FR3121233A1 - Optical system for reading radiochromic films and method of operation thereof. - Google Patents

Abstract

Optical system for reading radiochromic films and method of operation thereof. The invention relates to an optical system (1) for reading a radiochromic film, comprising: - a light source (2) comprising at least one light-emitting diode (LED) arranged to illuminate the radiochromic film, the wavelength d the emission of the LED diode being adapted to correspond to that of the maximum absorption peak of the radiochromic film, - a bi-telecentric lens (3) arranged to receive the optical beam emitted by the diode(s) (LED) and having passed through the radiochromic film, - an imager (4) optically coupled to the bi-telecentric lens. Figure for abstract: Fig. 1

Description

Optical system for reading radiochromic films and method of operation thereof.

The present invention relates to the general field of metrology of ionizing radiation.

It relates more particularly to the reading of radiochromic films. A radiochromic film is a 2-dimensional dosimeter considered as an ideal detector with properties allowing it to be used for any type of application involving ionizing radiation. In practice, a radiochromic film is composed of monomers which, under the action of ionizing radiation, will polymerize and create a darkening of the film, directly correlated to the amount of radiation to which it has been subjected.

Ionizing radiation is now widely used in many applications, such as in the medical field for example.

Monitoring the radiation dose delivered to patients during these medical procedures is of great importance. More particularly, the delivery of the dose during an external radiotherapy treatment must be controlled beforehand in a personalized manner.

This verification is particularly critical in radiotherapy techniques using small radiation fields and high dose levels.

To do this, a measurement using a matrix of detectors in a phantom simulating the patient is systematically carried out and compared with the provisional calculation carried out upstream as part of a treatment plan proposed by the medical staff.

Another approach consists in directly obtaining the two-dimensional map of the dose distribution from the irradiation of a radiochromic film.

Currently, the most commonly used radiochromic films are of the GAFchromic™ brand, and in particular the EBT3 and EBT-XD models. The use of this dosimeter is nevertheless under-exploited today. Quantification of absorbance change with dose must be reliable and accurate, which is problematic when using commercial scanners as 2D image densitometers.

Radiochromic films are currently read with commercial photographic scanners such as those marketed under the names Epson® 10000XL, 11000XL, 12000XL, V700, V750Pro or V800.

A large number of scientific publications have highlighted the difficulties of use, reading artifacts and the inadequacy of the flatbed photographic scanner to obtain reliable dosimetry and ensure the metrological traceability required for all other dosimetric detectors: [1] , [2], [3], [4], [5], [6].

These scanners were originally developed for photographic applications and are not well suited for scientific applications.

The associated software is a kind of black box that often hides useful information about the digital processing applied to the images for better visual rendering.

Moreover, these optical systems, each comprising a photographic scanner and its associated software, are not measuring instruments and are therefore not suitable for scientific applications.

The existing optical systems for reading radiochromic films can essentially be classified into three categories:

• X-ray microdensitometer systems with CCD camera (acronym Anglo-Saxon for “ Charge - Coupled Device ”), as described in patent US5623139;
• flatbed scanners;
• laser densitometry systems (LDS acronym for “ Laser Densitometry System ”).

Patent US9128053B2 and the related publication [6] describe such an LDS system which consists of the use of a laser coupled to a photodiode to perform point-by-point scanning of the radiochromic film.

The inventors have made an inventory of the main characteristics displayed and their drawbacks, summarized in the following table 1.

