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(BWA) A flexible surface coil for body imaging.
 
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Boil off Rate
 
Rate of cryogen (liquid helium) evaporation in superconducting magnets, usually measured in liters of liquid per hour. The boil off rate of cryogen increases during ramping of the magnet and with eddy currents induced in the cryoshields by pulsed field gradients. In calculating cryogen consumption additional transfer and filling losses have to be considered.

See also Cryoshielding and Helium.
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• View the DATABASE results for 'Boil off Rate' (4).Open this link in a new window

 
Further Reading:
  Basics:
Cryogenic Liquids and their Hazards
   by www.ccohs.ca    
Zero Boil Off Cryostats
Friday, 23 September 2011   by www.cryogenicsociety.or    
  News & More:
A hot time for cold superconductors
Tuesday, 9 December 2003   by www.brightsurf.com    
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Boltzmann Distribution
 
When a group of spins is placed in a magnetic field, each spin aligns in one of the two possible orientations. The relative numbers of spins with different alignments will be given by the Boltzmann distribution.
Definition: if a system of particles, which are able to exchange energy in collisions is in thermal equilibrium, then the relative number (population) of particles, N1 and N2, in two particular energy levels with corresponding energies, E1 and E2, is given by N1/N2 = exp [-(E1 - E2)/kT] where k is the Boltzmann constant and T is the absolute temperature.
For example, in NMR of protons at room temperature in a magnetic field of 0.25 tesla, the difference in relative numbers of spins aligned with the magnetic field and against the field is about one part in a million; the small excess of nuclei in the lower energy state is the basis of the net magnetization and the resonance phenomenon.
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• View the DATABASE results for 'Boltzmann Distribution' (4).Open this link in a new window

Bolus Injection
 
A bolus is a rapid infusion of high dose contrast agent. Dynamic and accumulation phase imaging can be performed after bolus injection. Since the transit time of the bolus through the tissue is only a few seconds, high temporal resolution imaging can be required to obtain sequential images during the wash in and wash out of the contrast material and, therefore, resolve the first pass of the tracer.
For the same injected dose of contrast agent the injection rate (and, consequently, the total injected volume) modifies the bolus peak profile. Increasing the injection rate produces a sharpening of the peak (Cmax increase, Tmax decrease, peak length decrease). At a low injection rate, the first pass presents a plateau form. Substantial changes in the gadolinium concentrations during signal acquisition induce artifacts. Furthermore, the haemodynamic parameters (cardiac output, blood pressure) influence the bolus profile. The characteristics of gadolinium agents are favorable in the early bolus phase, whereas the advantages of large complexes (e.g. blood pool agents) and ultrasmall superparamagnetic iron oxide (USPIO) are most evident in the distribution phase.
 
Images, Movies, Sliders:
 Left Circumflex Ischemia First-pass Contrast Enhancement  Open this link in a new window
      

Courtesy of  Robert R. Edelman

 Normal Lung Gd Perfusion MRI  Open this link in a new window
 
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• View the DATABASE results for 'Bolus Injection' (9).Open this link in a new window

 
Further Reading:
  News & More:
Contrast Bolus Timing and Scan Delay
2003   by www.med.nyu.edu    
Bolus Tracking
 
A method of tracking moving spins after tagging them (locally altering their magnetization). The moving spins are then imaged at some time after tagging to show where the bolus (the small tagged volume of spins) has moved in the imaging plane.
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• View the DATABASE results for 'Bolus Tracking' (4).Open this link in a new window

 
Further Reading:
  Basics:
Contrast Bolus Timing and Scan Delay
2003   by www.med.nyu.edu    
Contrast MRI Best at Finding Liver Trouble - But Timing Matters
Sunday, 6 March 2011   by www.searchmedica.com    
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