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Duty Cycle
 
Duty cycle is the time during which the gradient system can be run at maximum power. The duty cycle is based on the total time and includes the cool down phase. The duty cycle on the RF pulse during MRI is restricted based on the specific absorption rate (SAR) limit.
SAR limits restrict radio frequency heating effects. The specific absorption rate increases with field strength, radio frequency power and duty cycle, type of the transmitter coil and body size. The especially in high and ultrahigh magnetic fields, important SAR issue can be readily addressed by reducing the RF duty cycle due to longer repetition times (TR) and the use of parallel imaging techniques. A TR longer than the minimum needed provides time for the tissue to cool down, but for the cost of a longer scan time. A parallel imaging technique reduces the RF exposure and the scan time.
See also High Field MRI (3 tesla 3T).
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• Related Searches:
    • Tesla
    • Radio Frequency
    • Specific Absorption Rate
    • MRI Safety
    • Absorbed Dose
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Absorbed Dose
 
This dose means the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).
The absorbed dose is dependent on the duty cycle and transmitter-coil type and increases with field strength, radio frequency power and and body size.
The specific absorption rate (SAR) describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the MR signal.
See also Specific Absorption Rate, MRI Safety, and MRI Risks.
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Further Reading:
  Basics:
Commission delays electromagnetic fields legislation
Monday, 29 October 2007   by cordis.europa.eu:80    
Physics of MRI Safety
   by www.aapm.org    
  News & More:
Evaluation of Absorbed Dose by MRI Read-Out
Saturday, 18 November 2017   by www.jstage.jst.go.jp    
MRI Resources 
Breast Implant - Abdominal Imaging - Service and Support - Most Wanted - Devices - Jobs
 
GradientForum -
related threads
 
The word gradient (from grade) means the inclination of a surface along a given direction. In MRI, gradient stands for gradient field and/or gradient coil. Inside the main magnet are three gradient coils located, which produce the desired gradient (magnetic) fields. These fields are used to alter (collectively and sequentially) the influence of the static magnetic field B0 on the imaged object by inc- or decreasing the field strength and changing the direction. Through this influence selective spatial excitation and spatial encoding (each voxel resonate at a different frequency) is possible. Gradients are also utilized in another way for fast imaging sequences.
See also Slew Rate and Duty Cycle.
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Further Reading:
  Basics:
Motion Compensation in MR Imaging
   by ccn.ucla.edu    
Active Noise Control of Magnetic Resonance Imaging Scanner Using Inverse Modeling Technique
Monday, 19 April 2010   by www.acoustics.org    
Artifacts in MRI
Saturday, 1 October 2011   by www.huc.min-saude.pt    
  News & More:
On the Horizon - Next Generation MRI
Wednesday, 23 October 2013   by thefutureofthings.com    
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Magnetic ForcesMRI Resource Directory:
 - MRI Accidents -
 
Forces can result from the interaction of magnetic fields. Pulsed magnetic field gradients can interact with the main magnetic field during the MRI scan, to produce acoustic noise through the gradient coil.
Magnetic fields attract ferromagnetic objects with forces, which can be a lethal danger if one is hit by an unrestrained object in flight. One could also be trapped between the magnet and a large unrestrained ferromagnetic object or the object could damage the MRI machine.
Access control and personnel awareness are the best preventions of such accidents. The attraction mechanism for ferromagnetic objects is that the magnetic field magnetizes the iron. This induced magnetization reacts with the gradient of the magnetic field to produce an attraction toward the strongest area of the field. The details of this interaction are very dependent on the shape and composition of the attracted object. There is a very rapid increase of force as one approaches a magnet. There is also a torque or twisting force on objects, e.g. a long cylinder (such as a pen or an intracranial aneurysm clip) will tend to align along the magnet's field lines. The torque increases with field strength while the attraction increases with field gradient.
Depending on the magnetic saturation of the object, attraction is roughly proportional to object mass. Motion of conducting objects in magnetic fields can induce eddy currents that can have the effect of opposing the motion.
See also Duty Cycle.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'
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• View the DATABASE results for 'Magnetic Forces' (4).Open this link in a new window

