Direct measurement of mammographic x-ray spectra using a CdZnTe detector

Photon-counting x-ray detectors for CT

The introduction of photon-counting detectors is expected to be the next major breakthrough in clinical x-ray computed tomography (CT). During the last decade, there has been considerable research activity in the field of photon-counting CT, in terms of both hardware development and theoretical understanding of the factors affecting image quality. In this article, we review the recent progress in this field with the intent of highlighting the relationship between detector design considerations and the resulting image quality. We discuss detector design choices such as converter material, pixel size, and readout electronics design, and then elucidate their impact on detector performance in terms of dose efficiency, spatial resolution, and energy resolution. Furthermore, we give an overview of data processing, reconstruction methods and metrics of imaging performance; outline clinical applications; and discuss potential future developments.

Accuracy validation of incident photon fluence on detective quantum efficiency in mammography

X-ray image evaluation is commonly performed by determining the detective quantum efficiency (DQE). DQE is calculated with a presampled modulation transfer function (MTF), incident photon fluence, and digital noise power spectrum (NPS). Accurate evaluation of MTF, incident photon fluence, and NPS is important for precise DQE determination. In this study, we focused on the accuracy of the incident photon fluence in mammography. The incident photon fluence is calculated using the squared signal-to-noise ratio (SNR_in²) value as specified in the International Electrotechnical Commission (IEC) 62220-1-2 report. However, the reported SNR_in² values were determined using a computer program, and the reported values may differ from those calculated from an X-ray spectrum that is measured with actual mammography equipment. Therefore, we evaluated the error range of reported SNR_in² values in mammography to assess the accuracy of the incident photon fluence. First, X-ray spectra from various mammography systems were measured with a CdTe spectrometer. Six mammographic X-ray units were used in this study. Second, the SNR_in² values were calculated from the measured X-ray spectra. The calculated values were compared to the reported values. The results show that the percentage differences between the calculated and reported SNR_in² values were within - 4.1% of each other. The results obtained in this study indicate that the SNR_in² values provided in the IEC report are a robust and convenient tool for calculating the incident photon fluence for DQE evaluation in mammography.

Monochromatic mammography using scanning multilayer X-ray mirrors

A prototype system for breast imaging using monochromatic X-rays has been developed using a scanning multilayer X-ray mirror in combination with a conventional mammography tube and an imaging detector. The X-ray mirror produces a monochromatic fan beam tuned near 19 keV, with an energy bandpass of approximately 1.5 keV. Rotating the mirror about the tube's focal spot in synchronization with the X-ray generator and detector enables the acquisition of monochromatic X-ray images over large areas. The X-ray mirror also can be rotated completely out of the beam so that conventional polychromatic images can be acquired using a K-edge filter, facilitating direct comparison between the two modes of operation. The system was used to image synthetic, tissue-equivalent breast phantoms in order to experimentally quantify the improvements in image quality and dose that can be realized using monochromatic radiation. Nine custom phantoms spanning a range of thicknesses and glandular/adipose ratios, each containing both glandular- and calcification-equivalent features, were used to measure contrast and signal-difference-to-noise ratio (SDNR). Mean glandular dose (MGD) was computed from measured entrance exposure, and a figure-of-merit (FOM) was computed as FOM = SDNR²/MGD in each case. Monochromatic MGD ranges from 0.606 to 0.134 of polychromatic MGD for images having comparable glandular SDNR, depending on breast thickness and glandularity; relative monochromatic dose decreases with increasing glandularity for all thicknesses. Monochromatic FOM values are higher than the corresponding polychromatic FOM values in all but one case. Additionally, the monochromatic contrast for glandular features is higher than the polychromatic contrast in all but one case as well. These results represent important steps toward the realization of clinically practical monochromatic X-ray breast imaging systems having lower dose and better image quality, including those for digital mammography, digital breast tomosynthesis, contrast-enhanced spectral mammography and other modalities, for safer, more accurate breast cancer detection, diagnosis and staging.

