Digital Radiography Systems (DRS) M ARIO L ENDIN
FEATURES OF THE DRS IMAGE
After this short and deliberately simple view of the sensors available at present we can now examine the features of the images obtainable through the DRS analyzing them both in absolute terms and, whe- re possible, in relation to conventional radiographic
Fig. 6.9. The incident X rays on the surface of the sensor demarcate the latent image.
Fig. 6.10. Memorising phosphorous function as normal reinforcing screens, but a part of X-ray energy is stored in the phosporous crystals. Stimulated by the laser light the crystals discharge light in function of the memorised energy.
blue phosphor (Denoptix – Gendex) instead of white phosphor. Blue phosphor is more luminous and ena- bles the laser reader to read the latent image better, affecting, then, in the last analysis, the technical featu- res of the image itself. The main advantage, however, is for blue phosphor to be more sensitive to radiation allowing then, a reduction of X-rays.5
It has been observed that X-rays incident on these sensors cause the excitation of the phosphor atoms which can be stimulated and which delineate the la- tent image (Fig. 6.9).
The sensor must then be inserted into the reader, which performs a complete scanning of the whole surface of the sensor. The reading is performed by a very thin laser red beam which releases the latent energy by stimulating the phosphor atoms to emit either a white or a blue luminosity, depending on the quality of the sensitive material (Fig. 6.10).
The amount of released energy is measured at eve- ry pixel. At this stage an analog-digital converter co- mes into play and converts the electrical signal resul- ting in a digital signal. The converter sends the digi- tal signal to the computer through a connection cable and an interface electronic card. The actual acquisi-
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images.6 Usually the image acquired by the DRS sen- sor is digitalized (Tab. I, II, III and IV) under the form of 8-bit data. An 8-bit image has 256 levels of lumino- sity from 0 (black) to 255 (white). The contrast reso-
lution is measured as differences of level of gray, or pixel values, on a scale of grays of logarithm or semi- logarithm default.8
Table I
Requirements of a good sensor
– High sensitivity to X-rays – High resolution capacity
– Maximum depth of color (a high number of levels of gray)
– Compatibility with existing radiological systems – Acceptable costs
– Duration of sensor
Table III
Digitalization of an analog signal
Image luminosity and its sampling
Digital transformation of luminosity
The sampled value is calculated on the mean point of intervals.
Table II
Digitalization progress
– Consists of sampling a signal which varies conti- nuously at constant intervals
– One will obtain a series of whole numbers which approximate the starting function and allow its re- construction
– This process, then, represents the transformation
of an analog function into a more or less long se- quence of discrete values.
Table IV
Digitalization
– In the digitalization process not only is the sam- pling frequency important, but also the amplitude of the digitalized signal.
– This value represents the level of gray which is as- signed to the point under examination.
– We can think of digital images as of a set of ele-
ments memorized in a numerical form, so as to
maintain the biunique correspondence between the image and the element which represents it.
Reading and processing of DRS images
The factors which may affect the ease of reading of both a traditional and a digital radiographic image could be defined as intrinsic and extrinsic to the cha- racteristics of the image itself.
Among extrinsic factors we have to consider the pro- perties of the means we use for viewing the image. In the case of a traditional image the X-ray exami- nation table must feature a white, homogeneous and sufficiently powerful light.
Table V
Digitalization process
– In the image digitalization process a biunique cor-
respondence between the original image and its digitalized form is maintained.
– Xi = X/px , where px stands for the size of the pixel.
– Each element of the matrix contains the “depth” of the point as a level of gray.
Whereas in the case of a digital image the technical features of the monitor are very important. Particular attention must, then, be paid to resolution, definition, chromatic output, but also, not a minor detail, to ima- ge stability; a flickering screen or continuous and sud- den changes of luminosity and contrast will fatigue our visual ability and distract our attention from the particulars of interest.
The intrinsic factors of the image which affect the ea- se of reading are mostly its size; also the image defi- nition is very important of course. This, however, is a parameter which, having its own absolute specific im- portance, will be analyzed at a later stage.
are strictly linked with carrying out the investigation (emission time of X-rays and their quality and quan- tity) and the transition of developing and fixing (con- centration, effectiveness and temperatures of liquids, absence of luminous pollution, adequate washing and drying procedures).
Fig. 6.11. Screened X-ray examination table and optical magnification sy- stems.
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Digital radiographic images are, on the contrary, cha- racterized by the possibility of modifying luminosity and contrast in a continuative way, with no limits of range and in a fully reversible way, even though they maintain unaltered their basic properties (resolution and definition).31
This option, managed through a graphic processing program (one of the software components of DRS) al- lows an easy and fast manipulation of the image, op- timizing its final output.7
There are, in addition, some more functions offered by the graphic processing programs. These functions allow:
– image enlargement, enhancing in this way impor- tant details;
– to see the image as a negative through a complete inversion of chromatic components;
– to modify the histogram of the levels of gray to im- prove the dynamics of the image;
– to apply a three-dimensional filter to the image to obtain a “bas relief” effect (Fig. 6.13).
present and to take length measurements with a pre- cision up to 1/10 mm.14
This is also possible on curved stretches by approxi- mation with a line of measurement subdivided in mo- re segments angled among themselves even with de- gree fractions. It is obvious that the more detailed and close to reality the computer reading scales are, the more effective the interpretation may be (Fig. 6.15).
