Choosing the Right Flat Panel Detector

The flat panel detector has advantages over film such as sensitivity and better process reproducibility, but it also has disadvantages such as cost and lower spatial resolution. Here is a more comprehensive list of the advantages and disadvantages:

• Advantages:
  • Sensitivity: the sensitivity corresponds to the smallest measurable variation in X-ray absorption. A flat panel detector is more sensitive than film and therefore enables the reduction of the dose of X-rays necessary to make an image.
  • Reduction of the time required to acquire an image.
  • Reduced operating costs: in the medium to long term, savings can be made on film and chemicals (after a minimum of 2 to 3 years).
  • Efficiency: better reproducibility of the process.
  • Ability for software processing: some software can be installed to allow image enhancement, edge detection or diagnostic assistance.
  • Simplified image storage: the detectors generate images that do not exceed a few dozen MB per X-ray.
  • Portability: flat panel detectors are more practical and easy to install (in mobile and veterinary radiography in particular).

•  Disadvantages:
  • Cost: for infrequent use, choosing a flat panel detector will be more expensive than traditional film.
  • Obsolescence: some technologies become obsolete and no longer have technical support.
  • Lower spatial resolution compared to film: film offers about 10 pl/mm (up to 20 in mammography) while a flat panel detector offers 2.5 to 3.5 pl/mm.
  • Artifacts in the digitization of the signal.
  • Dose creep: unlike film, a digitized image is of better quality when it is overexposed. In order to avoid the appearance of detector-induced noise related to the under-exposure of images, technicians may be driven to increase the radiation exposure dose, leading to the phenomenon of “dose creep”.

Canon portable flat panel detector

In order to make the best possible choice when purchasing a flat panel detector, several criteria such as image quality, application and the size of the device must be taken into account. Here is the list of criteria:

• Image quality: look for the best “contrast-to-noise ratio”, i.e. very good contrast with little noise.
• Applications: there are several applications for a flat panel detectors, you will find in particular ones for mammography, flat panel detectors for conventional digital radiology, for interventional digital radiology (angiography in particular), or mobile/outdoor radiology (flat panel detectors for veterinary radiography).
• Device size: several formats are available, including in inches 14×14, 17×14, 11×11, etc.
• Cost
• Durability / Fragility
• Available options: several options exist such as portability, wireless connection, waterproof, etc.

It is difficult to determine exactly how the performance of flat panel detectors will change over time, even though some studies have been carried out on their ageing process.

It should simply be noted that the sensitivity of scintillator detectors decreases over time and depends primarily on the number of images taken

For SwissRay detectors, for example, the service life is at least five years. Canon detectors are guaranteed for 210,000 shots and seven years of service life.

The principle of a flat panel detector is to transform an X-ray beam of energy ranging from 20 to 120 keV into an electrical signal, which is then digitized. There are three main operating techniques for direct conversion and indirect conversion flat panel detectors. These three technologies depend on the number of steps required to transform an X-ray photon beam into an electrical signal.

• Direct conversion flat panel detectors with “Amorphous Selenium + TFT Matrix”:
  • Step 1 (capture): an amorphous selenium plate used as a photoconductor directly converts X-photons into electrical charges.
  • Stage 2 (collection): the electrical charges produced are recovered without further conversion by a TFT transistor array. These analog electrical values are then digitized, and it becomes possible to form a digital image.

• Indirect conversion flat panel detectors with “CsI + Photodiode” (1st type):
  • Step 1 (capture): a layer of cesium iodide (used as a scintillator) converts X-ray photons into light photons.
  • Step 2 (conversion): an amorphous silicone layer (photodiode) converts light photons into an electron beam.
  • Stage 3 (collection): the electrical charges produced are recovered without further conversion by a TFT transistor array.

• Indirect conversion flat panel detectors with “Csl + CCD Matrix” (2nd type):
  • Step 1 (capture): a layer of cesium iodide (used as a scintillator) converts X-ray photons into light photons.
  • Step 2 (conversion and collection): these light photons are then converted into electrical charges by CCD sensors.

Please note: we have not presented the following in this buying guide:

• CR sensors: the conversion of X-rays into an electrical signal is not immediate. The X-photons create charges trapped in the material (a scintillator) which are then released by the stimulation of a laser beam.
• Photomultipliers: these are used for image intensification and are often integrated into brightness amplifiers.

Each of the three techniques discussed in the previous section has its advantages and disadvantages. They are outlined below:

• Direct conversion flat panel detectors with Amorphous Selenium + TFT Matrix:
  • Advantages: excellent spatial resolution.
  • Disadvantages: low absorption of X-rays by selenium (except in mammography, where absorption is greater than 90%); cannot be used for dynamic examinations (selenium is remanent, so it must be wiped off between two exposures); there is a risk of destroying the TFT transistor array if you try to increase the dose to compensate for the low absorption of X-rays by selenium.

• Indirect conversion flat panel detectors with CsI + Photodiode:
  • Advantages: good X-ray absorption by the scintillator in conventional radiology; non remanent detector; low cost and low consumption compared to CCDs (the array of transistors coupled to the photodiodes is generally made of CMOS: they are cheaper than CCDs and consume ten times less).
  • Disadvantages: significant heating of the detector (because of a very fast reading frequency, it needs to be cooled).

• Indirect conversion flat panel detectors with Csl + CCD Matrix:
  • Advantages: very fast sensors (useful for dynamic applications such as angiography); good sensitivity (dose reduction required); good linearity between response and intensity; CCDs are good scintillator light sensors and less sensitive to noise (compared to CMOS).
  • Disadvantages: CCDs are smaller than CMOS which means there is some loss of X-photons when a large FOV is required.