A Single Scatter Model for X-ray CT Energy Spectrum Estimation and Polychromatic Reconstruction

To improve the quantitative accuracy of linear attenuation coefficients measured by computed tomography (CT), we used a single scatter model to estimate the Compton scatter distribution and then a polychromatic image reconstruction algorithm, namely the iterative maximum-likelihood polychromatic algorithm for CT (IMPACT), was implemented to include scatter correction (SC). To perform the IMPACT, the X-ray spectra of a tube were estimated via an expectation-maximization (EM) algorithm with SC. To test the accuracy of the estimated spectra, the projection images of cubic phantoms containing different depths of polymethylmethacrylate (PMMA) were acquired. The percentage of root mean square errors (%RMSE) of the measured transmission data and calculated transmission values were used to evaluate the accuracy of the estimated spectra. In addition, a hydroxylapatite (HA) phantom was used to study streak artifacts and evaluate the accuracy of the linear attenuation coefficients estimated using the IMPACT with SC. The %RMSE of the EM-with-SC estimated spectra were all lower than 1% and were also smaller than that without SC. The error in the quantification of the HA linear attenuation was only about 3% after SC. Our results showed that the quantitative accuracy of the linear attenuation coefficients measured with a cone beam CT was improved when the IMPACT with SC was used.

Experimental measurements and Monte Carlo modelling of the XSTRAHL 150 superficial X-ray therapy unit

Background

Superficial X-ray therapy units are used for the treatment of certain types of skin cancer and some severe dermatological conditions. The performance assessment and beam characteristics of the superficial unit are very important to ensure accurate dose delivery during patient treatment. Both experimental measurements and Monte Carlo calculations can be used for this purpose.

Purpose

This study aims to investigate whether it is possible to reproduce experimentally measured data for the XSTRAHL 150 superficial X-ray unit with simulations using the BEAMnrc Monte Carlo code.

Materials and Methods

The experimental procedure applied in this study included the following: experimental measurements of different X-ray spectra, half-value layers, percentage depth dose and beam profiles. Monte Carlo modelling of the XSTRAHL 150 unit was performed with the BEAMnrc code. The validity of the model was checked by comparing the theoretical calculations with experimental measurements.

Results

There was good agreement (∼1%) between experimentally measured and simulated X-ray spectra. Results of half-value layers obtained from simulated and measured spectra showed that there was a maximum of 3·6% difference between BEAMnrc and measurements and a minimum of 2·3%. In addition, simulated percentage depth dose and profile curves have been compared against experimental measurements and show good agreement (within 2% for the depth dose curves and 3–5% for beam profile curves, depending on the applicator size).

Conclusion

The results of this study provide information about particles’ interaction in different kilovoltage and filter combinations. This information is useful for X-ray tube design and development of new target/filter combinations to improve beam quality in superficial X-ray radiotherapy. The data presented here may provide a base for comparison and a reference for other or potential new users of the XSTRAHL 150 X-ray unit.

Energy calibration of the pixels of spectral X-ray detectors

The energy information acquired using spectral X-ray detectors allows noninvasive identification and characterization of chemical components of a material. To achieve this, it is important that the energy response of the detector is calibrated. The established techniques for energy calibration are not practical for routine use in pre-clinical or clinical research environment. This is due to the requirements of using monochromatic radiation sources such as synchrotron, radio-isotopes, and prohibitively long time needed to set up the equipment and make measurements. To address these limitations, we have developed an automated technique for calibrating the energy response of the pixels in a spectral X-ray detector that runs with minimal user intervention. This technique uses the X-ray tube voltage (kVp) as a reference energy, which is stepped through an energy range of interest. This technique locates the energy threshold where a pixel transitions from not-counting (off) to counting (on). Similarly, we have developed a technique for calibrating the energy response of individual pixels using X-ray fluorescence generated by metallic targets directly irradiated with polychromatic X-rays, and additionally γ-rays from (241)Am. This technique was used to measure the energy response of individual pixels in CdTe-Medipix3RX by characterizing noise performance, threshold dispersion, gain variation and spectral resolution. The comparison of these two techniques shows the energy difference of 1 keV at 59.5 keV which is less than the spectral resolution of the detector (full-width at half-maximum of 8 keV at 59.5 keV). Both techniques can be used as quality control tools in a pre-clinical multi-energy CT scanner using spectral X-ray detectors.

