality of “Quiet” MR FSE acquisitions in comparison to techniques in current day to day practice in imaging of the brain.