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British Journal of Radiology (2004) 77, 751-758
© 2004 British Institute of Radiology
doi: 10.1259/bjr/16707499
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Full Paper

Threshold contrast detail detectability curves for fluoroscopy and digital acquisition using modern image intensifier systems

D S Evans, MSc A Mackenzie, MSc C P Lawinski, MPhil and D Smith, MSc

KCARE (King's Centre for the Assessment of Radiological Equipment), King's College Hospital NHS Trust, London SE5 9RS, UK


   Abstract
Top
 Abstract
Introduction
 Method
 Results and discussion
 Conclusions
 References
 
Threshold contrast detail detectability (TCDD) test objects are a commonly used tool to assess image quality of imaging systems. FAXIL (The Facility for the Assessment of X-ray imaging, Leeds) produced updated standard TCDD curves, for fluoroscopy systems in good adjustment, in 1992. Fluoroscopy curves can be corrected to account for the effect of image intensifier input air kerma rate and field size. This paper presents updated TCDD curves for fluoroscopy and new curves for digital acquisition. The results for digital acquisition suggest that the TCDD curves should not be corrected for input air kerma, as the quantum noise is not dominant and system noise is significant. These curves will prove useful for accepting new equipment, to give an indication of the expected image quality for a new image intensifier system.


   Introduction
Top
 Abstract
 Introduction
Method
 Results and discussion
 Conclusions
 References
 
Routine quality assurance is important in ensuring the optimal clinical performance of radiological imaging equipment. FAXIL (The Facility for the Assessment of X-ray Imaging, Leeds) designed a set of test objects for serial testing of fluoroscopic equipment. These are commonly used in the UK for serial testing of X-ray imaging systems and they have been shown to be useful for assessment of the overall imaging performance of image intensifier based X-ray imaging systems [1–3]. The threshold contrast detail detectability (TCDD) test objects (e.g. TO10, TO12, TO20) are a good overall measure of image quality for fluoroscopic equipment. Current guidelines for the assessment of the performance of radiological equipment in the UK recommend TCDD test object data as one of the assessment criteria [4]. TCDD measurements are not limited to fluoroscopic imaging and can be applied to other modes of image acquisition. A number of test objects designed for assessing TCDD properties of a range of imaging modalities (such as digital fluorography, mammography and digital radiography) are available [5–7].

TCDD test objects all have the same basic design philosophy and scoring methodology. They comprise a series of groups of circular details with the details in each group having the same diameter but a range of radiographic contrast. A TCDD test object can contain either positive or negative details. Positive details comprise of discs of attenuating material positioned on a uniform base plate (often Perspex). Negative details are holes drilled in a uniform base plate (again often Perspex).

The images of the test object are acquired under standard reproducible exposure conditions, which allow the contrast of the details to be known and which facilitate consistency. The image reader scores the images by visually assessing the lowest contrast detail visible for each group of the same diameter. Details can be scored as half visible if the detail is partially distinguishable. Ideally, at least two readers score a number of images to allow results to be averaged. A variable viewing distance protocol has since been published [8]. This demonstrates that variable distance scoring allows for optimal viewing of every detail size. However, many users of these test objects continue to use fixed viewing distance protocols, to maintain consistency and facilitate a quick scoring process, as variable viewing requires considerably more time to ensure that the distance is optimized for each detail size and for each user.

TCDD data are presented graphically as a contrast resolution diagram or more commonly as the threshold detection index (HT) as a function of the square root of the detail area on log–log scales. HT(A) is calculated using Equation 1Go.Go


{770751E001}

where CT=threshold contrast (%) and A=area of detail (cm2).

HT curves allow for an assessment of the overall imaging performance of the system, by considering both the shape and the position of the curve. The higher the value of HT(A), the more easily a detail of that size will be seen.

A number of factors may affect the TCDD curve, including input air kerma rate, receptor sensitivity, magnification, lag and spatial resolution. We wish to average the curves from different systems to produce representative curves; therefore some of the above factors need to be considered in the measurement of TCDD.

The TCDD is primarily dependent on the sensitivity of the image intensifier and television system. The sensitivity describes the ability of the imaging system to absorb photons and to use them to form the image. The higher the sensitivity the lower the noise and therefore threshold contrast will potentially be improved. The sensitivity is not the only contributor to the signal to noise ratio of the imaging system, as further stochastic and non-stochastic noise sources may be present [9, 10].

