LASER SCANNING OF RADIOGRAPHS*

William F. Schreiber and Uri Gronemann Massachusetts Institute of Technology

Cambridge, Massachusetts 02139

INTRODUCTION

This work is concerned with the development of a low cost, high quality, fac- simile system for medical radiographs. In this context, high quality means unim- paired diagnostic usefulness, and low cost means low enough so as not t o inhibit its usefulness. At today’s prices this may be estimated t o be about $10,000 per unit.

The existence of such a system might lead t o its use in the following appli- cations:

1. Accurate display with the possibility of correcting minor exposure errors. With present duplicating methods there is difficulty maintaining tone scale accuracy.

2. Remote diagnosis, consultation, and study. At present, a substantial amount of this is done by mail. In any event there is a good deal of reluctance t o part with the originals. Transmission would be via the Bell system’s DDS network at 56 kbps, o r equivalent digital channels.

3 . Computer processing. The present generation of input and output devices suffers from high cost and/or poor quality. It is conceivable that computer pro- cessing of radiographs might be used diagnostically if it were cheap enough and good enough.

4. Digital storage and retrieval. A great deal of interest is being shown in this problem, the major barriers being cost and quality.

BACKGROUND

Previous work in our laboratory, supported by the Associated Press, led t o the development of a laser scanner facsimile system for the transmission of news photographs. These machines are now in production by Harris Electronics. By the Fall of 1976, the Laserphoto system, as it is called, will have completely re- placed the existing mechanical devices. Laserphoto machines are comparable in cost t o older mechanical facsimiles but offer great advantages in image quality, reliability and simplicity of use and maintenance. They are a t least an order of magnitude cheaper than previous laser scanner systems which have been designed for very high resolution and high speed applications, such as military reconnais- sance.

The medical radiograph system is a version of the newspaper system, upgraded to a resolution and speed level appropriate for that task but still well below the ultimate resolution and speed performance, which had previously resulted in very high cost. The results of our initial development indicate that the price and performance goals for this application can readily be met.

*This work was supported by NIGMS Grant 5 POI-GM19428-03Sl

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-. FOCAL PLANE

POSITION OF PINHOLE FILTER

DESCRIPTION OF SYSTEM

Optical-Mechanical Assembly

The optical system of the radiograph scanner, in both the transmitter and the receiver, is similar to that of the Laserphoto machine except that we have selected a higher quality standard lens and operate the system at a lower f number to obtain higher resolution. The double-pass optical scan principle is shown in F I G U R E 1 and discussed in greater detail in References 1 and 2. The complete optical system of the scanner is shown schematically in F I G U R E 2. Notice that, unlike the system of F I G U R E 1, the first pass beam is incident on the objective lens outside the (horizontal) plane of scan. While the bottom part of the figure shows a sectional side view of the whole system, the top part shows a top view of the portion of the beam path past the scanning mirror. In this top view, the light-collecting portion is shown, for clarity, unfolded-that is, as though the path were not folded by the diagonal mirror (the position of the latter being indicated by a dashed line). The objective lens is of 16 in. focal length. As the f number is lowered, by further expanding the beam, the on-axis resolution rises while the edge resolution falls. 250 lines/in. is obtainable over the 14 in. width required for a full sized chest film, while more than 1000 lines/in. is achieved over the central 4 in. Preliminary experience indicates that most x-ray films would require no more than 250 lines/in. and this is the sampling density used at present.

Since the optical system has a f la t field, flat-bed film drives are possible for both the transmitter and receiver. This is a major contributor t o the low cost of the system, as well as t o its ease of operation. The transmitter uses a double pinch roller drive, powered by a stepping motor, to transport the film past the scanning aperture. In the receiver, a self-contained assembly includes a roll of unexposed film, a main drive capstan, powered by a similar stepping motor, and a small oven. The film is exposed as it passes over the capstan. With the aid of a set of rollers, it then passes through the oven without touching anything but a soft air seal, emerging at the top fully processed. The optical-mechanical as-

h 1

1 COLLIMATED BEAM

IIGURE 1. Principle of the double-pass optical system. Since the lens has a flat field and the incident beam is collimated at the scanning mirror, the reflected beam is in focus in a plane perpendicular to thc lens axis and containing the focal point of the first (beam expan- sion) lens. The size of the focussed spot depends on the f number of the system and the aberrations of the objective lens. For all but the very highest resolution, the quality of the beam expansion lens does not affect the spot size.

