Radiology worldwide – the WHO approach

Solutions

Radiography

When WHO had ascertained that the vast majority of x-ray examinations were going to be "plain", non-fluoroscopic radiography, the following design requirements were established for the radiographic system.

1. High quality images, as good as or better than most radiographs available in major centres.
2. Standard radiographic projections, which will easily be recognized by any doctor.
3. Safety for staff and patients.
4. Easy installation and maintenance. Easy operation.
5. Ability to operate with poor main electrical supply.

Fulfilling these requirements is not difficult, but WHO chose technical solutions which are unconventional. The explanation which follows does not indicate any order of priority, because all requirements are interdependent.

Technical solutions

Power supply
Developing countries, and indeed almost any country, may have a highly variable and often intermittent main electric supply. Batteries become the logical method of power storage, because power from batteries is available at any time. Batteries are rechargeable from many different power sources and are insensitive to voltage or frequency fluctuations. The power from a simple grounded "kitchen" or similar outlet or from a small 230 V, 10 A alternating current (AC) generator is all that is required. Sealed and maintenance- free lead-acid batteries are recommended.

The design of high tension generators using store d direct current (DC) energy must include inverter technology: the use of a medium or high frequency converter x-ray generator is essential.

X-ray generator
The x -ray unit must be able to produce good radiographs of a finger (low power), a child' s chest (medium power, very short exposure time), a large adult chest (high power, short exposure time), and a lateral view of the lumbo-sacral junction of a heavy patient (very high power, long exposure time).

To accomplish this the x-ray tube voltage must range from about 50 kV to 120 kV and these voltage values should be reached in 2-3 ms (milliseconds). The current-time product (milliampere-second value) must be variable between 0.8 and 200 mAs in 26% increments. The instantaneous output of the generator must be more than 12 kW and the total energy output must be no less than 25 kWs (kilowatt-seconds). If a "green" screen-film system is used, a total energy output of 12-15 kWs may be satisfactory .
Given the almost constant output from a converter generator, the image quality will depend on focus-film distance, object-film distance, the collimation and the use of an anti-scatter grid.

Focal spat and imaging geometry
WHO recommends the use of a focal spot of 1 mm or less. The focus-film distance is 140 cm for ALL examinations. The patient - film distance is 2-3 cm. Thus, a chest film exposed at 140 cm will have the same

Figure 3. Patients can be examined standing, sitting or lying down; the beam can be angled as required. Cross-table decubitus projections of the skull, chest or abdomen are easily obtained. (From WHO-BRS: Manual of Radiographic Technique)

 

magnification of the heart as one made at the more usual 180-200 cm and a patient-film distance of 6-8 cm.

Grid
Because the focus-film distance is fixed, an accurately focused anti-scatter grid with a ratio of 10:1 can be used. The line density should be 4060 lines/cm. A correctly focused high-quality grid with more than 40 lines/cm is almost invisible on the radiograph when the film is viewed at a distance of 30 cm or longer. A bucky mechanism is not necessary, which saves space in the cassette holder, reduces the complexity of the unit and saves maintenance and money.

Figure 4. The two pages of the WHO Technique Manual which show how to take an "erect" PA chest radiograph. The operator follows the instructions given in the diagrams and, at the bottom of the second page, can see the result which should be obtained.

 

The specified range of movements
This permits a WHO-designed x-ray unit to function equally well as a chest unit, a vertical bucky, or a floating-top table. Although there is a fixed tube-film relationship, it is possible to use a vertical or horizontal beam +/- 30°. The range of movements is best shown by illustrations (se e Fig. 3). As part of the imaging system, WHO has produced a "Manual of Radiographic Technique", designed for this type of unit and demonstrating over 100 standard projections (see Fig. 4).

Exposure values
Based on the use of a 200 speed screen- film combination, exposures can be obtained from the technique manual, using measurements of the patient’s thickness. If necessary, it is easy to recalculate the mAs-values for other screen-film combinations which have different speeds. All exposure values in the WHO manual are valid for 3-phase or multipulse converter generators. If an outdated single-phase generator is used, the mAs-values will probably have to be doubled.