One of the most important factors affecting the measured TCDD is the input air kerma rate to the image intensifier. While quantum noise dominates in an image, the TCDD will improve with increasing air kerma [10]. A smaller image intensifier field size will generally enable the smaller details to be more easily seen due to increased magnification of the image. This has the effect of shifting the TCDD curve along the x-axis in proportion to the ratio of the diameters of the field sizes. Ideally, when measuring TCDD, the test object should be as close to the image intensifier input face as possible, normally 5 cm or less. However, in some cases this will not be practical and the details will then be magnified.

Other equipment related factors that will affect the TCDD curve are those of camera lag and spatial resolution. These factors can differ greatly between systems as a range of camera tubes and charge-coupled device (CCD) cameras are used in image intensifier television systems. Lag is a measure of signal retention between frames [11]. Although a great deal of lag can be detrimental to clinical image quality when imaging rapidly moving objects, a degree of lag can reduce noise when imaging static or more slowly moving objects. In fact this can be added for this purpose in the form of frame averaging. Therefore, for two systems which are identical except for the degree of lag, the system with the greater lag could potentially have improved TCDD performance. The television system is generally the limiting factor in an image intensifier chain in terms of spatial resolution [3]. Therefore, improved camera resolution may result in improved spatial resolution within the image, other factors being constant. It is expected that improved camera resolution will result in improved visualization of small details.

It is well known that TCDD tests are subjective in nature and as such, individual observers may have different criteria for positively identifying details. Also, the details can be scored as being "half-visible" and again individuals will have their own threshold criteria. Therefore, it is important that all readers of test object images have undergone appropriate training and a familiarization period with the test object being used. Experience suggests that trained observers with the same scoring criteria will tend to score consistently. Additionally, viewing conditions such as the ambient lighting levels and the viewing distance to the monitor screen may affect the scoring [8]. Standard scoring arrangements should be used.

In the UK, the Leeds TO10 TCDD test object is commonly used for assessing fluoroscopic systems. TO12 and TO20 TCDD test objects are additionally available for assessing digital acquisition and digital subtraction angiography systems. The instruction manual for TO10 includes standard TCDD curves for systems that were considered, at the time of testing, to be in good adjustment. These were scored using a fixed distance viewing protocol. The standard curves were averaged for 10 representative X-ray systems. There were three curves presented for three field size ranges (16–17 cm, 22–25 cm, 30–33 cm). These curves were also included in a Department of Health Document which considered the technical testing of X-ray Image Intensifier Systems [3]. The input air kerma rate at which the data were measured is not made clear. This leads to uncertainty when comparing results measured in the field with the reference curves.

As stated above, TCDD is normally interpreted graphically. Gallacher et al [12] proposed the use of a single number, the quality index, Q, to indicate the quality of a system. This is calculated by averaging the ratio of HT(A) for the tested system to a reference value for all detail sizes (Equation 2Go). Currently the TCDD curve from the correct field size range from [3] is used to provide HTref(A).Go


{770751E002}

where Q=quality index, n=number of detail diameter groups, HT(A)=threshold detection index from tested system and HTref(A)=threshold detection index from reference curve.

As discussed above, there are a number of factors which can affect the TCDD. Gallacher et al proposed correction factors to remove some of the effects of the measurement conditions (Equation 3Go). Q is normalized for the input air kerma rate, use of pulsed fluoroscopy and the radiation beam quality. Of particular interest is the exposure normalization, based on the quantum noise which follows Poisson statistics [8, 9]. The noise is therefore inversely proportional to the square root of the number of detected photons or their input air kerma rate [13]. The threshold detection index is itself proportional to the signal-to-noise ratio (SNR) of the system and hence modifying factors applied to this index must reflect the effect of quantum statistics on noise level [8]. Hence, the modifying factor applied to the threshold detection index values will be the square root of the ratio of the actual measured input air kerma rate against a standard reference value of input air kerma rate.Go


{770751E003}

where Qn=normalized quality index, Km=measured input air kerma rate of the system in nGy s–1, Kref=reference input air kerma rate of the system in nGy s–1, Rn(V,X)=radiation beam quality normalized factor and isa function of V (tube potential) and X (beam filtration in mm copper) and {Lambda}(f)=pulsed fluoroscopy correction factor.

It must be noted that the quality index does not replace the contrast threshold detection index (CTDI) curve, as the shape of the curve is important; but it can simplify interpretation.

