Magnus Hemmendorff

Contrast-enhanced spectral mammography with a photon-counting detector

PURPOSEnSpectral imaging is a method in medical x-ray imaging to extract information about the object constituents by the material-specific energy dependence of x-ray attenuation. The authors have investigated a photon-counting spectral imaging system with two energy bins for contrast-enhanced mammography. System optimization and the potential benefit compared to conventional non-energy-resolved absorption imaging was studied.nnnMETHODSnA framework for system characterization was set up that included quantum and anatomical noise and a theoretical model of the system was benchmarked to phantom measurements.nnnRESULTSnOptimal combination of the energy-resolved images corresponded approximately to minimization of the anatomical noise, which is commonly referred to as energy subtraction. In that case, an ideal-observer detectability index could be improved close to 50% compared to absorption imaging in the phantom study. Optimization with respect to the signal-to-quantum-noise ratio, commonly referred to as energy weighting, yielded only a minute improvement. In a simulation of a clinically more realistic case, spectral imaging was predicted to perform approximately 30% better than absorption imaging for an average glandularity breast with an average level of anatomical noise. For dense breast tissue and a high level of anatomical noise, however, a rise in detectability by a factor of 6 was predicted. Another approximately 70%-90% improvement was found to be within reach for an optimized system.nnnCONCLUSIONSnContrast-enhanced spectral mammography is feasible and beneficial with the current system, and there is room for additional improvements. Inclusion of anatomical noise is essential for optimizing spectral imaging systems.

Development of Low-Dose Photon-counting Contrast-enhanced Tomosynthesis with Spectral Imaging

PURPOSEnTo demonstrate the feasibility of low-dose photon-counting tomosynthesis in combination with a contrast agent (contrast material-enhanced tomographic mammography) for the differentiation of breast cancer.nnnMATERIALS AND METHODSnAll studies were approved by the institutional review board, and all patients provided written informed consent. A phantom model with wells of iodinated contrast material (3 mg of iodine per milliliter) 1, 2, 5, 10, and 15 mm in diameter was assessed. Nine patients with malignant lesions and one with a high-risk lesion (atypical papilloma) were included (all women; mean age, 60.7 years). A multislit photon-counting tomosynthesis system was utilized (spectral imaging) to produce both low- and high-energy tomographic data (below and above the k edge of iodine, respectively) in a single scan, which allowed for dual-energy visualization of iodine. Images were obtained prior to contrast material administration and 120 and 480 seconds after contrast material administration. Four readers independently assessed the images along with conventional mammograms, ultrasonographic images, and magnetic resonance images. Glandular dose was estimated.nnnRESULTSnContrast agent was visible in the phantom model with simulated spherical tumor diameters as small as 5 mm. The average glandular dose was measured as 0.42 mGy per complete spectral imaging tomosynthesis scan of one breast. Because there were three time points (prior to contrast medium administration and 120 and 480 seconds after contrast medium administration), this resulted in a total dose of 1.26 mGy for the whole procedure in the breast with the abnormality. Seven of 10 cases were categorized as Breast Imaging Reporting and Data System score of 4 or higher by all four readers when reviewing spectral images in combination with mammograms. One lesion near the chest wall was not captured on the spectral image because of a positioning problem.nnnCONCLUSIONnThe use of contrast-enhanced tomographic mammography has been demonstrated successfully in patients with promising diagnostic benefit. Further studies are necessary to fully assess diagnostic sensitivity and specificity.

A photon-counting detector for dual-energy breast tomosynthesis

We present the first evaluation of a recently developed silicon-strip detector for photon-counting dual-energy breast tomosynthesis. The detector is well suited for tomosynthesis with high dose efficiency and intrinsic scatter rejection. A method was developed for measuring the spatial resolution of a system based on the detector in terms of the three-dimensional modulation transfer function (MTF). The measurements agreed well with theoretical expectations, and it was seen that depth resolution was won at the cost of a slightly decreased lateral resolution. This may be a justifiable trade-off as clinical images acquired with the system indicate improved conspicuity of breast lesions. The photon-counting detector enables dual-energy subtraction imaging with electronic spectrumsplitting. This improved the detectability of iodine in phantom measurements, and the detector was found to be stable over typical clinical acquisition times. A model of the energy resolution showed that further improvements are within reach by optimization of the detector.

