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newnamespace:acoustic_frequencies_visualization_tool [2010/11/24 11:30] diana |
newnamespace:acoustic_frequencies_visualization_tool [2010/11/24 16:38] (current) diana |
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During a scan a preselected set of defined radio frequencies and gradient pulses are repeated many times, wherein the time interval between pulses and the amplitude and shape of the gradient waveforms will control the NMR signal reception. Specific pulse sequences weightings are dependent on the field strength. | During a scan a preselected set of defined radio frequencies and gradient pulses are repeated many times, wherein the time interval between pulses and the amplitude and shape of the gradient waveforms will control the NMR signal reception. Specific pulse sequences weightings are dependent on the field strength. | ||
- | **Lorenz force and sound level pressure** | + | ===== Lorenz force and sound level pressure |
The creation of large magnetic fields within whole body MRI scanners is exacerbated by the need to switch the gradients rapidly, particularly in faster imaging methods (echo planar imaging, EPI). The resulting Lorentz forces and consequent torques applied to the coil structure together with individual wire movements can advance high level of undesirable acoustic noise. [Chapman 1995] | The creation of large magnetic fields within whole body MRI scanners is exacerbated by the need to switch the gradients rapidly, particularly in faster imaging methods (echo planar imaging, EPI). The resulting Lorentz forces and consequent torques applied to the coil structure together with individual wire movements can advance high level of undesirable acoustic noise. [Chapman 1995] | ||
Various approaches have been used to reduce acoustic noise emissions since the loud noises might be amplified and become dangerous for the safety of the patient (possibly hazardous to hearing, physiological consequences, | Various approaches have been used to reduce acoustic noise emissions since the loud noises might be amplified and become dangerous for the safety of the patient (possibly hazardous to hearing, physiological consequences, | ||
- | ** Important echo planar imaging (EPI) safety issue** | + | ===== Important echo planar imaging (EPI) safety issue ===== |
Gradient systems of an MR scanner have __// | Gradient systems of an MR scanner have __// | ||
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* may generate loud gradient noise | * may generate loud gradient noise | ||
* may cause hearing damages (patient/ | * may cause hearing damages (patient/ | ||
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+ | __Write sequences in a way that keeps the gradients away from acoustic resonance frequencies__: | ||
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+ | ===== MATLAB script to check whether your pulse sequence peaks fall into acoustic ranges ===== | ||
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+ | The Matlab script we created is called gradientacoustic at in can be used to import your Siemens pulse sequence files (*.dsv). After you select your files to import, the script displays the time domain shape of your x, y, and z Gradients. | ||
+ | The MATLAB script can be used for following main functions: | ||
+ | - simulation of trapezoidal sequences | ||
+ | - load simulated gradient files (DSV, Siemens IDEA VA25, VB15) | ||
+ | - enables to zoom-in into critical parts of the sequence | ||
+ | - selection of gradient engines (e.g.TQ-engine) | ||
+ | - single sided FFT | ||
+ | - peak picking and peak labeling in micro-second | ||
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+ | {{xarxa: | ||
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- | **References** | + | ===== References |
**[Tomasi 2003]** Dardo G. Tomasi, Thomas Ernst. Echo Planar Imaging at 4 Tesla With Minimum Acoustic Noise. J. Magn. Reson. Imag. 18 (2003) 128-130. | **[Tomasi 2003]** Dardo G. Tomasi, Thomas Ernst. Echo Planar Imaging at 4 Tesla With Minimum Acoustic Noise. J. Magn. Reson. Imag. 18 (2003) 128-130. |