Since Mount St. Helens blew its top in 1980, the U.S. Forest Service and the U. S. Geological Survey have kept a pretty close eye on this particular piece of Washington State. As far as I know, they aren't using X-rays on the volcano, but the images they are producing are still quite wonderful.
The U. S. Forest Service mounted a real-time web cam in September, 2004, aimed right down the throat of the volcano. In June 2007, they added a High Definition webcam to the site, with significantly improved images.
In 1992, Johnathan Lees (now a professor of geolocial sciences at the University of North Carolina, Chapel Hill) obtained tomographic images of Mount St. Helens. Instead of X-rays, he used P-wave seismic tomography to produce 3D velocity maps of the magma system beneath the mountain. Another big imaging difference: pixel size. In medical imaging, an image pixel typically represents a piece of tissue less than a millimeter in width. Lees' pixels were much less wimpy -- the smallest of them represented a chunk of the Earth 0.5 kilometer in width! The image below shows a tomographic slice from a 27.5 x 21 x 20 km target volume. For scale, the above-the-ground profile of the volcano has been drawn in green at the top of the image.
Lees, J.M., The magma system of Mount St. Helens: Non-linear high resolution P-wave tomography, J. Volc. Geoth. Res., 53 (1-4), 103-116 1992.
Since the big boom in 1980, St. Helens has continued to erupt. In the past 3 years, the volcano has progressively added 100 million cubic meters to the lava dome in its crater. However, this latest eruption has been so gradual that normal movies of it would be like watching grass growing, only not as exciting. Instead, the USGS created a time-lapse movie of this eruption from data collected between October 2004 and July 2007. (via Ars Technica).
What's in store for Mount St. Helens next? Will someone figure out a way to use MR or PET on the mountain? Perhaps we can harness gravity waves or neutrinos to create tomograms of the whole darned planet. If so, I'd pay cash money to see that.