 
Further Reading:
  Basics:
How strong are magnets?
   by my.execpc.com    
Magnetic Field of the Strongest Magnet
2003   by hypertextbook.com    
  News & More:
New imaging project for new applications in cancer diagnostics
Monday, 27 March 2017   by www.news-medical.net    
Two stuck to MRI machine for 4 hrs
Tuesday, 11 November 2014   by www.mumbaimirror.com    
Technical Assessment of Artifact Production from Neuro Endovascular Coil At 3 Tesla MRI: An In Vitro Study
2012   by www.tmps.or.th    
Scientists improve MRI sensitivity
Wednesday, 28 January 2009   by news-service.stanford.edu    
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Radiology  (2) Open this link in a new window
Specific Absorption Rate
 
(SAR) The Specific Absorption Rate is defined as the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).
The SAR describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the MR signal. Inhomogeneity of the RF field leads to a local exposure where most of the absorbed energy is applied to one body region rather than the entire person, leading to the concept of a local SAR. Hot spots may occur in the exposed tissue, to avoid or at least minimize effects of such theoretical complications, the frequency and the power of the radio frequency irradiation should be kept at the lowest possible level. Averaging over the whole body leads to the global SAR.
It increases with field strength, radio frequency power and duty cycle, transmitter-coil type and body size. The doubling of the field strength from 1.5 Tesla (1.5T) to 3 Tesla (3T) leads to a quadrupling of SAR. In high and ultrahigh fields, some of the multiple echo, multiple-slice pulse sequences may create a higher SAR than recommended by the agencies. SAR can be reduced by lower flip angle and longer repetition times, which could potentially affect image contrast.
Normally no threatening increase in temperature could be shown. Even in high magnetic fields, the local temperature increases not more than 1°C. 2.1°C is the highest measured increase in skin temperature. Eddy currents may heat up implants and thus may cause local heating.

FDA SAR limits:
Whole body: 4W/kg/15-minute exposure averaged;
Head: 3W/kg/10-minute exposure averaged;
Head or torso: 8W/kg/5 minute exposure per gram of tissue;
Extremities: 12W/kg/5 minute exposure per gram of tissue.
IEC (International Electrotechnical Commission) SAR limits of some European countries:
All limits are averaged over 6 minutes.
Level 0 (normal operating mode): Whole body 2W/kg; Head 3.2W/kg; Head or Torso (local) 10W/kg; Extremities (local) 20W/kg;
Level I (first level controlled operating mode): Whole body 4W/kg; Head 3.2W/kg; Head or Torso (local) 10W/kg; Extremities (local) 20W/kg;
Level II (second level controlled operating mode): All values are over Level I values.
(For more details: IEC 60601-2-33 (2002))

In most countries standard MRI systems are limited to a maximum SAR of 4 W/kg, so most scanning in level II is impossible.
For Level I, in addition to routine monitoring, particular caution must be exercised for patients who are sensitive to temperature increases or to RF energy.
For Japan different SAR limits are valid.
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• View the NEWS results for 'Specific Absorption Rate' (1).Open this link in a new window.
 
Further Reading:
  Basics:
On the estimation of the worst-case implant-induced RF-heating in multi-channel MRI.
Thursday, 2 March 2017   by www.ncbi.nlm.nih.gov    
What MRI Sequences Produce the Highest Specific Absorption Rate (SAR), and Is There Something We Should Be Doing to Reduce the SAR During Standard Examinations?
Thursday, 16 April 2015   by www.ajronline.org    
Numerical investigations of MRI RF field induced heating for external fixation devices
Thursday, 7 February 2013   by 7thspace.com    
Evaluation of Specific Absorption Rate as a Dosimeter of MRI-Related Implant Heating
2004   by www.imrser.org    
  News & More:
MRI in Patients with Implanted Devices: Current Controversies
Monday, 1 August 2016   by www.acc.org    
Accounting for biological aggregation in heating and imaging of magnetic nanoparticles
Tuesday, 2 September 2014   by www.ecnmag.com    
Commission delays electromagnetic fields legislation
Monday, 29 October 2007   by cordis.europa.eu:80    
Guidance for Industry and FDA Staff, Criteria for Significant Risk Investigations of Magnetic Resonance Diagnostic Devices
Monday, 14 July 2003   by www.fda.gov    
MRI Resources 
Pregnancy - Contrast Agents - Absorption and Emission - Nerve Stimulator - Liver Imaging - MRI Centers
 
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