Discrete Algebraic Reconstruction Technique (DART)

DART is an iterative reconstruction algorithm for discrete tomography. The original publication in [1] was used as reference to create this library. What this repository consists of, is an implementation of the DART algorithm together with a framework to generate phantoms and measurements, to test the algorithm itself.
The DART algorithm alternates between continuous and discrete reconstruction steps. For the continuous step, many reconstruction algorithms were implemented with astra-toolbox. Publications relevant to this library can be found in [2], [3] and [4]. For the original publication of SART, which is the main reconstruction algorithm presented in the original DART publication, you can refer to [5].

Prerequisites

An environment with Python 3 >= 3.6 is required to use this library with the astra-toolbox installed. Astra-toolbox can be installed with anaconda with thwe following command:

conda install -c astra-toolbox astra-toolbox

Or you can follow the instructions available here.

To install the DART pip package run the following command:

pip install DART-OhGreat

Usage

To run DART, data (in the form of phantoms) and measurements (in the form of projections) need to be artificially constructed. Therefore, three main wrappers have been created around the code to solve the following tasks:

• phantom creation
• projection and measurements acquisition
• running DART reconstruction algorithm

Usage of the package for each of this tasks is described in detail in the following sections.

If instead of using the pip package you chose to clone the repository, you need to add src. before every import as such:

from src.phantoms.creator import create_phantoms

Generating phantoms

Semilunar phantoms

To generate semilunar like phantoms you can use the create_phantoms function. It can be imported from phantoms.creator.py and used as below:

from phantoms.creator import create_phantoms
phantom_list = create_phantoms(phantoms="semilunars",img_size=512, gray_values=[255, 200, 150], n=3, overlap=False, seed=None, img_name="dir/to/save/filename")

Parameters:

• phantoms: (string), name of the phantom family. Shoud be one between "semilunars", "aliens", "clouds", "paws".
• img_size: should be an integer of value 256 or 512 and represents the size of the phantoms to generate.
• gray_values: list of three integers between 0 and 255 representing the intensities to use for the various phantom layers.
• n: integer representing the number of phantoms to generate.
• overlap: defines wether the shape layers should overlap between them.
• seed: integer to be passed to have reproducible results.
• img_name: string defining the path and filename to use. The filename should not have the extension, it will be created as a png by default.

Output:

• python list of phantoms. (Phantoms as numpy arrays of type np.uint8)

Generating projections

From 2D phantoms

To generate measurements in the form of 1D projections, the function project_from_2D has been created. You can import it and use it with the following commands:

from projections.project import project_from_2D
proj_id, sino_id, sinogram = project_from_2D(phantom_id, vol_geom, n_projections, detector_spacing, apply_noise=False, save_dir=None, use_gpu=False)

Parameters:

• phantom_id: Phantom as astra-toolbox object.
• vol_geom: geometry of the output image. Used to define the number of detectors as the first dimension of the vol_geom.
• n_projections: is an integer value representing the number of projections as the number of angles to make measurements from.
• detector_spacing: defines the size of the pixel.
• angles: angles to use for the measurements. (np.array, should be created as a np.linspace of values)
• noise_factor: factor that adds Poisson distributed noise to the image, when defined.
• save_dir: string representing the directory to save png images that represent the measurements. Images won't be saved if this parameter is not set.
• use_gpu: creates a projector that can use GPU

Output:

• The function will return proj_id, sino_id and sinogram. The first is a reference to the astra toolbox projector object, the second is a reference to the astra toolkit sinogram object and the former is the sinograms' actual measurements.

From 3D phantoms

to be added

DART and substeps

All the steps required to run the DART algorithm have been broken down and can be used separately. A detailed desctiption for the usage of all the functions available in the library will follow in this section.