Response functions of Si(Li), SDD and CdTe detectors for mammographic x-ray spectroscopy

In this work, the energy response functions of Si(Li), SDD and CdTe detectors were studied in the mammographic energy range through Monte Carlo simulation. The code was modified to take into account carrier transport effects and the finite detector energy resolution. The results obtained show that all detectors exhibit good energy response at low energies. The most important corrections for each detector were discussed, and the corrected mammographic x-ray spectra obtained with each one were compared. Results showed that all detectors provided similar corrected spectra, and, therefore, they could be used to accurate mammographic x-ray spectroscopy. Nevertheless, the SDD is particularly suitable for clinic mammographic x-ray spectroscopy due to the easier correction procedure and portability.

Possible use of CdTe detectors in kVp monitoring of diagnostic x-ray tubes

It has been suggested that kVp of diagnostic X-ray devices (or maximal energy of x-ray photon spectra) should be monitored routinely; however a standardized noninvasive technique has yet to be developed and proposed. It is well known that the integral number of Compton scattered photons and the intensities of fluorescent x-ray lines registered after irradiation of some material by an x-ray beam are a function of the maximal beam energy. CdTe detectors have sufficient energy resolution to distinguish individual x-ray fluorescence lines and high efficiency for the photon energies in the diagnostic region. Our initial measurements have demonstrated that the different ratios of the integral number of Compton scattered photons and intensities of K and L fluorescent lines detected by CdTe detector are sensitive function of maximal photon energy and could be successfully applied for kVp monitoring.

A comparison of mammographic x-ray spectra: simulation with EGSnrc and experiment with CdTe detector

Mammographic x-ray spectra simulated by BEAMnrc/EGSnrc Monte Carlo code were qualitatively compared with the results obtained from the direct measurement using a cadmium telluride x-ray spectroscopy system and from the generation of IPEM report number 78. Generally, there is good agreement between the simulated and measured spectra, though there are slight differences at low energy in which the K-characteristic x-ray intensity is relatively higher for IPEM spectra. In addition, transmission curves were measured and simulated using a breast tissue-equivalent phantom (BR-12) as filtration. Comparison of the transmission curves shows good agreement. Moreover, the first half value layer (HVL) from direct measurement using ion chamber was consistent with the first HVL calculated by simulated spectra. Therefore, Monte Carlo may be used as an alternative tool for obtaining x-ray spectra when direct measurement is not available.

An analytic model for the response of a CZT detector in diagnostic energy dispersive x-ray spectroscopy

A CdZnTe detector (CZTD) can be very useful for measuring diagnostic x-ray spectra. The semiconductor detector does, however, exhibit poor hole transport properties and fluorescence generation upon atomic de-excitations. This article describes an analytic model to characterize these two phenomena that occur when a CZTD is exposed to diagnostic x rays. The analytical detector response functions compare well with those obtained via Monte Carlo calculations. The response functions were applied to 50, 80, and 110 kV x-ray spectra. Two 50 kV spectra were measured; one with no filtration and the other with 1.35 mm Al filtration. The unfiltered spectrum was numerically filtered with 1.35 mm of Al in order to see whether the recovered spectrum resembled the filtered spectrum actually measured. A deviation curve was obtained by subtracting one curve from the other on an energy bin by bin basis. The deviation pattern fluctuated around the zero line when corrections were applied to both spectra. Significant deviations from zero towards the lower energies were observed when the uncorrected spectra were used. Beside visual observations, the exposure obtained using the numerically attenuated unfiltered beam was compared to the exposure calculated with the actual filtered beam. The percent differences were 0.8% when corrections were applied and 25% for no corrections. The model can be used to correct diagnostic x-ray spectra measured with a CdZnTe detector.

