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Postprocessing
 
MR images can be manipulated for evaluation in various ways. Postprocessing includes: Subtraction, addition, rotation, inversion, multiplanar reconstruction (MPR), maximum intensity projection (MIP), etc.
Subtraction is particularly useful in contrast enhanced MRI examinations (for example breast MRI, brain MRI). The pre contrast images are subtracted from the images after an injection of contrast agents (sometimes also called dye) for a better tumor detection.
See also Computer Aided Detection
 
Images, Movies, Sliders:
 PCA-MRA 3D Brain Venography Colored MIP  Open this link in a new window
    

 TOF-MRA Circle of Willis Inverted MIP  Open this link in a new window
    

 
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Further Reading:
  Basics:
3D-DOCTOR Tutorial
   by www.ablesw.com    
  News & More:
CAD System Designed for MRI
Friday, 3 October 2003   by www.hospimedica.com    
MRI Resources 
Pediatric and Fetal MRI - Resources - Safety Products - Equipment - Case Studies - Cochlear Implant
 
Baseline Correction
 
Postprocessing of the spectrum to suppress baseline deviations from zero that may be superimposed on desired spectral lines. These deviations may be due either to various instrumental effects or to very broad spectral lines.
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• View the DATABASE results for 'Baseline Correction' (3).Open this link in a new window

MRI Resources 
Corporations - Software - Service and Support - MRI Technician and Technologist Career - Research Labs - Stimulator pool
 
Contrast Enhanced Magnetic Resonance AngiographyInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.MRI Resource Directory:
 - MRA -
 
(CE MRA) Contrast enhanced MR angiography is based on the T1 values of blood, the surrounding tissue, and paramagnetic contrast agent.
T1-shortening contrast agents reduces the T1 value of the blood (approximately to 50 msec, shorter than that of the surrounding tissues) and allow the visualization of blood vessels, as the images are no longer dependent primarily on the inflow effect of the blood. Contrast enhanced MRA is performed with a short TR to have low signal (due to the longer T1) from the stationary tissue, short scan time to facilitate breath hold imaging, short TE to minimize T2* effects and a bolus injection of a sufficient dose of a gadolinium chelate.
Images of the region of interest are performed with 3D spoiled gradient echo pulse sequences. The enhancement is maximized by timing the contrast agent injection such that the period of maximum arterial concentration corresponds to the k-space acquisition. Different techniques are used to ensure optimal contrast of the arteries e.g., bolus timing, automatic bolus detection, bolus tracking, care bolus. A high resolution with near isotropic voxels and minimal pulsatility and misregistration artifacts should be striven for. The postprocessing with the maximum intensity projection (MIP) enables different views of the 3D data set.
Unlike conventional MRA techniques based on velocity dependent inflow or phase shift techniques, contrast enhanced MRA exploits the gadolinium induced T1-shortening effects. CE MRA reduces or eliminates most of the artifacts of time of flight angiography or phase contrast angiography. Advantages are the possibility of in plane imaging of the blood vessels, which allows to examine large parts in a short time and high resolution scans in one breath hold. CE MRA has found a wide acceptance in the clinical routine, caused by the advantages:
3D MRA can be acquired in any plane, which means that greater vessel coverage can be obtained at high resolution with fewer slices (aorta, peripheral vessels);
the possibility to perform a time resolved examination (similarly to conventional angiography);
no use of ionizing radiation; paramagnetic agents have a beneficial safety.


 
Images, Movies, Sliders:
 CE-MRA of the Carotid Arteries  Open this link in a new window
    
SlidersSliders Overview

 CE MRA of the Aorta  Open this link in a new window
    
SlidersSliders Overview

 CE-MRA of the Carotid Arteries Colored MIP  Open this link in a new window
    
SlidersSliders Overview

 
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• View the DATABASE results for 'Contrast Enhanced Magnetic Resonance Angiography' (14).Open this link in a new window


• View the NEWS results for 'Contrast Enhanced Magnetic Resonance Angiography' (2).Open this link in a new window.
 
Further Reading:
  Basics:
Contrast-Enhanced MR Angiography(.pdf)
   by ric.uthscsa.edu    
CONTRAST ENHANCED MR ANGIOGRAPHY – PRINCIPLES, APPLICATIONS, TIPS AND PITFALLS(.pdf)
  News & More:
CONTRAST-ENHANCED MRA OF THE CAROTIDS(.pdf)
PERIPHERAL VASCULAR MAGNETIC RESONANCE ANGIOGRAPHY(.pdf)
CONTRAST ENHANCED MRI OF THE LIVER STATE-OF-THE-ART(.pdf)
Searchterm 'Postprocessing' was also found in the following service: 
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DixonInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.
 