One aim of this work was to provide updated TCDD curves for fluoroscopy systems based on a number of evaluations on a wide range of image intensifier systems. As systems in clinical use cover a range of input air kerma rates and field sizes, it is proposed to use some of the methods described by Gallacher et al to correct for air kerma rate and produce normalized curves. Therefore, we can produce average fluoroscopy curves for a typical image intensifier. Additionally, TCDD curves are produced for digital image acquisition through the image intensifier performance as previously these have not been published.

Some of the reasons for updating these curves is that equipment has improved, as shown by Gallacher et al [12]. Also the dosimetry data for the Leeds reference curves were not given and had to be estimated. In addition, a curve for the field size 38–40 cm was not available.


   Method
Top
 Abstract
 Introduction
 Method
Results and discussion
 Conclusions
 References
 
Fluoroscopy and fluoroscopy-based digital acquisition
The modern image intensifier systems used in this work were based on KCARE evaluation reports covering radiography/fluoroscopy (R/F) [14], which included 11 systems, together with multipurpose fluoroscopy systems [15], which included 5 systems. The systems (Table 1) were all less then 2 years old and were, according to the supplier, in good adjustment. The tests included a full evaluation of image quality using a range of standard test objects and standard protocols [4, 16].

The image intensifier input air kerma was measured using a calibrated MDH 2025 electrometer (Radcal, Monrovia, USA) with a 60 cm3 circular ionization chamber positioned as close to the input face of the image intensifier as possible. Copper filtration was positioned at the collimator port (1 mm for fluoroscopy, 1.5 mm for digital acquisition). If possible, the anti-scatter grid was removed, otherwise a nominal grid attenuation factor was estimated or taken from the manufacturers' data. The image intensifier input air kerma rate was measured under automatic conditions and where possible additional spectral filters were removed. Sufficient paper was then placed in the beam to force the tube potential to 70 kV for fluoroscopy and 75 kV for acquisition. This was positioned as far from the test object as possible to minimize the effects of scatter. The measurement was undertaken for each field size and each input air kerma rate or exposure setting if provided.

The TCDD test object (TO10 for fluoroscopy, TO12 for acquisition; University of Leeds, Leeds, UK) was placed as close as possible to the image intensifier. The test object was imaged using the same conditions as the input air kerma rate was measured. Paper is of a low atomic number relative to copper and so will have a slight effect on the X-ray spectrum and have minimal effect on the measured TCDD. The TO10 was imaged for the different field sizes, input air kerma rates, and pulse rates. However, pulsed fluoroscopy is not considered in this paper.

To maintain consistency, the test objects were scored according to the original test object manual, using a fixed viewing distance protocol and with low ambient light conditions. The results were used to calculate HT curves for each system and for each field size and input air kerma level for fluoroscopy and digital image acquisition.

The quality index was then corrected according to the method indicated by Gallacher et al [12]. The pulsed fluoroscopy and radiation beam quality correction factors are both unity in this case, as we are not considering pulsed fluoroscopy and 70 kV and 1 mm of copper were used. Therefore the only correction is that for input air kerma (Equation 4Go).Go


{770751E004}

The air kerma correction was initially investigated on those systems which can operate at multiple exposure levels, to test if this method was valid. The aim of this work is to produce reference curves for modern equipment in good adjustment. Some systems were significantly poorer in performance; therefore it was felt that they should be removed from the study. The removal criterion for a system was a normalized quality index (Qn) in excess of one standard deviation less than the mean Qn for that particular field size range.


   Results and discussion
Top
 Abstract
 Introduction
 Method
 Results and discussion
Conclusions
 References
 
Fluoroscopy
The dosimetric measurements showed that current fluoroscopy systems operate over a wide range of input air kerma rates (Table 2Go). Additionally, a number of systems can be operated at more than one air kerma rate level. On these systems, as expected a higher input air kerma level resulted in an improvement in TCDD performance.


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  		Table 2. Input air kerma rate for all systems and exposure modes

 
Correction for input air kerma rate was investigated on some of those systems that operate at multiple air kerma levels. Table 3Go shows that air kerma correction reduces the spread of the data to within uncertainties. The remaining spread in the data is thought to be due to the non-stochastic noise processes that contribute to the image formation, the discrete nature of the details on the test objects and scorer variability. This therefore suggested that fluoroscopy TCDD curves can be corrected to an average input air kerma rate level.


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  		Table 3. Systems with multiple input air kerma rates

 
Overall, the systems provided a range of field sizes, from 16 cm to 40 cm, with either three or four field sizes per system. These were separated into four groups in terms of nominal field size.

