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NEXT ROOST NEXT PAGE UP CATEGORY PREVIOUS PAGE PREVIOUS ROOST Carl's Roost:
Getting the most from MRI
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To get the most out of the Magnetic Resonance Images of the egg, you need to have a strategy. Here are some ideas:


Get your bearings - scout the territory

The first step is to get the "lay of the land". Professional radiologic technologists (who do MRI on humans every day) do this by obtaining scout images first. A good start is to do one each, of side, front and top views of the egg. Use "t1_contrast" and a 4mm slice (relatively thick). You may want to do the top view at a slice offset of +15-20mm (near the top of the egg).


Plan your study

1) Note down the field of view (mm) in the scout images (above), and use that number with the scale on the images to determine slice positions for detailed examination. For example, if the field of view is 56mm (a good choice), and there are 8 scale divisions in each direction, then each division is 7 mm (7x8=56).

2) Now choose the views and slice offsets (in mm) you will concentrate on. For example, suppose there is an interesting structure in the "front" view, about 2.5 divisions left of the center. You will want to do a "side" view, with slice offset 18mm (2.5 x 7 = 17.5), and maybe a slice on either side of this ("forward" or "back", I.e. 16, 18 , 20mm). Remember that the maximum offset is limited to 8 times the thickness. So you'll want to use at least a 3mm slice for this example.


Carry out the study plan

Be patient and thorough. Start at one end of your range (16mm in our example). Repeat that image until you get a good one. Note the frame number. Then advance to the next one in the series, and so on. You can review and analyze the images after your experiment time is up, or have a classmate do the review while you work on the acquisition.

Coordinate your work with others in your class. If two people independently request acquisitions within a few seconds (without each waiting for the other to finish), the results may be useless for both students.

Note: Professional scientists and engineers usually work as a team, rather than individually. Each one has a specific set of jobs to do during the project. This helps them stay out of each other's way, and makes much better use of limited resources such as observing time.



Review your results

Use the MRI Database Table to get a list of all the images from your session (print it out and keep it in your notebook for reference). Mark the most interesting images. Use the MRI Database to retrieve these as a group. Print them out or look at them carefully (save to disk on your computer for faster retrieval).

Try to identify points of intersection between perpendicular planes, or structures that continue from one plane to the next, in a series. Look for "landmarks". Compare with similar views from previous days.


Getting good images

You'll want to get the best images you can. While it is tempting to decrease the field of view and slice thickness to get high resolution, you may have noticed that this doesn't work very well. Here are some recommendations:

Field-of-view: stay above 50 mm. The resolution for 50mm fov is about 200 micrometers in the plane.

T1_contrast: use 4mm slices or thicker.
T2_contrast: use 2mm slices (up to 16mm offset).

Making changes: Don't change things too rapidly. A 10-20% change in field of view makes a big difference in the appearance of the image. Don't type in random numbers - that's just wasting your time.


Reasons for bad images:

Field of view too small. Causes "wrap-around" and "snowy" images.

Slice too thin. Causes "snowy" images. Slice position may be unexpected if offset is more than 8 times the thickness!

Motion. Causes "blurring" or vertical streaks. Repeat the image as necessary (use the "Reacquire" link for this purpose).

Conflicting commands. If two or more requests are received at the same time, the system may switch between views during the acquisition. The result can look like a "double-exposure", or like motion.

Wrong slice position. This one is obvious!

C.D.Gregory: April 22, 1996



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