An iterative three-dimensional electron density imaging algorithm using uncollimated compton scattered x rays from a polyenergetic primary pencil beam

Compton scatter imaging: A promising modality for image guidance in lung stereotactic body radiation therapy

PURPOSE

Lung stereotactic body radiation therapy (SBRT) requires delivering large radiation doses with millimeter accuracy, making image guidance essential. An approach to forming images of patient anatomy from Compton-scattered photons during lung SBRT is presented.

METHODS

To investigate the potential of scatter imaging, a pinhole collimator and flat-panel detector are used for spatial localization and detection of photons scattered during external beam therapy using lung SBRT treatment conditions (6 MV FFF beam). MCNP Monte Carlo software is used to develop a model to simulate scatter images. This model is validated by comparing experimental and simulated phantom images. Patient scatter images are then simulated from 4DCT data.

RESULTS

Experimental lung tumor phantom images have sufficient contrast-to-noise to visualize the tumor with as few as 10 MU (0.5 s temporal resolution). The relative signal intensity from objects of different composition as well as lung tumor contrast for simulated phantom images agree quantitatively with experimental images, thus validating the Monte Carlo model. Scatter images are shown to display high contrast between different materials (lung, water, bone). Simulated patient images show superior (~double) tumor contrast compared to MV transmission images.

CONCLUSIONS

Compton scatter imaging is a promising modality for directly imaging patient anatomy during treatment without additional radiation, and it has the potential to complement existing technologies and aid tumor tracking and lung SBRT image guidance.

Snapshot 2D tomography via coded aperture x-ray scatter imaging

This paper describes a fan beam coded aperture x-ray scatter imaging system that acquires a tomographic image from each snapshot. This technique exploits the cylindrical symmetry of the scattering cross section to avoid the scanning motion typically required by projection tomography. We use a coded aperture with a harmonic dependence to determine range and a shift code to determine cross range. Here we use a forward-scatter configuration to image 2D objects and use serial exposures to acquire tomographic video of motion within a plane. Our reconstruction algorithm also estimates the angular dependence of the scattered radiance, a step toward materials imaging and identification.

Evaluation of the Feasibility and Quantitative Accuracy of a Generalized Scatter 2D PET Reconstruction Method

Scatter degrades the contrast and quantitative accuracy of positron emission tomography (PET) images, and most methods for estimating and correcting scattered coincidences in PET subtract scattered events from the measured data. Compton scattering kinematics can be used to map out the locus of possible scattering locations. These curved lines (2D) or surfaces (3D), which connect the coincidence detectors, encompass the surface (2D) or volume (3D) where the decay occurs. In the limiting case where the scattering angle approaches zero, the scattered coincidence approaches the true coincidence. Therefore, both true and scattered coincidences can be considered similarly in a generalized scatter maximum-likelihood expectation-maximization reconstruction algorithm. The proposed method was tested using list-mode data obtained from a GATE simulation of a Jaszczak-type phantom. For scatter fractions from 10% to 60%, this approach reduces noise and improves the contrast recovery coefficients by 0.5–3.0% compared with reconstructions using true coincidences and by 3.0–24.5% with conventional reconstruction methods. The results demonstrate that this algorithm is capable of producing images entirely from scattered photons, eliminates the need for scatter corrections, increases image contrast, and reduces noise. This could be used to improve diagnostic quality and/or to reduce patient dose and radiopharmaceutical cost.

An energy-dispersive technique to measure x-ray coherent scattering form factors of amorphous materials

The material-dependent x-ray scattering properties of amorphous substances such as tissues and phantom materials used in imaging are determined by their scattering form factors, measured as a function of the momentum transfer argument, x. Incoherent scattering form factors, F(inc), are calculable for all values of x while coherent scattering form factors, F(coh), cannot be calculated except at large x because of their dependence on long-range order. As a result, measuring F(coh) is very important to the developing field of x-ray scatter imaging. Previous measurements of F(coh), based on crystallographic techniques, have shown significant variability, as these techniques are not optimal for amorphous materials. We have developed an energy-dispersive technique that uses a polychromatic x-ray beam and an energy-sensitive detector. We show that F(coh) can be measured directly, with no scaling parameters, by computing the ratio of two spectra: the first, measured at a given scattering angle and the second, the direct transmission spectrum with no scattering. Experiments have been constructed on this principle and used to measure F(coh) for water and polyethylene to explore the reliability of the technique. A 121 kVp x-ray spectrum and seven different scattering angles between 1.67 and 15.09 degrees were used, resulting in a measurable range of x between 0.5 and 9.5 nm(-1). These are the first measurements of F(coh) made without the need for a scaling factor. Resolution in x varies between 10% for small scattering angles and 2% for large scattering angles. Accuracy in F(coh) is shown to be strongly dependent on the precision of the experimental geometry and varies between 5% and 15%. Comparison with previous published measurements for water shows values of the average absolute relative difference between 8% and 14%.