Using the criterion described in the method for exclusion of data from this study, 13 systems remained in the study. The systems removed from this study agreed well with the evaluators general opinions regarding suboptimal image quality.

In each field size group, the data for each system were averaged after correcting for input air kerma rate to the image intensifier. Figure 1Go and Table 5 shows the four field size groups that were considered and the best fit curves that have been plotted. The error (standard deviation) bars indicate the variation introduced through averaging the data for the various systems.



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  		Figure 1. Average fluoroscopy threshold contrast detail detectability (TCDD) curves for nominal field size ranges (numerical values for these data are included in Table 5Go): (a) 16–17 cm, (b) 20–25 cm, (c) 29–33 cm and (d) 38–40 cm.

 
Correction of field size
Launders et al [8] and Gallacher et al [12] both showed that the TCDD could be corrected for field size. Each of the average curves is corrected for the air kerma rate and field size as described previously (Figure 2Go). There is very good correlation between the curves for the smaller details, but poorer for the larger details.



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  		Figure 2. Correction of fluoroscopy HT for exposure rates and field sizes.

 
Digital acquisition
The same method was applied to the analysis of digital images acquired through the image intensifier for the multipurpose systems.

If the data for those units that operated at a range of exposure settings were normalized to a mean exposure level, it was found that there was an increase in spread of the data (Tables 4aGo and 4bGo). A similar relationship was observed for the remaining systems. This suggests that TCDD data for digital acquisition cannot be corrected for air kerma. This is due to the quantum noise not being dominant at values of air kermas used for digital acquisition, rather, additional noise sources such as intensifier structure mottle dominate [10].


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  		Table 4a. GE Advantx TC: HT(A) values for different input exposure

 

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  		Table 4b. GE Advantx TC: HT(A) values corrected for different input air kerma

 
The reference curves are shown in Figure 3 and Table 6Go. The systems have been grouped as for the fluoroscopy data. The reference curves are the mean of the measured curves without normalization.



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  		Figure 3. Average digital acquisition threshold contrast detail detectability (TCDD) curves for nominal field size ranges (numerical values for these data are included in Table 6Go): (a) 16–17 cm, 1.4–4.6 µGy; (b) 20–25 cm, 0.39–3.9 µGy; (c) 29–33 cm, 0.46–3.9 µGy; and (d) 38–40 cm, 0.54–1.4 µGy.

 

   Conclusions
Top
 Abstract
 Introduction
 Method
 Results and discussion
 Conclusions
References
 
The generation of TCDD curves from images of standard test objects is a simple procedure that allows analysis of image quality for a range of imaging modalities; including image intensifier systems.

Threshold contrast data for a number of image intensifier systems have been presented for fluoroscopy images. As for previous work [12, 13], we have shown that it is possible to correct TCDD curves for input air kerma and field size. This allows for comparison of systems operating at different field sizes and input air kerma rates. Normalized data can be compared with previous results when performing routine quality control testing or to reference data which have been acquired from a number of well optimized, modern systems. Unlike the 1992 reference data, these data are useful for those who wish to use the Gallacher quality index, as it has known dosimetry. Furthermore, the data include a larger range of field size compared with the 1992 data.

The TCDD data presented demonstrate that improvements in imaging characteristics have been made since the 1992 data. This is thought to be due to the technology used, e.g. improved sensitivity results in TCDD curves of a higher magnitude. These improvements are mainly due to the reduced veiling glare, better electronics, improvements in output phosphor and improved manufacture [17]. Also, TCDD curves can demonstrate any reduction in image quality, which, for example, may be due to ageing of the image intensifier or faults in the system.

Previously, reference TCDD curves for digital acquisition systems have not been published. It has been demonstrated that the air kerma level–image noise relationship is not dominated by quantum noise, as non-stochastic noise sources (structural noise) have a significant impact on image noise. Therefore, a simple correction based on quantum noise is not appropriate; this agrees with Marshall et al [10]. Therefore due to the variability in the results for digital acquisition and the inability to correct for input air kerma, there is a spread of data that could describe these systems. This means that the exposure correction for the Gallacher quality index should not be used for digital acquisition. Cardiac systems have not been included in this work. They operate at lower exposure rates and it should be possible to undertake air kerma correction.

Although a number of the systems included in this work employ CCD-based cameras, a majority use Saticon type camera tubes in the television imaging system. Due to the lack of lag related to the use of CCDs and Plumbicon camera types, these are used in angiography, vascular fluoroscopy and neurofluoroscopy systems due to the requirement of imaging moving structures and objects. Therefore, these systems will require more noise suppression software to produce images with similar noise levels as standard fluoroscopy systems.