A semianalytic model to extract differential linear scattering coefficients of breast tissue from energy dispersive x-ray diffraction measurements

The goal of this work is to develop a technique to measure the x-ray diffraction signals of breast biopsy specimens. A biomedical x-ray diffraction technology capable of measuring such signals may prove to be of diagnostic use to the medical field. Energy dispersive x-ray diffraction measurements coupled with a semianalytical model were used to extract the differential linear scattering coefficients [mus(x)] of breast tissues on absolute scales. The coefficients describe the probabilities of scatter events occuring per unit length of tissue per unit solid angle of detection. They are a function of the momentum transfer argument, x=sin(theta/2)/X, where theta=scatter angle and lambda=incident wavelength. The technique was validated by using a 3 mm diameter 50 kV polychromatic x-ray beam incident on a 5 mm diameter 5 mm thick sample of water. Water was used because good x-ray diffraction data are available in the literature. The scatter profiles from 6 degrees to 15 degrees in increments of 1 degrees were measured with a 3 mm x 3 mm x 2 mm thick cadmium zinc telluride detector. A 2 mm diameter Pb aperture was placed on top of the detector. The target to detector distance was 29 cm and the duration of each measurement was 10 min. Ensemble averages of the results compare well with the gold standard data of A. H. Narten ["X-ray diffraction data on liquid water in the temperature range 4 degrees C-200 degrees C," ORNL Report No. 4578 (1970)]. An average 7.68% difference for which most of the discrepancies can be attributed to the background noise at low angles was obtained. The preliminary measurements of breast tissue are also encouraging.

Some aspects involving the use of CdTe for finding end-point energies in diagnostic radiology

We have considered the use of CdTe detectors and commercially available spectroscopy systems for the determination of the end-point energies of diagnostic X-ray devices. A simple technique based upon filtered spectra and simple fitting procedures was used to deduce end-point energies. We observed some unexpected detector-related effects such as significant shifts in the energy calibration at high count rates. When CdTe detectors operate at high counting rates drifts in the DC level, associated with collection rates of the positive charge (holes) can yield a corresponding shift in the spectrum towards lower energies. Only for relatively low counting rates and long detection periods, in excess of 10 min, can end-point energies be obtained with CdTe that reach levels of precision found in recent protocols. High rates also decrease energy resolution and lower the accuracy of recalibrations using characteristic X-rays from the target.

Physical imaging properties and low-contrast performance of a newly developed flat-panel digital radiographic system

We investigated the clinical usefulness of a newly developed flat-panel detector (FPD) system by comparing its physical imaging properties and low-contrast detectability with those of a current FPD system. The newly developed CsI-based indirect FPD (Canon, CXDI-40C) and current Gd(2)O(2)S-based FPD (Canon CXDI-11) systems were used. Characteristic curves, resolution properties, radiographic noise, detective quantum efficiencies (DQEs) and low-contrast detectability for both systems were measured. The new FPD system showed considerably lower noise levels than those of the current FPD system. DQE (0) s of the new and current FPD systems were 75% and 35%, respectively. Observer performance tests of the contrast-detail (C-D) phantom indicated that the new FPD system can significantly improve low-contrast performance over that obtainable with the current FPD system under the same conditions of exposure. The new FPD system provided approximately 50% reduction in exposure while providing comparable detectability. The newly developed FPD system provides radiographic images with excellent inherent physical image quality and low-contrast performance.

