The X-ray system: what is in the room and how it fits together

Name the subsystems of any X-ray system, recognise the main system types, and follow the power, radiation, image and control chains well enough to decide where a reported fault most likely sits.

Why start with the whole system

When a radiographer calls and says 'the X-ray is not working', that sentence can mean a dozen different things: the console does not boot, the tube stand does not move, the exposure button does nothing, an exposure happens but no image arrives, or the image arrives but looks wrong. Each of these points at a different part of the machine. An engineer who knows only the part he repaired last time will start in the wrong place.

Every X-ray system, from a ward mobile unit to an angiography suite, is built from the same blocks. The size, the software and the brand change; the logic does not. If you can name the blocks and say what each one needs from the others, you can walk into a room with a machine you have never seen and still ask the right first questions. This lesson gives you that map. The following lessons go deeper into each block.

The blocks of an X-ray system

Think of the system as a chain from the wall socket to the radiologist's screen. Power enters the generator. The generator produces high voltage for the tube and heats its filament. The tube produces X-rays. The collimator shapes the beam, and filters harden it. The beam passes through the patient and, usually, through an anti-scatter grid. The detector converts what is left into an image. The workstation processes the image and sends it to PACS. Around all this, the stand and table hold the tube and detector in position, and a control system ties everything together with interlocks.

Each block depends on the ones before it, and each block also reports back. A generator will not expose if the tube anode is not spinning, if a door contact is open, or, on digital systems linked to the generator, if the detector has not reported that it is ready. A retrofit panel that uses automatic exposure detection is different: the generator does not know about it, so if the panel is not armed the exposure still happens and the image is lost. That is why a fault in one block often shows up as a complaint about another.

• Generator: takes mains power and produces tube voltage (kV), tube current (mA) and exposure timing; also drives the anode rotor and the filament.
• Tube and housing: the tube produces X-rays; the housing holds it in oil, shields leakage radiation and carries the HV connections.
• Collimator: adjustable shutters, a light field showing where the beam will fall, added filtration, and often a dose-area product (DAP) chamber.
• Grid and bucky: absorbs scattered radiation before it reaches the detector; may be fixed or moving.
• Detector: film-screen, CR imaging plate, image intensifier with camera, or DR flat panel.
• Stand and table: column, ceiling suspension or C-arm; table top with brakes; motors, counterweights and position sensors.
• Workstation and network: acquisition software, image processing, worklist, DICOM transfer to PACS and printers.
• Control and safety chain: exposure switches, interlocks, emergency stops, warning lights, door contacts.

Film, CR and DR: three ways to catch the image

You will still meet all three detector technologies in the field, sometimes in the same hospital. With film-screen, the image is a chemical process in a cassette and a processor; the X-ray system itself does not know whether an image was made. With computed radiography (CR), the cassette holds a reusable phosphor plate that stores the image as trapped energy; a separate reader scans it with a laser, digitises it and erases the plate for reuse. The X-ray system and the CR reader are often from different manufacturers and only meet through the worklist and the network.

With direct radiography (DR), a flat-panel detector reads the image electronically within seconds. A DR panel can be fixed in a bucky or used as a portable cassette-sized unit, wired or wireless, with its own battery. DR is tightly linked to the generator: the panel must be armed before the exposure and must tell the system it is ready, either through a hardware signal or through automatic exposure detection, where the panel senses the start of radiation itself. Many 'the X-ray does not expose' calls on DR systems are really 'the detector is not ready' calls. Lesson 5 covers detectors in detail.

System types you will meet

The same blocks are arranged differently for different clinical jobs. Knowing the type tells you where the weak points usually are.

Fixed radiography rooms have a floor or ceiling-mounted tube stand, a patient table with a bucky and often a wall stand for chest work. They draw a heavy current in short bursts, so supply quality matters. Mobile radiography units are self-contained trolleys with a motorised drive, a battery or a capacitor bank, and a folding arm; they suffer from being driven into door frames and lifts, and from battery problems. C-arms, used in theatres, carry the tube and the detector on opposite ends of a C, provide fluoroscopy and are moved between rooms, so cables, brakes, wheels and connectors take a beating. Fluoroscopy rooms (remote-controlled tables) combine a tilting table, a spot-film device or panel and continuous low-dose exposure. Angiography systems are large ceiling or floor-mounted C-arms with powerful generators, high-heat-capacity tubes and complex collision protection. Mammography systems use low kV, special tube targets and compression, and are covered by their own quality rules.

This course is about principles shared by all of them. Where a type has a specific risk, it is pointed out, but model procedures always come from the manufacturer's documentation.

Four chains to follow

When you are not sure where a fault is, follow one of four chains, and decide which one the complaint belongs to before you open anything.

The power chain runs from the building supply through the main disconnect, the generator's input stage and its internal power supplies to the tube, the motors and the computers. Symptoms: nothing powers up, a breaker trips, the system reboots during exposure.

The radiation chain is the exposure request travelling from the hand switch or foot switch through the control system and its interlocks to the generator and tube, and the beam leaving the collimator. Symptoms: the exposure button does nothing, preparation runs but no exposure, the exposure is aborted, the dose or density is wrong.

The image chain runs from the detector through acquisition, processing and display to the archive. Symptoms: no image, a dark or noisy image, artefacts, images not arriving in PACS or arriving under the wrong patient.

The control and mechanical chain covers motors, brakes, locks, position sensors and the communication links between boards. Symptoms: a movement does not work, a brake does not release or does not hold, a communication error between console and generator.

Your first questions on site

Before you touch the machine, collect facts. Ask the user what exactly happened, what the screen showed, and whether anything changed recently: a power cut, a software update, a move, a new network switch, a new member of staff. Read the error message or code and write it down exactly as displayed. Look at the error log if you have access to it; the first error in a sequence is usually more important than the last. Look at the room: is the tube stand where it should be, are cables trodden on, is anything wet, is the air conditioning working.

Then decide which chain the complaint belongs to and start with the cheapest, most likely checks in that chain. A two-minute check of an emergency stop, a door contact or a detector battery saves hours of board-level work. The rest of this course gives you the checks for each chain.

On-site checklist

• Write down the error message or code exactly as shown, and the time.
• Ask what changed recently: power, software, network, move, staff.
• Read the error log and note the first error in the sequence.
• Decide which chain the complaint belongs to: power, radiation, image, control/mechanical.
• Check the cheap things first: emergency stops, door contacts, detector battery and link.
• Look at the room: cables, liquids, air conditioning, signs of collision.

Check yourself