MTF of a 210-70-mm fiber-optic output x-ray image intensifier tube Clayton W. Bates, Jr., and Stephen D. Sparks

C. W. Bates is with Stanford University, Stanford, Cali­fornia 94305; and S. Sparks is with Varian Associates, Palo Alto, California 94303. Received 28 February 1974. At the third SPIE Technology Utilization Program on

Applications of Optical Instrumentation in Medicine in Kansas City, 1-2 August 1974,1 we presented some prelimi­nary results on the performance of an experimental x-ray image intensifier tube with a 210-mm input and a 70-mm fiber-optic output. This tube was a step in the direction of developing x-ray image intensifiers with larger outputs (100 mm), which will allow most radiographic examinations to be performed using 100-mm spot film, which is easier to handle, store, and retrieve than the present standard films, at a considerable reduction in x-ray dosage. At that time the modulation transfer function (MTF) of this tube was not available. This Letter provides this information and compares it with the MTF of standard screen-film combi­nations.

Figure 1 gives a block diagram of the apparatus used to measure the CTF (contrast transfer function). This was converted to MTF using the Coltman formula.2 Figure 2 shows the results of these measurements. The effects of the various lenses involved in the measurements were taken into consideration. It is interesting to compare the MTF of the 210/70-mm tube with that of a standard par screen-film combination.3 These curves show that up to about 1 lp/mm the par screen-film MTF is higher than that of the 210/70-mm tube, after which the tube is superi­or. This means that the large area contrast of the par screen-film combination is better than that of the 210/70-mm tube, but the limiting resolution of the tube is greater.

Since this tube was built, we have developed a process for depositing the CsI(Na) input scintillator with vastly im­proved information transmitting ability.4 From measure­ments on several similar x-ray image intensifier tubes (without fiber-optic outputs), we expect an average im­provement in MTF as given by the uppermost curve in Fig. 2. This is well above the par screen-film combination over the complete range of spatial frequencies of interest. This is an important criterion for this tube with a minification of 3:1, as the ultimate tube we would like to consider is one with an input of 17 in. and an output of 4 in. (~100-mm). This corresponds to a minification of 4.25:1. If we consider that our improved MTF characteristics have come from a combination of using a fiber optic output and a smaller minification than is commonly used (8:1), we can see from the curves that an x-ray image intensifier tube with the im­proved input scintillator and a minification of 4.25:1 will still be superior to the par screen-film combination over

1484 APPLIED OPTICS / Vol. 14, No. 7 / July 1975

Fig. 1. CTF measuring scheme for 210/70-mm intensifie.

Fig. 2. MTF of 210/70-mm tube and par speed screen-film combination.

the range of spatial frequencies of interest. Work is pro­ceeding in this direction.

C. W. Bates, Jr., is associated with both the Department of Materials Science & Engineering and the Department of Electrical Engineering of Stanford University. S. Sparks is with the Light Sensing & Emitting Division of Varian.

References 1. C. W. Bates, Jr., "New Trends in X-Ray Image Intensification"

(Special Topics Section), presented at SPIE Technology Utili­zation Program, Kansas City, Mo., 1-2 August 1974.

2. J. W. Coltman, J. Opt. Soc. Am. 44, 468 (1954). 3. The MTF data for the par screen-film combination was mea­

sured by Lloyd Bates of Johns Hopkins University. 4. C. W. Bates, Jr., unpublished results.

July 1975 / Vol. 14, No. 7 / APPLIED OPTICS 1485