Digital breast tomosynthesis is a new technique intended to overcome the limitations of conventional projection mammography by reconstructing slices through the breast from projection views acquired from different angles with respect to the breast. We formulate a general theory of filtered backprojection reconstruction for linear tomosynthesis. The filtering step consists of an MTF inversion filter, a spectral filter, and a slice thickness filter. In this paper the method is applied first to simulated data to understand the basic effects of the various filtering steps. We then demonstrate the impact of the filter functions with simulated projections and with clinical data acquired with a research breast tomosynthesis system.** With this reconstruction method the image quality can be controlled regarding noise and spatial resolution. In a wide range of spatial frequencies the slice thickness can be kept constant and artifacts caused by the incompleteness of the data can be suppressed.

Optimizing filtered backprojection reconstruction for a breast tomosynthesis prototype device

Diagnostic and interventional flat detector X-ray systems are penetrating the market in all application segments. First introduced in radiography and mammography, they have conquered cardiac and general angiography and are getting increasing attention in fluoroscopy. Two flat detector technologies prevail. The dominating method is based on an indirect X-ray conversion process, using cesium iodide scintillators. It offers considerable advantages in radiography, angiography and fluoroscopy. The other method employs a direct converter such as selenium which is particularly suitable for mammography. Both flat detector technologies are based on amorphous silicon active pixel matrices. Flat detectors facilitate the clinical workflow in radiographic rooms, foster improved image quality and provide the potential to reduce dose. This added value is based on their large dynamic range, their high sensitivity to X-rays and the instant availability of the image. Advanced image processing is instrumental in these improvements and expand the range of conventional diagnostic methods. In angiography and fluoroscopy the transition from image intensifiers to flat detectors is facilitated by ample advantages they offer, such as distortion-free images, excellent coarse contrast, large dynamic range and high X-ray sensitivity. These characteristics and their compatibility with strong magnetic fields are the basis for improved diagnostic methods and innovative interventional applications.

Flat detectors and their clinical applications

A prototype breast tomosynthesis system has been developed, allowing a total angular view of ±25°. The detector used in this system is an amorphous selenium direct-conversion digital flat-panel detector suitable for digital tomosynthesis. The system is equipped with various readout sequences to allow the investigation of different tomosynthetic data acquisition modes. In this paper, we will present basic physical properties -- such as MTF, NPS, and DQE -- measured for the full resolution mode and a binned readout mode of the detector. From the measured projections, slices are reconstructed employing a special version of filtered backprojection algorithm. In a phantom study, we compare binned and full resolution acquisition modes with respect to image quality. Under the condition of same dose, we investigate the impact of the number of views on artifacts. Finally, we show tomosynthesis images reconstructed from first clinical data.

Digital breast tomosynthesis using an amorphous selenium flat panel detector

In breast tomosynthesis a rapid sequence of N images is acquired when the x-ray tube sweeps through different angular views with respect to the breast. Since the total dose to the breast is kept the same as that in regular mammography, the exposure used for each image of tomosynthesis is 1/N. The low dose and high frame rate pose a tremendous challenge to the imaging performance of digital mammography detectors. The purpose of the present work is to investigate the detector performance in different operational modes designed for tomosynthesis acquisition, e.g., binning or full resolution readout, the range of view angles, and the number of views N. A prototype breast tomosynthesis system with a nominal angular range of +/-25 degrees was used in our investigation. The system was equipped with an amorphous selenium (a-Se) full field digital mammography detector with pixel size of 85 microm. The detector can be read out in full resolution or 2 x 1 binning (binning in the tube travel direction). The focal spot blur due to continuous tube travel was measured for different acquisition geometries, and it was found that pixel binning, instead of focal spot blur, dominates the detector modulation transfer function (MTF). The noise power spectrum (NPS) and detective quantum efficiency (DQE) of the detector were measured with the exposure range of 0.4-6 mR, which is relevant to the low dose used in tomosynthesis. It was found that DQE at 0.4 mR is only 20% less than that at highest exposure for both detector readout modes. The detector temporal performance was categorized as lag and ghosting, both of which were measured as a function of x-ray exposure. The first frame lags were 8% and 4%, respectively, for binning and full resolution mode. Ghosting is negligible and independent of the frame rate. The results showed that the detector performance is x-ray quantum noise limited at the low exposures used in each view of tomosynthesis, and the temporal performance at high frame rate (up to 2 frames per second) is adequate for tomosynthesis.

