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33 Cards in this Set
- Front
- Back
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Problems Associated With the Noise of MRI Systems |
-annoyance -difficulty communicating with patient/technologist -anxiety or discomfort -potential hearing impairment -possible impact on quality of images |
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Patients More Sensitive to Noise |
-psychiatric patients -young children/ infants (neonates at a higher risk - elderly -patients taking medication |
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The Main Causes of Noise in the MRI System |
-GRADIENTS -RF "hearing" -subsidiary system noise |
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RF "Hearing" (Noise) |
-if the head is subjected to certain radio-frequency (RF) radiation, an audible sound can be perceived as a click, chirp, buzz, or knocking noise may be heard -reversible effects -RF energy produces a minute temperature elevation in the tissue in the head -pressure wave detected by the hair cells of the cochlea via bone conduction -usually masked and not detected at all |
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Subsidiary System Noise |
cold head and fans *cause no issue for the patient |
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Human Hearing |
-detects frequencies in the range of: 20Hz - 20000 Hz |
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Acoustic Noise |
-"unwanted' noise or noise that is unintended -measured by: frequency (Hz), Intensity (dB), time and pressure -can be consist, intermittent and explosive -given a decibel and sound pressure rating |
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Sound Pressure |
the deviation from the average (normal) atmospheric pressure |
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Threshold for Pain |
140 dB -noise can alter the threshold either permanently or temporally |
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Permanent Threshold Shift |
-inner hair cells become damaged or are destroyed --> they are not able to regenerate -some are normal due to the aging process |
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Gradient Magnetic Field |
-the main source of acoustic noise associated with MRI -Occurs during the rapid alterations of currents within the gradient coils -currents produce significant forces on the gradients -force on the gradients creates a vibration of the coils impacting their mountings (that also vibrate) -heard as a loud tapping, knocking or even has been compared to a jackhammer - |
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Certain Pulse Sequences |
-increase the acoustic noise within the scanner -loudest sequences:fast spin echo (FSE), fast gradient echo (FGRE), and echo planar imaging -using conventional spin echo sequences over gradient echo sequences can decrease sound |
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Echo Planar Imaging (EPI) |
-extremely fast imaging technique -requires more rapid gradient magnet field which increases auditory effects -typical gradient switching time for EPI: 50 um -decrease in acquisition time = shorter duration |
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The Gradient Noise is Enhanced by These Certain Modifications of the Parameters for Certain Pulse Sequences |
-field strength -decrease in Field of view -shorter time of echo and time of repetition -thinner slices (steeper gradient = louder noise) -patient position -slice orientation |
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Field Strength |
-using and identical sequence on a scanner with identical specifications apart from field strength -with an increase of field strength there will be an increase in acoustic noise (increase of the median sound pressure level of 5) |
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Adjustment of These Certain Parameters Will Cause Change in Acoustic Noise Levels |
Greatest Effect: decreasing the field of view and time of repetition Less Influential: decreasing time of echo and slice thickness |
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Patient Position |
-the noise measurement at various positions in the bore along the long axis where measured *isocenter = lowest *edge of bore = highest * Size of the patient may also affect the amount of acoustic noise |
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Physical Features of the Magnet |
-affect the level of acoustic noise -whether or not there is sound insulation |
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Acoustic Noise Level of the Worst Case Pulse Sequences |
3D and fast gradient echo: 103-113 dB |
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EPI Sequences |
-have extremely fast switching times and high gradient magnitudes ranged from: *on 1.5T: 114-115 dB *on 3T: 126-131 dB |
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One of the Most Important Physical Factors That Determines the Effect of Noise |
time |
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3 Possible Effects of Stimulation of the Ear with Noise |
1) adaptation 2) Temporary threshold shift (TTS) 3) Permanent threshold shift (PTS) |
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Factors that Increase the Risk of Hearing Loss |
-noise level -duration -number of exposures -susceptibility of the individual |
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MRI Difficulities Caused by Acoustic Noise |
-minor problems: simple annoyance, difficulties in verbal communication -severe problems: temporary hearing loss, permanent hearing impairment -patient being startled or uncomfortable: causes motion artifacts on the images |
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Controlling Acoustic Noise |
-at the source -control along the path of noise -at the receiver Generally we try to cover noise at either the source or receiver -cheapest: earplugs (decrease by 33dB) *hamper communication during the exam -MRI headphones usually supplied |
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MRI Headphones |
-allow you to talk to the patient during the exam -allows music to be played during the exam to help the patient relax -allows for instructions to the patient |
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Anti-Noise Teschniques |
aka: active noise cancellation -interferes destructively with the noise source and will therefore reduce the noise levels -not commonly used since headphones |
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Toshiba Medical Systems |
Pianissimo: - A vacuum sealed space surrounding the gradient coil is mounted separately from the magnetic structure -provides independent suspension for the vibrating gradient coil -claims to reduce up to 90% |
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Siemens Medical Systems |
MAGNETOM Avanto: -special casting for gradient coils, special magnet suspension, and unique MR sequence design -claims to reduce up to 97% (wouldn't need headphones) |
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GE Medical Systems |
GE Silent Scan Technology: -gradients are used continuously and are not rapidly switched on or off -mechanical vibration is eliminated and no noise is generated *need RF coils to be capable of switching in a matter of nanoseconds |
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Quiet or Whisper Sequences |
-decrease the amount of acoustic noise -used on: slightly sedated patients or pediatrics who are just asleep -helps minimize noise and anxiety |
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Rules for Being Within When a Scan is Happening |
-hearing protection = required |
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The Cold Head |
-constant pumping or chirping sound -noise is substantially less and considered more of an annoyance |