Compton-scattering measurement of diagnostic x-ray spectrum using high-resolution Schottky CdTe detector

The analysis of x-ray spectra is important for quality assurance (QA) and quality control (QC) of radiographic systems. The aim of this study is to measure the diagnostic x-ray spectra under clinical conditions using a high-resolution Schottky CdTe detector. Under clinical conditions, the direct measurement of a diagnostic spectrum is difficult because of the high photon fluence rates that cause significant detector photon pile-up. An alternative way of measuring the output spectra from a tube is first to measure the 90 deg Compton scattered photons from a given sample. With this set-up detector, pile-up is not a problem. From the scattered spectrum one can then use an energy correction and the Klein-Nishina function to reconstruct the actual spectrum incident upon the scattering sample. The verification of whether our spectra measured by the Compton method are accurate was accomplished by comparing exposure rates calculated from the reconstructed spectra to those measured with an ionization chamber. We used aluminum (Al) filtration ranging in thickness from 0 to 6 mm. The half value layers (HVLs) obtained for a 70 kV beam were 2.78 mm via the ionization chamber measurements and 2.93 mm via the spectral measurements. For a 100 kV beam we obtained 3.98 and 4.32 mm. The small differences in HVLs obtained by both techniques suggest that Compton scatter spectroscopy with a Schottky CdTe detector is suitable for measuring the diagnostic x-ray spectra and useful for QA and QC of clinical x-ray equipment.

X-ray spectroscopy in mammography with a silicon PIN photodiode with application to the measurement of tube voltage

In this work a silicon PIN photodiode was employed in mammographic x-ray spectroscopy under clinical and nonclinical conditions. Measurements have been performed at a constant potential tungsten anode tube, adapted in this work with molybdenum filters to produce a beam like that used in mammography, and at a clinical equipment with a molybdenum anode tube by using an additional aluminum filtration. The corrected x-ray spectra were in full agreement with those generated by theoretical models published in the literature and agree well with those measured with a CdZnTe detector for tube voltages less than 30 kV. The half value layer and the relative exposure values calculated from the corrected silicon PIN photodiode spectra were in agreement with those measured with an ionization chamber. These results indicate that a silicon PIN photodiode are very suitable for mammographic x-ray spectroscopy. As an application, the voltage (kV) applied to mammographic x-ray equipment has been measured through the evaluation of the spectra high energy cut off. Uncertainties evaluated for the voltage values calculated from the measured spectra are less than 0.13% for voltages in the range 20-35 kV. The low uncertainties associated with the obtained results in this work point out that the method employed can be accurately used for calibration of noninvasive mammographic kVp meters.

Direct analysis of molybdenum target generated x-ray spectra with a portable device

In routine applications, information about the photon flux of x-ray tubes is obtained from exposure measurements and cataloged spectra. This approach relies mainly on the assumption that the real spectrum is correctly approximated by the cataloged one, once the main characteristics of the tube such as voltage, target material, anode angle, and filters are taken account of. In practice, all this information is not always available. Moreover, x-ray tubes with the same characteristics may have different spectra. We describe an apparatus that should be useful for quality control in hospitals and for characterizing new radiographic systems. The apparatus analyzes the spectrum generated by an x-ray mammographic unit. It is based on a commercial CZT produced by AMPTEK Inc. and a set of tungsten collimator disks. The electronics of the CZT are modified so as to obtain a faster response. The signal is digitized using an analog to digital converter with a sampling frequency of up to 20 MHz. The whole signal produced by the x-ray tube is acquired and analyzed off-line in order to accurately recognize pile-up events and reconstruct the emitted spectrum. The energy resolution has been determined using a calibrated x-ray source. Spectra were validated by comparison of the HVL measured using an ionization chamber.

Evaluation of the use of six diagnostic X-ray spectra computer codes

A knowledge of photon energy spectra emitted from X-ray tubes in radiology is crucial for many research domains in the medical field. Since spectrometry is difficult because of high photon fluence rates, a convenient solution is to use computational models. This paper describes the use of six computer codes based on semiempirical or empirical models. The use of the codes was assessed, notably by comparing theoretical half value layers and air kerma with measurements on five different X-ray tubes used in a research hospital. It was found that three out of the six computer codes give relative spectra very close to those produced by X-ray units equipped with constant potential generators: the mean difference between measured and modelled half value layer was less than 3% with a standard deviation of 3.6% whatever the tube and the applied voltage. Absolute output is less accurate: for four computer codes, the mean difference between the measured and modelled air kerma was between 18% and 36%, with a standard deviation of 9% whatever the tube (except for the single phase generator) and the applied voltage. One of the codes gives a good output and beam quality for X-ray units equipped with 100% ripple voltage generators. The use of computational codes as described in this paper provides a means of modelling relative diagnostic X-ray spectra, the usefulness of the tube output data depending on the accuracy required by the end user.

