Physical characteristics of four roentgenographic chains proposed for chest radiography

Transcription

1 1982, British Journal of Radiology, 55, 1-6 DECEMBER 1982 Physical characteristics of four roentgenographic chains proposed for chest radiography By H. Manninen, M.D., M.Sc, M. Pitkanen, M.Sc. and M. Wiljasalo, M.D. Department of Diagnostic Radiology, University Central Hospital, SF Kuopio 21, Finland (Received January 1982 and in revised form June 1982) ABSTRACT Four types of roentgenographic chains have been studied in the present work: the ordinary grid technique with 10 screenfilm combinations, the air gap technique with 8 screen-film combinations, the roentgen photofluorography (RP technique) and the spotfilm technique. To study resolution and contrast properties a gold plate was radiographed in a 20 cm thick water phantom and the equivalent passband and the inherent subject contrast were calculated. The effect of motion unsharpness on resolution was examined theoretically. The RMS noise and the speed of each technique were evaluated. The air gap technique proved to be a good alternative to the ordinary grid technique. Rare earth screens decrease the radiation dose to at least one third of that of a medium-speed film/calcium tungstate screen combination without noticeably impairing resolution, contrast or noise properties, but reducing motion unsharpness considerably. Differences between various screen-film combinations of equal speed were not very marked. The RP technique proved to be poorest in almost all respects. The spotfilm technique seems worth developing for chest radiography. General requirements for radiographic chains used in thorax roentgenography are good resolution, contrast and noise properties, a small radiation dose and a short exposure time. In mass chest radiography, roentgen photofluorography (the RP technique) or the grid technique with medium speed film/calcium tungstate screen have ordinarily been used. A remarkable dose reduction can be achieved with the new rare-earth screens (Ardran et al, 1975). The air gap technique has also been used in chest radiography (Trout et al, 1975), as have modern image intensifier systems using spotfilm technique. The study presented here was undertaken to evaluate physical properties of four different roentgenographic chains used or proposed for chest radiography. TECHNIQUES AND METHODS The systems considered The four types of roentgenographic chains considered are: A the ordinary grid technique with 10 screen-film combinations B the air gap technique with eight screen-film combinations *Present address: Department of Radiotherapy, University Central Hospital of Tampere, SF-362 Pikonlinna, Finland. C the RP technique D the image intensifier spotfilm technique For the grid and air gap techniques a three phase, sixpulse generator (Philips DA 0) with Philips Rotalix SR0 03 tube was used. In the air gap technique, instead of the grid there was an 18 cm thick air space between the object and thefilm.the RP system consisted of the Pleophos 4S/Odelca system. For the spotfilm technique, a three-phase, twelve-pulse generator (Valmet BR 1) with Philips Diagnost 62 image intensifier was used. Characteristics of the systems are listed in Table I. Focal spot measurements The focal-spot dimensions were measured according to recommendations of the International Electrotechnical Commission (IEC, 1970) with a 30 fxm pinhole. A multi-exposure technique at a high voltage of 120 kv and with ma-values typical for a chest radiograph was used. Film development The films were developed at 33 C. The developing time was s in the grid, air gap and spotfilm techniques, and 1 s in the RP technique. The developing conditions were controlled with test films (Agfa Gevaert System Control, RP 701 A). Film characteristics The characteristic curves (H and D curves) were obtained from 20 exposures in the density range from 0.4 to 3.4 for each screen-film combination and for the RP and spotfilm systems, all with a 10 cm thick water phantom. A dosemeter (Ionex 2500/3) with a 0.6 cm 3 ionisation chamber was used to obtain the corresponding exposures. The gamma of each characteristic curve was calculated, it being defined as the slope of the straight portion of the logarithmic H and D curve. The gradient of the characteristic curve was calculated by numerical differentiation. The area under the gradient-density curve, A gd, characterizes the contrast properties of the combination in question over the whole density area. The latitude, L (defined as L = log E 2 log E t, where E 2 and E t are the exposures required to produce a film density of 2.0 and 0.25 above base plus fog density, respectively) was calculated to determine the range of exposures over which an acceptable radiograph can be obtained in ordinary clinical radiography / $2.00/0 1

