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Fat Saturation
(FAT SAT) A specialized technique that selectively saturates fat protons prior to acquiring data as in standard sequences, so that they produce a negligible signal. The presaturation pulse is applied prior to each slice selection. This technique requires a very homogeneous magnetic field and very precise frequency calibration.
Fat saturation does not work well on inhomogeneous volumes of tissue due to a change in the precessional frequencies as the difference in volume affects the magnetic field homogeneity. The addition of a water bag simulates a more homogeneous volume of tissue, thus improving the fat saturation. Since the protons in the water bag are in motion due to recent motion of the bag, phase ghosts can be visualized.
Fat saturation can also be difficult in a region of metallic prosthesis. This is caused by an alteration in the local magnetic field resulting in a change to the precessional frequencies, rendering the chemical saturation pulses ineffective.
See also Fat Suppression, and Dixon.

Images, Movies, Sliders:
 Shoulder Sagittal T2 FatSat FRFSE  Open this link in a new window

Courtesy of  Robert R. Edelman
 Shoulder Coronal T2 FatSat FRFSE  Open this link in a new window
 Shoulder Axial T2 FatSat FRFSE  Open this link in a new window

Courtesy of  Robert R. Edelman
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Further Reading:
Techniques of Fat Suppression(.pdf)
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Fat SuppressionForum -
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Fat suppression is the process of utilizing specific MRI parameters to remove the deleterious effects of fat from the resulting images, e.g. with STIR, FAT SAT sequences, water selective (PROSET WATS - water only selection, also FATS - fat only selection possible) excitation techniques, or pulse sequences based on the Dixon method.
Spin magnetization can be modulated by using special RF pulses. CHESS or its variations like SPIR, SPAIR (Spectral Selection Attenuated Inversion Recovery) and FAT SAT use frequency selective excitation pulses, which produce fat saturation.
Fat suppression techniques are nearly used in all body parts and belong to every standard MRI protocol of joints like knee, shoulder, hips, etc.

Image Guidance
Imaging of, e.g. the foot can induce bad fat suppression with SPIR/FAT SAT due to the asymmetric volume of this body part. The volume of the foot alters the magnetic field to a different degree than the smaller volume of the lower leg affecting the protons there. There is only a small band of tissue where the fat protons are precessing at the frequency expected, resulting in frequency selective fat saturation working only in that area. This can be corrected by volume shimming or creating a more symmetrical volume being imaged with water bags.
Even with their longer scan time and motion sensitivity, STIR (short T1//tau inversion recovery) sequences are often the better choice to suppress fat. STIR images are also preferred because of the decreased sensitivity to field inhomogeneities, permitting larger fields of views when compared to fat suppressed images and the ability to image away from the isocenter.
See also Knee MRI.
Recently introduced Dixon turbo spin echo (fast spin echo) sequences can deliver a significant better fat suppression than conventional TSE//FSE imaging.

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 Shoulder Axial T2 FatSat FRFSE  Open this link in a new window

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 MRI Orbita T2 FatSat  Open this link in a new window
 Knee MRI Sagittal STIR 001  Open this link in a new window
 MRI - Anatomic Imaging of the Ankle 3  Open this link in a new window
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Further Reading:
Techniques of Fat Suppression(.pdf)
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Field of View
(FOV) Defined as the size of the two or three dimensional spatial encoding area of the image. Usually defined in units of mm˛. The FOV is the square image area that contains the object of interest to be measured. The smaller the FOV, the higher the resolution and the smaller the voxel size but the lower the measured signal. Useful for decreasing the scantime is a field of view different in the frequency and phase encoding directions (rectangular field of view - RFOV).
The magnetic field homogeneity decreases as more tissue is imaged (greater FOV). As a result the precessional frequencies change across the imaging volume. That can be a problem for fat suppression imaging. This fat is precessing at the expected frequency only in the center of the imaging volume. E.g. frequency specific fat saturation pulses become less effective when the field of view is increased. It is best to use smaller field of views when applying fat saturation pulses.

