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Result: Searchterm 'Frequency'
found in 14 messages |
Result Pages: 1 [2] 3 |
More Results: Database (251) News Service (11) Resources (6) |
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Jill Paplow
Tue. 12 Dec.17, 23:01
[Start of: 'Content specifications for ARRT' 0 Reply]
Category:
Applications and Examinations |
Content specifications for ARRT |
In the ARRT content specifications under Image production, radiofrequency...it lists pulse profile. What is this referring to?
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meike carr
Mon. 23 Oct.17, 15:41
[Reply (2 of 3) to: 'GE 8 channel Body Coil Artifact' started by: 'Arnold Somereville' on Sun. 23 Apr.17]
Category:
Artifacts |
GE 8 channel Body Coil Artifact |
It is hard to see the exact artifact in this picture. It almost looks like wrap to me, but again hard to determine with pic provided. What else has been done to fix this issue? Have you changed the frequency direction or opened up your FOV?
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Martin Rosellen
Wed. 19 Oct.16, 15:30
[Start of: 'k space explanation' 2 Replies]
Category:
General |
k space explanation |
dear all,
I am trying to get a feel for what k-space is. So far I have not got an explanation that I understand. My idea to picture the frequencies that get emmitted from a slice as follows: - We have a box filled with still waterrn - a set of objects get dropped into the water simultaneously - every object emmits waves that are specific to its shape. To fill k space, what is measured and how? All I know is that the sampling frequency stays the same. Sorry for the ugly formatting. Did not find help on that. Thanks Martin
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Kerstin Amort
Thu. 9 Aug.12, 09:47
[Start of: 'Flip angle' 0 Reply]
Category:
Basics and Physics |
Flip angle |
A very basic question about flip angle:
How can I calculate the duration of the RF pulse when flip angle (90°) and radiofrequency (1 mT) are given?
Unfortunately I do not have an equation that fits perfectly...
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Steven Ford
Tue. 31 Jan.12, 08:19
[Reply (1 of 2) to: 'RF shimming' started by: 'Reader Mail' on Thu. 1 Oct.09]
Category:
Basics and Physics |
RF shimming |
For Magnetic fields, the overall field is adjusted to push it up a little bit in one spot and push it down a little bit in another area. The goal is to create a field that's perfectly homogenous.
The RF field created by the transmit coil likewise must be as homogenous as possible, so that the flip angle is constant throughout the imaging volume. In the past, designers have solved this problem by building coils such as the 'birdcage' style that would create a very even amount of energy inside. This is one reason why the transmit coils tend to be large.
With the advent of 3 Tesla and stronger magnets, the RF resonant frequency also rises. RF energy absorbed in the patient rises with the higher frequencies also, and another problem raises its head: it's a lot harder to make a very homogenous RF field. Even if you are scanning phantoms, the inside tends to be subject to different energy than the edges.
But in the human body, there are all sorts of irregular lumps and bumps that absorb RF differently, further complicating matters.
Now, on modern scanners it's possible to perform a magnetic field shim with the patient actually in the magnet in order to compensate for minute changes in the magnet from one exam to another. For super-high field magnets, an RF shim is also a handy thing to do.
If you have a Multi element RF transmit coil (regular phased array coils are just for receiving) you can run a program which selectively turns up the power in some elements so that the overall signal received is maximized. That's an RF shim.
Steven Ford
Professional Imaging Services, Inc.
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