Tomography#

class deepinv.physics.Tomography(angles, img_width, circle=False, parallel_computation=True, adjoint_via_backprop=False, fbp_interpolate_boundary=False, normalize=False, fan_beam=False, fan_parameters=None, device=torch.device('cpu'), dtype=torch.float, **kwargs)[source]#

Bases: LinearPhysics

(Computed) Tomography operator.

The Radon transform is the integral transform which takes a square image \(x\) defined on the plane to a function \(y=\forw{x}\) defined on the (two-dimensional) space of lines in the plane, whose value at a particular line is equal to the line integral of the function over that line.

Note

The pseudo-inverse is computed using the filtered back-projection algorithm with a Ramp filter. This is not the exact linear pseudo-inverse of the Radon transform, but it is a good approximation which is robust to noise.

Note

The measurements are not normalized by the image size, thus the norm of the operator depends on the image size.

Warning

The adjoint operator has small numerical errors due to interpolation. Set adjoint_via_backprop=True if you want to use the exact adjoint (computed via autograd).

Parameters:

angles (int, torch.Tensor) – These are the tomography angles. If the type is int, the angles are sampled uniformly between 0 and 360 degrees. If the type is torch.Tensor, the angles are the ones provided (e.g., torch.linspace(0, 180, steps=10)).
img_width (int) – width/height of the square image input.
circle (bool) – If True both forward and backward projection will be restricted to pixels inside a circle inscribed in the square image.
parallel_computation (bool) – if True, all projections are performed in parallel. Requires more memory but is faster on GPUs.
adjoint_via_backprop (bool) – if True, the adjoint will be computed via deepinv.physics.adjoint_function(). Otherwise the inverse Radon transform is used. The inverse Radon transform is computationally cheaper (particularly in memory), but has a small adjoint mismatch. The backprop adjoint is the exact adjoint, but might break random seeds since it backpropagates through torch.nn.functional.grid_sample(), see the note here.
fbp_interpolate_boundary (bool) – the filtered back-projection usually contains streaking artifacts on the boundary due to padding. For fbp_interpolate_boundary=True these artifacts are corrected by cutting off the outer two pixels of the FBP and recovering them by interpolating the remaining image. This option only makes sense if circle is set to False. Hence it will be ignored if circle is True.
normalize (bool) – If True, the outputs are normlized by the image size (i.e. it is assumed that the image lives on [0,1]^2 for the computation of the line integrals). In this case the operator norm is approximately given by \(\|A\|_2^2 \approx \frac{\pi}{2\,\text{angles}}\), If False, then it is assumed that the image lives on [0,im_width]^2 for the computation of the line integrals
fan_beam (bool) – If True, use fan beam geometry, if False use parallel beam
fan_parameters (dict) –
Only used if fan_beam is True. Contains the parameters defining the scanning geometry. The dict should contain the keys:
- ”pixel_spacing” defining the distance between two pixels in the image, default: 0.5 / (in_size)
- ”source_radius” distance between the x-ray source and the rotation axis (middle of the image), default: 57.5
- ”detector_radius” distance between the x-ray detector and the rotation axis (middle of the image), default: 57.5
- ”n_detector_pixels” number of pixels of the detector, default: 258
- ”detector_spacing” distance between two pixels on the detector, default: 0.077
The default values are adapted from the geometry in https://doi.org/10.5281/zenodo.8307932, where pixel spacing, source and detector radius and detector spacing are given in cm. Note that a to small value of n_detector_pixels*detector_spacing can lead to severe circular artifacts in any reconstruction.
device (str) – gpu or cpu.

Examples:

Tomography operator with defined angles for 3x3 image:

>>> from deepinv.physics import Tomography
>>> seed = torch.manual_seed(0)  # Random seed for reproducibility
>>> x = torch.randn(1, 1, 4, 4)  # Define random 4x4 image
>>> angles = torch.linspace(0, 45, steps=3)
>>> physics = Tomography(angles=angles, img_width=4, circle=True)
>>> physics(x)
tensor([[[[ 0.0000, -0.1791, -0.1719],
          [-0.5713, -0.4521, -0.5177],
          [ 0.0340,  0.1448,  0.2334],
          [ 0.0000, -0.0448, -0.0430]]]])

Tomography operator with 3 uniformly sampled angles in [0, 360] for 3x3 image:

>>> from deepinv.physics import Tomography
>>> seed = torch.manual_seed(0)  # Random seed for reproducibility
>>> x = torch.randn(1, 1, 4, 4)  # Define random 4x4 image
>>> physics = Tomography(angles=3, img_width=4, circle=True)
>>> physics(x)
tensor([[[[ 0.0000, -0.1806,  0.0500],
          [-0.5713, -0.6076, -0.6815],
          [ 0.0340,  0.3175,  0.0167],
          [ 0.0000, -0.0452,  0.0989]]]])

A_adjoint(y, **kwargs)[source]#

Computes adjoint of the tomography operator.

Warning

The default adjoint operator has small numerical errors due to interpolation. Set adjoint_via_backprop=True if you want to use the exact adjoint (computed via autograd).

Parameters:: y (torch.Tensor) – measurements
Return torch.Tensor:: noisy measurements

A_dagger(y, **kwargs)[source]#

Computes the filtered back-projection (FBP) of the measurements.

Warning

The filtered back-projection algorithm is not the exact linear pseudo-inverse of the Radon transform, but it is a good approximation that is robust to noise.

Tip

By default, the FBP reconstruction can display artifacts at the borders. Set fbp_interpolate_boundary=True to remove them with padding.

Parameters:: y (torch.Tensor) – measurements
Return torch.Tensor:: noisy measurements

Examples using `Tomography`:#

A tour of forward sensing operators

Patch priors for limited-angle computed tomography

Vanilla PnP for computed tomography (CT).

Self-supervised learning with measurement splitting

Learned Primal-Dual algorithm for CT scan.

Tomography#

Examples using Tomography:#

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Examples using `Tomography`:#