Detector types and what is inside a flat panel
Identify the type of any digital X-ray detector on site - CR, indirect or direct DR, tethered or wireless - and name its main parts, what each part does and how each one typically fails.
Why a Level 2 course on one part
X-ray Service Level 1 treated the detector as one block in the image chain: it receives the beam that leaves the patient and hands a raw image to the console. That is enough to locate a fault to 'the detector side'. It is not enough when the detector is the part being blamed. A flat panel is usually the most expensive single item in a radiography room, it is sealed, and in most cases it cannot be repaired in the field. The decision 'replace the panel' therefore carries a large cost, and it is the decision most often made without proof.
In practice a large share of detector calls are not panel failures at all. The images in the case collection behind this course show the pattern clearly: a calibration taken with the grid in, a wireless link that times out, a battery the panel does not recognise, a cable broken inside its connector, a disk filled with temporary files, a wrong detector type selected in the software. The engineer who knows how a panel is built, how it talks to the generator and the console, and what each calibration does, finds these in an hour. The engineer who does not, orders a panel.
This lesson builds the picture you will use for the rest of the course: the types of detector you will meet, and what sits inside the enclosure of a flat panel.
CR and DR: two different machines
Computed radiography (CR) uses a cassette with a storage-phosphor plate. The exposure leaves a latent image trapped in the phosphor, and a separate reader scans the plate with a laser, collects the emitted light, digitises it and erases the plate. The detector itself has no electronics, no battery and no link; everything active is in the reader. CR faults are mostly mechanical and optical: rollers, plate transport, dirt, scratches, a weak erase lamp, a laser or light-collection path that has aged. CR is covered at overview level in X-ray Level 1 and is not the subject here, but you will still meet it, often in the same department as DR.
Digital radiography (DR) reads the image directly from the detector, a few seconds after the exposure, with no plate handling. The DR detector is an active electronic device: a large sensor array, readout electronics, a processor, memory, a power supply, and a link to the console. When we say 'flat panel' in this course we mean a DR detector of this kind. Some older DR systems used a scintillator screen viewed by one or more cameras through optics or fibre; you may still see them, and some engineers still call any DR detector 'the camera'. The principles of calibration and artefacts below apply to them too, but their failure modes include the optics and the camera electronics.
A quick way to tell what you are looking at: a CR cassette is light, has no indicator lights and no connector, and goes into a reader. A DR panel has a status display or LEDs, a battery compartment or a cable, and appears in the console's detector status screen.
Indirect and direct conversion
Most general radiography panels use indirect conversion. A scintillator layer on top of the sensor turns X-ray photons into visible light, and a matrix of photodiodes under it turns the light into charge. Two scintillators dominate. Gadolinium oxysulfide (GOS, Gd2O2S:Tb) is a powder screen: robust and cheaper, but the light spreads sideways in it, so sharpness and efficiency are lower. Caesium iodide doped with thallium (CsI:Tl) is grown as fine needle-like columns that guide the light towards the photodiode, so the layer can be thicker and still sharp; it gives a better dose efficiency at a higher price. CsI is hygroscopic, which is one reason panels are sealed and why liquid inside a CsI panel is serious. Manufacturers often sell the same panel in both versions; one manufacturer's manual for a 43 x 43 cm tethered panel lists a GOS model and a CsI model side by side with the same pixel pitch.
Under the scintillator sits the sensor array. In most panels it is amorphous silicon (a-Si) on glass: each pixel has a photodiode and a thin-film transistor (TFT) that acts as a switch. Newer designs use CMOS sensors or oxide TFTs such as IGZO; manufacturers present them as faster, with lower noise or leakage than a-Si, and how much this matters depends on the model. Some manufacturers have moved to flexible substrates instead of glass, which makes the panel lighter and more tolerant of shock; the Vieworks VIVIX-S FW service manual, for example, describes a flexible a-Si TFT array with PIN photodiodes under CsI. From the service point of view the sensor technology matters because it changes speed, noise, temperature behaviour and robustness, but the calibration principles stay the same.