The Dixon technique is a MRI method used for fat suppression and/or fat quantification. The difference in magnetic resonance frequencies between fat and water-bound protons allows the separation of water and fat images based on the chemical shift effect.
This imaging technique is named after Dixon, who published in 1984 the basic idea to use phase differences to calculate water and fat components in postprocessing. Dixon's method relies on acquiring an image when fat and water are 'in phase', and another in 'opposed phase' (out of phase). These images are then added together to get water-only images, and subtracted to get fat-only images. Therefore, this sequence type can deliver up to 4 contrasts in one measurement: in phase, opposed phase, water and fat images. An additional benefit of Dixon imaging is that source images and fat images are also available to the diagnosing physician.
The original two point Dixon sequence (number of points means the number of images acquired at different TE) had limited possibilities to optimize the echo time, spatial resolution, slice thickness, and scan time; but Dixon based fat suppression can be very effective in areas of high magnetic susceptibility, where other techniques fail. This insensitivity to magnetic field inhomogeneity and the possibility of direct image-based water and fat quantification have currently generated high research interests and improvements to the basic method (three point Dixon).
The combination of Dixon with gradient echo sequences allows for example liver imaging with 4 image types in one breath hold. With Dixon TSE/FSE an excellent fat suppression with high resolution can be achieved, particularly useful in imaging of the extremities.
For low bandwidth imaging, chemical shift correction of fat images can be made before recombination with water images to produce images free of chemical shift displacement artifacts. The need to acquire more echoes lengthens the minimum scan time, but the lack of fat saturation pulses extends the maximum slice coverage resulting in comparable scan time.
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• View the DATABASE results for 'Dixon' (8).Open this link in a new window

 
Further Reading:
  Basics:
Separation of fat and water signal in magnetic resonanace imaging
2011   by www.diva-portal.org    
Direct Water and Fat Determination in Two-Point Dixon Imaging
April 2013   by scholarship.rice.edu    
MRI evaluation of fatty liver in day to day practice: Quantitative and qualitative methods
Wednesday, 3 September 2014   by www.sciencedirect.com    
Measurement of Fat/Water Ratios in Rat Liver Using 3DThree-Point Dixon MRI
2004   by www.civm.duhs.duke.edu    
  News & More:
Liver Imaging Today
February 2013   by www.healthcare.siemens.it    
mDIXON being developed to simplify and accelerate liver MRI
September 2010   by incenter.medical.philips.com    
MRI Resources 
Research Labs - Education pool - Safety pool - Collections - Breast MRI - Resources
 
Inhomogeneity
 
Inhomogeneity is the degree of lack of homogeneity, for example the fractional deviation of the local magnetic field from the average value of the field. Inhomogeneities of the static magnetic field, produced by the scanner as well as by object susceptibility, is unavoidable in MRI. The large value of gyromagnetic coefficient causes a significant frequency shift even for few parts per million field inhomogeneity, which in turn causes distortions in both geometry and intensity of the MR images.
Manufacturers try to make the magnetic field as homogeneous as possible, especially at the core of the scanner. Even with an ideal magnet, a little inhomogeneity is always left and is caused in addition by the susceptibility of the imaging object. The geometrical distortion (displacement of the pixel locations) are important e.g., for some cases as stereotactic surgery. Displacements up to 3 to 5 mm have been reported. The second problem is the undesired changes in the intensity or brightness of pixels, which may cause problems in determining different tissues and reduce the maximum achievable image resolution.


Image Guidance
General strategies for reducing field inhomogeneity induced artifacts:
Increasing the strength of the gradient magnetic field.
Decreasing the echo time.
Improving the image resolution. Phase encoding.
Postprocessing.
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• View the DATABASE results for 'Inhomogeneity' (21).Open this link in a new window

 
Further Reading:
  News & More:
Implementation of Dual-Source RF Excitation in 3 T MR-Scanners Allows for Nearly Identical ADC Values Compared to 1.5 T MR Scanners in the Abdomen
Wednesday, 29 February 2012   by www.plosone.org    
Optimizing SPIR and SPAIR fat suppression
Tuesday, 30 November 2004   by clinical.netforum.healthcare.philips.com    
MRI Resources 
Musculoskeletal and Joint MRI - Spine MRI - Fluorescence - Portals - Blood Flow Imaging - Colonography
 
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