[Table 1]: Category of existing optical systems for reading radiochromic films Features X-ray microdensitometer with CCD camera according to US5623139 Flatbed scanners LDS laser densitometers according to [6] Light source spectrum Red light emitting at the absorption peak of the radiochromic film White light (continuous spectrum or spectral line) Laser emitting at 635nm Homogeneity of film illumination Continuous lighting by a number of 60 low intensity LEDs (50 mcd / LED) Very inhomogeneous due to the light source used, the optics and the scanning. Partial film illumination, scanned in two dimensions by moving it under the laser spot Sensor size Low resolution CCD sensor (375x 242 pixels), with 23 x 27 µm pixels. Linear color CCD sensor Size of a photodiode with pinhole collimator optical filter No Filters integrated in CCD sensor and very broad in wavelength (red, green and blue). Not optimized with respect to the absorption peak of radiochromic films. Cooling Yes No No Dynamic range in the image 16 bit A/D converter for camera 16 bit ADC per channel. Rather 8-9 bits usable in practice due to the sensor used and the electronics. 12 bit Polarization Use of mirrors, modifying the polarization according to the angle of incidence.
Sensitivity of the sensor depends on the incidence angle of the incoming light Due to the polarization of the laser light, the measured response of the film turns out to be orientation-dependent: care must be taken to strictly maintain the same alignment for the calibration and application films (angle dependence) Stability and reproducibility Significant drift over time.
Uncontrolled internal recalibration process Stability of laser beam and photodiode current Resolution Low Important in theory, but limited in practice due to the scanning, optics and sensor used.
Artificial high resolution by software processing of pixel overlap and most scanners have rectangular pixels. The software tries to compensate for this but accuracy is lost.
Only a part of the pixels are used in practice when extracting data from the red color channel Resolution up to 34 µm in theory, but it already takes 2 min to scan a 5x5 cm² film with a resolution of 352 µm Optical defects Geometric image distortions Possible chromatic and geometric distortions related to the scanning phenomenon during digitization and the optics used.
Sensor saturation Spots in laser light create small, non-uniform, time-varying spots of light that are uncontrolled and can introduce noise into
the absorption reading at the level of the photo sensor. Exposure time Greater than 100ms Managed internally by the scanner, risk of overexposure and saturation of the sensor, which prevents the measurement of optical densities. 6 ms per measurement point (i.e. 2 min to acquire 142 x 142 measurement points, without counting the travel times of the spot).
This integration time is necessary to saturate each pixel. This time limits the image resolution achievable with this system. Laser dot size and smudges are an issue. Acquisition time Approximately 1 min per film depending on the chosen resolution and the need to scan the film several times The 2D scan is long: 2 min to scan a 5x5cm² film with a resolution of 352 µm Sensitivity to the T° of the radiochromic film No thermal regulation Long term overheating. No thermal regulation Localized overheating at the laser point.
The densitometer is placed in a thermal enclosure. Image acquisition No Obligation to use the acquisition software supplied with the scanner. No control or information about corrections, interpolations and calibrations applied to images All image parameters are controllable. Point-by-point image reconstruction

If we are specifically interested in laser densitometry systems, the high time required to acquire an image (2 min for a 5x5cm² film and a resolution of 72dpi) makes it difficult to transpose it to a hospital environment for routine clinical use. The use of a laser brings up issues of speckle, shape and overlapping of the spot.

Scanning increases the acquisition area but causes overlapping and mosaic problems. Scanning during digitization can also generate geometric distortions. Lasers are more expensive and light intensities are more difficult to control. It is difficult to ensure uniform illumination of the film when scanning the area.

There is therefore a need to improve optical systems for reading radiochromic films, in particular in order to overcome the drawbacks of X-ray microdensitometers with CCD camera, flat optical scanners and existing laser densitometers.

The general object of the invention is then to meet this need at least in part.

To do this, the invention firstly relates to an optical system for reading a radiochromic film, comprising:

- a light source comprising at least one light-emitting diode (LED) arranged to illuminate the radiochromic film, the emission wavelength of each LED diode being adapted to correspond to that of the absorption peak(s) ( s) principal(ux) of sensitivity of the radiochromic film,

- a bi-telecentric lens arranged to receive the optical beam emitted by the diode(s) (LED) and having passed through the radiochromic film,

- an imager optically coupled to the bi-telecentric lens.

