In the classical transmission radiographic arrangement the investigated specimen is penetrated by a collimated beam of radiation modifying its properties (intensity, particle composition, energy spectrum, direction of propagation, polarization …). The radiation imaging detector is placed behind a specimen recording the modified beam properties. Quality of the irradiating beam together with the performance of radiation imaging detector is crucial for amount of information about structure of the investigated specimen which can be retrieved.

Radiogram of a hand of Mrs. Anna Bertha Roentgen taken by William Conrad Roentgen in 1895.

X-ray transmission radiography

The basic principle of X-ray transmission radiography consists in recording of modifications in intensity of X-ray beam penetrating an investigated specimen. Typical X-ray sources are X-ray tubes (providing polychromatic radiation), more rarely synchrotron facilities or X-ray lasers (providing monochromatic radiation). However, if the inner structure of a pictured sample shows a similar value of the absorption coefficient within the whole volume (e.g. soft tissues) then this technique can be not sufficient. In such a case it is recommended to adopt a different method e.g. Phase Enhanced Imaging or to use a different radiation type e.g. slow neutrons.

Experimental setup for high resolution X-ray radiography with microfocus or nanofocus X-ray tube. Projection enlargement is determined by object position.

There are two main approaches in the field of imaging detectors with the digital output in the field of transmission radiography:

Charge-integrating devices

Ionizing radiation creates a charge (ussualy idirectly) which is collected and integrated in analog memories (capacitors, potential wells) of pixels (CCD, CMOS sensors, Flat panels …).

Particle-counting pixel detectors

Ionizing particle creates a charge in a sensitive volume which is compared with a threshold and digital counter is incremented.

Both approaches offer advantages and disadvantages making them suitable for different application fields. Generally speaking: charge-integrating devices are good for measurements with high radiation intensity. If radiation intensity is lower then long exposure time is required for statistical noise suppression. With long exposure time the contribution of the useful signal can be low in comparison with the integral of the so called dark current which is always present in this type of devices. Therefore, the maximal signal to noise ratio (SNR) is limited.

In cases when the maximum count rate is lower than about 106 counts per pixel per second the particle-counting pixel detectors offer better results than charge-integrating devices. All false signals (leakage current and noise) are separated from the useful signal by threshold settings so that there is no false counting.  Thus, dynamic range of pixel detectors is in fact unlimited and it is possible to reach almost arbitrary SNR just by exposure time prolongation. Moreover response of particle-counting devices is perfectly linear (twice intensity - twice counts).

The maximal count rate of 106 counts per pixel per second is more than sufficient for most radiographic applications. Moreover, new designs of pixel detectors such as Timepix offer an ability of CCD like operation with a digital integration of charge quanta which have higher energy then certain threshold level. There is be no dead time present in this mode and the limit of the maximal detectable intensity is eliminated.

X-ray transmission radiography with pixel detectors

A pixel detector of Medipix type used in X-ray radiography with polychromatic radiation must be first calibrated. The reason is that a detection efficiency of individual pixels is never uniform and so is their energy dependence. Such non-uniformities incorporate a fix-pattern-noise into raw images.

X-ray transmission radiography of a daisy blossom is particularly difficult due to low X-ray absorption.
Raw image is damaged by pixel efficiency non uniformity. Source: Microfocus Tungsten X-ray tube at 40kV.

The simplest method suppressing image distortion due to pixel efficiency non uniformity is based on using a matrix of multiplicative coefficients characterizing the relative efficiency of each pixel to the efficiency of an average pixel, sc. flat-field correction. A raw image of an unknown sample can be corrected by multiplication of the measured pixel value and its respective correction coefficient to produce the corrected image (see left part of following figure).

The attenuation of X-ray beam intensity in a material depends on the beam energy as well as on the material properties. Therefore, in a case of polychromatic irradiation, each energy fraction of the incident beam is attenuated with different attenuation coefficient. Generally it can be stated that harder components of spectrum are attenuated less then softer components. This effect is often called beam hardening (BH). Detection efficiency of individual pixels of a detector is uniquely dependent on energy, therefore it is dependent also on sample absorption. As a result an image distortion occurs. By careful calibration of response of each individual pixel it is possible to correct data and suppress the noise as demonstrated in following figure.

Left: Image corrected by flat-field correction technique. Fix pattern noise is reduced but not fully.
Right: Per pixel BH corrected image. Constant pattern noise is completely removed

The correcting method as shown above works well. The noise remaining in corrected image is caused only by fluctuations of number of trapped photons which follows the Poisson statistics. This noise can be reduced by using a more intensive source of radiation or by exposition time prolongation. This way it is possible to obtain in principle arbitrary signal to noise ratio (SNR) and reach almost arbitrary contrast in taken images. Example of such extreme contrast is shown in following figure.

Mouse backbone and pelvis

In vivo X-ray radiograph of a mouse pelvis and surrounding soft tissue structures demonstrates extremely high contrast of the corrected images taken by Medipix2 (with microfocus tungsten X-ray tube at 50 kV).

Even structure of overlaying hair (circular inset) is seen through full thickness of mouse body!

Apparatus

We have built two systems for X-ray transmission microradiography with the state-of-the-art semiconductor hybrid pixel detector Medipix2 (512 x 512 pixels with 55 um pitch) and a commercial micro- and nano-focus X-ray tubes (Hamamatsu and FeinFocus) placed inside of shielded cabinets.


Medipix2/Timepix detectorMedipix2 device
Experimental setup for high resolution X-ray radioscopy and tomography consisting of Microfocus X-ray tube (5um spot size); sample holder with adjustable temperature stabilization, 3-fold translation and rotation; motorized revolver wheels with calibrating foils; pixel detector Medipix2 with 2-fold translation. Spatial resolution of the system is about 3 um. Silicon pixel detector Medipix2 (single). In the X-ray radiographic setup we  use a quad version of this device (4 devices tiled together, 30 x 30 mm active area). Device consists of two chips connected by bomp-bonding technique. The upper chip is pixelated semiconductor detector (ussualy Silicon). The bottom chip is ASIC read-out containing matrix of 256 x 256 of preamplifiers comparators and counters.

X-ray setup with nanofocus tube
Experimental setup for very high resolution X-ray radioscopy and tomography consisting of Nanofocus X-ray tube FeinFocus (0.5um spot size); sample holder with adjustable temperature stabilization, 4-fold translation and rotation; motorized revolver wheels with calibrating foils; pixel detector Medipix2 with 2-fold translation. Spatial resolution of the system is better than 0.5 um.