The detection of symptomless or latent pulmonary tuberculosis by mass radiography has been the subject of much careful investigation over the past twenty-five years.  The results of chest surveys of apparently healthy groups, carried out in many countries, with full-size films, photographic paper or by screen examination (fluoroscopy), have clearly demonstrated the value of mass radiography, but all these methods have disadvantages which make their application on a large scale impracticable.  The expense of full-size X-ray films is considerable, and the time required for processing and then interpreting them is too long; storage also presents a difficulty.  Photographic paper is cheaper than film, but is has the same disadvantages of lengthy processing, slow interpretation and difficulty in storage, and the additional failings of poor detail and limited latitude for exposure.  Large scale screen examination is cheap and easy to conduct, but fatiguing and dangerous for the operator; when carried out rapidly, it may result in minimal tuberculous lesions being overlooked; and no permanent record is provided.

It has long been obvious that all these disadvantages could be overcome by miniature photography of the chest image on the fluorescent screen, and, as far back as 1912, Caldwell not only predicted mass radiography on a national scale but described the technical developments necessary for its successful introduction.  Approximately between 1925 and 1935 all the technical prerequisites for successful mass radiography were produced.  Holland developed fine focus, rotating anode tubes, England contributed the highly luminescent Levy West screen, and Germany wide aperture lenses; while America, England and Germany all developed the necessary fine grain, fast film.  It was left to de Abreu of Brazil to combine all these, and to employ mass radiography as we know it, on a large scale.  He worked ceaselessly on the problem from 1924 onwards, and saw his labours justified in 1936.  The pre-war literature of miniature radiography has been ably reviewed by Bentley and Leitner (1940).

Since the outbreak of war, miniature radiography has made great strides in tits application to the examination of large groups of the apparently healthy, and its technique has been improved by intensive research, in which this country has had a full share.  Both in the British Empire and in the U.S.A. the main field work has been government-sponsored on Service personnel and war workers but individual surveys have also been reported.  In Britain, the Royal Navy and Royal Air Force led the way; because of shortage of medical personnel and the demands of the Services on X-ray manufacturers, it was not until 1943 that routine work on civilian groups was begun.

One of us has elsewhere outlined the origin of the British national civilian mass radiography scheme (Hart, 1944).  The Committee on Tuberculosis in War-Time, set up by the Medical Research Council at the request of the Ministry of Health, reported in October, 1942 (M.R.C. Special Report Series No.246).  One of its proposals was for the routine use of mass radiography in civilians by means of the miniature method, which its Technical Sub-Committee had reported as having reached a suitable standard of radiographic technique and apparatus design.  The Ministry of Health responded by promptly implementing this recommendation, and in December, 1942, the Minister announced (Circular 2741) that mobile miniature radiography sets, in accordance with the specification approved by the Committee’s experts, would be made available for the use of a limited number of local authorities, the limits being set by the restrictions of war conditions on production and staffing.  These sets have been produced during 1943 and 1944 and, at the time of writing, 13 local authorities have commenced operations.  Meanwhile, the team under the Medical Research Council, with which we have been concerned, has carried out field trials with one of these sets, and the experience gained has been drawn upon extensively in the present report.

The following is a summary of the main features embodied in a mobile apparatus.  This form of apparatus was used for the Medical Research Council survey, and is also being supplied for the national scheme to the local authorities through the Ministry of Health.  We regard the majority of these features as essential in any similar type of apparatus.  Some of the features are described in greater detail in subsequent sections of this report.*

(i)   Power-unit.

The high tension power-unit, embracing four valve rectification, has an output of from 100 to 400mA, with a maximum kilovoltage of 100 kVp and 91 kVp respectively.  Provision is made also for screening at 3 mA at up to 85 kVp.  When the electric mains supply is satisfactory, miniature radiographs are taken at 200mA and large films at 300 mA.  The exposure time for subjects of average chest thickness 8 to 10 inches is then 0.1 sec. for miniature, and 0.05-0.07 sec. for large films.  Unsatisfactory mains supply (not met with on the M.R.C. survey) may necessitate exposures being made at 100 and 150 mA for miniature and large films, respectively, with appropriate increases in exposure time.

(ii)  X-ray Tube.