Schreiber & Gronemann: Laser Scanning of Radiographs 47 1

OBJECTIVE

CYLINDRICAL zz:E OPTICS

' r , REFLECTOR

FEEDBACK

GALVANOMETER

15% REFLECTOR

VIEW OF ENTIRE SYSTEM

FIGURE 2. Schematic of optical system of the radiograph scanner.

semblies of the transmitter and the receiver are shown in FIGURES 3 and 4, respectively.

Dry Silver Film

As a result of our very satisfying experience with 3M dry silver paper in the newspaper facsimile project, we are using dry silver film for the x-ray receiver. This film has an appearance, resolution, and graininess comparable t o conven- tional film, although the density range is somewhat less. At the same time it offers the great advantage of dry processing, so that the film emerges from the receiver fully processed. Of course any red-sensitive film of adequate speed and photographic quality could be used, if one were willing t o use a wet process.

Present types of dry silver film are easily damaged when hot. For this reason we use an oven processor, in which the film comes in contact only with hot air. This in turn requires keeping film threaded through the oven and wastes several inches between pictures. It is expected that eventually a harder film will be avail-

472 Annals New York Academy of Sciences

FIGURE 3 . Transmitter optical-mechanical assembly.

able, capable of being contact processed. Such film would be accumulated during exposure, then cut and processed as a single sheet.

B b c k Diagrams

Operation of the system is best understood by reference t o the block diagrams, FIGURES 5 and 6.

In the transmitter an internally modulated He-Ne laser is switched on and off at 56 kHz. A feedback system monitors the first harmonic of laser beam inten-

FIGURE 4. Receiver optical-mechanical assembly.

Schreiber & Gronemann: Laser Scanning of Radiographs 473

MODULATOR BANOPASS AMP

- 1 CONVERTER [-c.NTRz CONTROL

PANEL +

DATA ENCODER

GALVANOMETER DRIVER -t

PARAMETER ENCODER

CLOCK OUT

DATA OUT

START

FIGURE 5. Transmitter block diagram.

sity and compares it t o a reference. The laser modulator is driven by the error signal.

Light falling on the film may be picked up either by reflection ( to permit oc- casional use of reflection copy, such as test charts) or transmission. In the latter case, the light passing through the film is collected on a single photodiode by

MODULATOR DRIVER CONTROL

MARKING POWER CONTROL CONTROL

CLOCK IN DATA IN

FIGURE 6. Receiver block diagram

474 Annals New York Academy of Sciences

means of a large circular cylindrical mirror. A transconductance-type low-noise preamplifier raises the signal t o a suitable level. It is then filtered by a band- pass amplifier tuned t o 56 kHz, demodulated in a coherent detector and applied t o a logarithmic amplifier. The output of the log amplifier is a voltage propor- tional t o the optical density of the film. The goal is to cover a density range of 3.5 optical density units. This voltage is sampled and converted t o a digital code.

By means of the front panel switches, seen in FIGURE 7, the operator selects the number of bits per elements (1-8) as well as the width and length of the picture. The setting of these switches is entered into registers when the start button is pressed, and thereafter all of the signals, pulse trains, control levels, and analog deflection voltages, are generated automatically. In particular, the analog/digital converter is operated at a submultiple of the 56 kbps clock rate as determined by the number of bits per element.

The parameter encoder transmits the scan format information; the first few words transmitted comprise the contents of the parameter registers. Thereafter video data is transmitted via the data encoder in serial binary code until the picture is finished. In addition; clock pulses are supplied from the transmitter t o the communication channel, or, in the case of the DDS channel, the clock is provided to the transmitter from the channel service unit (CSU).

