 | Info
Sheets |
| | | | | | | | | | | | | | | | | | | | | | |
 | Out-
side |
| | | | |
|
| | | | |
Result : Searchterm 'Inhomogeneity' found in 2 terms [ ] and 19 definitions [ ]
| | 1 - 5 (of 21) nextResult Pages : [1] [2 3 4 5] | |  | |
 |
Searchterm 'Inhomogeneity'
was also found in the following service of MR-TIP.com: |
|
| | | | | | |
 |
MRI Resources |
|
| | | | | |  |
| Field Inhomogeneity Artifact |   |
| |
|
Quick Overview
|
|
A disturbance of the field homogeneity, because of magnetic material (inside or outside the patient), technical problems or scanning at the edge of the field.
When images were obtained in a progression from the center to the edge of the coil, the homogeneity of the field observed by the imaged volume, changes when the distance from the center of the volume increase.
The same problem appears by scanning at a distance from the isocenter in left-right direction or too large field of view.
There are different types of bad image quality, the images are noisy, distorted or the fat suppression doesn't work because of badly set shim currents.
E.g. by using an IR sequence, changes in the T1 recovery rates of the tissues are involved. The inversion time at the center of the imaged volume is appropriate to suppress fat, but at the edge of the coil the same inversion time is sufficient to suppress water. Since the inversion time is not changed, the T1 recovery rates will increase.
Image Guidance
Take a smaller imaging volume (and for fat suppression a volume shimming), take care that the imaged region is at the center of the coil and that no magnetic material is inside the imaging volume. |
| | | |
 Further Reading: |
Basics:
|
|
News & More:
| | | | |
| |
|
Searchterm 'Inhomogeneity'
was also found in the following service of MR-TIP.com: |
|
| | | |
|
|  |
| Metal Artifact | |
| |
|
Quick Overview
|
|
Ferromagnetic metal will cause a magnetic field inhomogeneity, which in turn causes a local signal void, often accompanied by an area of high signal intensity, as well as a distortion of the image.
They create their own magnetic field and dramatically alter precession frequencies of protons in the adjacent tissues. Tissues adjacent to ferromagnetic components become influenced by the induced magnetic field of the metal hardware rather than the parent field and, therefore, either fail to precess or do so at a different frequency and hence do not generate useful signal. Two components contribute to susceptibility artifact, induced magnetism in the ferromagnetic component itself and induced magnetism in protons adjacent to the component.
Artifacts from metal may have varied appearances on MRI scans due to different type of metal or configuration of the piece of metal.
The biocompatibility of metallic alloys, stainless steel, cobalt chrome and titanium alloy is based on the presence of a constituent element within the alloy that has the ability to form an adherent oxide coating that is stable, chemically inert and hence biocompatible. In relation to imaging titanium alloys are less ferromagnetic than both cobalt and stainless steel, induce less susceptibility artifact and result in less marked image degradation.
Image Guidance
Remove the metal when possible or take a not so sensitive sequence (a SE or another sequence with a rephasing 180° pulse).
See also Susceptibility Artifact. |
| | | |
| Further Reading: |
News & More:
|
| | |
| |
|
MRI Resources |
|
| | | | | | |
| Adiabatic Radio Frequency Pulses | |
| |
|
| Adiabatic RF pulses are amplitude and frequency modulated pulses that are insensitive to the effects of B1-inhomogeneity and frequency offset (conventional RF pulses used in MRI are only amplitude modulated). Due to an extended application time adiabatic RF pulses are mostly used in NMR imaging applications. |
| | | |
| Further Reading: |
Basics:
|
| | |
| |
|
Searchterm 'Inhomogeneity'
was also found in the following service of MR-TIP.com: |
|
| | | |
|
| |
| Coherent Gradient Echo | |
| |
|
Coherent gradient echo sequences can measure the free induction decay (FID), generated just after each excitation pulse or the echo formed prior to the next pulse. Coherent gradient echo sequences are very sensitive to magnetic field inhomogeneity.
An alternative to spoiling is to incorporate residual transverse magnetization directly into the longitudinal steady state.
These GRE sequences use a refocusing gradient in the phase encoding direction during the end module to maximize remaining transverse (xy) magnetization at the time when the next excitation is due, while the other two gradients are, in any case, balanced.
When the next excitation pulse is sent into the system with an opposed phase, it tilts the magnetization in the -a direction. As a result the z-magnetization is again partly tilted into the xy-plane, while the remaining xy-magnetization is tilted partly into the z-direction.
A fully refocused sequence with a properly selected and uniform f would yield higher signal, especially for tissues with long T2 relaxation times (high water content) so it is used in angiographic, myelographic or arthrographic examinations and is used for T2* weighting.
The repetition time for this sequence has to be short. With short TR, coherent GE is also useable for breath hold and 3D technique. If the repetition time is about 200 msec there's no difference between spoiled or unspoiled GE. T1 weighting is better with spoiled techniques.
The common types include GRASS, FISP, FAST, and FFE.
The T2* component decreases with long TR and short TE. The T1 time is controlled by flip angle. The common TR is less than 50 ms and the common TE less than 15 ms
Other types have stronger T2 dependence but lower SNR. They include SSFP, CE-FAST, PSIF, and CE-FFE-T2.
Examples of fully refocused FID sequences are TrueFISP, bFFE and bTFE.
|
| | | |
| Further Reading: |
Basics:
|
| | |
| |
|
|
MRI Resources |
|
| | | | | | | 1 - 5 (of 21) nextResult Pages : [1] [2 3 4 5] |
| |
|
| |
 | Look
Ups |
| |
bottom
(.pdf)
Random Page
top