Analysis of variations in contrast-detail measurements performed on image intensifier-television systems

A statistical alternative to current measures of image quality in digital mammography

Objective. Mammogram image quality in European breast screening systems is defined by threshold gold thickness (T) assessment of the CDMAM contrast-detail phantom. Previous studies have outlined several limitations of the phantom including expense, number of images required and inter-phantom manufacturing variability. Two alternative approaches to image quality assessment for routine quality control are examined and compared to the CDMAM technique: (i) A detectability index (d′) based on a non-prewhitened model observer with an eye filter (NPWE) and (ii) A statistical estimate of contrast based on image noise levels (CSTAT ). Approach. The d′ calculation follows a previously published methodology based on the NNPS and contrast, both measured from an image of 5 cm of PMMA containing a 0.2 mm Al target, as well as the MTF measured under standard conditions. For the proposed statistical method, pixels in the centre of the same NNPS image were re-binned into a range of equivalent CDMAM target areas. For any area, the minimum contrast necessary to distinguish a signal from the background, CSTAT , is 3.29σ at a 95% level of confidence, where σ is the standard deviation of the background pixels. Theoretical analysis predicts a simple relationships between CSTAT , T and d′. Measured values of CSTAT were compared to T and d′ as a function of air kerma at the detector for ten digital mammography systems from three different manufacturers. Main Results. Theoretical relationships between CSTAT , d′ and T were demonstrated. Minimum acceptable image quality performance for 0.10 and 0.25 mm diameter discs, defined by the European Guidelines in terms of T, are equivalent to d′ values of 0.85 and 5.36 and threshold CSTAT values of 0.055 and 0.022. Significance. Strong correlations between log(T), log(d′) and log(CSTAT ) suggest that either alternative approach produces information corresponding to that obtained using the CDMAM. CSTAT should be considered as a simple, objective and cost-effective alternative to routine image quality assessment in mammography.

SIGNAL-TO-NOISE RATIO RATE MEASUREMENT IN FLUOROSCOPY FOR QUALITY CONTROL AND TEACHING GOOD RADIOLOGICAL IMAGING TECHNIQUE

Visibility of low-contrast details in fluoroscopy and interventional radiology is important. Assessing detail visibility with human observers typically suffers from large observer variances. Objective, quantitative measurement of low-contrast detail visibility using a model observer, such as the square of the signal-to-noise ratio rate (SNR2rate), was implemented in MATLAB™ and evaluated. The expected linear response of SNR2rate based on predictions by the so-called Rose model and frame statistics was verified. The uncertainty in the measurement of SNR2rate for a fixed imaging geometry was 6% based on 16 repeated measurements. The results show that, as expected, reduced object thickness and x-ray field size substantially improved SNR2rate/PKA,rate with PKA,rate being the air kerma area product rate. The measurement precision in SNR2rate/PKA,rate (8-9%) is sufficient to detect small but important improvements, may guide the selection of better imaging settings and provides a tool for teaching good radiological imaging techniques to clinical staff.

Extended detail contrasts for TO.10 use on flat panel detector fluoroscopy systems

OBJECTIVES

Leeds Test Object Ten (TO.10) is routinely used to subjectively estimate Threshold Contrast Detail Detectability (TCDD) as a measure of image quality in fluoroscopy. However, manufacturer guidance provides calibrated contrasts for only limited peak voltage and copper filtration thickness combinations. Prescribed testing conditions are often difficult to attain as modern flat panel fluoroscopic systems independently determine voltage and copper filtration thickness settings. This work aims to extend the range of TO.10 contrasts available for routine testing at peak voltage and copper thickness settings likely to be encountered.

METHODS

Two methods are described for generalising the calculation of target contrasts: a three-dimensional interpolation/extrapolation model in MATLAB^®, and a multivariate log-polynomial function. Both methods utilise the available calibrated contrasts to estimate contrasts at voltage and copper thickness combinations routinely encountered.