Contrast-enhanced dual-energy mammography using a scanned multislit system: evaluation of a differential beam filtering technique

This paper describes a method for single-exposure, contrast-enhanced dual-energy imaging of tumors utilizing a scanned multislit system for digital mammography. This photon-counting system employs an array of silicon strip detectors mounted in an edge-on geometry. The line detectors and pre- and post-collimator slits are carefully aligned, and the multislit setup allows differential filtering of the x-ray beam in the pre-collimator slits. A high-energy image is constructed from those lines where the filter material has been chosen to harden the x-ray beam and the low-energy image from the lines with a filter producing softer beams. Both images are obtained in the same scan, eliminating the need to change tube voltages and anode materials and minimizing the risk of motion artifacts. The method is illustrated on a purpose-built phantom, and logarithmic subtraction of the images produces images essentially free of anatomical clutter with the contrast-enhanced targets clearly visible.

Dual-energy imaging using a digital scanned multi-slit system for mammography: evaluation of a differential beam filtering technique

This paper describes a method for single exposure contrast-enhanced dual-energy imaging of tumors utilizing a scanned multi-slit system for digital mammography. This photon counting system employs an array of silicon strip detectors in an edge-on geometry. In the multi-slit setup, the line detectors and pre-collimator slits are aligned orthogonal to the scan direction. This geometry is advantageous to dual-energy imaging, since it allows differential filtering of the x-ray beam in the pre-collimator slits. A high-energy image is constructed from those lines where the filter material has been chosen to harden the x-ray beam and the low-energy image from the lines with a filter producing softer beams. Both images are obtained in the same scan, eliminating the need to change tube voltages and anode materials and minimizing the risk of motion artifacts. The method is illustrated on a purpose-built phantom and logarithmic subtraction of the images produces images essentially free of anatomical clutter with the contrast-enhanced targets clearly visible.

IMAGING OF SMALL CHILDREN WITH A PROTOTYPE FOR PHOTON COUNTING TOMOSYNTHESIS

We present data on a first prototype for photon counting tomosynthesis imaging of small children, which we call photoncounting tomosynthesis (PCT). A photon counting detector can completely eliminate electronic noise, which makes it ideal for tomosynthesis because of the low dose in each projection. Another advantage is that the detector allows for energy sensitivity in later versions, which will further lower the radiation dose. In-plane resolution is high and has been measured to be 5 lp/mm, at least 4 times better than in CT, while the depth resolution was significantly lower than typical CT resolution. The image SNR decreased from 30 to 10 for a detail of 10 mm depth in increasing thickness of PMMA from 10 to 80 mm. The air kerma measured for PCT was 5.2 mGy, which leads to an organ dose to the brain of approximately 0.7 mGy. This dose is 96 % lower than a typical CT dose. PCT can be appealing for pediatric imaging since young children have an increased sensitivity to radiation induced cancers. We have acquired post mortem images of a newborn with the new device and with a state-of-the-art CT and compared the diagnostic information and dose levels of the two modalities. The results are promising but more work is needed to provide input to a next generation prototype that would be suitable for clinical trials.

The effect of slab size on mass detection performance of a screen-film CAD system in reconstructed tomosynthesis volumes

In the development of a computer-aided detection (CAD) system a large database of training samples is of major importance However digital breast tomosynthesis (DBT) is a relatively new modality and no large database of cases is available yet To overcome this limitation we are developing a CAD system for mass detection in DBT that can be trained with regular 2D mammograms, for which large datasets are available We trained our system with a very large database of screen-film mammograms (SFM) Our approach does not use projection images, but only reconstructed volumes, because it is expected that manufacturers of tomosynthesis systems will only store the reconstructed volumes In this study we developed a method that converts reconstructed volumes into a series of SFM-like slices and combinations of slices, called slabs By combining slices into slabs, more information of a whole mass, which usually spans several slices, is used and its appearance becomes more similar to a 2D mammogram In this study we investigate the effect of using slabs of different sizes on the performance of our CAD system For validation we use a dataset of 63 tomosynthesis cases (245 volumes) consisting of 42 normal cases (163 volumes) and 21 abnormal cases (82 volumes) with a total of 47 malignant masses and architectural distortions The volumes are acquired with a tomosynthesis system from Sectra and are reconstructed into 0.3 cm thick slices Results show that performance of our CAD system increases significantly when slices are combined into larger slabs Best performance is obtained when a slab thickness of 1.5 cm (5 slices) is used, which is significantly higher than using slabs of a single slice, two slices and all slices.