Direct conversion panels use a photoconductor, almost always amorphous selenium (a-Se), that turns X-rays straight into charge under a high bias voltage, with no light stage. There is little sideways spread, so resolution is very high; this is why a-Se is common in mammography. The price is sensitivity to temperature (selenium can be damaged by heat and by cold storage outside the manufacturer's range), a high-voltage bias supply inside the detector, and characteristic lag and ghosting behaviour that we meet in lesson 5. If you service mammography, know which of your units have a-Se panels and read their storage and transport rules with special care.
Tethered, wireless and built-in
Fixed panels are built into a table bucky or a wall stand and connected by cable for power, data and, often, a synchronisation signal. Portable panels are cassette-sized, so they can be placed under a patient on a trolley or in a bed. A portable panel can be tethered by a cable or wireless. Wireless panels run on a rechargeable battery, send images over a wireless LAN, and usually have a way to work tethered as well, either through a docking cable or a charging cradle, which is useful when the radio link is in doubt.
Sizes follow the old film cassette formats, so that a panel can fit an existing bucky: the manuals for this course list panels of about 25 x 30, 36 x 43 and 43 x 43 cm (17 x 17 inch), and 35 x 43 cm (14 x 17 inch) repeats the classic cassette, with smaller formats for mammography and dental, and larger or special shapes for long-leg and spine work. The thickness of a cassette-sized panel is usually close to the standard cassette thickness for the same reason. Pixel pitch differs by model: the manuals available for this course give 99, 124, 127 and 140 micrometres for different general radiography panels. Smaller pixels mean more data per image and therefore longer transfer times, which matters for wireless links.
Note also that the same physical panel may be sold under several brand names. Detector manufacturers supply panels to system manufacturers, who integrate them with their own software. Knowing who really made the panel tells you which manuals, error lists and calibration tools actually apply.
What is inside the enclosure
Open the manufacturer's exploded view of almost any flat panel and you will find the same layers. From the tube side: a carbon-fibre front cover that is strong but transparent to X-rays; a foam or honeycomb layer that spreads load; the scintillator, sealed against moisture; the sensor array on glass or a flexible substrate; a lead or metal shield behind the array to protect the electronics from the beam and stop backscatter; the readout electronics, connected to the edges of the array by thin flexible circuits; a main board with processor, memory, the link interface and the exposure-detection circuit; and, in a wireless panel, the battery, antennas and charging contacts. Everything sits in a frame designed to keep the glass from bending.
The flexible connections at the edge of the array and the glass itself are the weak points. A local load or a drop can crack the glass or break a few contacts in the flexible circuits, and the result is a line, a block of lines or a dead region. This is why manufacturers give two load limits: one for weight spread over the whole surface and a lower one for a small area. As an example, the Rayence 1717 manual gives limits for an even load over the window and for a load on a 40 mm circle, and the two Vieworks manuals give different limits for different panel families. The numbers vary by model; what is constant is that a knee, an elbow or the edge of a bed rail is a local load.
Two more blocks deserve attention because they cause faults that look like a broken panel. The automatic exposure detection (AED) circuit watches the signal for the start of an exposure when there is no wired synchronisation; if it is damaged or set too sensitive, the panel may take blank images on its own or miss real exposures. Many wireless panels also include shock and temperature sensors whose records can be read in the service tool; one manufacturer's self-diagnosis lists the impact sensor, temperature sensor and AED sensor as separate test items. These records are valuable evidence when you need to know whether a panel has been dropped.
Reading the label and the documents before anything else
Before any detector job, identify exactly what you have: the system brand, the panel manufacturer and model, the scintillator type, the serial number, the firmware version shown by the console, the type of link (tethered, wireless, both) and the synchronisation mode in use. Write them in the job record. Then find the manufacturer's documents for that model: the user manual for handling, cleaning and calibration intervals, and the installation or service manual for the environment, interface and diagnostic tools. Many problems in the case collection were solved simply by someone recognising that the panel was a rebadged model from a different maker, whose tools and settings then applied.