Fig. 6.13. Image processing with 3-D effect application.
Fig. 6.14. The software of the image graphic management can process it with different “effects”.
Fig. 6.15. Length and angle measurements may be useful for a preventive ap- praisal of the level of difficulty of the planned endodontic treatment.
These and other functions are meant to offer the professional more reading scales so as to identify each time the most effective one in terms of global comprehension of the image (Fig. 6.14).24,25
Other interesting options are the ones which allow the computer - basing itself on pre-set reading scales - to carry out a rough analysis of the bone density of the sectors investigated by the radiovideographic image, interpolating the quality of the tones of gray there
In this case the following are particularly important: not only the monitor, but also the computer features, then the quality of the graphic card (which physical- ly allows the computer to acquire and process any type of image), the type of processor (the computer “brains”), the size of the memory on the fixed disk (“the store”) and the so called RAM memory (which allows the computer to keep programs and images “alive” and active). All these components, affecting not only the image quality, but also the speed and the efficiency of the processing, are so strictly correlated that the inadequacy of a single one of these may de- termine a remarkable worsening of the quality of the end product.32
Acquisition Sensors
The two main groups of DRS acquisition sensors and their respective features have already been described. Needless to say that this component of the system has a decisive influence on the definition quality of the acquired image.3 It is, however, also necessary to em- phasize that sensors, in particular silicon cell ones, ha- ve seen a very rapid evolution, both on terms of phy- sical size and on terms of quantity and quality of the acquisition points, total surface being the same. It is likely that this rapid technical progress is far from en- ding and that there may be important improvements in methodology also in the short term.
I would like, anyway, to quote some important pa- rameters which directly affect the definition of the fi- nal image.
1) The resolution of an “imaging” system is defined by its ability to “resolve” different density structu- res placed one next to the other.
It is usually measured through a “grid test” which consists of a series of lead bars alternate with spa- ces subdivided into groups of decreasing amplitu- de. In each group, the full and the empty ones are equally spaced.
The whole set of a bar and a space is defined as a
Fig. 6.16. The set of a bar and a space is defined as a “pair of lines”.
Fig. 6.17. Examples of a subsampling. If the acquisition system does not featu- re sufficient resolution, the resulting image will not be sufficiently defined.
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ve all, to the possibility of magnifying smaller and smaller details keeping high the quality itself of the magnified image.11
2) The signal-to-noise ratio (SNR = signal-to-noise ra- tio) (Fig. 6.18) quantifies the performance of the system relating the technical qualities of the sen- sors (both DRS and traditional) - in keeping with their characteristic “background noise” - to quality and quantity of X-rays necessary to obtain a good image.
For simplification’s sake, one can say that the greater the disturbance (background noise) of the sensor or devices to it correlated (for instance: la- ser readers or electronic cards), the greater will have to be the quantity of X-rays necessary to ou- tdo them and obtain a good image (Fig. 6.19). A system which may boast a high SNR will then
be able to supply images with a better definition, X-ray dosages being the same.
3) The frequency interval of data collection (or dy- namic range) is the parameter which quantifies the ability to collect faithfully small details without the “luminous tail” effects, that is the quantity and quality of the images obtainable through the sy- stem.
Also in this case a better technical setting corre- sponds to a high value. In particular the ability to acquire images both with a low and a high radiant energy level of an equal quality standard, with the reduction of over- and underexposure risks of the images themselves, corresponds to a wide spectrum of dynamic range.
Software
The programs which run the transition of information from the sensor to the computer memory and which, then, in the last analysis, control first the production and, then, the management, processing and filing of the image, should hardly affect the determination of definition, meant as amount of dots for a given sur- face.2,26
Software should be, then, particularly in the “produc- tion” stage of the image, only and exclusively as a means, an intermediary between the sensor percei- ving the object, as it is drawn by the X-rays, and hard- ware which then physically shows it.
Actually software – particularly in DRS with silicon cell sensors – still plays an important role at this sta- ge.
One can imagine the sensitive surface of a silicon cell sensor subdivided by a sort of grid into a number of
Fig. 6.18. Image quality is measured not so much by noise but by the ratio between the useful signal and the noise itself. SNR = SIGNAL TO NOISE RATIO
B A
Fig. 6.19. Images with a good contrast can be interpreted with greater confi- dence.
facturing faults (here manufacturers must take action for the accuracy of their quality control) and wear and tear as well as fatigue.
At this stage of the process software comes into play and reads each pair of neighboring dots, gives them a value, corresponding to a scale of “grays”, and fills the empty space with a few dots with an intermediate va- lue between the two. In short, what we could see at the end of the process is an image which is artificial in a small part: this fact shows as an insufficient defi- nition of the image itself.
The above-mentioned process was particularly inci- dental with the first generation DRS.
The technical evolution of the sensors has allowed to reduce the distance between each pair of dots more and more and to have an accurate check on the cri- tical phases of production. The last generation DRS – equipped with silicon cell sensors – supply images with a technical definition close to the one of traditio- nal radiographic images. The DRS images, however, present final reading features which can nearly over- lap.13
A different consideration must be made for DRS which use excitable phosphor sensors: in this case the ba- sic acquisition units feature a molecular size as in tra-
logical evolution of the systems based on this type of sensors allows today to obtain DRS images which are both strictly faithful to reality and with a definition which can practically overlap the one given by tradi- tional X-rays (Fig. 6.20).