RESULTS

Results are presented as Threshold Detection Index [Formula: see text] curves fit by a second-order polynomial of log [Formula: see text] to log [Formula: see text] . Results are found to be more accurate at unprescribed conditions while also reproducible for relatively consistent input air kerma rate (IAKR) expected from automatic dose rate controls (ADRC).

CONCLUSIONS

The calculation of TO.10 contrasts at non-standard conditions aids in the determination of an absolute estimate of image quality in fluoroscopy with greater accuracy, reproducibility and efficiency.

ADVANCES IN KNOWLEDGE

TO.10 detail contrasts for TCDD testing of fluoroscopy units have been significantly extended beyond those previously available. The described methods will aid the clinical physicist in absolute assessments of fluoroscopic image quality and facilitate inter system comparisons.

ASSESSING THE USEFULNESS OF THE QUASI-IDEAL OBSERVER FOR QUALITY CONTROL IN FLUOROSCOPY

The aim of this work was to evaluate the reliability of the square of the signal-to-noise ratio rate, [Formula: see text], as a precise measurement for quality control test in a digital fluoroscopy system. The quasi-ideal model observer was used to measure [Formula: see text] The dose rate, pulse rate and field of view were varied, and their effect on dose efficiency, defined as [Formula: see text], was evaluated (where [Formula: see text] is the air kerma-area product rate). Measurements were repeated to assess reproducibility. The relative standard deviation in [Formula: see text] over seven consecutive measurements was 5 %. No significant variation in [Formula: see text] was observed across different pulse rates (10-30 pulses s(-1)). The low-dose-rate setting had a superior dose efficiency compared with the medium- and high-dose-rate settings. A smaller field of view resulted in higher dose efficiency.The results show that [Formula: see text] measurements offer the high precision required in quality control constancy tests.

Synchronized, interactive teleconferencing with digital cardiac images

St James's Hospital is a tertiary referral center for percutaneous intervention and cardiothoracic surgery for a number of referring hospitals. This article reports on the development and implementation of a synchronized, interactive teleconferencing system for cardiac images that links St. James's Hospital with a remote site (Sligo General Hospital) and overcomes the problems of transmission of large image files. Teleconferencing was achieved by setting up lossless auto transmission of patient files overnight and conferencing the next morning with linked control signals and databases. As a suitable product was not available, a commercially new software was developed. The system links the imaging databases, monitors and synchronizes progress through imaging sequences, and links a range of image processing and control functions. All parties to the conference are ensured that they are looking at the same images as they are played or at specific aspects of an image that the other party is highlighting. The system allows patient management decisions to be made at a weekly joint teleconference with cardiothoracic surgeons and interventional cardiologists from both sites. Rapid decision making was facilitated with 70% of decisions obtained within 24 h, and 88% within 1 week of their procedure. In urgent cases, data can be transmitted within 20 min of the diagnostic procedure. The system allows increased access to angiography for patients living in rural areas, and provides a more focused referral for revascularization. Participation of the referring cardiologist has improved the quality of decision making.

Quantification of motion unsharpness in digital fluoroscopy

The objectives of this work were first to develop a convenient method to quantify persistence in digital fluoroscopy systems, then to quantify the effect of variable temporal averaging on the detection of moving low-contrast test details within digital fluoroscopic and pulsed fluoroscopic images. The results were analysed to clarify the relationship between the optimum persistence required to see the lowest contrast for circular test details for a range of diameters and their speed of movement. The optimum persistence values obtained are compared with the limited data available on speeds of movement of patient organs during fluoroscopy. It is tentatively concluded that for imaging the abdomen, the optimum imaging system persistence time constant is approximately 0.15 s. For the much greater speeds associated with cardiac motion, no additional frame averaging is necessary, i.e. just the persistence provided by the observer's visual system appears to be optimal for small objects.

