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(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:
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Whole body: 4W/kg/15-minute exposure averaged;
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Head: 3W/kg/10-minute exposure averaged;
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Head or torso: 8W/kg/5 minute exposure per gram of tissue;
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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.
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Level 0 (normal operating mode): Whole body 2W/kg; Head 3.2W/kg; Head or Torso (local) 10W/kg;
Extremities (local) 20W/kg;
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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;
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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. | | | | | | | | | Further Reading: | | Basics:
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From Hitachi Medical Systems America Inc.;
the AIRIS II, an entry in the diagnostic category of open MR systems, was designed by Hitachi
Medical Systems America Inc. (Twinsburg, OH, USA) and Hitachi Medical Corp. (Tokyo) and is manufactured by the Tokyo branch. A 0.3 T field-strength magnet and phased array coils deliver high image quality without the need for a tunnel-type high-field system, thereby significantly improving patient comfort not only for claustrophobic patients.
Device Information and Specification
CLINICAL APPLICATION
Whole body
QD Head, MA Head and Neck, QD C-Spine, MA or QD Shoulder, MA CTL Spine, QD Knee, Neck, QD TMJ, QD Breast, QD Flex Body (4 sizes), Small and Large Extrem., QD Wrist, MA Foot and Ankle (WIP), PVA (WIP)
SE, GE, GR, IR, FIR, STIR, FSE, ss-FSE, FLAIR, EPI -DWI, SE-EPI, ms - EPI, SSP, MTC, SARGE, RSSG, TRSG, MRCP, Angiography: CE, 2D/3D TOF
IMAGING MODES
Single, multislice, volume study
TR
SE: 30 - 10,000msec GE: 20 - 10,000msec IR: 50 - 16,700msec FSE: 200 - 16,7000msec
TE
SE : 10 - 250msec IR: 10 -250msec GE: 5 - 50 msec FSE: 15 - 2,000
0.05 sec/image (256 x 256)
2D: 2 - 100 mm; 3D: 0.5 - 5 mm
Level Range: -2,000 to +4,000
POWER REQUIREMENTS
208/220/240 V, single phase
COOLING SYSTEM TYPE
Air-cooled
2.0 m lateral, 2.5 m vert./long
| | | | • View the DATABASE results for 'AIRIS II™' (2).
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From Hitachi Medical Systems America, Inc.;
the AIRIS made its debut in 1995. Hitachi followed up with the AIRIS II system, which has proven equally successfully. 'All told, Hitachi has installed more than 1,000 MRI systems in the U.S., holding more than 17 percent of the total U.S. MRI installed base, and more than half of the installed base of open MR systems,' says Antonio Garcia, Frost and Sullivan industry research analyst.
Now Altaire employs a blend of innovative Hitachi features called VOSIâ„¢ technology, optimizing each sub-system's performance in concert with the
other sub-systems, to give the seamless mix of high-field performance
and the patient comfort, especially for claustrophobic patients, of open MR systems.
Device Information and Specification
CLINICAL APPLICATION
Whole body
DualQuad T/R Body Coil, MA Head, MA C-Spine, MA Shoulder, MA Wrist, MA CTL Spine, MA Knee, MA TMJ, MA Flex Body (3 sizes), Neck, small and large Extremity, PVA (WIP), Breast (WIP), Neurovascular (WIP), Cardiac (WIP) and MA Foot//Ankle (WIP)
SE, GE, GR, IR, FIR, STIR, ss-FSE, FSE, DE-FSE/FIR, FLAIR, ss/ms-EPI, ss/ms EPI- DWI, SSP, MTC, SE/GE-EPI, MRCP, SARGE, RSSG, TRSG, BASG, Angiography: CE, PC, 2D/3D TOF
IMAGING MODES
Single, multislice, volume study
TR
SE: 30 - 10,000msec GE: 3.6 - 10,000msec IR: 50 - 16,700msec FSE: 200 - 16,7000msec
TE
SE : 8 - 250msec IR: 5.2 -7,680msec GE: 1.8 - 2,000 msec FSE: 5.2 - 7,680
0.05 sec/image (256 x 256)
2D: 2 - 100 mm; 3D: 0.5 - 5 mm
Level Range: -2,000 to +4,000
COOLING SYSTEM TYPE
Water-cooled
3.1 m lateral, 3.6 m vertical
| | | | • View the DATABASE results for 'Altaire™' (2).