Image Guidance
Smaller FOV required higher gradient strength and concludes low signal. Therefore you have to find a compromise between these factors. The right choice of the field of view is important for MR image quality. When utilizing small field of views and scanning at a distance from the isocenter (more problems with artifacts) it is obviously important to ensure that the region of interest is within the scanning volume.
A smaller FOV in one direction is available with the function rectangular field of view (RFOV).
See also Field Inhomogeneity Artifact.
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 MRI - Anatomic Imaging of the Foot  Open this link in a new window
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 MRI - Anatomic Imaging of the Ankle 1  Open this link in a new window
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Further Reading:
Image Characteristics and Quality
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Breast MRIMRI Resource Directory:
 - Breast MRI -
(MR mammography) Magnetic resonance imaging of the breast is particularly useful in evaluation of newly diagnosed breast cancer, in women whose breast tissue is mammographically very dense and for screening in women with a high lifetime risk of breast cancer because of their family history or genetic disposition.
Breast MRI can be performed on all standard whole body magnets at a field strength of 0.5 T - 1.5 Tesla. Powerful gradient strengths over 15 mT/m will help to improve the balance between spatial resolution, scanning speed, and volume coverage. The use of a dedicated bilateral breast coil is obligatory.
Malignant lesions release angiogenic factors that increase local vessel density and vessel permeability. Breast cancer is detectable due to the strong enhancement in dynamic breast imaging that peaks early (about 1-2 min.) after contrast medium injection. If breast cancer is suspected, a breast biopsy may be necessary to secure the diagnosis.
See also Magnetic Resonance Imaging MRI, Biopsy and MR Guided Interventions.

Requirements in breast MRI procedures:
Both breasts must be measured without gaps.
Temporal resolution should be sufficient to allow early imaging after contrast agent with dynamic imaging every 60-120 sec.
For the best possible detection of enhancement fat signal should be eliminated either by image subtraction or by spectrally selective fat saturation.
Thin slices are necessary to assure absence of partial volume effects.
Imaging should be performed with a spatial resolution in plane less than 1 mm.

For Ultrasound Imaging (USI) see Breast Ultrasound at

See also the related poll result: 'MRI will have replaced 50% of x-ray exams by'
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 Breast MRI Images T2 And T1  Open this link in a new window
 Breast MRI Images T2 And T1 Pre - Post Contrast  Open this link in a new window
 Breast MRI Images T1 Pre - Post Contrast  Open this link in a new window
Radiology-tip.comMammography,  Breast Imaging
Radiology-tip.comBreast Ultrasound

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• View the NEWS results for 'Breast MRI' (41).Open this link in a new window.
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Chemical Shift ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
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Quick Overview
Please note that there are different common names for this artifact.

Artifact Information
NAME Chemical shift, black boundary, spatial misregistration, relief
DESCRIPTION Black or bright band
REASON Chemical shift, opposed phase image
HELP Fat suppression, smaller water fat shift (high bandwidth), in phase image, SE sequences

During frequency encoding, fat protons precess slower than water protons in the same slice because of their magnetic shielding. Through the difference in resonance frequency between water and fat, protons at the same location are misregistrated (dislocated) by the Fourier transformation, when converting MRI signals from frequency to spatial domain. This chemical shift misregistration cause accentuation of any fat-water interfaces along the frequency axis and may be mistaken for pathology. Where fat and water are in the same location, this artifact can be seen as a bright or dark band at the edge of the anatomy.
Protons in fat and water molecules are separated by a chemical shift of about 3.5 ppm. The actual shift in Hertz (Hz) depends on the magnetic field strength of the magnet being used. Higher field strength increases the misregistration, while in contrast a higher gradient strength has a positive effect. For a 0.3 T system operating at 12.8 MHz the shift will be 44.8 Hz compared with a 223.6 Hz shift for a 1.5 T system operating at 63.9 MHz.

Image Guidance
For artifact reduction helps a smaller water fat shift (higher bandwidth), a higher matrix, an in phase TE or a spin echo technique. Since the misregistration offset is present in the read out axis the patient may be rescanned with this axis parallel to the fat-water interface. Steeper gradient may be employed to reduce the chemical shift offset in mm. Another strategy is to employ specialized pulse sequences such as fat saturation or inversion recovery imaging. Fat suppression techniques eliminate chemical shift artifacts caused by the lack of fat signal.
See also Black Boundary Artifact and Magnetic Resonance Spectroscopy.


• View the DATABASE results for 'Chemical Shift Artifact' (7).Open this link in a new window

Further Reading:
MRI Artifact Gallery
  News & More:
What is chemical shift artefact? Why does it occur? How many Hz at 1.5 T?
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Wednesday, 20 July 2005   by    
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