2 VOL. 55, No. 660 H. Manninen, M. Pitkdnen and M. Wiljasalo TABLE I THE ROENTGENOGRAPHIC CHAINS AND THEIR CHARACTERISTIC FEATURES System Focal spot mm Focus-to-film distance cm System characteristic A. Grid technique B. Air gap technique C. RP technique D. Spotfilm technique : 1 Stationary grid, 40 lines/cm 18 cm air gap x mm film x mm film, 9" image intensifier Resolution measurements For the resolution studies the polished edge of a 170/mi thick gold plate was radiographed in a 20 cm thick water phantom made of 5 mm thick perspex. The edge was radiographed at the back of the phantom {i.e. nearest the film = 0 cm), at the front of the phantom {i.e., nearest the tube = 21 cm) and inside the phantom, 12 cm from the film side. The experimental arrangement for the air gap technique is shown in Fig. 1. The edge was placed perpendicular to the tube axis in the central ray, and, in the grid technique, it was also positioned to be perpendicular to the grid laminae. The central ray was found by radiographing a 30 cm long hollow metal cylinder with a 3 mm hole in the end-plate facing the film and a 1 mm hole in the opposite end. When aligned with the central ray the images of the holes are concentric. The field size used was 35 x 35 cm, except for the spotfilm technique when it was 23 x 23 cm. The edge images were scanned with a double beam micro-densitometer (Joyce Loebl & Co, MK III CS) with a light aperture of 0.04x1.3 mm. The density distribution was converted into a radiation distribution by the characteristic curve of each film-screen combination to obtain the edge response function (ERF). The density over the edge was from 1.2 to 1.8. So the density of the image fell on the monoexponential portion of the characteristic curve. The ERF was numerically differentiated with a sample interval of 60 nm to obtain the line spread function (LSF). The modulation transfer function (MTF) was further calculated as a Fourier transformation of the LSF. The equivalent passband, N e, defined as the integral over the squared MTF (Rao et al, 1978), was used as a single measure of the resolution. Integration was done down to the 1% level of the MTF curve. To study the effect of motion unsharpness on resolution, a uniform velocity of lomms" 1 of the object was assumed. The MTF of motion unsharpness (MTF mo ) was calculated by the technique suggested by Morgan (1962). The equivalent passband corrected for motion unsharpness (N em ), was derived from the MTF mo. The total MTF (MTF m ) was obtained by multiplying the original MTF by the MTF mo. To obtain the exposure time for each technique, radiographs of equal density were taken of an Alderson thorax phantom. In the study no attention has been paid to evaluating the effect of distortion in the periphery of the field on the resolution. Measurement of inherent subject contrast In order to measure contrast properties of the different techniques the inherent subject contrast, C s, of the gold plate was calculated using the formula : where D o is the density of an area of the image of the gold plate and D a is the density in an adjacent area. System speed The speed of each system was determined as the reciprocal of the film exposure in roentgens required to produce a film density of 1.0 above base plus fog, and is expressed relative to the speed of the screen/wicor-x RPfilmcombination as used in the grid technique. Image noise To measure noise properties of the different imaging systems, the root-mean-square (RMS) noise was calculated as the standard deviation of the density measurements of a uniformly exposed film. The SCATTERING MEDIUM (VATER) *- 13 CM X-RAY TARGET KV p - FOCAL SPOT 1.6*4x2.32 MM FIELD 35x35 CM SIZE MBER ^ / " ' ~~ - - bold PLATE CASETTE MTH FILM AND SCREEN -20 CM 8 12 CM 350 CM FIG. 1. Experimental arrangement for water phantom studies of the air gap technique.