Although caution should always be taken when comparing TCDD data due to potential set up differences, scoring criteria and experience of the scorers, these curves may be useful in the acceptance testing of new image intensifiers for fluoroscopy and digital acquisition.


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  		Table 5. Fluoroscopy reference curves HT(A) values

 

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  		Table 6. Acquisition reference curves HT(A) values

 

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  		Table 1. Systems evaluated

 

   Acknowledgments
 
The authors would like to acknowledge the Medicine and Healthcare products Regulatory Agency (MHRA) for funding these projects and the staff of the imaging departments, suppliers and manufacturers for their assistance and co-operation in providing access to the imaging systems.


   Footnotes
 
Address correspondence to A Mackenzie. Back

Current address for D S Evans, Imaging and Medical Physics, University Hospital Birmingham NHS Trust, Birmingham B15 2TH, UK. Back

Current address for D Smith, West Hertfordshire Hospital NHS Trust, Watford WD18 0HB, UK. Back

Received for publication April 23, 2002. Revision received January 5, 2004. Accepted for publication April 6, 2004.


   References
Top
 Abstract
 Introduction
 Method
 Results and discussion
 Conclusions
 References
 

  1. Hay GA, Clarke OF, Coleman NJ, Cowen AR. A set of X-ray test objects for quality control in television fluoroscopy. Br J Radiol 1985;58:335–44.[Abstract/Free Full Text]
  2. Cowen AR, Clarke OF, Coleman NJ, Craven DM, McArdle S, Hay GA. Leeds X-ray test objects instruction manual (4th Edn). The University of Leeds, 1992.
  3. Department of Health (DH) Working Party Report. The testing of X-ray image intensifier television systems: 1998. MDA/98/68 1998.
  4. The Institute of Physics and Engineering in Medicine report 77, Fitzgerald M et al. Recommended standards for the routine performance testing of diagnostic X-ray imaging systems. IPEM Report 77. York, UK: IPEM, 1997: 41–51.
  5. Workman A, Cowen AR. Signal, noise and SNR transfer properties of computed radiography. Phys Med Biol 1993;38:1798–808.
  6. De Paredes ES, Fatouros PP, Thunberg S, Cousins JF, Wilson J, Sedgwick T. Evaluation of a digital spot mammographic unit using a contrast detail phantom. Digital Mammography Conference, Nijmegen 1998.
  7. Funke M, Breiter N, Hermann KP, Oestmann JW, Grabbe E. Storage phosphor direct magnification mammography in comparison with conventional screen-film mammography – a phantom study. Br J Radiol 1998;71:528–34.[Abstract]
  8. Launders JH, McArdle S, Workman A, Cowen AR. Update on the recommended viewing protocol for FAXIL threshold contrast detail detectability test objects used in television fluoroscopy. Br J Radiol 1995;68:70–7.[Abstract/Free Full Text]
  9. Cohen G, Wagner LK, Rauschkolb EN. Evaluation of a digital subtraction angiography unit. Radiology 1982;144:613–7.[Abstract/Free Full Text]
  10. Marshall NW, Kotre CJ, Robson KJ, Lecomber AR. Receptor dose in digital fluorography: a comparison between theory and practice. Phys Med Biol 2001;46:1283–96.[CrossRef][Medline]
  11. Marshall NW, Kotre CJ. Measurement and correction of lag on contrast-detail test results in fluoroscopy. Phys Med Biol 2002;47:947–60.[Medline]
  12. Gallacher DJ, Mackenzie A, Batchelor S, Lynch J, Saunders JE. Use of a quality index in threshold contrast detail detection measurements in television fluoroscopy. Br J Radiol 2003;76:464–72.[Abstract/Free Full Text]
  13. Kotre CJ, Marshall NW, Faulkner K. An alternative approach to contrast-detail testing of X-ray image intensifier systems. Br J Radiol 1992;65:686–90.[Abstract/Free Full Text]
  14. KCARE MDA/2000/49. A comparative report of fluoroscopy systems (undercouch & overcouch). 2000.
  15. KCARE MDA 00079. A comparative report of multi-purpose C-arm fluoroscopy systems. 2000.
  16. IPEM report 32(II). Hiles PA, Starritt HC. Measurement of the performance characteristics of diagnostic X-ray systems used in medicine: X-ray image intensifier television systems. York, UK: IPEM, 1996.
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