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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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• Related Searches:
    • MRI Risks
    • Absorbed Dose
    • Contraindications
    • Magnetic Resonance Imaging MRI
    • High Field MRI
 
Further Reading:
  Basics:
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:
Specific Absorption Rate: A Specious Dosimetric Means of Characterizing MRI-Related Implant Heating?
Wednesday, 3 December 2003   by rsna2003.rsna.org    
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 
Collections - - Absorption and Emission - Musculoskeletal and Joint MRI - Functional MRI - Shielding
 
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:
Physics of MRI Safety
   by www.aapm.org    
  News & More:
Commission delays electromagnetic fields legislation
Monday, 29 October 2007   by cordis.europa.eu:80    
MRI Resources 
Spectroscopy pool - Universities - Guidance - Health - Crystallography - Breast Implant
 
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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MRI RisksMRI Resource Directory:
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The subacute risks and side effects of magnetic and RF fields (for patients and staff) have been intensively examined for a long time, but there have been no long-term studies following persons who have been exposed to the static magnetic fields used in MRI. However, no permanent hazardous effects of a static magnetic field exposure upon human beings have yet been demonstrated.
Temporary possible side effects of high magnetic and RF fields:
Varying magnetic fields can induce so-called magnetic phosphenes that occur when an individual is subject to rapid changes of 2–5 T/s, which can produce a flashing sensation in the eyes. This temporary side effect does not seem to damage the eyes. Static field strengths used for clinical MRI examinations vary between 0.2 and 3.0 tesla;; field changes during the MRI scan vary in the dimension of mT/s. Experimental imaging units can use higher field strengths of up to 14.0 T, which are not approved for human use.
The Radio frequency pulses mainly produce heat, which is absorbed by the body tissue. If the power of the RF radiation is very high, the patient may be heated too much. To avoid this heating, the limit of RF exposure in MRI is up to the maximum specific absorption rate (SAR) of 4 W/kg whole body weight (can be different from country to country). For MRI safety reasons, the MRI machine starts no sequence, if the SAR limit is exceeded.
Very high static magnetic fields are needed to reduce the conductivity of nerves perceptibly. Augmentation of T waves is observed at fields used in standard imaging but this side effect in MRI is completely reversible upon removal from the magnet. Cardiac arrhythmia threshold is typically set to 7–10 tesla. The magnetohydrodynamic effect, which results from a voltage occurring across a vessel in a magnetic field and percolated by a saline solution such as blood, is irrelevant at the field strengths used.
The results of some animal and cellular studies suggest the possibility that electromagnetic fields may act as co-carcinogens or tumor promoters, but the data are inconclusive. Up to 45 tesla, no important effects on enzyme systems have been observed. Neither changes in enzyme kinetics, nor orientation changes in macromolecules have been conclusively demonstrated.
There are some publications associating an increase in the incidence of leukemia with the location of buildings close to high-current power lines with extremely low-frequency (ELF) electromagnetic radiation of 50-60 Hz, and industrial exposure to electric and magnetic fields but a transposition of such effects to MRI or MRS seems unlikely.
Under consideration of the MRI safety guidelines, real dangers or risks of an exposure with common MRI field strengths up to 3 tesla as well as the RF exposure during the MRI scan, are not to be expected.
For more MRI safety information see also Nerve Conductivity, Contraindications, Pregnancy and Specific Absorption Rate.

See also the related poll result: 'In 2010 your scanner will probably work with a field strength of'
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• View the DATABASE results for 'MRI Risks' (9).Open this link in a new window


• View the NEWS results for 'MRI Risks' (3).Open this link in a new window.
 
Further Reading:
  Basics:
Specific Absorption Rate: A Specious Dosimetric Means of Characterizing MRI-Related Implant Heating?
Wednesday, 3 December 2003   by rsna2003.rsna.org    
Physics of MRI Safety
   by www.aapm.org    
  News & More:
Commission delays electromagnetic fields legislation
Monday, 29 October 2007   by cordis.europa.eu:80    
MRI Resources 
MRI Technician and Technologist Schools - Bioinformatics - Most Wanted - MRCP - Implant and Prosthesis - Knee MRI
 
High Field MRI
 
The principal advantage of MRI at high field is the increase in signal to noise ratio. This can be used to improve anatomic and/or temporal resolution and reduce scan time while preserving image quality. MRI devices for whole body imaging for human use are available up to 3 tesla (3T). Functional MRI (fMRI) and MR spectroscopy (MRS) benefit significantly. In addition, 3T machines have a great utility in applications such as TOF MRA and DTI. Higher field strengths are used for imaging of small parts of the body or scientific animal experiments. Higher contrast may permit reduction of gadolinium doses and, in some cases, earlier detection of disease.
Using high field MRI//MRS, the RF-wavelength and the dimension of the human body complicating the development of MR coils. The absorption of RF power causes heating of the tissue. The energy deposited in the patient's tissues is fourfold higher at 3T than at 1.5T. The specific absorption rate (SAR) induced temperature changes of the human body are the most important safety issue of high field MRI//MRS.
Susceptibility and chemical shift dispersion increase like T1, therefore high field MRI occasionally exhibits imaging artifacts. Most are obvious and easily recognized but some are subtle and mimic diseases. A thorough understanding of these artifacts is important to avoid potential pitfalls. Some imaging techniques or procedures can be utilized to remove or identify artifacts.
See also Diffusion Tensor Imaging.

See also the related poll result: 'In 2010 your scanner will probably work with a field strength of'
Radiology-tip.comMagnetic Resonance Guided Focused Ultrasound,  High Intensity Focused Ultrasound
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• View the DATABASE results for 'High Field MRI' (16).Open this link in a new window


• View the NEWS results for 'High Field MRI' (9).Open this link in a new window.
 
Further Reading:
  Basics:
CLINICAL WHOLE BODY MRI AT 3.0 T(.pdf)
2001
Dealing with Increased MRI Field Strength
Tuesday, 1 October 2013   by www.evaluationengineering.com    
Musculoskeletal MRI at 3.0 T: Relaxation Times and Image Contrast
Sunday, 1 August 2004   by www.ajronline.org    
  News & More:
Vascular Filters of Functional MRI: Spatial Localization Using BOLD and CBV Contrast
High-Resolution, Spin-Echo BOLD, and CBF fMRI at 4 and 7 T(.pdf)
October 2002   by otg.downstate.edu    
Turbo-FLASH Based Arterial Spin Labeled Perfusion MRI at 7 T
Thursday, 20 June 2013   by www.plosone.org    
MRI Resources 
Mobile MRI Rental - Safety Training - Pacemaker - Liver Imaging - Stimulator pool - Knee MRI
 
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