By "imager" is meant here and in the context of the invention, the usual technological meaning, namely any device with at least one photographic sensor which converts electromagnetic radiation from at least one LED into an analog electrical signal. This signal is then amplified, then digitized by an analog-digital converter and finally processed to obtain a digital image. An imager according to the invention can be at least one photographic sensor implementing any technology. It can be a CCD sensor (English acronym for “Charge-Coupled Device”) or a CMOS sensor (English acronym for “Complementary Metal-Oxide-Semiconductor”).

Advantageously, the imager is an sCMOS (acronym for “scientific Complementary Metal–Oxide–Semiconductor”) imager.

Preferably, the light source comprises at least one matrix of LED diodes. The light source can have high homogeneity and high intensity, typically up to 35500 mcd per LED array. In addition, one or more, in particular three, diffusing glass plates is (are) arranged above the LEDs to eliminate the hot spots of the lights emitted by the individual LEDs. The diffusing glass plates homogenize the lights into a single emission profile.

Preferably again, each LED diode has an emission length in the red, providing a quasi-monochromatic light which guarantees with the bi-telecentric lens the absence of geometric and chromatic distortions. In particular, the surface treatment of the bi-telecentric lens avoids chromatic aberrations, i.e. focusing at different distances depending on the wavelength.

The system comprising at least one suitable current controller for each row of the LED array and a sufficient voltage supply to make the currents constant for the entire LED array. Thus the output light is constant for all the LED matrices of the light source. Advantageously, the LEDs can be mounted in parallel with, for each row, a current controller which makes it possible to absorb voltage variations such that at the output, the intensity is constant for each row, and therefore for each LED of this row. . In addition, putting one controller per row makes it possible to have a more uniform intensity of all the LEDs.

According to an advantageous embodiment, the imager is a monochrome camera comprising sCMOS sensors.

According to this mode, and an advantageous embodiment variant, a microlens is arranged in front of each pixel of an sCMOS sensor of the camera. With microlenses, the effects of optical crosstalk are minimized.

According to an advantageous configuration, the optical system comprises:

- a support transparent to the wavelength of the diode(s) (LED), the support being adapted to support the radiochromic film and arranged to allow the optical beam emitted by the diode(s) (LED) to pass ,

- a compression plate with an optical index close to that of the radiochromic film, the plate being mounted to be applied against the support so as to flatten the radiochromic film.

The compression plate, in particular made of anti-Newton glass, makes it possible to guarantee the flatness of the radiochromic film during the image. By "anti-Newton glass" is meant here and in the context of the invention a frosted glass whose surface is not perfectly smooth so as to leave a presence of air between the compression plate and the radiochromic film, preventing the formation of interference fringes called Newton's rings.

According to another advantageous embodiment, the system comprises an enclosure impervious to external light in which are housed at least the light source, the bi-telecentric lens, and where appropriate the support with the compression plate.

Preferably, the walls of the enclosure are made of thermally insulating material.

The system includes a thermal control device for controlling the temperature of the light source.

The thermal regulation device is advantageously a heat exchanger connected to a circulation thermostat. Such a device makes it possible to obtain chromatic and thermal stability of the light source.

The invention also has a method of operating the optical system as described previously, according to which the piloting of the LED diode(s) is independent of that of the camera.

According to an advantageous embodiment, the method comprises the following steps:

- control of the LED diode(s) in pulsed mode,

- acquisition of images by the camera while the radiochromic film is illuminated by the LED diode(s).

The film is digitized with an exposure time of a few ms and an average can be obtained by acquiring several images at a high frame rate.

In practice, the exposure time is managed by the user so as not to saturate the sCMOS sensor, in order to be able to measure optical densities. Indeed, a saturation of the sensor does not make it possible to know the quantity of incident light.

The global shutter of the sCMOS sensor needs external light control. In the context of the invention, two main modes of shuttering of the sCMOS sensor can be envisaged:

• the first consists in carrying out a line reading but by ensuring that each line does not have the same quantity of light received,
• the second is that the entire sensor surface is exposed and read simultaneously. This second mode requires flash type lighting and protection against external light. If the flash dominates the exposure and the external light is suppressed, each pixel receives the same amount of light. The reading per line as well as the different exposure times of the lines no longer have any importance, since the external light no longer penetrates.