3 Physical characteristics of four roentgenographic chains proposed for chest radiography DECEMBER 1982 measurements were made at 300 independent locations on a film of mean density 1.0 ±0.1 using the 0.3 mm diameter aperture of a Repromaster RM 21 densitometer. In the RP and spotfilm techniques the RMS noise was normalised relative to the image size of 35x35 cm. Figure of merit In order to consider spatial resolution, large object contrast, exposure time and the radiation dose of different techniques as a whole, an overall figure of merit, F m, was defined as a sum of the N em, C s and speed and is expressed as a percentage of the best combination. The reproducibilities of the MTF and C s determinations were tested by scanning and calculating twelve edge images twice. RESULTS The gradient-density curves for different screen-film combinations and the RP and spotfilm techniques are shown in Fig. 2. The values of the gamma, the area under the gradient-density curve, A gd, latitude and RMS noise for the corresponding combinations are given in Table II. Table III shows the equivalent passband, N e, the equivalent passband corrected by motion unsharpness, N em, with the associated exposure time, the inherent subject contrast, C s, the speed and the overall figure of merit, F m, for the different techniques. The N e, N em and C s values have been averaged over the depths 0, 12 and 21 cm and normalised to % for the best combination. The resolution measured by the equivalent passband was similar in the grid and air gap techniques. The average ratio of the N e values for the grid and air gap techniques was 0.98 (±0.05 ± 1SD, n = 8). The differences in values for equivalent passband between the various screen-film combinations were within 20%. The RP and spotfilm techniques proved noticeably poorer in this respect compared with the grid and air gap techniques. The inherent subject contrast of the average air gap, spotfilm and RP techniques was, respectively about 70%, 55%, and 20% of that for the grid technique (Table III). A highly significant correlation was obtained between the C s and the latitude of corresponding combinations in the grid, RP and spotfilm techniques (r = 0.9, n = 12). The correlation between the C s and gamma was of the same order (r = 0.910, n 12). The correlations between the grid and air gap techniques for N e and C s of the corresponding screen-film combinations (n = 8) were 0.8 (p < 0.01) and (p < 0.001), respectively. A highly significant correlation was obtained between the RMS noise and the speed of each screen-film combination (r = 0.0, n = 10). In the RP technique the radiation dose was about five times as great as, and in the spotfilm technique about one-fiftieth of, the dose for a combination of medium speed film/calcium tungstate screens when used in the grid technique (Table III). The radiation dose in the grid technique was about three times that in the air gap technique. Due to the long focus-to-film distance used in the air gap technique the exposure times were about 30% longer than in the grid technique. In the RP technique the exposure time was about three times as long as, and in the spotfilm technique about one-twentieth of, that in the /Cea Wicor-X RP film-screen CONTRAST 3.3 \ CAWO UNIV./Fun RX N!F -> / / 2.5 SE 2/FUJI RX A\F SE 6/FUJI RX 2.0 ALPHA "/TRIMAX XD 1.5 l.'j SPOT-FILM TECHNIQUE \ \ \> > \\ \ * \\ RP - TECHNIQUE \ \ \ \ idensity l. 3.0 FIG. 2. The gradient of the characteristic curve as a function of film density for the different screen-film combinations. DlNfrlJY 3. 3

4 VOL. 55, No. 660 H. Manninen, M. Pitkanen and M. Wiljasalo TABLE II THE GAMMA VALUE, AREA UNDER THE GRADIENT-DENSITY CURVE G4 gd), LATITUDE AND RMS NOISE OF VARIOUS SCREEN-FILM COMBINATIONS AND OF THE RP AND SPOTFILM TECHNIQUES. THE BEST COMBINATION FOR A %i HAS BEEN NORMALISED TO % Film Screen Gamma value ^gd Latitude RMS noise x 10" 3 CEA Wicor-X RP Kodak Ortho G Kodak Ortho H Cawo SE 2 Cawo SE RP technique Spotfilm technique TABLE III THE EQUIVALENT PASSBAND (N e), THE EQUIVALENT PASSBAND CORRECTED BY MOTION UNSHARPNESS (jv em) AND ASSOCIATED EXPOSURE TIME IN ms, THE INHERENT SUBJECT CONTRAST (C s), THE SPEED AND OVERALL FIGURE OF MERIT (F