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| Result: Searchterm 'Signa'
found in 78 messages |
| Result Pages: 1 2 3 4 5 6 7 8 9 10 11 12 13 [14] 15 16 |
More Results:  Database (382) News Service (60) Resources (16) |
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peter hobden
Wed. 14 Feb.07,
15:35
[Reply (1 of 3) to:
'Geometric distortion'
started by: 'ergun ahunbay'
on Tue. 2 Jan.07]
 Category:
Artifacts
|
| Geometric distortion |
Dear Listers,
I have some questions about the image distortions in MRI:
1) Are the magnetic field inhomogeneities the only cause of image distortions? Does RF-inhomogeneities cause distortions or just signal inaccuracies?
There are other factors that can affect the /amount/ of distortion. Run an EPI sequence and change the bandwidth and see the image change shape.
There are other causes of image distortion which are not necessarily directly related to field homogeneity.
It is a complicated question you are asking here!
2) Is it accurate to say that a spiral sequence would not have distortion due to magnetic field inhomogeneity(, but it would have blurring). Would there be image geometric distortions with a spiral imaging?
I guess you are talking about spiral filling of k-space?? The sampling time reduction will help to reduce distortion to some extent.
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Tian Ming
Mon. 22 Jan.07,
10:23
[Start of:
'MRI Marker'
1 Reply]
Category:
Applications and Examinations
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| MRI Marker |
Are there any MRI markers that are visible on all sequences, small, flat, cheap and self-adhesive?
Vitamin E or fish oil capsules work mostly well, but the signal is suppressed on sequences with fat saturation.
To mark a small region they are too big and
dent the skin where fixed.
Thankful for every suggestion and friendly greetings to all.
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ergun ahunbay
Tue. 2 Jan.07,
19:20
[Start of:
'Geometric distortion'
2 Replies]
Category:
Artifacts
|
| Geometric distortion |
Dear Listers,
I have some questions about the image distortions in MRI:
1) Are the magnetic field inhomogeneities the only cause of image distortions? Does RF-inhomogeneities cause distortions or just signal inaccuracies?
2) Is it accurate to say that a spiral sequence would not have distortion due to magnetic field inhomogeneity(, but it would have blurring). Would there be image geometric distortions with a spiral imaging?
thank you very much in advance
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Reader Mail
Mon. 30 Oct.06,
16:25
[Reply (1 of 2) to:
'interleaved slices and mean intensity difference in adjacent slices'
started by: 'Reader Mail'
on Wed. 4 Oct.06]
Category:
General
|
| interleaved slices and mean intensity difference in adjacent slices |
With interleaved slice acquisition for example in each TR first the odd slices, and then the even slices are acquired. Differences in signal intensity may occur by changes between these acquisitions (like movement), especially with a long TR. In particular, for fMRI sequences with interleaved slice acquisition a slice time correction is recommended to get accurate measurements.
However, I think "large" signal intensity differences with interleaved slices (except fMRI) are rarely seen with usual sequences and sufficient processing (interpolation) algorithms.
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Jennifer Jackson
Fri. 21 Jul.06,
04:44
[Reply (1 of 2) to:
'Phase encoding'
started by: 'guillaume bierry'
on Fri. 7 Jul.06]
Category:
General
|
| Phase encoding |
Wow this is a long subject, First you have to under stand how the 3 main gradients are used. (slice select, Freq, then phase)
So you are use 1 of the gradients for your phase encoding. You need to fill the k-space which depends on you matrix. Lets assume you are using 256 (phase). In order to fill K-space you have to have 256 aquistions of the signal. Meaning for each picture (or slice) the signal must be acquire(sampled) 256 times each at a differnt amplitude to put it in a different Line of k-space.
Example- If you are performing a SE(spin echo) you have 1 TR period and 1 TE period (TE=is when the signal is sampled) so you will have to do the TR period 256 times to fill K-space with diffent amplitudes.
There is alot more that I left out. If this did not answer your question let me know
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