The inventors believe that this second mode of synchronizing the light source with the flash sCMOS sensor is preferable, with the following additional advantages:

• the fact of generating pulses of light makes it possible to dissipate less heat than in the case of a continuous emission of light. Cooling is therefore easier to achieve,
• the lifetime of the light source is increased because it operates only a few ms per frame.

According to an advantageous variant embodiment, the acquisition step comprises the acquisition of correction images called offset, flat and dark (respectively “offset”, “flat” and “dark”).

According to another advantageous embodiment variant, the method comprises a pre-processing step by acquiring correction images in order to obtain reference images called master images of the film before irradiation and after irradiation.

Preferably, the method comprises a processing of the master images of the film comprising the following steps:

- conversion into optical density of the pixel values of the master images of the film before irradiation and after irradiation,

- correction of the optical density of the master images of the film before irradiation and after irradiation by means of calibrated neutral filters so as to obtain the images of the film calibrated in optical density, before irradiation and after irradiation,

- sub-pixel registration of the master images of the film before irradiation and after irradiation,

- subtraction of the two images of the film, calibrated in optical density before and after irradiation, taking into account the image of the film before irradiation as background noise,

- application of a calibration curve to images established from films, irradiated at known doses traceable to established primary references, so as to obtain the dosimetry of the film.

Any organization with appropriate irradiation means and calibrated dosimetric measurement equipment can produce such a calibration curve for radiochromic films.

It is the realization of a calibration curve of the optical density according to the dose absorbed in a phantom, generally in water or equivalent under conditions in which the dose can be traced metrologically, and its application to the subsequent measurements carried out with films from the same manufacturing batch, which makes it possible to obtain dosimetric information from the films.

Thus, the invention essentially consists of an optical system comprising a uniform LED source, an imager, advantageously an sCMOS imager, preferably monochrome, coupled to a bi-telecentric optical lens.

The use of a bi-telecentric lens makes the imager insensitive to film polarization.

The method of operation of the system allows the determination of the dose after irradiation of the film by measuring the two-dimensional optical density on the film.

The use of calibrated neutral filters makes it possible to restore metrological traceability in optical density compared to flatbed scanners according to the state of the art. In other words, the invention allows reliable measurements of the optical density of radiochromic films.

By construction, flatbed optical scanners according to the state of the art do not allow reliable measurements of optical densities for various reasons, including the use of incident white light, the inhomogeneity of the light source, the saturation of the sensor and internal recalibration.

By using dose measurements traceable to primary references established by an organization, as well as calibrated neutral filters, the system according to the invention is a fully metrologically traceable instrument whose performance can be quantified in terms of uncertainties. Its performance greatly exceeds that of state-of-the-art optical flatbed scanners.

The advantages of the invention are numerous, among which we can cite:

• a dedicated scientific instrument, sized for reading radiochromic films, in order to ultimately have reliable dosimetric information, while obtaining double metrological traceability of the optical density and the radiation dose,
• the implementation of the optical components of the system, whose characteristics can be controlled precisely and independently, makes it possible to avoid reading artifacts known with existing flatbed scanners, with rigorously qualified metrological performance,
• the system is large enough to reproduce the full size of a 6x6 cm² film, or even greater dimensions if the support is movable on two axes, the sCMOS imager having a resolution carefully matched to the objective to provide a pixel size ideal on film for capturing fine detail in the exposed pattern,
• a very stable instrument thanks to the addition of current controllers and reproducible by the periodic calibration of the system in optical density,
• the independent control of each optical component of the system allows greater flexibility for the acquisition (image sequences, exposure time control) and image processing,
• a real quantification of the uncertainties on the result is achievable, which is currently lacking with flatbed scanner systems according to the state of the art.

The invention can be considered for all applications and industrial fields implementing 2D dose measurements in medical applications of ionizing radiation, as well as for the estimation of the dose to patients for the characterization of the radiation field.