Finally, remember the scope. This course teaches you to understand the detector, keep it working, and prove where a fault is. Opening a sealed panel, replacing internal boards or reloading detector firmware is the manufacturer's or its authorised service's work, with their tools and their calibration files. Doing it without them usually voids the warranty and may make the panel unusable.
| Part | What it does | How it fails | |
|---|---|---|---|
| 01 | Front cover (carbon fibre) | Protects the sensor, carries the patient load, transparent to X-rays. | Cracks, dents or delamination after drops and local loads; liquid path at damaged edges. |
| 02 | Scintillator (CsI:Tl or GOS) | Converts X-ray photons to visible light over the sensor (indirect panels). | Moisture damage of CsI, local degradation, cracks after shock; ages with dose. |
| 03 | Photoconductor (a-Se, direct panels) | Converts X-rays straight to charge under a high bias voltage. | Damage from temperature outside limits, ghosting and lag, bias supply faults. |
| 04 | Sensor array (a-Si/IGZO TFT or CMOS) | Matrix of photodiodes and switches that stores and releases each pixel's charge. | Cracked glass, dead pixels, rows and columns, rising leakage with age and heat. |
| 05 | Gate drivers and readout ICs on flex circuits | Switch the rows on in turn and amplify and digitise the columns. | Broken flex contacts after bending or drops: lines, blocks, half-image faults. |
| 06 | Main board (processor, memory, FPGA) | Controls the readout, stores images and calibration data, runs the link. | Memory or calibration-data errors, firmware faults, failed capacitors, no boot. |
| 07 | Exposure detection and sync circuit | Detects the start of exposure (AED) or handles generator sync signals. | Missed exposures, self-triggered blank images, sensitivity to shock and noise. |
| 08 | Battery pack and charging contacts | Powers a wireless panel; contacts connect it to the panel, cradle or charger. | Capacity loss with cycles and heat, swelling, dirty contacts, pack not recognised. |
| 09 | Wireless module and antennas | Send images and status to the access point or console. | Slow or dropped links, damage at the antenna area, interference-related errors. |
| 10 | Tether cable and connector | Power, data and sync for fixed or tethered panels. | Broken conductors at the connector or strain relief, bent pins, intermittent errors. |
The 'new' detector that was already known
A hospital was offered a wireless detector sold under a large imaging brand, to be fitted to an older domestic radiography unit, and the engineer was asked whether the two could work together at all. Instead of answering from the brochure, he read the detector's label and documents and found that the panel was produced by a detector manufacturer whose panels and integration kit he already knew. That changed the question: integration used the panel maker's standard software development kit and generator interface, the original installation media had to come with the detector, and any authorisation had to be arranged through the supplier. The job was planned with the correct manuals from the start instead of by trial and error.
Find out who actually made the panel. The right manuals, tools, error lists and integration path follow from the real manufacturer, not from the name on the box.
A black square instead of an image
A digital radiography system began to show a uniformly black square instead of the image, and the same happened when images were imported, while the software reported no error at all. The first idea was a fault in the image processing on the workstation. Since the console and the software looked healthy, colleagues pointed at the detector's own electronics: on that design a failed tantalum capacitor on the camera board can often be seen by eye. Board work inside a detector is for a repair centre, so the engineer's part was to rule out the console and software, record the symptom and send the detector to a specialist repairer. The reported repair was replacing that capacitor; the final result was not confirmed. Recalibration or reinstalling software would not have found it.
No error message does not mean no hardware fault. A blank or uniform image with a healthy console points at the detector's electronics: diagnose to that point, then send the detector to a repair centre instead of opening it.
On-site checklist
- Record system brand, panel maker and model, scintillator type, serial number and firmware version.
- Record the link type (tethered, wireless, both) and the synchronisation mode in use.
- Find the user manual and the installation or service manual for that exact model.
- Note the manufacturer's environment, load and drop limits for the panel in your job file.
- Check whether the panel is a rebadged model from another maker before choosing tools.
- Look for the shock and temperature records if the panel's service tool provides them.