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19 Cards in this Set
- Front
- Back
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Gradient Amplitude |
-the strength of the gradient -vary depending on the gradient package for the system -gradient strengths range from: 10-60 mT/m |
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Gradient Rise Time |
-time it takes the gradient to reach the max strength of amplitude -typical values from 200-1000us -the shorter the rise time, the shorter the sequence and less overall time is needed to complete the scan |
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Slew Rate |
-the strength of the gradient over distance slew rate = amplitude/ rise time -units: T/m/s -typical slew rates are in the range of: 20-150 T/m/s |
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Duty Cycle |
the amount of time the gradient is active |
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Gradient Magnetic Fields: Possible Bioeffects |
1. peripheral nerve stimulation 2. magnetophosphenes 3. hearing damage due to acoustic nose |
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Peripheral Nerve Stimulation: Time Varying Magnetic Fields (TVMF) |
-variable because of he speed, strength and duration of gradient pulse changes
-increase amplitude and speed = increase TVMF -certain pulse sequences pose more risk: *diffusion *blood oxygen level dependent (BOLD) |
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Peripheral Nerve Stimulation |
-occur when the rapidly changing gradient coils induce electric fields in human tissues -causes a stimulation and sensation in peripheral nerves -feelings of mild vibrations, poking and involuntary muscle contraction -these sensations are not considered a significant risk in MRI -no danger of cardiac, brain or eye stimulation -mean sensation threshold was 45T/s |
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Peripheral Nerve |
portion of the spinal nerve distal to the root and plexus |
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Areas of Stimulation |
-scapula and other parts of the back -nose -upper arms -legs -hands -buttocks -head -typically at areas of bony prominences *Since bone is less conductive than surrounding tissue it may increase densities of the current in narrow regions of tissue between bone and skin, resulting in lower stimulation threshold |
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Peripheral Nervous System: X Gradient |
-bridge of the nose -left side of the thorax -iliac crest -left thigh -buttock -lower back |
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Peripheral Nervous System: Y Gradient |
-scapula -upper arm -shoulder -right side of thorax -iliac crest -hip -hands -upper back |
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Peripheral Nervous System: Z Gradient |
-scapula -xiphoid -thorax -abdomen -iliac crest -upper and lower back |
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Peripheral Nervous System: normal operating mode |
suitable for all patients |
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Peripheral Nervous System: First Level Control Mode |
-levels could cause physiological reactions *monitoring required |
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Peripheral Nervous System: Second Level Controlled Mode |
restricted, may produce significant risk |
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Peripheral Nervous System: Technologists Role |
-inform the patient and ask if they would like to proceed -must maintain constant contact with the patient -patient must not clasp hands -if the patient feels and pain or is uncomfortable the procedure should be stopped |
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Magnetophosphenes |
-visual sensation --> flashes of light -results from the direct excitation of the optic nerve and/or the retina -due to rapid eye movement within a strong magnetic field and/or TVMF |
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FDA Limitations: Gradients |
-basically if it hurts don't do it |
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Health Care Canada Limits: Gradients |
-limit: 3 T/s rate of change |
