Digital X-ray Detector Control
Rob van den Berg
(Philips Medical Systems)
Traditionally X-ray devices used photographic film to capture images. Nowadays
X-ray machines are using digital devices (e.g. CCD) for capturing images.
These devices are used to perform examinations using a lower X-ray dose,
as well as to offer easier archiving and off-line processing than traditional
photographic film material. However to be able to use such a device, software
is needed to:
The task of this assignment is to design a framework of classes to support
these requirements. The remainder of this document will give some more
detail on the task. Figure 1 gives an idea on how such a system might look
control the settings of the Digital Imaging Device (DID) and
readout the DID and process the resulting image
- Detector Imaging Device: consists of Detector and other hardware and
software to embed it into an XRS
used for the detection of X-rays, part of a DID
X-ray system; the composite of all parts that make up an X-ray system,
e.g. DID, table, X-ray Generator
The task is to design a framework of classes that will handle:
The resulting framework will enable developers of a specific X-ray machine
to integrate a DID into this machine. The responsibility of the XRS is
to control the DID. The DID itself is responsible for the technical control
of a device. The XRS is responsible for medical application knowledge/control:
the XRS will select parameter values for e.g. a colon examination, whereas
the DID will interpret these in values needed to control the device. No
medical knowledge is embedded in the DID. The remaining sections will detail
the above listed requirements
control of the DID: e.g.. select the right configuration, handle start/stop
image readout: read the values of the detector pixels
image pre-processing: remove detector artefacts (non-linearity, pixel defects
from the image)
1. DID Configuration
An X-ray detector can be set in a number of modes. These modes can be characterized
by the following parameters:
The software should select the right detector mode, requested by the control
system of the X-ray modality, as well as retrieve the right set of calibrations
needed for the Image Processing. Also the right correction Algorithms and
Algorithm parameters will be selected.
Image size: in pixels * pixels (e.g. 1K by 1K)
Frame speed: in number per second
pixel binning: one can combine the result of multiple pixels into 1. (e.g.
a bin of 2x2 pixels)
2. DID Control
The detector needs to be read-out with a fixed frequency. When acquiring
images, this frequency is equal to the frame speed, otherwise the detector
will be in idle mode, in which it still needs to read out every n mSec.
Mode changes have to be synchronized with the read-out request: A 'Change
Mode' command will be send during read-out, after which the detector will
switch to the new mode.
If the detector is idle for a number of minutes, the subsystem will execute
calibration: the dark offset will be calculated. The dark offset is the
pixel value if the Detector is not radiated. In an ideal detector
equal to 0. The result of this calibration needs to be stored as it will
be used for Image Processing.
to comply with radiation regulations it is imperative that the X-ray generator
can only fire when the detector is ready (every radiation should result
in an image). The sequence of events leading to an image look as follows:
DID is configured according to the XRS' request,
the DID informs XRS that it is ready for Xray,
and starts reading out the detector.
The XRS will start radiating when it has received an OK signal from all
involved subsystems (Xray generator, geometry etc.).
3. Image Pre-processing
Images generated by the detector have to be corrected before being displayed
to the radiologist. As these images will be used in an Operating Room situation,
the calculations have to be in real-time. Therefore, the correction algorithms
are designed in such a way to allow processing on a multi-processor machine.
Some algorithms can be calculated on parts of the image (noise reduction,
pixel defect interpolation) usually in strips. Other algorithms can only
be done on the full image (rotation, flip and tilt). The diagram below
depicts how such a pipeline could look like. In this case all the algorithms
can be done on a strip basis.
As mentioned in 1. Prior to radiation, the XRS informs the DID of a
mode change. The required calibration data needed for the image processing
is loaded in memory from disk. The detector is properly set-up after this
step has been finished. However, ultimately the choice of algorithm is
determined by the information contained in the image header.
Further Image handling will be done by the XRS.
Last updated by Torsten Layda,
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