Subjective and objective measures of image quality in digital fluoroscopy

There are numerous methods for assessing image quality in diagnostic X ray. In our study we assessed how imaging quality assurance methods perform in practice. Physics assessments were based on IPEM protocols using Leeds test objects. Clinical assessment was based on a questionnaire. A total of 15 systems in three European locations were assessed, covering a range of image intensifier-TV digital fluoroscopy units. Analysis of 274 clinical questionnaires showed that clinical and physics assessments did not place systems in the same order, based on a given image quality parameter. In almost all the comparisons, low level correlation was measured for statistical comparison of rank order (rs < 0.3). However, broad agreement was observed between physics and clinical assessments for image quality associated with contrast and noise. This study emphasises the importance of maintaining links with clinical assessment, when developing quality assurance metrics, and measuring the mutual performance of clinical and physical assessments of image quality.

A novel method for producing x-ray test objects and phantoms

A novel method for producing customized x-ray test objects and clinically realistic phantoms has been developed. Test objects can be created with a drawing software package and the digital images can be printed on a standard inkjet printer but using potassium iodide solution in place of the cartridge's ink. The reproducibility and the consistency, the limiting spatial resolution, the uniformity as well as the potassium iodide thickness per print have been evaluated. The relationship between the number of prints, grey levels and the radiation contrast was investigated and quantified. A copy of the Leeds TO10 contrast detail test object was printed and the x-ray images of the Leeds TO10 and of the printed Leeds TO10 were compared. In addition, the potential use of this method was demonstrated by reproducing a percutaneous transluminal coronary angioplasty clinical digital image. The reproducibility and consistency of this method was found to be better than 0.1%. The limiting spatial resolution of the printer using ink was found to be 3.55 1p mm(-1) but it deteriorated when the ink was replaced with potassium iodide and as the print density increases. The uniformity across the printed area was found to be satisfactory although an artefact due to the printer was present in the x-ray images. The comparison between the Leeds TO10 and the printed Leeds TO10 gave differences less than 10%. A good agreement between the clinical image and the printed clinical image was found. In conclusion, the method is a reliable, cost-effective, flexible and alternative way for producing x-ray test objects and clinically related phantoms.

How should low-contrast detail detectability be measured in fluoroscopy?

The relationship and precision of four methods for measuring the low-contrast detail detectability in fluoroscopic imaging were studied. These included the physical measurement of the accumulation rate of the square of the signal-to-noise ratio (SNR(rate)2), two-alternative forced-choice (2-AFC) experiments, sixteen-alternative forced-choice (16-AFC) experiments and subjective determination of the threshold contrast. The precision and sensitivity of the threshold contrast measurement were seen to be modest in the constancy testing of fluoroscopic equipment: only large changes in system performance could be reliably detected by that method. The measurement of the SNR(rate)2 is suggested instead. The relationship between the results of the various methods were studied, and it was found that human performance can be related to SNR(rate)2 by introducing the concept of the effective image information integration time (t(eff)). When measured for an unlimited observation time, it depicts the saturation of human performance in detecting a static low-contrast detail in dynamic image noise. Here, t(eff) was found to be about 0.6 s in 2-AFC tests and 0.3 s in 16-AFC tests.

Optimization of variable temporal averaging in digital fluoroscopy

In modern X-ray fluoroscopy systems, the amount of temporal averaging (i.e. persistence) applied to the image is often user selectable. The objective of this work is to quantify the effect of variable temporal averaging on the detection of low contrast test objects moving at a range of known speeds within the digital fluoroscopic image. An image intensifier system with a short-persistence television camera was used to record image sequences of a moving threshold contrast-detail diameter test object onto broadcast-standard U-matic videotape. The image sequences were replayed through an image processing system allowing different amounts of temporal averaging to be applied. The test images were scored by an experienced observer. The temporal averaging time constants produced by the added image processing were measured using a method based on noise correlation. Results are presented showing the trends of threshold contrast with test detail diameter and movement speed. The optimum value of temporal averaging time constant is presented as a function of detail diameter for a range of speeds. By comparison with the limited information available in the literature on organ movement, it is tentatively concluded that for the organ movement speeds expected in the abdomen, the optimum imaging system persistence time constant should be approximately 0.15 s. For the much greater speeds associated with cardiac motion no additional frame averaging, i.e. just the persistence provided by the observer's visual system, appears to be optimal.