The analog deflection voltage is of the appropriate amplitude and frequency as selected by the switch setting. The galvo driver generates the current required t o operate the galvanometer, while the stepping motor driver generates the cur- rent necessary t o cause the stepping motor t o advance the film at the end of each line.

In the receiver, data and clock pulses are received from the communication channel. The first words, containing the scan parameters, enter appropriate re- gisters, where they control decoding and generation of deflection voltages just as in the transmitter. Subsequent words are converted into analog voltages by the digital-analog converter and applied t o a nonlinear amplifier. The NLA re- converts the logarithmic signal scale t o that of light intensity and compensates for the &log E curve of the film in such a way that the ouput film density would be equal t o the input film density. The “marking power” knob controls the final signal amplitude and essentially calibrates the system for the film sensi- tivity.

An internally modulated laser, similar t o the one in the transmitter, is used t o produce the modulated light beam t o expose the film. In this case, the error signal which drives the modulator is the difference between the laser beam inten- sity as measured by the light monitor and the signal intensity as obtained from the marking power control.

The temperature control of the oven maintains an adjustable constant proces- sing temperature for the dry silver film, to achieve a suitable film characteristic. The oven temperature is adjusted t o compensate for varying film speeds, which may result from changing the scan parameters.

PHYSICAL CONSTRUCTION AND PRESENT STATUS

The two units are constructed in separate, independent, cabinets with the optical-mechanical assemblies on top, as shown in FIGURES 7 and 8. The mechanical and electronic construction was done in an expansive “breadboard” fashion, to facilitate development work and t o permit extensive modifications. An operational system could be made appreciably more compact and “stream- lined.” The transmitter and receiver have so far been connected by a short

Schreiber & Gronemann: Laser Scanning of Radiographs 47 5

476 Annals New York Academy of Sciences

shielded cable and operated in a single room. The light-collecting optics (con- densing reflector) in the transmitter has not yet been implemented. Instead, a long array of solar-cell-type photodiodes, in conjunction with a diffusing reflec- tor, has been used.

RESULTS AND FUTURE WORK

Images of fairly good diagnostic quality have been produced by the system. They are not shown here, as the offset printing process could not adequately show the difference between original and copy. Owing t o the simpler light col- lector in the transmitter (described in the previous paragraph), the signal-to- noise ratio is not as high as aimed at, with the result that descriminable source density is limited t o 2.5 optical density units. The spatial resolution of the trans- mitter is commensurate with the sampling density, i.e. 250 to the inch. Measure- ments of spot size show that a resolution twice as fine is easily achievable, and that a resolution commensurate with 1000 samples per inch over 4 inches of width is feasible with slight modification of the optics.

The reproduced image on the film shows two defects that are thought to be due to the lack of antihalation backing, namely some diffusion, with a resultant reduction of resolution, and interference between the two film surfaces, causing a visible random fringe pattern. A solution t o this problem is sought in coopera- tion with the film manufacturer. Of course, a normal wet-process film would not exhibit these problems. In addition there appear some horizontal and vertical streaks due t o minor defects in the circuitry and in the oven, respectively.

Current and near-future work (which is partly supported by the radiology departments of the Peter Bent Brigham and Beth Israel Hospitals) addresses it- self t o (a) correcting the above-mentioned defects (including completion of the light-collecting optics), (b) incorporating variable sampling density-up to 1000 per inch, (c) interfacing the instruments t o a PDP-11 computer, (d) experimen- tally operating the system within the radiology department quarters of the Peter Bent Brigham Hospital, (e) planning an experimental long distance transmission link, and (f) building another, more compact, version of the improved equipment.

REFERENCES

I . SCHREIBER, W. F. 1974. Laser/dry silver recorder. Proc. SOC. Photo-Opt. Instrum. Eng.

2. SCHREIBER, W. F. 1974. A laseridry silver facsimile system. J. Tech. Assoc. Pulp and Paper 53: 116-122.

Ind. 57(4): 91-93.