US3482093A

US3482093A - Fluoroscopy

Info

Publication number: US3482093A
Application number: US597986A
Authority: US
Inventors: Yoram Palti
Original assignee: Yissum Research Development Co of Hebrew University of Jerusalem
Current assignee: Yissum Research Development Co of Hebrew University of Jerusalem
Priority date: 1965-12-03
Filing date: 1966-11-30
Publication date: 1969-12-02
Anticipated expiration: 1986-12-02
Also published as: GB1155088A

Description

Dec. 2, 1969 .Y. PAL'r A 3,432,093

FLUOROSCOPY Filed Nov. so. 4lease FIG. 1

BRIGHTNESS ENHANCEMENT FIG. 2 1.5.

Loi

o'5500 250 |25 IOO 75 60 50 O 30 YORAM PALTI United States Patent O 3,482,093 FLUOROSCOPY Yoram Palti, Jerusalem, Israel, assignor to Yissum Research Development Company, Hebrew University, Jerusalem, a corporation of Israel Filed Nov. 30, 1966, Ser. No. 597,986 Claims priority, application Great Britain, Dec. 3, 1965, 51,446/ 65 Int. Cl. G01n 23/04; H01j 37/22 U.S. Cl. 250-53 1 Claim ABSTRACT F THE DISCLOSURE BACKGROUND Fluoroscopy of humans by means of X-ray is very fequently used in various diagnostic and surgical procedures. Recently the use of this convenient and important examination tool has been drastically curtailed because of the fear of cumulative effects of radiation, which may cause severe damage. As the method of uoroscopy is of considerable diagnostic value, any method which is adapted to decrease the amount of radiation applied to the examined person is of considerable value.

During the conventional uoroscopy of still patients some of the radiation applied to the patient reaches the examiner who may be exposed to dangerous cumulative effects. A similar problem arises in the X-ray examination of welded, cast and other articles for the detection of manufacturing flaws. Any method which decreases the required output of an X-ray machine, makes possible the use of such machines which are safer and of simpler and less expensive construction.

In order to decrease the quantity of applied radiation, various optical and electronic devices have been developed. Among these there may be mentioned the use of a uoroscent screen in conjunction with an image intensifier, adapted to result in an image of adequate brightness with substantially decreased intensity of radiation. Such intensifiers are rather expensive, and devices of equivalent performance, but of substantially decreased cost are widely sought.

Conventioinal X-ray machines make use of continuous radiation (which may be of the frequency of the current used, such as 50 or 60 cycles), and after passing through the tissue the radiation impinges on a fluoroscent screen, where an image is obtained which corresponds roughly to the intensity of the impinging radiation.

SUMMARY that the fluoroscope employed is a fluoroscope as aforesaid.

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The source of X-rays may be an X-ray tube provided with a grid or other control electrode (eg. an electrode arranged to deflect the electron beam away from the target) by which the generation of the X-rays is rendered intermittent, or by supplying the tubes anode with pulsatile potential. For example, a high frequency pulse generator may be placed between the power source and the X-ray tube of the uoroscope. Alternatively the stream of X-rays may be interrupted with a mechanical shutter. Advantageously the repetition frequency at which the X-rays are emitted in the direction of the screen and the time fraction of the cycle for which the X-rays are emitted in the direction of the screen are both variable. The intensity of the X-rays emitted in the direction of the screen is preferably also variable.

DRAWING FIG. 1 is a schematic circuit diagram of a pulse generating circuit for providing X-ray pulses in accordance with the present invention; and

FIGS. 2 and 3 are graphical representations of screen brightness based on various pulse frequencies.

DESCRIPTION It will be noted that when the actual generation of the X-rays is rendered intermittent, all parts of the screen tend to be affected simultaneously by the intermittent effect. However Where a mechanical shutter is provided, there will often be a tendency for the different parts of the screen to be affected in sequence; this is especially true where the shutter is a rotary device having parts which are substantially opaque (eg. 'formed of lead), and apertures or other parts which are transparent, to the X-rays or having parts of markedly different transparencies to the X-rays. A mechanical shutter in the form of a disc, with a variable speed drive, which has a transversely movable axis of rotation and which is divided into transparent and opaque parts bounded by generally radial, but appropriately non-linear boundaries, may be employed to vary both the repetition frequency and the time fraction of the cycle for which the X-rays are emitted in the direction of the screen.

In place of a mechanical shutter, rotating disc or otherwise, an electronic system may be used. Such a system may merely comprise a variable pulse generator in the X-ray machine circuit. Such a pulse generator may be located immediately before the X-ray tube in the circuit, or, if desired, the entire power input to the X-ray machine may be controlled by such a pulse generating circuit.

Various other alternatives are also possible including mechanical switching devices. For example, power to the X-ray tube or the machine may be controlled with a rotating disc switch having electrically conductive and insulating areas thereon and an electrically conductive element sliding along the surface of such rotating disc.

For convenience of description, reference will be made hereinafter to pulses of the radiation. Actual pulses are obtained only when the radiation from the X-ray tube is cut off or reduced and then returned to its former level in each cycle. They are not obtained with the mechanical devices such as shutters which do not affect all parts of the screen simultaneously but it will be understood that references to pulses are to be taken as including such shutters unless the context otherwise demands.

In using the apparatus the frequency, duration and intensity of the pulses will be adjusted so as to result in a bright image, yet with substantial decreased total quantity of radiation as compared with conventional methods.