Use of a quality index in threshold contrast detail detection measurements in television fluoroscopy

The use of a single index to assist in quality control procedures of X-ray television fluoroscopy systems was investigated. A single quality index was devised incorporating a measure of threshold contrast detail detectability (TCDD) performance and taking into account image intensifier input kerma rate, field size, differences in radiation beam quality, and pulsed fluoroscopy. This was applied to a number of clinical systems to investigate changes in image quality index quantified over time. Accepted measurement protocols were used to obtain these measurements. The results show system performance for different systems and can establish the decline in performance parameters over time or assess non-optimal image quality with clinical systems in field measurements. The systems studied were assessed with a variety of performance parameters including TCDD results, low contrast sensitivity, limiting resolution, and image intensifier input kerma rate under clinical modes of operation. The TCDD quality index, and dose normalized quality index, were found to be useful image quality assessment parameters for serial testing of systems, which augment the use of graphical methods for the display of TCDD curves.

Comparison of an amorphous silicon/cesium iodide flat-panel digital chest radiography system with screen/film and computed radiography systems--a contrast-detail phantom study

Flat-panel (FP) based digital radiography systems have recently been introduced as a new and improved digital radiography technology; it is important to evaluate and compare this new technology with currently widely used conventional screen/film (SF) and computed radiography (CR) techniques. In this study, the low-contrast performance of an amorphous silicon/cesium iodide (aSi/Csl)-based flat-panel digital chest radiography system is compared to those of a screen/film and a computed radiography system by measuring their contrast-detail curves. Also studied were the effects of image enhancement in printing the digital images and dependence on kVp and incident exposure. It was found that the FP system demonstrated significantly better low-contrast performance than the SF or CR systems. It was estimated that a dose savings of 70%-90% could be achieved to match the low-contrast performance of the FP images to that of the SF images. This dose saving was also found to increase with the object size. No significant difference was observed in low-contrast performances between the SF and CR systems. The use of clinical enhancement protocols for printing digital images was found to be essential and result in better low-contrast performance. No significant effects were observed for different kVps. From the results of this contrast-detail phantom study, the aSi/CsI-based flat-panel digital chest system should perform better under clinical situations for detection of low-contrast objects such as lung nodules. However, proper processing prior to printing would be essential to realizing this better performance.

The practical application of signal detection theory to image quality assessment in x-ray image intensifier-TV fluoroscopy

This paper applies a published version of signal detection theory to x-ray image intensifier fluoroscopy data and compares the results with more conventional subjective image quality measures. An eight-bit digital framestore was used to acquire temporally contiguous frames of fluoroscopy data from which the modulation transfer function (MTF(u)) and noise power spectrum were established. These parameters were then combined to give detective quantum efficiency (DQE(u)) and used in conjunction with signal detection theory to calculate contrast-detail performance. DQE(u) was found to lie between 0.1 and 0.5 for a range of fluoroscopy systems. Two separate image quality experiments were then performed in order to assess the correspondence between the objective and subjective methods. First, image quality for a given fluoroscopy system was studied as a function of doserate using objective parameters and a standard subjective contrast-detail method. Following this, the two approaches were used to assess three different fluoroscopy units. Agreement between objective and subjective methods was good; doserate changes were modelled correctly while both methods ranked the three systems consistently.

The contrast-detail behaviour of a photostimulable phosphor based computed radiography system

Contrast-detail measurements were performed on a computed radiography imaging system as a function of detector entrance air kerma over the dose range from 0.743 microGy (0.085 mR) to 277 microGy (31.8 mR). A theoretical model of contrast-detail behaviour for a photostimulable phosphor computed radiography system has been derived, which is based on a modified version of the Rose theory of threshold detection. Included in the model are both system and x-ray quantum noise terms, as well as the response of the eye. The zero-frequency noise power of the computed film images was measured with a double-beam scanning microdensitometer. For a given detector dose, good agreement was found between the predicted and measured data when this measurement of system noise was included in the model. The contrast-detail results obtained for the computed radiography system were also compared with contrast-detail results for an image intensifier-TV based digital imaging system and a conventional film-screen system.

