Discussion of using an MRI to measure the size of a tumor

After having a second MRI performed on me and the Radiologist's report that the schwannoma had shrunk, I started to think about whether this statement could be made with 100% certainty given the method chosen to view the schwannoma.  For those of you who work with me in Optical Proximity Correction, you will see why it is not hard to question this statement as the sampling is not so much different from OPC or ORC.

In a conventional xray film imaging, the photons are shot through the body and collected on a film. The film is really just a macro-scopic photon counter. The more photons received, the darker the film. Your bone structure is an excellent blocker of xray photons, and as a result, the bones "appear" white on these films. The important aspect here is that by its very nature, given the quality of absorption or blocking of xray photons, the maximum extent of the bone is shown.

This sketch can help describe what I mean:

In this sketch the xray photons are shown in red and blue arrows traveling from left to right. The oval in the middle is blocking the "blue" xray photons from reaching the film. The red xray photons reach the film without interference. If this were real, where the red photons hit the film, the film would be black, but where the blue photons were blocked, the film would be white. The dotted green arrow could then give you an accurate length of the oval. Note, though this is highly 2 dimensional in nature and would not be able to give you any information on the width or depth of the oval. The xrays would have to come in from multiple directions in order to extract that information. Presumably this is what a CAT scan is doing.

However this technique is not very useful if you need to probe the interior of the oval.

The MRI is working on a different principle. Instead of an electron - photon scattering principle shown above, it is looking for a resonance of  the nucleus of an atom .  To a physicist, MRI is just a political spin term. The real term is Nuclear Magnetic Resonance. But Nuclear is thought of negatively, hence the replacement name of Magnetic Resonance Imaging, which is not as precise, as there is also Electron Spin Resonance based on the same principles but looking at electrons instead of  the nucleus of an atom (not as useful for medical imaging though). In any case, just google on nmr and you can learn more than you ever wanted to know about this.

I will, however, start some gross simplifications on what is going on. As the image is acquired, it is acquired in a 3 dimensional space (vs the 2 dimensional space of a conventional xray).  When you look at a series of  MRI images, each image represents a 2 dimensional view at a specific third dimension coordinate. Or a series of planes separated by some amount.

You could think if it this way

Here, the images are gathered along the red lines and the radiologist is presented with 7 slices along the artifact. While there must be some "depth" each slice, I think we can visualize them as infinitely thin. While this is much much more powerful than the older conventional xray, it is by no means perfect. It has its limitations as well

In particular, since it is only taking a finite number of infinitely thin slices, information can be missed in the areas not sliced.

Imagine for a moment that you had a way to capture without loss the full schwannoma. And then measured its diameter in the x and y plane as a function of its position in the  z plane. It might look something like this:

(This is actually just y=sin(x) from 0 to 180 degrees)

Now imagine that the MRI sampled in its finite method only a fraction of this curve (shown in the red points):

In this case we were lucky and hit the peak diameter.

But what if  we shifted where the MRI sampled?

If we only sampled on the blue points and not the red points, we would miss the maximum and the measurement of the size of the schwannoma would not be accurate, and therefore saying that the schwannoma grew or shrunk may also not be accurate. My schwannoma only shows up in three images in a series, indicating a coarse sampling. Of course there are other types of slices taken,  some alone a different z-axis, and perhaps in the hands of someone qualified a more accurate reading of the size can be given.

This then brings up the following discussion. I can understand, how on an initial MRI when one is not sure what one is looking for, doing a coarse sampling in the z-axis. However, once something like my schwannoma is found, why, on follow up MRIs, when the technician injects the contrast enhancing dye, and sees the image come up on the screen, the stepping in the z-axis is not taken down to the 3 dimensional pixel size of each data point or even lower?

Back to schwannoma home page

Back to snowedunder home page

This page last updated on Feb 26 at 10:31 am MST

Copyright 2006 - D. Samuels