m) OF THE EQUIPMENT CHAINS. N e, N em, C AND F m HAVE BEEN NORMALISED AS A PERCENTAGE OF THE BEST COMBINATION System Film Screen % /o Exposure time ms Q Speed F m A B CEA Wicor-X RP Kodak Ortho G Kodak Ortho H Fuji RXNIF Kodak Ortho G Cawo SE 2 Cawo SE 6 Cawo SE 2 Cawo SE C Agfa Scopix RP D Agfa Scopix RP combination in the grid technique. If the motion unsharpness is taken into consideration, the resolution is strongly dependent on the exposure time (Table III). In Figs. 3 and 4 the N e and C s are shown as functions of depth in the water phantom. The resolution gets poorer with longer object-to-film distances, or when depth in the water phantom increases (Fig. 3). The slopes of the jv e -depth curve and the speed of each combination showed some correlation in the grid technique (r = , n= 10). DISCUSSION Our method of calculating resolution from the ERF proved to be reliable. By choosing appropriate values for the thickness of the gold plate and the exposure, giving a density in the adjacent area to the plate of 1.7 ± 0.1, then the density across the image of the edge fell on the monoexponential portion of the characteristic curve, 4

5 Physical characteristics of four roentgenographic chains proposed for chest radiography DECEMBER 1982 EQUIVALENT PASSBAND LP/MM 1.5 THF GRID TECHNIQUE THE AIR GAP TECHNIQUE l.t "CAWO UNIV./WICOR-X RP \-C -CAWO UNIV./FUJI RX NIF -SE 2/FUJI RX NIF SE 6/FUJI RX NIF ALPHA 4/TRIMAX ALPHA 4/TRIMAX ALPHA 8/TRIMAX XD XM XD ALPHA 8/TRIMAX Xfl -LANEX REGULAR/ORTHO G 0.5 LANEX RPCULAR/ORTHO H RP - TECHNIQUE SPOT-FILM TECHNIQUE FIG The equivalent passband as a function of CM CM depth in water phantom (0 cm = nearest DEPTH IN WATER PHANTOM to film). and thus the accuracy of the transformation from the a relatively slow velocity of lomms" 1 was used. In density distribution to absorbed dose was optimised. A clinical practice the peak velocities in the lungs near the numerical differentiation proved to be sufficiently stable heart can be as high as 300 mm s" 1 (Berger, 1963). in the calculation of the LSF from the ERF. However, even at 10 mm s" 1 a very strong reduction in To consider the effect of focus size, magnification and scattered radiation, the measurements were done at three depths of the water phantom. Figure 3 shows that the resolution, particularly in the RP technique is rapidly impaired when the object-to-film distance increases. This is mostly due to the increasing object-to-film distance when the effect of the focal-spot size becomes important. In the grid, air gap and spotfilm techniques, a focal spot blurring of about 0.2 mm is expected at a depth of 21 cm, while in the RP technique the unsharpness is about the resolution of the slowest systems was noticed. This indicates the significance of short exposure times in chest imaging systems. Highly significant correlations between C s and latitude and between C s and gamma shows the close relationship between large object contrast and microscopic contrast. Significant correlations between the grid and air gap techniques for N e and C s for corresponding screen-film combinations indicate uniform behaviour within the techniques. 0.4 mm. The focal spot size actually changes with tube To obtain a single measure of parameters current. Geometric unsharpness due to the focal spot characterising the physical image quality of chest will thus vary in different screen-film combinations, but radiography an overall figure of merit, F m, was defined. the effect is quite small in the exposure range used Because of its extremely small radiation dose and short (Chaney & Hendee, 1974). At the periphery of the image exposure time the spotfilm technique unexpectedly field geometric distortion is more important in proved to be the best, but fast screen-film combinations techniques with short focus-to-film distance. In the grid both in the grid and air gap techniques had high scores, technique the resolution apparently becomes worse more too. On the whole F m, as calculated, emphasises the slowly when the speed of the system increases (Fig. 3). speed of an imaging system at the cost of resolution and The reason for this is not obvious. constrast. In the theoretical calculations of motion unsharpness System noise will interfere with the visual detection of 5

