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British Journal of Radiology (2004) 77, 52-56
© 2004 British Institute of Radiology
doi: 10.1259/bjr/62332261
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Short communication

Quality control of a laser camera with the SMPTE test pattern: optical density variations with printing format and frame position

I A Tsalafoutas, MSc, PhD 1 V Tsapaki, MSc, PhD 1 E Koulentianos, MD 2 and C H Triantopoulou, MD 3

1 Medical Physics Unit, 2 Radiology Department and 3 Computed Tomography Department, ‘Konstantopoulio-Agia Olga’ Hospital, 3–5 Agias Olgas, Nea Ionia, 142 33, Athens, Greece


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 References
 
The purpose of this study was to investigate the use of the Society of Motion Picture and Television Engineers (SMPTE) test pattern in the quality control of a modern laser camera and the variations in the optical density (OD) of the film when different formats are used. The SMPTE pattern was printed on all the available frames in eight different formats. Furthermore, six films were produced using the same format to check for any reproducibility problems. The OD values of the 11 step greyscale of the SMPTE patterns were measured with a densitometer, as well as the OD of steps 10 and 11 of the 16 step monitor greyscale printed to the left of each frame along with the SMPTE pattern. Variations up to 0.2 were observed in the OD of the same step when different formats and different frames within the same film were compared. Furthermore, the OD variations with frame position were found to follow a specific pattern. The OD variations observed with printing format and frame position can not be explained with certainty. They may indicate a laser camera malfunction and, if this is the case, limits to the maximum variation allowed should be set.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 References
 
Since the introduction of CT into clinical practice in the early 1970s, there has been a world-wide steady increase in the number of CT systems. The National Radiation Protection Board (NRPB) reported in 1992 [1] that 200 CT scanners were in clinical use in UK while in 1997 this number increased to 350 CT scanners [2]. A similar study conducted in Greece in 1999, reported that there were 152 CT scanners in operation [3]. Nowadays, CT plays a very important role in clinical practice and has become one of the most useful diagnostic tools. Therefore, it is essential that a quality control (QC) program is applied so as to monitor the condition of all the components in the imaging chain (from data acquisition to image recording), in order to reveal promptly subtle variations in performance which can degrade the overall presentation of the CT images.

Monitors and films are currently the main way of viewing the CT images. However, in many cases diagnosis is based solely on film reading. Therefore, when a specific window and level is used for the best presentation of a CT image on the monitor it is mandatory that the image characteristics are maintained when printed on film.

In 1985, the Society of Motion Picture and Television Engineers (SMPTE) published a Recommended Practice (RP-133) "Specifications for Medical Diagnostic Imaging Test Pattern for Television Monitors and Hard-copy Cameras" [4]. Since then, the SMPTE monochrome test pattern has been applied to the acceptance testing and QC of image display systems and laser cameras, providing both qualitative and quantitative information. The available information on the use of the SMPTE pattern, for the QC of hard-copy and laser cameras is summarized in the report of American Association of Physicists in Medicine (AAPM) Diagnostic X-ray Imaging Committee Task Group No. 1 [5], where technical data on the design of these systems are also given.

In the AAPM report [5], data on the optical densities (ODs) of the 11 step greyscale included in the SMPTE pattern were given, for one film used in video hard-copy cameras and one used for laser cameras. These data included the OD variation tolerances for the QC of these systems. However, apart from that study, little information on this subject can be found in the current literature, while no data exist for the various types of laser cameras and films commercially available. Furthermore, tolerances for the variation of the OD of the SMPTE pattern greyscale when different formats are used, have not been set.

In the present study the SMPTE test pattern was used for the QC of a modern laser camera interfaced to a spiral CT system. Our main interest was focused on the investigation of the variations of the SMPTE test pattern characteristics when this is printed using different formats and for different frame positions within a film.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 References
 
This study was carried out in the CT department of the Konstantopoulio-Agia Olga Hospital, where a spiral CT unit (Prospeed SX Power; GE Medical Systems, Milwaukee, WI) is interfaced to a laser camera (Scopix LR 3300; Agfa-Gevaert, Belgium). The film routinely used is Agfa Scopix LT 2B.

In Figure 1Go the SMPTE test pattern used in our study is shown. The image background is at a uniform grey (at the 50% grey level) and a cross-hatch pattern and border (at the 75% grey level) are used to reveal any spatial distortion or artefacts and to ensure that all the area is displayed. A white and black window with black and white inserts, respectively, are provided to stress the system and test for transient and low frequency response. High and low contrast bar patterns are positioned at the centre and at the four corners of the image. The entire dynamic range of the image is represented in a greyscale positioned around the centre of the image, with 11 steps (patches) from 0% to 100% (in 10% increments). Next to the 0% patch, there is a 5% patch insert in a 0% patch and next to the 100% patch, there is a 95% patch insert in a 100% patch. A more detailed description of the SMPTE pattern characteristics is given in the AAPM report [5]. The latter two patches along with the high and low contrast bar patterns can be used to adjust the brightness and contrast of the image so that all the information available in the test pattern is visible on the image display. When the camera is properly adjusted this information should be maintained when the SMPTE test pattern is printed on film.