Imaging performance of an amorphous selenium digital mammography detector in a breast tomosynthesis system.

Imaging detectors for medical X-ray and computed tomography (CT) applications have undergone many improvements and technology changes over time. But most (dynamic) detectors sold in this field still rely on indirect conversion, using scintillators and photodiodes to convert the X-ray quanta ultimately into electrical signals. Direct conversion detectors promise very high spatial resolution and high signal-to-noise ratios. Some direct conversion materials may allow for counting or even energy resolving detection of the X-ray quanta. Based on this, for example spectrally resolving CT systems are becoming an interesting option for the next decade. This contribution highlights the requirements of advanced medical X-ray and CT imaging and reviews examples of status and progress in the field. The emphasis is on the direct conversion sensors for pixelated detectors, but considerations on read-out concepts and on associated challenges such as interconnects will also be presented. Finally, the most burning issues, such as count rate limitations and polarization effects, will be discussed from an application point of view.

Status of Direct Conversion Detectors for Medical Imaging With X-Rays

In this paper, two new selenium large area detectors are introduced. The first detector is a 24x30cm detector suitable for full-field digital mammography applications. The second detector is a new 43x43cm detector for general radiographic applications. Both detectors are capable of static and dynamic imaging, and are quantum noise limited over the exposure ranges intended for their typical use. For static imaging applications, ghost and lag were compared on both detectors, and no measurable artefacts were reported. On dynamic imaging sequences, lag was shown to be significant on both detectors, and a method for reducing the artifact due to lag was presented.

Ghosting comparison for large-area selenium detectors suitable for mammography and general radiography

A selenium-based flat-panel direct converter detector suitable for digital mammography was developed. The detector is based on a TFT-array with a resolution of 2816 X 2048 pixels, and a pixel pitch of 85 micrometers . Although the geometric fill factor for each pixel is around 70% , the effective fill factor for the detector is closer to 88% due to internal electric field shaping within the selenium layer. A selenium multilayer p-i-n structure of 200 micrometers was deposited onto the array by selectively doping the regions near each contact to produce unipolar conducting blocking layers. This structure absorbs more than 95% of a typical mammography beam.

Se-based flat-panel detector for screening mammography

The advent of digital detectors will enable several advanced imaging applications to be used in the fight against breast cancer. For example, dynamic imaging applications such as tomosynthesis, contrast enhanced and dual energy mammography have demonstrated promising results. In this paper, we will assess the suitability of this detector for these advanced applications. MTF and DQE measurements were performed on a selenium FFDM detector to assess image quality. Ghosting properties of a digital detector are also an important factor, since it can strongly degrade image quality. In this paper, we will also report on the ghosting characteristics of the selenium detector, using typical exposures envisioned to be used in tomosynthesis exams. The physical mechanisms that create ghost images will be discussed and will be quantified.

Imaging performance of a clinical selenium flat-panel detector for advanced applications in full-field digital mammography

In a method for producing a stereotactic image of a patient fixed in a mammography device for a biopsy, a computerized control and evaluation unit waits for a start signal to be given by an operator, and after receipt of the start signal, automatically implements the following steps. An x-ray apparatus of the mammography device is positioned in a first predetermined imaging position, and a first image of the patient is acquired with the x-ray apparatus in this position. The x-ray apparatus is then pivoted into a second imaging position, different from the first, and a second image of the patient is acquired at the second position. The first and second images are acquired to form a stereotactic image of the patient.

Marc Hansroul

Papers

Ghosting comparison for large-area selenium detectors suitable for mammography and general radiography

Se-based flat-panel detector for screening mammography

Imaging performance of a clinical selenium flat-panel detector for advanced applications in full-field digital mammography

Method for producing a stereotactic image in a mammography device