Electronic Portal Imaging Devices

Electronic portal imaging devices – EPID- can use for daily imaging for treatment localization and verification. The portal image can be analyzed with computer software before the treatment, no significant additional dose is required for patient. The image can be saved in DICOM file format and it can be reviewed at any time.
The first EPID system was video based.The beam transmitted through the patient excites a metal fluorescent screen. A front-silvered mirror, placed diagonally, reflects the fluorescent light by 90 degree into the video camera (commonly using CCD). The analog output of the video camera is converted into a digital array with ADC known as a “frame grabber”. The special resolution depends from phosphor thickness.

 

Another kind of EPID has a matrix ionization chamber system, the electronic imaging system that consists of 256 × 256 liquid ionization chamber (scanning liquid ionization chamber – SLIC). This system was developed in Amsterdam with the next construction: the 1 mm gap between the boards is filled with isooctane liquid, which serves as an ionization medium with excellent x-ray detection efficiency. An electronic circuit performs the rapid switching and analog-to-digital conversion. Essers, van Herkand his colleagues investigated this devise and gave its detailed analysis.

 
The new generation of EPID system is amorphous silicon based system. Within this unit a scintillator converts the radiation into visible light. The light is detected by an array of photodiodes implanted on an amorphous silicon panel. The photodiodes integrate the light into charge captures. Resolution and contrast are greater than that of other system.

In this method shown on the Figure 3, a phosphor layer (screen such as Gd2O2S:Tb, or a structured scintillator such as CsI:Tl) is enlaced in intimate contact with an active-matrix array. The intensity of the light emitted from a particular location of the phosphor is a measure of the intensity of the x-ray beam incident on the surface of the detector at that point. Each pixel on the active matrix has a photosensitive element that generates an electrical charge whose magnitude is proportional to the light intensity emitted from the phosphor in the region close to the pixel. Each pixel consists of an a-Si (amorphous silicon) photodiode connected to a TFT (thin film transistor). Each photodiode is connected to a common bias line. Amorphous silicon photodiodes are sensitive to visible light, with a response curve comparable to human vision. All signals of the columns are amplified in charge amplifiers and converted to the digital format by ADCs (analogue to digital converters). The digital data are transmitted to the data acquisition unit or frame grabber. The frame grabber utilizes the PCI bus for direct image acquisition into the PCIs main memory and imager control functions. The acquired images after corrections are displayed on PC monitor.
Three corrections are performed which are Offset, Gain, and Dead pixel correction.
1. Offset correction is used to correct the dark current of each pixel for a specified frame time.
2. The Gain correction is used to homogenize different pixel sensitivities.
3. The Dead pixel correction allows a software repair of defected pixels to enhance image quality. Improper pixel values are replaced with the averaged value of the eight adjacent pixels, while dead pixels are not averaged.

 

Clinical approach of portal imaging

 
Different image acquisition modes are available in clinical approach:
1. Single exposure: a single image is acquired for a short period of time at the beginning of the treatment. More than one image can made during the one fraction of treatment.
2. Double exposure: one image is the single exposure image, and the second is an “larger open field” image, it’s give more information about patient anatomy.
3. Movie loops: EPID allows movie loops or on-line fluoroscopy to be acquired during treatment. This acquisition mode is very useful for investigation of movements of internal organs.
The purpose of portal imaging is:
1. To verify the patient cross-section on the CT images is the same as the real patient (in some radiotherapy department the patient have to wait for treatment more than two months, and during this time they lost their weight).
2. To verify the field placement, characterized by the isocentre relative to anatomical structures of the patient during the treatment or to verify that the beam aperture by MLS or by blocks has been properly produced.
3. With the movie loops acquisition mode have to follow the patient’s organ motion during respiration, and on the base of image analysis can make a patient set-up correction.
4. The whole process should be repeated several times during treatment, but at least once a week.
The clinical application of EPIDs can be separated:
1. Off-line analysis: quantification and separation the random and systematic uncertainties for patient set-up.
2. On-line analysis: fined the unacceptable discrepancies between the portal image and reference image (simulator or DRR images), and make a decision about continuation of treatment.

 

Literature

 
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