The rotating anode X-ray tube supplied with the apparatus has a dual focus.  The focal-spot sizes are 1mm and 2mm square; for the higher milliamperages employed in mass radiography, it is necessary to use the broad focus.  It is desirable to have a reserve tube.  A multiple diaphragm fitted to the tube aperture serves to confine the X-ray beam to the area of the fluorescent screen at each of four distances 36, 48, 60 and 72 inches.  The alignment between tube and screen is checked by means of an optical centring device.

(iii)  Fluorescent Screen and Grid.

Experience has shown that 16 by 16 inches is the optimum size for the fluorescent screen, the viewing side of which is covered by a sheet of protective lead glass.  The yellow-green type of screen† used with the appropriate film forms the fastest combination as yet available.  Important features of the radiographic grid are even spacing of the lead slat, and the highest possible radiographic translucency.

(iv)  The Camera.

The control of electrically operated camera is linked with the X-ray exposure switch.  The camera magazine is made to take 82 feet (25 metres) of 35-mm film, which is removed in exposed lengths of from 5 to 25 feet; a cutter to divide the film is incorporated in the camera.

(v)  The Lens.

Experiments have been made with a number of different lenses, coated and uncoated, and it has been found that a new British-made lens of 2-inch focus and f/1.5 aperture, which is fluoride-coated, gives superb quality in the radiographs and is also faster than other lenses of this type tested.‡

(vi)  Identification.

The identification system enables the number on the individual record card to be photographed on to the lower border of the miniature chest radiograph.  The illumination of the card is controlled by the X-ray exposure switch.  The mechanical position of the card is a factor in operating the electrical system; thus, the exposure cannot be made unless the card is place correctly in the slot provided.

(vii)  Direct Radiographs.

High quality, full-size chest radiographs can be taken on this apparatus at distances up to 72 inches, although a 60-inch distance has been adopted for routine postero-anterior views.

(viii)  Screening.

Facilities for screen examinations are limited to positioning as a preliminary to taking full-size radiograph.

(ix)  Adjustment for Height.

The use of a flexible cable system allows for a good range of subject heights, and only on rare occasions is it necessary for an examinee to stoop slightly to enable the whole of the chest to be included.  Occasionally, an examinee is asked to stand on the higher portion of the camera-tunnel base in order to reach the screen, and persons under the height of 4 feet 8 inches stand on a low footstool.

(x)  Passage of Examinees.

It is unnecessary for examinees to pass through the apparatus between the tube and the camera tunnel, as was originally anticipated.  They step in and out from one side only, passing by the apparatus, so that the operators, with their controls, work completely away from the side used by the examinees.

(xi)  Protection.

Protection from excessive radiation for the radiographers during routine operations is quite satisfactory.  Metal protective screens, with lead glass viewing windows, are provided for the positioner and control table operators.

(xii)  Mobility.

The apparatus is mobile and can be dismantled or reassembled by two women in 12 minutes.  Collection from the X-ray room and loading into the van (or vice versa) takes approximately one hour.  The carrying van should be fitted with tackle to raise and lower the three heavier parts of the apparatus.

(xiii)  Fitted Darkroom Van.

It is convenient for the carrying van to be fitted as a darkroom, thus rendering unnecessary a general purpose darkroom in the survey premises.

(xiv)  Generator.

The M.R.C. Unit has been fortunate in that suitable electric mains supplies have been available, but there is no doubt that a van fitted with a specially designed 20 kVA generator is desirable as anj alternative to an unsatisfactory mains supply.

Further details of this apparatus are provided by its manufacturer in booklet “Installation and Operating Instructions”, and a general discussion of the principles involved in its use has been given by Minns (1943).  It is imperative, however, for the radiographer to be able to deal with minor breakdowns; special instruction to this end will always be needed, and is at present included as part of the Ministry of Health’s mass radiography training course.

A considerable number of improvements has been made in this apparatus as a result of our experience.  These are incorporated in all new apparatus and, as far as practicable, will be added to all apparatus already issued.

*  The manufacturer of the miniature radiography apparatus described throughout this report is Watson and Sons (Electro-Medical), Ltd.  The apparatus was designed in accordance with a specification approved by the Technical Sub-Committee of the Committee on Tuberculosis in War-Time.

†  This is a Levy-West Mark 39 screen.