Safety

The WHO-specified frequency converter generator produces a high quality x-ray beam at almost constant potential. The x-ray tube should have a total filtration equivalent to about 4 mm aluminium (Al). Thus, the amount of soft radiation likely to be absorbed in the patient is very low.

The collimator cannot be removed and in the simplest WHO specification the collimated x-ray fields are matched to the film sizes. Even when the collimator is wide open, the x-ray beam cannot bypass the cassette holder, which has a radiation-absorbing back.

The focus-film distance is fixed at a longer distance than is conventional for most general radiography and the grid is accurately focused to this non-variable focus-film distance.

These factors added together result in an average patient surface dose of about 50% when compared with conventional equipment using single phase generators, poorly filtered x-ray tubes, primitive collimators and variable focus-film distance. It is possible to reduce doses further by replacing the common "blue" screen-film system with a "green" screenfilm system, doubling the sensitivity in the 90-120 kV range. Because correct alignment is assured and the scattered radiation is low, this WHO design is one of the safest x-ray units ever produced.

Design and installation

The examination stand of the WHO-designed x-ray unit consists of a long arm, shaped as a question mark, carrying an x-ray tube and a cassette holder at opposite ends (see Figure 3). This arm is mounted on a fixed vertical column in such a way that the assembly can be moved up and down and rotated to provide different beam directions in a single plane. The entire stand weighs 200-300 kg. The column has a rectangular base (about 20x30 cm) and may be installed standing on a flat, strong floor, held steady by a 40-50 cm long support to the nearest wall. Instead of using a wall support, the unit can be mounted on a large (about 1m2) base plate of steel.

The patient may be examined standing, sitting, or lying down on a trolley with large wheels and x-ray translucent top. Cross-table decubitus projections of any part of the patient are easy to obtain (Fig. 3). Any WHO-specified x-ray unit can be installed by two people in a single day.

There is no need to discuss the details of installation, room size or layout any further. One possible layout for a very small x-ray department

Figure 5. Layout of a 42 m2 x-ray department for 2000-3000 examinations per year. (Scale 1:100). There are many alternative designs.

(2000-3000 exams/year) is shown in Fig. 5. Many variatations are possible. The specifications of equipment, room sizes, films, darkroom equipment, protection requirements and everything else required can be obtained from the Chief of Radiation Medicine, WHO/OMS, 1211 Geneva 27, Switzerland.

How does the WHO-designed equipment work in practice?

The WHO Basic Radiological System, originally abbreviated to WHOBRS, was recently (1993) renamed to include ultrasound. It is now known as the WHO Imaging System - WHIS - with components of Radiography (RAD), Ultrasound (US), Darkroom (DR), and Manuals (MAN). All the sub-systems have been well tested, particularly the WHIS-RAD and the radiographic manuals. Extensive experience in the use of the WHISRAD has been documented by several primary care centres in Sweden, at a mass-chest centre in Moscow, and in nine rural hospitals in Africa.

In and around Lund, Sweden, there are four installations supervised by the University Radiology Department and two private installations. More than 250,000 examinations have been made with an average of four films per examination. The results completely meet the needs of patients and staff, the radiographers and radiologists from the University Hospital in Lund, and the referring physicians.

In 1990, in Moscow, the International Organization for Migration (10M) needed to have chest x-rays of about 200,000 emigrants within 3 years. A single WHO installation examined 300-400 chests every day, completing 50,000 examinations in the first nine months with a one-day breakdown - and this was due to an overload of the automatic processor, not the x-ray unit. The film quality was consistently high. After three years, with the addition of a second WHO-unit, more than 300,000 examinations have been made. The only repair has been the replacement of the x-ray tube in the oldest unit.

In different countries in Africa, nine WHO-units were installed as gifts from the Netherlands Charity "SIMA VI", all in small rural hospitals. After they had been working for two years, an independent team composed of a radiologist, a radiographer and a radiation physicist was sent to inspect each unit, check radiation output, film quality, clinical results and "customer satisfaction". While the teams found they could further improve image quality, most of the faults had occurred in the darkroom. The overall conclusion was that the WHO design was the best, and ideally suited for general purpose radiography, especially in a rural setting.

