Robert M. Gagne

A method for describing the doses delivered by transmission x‐ray computed tomography

A method for describing the absorbed dose delivered by x-ray transmission computed tomography (CT) is proposed which provides a means to characterize the doses resulting from CT procedures consisting of a series of adjacent scans. The dose descriptor chosen is the average dose at several locations in the imaged volume of the central scan of the series. It is shown that this average dose, as defined, for locations in the central scan of the series can be obtained from the integral of the dose profile perpendicular to the scan plane at these same locations for a single scan. This method for estimating the average dose from a CT procedure has been evaluated as a function of the number of scans in the multiple scan procedure and location in the dosimetry phantom using single scan dose profiles obtained from five different types of CT systems. For the higher dose regions in the phantoms, the multiple scan dose descriptor derived from the single scan dose profiles overestimates the multiple scan average dose by no more than 10%, provided the procedure consists of at least eight scans.

An energy‐ and depth‐dependent model for x‐ray imaging

In this paper, we model an x-ray imaging system, paying special attention to the energy- and depth-dependent characteristics of the inputs and interactions: x rays are polychromatic, interaction depth and conversion to optical photons is energy-dependent, optical scattering and the collection efficiency depend on the depth of interaction. The model we construct is a random function of the point process that begins with the distribution of x rays incident on the phosphor and ends with optical photons being detected by the active area of detector pixels to form an image. We show how the point-process representation can be used to calculate the characteristic statistics of the model. We then simulate a Gd2O2S:Tb phosphor, estimate its characteristic statistics, and proceed with a signal-detection experiment to investigate the impact of the pixel fill factor on detecting spherical calcifications (the signal). The two extremes possible from this experiment are that SNR2 does not change with fill factor or changes in proportion to fill factor. In our results, the impact of fill factor is between these extremes, and depends on the diameter of the signal.

Evaluation of x-ray sources for mammography

A computational approach is being developed for the evaluation of mammographic imaging system performance. This approach takes into account both the spatial frequency properties and the x-ray spectral characteristics of the system being evaluated. The initial version of the program that implements the approach has been used to evaluate a conventional mammography source assembly for several breast thicknesses, and to compare the conventional tube and filter combination to alternatives that have been suggested for the imaging of breasts that are thicker or more dense than average. It has also been used to study the effect of varying the thickness of the molybdenum filter in the conventional system. The parameters calculated include contrast, average glandular dose, tube load, and a figure of merit, SNR2/Dose. The calculations confirm the strong dependence of system performance on both tube potential and breast thickness for the standard system, and indicate that alternative designs can improve performance in the imaging of thicker or more dense breasts. The study of filter thickness shows that, of the four parameters calculated, only tube load is strongly affected by filter thickness.

Standardization of NPS measurement: interim report of AAPM TG16

This article reviews the state of the Noise Power Standard being drafted by Task Group No. 16 for the American Association of Physicists in Medicine. The Standard is intended to represent a consensus on acceptable practices in the measurement and reporting of noise power spectra for digital radiographic imaging devices based on single projections and to contain informative sections which will be of use to those not completely familiar with the measurement and interpretation of noise power spectra. Several of the issues considered by the committee are reviewed, including issues of conditioning and windowing data, issues specific to several modalities, and various methods of data presentation. A note on the historical background of noise power measurements and a brief discussion of possible avenues for future research is included.

Laboratory measurements of sensitometry, MTF, veiling glare, Wiener spectrum and DQE for image intensifier tubes

Several different test protocols were used to measure the sensitometric response, the modulation transfer function (MTF), the veiling glare and the Wiener spectrum of two x-ray image intensifier (II) tubes. These data provided the means to calculate summary measures of imaging performance, i.e., the noise equivalent quanta, NEQ, or the detective quantum efficiency, DQE, as a function of spatial frequency. Results are presented to show the differences between an older versus a newer generation II tube, i.e., DQE equals 0.3 +/- 0.03 and 0.6 +/- 0.06 at 0.5 lp/mm, respectively. The eventual goal of this work is to achieve some consensus on methodology for these measurements and to validate the use of algorithmic observers in quantitating imaging performance in the clinical environment.

