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Signal Intensity
 
Signal intensity interpretation in MR imaging has a major problem.
Often there is no intuitive approach to signal behavior as signal intensity is a very complicated function of the contrast-determining tissue parameter, proton density, T1 and T2, and the machine parameters TR and TE. For this reason, the terms T1 weighted image, T2 weighted image and proton density weighted image were introduced into clinical MR imaging.
Air and bone produce low-intensity, weaker signals with darker images. Fat and marrow produce high-intensity signals with brighter images.
The signal intensity measured is related to the square of the xy-magnetization, which in a SE pulse sequence is given by
Mxy = Mxy0(1-exp(-TR/T1)) exp(-TE/T2) (1)
where Mxy0 = Mz0 is proportional to the proton or spin density, and corresponds to the z-magnetization present at zero time of the experiment when it is tilted into the xy-plane.

See also T2 Weighted Image and Ernst Angle.
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Further Reading:
  Basics:
Contrast mechanisms in magnetic resonance imaging
2004   by www.iop.org    
Image Characteristics and Quality
   by www.sprawls.org    
  News & More:
Specialized MRI sensor can detect light deep within tissues
Thursday, 22 December 2022
Study compares effect of food intake on manganese-based MRI contrast agent absorption
Saturday, 3 December 2022   by www.itnonline.com    
Kidney stone imaging with 3D ultra-short echo time (UTE) magnetic resonance imaging. A phantom study - Abstract
Wednesday, 11 March 2015   by www.urotoday.com    
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Radiology  (1) Open this link in a new window
Normalization Filter
 
This function equalizes the signal intensity when surface coils are used. By using this filter, the signal intensity of the region close to the coil is reduced, and the signal intensity of the area remote to the coil is increased. Also called homogeneity correction.
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Shielding - Mobile MRI - Health - Jobs - Knee MRI - Crystallography
 
Signal to Noise Ratio
 
(SNR or S/N) The signal to noise ratio is used in MRI to describe the relative contributions to a detected signal of the true signal and random superimposed signals ('background noise') - a criterion for image quality.
One common method to increase the SNR is to average several measurements of the signal, on the expectation that random contributions will tend to cancel out. The SNR can also be improved by sampling larger volumes (increasing the field of view and slice thickness with a corresponding loss of spatial resolution) or, within limits, by increasing the strength of the magnetic field used. Surface coils can also be used to improve local signal intensity. The SNR will depend, in part, on the electrical properties of the sample or patient being studied. The SNR increases in proportion to voxel volume (1/resolution), the square root of the number of acquisitions (NEX), and the square root of the number of scans (phase encodings). SNR decreases with the field of view squared (FOV2) and wider bandwidths. See also Signal Intensity and Spin Density.

Measuring SNR:
Record the mean value of a small ROI placed in the most homogeneous area of tissue with high signal intensity (e.g. white matter in thalamus). Calculate the standard deviation for the largest possible ROI placed outside the object in the image background (avoid ghosting/aliasing or eye movement artifact regions).
The SNR is then:
Mean Signal/Standard Deviation of Background Noise
 
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• View the DATABASE results for 'Signal to Noise Ratio' (48).Open this link in a new window


• View the NEWS results for 'Signal to Noise Ratio' (2).Open this link in a new window.
 
Further Reading:
  Basics:
Musculoskeletal MRI at 3.0 T: Relaxation Times and Image Contrast
Sunday, 1 August 2004   by www.ajronline.org    
  News & More:
Picture-Perfect Particles Enhance MRI Signal
Friday, 13 April 2012   by cen.acs.org    
Researchers design 'intelligent' metamaterial to make MRIs affordable and accessible
Tuesday, 5 November 2019   by phys.org    
Metamaterials boost sensitivity of MRI machines
Thursday, 14 January 2016   by www.eurekalert.org    
Optimizing Musculoskeletal MR
   by rad.usuhs.mil    
Searchterm 'Signal Intensity ' was also found in the following services: 
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Combination Oral Contrast AgentsInfoSheet: - Contrast Agents - 
Intro, Overview, 
Characteristics, 
Types of, 
etc.MRI Resource Directory:
 - Contrast Agents -
 
Biphasic oral contrast agents may produce either high or low signal intensities depending on the pulse sequence used, for example low signal intensity on T1 weighted MR images and high signal intensity on T2 weighted images. The combination of different oral contrast agents can generate a macroscopic cancellation of negative and positive magnetic susceptibility, thereby eliminating susceptibility artifacts.
Possible combinations are e.g., ferric ammonium citrate and corn oil, or ferrous sulfate emulsified with baby formula. Paramagnetic agents combined with oil emulsion may be used in MRI as positive abdominal contrast agents. The combination of diamagnetic barium sulfate and superparamagnetic iron oxide (SPIO) in one suspension may be a useful negative contrast agent.

See also Gastrointestinal Paramagnetic Contrast Agents, Gastrointestinal Superparamagnetic Contrast Agents, Gastrointestinal Diamagnetic Contrast Agents, Gastrointestinal Imaging.
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• View the DATABASE results for 'Combination Oral Contrast Agents' (3).Open this link in a new window

 
Further Reading:
  Basics:
Usefulness of MR Imaging for Diseases of the Small Intestine: Comparison with CT
2000   by www.ncbi.nlm.nih.gov    
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Radiology  (1) Open this link in a new window
Contrast to Noise Ratio
 
(CNR) In Magnetic Resonance Imaging MRI, Contrast to noise ratio is the relationship of signal intensity differences between two regions, scaled to image noise. Improving CNR increases perception of the distinct differences between two clinical areas of interest. A contrast to noise ratio is a summary of SNR and contrast. It is the difference in SNR between two relevant tissue types.
(A and B): CNR = SNRA - SNRB

See also Signal Intensity, Signal to Noise Ratio and Medical Imaging.
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• View the DATABASE results for 'Contrast to Noise Ratio' (2).Open this link in a new window


• View the NEWS results for 'Contrast to Noise Ratio' (1).Open this link in a new window.
 
Further Reading:
  Basics:
Vascular Filters of Functional MRI: Spatial Localization Using BOLD and CBV Contrast
Contrast mechanisms in magnetic resonance imaging
2004   by www.iop.org    
Optimal k-Space Sampling for Dynamic Contrast-Enhanced MRI with an Application to MR Renography
Thursday, 5 November 2009   by www.ncbi.nlm.nih.gov    
MRI Resources 
Health - Collections - Software - MRI Reimbursement - Non-English - Resources
 
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