Optimal technique factors for magnification mammography

RATIONALE AND OBJECTIVES

Use of small focal spots with low x-ray tube currents may result in very long exposure times and thus result in motion blur in magnification mammography. The authors investigated the reduction in exposure time with increasing x-ray tube kVp and the corresponding decrease in perceived visibility of low-contrast objects in phantom images.

METHODS

Exposure times required to radiograph an RMI 156 phantom in a magnification geometry were measured as a function of x-ray tube kVp when operated under automatic exposure control. Magnification images of the RMI 156 phantom were obtained at x-ray tube voltages ranging from 28 to 34 kVp. Five radiology residents ranked the visibility of two borderline fibers and six borderline microcalcification specks using a 5-point scale ranging from excellent to barely visible.

RESULTS

Between 28 and 34 kVp, the density of the RMI phantom images was nearly constant with a mean value of 1.32 +/- 0.04. Increasing the x-ray tube voltage from 28 kVp to 34 kVp reduced the exposure time from 1.27 seconds to 0.66 seconds. Image quality at 30 and 32 kVp was not significantly worse than that achieved at 28 kVp. Increasing the x-ray tube voltage to 34 kVp, however, resulted in a statistically significant (P < 0.001) deterioration in the relative visibility of fibers and microcalcification specks.

CONCLUSIONS

Magnification mammography performed at 32 kVp will decrease exposure times significantly and result in a microcalcification and fiber visibility that is similar to that achieved at 28 kVp.

Efficiency of low-contrast detail detectability in fluoroscopic imaging

The detectability of a static low-contrast detail in the dynamic fluoroscopic image of a homogeneous phantom was assessed by physical measurement of the signal-to-noise ratio (SNR) and by psychophysical measurement of the human observer detectability index d'. The two-alternative forced-choice method was used for human observer tests. The image data consisted of digitally recorded fluoroscopic image sequences which were displayed in a continuous loop of varying length (1-50 frames) at a rate of 25 frames/s. Human detection performance was seen to improve with the SNR in all cases studied: when the signal was made stronger, the image noise lower, or when the SNR in the image sequence was made higher by increasing the length of the image sequence. The results imply that the statistical efficiency of humans decreases slowly when the number of frames in the displayed loop is increased. This decrease of efficiency with loop length was not seen in all test series, however, and it is possible that the phenomenon is partly related to the high d' values found at the greatest loop lengths studied. When the display contrast was high, the statistical efficiency of the human observer was 30%-40% for both static and dynamic images. The efficiency was somewhat lower, 15%-25%, for images that were displayed with a display contrast gain setting more typical of fluoroscopy. The accumulation rate of SNR2 is a suitable quantity for the measurement of fluoroscopic image quality as related to a given static signal detection task. In contrast to this, visibility measurement by determination of the threshold contrast was seen to be unacceptably imprecise if the test is based on only one observer's opinion, as is often the case in practical quality assurance testing. The precision of the threshold contrast measurement could, however, be improved by using several observers and test objects with a smaller step between details than is usual.

Update on the recommended viewing protocol for FAXIL threshold contrast detail detectability test objects used in television fluoroscopy

The significance of varying the viewing conditions that may affect the perceived threshold contrast of X-ray television fluoroscopy systems has been investigated. Factors investigated include the ambient room lighting and the viewing distance. The purpose of this study is to find the optimum viewing protocol with which to measure the threshold detection index. This is a particular problem when trying to compare the image quality of television fluoroscopy systems in different input field sizes. The results show that the viewing distance makes a significant difference to the perceived threshold contrast, whereas the ambient light conditions make no significant difference. Experienced observers were found to be capable of finding the optimum viewing distance for detecting details of each size, in effect using a flexible viewing distance. This allows the results from different field sizes to be normalized to account for both the magnification and the entrance air kerma rate differences, which in turn allow for a direct comparison of performance in different field sizes.