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Magnetic Field Mapping
 
The mapping of the magnetic field by measuring or imaging the spatial distribution of magnetic field strength, can be performed by scanning with a probe and handles a large range of field strengths, but is slow and tedious. Accurate field maps can be made by measuring the Larmor frequency as a function of position.
The field must be homogeneous enough to allow MR imaging to be performed, than the magnetic field can be mapped by different methods.
1. The adaptation of chemical shift imaging.
2. The faster one measures the change in signal phase in an image obtained with a gradient echo pulse sequence resulting from a change in echo time TE, which is proportional to the local field strength.
Also useful is a spin echo pulse sequence with data collection from two time locations of the readout gradient and the data acquisition interval, where each having a known shift of the acquisition center away from the spin echo.
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Further Reading:
  News & More:
sPHENIX assembly update: magnet mapped, detectors prepared
Friday, 23 December 2022   by www.eurekalert.org    
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:
Imaging chain faces regulators after inmate, guard get stuck to MRI machine
Friday, 1 December 2023   by healthimaging.com    
Measuring magnetic force field distributions in microfluidic devices: Experimental and numerical approaches
Saturday, 2 December 2023   by analyticalsciencejournals.onlinelibrary.wiley.com    
Two stuck to MRI machine for 4 hrs
Tuesday, 11 November 2014   by www.mumbaimirror.com    
New imaging project for new applications in cancer diagnostics
Monday, 27 March 2017   by www.news-medical.net    
MRI Resources 
Blood Flow Imaging - Universities - Mobile MRI - Bioinformatics - Absorption and Emission - Brain MRI
 
Magnetic Fringe FieldInfoSheet: - Coils -
Intro, 
Overview, 
etc.
 
The region surrounding a magnet and exhibiting a magnetic field strength, which is significantly higher than the earth's magnetic field (typically 0.05-0.1 mT, depending on geographical location). Initially the most magnets had very extensive fringe fields. Magnets with iron have reduced the fringe field substantially (passively shielded magnets). At least, adding appropriate additional superconducting coils to superconducting magnets has resulted in a drastic reduction of the extent of the fringe fields (actively shielded magnets).
Due to the physical properties of magnetic fields, the magnetic flux, which penetrates the useful volume of the magnet will return through the surroundings of the magnet to form closed field lines. Depending on the magnet construction, the returning flux will penetrate large open spaces (unshielded magnets) or will be confined largely to iron yokes or through secondary coils (shielded magnets).
Fringe fields constitute one of the major hazards of MR scanners as these fields acting over extended distances outside the magnet produce strong attractive forces upon magnetic objects. These can thus 'fly' into the magnet when loose nearby acting like projectiles. Fringe fields also exert unwanted forces on metallic implants in patients.
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• View the DATABASE results for 'Magnetic Fringe Field' (3).Open this link in a new window

 
Further Reading:
  Basics:
Magnetic Field
   by hyperphysics.phy-astr.gsu.edu    
Magnetic Gradient
 
One of three linear magnetization waveforms superimposed on the main magnetic field at specific times within a pulse sequence to select the imaging region or provide necessary spatial localization information. A magnetic gradient is defined as the amount and direction of the linear rate of change of the magnetic field in space.
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• View the NEWS results for 'Magnetic Gradient' (1).Open this link in a new window.

• View the DATABASE results for 'Magnetic Gradient' (5).Open this link in a new window

 
Further Reading:
  News & More:
Magnetic fields drive drug-loaded nanoparticles to reduce blood vessel blockages in an animal study
Monday, 19 April 2010   by www.eurekalert.org    
Active Noise Control of Magnetic Resonance Imaging Scanner Using Inverse Modeling Technique
Monday, 19 April 2010   by www.acoustics.org    
Magnetic Induction
 
(B) Also called magnetic flux density with the SI unit tesla (T) usually denoted by the symbol B. The magnetic induction is the net magnetic effect from an externally applied magnetic field and the resulting magnetization.
The symbol H was used for the magnetic field (measured in amperes per meter (A/m)). However, this distinction is often ignored, and both quantities are often referred to as the magnetic field.
B is proportional to H (B = μH).
(μ is the magnetic permeability (in henries per meter) of the medium)
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• View the DATABASE results for 'Magnetic Induction' (5).Open this link in a new window

 
Further Reading:
  Basics:
Magnet basics
   by my.execpc.com    
  News & More:
Linear Collider Final FocusMagnet Construction
   by www.bnl.gov    
MRI Resources 
Safety Products - Non-English - Stimulator pool - Anatomy - PACS - Health
 
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MRI is trending to low field magnets :
reduced costs will lead to this change 
AI will close the gap to high field 
only in remote areas 
is only temporary 
never 

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