X-ray detectors and CT scanning

Recording the shadow

The beam that survives the patient has to be turned into an image, and the receptor chosen sets how much beam the patient had to be given in the first place. Photographic film on its own is a poor X-ray detector. The emulsion is thin and made of light elements, so only a per cent or two of the arriving photons interact with it at all. The rest cross the patient, deliver their dose and blacken nothing.

An intensifying screen is the fix. It is a layer of fluorescent material, calcium tungstate in the older ones and a rare-earth phosphor in the newer, held in contact with the film and usually one on each side of it. Its atoms have high atomic numbers, so it absorbs X-rays far more readily than the emulsion does, and each X-ray it absorbs releases thousands of visible-light photons, which the film is highly sensitive to. Almost all the blackening is then done by light rather than by X-rays directly.

Follow that through to the patient and the reason for the screen appears. The same darkening now needs perhaps a fiftieth of the photons through the body, so the exposure, and with it the dose, falls by that factor. What is lost is sharpness, because the light spreads sideways from the point where the X-ray was absorbed before it reaches the emulsion, so each photon blackens a small patch instead of a point. A thicker screen absorbs more and saves more dose, and blurs more. Dose against detail is the trade-off in every receptor.

Digital flat-panel detectors have largely replaced film and screen, and they begin the same way. A scintillator layer converts each X-ray to light, and in the better panels the caesium iodide is grown as fine parallel needles so the light is channelled down its own column rather than spreading sideways, which recovers much of the sharpness lost with a screen. Under the scintillator sits a matrix of photodiodes, one per pixel, each with its own thin-film transistor. Light frees charge in the diode, the charge is stored, and the array is read out row by row as numbers.

Reading numbers instead of developing a film changes what can be done next. The response stays proportional to the intensity over a far wider range, so one exposure covers bone and soft tissue where a film would be over-exposed in one and under-exposed in the other; the contrast can then be stretched electronically to suit whatever is being looked for; and the image exists at once and can be sent anywhere. The panel also absorbs a larger fraction of the arriving photons than film, so the same image can be made at a lower dose, provided the exposure settings are actually turned down to take the benefit.

Live screening needs a different instrument, and the image intensifier supplies it. An input phosphor converts the X-rays to light, a photocathode converts that light to electrons, and those electrons are accelerated through tens of kilovolts and focused down onto a small output phosphor. Accelerating them is what supplies the gain: each arriving X-ray ends as a far brighter flash than it began, so a surgeon can watch a catheter move in real time on a beam weak enough to be run continuously.

From shadows to slices

A CT scanner records X-ray attenuation from many angles and uses these projections to reconstruct a cross-sectional image. A plain X-ray superposes everything along each ray into a single value of transmitted intensity; a CT scanner's tube and detectors rotate around the patient, recording attenuation along thousands of directions, and a computer then solves for the map of μ across the slice that is consistent with all of those measurements at once.

Two pieces of hardware make that possible, and questions ask what each is for. The first is the shape of the beam. A collimator at the tube narrows the output to a thin fan, wide enough to cross the patient but only a few millimetres thick, and that narrowness does two jobs at once. It confines the dose to the slice being imaged, leaving the tissue above and below it alone, and it defines how thick that slice is, which sets the resolution along the length of the body.

The narrow beam also improves the reconstruction. Reconstruction assumes each reading is the attenuation along one straight ray, and a Compton-scattered photon breaks that assumption, since it arrives from the wrong direction but is counted as though it had travelled straight. Fewer photons are set scattering in the first place when only a thin slab is irradiated, and each detector carries its own collimator facing the tube, so what reaches it has come along its own ray or not at all.

The second is the detector array facing the tube across the patient, an arc of hundreds of small scintillator and photodiode elements in place of a sheet of film. Each element measures the intensity transmitted along one ray and reports it as a number, and the whole arc is read hundreds of times per rotation. That is what the computation needs, a table of transmitted intensities indexed by ray and by angle rather than a picture. The elements are small, which sets the detail across the slice, and they respond in microseconds, so a full set of angles is gathered while the patient holds one breath.

The answer is a cross-sectional image on which soft tissues are distinguishable, and stacking slices gives a three-dimensional reconstruction. Set against that, there are drawbacks. Many exposures mean a far larger radiation dose than a single plain film, and the machine is expensive, so the sharper picture has to be clinically justified. Detail against dose is the recurring theme of every ionising technique.