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  		Figure 1. The Society of Motion Picture and Television Engineers (SMPTE) test pattern.

 
For the purposes of this study, the SMPTE pattern was recalled from the CT hard disk and was presented on the monitor. The window was set at 100 (the numerical range of the pattern) and the window level at 50 (the middle of the pre-set window), as indicated in the AAPM report [5]. The contrast and brightness controls were adjusted in order to obtain the optimal presentation of the pattern characteristics on the monitor. The high and low contrast bar patterns were all visible and the 5% and 95% insert patches were clearly discerned from the 0% and 100% background patches, respectively. This procedure may be subjective since it is based on qualitative criteria. However, it is considered to be efficient for the proper adjustment of the monitor controls [5]. When the monitor was adjusted, all the 16 steps of the greyscale displayed on the left side of the monitor were clearly discernible. Unlike the SMPTE pattern, the monitor greyscale is not affected by window width and window level settings.

To check the photographic processor, a 21 step greyscale was produced by the laser camera. The target density (the OD of step 21) was set at the default value of 3.3. The OD of the 21 steps were measured with a calibrated optical densitometer (RMI 331 Densitometer; X-Rite, Granville, MI) and were manually entered into the laser camera for updating the look-up tables.

After the adjustment of the monitor and the QC of the processor, the SMPTE pattern was printed on film, using all the frames of 8 different formats available on the laser camera: x 1, x 2, x 6, x 12, x 15, x 16, x 20 and x 24. Furthermore, 5 additional films were produced in the x 6 format in order to check the reproducibility of the laser camera.

All the printed films were examined using a viewing box, to verify that all the SMPTE characteristics are visible on the films as on the monitor. Afterwards, the OD of the 11 patches of the SMPTE pattern greyscale were measured on all the printed frames of all formats. The difference of the 10% and 70% patches were taken as the contrast index of the film and the 40% patch as the speed index [5].

In addition to measurements of the SMPTE pattern greyscale patches, we measured the OD of steps 10 and 11 of the monitor greyscale printed alongside the SMPTE pattern, to the left side of each frame. The monitor greyscale was included in our study in order to test if it could be used in any way for the QC of those systems when a SMPTE pattern is not readily available. The choice of the steps 10 and 11 was somewhat arbitrary, based on the fact that these densities were around the density of 1+base+fog. We did not measured the rest of the steps in all formats because of the large number of measurements involved and because these two steps were adequate to confirm the observations made on the SMPTE pattern.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 References
 
In Table 1Go the mean values, standard deviation (SD) and the maximum variation (max-min) of the measured OD of the SMPTE pattern patches, steps 10 and 11 of the monitor greyscale and the contrast index are given for three groups. In group A only the upper left frame of each format was considered, in group B all the frames in each format were considered, whereas in group C all the frames in the most frequently used format ( x 20) were taken into account. In the final column the corresponding values reported for the Kodak SO-497 film (Eastman Kodak Co., Rochester, NY) and the acceptable variation limits for the periodic QC (where the same format and frame position is monitored), are also given [5].


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  		Table 1. Mean values and maximum variation (max-min) of the optical density (OD) of the Society of Motion Picture and Television Engineers (SMPTE) patches, the contrast index (CI) and the steps 10 and 11 of the monitor greyscale for groups A, B and C are given

 
In Table 1Go it can be seen that the mean values of group A are slightly larger than the corresponding values of groups B and C but the maximum variation is slightly smaller in A. In fact mean values for groups B and C are almost equal with the maximum variation being slightly larger in group B. For all groups however, the mean OD values and the contrast index for the Agfa Scopix LT 2B are similar to the reference values given for film Kodak SO-497, except the value of the 0% patch. This may be due to the fact that the maximum density in the processor has been set quite high (at 3.3), whereas the corresponding value in the referred study [5] is not clearly stated but is probably smaller (in this report is proposed that the maximum OD should not be set too high). It should also be noted that for the high density patches the maximum variation is quite large compared with the tolerance limits reported for the periodic QC of Kodak SO-497 laser camera film [5], given that in our study all films were printed on the same day and variations were observed even within the same sheet of images.

In Table 2Go the measured ODs of the six films acquired in the x 6 format (groups D & E) are given. In group D only the upper left frame was considered whereas in Group E all the frames were taken into account. The mean values of group D are larger than E but the maximum variation is larger in E (as when comparing groups A and B). Thus, from the 6 films acquired in the x 6 format it is clearly shown that, whereas the maximum variation in the OD of a patch when the same format and frame are considered does not exceed 0.05 (indicating good reproducibility), when the different frames in this format are considered the maximum variations are doubled and in the case of steps 10 and 11 become 4 times larger.