The brightness of the fluorescent screen depends on the intensity of the radiation and to a lesser degree on its fr equency, the eiciency of most screens being somewhat higher at higher frequencies. The total amount of emitted light depends also on the total duration of irradiation. It would be expected that the use of pulses totalling say half the time of observation would result in an image of about half or slightly more than the intensity of continu ous radiation of equal intensity. This would be found to be true if the light emitted by the fluorescent screen were measured with photometric means, but the perception of light-intensity (brightness) by a human observer depends on the intensity of the light and on its distribution in time. If the light is applied in pulses, there is obtained the physiological phenomenon called sommation, according to which the application of the stimuli results in an effect which is considerably greater than the time average of the individual stimuli. The sommation gives rise to a bigger and longer nervous response per pulse resulting in a perception by the observer of increased intensity and prolonged duration as compared with a continuous stimulus of integrally equal total radiation. The prolongation of perception is usually due to the development of a repetitive response, in this case at some levels beyond the obscrvers receptors themselves.

There exists a critical fusion frequency (CFF), which varies between about -80 pulses per `second depending upon the intensity of the light. At this, or at a higher frequency, there is obtained an apparently continuous image. The frequency of pulses used for fluoroscopy according to the present invention is preferably above the CFF, so as to give the observer the impression of a continuous image. It was found experimentally that when the duration of a light pulse is around 1 millisecond and the pulse repetition rate is around 50 pulses/sec. (which is over the CFF for light intensities obtained by uoroscopic screens) the sensation of brightness is enhanced (see FIG. 2). Such pulses give a sensation of brightness equivalent to the brightness of a continuous source of light, the energy output of which is from 2 to 3 times larger than the integral output of the pulsatile light.

The pulse duration and frequency which give the best brightness enhancement depend on the size and intensity of the light source, on the degree of dark adaptation of the observer and on his degree of fatigue. For this reason the repetition frequency at which the X-rays are emitted in the direction of the screen and the time fraction of the cycle for which the X-rays are emitted in the direction of the screen should preferably both be variable as also should the intensity of the X-rays so that the observer is able to choose the combination of parameters which gives the optimal sensation with the minimal total output of radiation.

Advantageously means are provided to give the observer information on the rate at which X-radiation is being received through the screen. Such means may, for example, be a radiation meter or a badge containing piece of film which is developed after a known period and compared with a standard.

The controls provided should conveniently enable the observer to adjust the pulse duration within the range from 0.3 to 3 milliseconds and preferably less than 1 millisecond. They should also enable the repetition frequency to be adjusted within the range of from to 150 pulses per second, e.g. from to 75 pulses per second in order to get the best results under the referred different conditions. Intensity control should, of course, also be provided. Under some conditions the image might actually be brighter than given by the unpulsed source since the efliciency of the uoroscopic screen is usually better for intermittent illumination.

FIG. 1 is a schematic diagram of a system which emits constant and pulsatile light. In the drawing:

G.T.-Sylvania type R1131C Glow Tube Pulse- Square pulse input from Tektronix type 161 pulse generator A, B-Manually operated switches R-Relay operated switch When the cathode (3rd leg) of a Glow Tube is connected to the plate of the penthode GL6, it generates square light pulses. When it is connected to the ground it generates a continuous light, the intensity of which can be controlled by a potentiometer.

With switch A in position 2 and switch B in position 1 GTI emits pulses of light while GT2 continuous light. On turning B to 2, GTI begins to emit continuous light and GT2 pulsatile light. When A is in position 1 the switch R replaces B in the circuit and it automatically switches from position 1 to 2 every 1 or 2 seconds. Under these conditions each Glow Tube emits alternatively every 1-2 seconds continuous or pulsatile light.

The mean light enhancement achieved by the use of the present invention may be illustrated by employing the Glow Tubes GT, and GT 2 of FIG. 1 to illustrate the response of a uoroscopic screen to continuous and pulsed X-ray beams. However, for actual X-ray tube control, one or both of these glow tubes could be replaced by a X-ray tube in the manner illustrated for the Glow Tubes in FIG. 1 so that the X-ray tube would be pulsed by the variable pulsingcircuit.

Light enhancement is the relationship between the total light energy emitted in pulsed illumination unit time and the total light energy emitted in continuous illumination. This may be charted, as illustrated by FIGS. 2 ad 3, by

making calculations according to the following equation:

enhancement Vp-Pdwhere Vp=the light intensity of one pulse;

Vc=the light intensity of a continuous source;

P=the duration of time between the beginning of a pulse and the beginning of the pulse which follows it;

Pd=the duration of the pulse The graph of FIG. 2 illustrates the mean light enhancement calculated at various pulse frequencies using a circuit similar to that of FIG. 1 including a continuous glow tube source and a pulsed Glow Tube source.

The graph of FIG. 3 illustrates the mean light enhancement calculated at various pulse frequencies using a single light source which alternately produced continuous and pulsed illumination in the manner described in connection with the circuit of FIG. 1. It will be understood that an image intensifier may be used with the apparatus and method of the present invention to give additionally improved results.

What is claimed is:

1. A method for minimizing radiation in a fluoroscope while maintaining brightness perception by achieving brightness enhancement at a frequency above the critical fusion frequency range for a fluoroscopic screen which includes directing an X-ray beam at a fluoroscopic screen and pulsating said beam at a frequency of 30-150 pulses per second but above a critical fusion frequency falling within the range of from 20 to 80 pulses per second for fluoroscopic screens to form square wave pulses having a pulse duration of from 0.3 to 3 milliseconds.

References Cited UNITED STATES PATENTS 2,617,046 11/1952 Douma et al. 250-102 2,790,085 4/ 1957 Fransen et al. 250-65 2,905,827 4/1959 Graves 250-65 2,937,277 5/ 1960 Euler, Jr. et al. Z50-65 3,277,302 10/ 1966 Weighart 250-102 RALPH G. NILSON, Primary Examiner S. C. SHEAR, Assistant Examiner