Experimental Setup


Experimental Test Setup

Our digital X-ray detector unit is based on a 1024 x 1024 pixels Charge Coupled Device (CCD) optically coupled via a Fiber Optic Plate (FOP) to a structured CsI(Tl) scintillator.

The choice of the CCD (Philips - FTT1010) has been mainly driven by the ability of the detector to operate at room temperature with no cooling with a very low dark current, efficient read-out noise suppression and appropriate full-well capacity.

In the picture on the right , the CCD is shown (top view) with the fiber optic plate mounted on it.

The scintillator (visible as a white square in the picture) is sealed with a (X-ray transparent) protective film which also covers the sides of the fiber optic plate.

The CCD is an MPP device with a very low dark current (10 pA / cm^2 @ 20° C). Although capable of providing 15 frames per second in frame-trasfer-mode with an internal 10 ms gate, the CCD allows a full-frame mode operation with an external gate. As an X-ray detector we operate the device in the latter mode and accumulate the charge in the CCD pixels during an integration time provided by an external gate (from several hundreds ms up to 1 s).

The pixel size is 12 micron. The quantum well capacity is close to 2 10^5 electrons, which allows 12 bit digitization with a sensitivity of about 50 electrons/ADU.

An advanced read-out system (based on an enhanced DCS technique) provides an extremely low read-out noise.

Our digital X-ray detector prototype is shown (on the left) mounted on its read-out board.

The fiber-optic plate is visible at the bottom of the picture standing up as a "pillar" from the CCD ( its real height is only 25 mm, though) .

The interface board to the frame grabber is also visible on the top left corner of the picture

The CCD and the electronics are housed into a box protected by an Al-Pb-Al cover during operation under the X-ray unit.

The metallic case containing the detector is shown in its real size in the picture on the left.

The computer monitor, visible in the picture on the left, displays a digital radiographic picture - taken by the detector - of a lead bar pattern.

The lines are getting thinner and closer to one another moving to the right of the picture.

The thin vertical slit visible on the right marks the end of the bar battern.

The maximum density provided by the above bar pattern was only 5 lp/mm. As the resolution of the system is almost three times better than that , the lines are well resolved.

Our test setup (on the left) during the assembly around a standard mammographic X-ray unit.

The collimator mask (above the breast phantom) and the anti-scatter grid (below) can be manually aligned with micrometers attached to precision linear stages.

The detector (hardly visible at the bottom of the picture) can be positioned in XYZ by means of 3 manual translation stages. It can be aligned with good accuracy and its vertical distance from the breast phantom can be varied during the measurements.

A precision XY translation stage, with optical feed-back on the actual position and driven by two step motors, allows a complete scan of the breast phantom under computer control.

The collimator and anti-scatter masks are visible in the picture below. To ensure the correct tolerances at the corners of the square holes the two masks have been cut by a laser under computer control.