System and method for statistical iterative reconstruction and material decomposition

Abstract

A method for imaging an object to be reconstructed includes acquiring projection data corresponding to the object. Furthermore, the method includes generating a measured sinogram based on the acquired projection data and formulating a forward model, where the forward model is representative of a characteristic of the imaging system. In addition, the method includes generating an estimated sinogram based on an estimated image of the object and the forward model and formulating a statistical model based on at least one of pile-up characteristics and dead time characteristics of a detector of the imaging system. Moreover, the method includes determining an update corresponding to the estimated image based on the statistical model, the measured sinogram, and the estimated sinogram and updating the estimated image based on the determined update to generate an updated image of the object. Additionally, the method includes outputting a final image of the object.

Description

BACKGROUND[0001]Embodiments of the present specification generally relate to photon-counting computed tomography imaging systems, and more specifically to a system and method for enhancing image quality in photon-counting computed tomography systems via use of a model that incorporates unique statistical properties of X-ray pile-up.[0002]Radiographic imaging systems, such as X-ray and computed tomography (CT) systems have been employed for observing interior aspects of objects. Typically, the imaging systems include an X-ray source that is configured to emit X-rays toward an object of interest, such as a patient, a work piece, a parcel, a piece of luggage, and so forth. A detecting device, such as an array of radiation detectors, is positioned on the opposite side of the object and is configured to detect the X-rays transmitted through the object.[0003]As will be appreciated, a CT scan is conducted by measuring a series of projection measurements from many different angles around a ...

AI Technical Summary

Problems solved by technology

A drawback of such detectors however is their inability to provide data or feedback regarding the number and / or energy of detected photons.

However, a drawback of these direct-conversion semiconductor detectors is their inability to count at the X-ray photon flux rates typically encountered with conventional CT systems.

Disadvantageously, the photon-counting CT systems are unable to efficiently count X-rays that arrive too close together in time.

This is typically a problem for measurements at high X-ray flux, and / or in regions of a sinogram including little attenuation through the patient and / or the X-ray source pre-patient filter (bowtie).

However, in addition to impacting the number of counts recorded, pile-up also impacts the noise in CT projection data.

Further, the very high X-ray photon flux rate has been known to cause pile-up and polarization in certain direct-conversion devices that ultimately leads to detector saturation.

If this happens sufficiently often, a significant distortion of the detected spectrum may result as piled-up events are shifted in the spectrum to higher energies.

In addition, pile-up leads to a more or less pronounced depression of efficiency in a projection area including lower attenuation, resulting in flux detection loss.

Above these levels, the detector response is less predictable and has degraded dose utilization.

That is, once a pixel is saturated (corresponding to higher values in the measured photon counts), additional radiation will not produce useful information in the measurements.

Further, as will be appreciated, detector saturation leads to corruption of imaging information and consequently results in noise and artifacts in X-ray projection data and reconstructed CT images.

Photon-counting direct-conversion detectors are known to suffer from decreased detector quantum efficiency (DQE) at high count rates mainly due to detector pile-up.

In particular, photon-counting direct-conversion detectors incur pile-up due to the intrinsic charge collection time (i.e., dead time) associated with each X-ray photon event.

Reduced DQE results in reduced image quality, i.e., a noisier image.

In addition, hysteresis and other non-linear effects occur at flux levels near and above detector saturation and lead to additional image artifacts.

However, if the count rate is increased to a point where the relationship between the true signal and the measured signal becomes non-monotonic, which is a characteristic of paralyzable electronics, correction of this non-monotonic relationship may no longer be practical.

However, the signal-to-noise ratio of the resulting signal may be reduced, and the level of crosstalk, both in terms of photons and deposited charge between neighboring detector pixels may be disadvantageously significant due to the increased perimeter between sub-pixels.

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