skin deep?

That's what us guys design ultrasound machines for :-)

And you think an x-ray is going to let you differentiate between the various types of soft tissue?

Scanning confocal microscopes can get images from a millimetre below the skin.

Frequency doubled fluorescence microscopy does at least as well - you focus a lot of photons at twice the wavelength that you want to excite at the point you want to image, and rely on two-photon absorbtion to excite the fluorescence you want to detect.

The wikipeia article sees it as an alternative to confocal microscopy, but you can use two-photon excitation in a confocal microscope.

-- Bill Sloman, Nijmegen

And very neatly too. I had a suspect freckle excised. Before removal an ultrasound scan was done. You could see the boundary between the suspect tissue and normal skin at

CAT scans do just that... 2d cross-section images.

John

X-ray absorption is approximately proportional to the square of the atomic number. There just is not much to be found under the skin near the Z = 20 of calcium. There is not enough potassium with Z=19. Iron in blood has Z = 28 but is only a small portion of hemoglobin. Iodine, often used as an x-ray contrast medium. is also scarcer in the body.

Bill

The more I drink, the more plausible this article gets!

Most people probably don't recall the discussion in physics class about how we humans see just a tiny portion of the EM spectrum and hence what's opaque in the visible light range can be anything but at other frequencies. (And as Joerg points out, there's also sound waves to consider for "probing" purposes.)

That being said, it sounds like the Bay Today article is about a guy who likely is just playing with smoke and mirrors.

There certainly are some cool systems out there to let one "see" through ordinary walls, but the ones I'm familiar with are still a ways from highly-detailed "X-ray vision" and cost a bundle besides. E.g.,

This is a very cool coffee-table book on the topic:

---Joel

One might try ultrasonics...ping and listen; build a picture and catch the fish.

As has been said, there's more than one way to see through soft tissues. But resolution is never quite the same as with visible light which would be ideal, yes?

Mark L. Fergerson

The problem is focus. Hold a page of text up at normal reading distance and focus on a few words. Note that background and peripheral objects, although noticeable, are out of focus. Now focus on a background object and note that the writing on the paper is no longer in focus. All lenses, including those in your eyes, have limited distances or ranges in which two objects on different planes (focal planes) may be simultaneously in focus. The solution has been MRI.

That was not was asked.

Bill

Was not was haven't made an album in ten years.

Indeed, looks quite nicely done, well-written and illustrated, and quite readable -- except doesn't seem to go into important modern laser-based techniques (e.g., no index entry for confocal microscopy).

For me at least, a more comprehensive, up to date version might be worth buying at $40; current version would have to be

Except infinite depth-of-field lenses (e.g. Frazier).

Tim

-- Deep Friar: a very philosophical monk. Website:

Ultrasound machine had dynamic focusing since the 70's :-)

What this means is "on-the-fly" focusing, like a lens that bends to the correct shape while the echoes are coming back from deeper and deeper regions. Easy on receive, but for transmit you need to do several shots and stitch the resulting horizontal image slices together. That's a whole science unto itself but nowadays very much standard procedure.

Yes, I wouldn't doubt it. I have no medical degree and very little knowledge in that field, my only use of ultrasound has been in an electronics cleaning bath. My experience with MRI is a yearly check up when I'm injected with something that makes me feel hot and want to pee! Thanks for the info.

Yearly? Wow! I was never in an MRI, so far. One reason why ultrasound is preferred is that MRI is hugely expensive while an ultrasound scan is typically reimbursed at the two-digit Dollar level. MRI is usually four-digit.

The underlying reason is equipment cost. A good MRI machine costs millions while a decent ultrasound scanner can be had for under $50k.

There are lots of solutions to this. In optical microscopy, you can take images at several depths and combine them, choosing the level that shows the most contrast at each position. Zeiss has been selling systems like that for years.

In confocal microscopy(*), you can just sum the images taken from different depths, since out-of-focus planes hardly contribute to the image at all.

There's also phase-coded imaging, which uses an artistically designed phase plate to allow 3D reconstruction from a single image. (It's really a beautiful technique, which I'd like to understand better than I do.)

So the old classical optics limitations are being overcome all over the place.

Cheers

Phil Hobbs

(*)My thesis was on a phase sensitive laser scanning confocal microscope, and I helped invent the modern real-time confocal.