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Effect of photon energy on X-ray images
I assume that everyone has had at some point the experience where less was more. Especially when dealing with computer based image postproduction. Software makes handy wonderful, or better: powerful, filters. Experienced artists know that only a pinch of something or homeopathy is a key to better results.
The same holds true in X-ray production. A maximum of energy does not provide better images. Let’s look closer at this point.
What is the influence of energy to X-ray images ?
Higher energies in X-rays mean shorter wavelengths and a higher resolution. Therefore it might seem reasonable to increase the energy in our X-ray tubes always to the maximum to produce incredible images based on a maximum resolution.
With four images below I show the influence of increased energy levels on X-ray images of a single rose. The applied energy levels are 40kV, 60kV, 90kV and 109kV. The steps of postproduction were the same in every image. Slight differences are owed to best contrast in each exposure.
Surprisingly to the novice we get an increasing loss of contrast (or less available contrast) in each image with higher energies. This effect of loosing contrast can easily be seen in this series of four X-rays and is highest at 109kV.
Rose digital X-ray photo at 40kV © Julian Köpke 
Rose digital X-ray photo at 90kV © Julian Köpke 
Rose digital X-ray photo at 60kV © Julian Köpke 
Rose digital X-ray photo at 109kV © Julian Köpke The explanation for less available contrast with higher energies is the following physical effect: the more photons have shorter wavelengths the more photons run unaffected through the object down onto the sensor. With all photons running through without any hindrance the sensor would show a homogenous gray value.
Every structure looses contrast when turning to higher energies. The optimum for a structure is found by experience and varies significantly.
In the medical field the applied energy strongly depends on the purpose of the examination and the structural demands to be diagnosed.
The above demonstrated meaningless low contrast for our single rose at 109kV doesn’t hold true at all in radiology. Radiologists use frequently 125kV for a chest film to get reproducibly valuable contrast in most patients.
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Fusion X-ray of tulips
Fusion imaging is beauty made of composite X-ray images and HDR images on a light box. The primary question is what energy fits best for flowers. To my experience 40 kV is often suitable. But: the proof of the pudding is in the eating.
Mammography systems start e.g. from 20 kV and reach 39 kV. The sensor is up to 24cm x 30cm. Conventional systems start from 40 kV and reach 125 kV. The sensor is up to 43cm x 43cm what makes them more attractive to floral compositions.
The higher resolution and the lower energies of a mammography will suit better for transparent objects. But the spatial limit of a composition (which is 24cm x 30cm) might put hard restrictions on the artist.
Floral compositions have more creative space with a bigger sensor. But the X-ray tube starts with 40 kV and this might lead to overexposure of tender structures.
Thus I performed today more than ten compositions to study this relation.
After four exposure of three tulips I found this composition with four dense blossoms attractive to go further. The composition might somehow resemble to a sketch of three angels. The image is nice due to very soft edges of their „wings“, technically blown out portions in the image. The inner structure of the nearly closed blossoms is well resolved. The stalks serve as „body“. There is no advantage with higher energies.
X-ray three tulips © Julian Köpke The same composition was done immediately after the X-ray as a bracketing series on a lightbox. After returning I processed a manual HDR, the colors not to warm.
Three purple tulips HDR photo © Julian Köpke The final fusion image is a composite of the preceding two images. Compared to the lightbox photo, the hidden stalks reappear naturally, the inner petals are outlined like a sketch.
Three purple tulips fusion X-ray photo © Julian Köpke -
X-ray fusion photo of a Nautilus
Fusion imaging can be done retrospective. My split Nautilus shell on a light box rendered with manual HDR shows already a nice structure of the inner parts.
Nautilus shell manual HDR photo on a light box © Julian Köpke The X-ray obtained a couple of days earlier easily fits onto the HDR with not a big deal of processing.
Nautilus X-Ray Energy Compressed © Julian Köpke The meaning of the fusion image may be different to the flowers. But it’s feasible to do it retrospectively.
Nautilus shell fusion X-ray photo and manual HDR photo on a light box. © Julian Köpke -
Nautilus shell X-ray fusion photo of energy levels
Different energies of X-ray radiation mean different transparency of an object. There is an example in my FAQ using a Nautilus shell.
Instead of compressing images of different energies to a single image today I subtracted the 70 kV image of a Nautilus shell from the 40 kV image.
The central parts of the Nautilus shell are more dense and show a significant higher difference. The core of the shell gets shiny. This is how it looks like:
Energy difference X-ray photo of a Nautilus shell. The image is the difference of a 70kV and a 40 kV image. © Julian Köpke In positive X-ray representation you can compare the results. Left hand is the compressed image of 4 different energy levels, right hand the difference image.
Nautilus X-Ray Energy Compressed © Julian Köpke 
Energy difference X-ray photo of a Nautilus shell. The fusion image is the difference of a 70kV and a 40 kV image in X-ray positive representation. © Julian Köpke -
Dahlias fusion X-ray HDR photo
Long time ago my friend Harold and I did these X-rays in my practice. There was so much to do. Today was a chance to process the fusion images. Some details can be found in my FAQs.
