Imagers in the imaging department: three technologies, one film path

Recognise which kind of imager you are standing in front of, name its main assemblies in the order the film meets them, and say which assembly a complaint most likely belongs to.

Why a hospital still prints film

Most departments now read images on screens and store them in PACS, yet the film printer is still on the call list of almost every service engineer. Surgeons want a film on the light box in theatre, patients are discharged with films to take to another hospital, some clinics have no viewing stations at all, and in many places the law or the payer still expects a hard copy. Mammography, orthopaedics and dental clinics often print every day. When the printer stops, the scanners keep working, but nobody leaves the department with images, and the call arrives as an emergency.

A medical imager (the industry also says laser camera, dry camera, hard-copy camera or film printer) receives an image over the network, almost always as a DICOM print job, turns pixel values into optical density on a sheet of film, and delivers a dry sheet ready to read on a light box. The engineering problem is always the same: put a controlled amount of blackening at each point of the sheet, at the right place, and do it the same way every day.

There is a second, smaller family you will meet at the same time: video printers next to ultrasound scanners and endoscopy towers. They print on thermal paper or small thermal film from a video or USB signal. They use the same thermal-head principle as the larger direct thermal imagers, so much of this course applies to them too.

Three technologies you will find on site

Direct thermal dry imagers write the image with a thermal print head: a long row of tiny resistive heating elements across the width of the film. The film carries a heat-sensitive layer; where an element heats it, the layer darkens, and more heat gives more density. The image is formed and fixed in one pass, with no light and no separate development step. Agfa Drystar models and Sony UP-DF thermal film printers are examples from the field cases in this course, and the small video printers work on the same idea with paper.

Laser photothermographic dry imagers work in two steps. First a laser diode, scanned across the sheet by a rotating polygon mirror while the film moves under it, exposes a light-sensitive layer and leaves an invisible latent image. Then the sheet is wrapped around a heated drum or carried over a heated plate, and the heat develops the latent image into a visible one. No liquid chemistry is used and no silver solution leaves the machine. Carestream (Kodak) DryView, Konica Minolta DRYPRO and Fujifilm DRYPIX imagers are typical examples; the DRYPRO SIGMA service manual is the example machine we refer to in this course.

Legacy wet laser imagers expose conventional silver-halide film with a laser and then pass it through a wet processor with developer, fixer, wash and dryer, either built in or docked to the camera. You will still find a few in service, usually old and short of parts. They add chemical handling, silver recovery and processor maintenance to everything in this course. If you meet one, treat the processor as a separate machine with its own procedures and safety data sheets.

• Direct thermal: heat writes the image directly; no light sensitivity in normal room light, but sensitive to heat and pressure.
• Laser photothermographic: light writes a latent image, heat develops it; the unexposed film must be protected from light.
• Wet laser: light writes, chemistry develops; add processor and chemical safety to your checklist.

Film: the consumable that causes most of the trouble

Each technology uses its own film, and the films are not interchangeable. A thermal film will not record a laser exposure, a laser film fed through a thermal printer produces nothing useful, and even within one brand there are different film types for general radiology and for mammography, on blue or clear base. The imager usually identifies the film from a tag on the box, the cartridge or the backing sheet: a barcode, an RF tag or a memory chip that tells it the film type, size, emulsion batch and sometimes how many sheets are in the pack. That tag is also how the imager chooses the right calibration data.

From this one fact come a whole set of field faults. Film loaded with the wrong side towards the head or laser prints nothing or only a faint silhouette. Film loaded without changing the backing sheet makes the imager think the pack is empty or overused. A box from another region, an unsupported type, or a non-genuine batch may be refused or misidentified. An expired or badly stored box gives high base density, which the imager often reports as a calibration failure. Before you suspect the head, the laser or a board, look at the box: type, side, expiry date, how it was stored and whether the tag or backing was replaced with the film.

Storage conditions are set by the film manufacturer and printed on the box or its data sheet: temperature, humidity, keeping away from heat sources, ionising radiation and chemical fumes, and for light-sensitive film protection from light until it is inside the imager. Check them on every visit. A film store next to a radiator or on a sunny windowsill causes faults that no repair will cure.

The main assemblies in the order the film meets them

Every dry imager, whatever the brand, follows the same path, so learn it as a chain. The film supply holds one or more trays or cartridges, each with a sensor that knows whether it is present and closed, and a tag reader. A pick-up mechanism, usually suction cups on an arm with a small vacuum pump, or a friction roller, lifts the top sheet and hands it to feed rollers. Transport rollers, belts and guides carry the sheet past sensors that confirm its leading and trailing edges at each stage.

In a thermal imager the sheet then passes between the thermal head and a rubber platen roller, line by line, while a drive motor and belt move it at a precisely controlled speed. In a laser imager the sheet is carried on an exposure transport under the optics module, where the laser beam sweeps across it, and then into the processor: a heated drum or plate held at a tightly controlled temperature, with rollers that keep the film in contact. After development the film passes a cooling section so that it does not curl or mark, and then a densitometer, which measures the density of test patches, before it drops into the output tray.

Around the path sit the electronics: a power supply, a machine control board for motors, heaters and sensors, an image processing computer with its storage (hard disk, flash card or similar) and network port, and a front panel or display. Many laser imagers also have an odour or charcoal filter and fans that remove the small amount of vapour released when the film is heat-developed. That filter is a PM item, and a forgotten one leaves deposits inside the machine.

Putting the complaint on the chain

When the department calls, your first job is to place the symptom on this chain before you open a cover. A message about the tray, the film count or the film type belongs to the supply and the tag. A jam with an area number belongs to transport. A line along the direction of travel in a thermal printer points at one or more head elements; a band across the film points at the drive and speed. Low or uneven density on a laser imager points at the processor temperature, the film or the calibration before it points at the laser. A printer that prints nothing at all while the modality says the job was sent belongs to the network and configuration side.

The same chain tells you who owns the fix. Film type, storage and loading belong to the department and the supplier, and the fix is training and purchasing. Transport, head, optics and processor belong to you or the authorised service. Network and DICOM settings belong partly to you and partly to whoever configures the modality and PACS. Write down which link you think is at fault and why: that note is the start of your service report.

On-site checklist

• Identify the technology: thermal head, laser plus heated processor, or wet processor.
• Write down make, model, serial number, total print count and prints since last PM.
• Check the film in use: type, size, expiry date, side, and whether the tag or backing was changed.
• Look at where the film is stored and compare with the conditions on the box.
• Ask what exactly is wrong: no print, jam, lines, bands, density, wrong size, wrong patient.
• Place the complaint on the film path before you open any cover.

Check yourself