The invention can even be implemented to reconstruct a dose in 3D, by stacking several radiochromic films alternately with slices so as to reconstitute a phantom, the reconstruction taking place by interpolation between the slices.

For example, a linear accelerator, X-ray generators or isotopic sources can generate these radiation fields not exclusively.

A particularly interesting application of the invention is for medical physicists in radiotherapy or radiology departments and for any producer of apparatus and instruments in the medical field.

Other advantages and characteristics will emerge better on reading the detailed description, given by way of illustration and not limitation, with reference to the following figures.

the is a perspective and partially exploded view of an optical system for reading radiochromic films according to the invention.

the is a longitudinal sectional view of part of the optical system according to the .

the is a block diagram of the protocol for preprocessing the images obtained using an optical system according to the invention.

the is a block diagram of the sequence of the process for processing images obtained using an optical system according to the invention.

the is an illustration of a radiochromic film irradiated by a beam of photons from an optical system according to the invention.

Claims (16)

Optical system (1) for reading a radiochromic film, comprising:

• a light source (2) comprising at least one light-emitting diode (LED) arranged to illuminate the radiochromic film, the emission wavelength of each LED diode being adapted to correspond to that of the peak(s) of main sensitivity absorption(s) of the radiochromic film,
• a bi-telecentric lens (3) arranged to receive the optical beam emitted by the diode(s) (LED) and having passed through the radiochromic film,
• an imager (4) optically coupled to the bi-telecentric lens.

Optical system (1) according to claim 1, the light source comprising at least one matrix of LED diodes. Optical system (1) according to Claim 1 or 2, each LED diode having an emission length in the red. An optical system (1) according to claim 2, comprising, at least one current controller suitable for each row of the LED array and a voltage supply sufficient to make the supply currents constant for any LED array. Optical system (1) according to one of the preceding claims, the imager being a so-called sCMOS monochrome camera. Optical system (1) according to claim 5, comprising a microlens arranged in front of each pixel of a CMOS sensor of the camera. Optical system (1) according to one of the preceding claims, comprising:
- a support (20) transparent to the wavelength of the diode(s) (LED), the support being adapted to support the radiochromic film and arranged to allow the optical beam emitted by the diode(s) to pass (LED),
- a compression plate (21), with an optical index close to that of the radiochromic film, the compression plate being mounted to be applied against the support so as to flatten the radiochromic film. Optical system (1) according to one of the preceding claims, comprising an enclosure impervious to external light in which are housed at least the light source, the bi-telecentric lens, and if necessary the support with the compression plate. Optical system (1) according to claim 8, the walls of the enclosure being made of thermally insulating material. An optical system (1) according to claim 8 or 9, comprising a thermal control device (5) for controlling the temperature of the light source. Optical system (1) according to claim 10, the thermal regulation device being a heat exchanger connected to a circulation thermostat. Method of operating the optical system according to one of the preceding claims, according to which the driving of the LED diode(s) is independent of that of the imager. Method of operation according to claim 12, comprising the following steps:

• control of the LED diode(s) in pulsed mode,
• acquisition of images by the imager during the illumination of the radiochromic film by the LED diode(s).

A method of operation according to claim 13, the step of acquiring comprises acquiring correction images called offset, flat and dark. Operating method according to claim 14, comprising a pre-processing step in order to obtain reference images called master images of the film before irradiation and after irradiation. Method of operation according to claim 15, comprising a processing of the master images comprising the following steps:

• conversion into optical density of the pixel values of the master images of the film before irradiation and after irradiation,
• correction of the optical density of master images of the film before irradiation and after irradiation by means of calibrated neutral filters, so as to obtain the images of the film calibrated in optical density, before irradiation and after irradiation,
• sub-pixel registration of the master images of the film before irradiation and after irradiation,
• subtraction of the two images of the film calibrated in optical density before and after irradiation taking into account the acquisition before irradiation as background noise,
• application of a calibration curve to the images established from films, irradiated at known doses traceable to established primary references, so as to obtain the dosimetry of the film.