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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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    • Radio Frequency
    • MRI Safety
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    • Low Field MRI
    • MRI Risks
 
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 
Developers - Pediatric and Fetal MRI - Case Studies - Musculoskeletal and Joint MRI - Pathology - MRI Centers
 
MAGNETOM Verio
 
From Siemens Medical Systems;
www.healthcare.siemens.com/magnetic-resonance-imaging/3t-mri-scanner/magnetom-verio Received FDA clearance in 2007.
The MAGNETOM Verio provides up to 102 integrated matrix coil elements and up to 32 independent radiofrequency channels that allow flexible coil combinations to make patient and coil repositioning virtually unnecessary. The Tim (total imaging matrix) technology also increases patient throughput due to a shorter scan time.
The open bore design offers great comfort for patients of all shapes and sizes.


Device Information and Specification
CLINICAL APPLICATION Whole Body
CONFIGURATION Ultra-short open bore
FIELD STRENGTH 3 Tesla
SURFACE COILS Head, spine, torso/ body coil, neurovascular, cardiac, neck and multi-purpose flex coils. Peripheral vascular, breast, shoulder, knee, wrist, foot//ankle, TMJ optional.
CHANNELS (min. / max. configuration) 8, 18, 32
SPECTROSCOPY Chemical shift imaging, single voxel spectroscopy
IMAGING TECHNIQUES iPAT, mSENSE and GRAPPA (image, k-space), noncontrast angiography, radial motion compensation, Dixon
MINIMUM TR 3-D GRE: 1.5 (256 matrix)
MINIMUM TE 3-D GRE: 0.63 (256 matrix)
FOV 0.5 - 50 cm
BORE DIAMETER
or W x H
At isocenter: L-R 70 cm, A-P 55 cm
TABLE CAPACITY 250 kg
MAGNET WEIGHT (gantry included) 8200 kg
DIMENSION H*W*D (gantry included) 173 x 230 x 222 cm
5-GAUSS FRINGE FIELD 2.6 m / 4.6 m
MAGNET TYPE Superconducting
CRYOGEN USE Zero boil off rate, refill approx. 10 years
COOLING SYSTEM Water
SLEW RATE up to 200 T/m/s
MAX. AMPLITUDE 45 mT/m
SHIMMING Passive, active; first order, second order standard
POWER REQUIREMENTS 380 / 400 / 420 / 440 / 460 / 480 V, 3-phase + ground; 110 kVA
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Liver ImagingForum -
related threadsMRI Resource Directory:
 - Liver Imaging -
 