New contrast media designed for x-ray energy subtraction imaging in digital mammography

RATIONALE AND OBJECTIVES

In contrast-enhanced dual-energy subtraction imaging 2 images acquired postcontrast media administration at different energies are subtracted to highlight structures hidden in the absence of contrast media. X-ray spectra of the newly developed digital full-field mammography units (GE Senographe 2000 D) are dominated by the emission lines of the Mo or Rh anodes. The K-edge of Zirconium (Zr) is flanked by these 2 emission lines. Thus, the attenuation of Zr should experience a pronounced change of attenuation in parallel with a change of anodes. Under clinically relevant conditions, the contrasting behavior of Zr should be compared with that of other elements having K-edge energies outside the window spanned by the 2 anode emission lines.

METHODS

Solutions containing the contrasting elements Br, Y, Zr, I, and Gd were investigated for dual-energy subtraction in digital mammography with the 2 anode/filter settings (Mo/Mo and Rh/Rh). These solutions were investigated in phantom studies in the energy range conventionally used in mammography. Additionally, the contrasting behavior of Zr and I was compared in an in vivo study in rats.

RESULTS

The sweeping over the K-edge by alternating between the Mo and Rh anodes increases the detection of Zr in energy subtraction imaging at constant high voltage. This procedure does not lead to sufficient contrast enhancement for iodine-based contrast media which become detectable by increasing the high voltage to 40-49 kV.

CONCLUSION

The instrumental and physical data outlined predestine Zr as contrasting element with a high potential for energy subtraction imaging in digital mammography in the energy range conventionally applied.

CdZnTe detector in mammographic x-ray spectroscopy

A CdZnTe (CZT) detector was utilized in mammographic x-ray spectroscopy under clinical conditions. First, the detector response was investigated using y-rays from 241Am. The escape of secondary (Compton scattered and K fluorescent) x-rays and tailing due to carrier trapping were minor in the mammographic energy range. In addition, the transmission of primary x-rays was minimal from the results calculated using the mass attenuation coefficients of CZT. Therefore, spectral distortion in this energy range was expected to be negligible. Secondly, x-ray spectroscopy was carried out with the CZT detector. The measured spectra were in good agreement with the spectra obtained with the Compton-scatter method with a high-purity germanium detector. Moreover, the half-value layers (HVLs) calculated from the CZT spectra were consistent with the HVLs measured with an ionization chamber. The results indicate that a CZT detector can be utilized in mammographic x-ray spectroscopy without any corrections.

An alignment method for mammographic X-ray spectroscopy under clinical conditions

This paper describes an alignment method for mammographic X-ray spectroscopy under clinical conditions. A pinhole, a fluorescent screen, a laser device and the case for a detector are used for alignment of the focal spot, a collimator and a detector. The method determines the line between the focal spot and the point of interest in an X-ray field radiographically. The method allows alignment for both central axis and off-axis directions.

CdZnTe detector in diagnostic x-ray spectroscopy

A method to utilize CdZnTe (CZT) detectors in diagnostic x-ray spectroscopy is described in this article. Spectral distortion due to transmission of primary x rays, the escape of cadmium- and tellurium-K fluorescent x rays, and tailing was severe in measured x-ray spectra. Therefore, correction for the distortion was performed with the stripping method using response functions. The response functions were calculated with the Monte Carlo method. The Hecht equation was employed to approximate the effects of carrier trapping in the calculations. The parameters in the Hecht equation, the mean-free path (lambda) of electrons and holes, were determined such that the tailing in calculated response functions fit that in measured gamma-ray spectra. Corrected x-ray spectra agreed well with the reference spectra measured with an HPGe detector. The results indicate that CZT detectors are suitable for diagnostic x-ray spectroscopy with appropriate corrections.