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  		Table 2. Mean values and maximum variation (max-min) of the optical density (OD) of the Society of Motion Picture and Television Engineers (SMPTE) patches, the contrast index (CI) and the steps 10 and 11 of the monitor greyscale for groups D and E are given

 
For the x 20 format, an illustrative description of the variation of the measured ODs is given in Figure 2Go. It can be seen that the variation in the OD of the SMPTE pattern patches follows a distinctive pattern that is more prominent for the 40% and for the patches with higher OD and also for the contrast index. A similar pattern to that of Figure 2Go was also observed for the x 24, x 16, x 15 and x 12 formats where 3 to 4 frames are printed in the same row. Starting from the upper left frame where the OD obtains its maximum value, when moving to the right the OD decreases, obtains its minimum value and then slightly increases again.



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  		Figure 2. The variation of the optical density (OD) values of selected patches of the Society of Motion Picture and Television Engineers (SMPTE), the contrast index (CI) and the step 11 of the monitor greyscale with frame number for the x 20 format. The frames have been numbered starting from the upper left corner and the vertical lines indicate the start of each one of the 5 rows of frames.

 
This pattern is more prominent for steps 10 and 11 of the monitor greyscale, probably because this greyscale is located at the edge of each frame (in contrast to the patches that are located around the centre of the SMPTE pattern), implying that the observed variation pattern may be enhanced at the edges of each frame. When we measured the 50% grey background of the x 1 format we observed that the OD increased from 0.8 in the centre to 0.88 close to the edges. However, no such variation was observed in the x 20 format frames.


   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
References
 
The results of this study have revealed that different formats and different frames in the same format present notable variations in the OD and contrast index that follow a specific pattern. It is apparent that even when comparing the upper left frame of each format, the OD variations across the image cannot be cancelled out, since the SMPTE patches are printed on a different position of the film, depending on the frame size.

If these variations are normal and expected due to the laser camera design or indicative of a malfunction it could not be determined, since no specifications or relevant bibliography are available. However, we obtained a x 20 format film from three other CT departments with different laser cameras (one Agfa LR 3300, one Agfa LR 5200 and one Kodak Ektascan 2180) and measured the OD of the steps of the monitor greyscale printed on all frames (no SMPTE pattern was available). OD variations were indeed observed in all films. However, the variation pattern and the maximum variations were different. It should be mentioned that the minimum variation (<0.1) was observed in the film from the LR 3300 (the same model as our laser camera), whereas in the film from the Kodak Ektascan 2180 a behaviour similar to our laser camera was observed.

In the AAPM report [5] it has been noted that OD variations of ±0.05 (or slightly wider) across the film could be accepted. However, it has not been determined if these variations should be random or follow a specific pattern. Furthermore, a limit above which these variations could indicate a certain malfunction of the laser camera and could be considered to affect clinical diagnosis, has not been set. Thus, when such variations occur, it is difficult to judge if a new camera should be rejected or if in an operating camera a service call and the subsequent cost are justifiable.

The OD differences observed in our study were large enough to be easily identified by visual inspection, when the SMPTE pattern or the same clinical image was printed on all frames of a large format (e.g. the x 20). However, all the SMPTE pattern characteristics associated with image contrast (as the insert patches, the high and the low contrast bar patterns) in all films studied remained discernible irrespective of format and frame position. Therefore, since no evident loss of image contrast was detected, variations in OD and contrast index with printing format and frame position less than ±0.1 can be considered acceptable. After all, the maximum variations observed in this study are within the limits proposed for day-to-day variations given in the last column of Table 1Go.

However, since variations with format and frame position can be combined with day-to-day variations (because of photographic processing or other reasons), resulting for example in a decrease in contrast index by 0.25, the overall tolerance for all types of variation should also be determined. We are not in the position to propose such limits. However, it would be reasonable that these should ensure that irrespective of format, frame position and day-to-day variations, all SMPTE characteristics should remain discernible.

Received for publication February 24, 2003. Revision received June 16, 2003. Accepted for publication September 18, 2003.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

  1. National Radiation Protection Board. Protection of the patient in the X-ray computed tomography. Documents of the NRPB 3 (4). Chilton: NRPB, 1992.
  2. Shrimpton PC. Reference doses for computed tomography. Radiological Protection Bulletin 193. Chilton: NRPB, 1997.
  3. Perris A, Hourdakis C, Manetou A, Iordanou I, Lyra M. Examination frequencies and patient doses from computed tomography examinations in the area of Athens, Greece. Health Phys 1999;77:192–5.[Medline]
  4. Specifications for medical diagnostic imaging test pattern for television monitors, hard-copy recording cameras» SMPTE Recommended Practice 1986, RP 133-1986, SMPTE J 1986;95:693–5.
  5. Gray JE, Anderson WF, Shaw CC, Shepard J, Zeremba LA, Lin PP. Multiformat video and laser cameras: history, design considerations, acceptance testing and quality control. Report of AAPM Diagnostic X-ray Imaging Committee Task Group No 1. Med Phys 1993;20:427–38.[Medline]



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