Image quality with WHO-specified radiographic units

It is difficult to define the "quality" of an image. The subjective impression of many observers is that the images produced with the WHO-specified units are equal or better than those produced in many university level hospitals. Generally, the image quality is superior to the image quality of most conventional x-ray installations.

Attempts have been made to quantify these impressions. In an assessment by the National Health Service of the United Kingdom, equipment following the WHO design was compared with conventional x-ray equipment at an 800 bed hospital. The images of the chest, abdomen, skull, spine, pelvis and extremities produced by the WHO unit were considered to be excellent by the radiologists in 20% of the examinations versus only 6% for conventional equipment. About 1 % of the radiographs were considered to be unsatisfactory for both types of equipment.

Radiation protection and patient dose with the WHO-BRS

The International Commission on Radiological Protection (ICRP) Publication 34, Protection of the Patient in Diagnostic Radiology, states that "The aim of the radiation protection of the patient has gradually shifted from a concern about population exposures and hereditary effects, to the ambition of limiting the risk to the individual patient". Consequently, during the past decade, authorities responsible for radiation protection have become increasingly involved with measuring the dose received by patients during x-ray examinations.

Surveys, each using essentially the same techniques, have been conducted in England, France, Italy and Sweden, and these results have been compared. Among the data reported was the mean entrance surface dose, which was obtained by placing thermoluminescent dosimeters on the skin according to a method developed by the National Radiological Protection Board (NRPB) of the United Kingdom.

For the majority of the examinations the entrance surface radiation dose from the WHO-unit tested in Sweden is several times lower than the mean values obtained in England, France and Italy, where conventional equipment was used.

WHO manuals for diagnostic imaging

The manuals which are part of the World Health Imaging System (WHIS), Radiographic Technique, Darkroom Technique, and Radiographic Interpretation for General Practitioners, have been accepted all over the world. The Manual of Radiographic Interpretation for General Practitioners had, in 1994, been published in nine languages, with a total estimated sale of more than 100,000 copies. The fourth WHIS-Manual, the WHO Manual of Diagnostic Ultrasound, will be published in 1995. The fifth manual, describing the design, layout and equipment of x-ray department buildings, the choice of equipment, and training, should also be published in 1995.

Ultrasound

There is an increasing need for ultrasound examinations, especially for the abdomen and in obstetrics and neonatal care: it has become a very popular way of imaging. Because there is no harmful radiation (so far as is known in 1994), many physicians and others have purchased ultrasound equipment. Some small ultrasound units seem attractive and cheap, but most do not give a good quality image. Too many ultrasound units are used by untrained and unqualified people, often to learn the sex of the fetus. In some societies this has led to abortions because the fetus is of the "wrong" gender.

WHO has reacted to this demand for ultrasonography in two ways. First, by providing specifications which include all necessary features for a general purpose ultrasound unit (WHIS-US or GPUS). Units meeting these minimum specifications will give high quality images and satisfy all the general diagnostic needs of a busy general or obstetric clinical practice. In many places the Doppler techniques will be unnecessary: there are neither the vascular diseases nor the vascular surgeons to warrant extra expenditure. These specifications were originally drawn up by an expert international group and have since been updated. Secondly, by emphasizing the need for training and by the publication (1995) of a Manual of Diagnostic Ultrasound, which is designed for those who work alone, especially those who have no experience with the clinical use of ultrasound.

General purpose ultrasound specifications are available, either in the Manual of Diagnostic Ultrasound or from the Radiation Medicine Unit at WHO, Geneva. These provide all the necessary information required to choose a unit and will be kept up-to-date. The quality of the image remains the major guiding principle behind the specifications.

Ultrasound equipment is probably physically safe to use, but there is potential danger because many diagnostic errors will occur if the operator is not properly trained. WHO has recommended minimum training requirements. Radiologists must assume the responsibility for insisting that these minimum standards (both professional and ethical) are met. Currently (1994), very few governments have laid down regulations governing the use of ultrasound, but there is hope that these will soon be forthcoming.

 

Philip E.S. Palmer, Thure Holm and Gerald P. Hanson