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FDA cleared and CE Mark 2011.
The Biograph mMR has a fully-integrated design for simultaneous PET/MRI imaging. The dedicated hardware includes solid-state, avalanche photodiode PET detector and adapted, PET-compatible MR coils.
The possibility of truly simultaneous operation allows the acquisition of several magnetic resonance imaging ( MRI) sequences during the positron emission tomography (PET) scan, without increasing the examination time.
See also Hybrid Imaging.
Device Information and Specification
CLINICAL APPLICATION
Whole Body
CONFIGURATION
Simultaneous PET/MRI
26 cm (typical overlap 23%)
A-P 45, R-L 50, H-F 50 cm
PET RING DIAMETER
65.6 cm
PATIENT SCAN RANGE
199 cm
HORIZONTAL SPEED
200 mmsec
PET DETECTOR
Solid state, 4032 avalanche photo diodes
DETECTOR SCINTILLATION MATERIAL
LSO, 28672 crystals
CRYSTAL SIZE
4 x 4 x 20 mm
DIMENSION H*W*D (gantry included)
335 x 230 x 242 cm (finshed covers)
COOLING SYSTEM
PET system: water; MRI system: water
Aautomatic, patient specific shim; active shim 3 linear and 5 non-linear channels (seond order)
POWER REQUIREMENTS
380 / 400 / 420 / 440 / 460 / 480 V, 3-phase + ground; Total system 110kW
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Ultrasound imaging is the primary fetal monitoring modality during pregnancy, nevertheless fetal MRI is increasingly used to image anatomical regions and structures difficult to see with sonography. Given its long record of safety, utility, and cost-effectiveness, ultrasound will remain the modality of first choice in fetal screening. However, MRI is beginning to fill a niche in situations where ultrasound does not provide enough information to diagnose abnormalities before the baby's birth. Magnetic resonance imaging of the fetus provides multiplanar views also in sub-optimal positions, better characterization of anatomic details of e.g. the fetal brain, and information for planning the mode of delivery and airway management at birth.
Indications:
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Examinations of the placenta
Modern fetal MRI requires no sedatives or muscle relaxants to control fetal movement. Ultrafast MRI techniques (e.g., single shot techniques like Half Fourier Acquisition Single shot Turbo spin Echo HASTE) enable images to be acquired in less than one second to eliminate fetal motion. Such technology has led to increased usage of fetal MRI, which can lead to earlier diagnosis of conditions affecting the baby and has proven useful in planning fetal surgery and designing postnatal treatments. As MR technology continues to improve, more advances in the prenatal diagnosis and treatment of fetal abnormalities are to expect. More advances in in-utero interventions are likely as well. Eventually, fetal MRI may replace even some prenatal tests that require invasive procedures such as amniocentesis.
For Ultrasound Imaging (USI) see Fetal Ultrasound at Medical-Ultrasound-Imaging.com. | | | | | | • View the DATABASE results for 'Fetal MRI' (5).
| | | • View the NEWS results for 'Fetal MRI' (2).
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Advances in medical imaging enable visualization of white matter tracts in fetuses Wednesday, 12 May 2021 by www.eurekalert.or | | |
Fetal CMR Detects Congenital Heart Defects, Changes Treatment Decisions Monday, 29 March 2021 by www.diagnosticimaging.com | | |
MRI scans more precisely define and detect some abnormalities in unborn babies Friday, 12 March 2021 by www.eurekalert.org | | |
Ultrasound and Magnetic Resonance Imaging of Agenesis of the Corpus Callosum in Fetuses: Frontal Horns and Cavum Septi Pellucidi Are Clues to Earlier Diagnosis Monday, 29 June 2020 by pubmed.ncbi.nlm.nih.gov | | |
MRI helps predict preterm birth Tuesday, 15 March 2016 by www.eurekalert.org | | |
3-T MRI advancing on ultrasound for imaging fetal abnormalities Monday, 20 April 2015 by www.eurekalert.org | | |
Babies benefit from pioneering 'miniature' MRI scanner in Sheffield Friday, 24 January 2014 by www.telegraph.co.uk | | |
Ultrasensitive Detector Pinpoints Big Problem in Tiny Fetal Heart Tuesday, 6 April 2010 by www.sciencedaily.com | | |
Real-time MRI helps doctors assess beating heart in fetus Thursday, 29 September 2005 by www.eurekalert.org |
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