Liver imaging can be performed with sonography, computed tomography (CT) and magnetic resonance imaging (MRI). Ultrasound is, caused by the easy access, still the first-line imaging method of choice; CT and MRI are applied whenever ultrasound imaging yields vague results. Indications are the characterization of metastases and primary liver tumors e.g., benign lesions such as focal nodular hyperplasia (FNH), adenoma, hemangioma and malignant lesions (cancer) such as hepatocellular carcinomas (HCC). The decision, which medical imaging modality is more suitable, MRI or CT, is dependent on the different factors. CT is less costly and more widely available; modern multislice scanners provide high spatial resolution and short scan times but has the disadvantage of radiation exposure.
With the introduction of high performance MR systems and advanced sequences the image quality of MRI for the liver has gained substantially. Fast spin echo or single shot techniques, often combined with fat suppression, are the most common T2 weighted sequences used in liver MRI procedures. Spoiled gradient echo sequences are used as ideal T1 weighted sequences for evaluating of the liver. The repetition time (TR) can be sufficiently long to acquire enough sections covering the entire liver in one pass, and to provide good signal to noise. The TE should be the shortest in phase echo time (TE), which provides strong T1 weighting, minimizes magnetic susceptibility effects, and permits acquisition within one breath hold to cover the whole liver. A flip angle of 80° provides good T1 weighting and less of power deposition and tissue saturation than a larger flip angle that would provide comparable T1 weighting.
Liver MRI is very dependent on the administration of contrast agents, especially when detection and characterization of focal lesions are the issues. Liver MRI combined with MRCP is useful to evaluate patients with hepatic and biliary disease.
Gadolinium chelates are typical non-specific extracellular agents diffusing rapidly to the extravascular space of tissues being cleared by glomerular filtration at the kidney. These characteristics are somewhat problematic when a large organ with a huge interstitial space like the liver is imaged. These agents provide a small temporal imaging window (seconds), after which they begin to diffuse to the interstitial space not only of healthy liver cells but also of lesions, reducing the contrast gradient necessary for easy lesion detection. Dynamic MRI with multiple phases after i.v. contrast media (Gd chelates), with arterial, portal and late phase images (similar to CT) provides additional information.
An additional advantage of MRI is the availability of liver-specific contrast agents (see also Hepatobiliary Contrast Agents). Gd-EOB-DTPA (gadoxetate disodium, Gadolinium ethoxybenzyl dimeglumine, EOVIST Injection, brand name in other countries is Primovist) is a gadolinium-based MRI contrast agent approved by the FDA for the detection and characterization of known or suspected focal liver lesions.
Gd-EOB-DTPA provides dynamic phases after intravenous injection, similarly to non-specific gadolinium chelates, and distributes into the hepatocytes and bile ducts during the hepatobiliary phase. It has up to 50% hepatobiliary excretion in the normal liver.
Since ferumoxides are not eliminated by the kidney, they possess long plasmatic half-lives, allowing circulation for several minutes in the vascular space. The uptake process is dependent on the total size of the particle being quicker for larger particles with a size of the range of 150 nm (called superparamagnetic iron oxide). The smaller ones, possessing a total particle size in the order of 30 nm, are called ultrasmall superparamagnetic iron oxide particles and they suffer a slower uptake by RES cells. Intracellular contrast agents used in liver MRI are primarily targeted to the normal liver parenchyma and not to pathological cells. Currently, iron oxide based MRI contrast agents are not marketed.
Beyond contrast enhanced MRI, the detection of fatty liver disease and iron overload has clinical significance due to the potential for evolution into cirrhosis and hepatocellular carcinoma. Imaging-based liver fat quantification (see also Dixon) provides noninvasively information about fat metabolism; chemical shift imaging or T2*-weighted imaging allow the quantification of hepatic iron concentration. See also Abdominal Imaging, Primovist™, Liver Acquisition with Volume Acquisition (LAVA), T1W High Resolution Isotropic Volume Examination (THRIVE) and Bolus Injection.

For Ultrasound Imaging (USI) see Liver Sonography at US-TIP.com.
 
Images, Movies, Sliders:
 Anatomic Imaging of the Liver  Open this link in a new window
      

 MRI Liver T2 TSE  Open this link in a new window
    
 
Radiology-tip.comAbdomen CT,  Biliary Contrast Agents
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Radiology-tip.comLiver Sonography,  Vascular Ultrasound Contrast Agents
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• View the DATABASE results for 'Liver Imaging' (13).Open this link in a new window


• View the NEWS results for 'Liver Imaging' (10).Open this link in a new window.
 
Further Reading:
  Basics:
Contrast MRI Best at Finding Liver Trouble - But Timing Matters
Sunday, 6 March 2011   by www.searchmedica.com    
MAGNETIC RESONANCE IMAGING OF FOCAL LIVER LESIONS(.pdf)
2002
  News & More:
MRI techniques evolving towards better assessment of liver fibrosis
Wednesday, 2 January 2008   by www.eurekalert.org    
Radiologists Offer Non-surgical Treatment For Early-stage Liver Cancer
Saturday, 5 March 2005   by www.sciencedaily.com    
Elastography: A Useful Method in Depicting Liver Hardness
Thursday, 15 April 2010   by www.sciencedaily.com    
Iron overload: accuracy of in-phase and out-of-phase MRI as a quick method to evaluate liver iron load in haematological malignancies and chronic liver disease
Friday, 1 June 2012   by www.ncbi.nlm.nih.gov    
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