Storage phosphor-based digital mammography using a low-dose x-ray system optimized for screen-film mammography

We are examining the feasibility of performing digital mammography by combining a storage- phosphor image receptor with a highly efficient x-ray system. The image receptor consists of Fuji series HR-V high resolution imaging plates and a Fuji 9000 reader. The x-ray system was developed using multiparameter optimization techniques, with the goal of reducing patient dose as much as possible while retaining acceptable imaging performance. We have measured sensitometric properties, modulation transfer function (MTF), and noise power spectrum (NPS) of the Fuji plates with low-energy x-ray spectra. We have used the measurements, along with information about the x-ray system, to estimate signal-to-noise ratios (SNRs) for objects in a contrast-detail (C-D) phantom. We present the results of our measurements on the Fuji plates, comparisons of calculated and observed C-D diagrams for this system and a conventional system, and comparisons of phantom images and doses for this system to images and doses for a conventional system. We conclude that digital mammography with the system studied is at least feasible since phantom image quality is comparable to that of a conventional system at dose levels that are somewhat lower.

Lesion detectability in digital radiography

The usefulness of Fourier-based measures of imaging performance has come into question for the evaluation of digital imaging systems. Figures of merit such as detective quantum efficiency are relevant for linear, shift-invariant systems with stationary noise. However, no digital imaging system is shift invariant, and realistic images do not satisfy the stationarity condition. Our methods for task- based evaluation of imaging systems, based on lesion detectability, do not require such assumptions. We have computed the performance of Hotelling and nonprewhitening matched-filter observers for the task of lesion detection in digital radiography.

Effect of shift invariance and stationarity assumptions on simple detection tasks: spatial and spatial frequency domains

A lens-coupled digital radiography system was used to investigate the impact of shift invariance and stationarity assumptions on task-based measures of performance. Experimental data are available on the imaging systems large area gray-scale transfer (sensitivity), resolution (modulation transfer function) and noise (variance map, autocovariance function, and noise power spectrum) properties in different regions of the field of view. These data were obtained for two different values of optical demagnification factor and three levels of x-ray exposure. Evidence of shift variant and non-stationary behavior is seen in the position dependence of the sensitivity, resolution and noise properties of the imaging system. Estimates of observer performance in terms of signal-to- noise-ratio were obtained from one algorithmic observer (DC- suppressed matched filter) and from one analytic observer (pre-whitening matched filter) for the task of detecting a known object (CDMAM phantom) on a flat background. For the special case of a large object at the center of the field- of-view, estimates of observer performance were essentially the same whether based on spatial or spatial frequency domain measures.

Assessment of lesion detectability of Monte Carlo modeling of digital radiography systems

Previously we used a simple 2-D model to evaluate the imaging performance of a digital radiographic system while varying input parameters such as transducer blur and signal size. We extend this work using a realistic phosphor simulation to explore the effect of the incident x-ray spectrum and the depth dependence of the point spread function and optical collection efficiency. Initially we investigate one Swank screen type representative of modern powder phosphor design. Images resulting from these simulations are used to get an estimate of the impact of these factors on lesion detectability. Results show that the simple 2-D model gives optimistic estimates of detectability.

Uncertainties in estimates of lesion detectability in diagnostic ultrasound

Statistical properties of estimates of focal lesion detectability for medical ultrasonic imaging systems are investigated. Analytic forms for bias and variance of estimates of detectability of a lesion consisting of fully developed speckle embedded within a speckle background are derived. Bias and variance of estimates of detectability are investigated using a computer simulation and experiments on tissue-mimicking phantoms. This work offers a systematic methodology for interpreting measurements on phantoms in order to assess lesion detectability. In addition, it provides useful results which may be used to improve design of phantoms and experiments for imaging-system performance assessment.