X-Ray Tomography

Autoradiography is a methodology used for different applications, especially in the biological ones. Its aim is to detect spatial distribution of a radioisotope and calculate its concentration in a particular region of the sample. Since the beginning of the last century autoradiography is known as a research method. After a long period characterized by conventional techniques based on photographic principles and screen films, in recent years it evolved towards digital acquisition, together with the development of new materials as scintillators and semiconductors.
Today, conventional autoradiography is still the most used technique, because of its practicality, lower costs and best resolution.The disadvantage related with this traditional technique is the very long time requested for an acceptable autoradiographic representation (up to several hours). The main task of the new research in digital autoradiography is the analysis in presence of low quantities of radioactivity and faster image acquisition time.
In particular this feature is very important in dynamic studies with radiotracers in biological tissues. The opportunity to reach this goal is now possible with the use of a new kind of image intensifier – the electron bombarded CCD.
The aim of our work is to develop a system for digital autoradiography based on this new device. Preliminary tests with our equipment have shown that real time imaging with commonly used radiotracers is feasible.

The Autoradiographic set-up

The autoradiographic set-up used for our experiment is quite simple. A special made fiber optics plate with a GOS layer deposited on the surface constitutes the sensitive part of the detector.
It consists in a 5 mm thick disk with a diameter of about 80 mm. Beta particles emitted by the radioisotopes located in the sample interact with the scintillating layer producing a very weak, visible light signal. To get such a weak signal from the back side of the plate an image intensifier is required. This is the most important part of our detector. A big electron bombarded CCD (EBCCD) camera has been used. The photocathode of the intensified camera has been put just in contact with the fiber optics plate, as in Figure.

Figure (right): (a) Fiber optics plate with GOS, (b) sample, (c) photocathode, (d) HV tube, (e) CCD, (f) electronics.

Images of the neutron activated sheet of gold taken in different dates.

Decay curve measured at different positions on the sheet of gold.

Real time autoradiography

A very interesting result has been achieved with our system: real time autoradiography of carbon-14 labeled radiotracers has been made. This is mainly due to the system capability to detect single beta particles. In fact the detector has a very low noise and the weak signal of carbon-14 beta can be detected. It is possible to reveal the shape of a radiotracer drop put on a millipore filter in only about 1 s, including readout time (Fig. 5(a)).
With the integration of some images (an average of 8, 16 or more frames) a very clear picture of the drop border has been obtained (Fig. 5(b)). A study on this sample has been made showing a diffusion phenomenon on the time scale of hours, as reported in the picture sequence below.


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