DART algorithm

DART can be used in the following way:

from algorithms.DART import *
# create DART instance
dart = DART(gray_levels=[0, 40, 150], p=0.85, rec_shape=img.shape,
            proj_geom=proj_geom, projector_id=projector_id,
            sinogram=sinogram)
# run DART algorithm
rec = dart.run(iters=10, rec_alg="SART_CUDA", rec_iter= 1000)

Instance parameters:

• gray_levels: gray levels known a priori used in the segmentation step. (list)
• p: probability of a pixel to not be sampled as a free pixel. (float)
• rec_shape: shape of the volume to create as output. (tuple)
• proj_geom: astra-toolbox projection geometry used for the measurements.
• projector_id: reference to the astra toolbox projector used to make the projections. (Can be created with the project_from_2D fucntion described above)
• sinogram: sinorgam of measurements. (np.array) (Can be created with the project_from_2D fucntion described above)

Run parameters:

• iters: number of DART iteration to perform. (int)
• p: as above, can be used to run multiple experiments without reistanciating DART.
• gray_levels:same as above.
• rec_alg: algebraic reconstruction algorithm to use: can be 'SART', 'SIRT' or 'FBP'. To run the GPU implementations just add '_CUDA' to the algorithn name (e.g. 'SART_CUDA').
• rec_iter: number of reconstruction subrutine iterations to run.

Output:

• (np.array), returns the reconstructed image.

Segmentation

The method segment can be used to segment an image at the defined gray values, once DART has been instanced as defined above.

new_gray_vals = [0, 130, 240]
dart.gray_values = new_gray_vals
# function to update gray values thresholds
dart.update_gray_thresholds()
segmented_img = dart.segment(img)

Parameters:

• img: is the grayscale input phantom to segment as a 2D numpy matrix.
• gray_levels : array of gray levels to compute the thresholds for the segmentation from.

Output:

• returns the segmented image. (np.array)

Pixel neighborhood

To calculate the indexes of neighbours of a specific pixel, you can use the method pixel_neighborhood as below:

neighbours = dart.pixel_neighborhood(img_shape, x, y)

Parameters:

• img_shape : is the shape of the reconstructed image.
• x,y: are the coordinates of the pixel to calculate the neighbours for.

Output:

• The method returns a list with all the indexes of the neighbours.

Boundary pixels

To calculate the boundary pixels of the phantom image, the method boundary_pixels takes as input the reconstructed image and calculates the boundary pixels with the help of the pixel_neighborhood method described above. You can use it as follows, after having created a DART instance:

b_pixels = dart.boundary_pixels(img):

Parameters:

• img: (2D np.array), image to calculate boundary pixels for.

Output:

• (np.array), mask of boundary pixels.

Free pixels

To calculate the free pixels, the following method is available:

free_pixels = dart.free_pixels()

The method takes into consideration the p value defined in the DART instance to calculate the free pixels.

Output:

• The output free_pixels is a binary 2D np.array, where the True values represent the free pixels.

Algebraic Reconstruction Algorithms

For the continous reconstruction step of DART, various algorithms have been implemented with the ASTRA-toolbox. Specifically, SART, SIRT and FBP are available for experimentation. You can call the functions detatched from DART as below:

The following example demostrates how to use SART:

from algorithms.SART import *
sart_res_id, sart_res = SART(vol_geom, vol_data, projector_id, sino_id, iters, use_gpu=True)

SIRT and FBP can be used similarly to the SART example above.

Parameters:

• vol_geom: represents the volume geometry for the output.
• vol_data: starting values to use for the reconstructed image.
• projector_id: specifies the projector to use for the measurements.
• sino_id: is the sinogram id of the projections.
• iters: number of dart iterations to run.
• use_gpu: set to True to run Astra on GPU. You also need to use a gpu capable projector.

Output:

• The algorithm will return sart_res_id which is the astra-toolbox reference to the reconstructed phantom, and sart_res, a numpy array with the actual values of the reconstructed phantom.

Examples and Results

Examples on how to use the repository are available in the notebook examples under the notebook_examples directory. To run experiments on various algorithms and measurement configurations you can check the examples in the experiment_scripts directory.

Issues

If you encounter any problems while using the framework you can notify us by opening an issue here: https://github.com/OhGreat/DART_python/issues

References