The manual HDR is already appealing to our eyes.
Dahlias using manual HDR in visible light There is some charm in the X-ray image of the same composition. The hidden parts of the stalks can be clearly seen.
Five Dahlias X-ray photo © Julian Köpke The fusion image of this composition shows both color and hidden structures.
Dahlias fusion digital X-ray with manual HDR photo in visible light Finished image with a background:
Dahlias fusion digital X-ray and manual HDR photo with background © Julian Köpke -
Sunflower X-ray photos revisited
How to show the sun in the middle of a sunflower ? For astronomers it is quite common to look at the sun in hydrogen alpha light, which is a pure red at 635nm. With artistic eyes, a red center might be overdone.
So I tried two different representations, one in BW that is close to the natural look and feel of a sunflower and one with a light blue in the center as complementary color to the yellow petals.
This sunflower is a fusion photo of X-ray, monochromatic Hα light of the sun converted to BW and a sunflower on a lightbox. © Julian Köpke 
This sunflower is a fusion photo of X-ray, monochromatic Hα light of the sun converted to blue and a sunflower on a lightbox. © Julian Köpke The surface structure of our sun can be seen like astronomers see it.
There is no photo of the next digital X-ray image of a sunflower with its stalk and a leaf:
Digital X-ray photo of a sunflower (inverted representation). © Julian Köpke -
Shells fusion X-ray photo
Long time I dreamed of this fusion image of shells. Because already on a lightbox some of the shells are transparent and have nice colors. I like the shining through effect very much.
The X-ray image is a compromise of structure and density resolution, depending on the maximum energy the mammography system is able to produce.
Today I’m not at all in a stable state due to a recurrent infection. So I allowed me to do this image instead of hard working.
Shells fusion X-ray photo © Julian Köpke It is the light inversion in Lab color mode that shows more of a X-ray look and feel. The colors are pretty close to the bright image.
Shells fusion X-ray photo with inversion of the L-channel © Julian Köpke -
FAQ: X-Ray of flowers
Are you already high-brow ? You don’t want physics, because you didn’t like it at school ? Then take a look at a well understandable FAQ-sheet for x-rays of flowers given by Harold Davis. The doctor advices you to stop reading here !
Those who like some more background may read the following paragraphs.
Our eyes are sensitive to visible light. The wavelengths of visible light range from 400nm to 750 nm. Digital sensors for photography are modified in their sensitivity to gain a pleasing image for human eyes. E.g. we like green tones. A digital sensor for photography can be modified in its sensitivity within the range of visible light and over a wider range of wavelengths than visible light. The energy of visible light ranges between 1.6 eV (750nm) and 3.2 eV (400nm). Typical spatial resolutions of photographic sensors in the consumer section are between 4µm and 8µm.
A digital x-ray sensor works with spatial resolutions between approximately 70µm and 140µm. Using a medical x-ray machine the available energy levels of x-rays depend on the purpose of a human examination. Energy levels of mammography systems vary between approximately 20 keV and 45 keV, depending on manufacturer. Energy levels of conventional x-rays for bones or chest vary between approximately 80 keV and 125 keV. The corresponding wavelengths are under these conditions 0.06nm (20 keV) down to 0.01 nm (125keV).
As you may know, visible light and x-rays are part of the electromagnetic spectrum. Visible light and x-ray differ in their energy. Higher energy of a radiation means higher frequency and shorter wavelengths. Our eyes don’t see other light than visible light. X-rays are a special light then, not to be seen with our eyes – but with a digital sensor.
A substantial property of x-rays is their ability to run through objects with mainly no interaction. The x-ray sensor „sees“ only a small percentage of less radiation coming from the x-ray source when an object is placed near the sensor.
The left hand image appears normal to your eyes when thinking of an x-ray. Before the digital era, radiologists were using films, an analog medium to produce an x-ray. As x-rays run through an object with mainly no interaction, the dark parts of the left image were fully exposed to radiation. A dark part in an x-ray image therefore was called transparent by radiologists. The parts with lighter grey or white in it were called „opaque“ or „dense“ or „attenuated“ areas. The brighter parts result from the attenuation of radiation by an object. As a matter of convenience, digital x-ray images are shown like the left image. You see already details of the inner structure of our flower, a Bird of Paradise.
X-Ray Bird of Paradise © Julian Köpke 
X-Ray (inverted) Bird of Paradise © Julian Köpke The right hand image is an inverted grey scale image. Black turns into white, 50% grey stays unaffected and white turns into black. A 75% grey turns into a 25% white. In every photo editor that’s just a simple and easy action to do. The inverted image is more pleasant to the perceptive habits of our eyes. To our experience, the inverted image is preferable for fusion imaging.
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X-ray fusion images
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