6 VOL. 55, No. 660 INHERENT SUBJECT CONTRAST % I HE GRID TECHNIQUE^" 5 "^7 --=>S--«-:«L=.. THE AIR GAP 'TECHNIQUE THE 'THE SPOTFILM- RP-TECHNIOUE CAWO H. Manninen, M. Pitkdnen and M. Wiljasalo UNIV./WICOR-X CAWO UNIV./FUJI RX NIF SE 2/FUJI RX NIF SE 6/FUJI RX NIF ALPHA 1/TRIMAX XD ALPHA 4/TRIMAX XM 1 ALPHA 8/TRIMAX XD» - ALPHA 8/TRIMAX XM '- LANEX REGULAR/ORTHO G This is probably due to the fact that the scattered radiation in the plane of the gold plate is preferentially reduced by the absorption in the water phantom and by the inverse square law (Trout et al, 1975). In the grid technique C s is slightly reduced when the object-to-film distance increases. This is due to the grid. Photons scattered in the area adjacent to the gold plate and falling on the plate image are more effectively absorbed by the grid when the plate is near the grid because the angle between the scattered radiation and the lead laminae becomes more acute. In the study a high-kilovoltage technique of 120 kv was used. Since the contrast, speed and noise properties of screen-film combinations are dependent on high voltage (Rao et al, 1978) the results are not directly applicable to low-kilovoltage technique. In the spotfilm technique a 9-inch image intensifier was used. It is clear that the image field is too small to be used in routine chest radiography. However, the technique has moderate resolution, contrast and noise properties with a minimal dosage and motion unsharpness. Therefore, it may be worth considering the use of the new 15-inch image intensifier systems for chest radiography. Our results show that the RP technique is now outdated for chest radiography from the viewpoint of physical imaging characteristics and radiation dose. There are new methods which can now be used for mass chest radiography and which give a lower radiation dose without markedly impairing resolution, contrast or noise properties. ACKNOWLEDGMENTS This work was supported by a grant from the Finnish Cultural Foundation and the Finnish Academy. DEPTH IN WATER FIG. 4. PHANTOM The inherent subject contrast as a function of depth in water phantom (0 cm = nearest to film). low contrast objects (Pollehn & Roehrig, 1970). RMSnoise, as we have used it, is a relative measure and depends on aperture size and density. A small circular aperture of diameter 0.3 mm was chosen to give high values for the standard deviation of the density measurements and thus diminish errors due to the densitometer. Three screen-film combinations were also measured using a one mm aperture, and a linear relationship with the RMS-values for the 0.3 mm aperture was noticed. Table II shows that the noise properties of the RP and spotfilm techniques are noticeably poorer than those of the screen-film systems. Furthermore, a highly significant correlation was found between the speed and the RMS-noise of the screen-film systems. Figure 4 shows that the C s in the air gap technique improves with the increasing object-to-film distance. 6 REFERENCES ARDRAN, G. M., LANGMEAD, W. A. & CROOKES, H. E., Exposure reduction using new screen/film combinations. British Journal of Radiology, 48, BERGER, A., Zum Problem der Bewegungsunscharfe im Rontgenbild der Lunge und des Herzens. Rontgenblatler, 16, CHANEY, E. L. & HENDEE, W. R., Effects of X-ray tube current and voltage on effective focal-spot size. Medical Physics, 1, INTERNATIONAL ELECTROTECHNICAL COMMISSION, Measurement of the dimensions of focal spot of diagnostic X- ray tubes using a pinhole camera. IEC Publication 336. MORGAN, R. H., The frequence response function, a valuable means of expressing the informational recording capability of diagnostic X-ray systems. American Journal of Roentgenology, 88, POLLEHN, H. & ROEHRIG, H., Effect of noise on the modulation transfer function of the visual channel. Journal of the Optical Society of America, 60, RAO, G. U. V., FATOUROS, P. P. & JAMES, A. E., Physical characteristics of modern radiographic screen-film systems. Investigative Radiology, 13, TROUT, E. D., KELLEY, J. P. & LARSON, V. L., A comparison of an air gap and a grid in roentgenography of the chest. American Journal of Roentgenology, 124,