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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 -
FAQ: High Dynamic Range
X-rays were initially used for research in atomic physics and medical diagnostics and therapy. Their ability to reveal structures inside an object even with an opaque surface was the driving feature of scientific and technical development of X-rays. Nowadays, beside its proven medical usefulness, X-rays are used to examine technical structures and there are telescopes to map X-rays from our Galaxy and the universe. Every radiological technician who starts in its profession learns to do X-rays of common structures like flowers, animals or teddy bears.
In the digital era, X-ray images are obtained using sensors, while film was used historically. The sensors in the medical radiological field have dimensions such as 24cm x 30cm or 43cm x 43cm. The corresponding spatial resolution for these sensors is between 70µm and 140µm. A typical high-end camera used by a professional or advanced amateur photographer might have a pixel resolution between 4 and 8 µm. Therefore, photographers might well wonder if there is any precise imaging possible with such pixel size. Let’s look at this a little more closely.
X-rays, like visible light, can be characterized by their energy or wavelength. Shorter wavelengths correspond to higher energy. The capability to penetrate an opaque structure increases with energy. If you think of a photon as a particle, smaller particles with higher energies penetrate an object more easily. An overview of this relationship is given in the table shown here:
To make this more clear, here is a series of X-Ray images with increasing energy. The first image was obtained with 40 kV which corresponds to a wavelength of 0.031nm. Our eyes are only able to see wavelengths between 400nm (blue) and 750 nm (red). Therefore, photons with a wavelength at 40 kV cannot be seen with the naked eye. The peripheral parts of the Nautilus shell are clearly depicted. A photographer would classify the circle at the center of the shell as „blown out“. In fact, they are not blown out. The radiation is not able to resolve the structure, because the wavelength of the X-rays is too long in this case to penetrate the shell.
40kV 10mAs 0.031nm Let’s go to shorter wavelengths (implying higher energies). Using 50 kV or a wavelength of 0.024nm gives more structure to the central parts. The photographic impression of a „blown out“ center is reduced. However, looking at the peripheral parts of the Nautilus there is a loss of intensity and a more grayish impression. It is conceivable that this might be regarded as an overall acceptable but subtle effect.
50kV 2mAs 0.024nm To take this further, we can go up to 60 kV or down to 0.0207nm. The center is now close to perfectly „exposed“ with some detail apparent, although some smaller structures are still not resolved. The intensity loss at the peripheral parts increases and is now pronounced. A photographer would clearly regard the periphery as „underexposed“.
60kV 2mAs 0.0207nm The last example of this direction of higher energy and shorter wavelengths is 70 kV or a wavelength of 0.0177nm. The photons wavelength is now 57% compared to 40 kV. You may think of this as „smaller“ photons. The result is a clearly depicted core with a complete loss of peripheral structure. A photographer would have every reason to be worried about „underexposure“ and loss of detail everywhere but the center.
70kV 10mAs 0.0177nm What we’ve seen here is that the capability of X-rays to penetrate an object and to go through an object is dependent on energy levels. With shorter wavelengths X-rays go through an object without disturbance but our sensor is „blown out“ at the peripheral parts of the Nautilus shell. Using 70 kV the central parts are much better resolved, but the periphery is too dark. The energy to use for any X-ray image therefore often depends on the primary goal of which portion of the subject is most important to capture.
In a certain sense, the compositing of images with different energies into one image can be compared to the HDR process of photography in the field of visible light. Whereby in visible light the intensity plays the essential role and not the light energy, which determines the color. When X-raying an object that requires different energy levels for accurate representation, I would therefore also speak of a High Dynamic Range image.
If combined, the four exposures shown provide a beautiful, nearly weightless image:
Nautilus X-Ray Energy Compressed © Julian Köpke -
Transparency and Energy in X-Rays
You always need some time to find out the best exposure values for a photo. Same idea holds in X-Ray imaging.
Today I did an x-ray series with my biggest Nautilus shell on a conventional radiography sensor, not a film. Starting from the lowest possible value 40kV an increment of 10 kV up to 70 kV can be seen in the images:
40kV 10mAs Black regions in the image a transparent, white are opaque. The center of the Nautilus has a loss of structure.
With 50 kV the structure in the center of the Nautilus is better depicted wheras the edge gets more transparent:
50kV 2mAs Same effect for the center and the edge can be seen with 60 kV:
60kV 2mAs With 70 kV it’s an exaggeration for the edge and best depiction for the center:
70kV 10mAs Higher kV means more transparency for denser structures but a loss of structure in transparent areas.
At fixed energy, X-Ray imaging behaves like a shadow related to visible light. When photographing, there is not chance to look through an opaque object. With higher energies, x-rays go through opaque objects and can be collected on a sensor.
Composing the images obtained at different energies is an X-Ray HDR image:
Nautilus X-Ray Energy HDR © Julian Köpke The representation of an X-Ray with white on black is a reminiscence of the film era. Radiologists just looked at the negatives ! Inverting black and white shows the positive image, like a print. Here I show the same image as positive, but rotated and flipped horizontally. Look how ethereal it appears now:
Nautilus X-Ray Energy Compressed © Julian Köpke -
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