Talk:Magnetic resonance imaging: Difference between revisions
imported>David E. Volk (Main difference MRI/NMR sentence is not true) |
imported>David E. Volk m (→Gradients) |
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== Gradients == | == Gradients == | ||
The following | The following quoted sentence is not true: | ||
:"The main difference between MR spectroscopy and MR imaging (the N is often dropped to avoid confusion with nuclear energy) is that the static magnetic field used in the former is supplemented in the latter by space-encoding magnetic field gradients which allow to combine the chemical information (or parts thereof) with spatial information to generate isotope-specific images." | :"The main difference between MR spectroscopy and MR imaging (the N is often dropped to avoid confusion with nuclear energy) is that the static magnetic field used in the former is supplemented in the latter by space-encoding magnetic field gradients which allow to combine the chemical information (or parts thereof) with spatial information to generate isotope-specific images." | ||
NMR Spectroscopy uses space encoding gradient fields, sometimes in only the Z-axis, but also often in the X, Y and Z-axes to defocus and refocus magnetization. In fact, the normal room temperature shims are in fact space-encoding gradients, the first is linear in Z-axis height, the second is quadratic in Z-axis, the third is cubic, etc. The non-spin shims are in the X & Y direction. So, we need to find a better way to state this sentence or remove it. [[User:David E. Volk|David E. Volk]] 02:36, 1 November 2009 (UTC) | NMR Spectroscopy uses space encoding gradient fields, sometimes in only the Z-axis, but also often in the X, Y and Z-axes to defocus and refocus magnetization. In fact, the normal room temperature shims are in fact space-encoding gradients, the first is linear in Z-axis height, the second is quadratic in Z-axis, the third is cubic, etc. The non-spin shims are in the X & Y direction. So, we need to find a better way to state this sentence or remove it. [[User:David E. Volk|David E. Volk]] 02:36, 1 November 2009 (UTC) | ||
Latest revision as of 20:36, 31 October 2009
neuroimaging
I added this red link (as an uncreated article - is it the best term?) - I won't be creating it though. If anyone else wants to... --Matt Lewis 00:44, 31 March 2008 (CDT)
The piece of the pictue that I miss
Your section on how various weightings and gradients return information of different brighness or color is very helpful. What still puzzles me about MRI, vs. CT ad SPECT, is that in the latter, there is a narrow collimated beam that is "spread" over a limited angular range--i.e., tomography. I have at least a conceptual grasp of how the image is built from the set of tomographic images.
As far as I know, magnetic and RF fields cannot be collimated to anywhere close to the precision of collimated ionizing ratioactive. The thing I don't yet grasp in MRI is how the image is formed form the individual images. Can you help define that, preferably comparing it with SPECT and CT? Howard C. Berkowitz 03:46, 29 July 2008 (CDT)
- By using gradient excitation fields in 2 or more directions, you can effectively wipe out all signals except for those in a small region, say a 1 mm squared point where the X-axis and Y-axis radio frequencies meet. Then, by collecting data for many small regions, overlapped a bit, you can build up the image. The greatest natural contrast occurs between aqueous and lipidic regions, because water moves very fast and large lipids move very slowly, contributing to very different relaxation rates. Contrasts can be artificially increased by using specially labeled compounds, which for example, might bind to certain receptors, or nerves, and so on. David E. Volk 23:11, 29 July 2008 (CDT)
Gradients
The following quoted sentence is not true:
- "The main difference between MR spectroscopy and MR imaging (the N is often dropped to avoid confusion with nuclear energy) is that the static magnetic field used in the former is supplemented in the latter by space-encoding magnetic field gradients which allow to combine the chemical information (or parts thereof) with spatial information to generate isotope-specific images."
NMR Spectroscopy uses space encoding gradient fields, sometimes in only the Z-axis, but also often in the X, Y and Z-axes to defocus and refocus magnetization. In fact, the normal room temperature shims are in fact space-encoding gradients, the first is linear in Z-axis height, the second is quadratic in Z-axis, the third is cubic, etc. The non-spin shims are in the X & Y direction. So, we need to find a better way to state this sentence or remove it. David E. Volk 02:36, 1 November 2009 (UTC)