- Overview
- Where to find further information
- General notes of usage
- Framework basic functionality
- Appendix
The SIRF (Synergistic Image Reconstruction Framework) software is an Open Source toolkit for the reconstruction of PET and MRI raw data. The aim is to provide code simple enough to easily perform a reconstruction, yet powerful enough to be able to handle real, full-size datasets. Our strategy in achieving this aim is to employ available Open Source reconstruction software written in advanced programming languages such as C++ and provide basic-user-friendly interfaces to it written in script languages, primarily Matlab and Python. The interface style permits a reconstruction to be performed in stages, allowing the user to inspect or modify data, or insert their own code.
This User’s Guide describes version 2.0 of SIRF. The software can be found on https://github.com/CCPPETMR.
The code builds upon existing Open Source software packages for medical image reconstruction. At the outset, these packages are STIR for PET reconstruction, Gadgetron for MRI and NiftyReg for registration/resampling. SIRF provides MATLAB and Python interfaces to these underlying reconstruction engines. This is done by wrapping the engines in a C++ layer, and then placing a C-interface between the wrapped C++ engines and the MATLAB and Python interfaces.
At present, you should only use the C++, MATLAB and Python interfaces. The underlying C library is internal and likely to change over the next few releases.
SIRF expects raw MR data in the ISMRMRD format. We currently provide a clone of the siemens_to_ismrmrd Git repository. This enables raw data from Siemens mMR Biograph PET-MR scanners to be converted to ISMRMRD format. For more details of how to export the raw MR data from Siemens PET-MR scanners and how to convert the data to ISMRMRD please see the wiki: https://github.com/CCPPETMR/SIRF/wiki/MR-raw-data.
Converters for data from other scanners are available from https://github.com/ismrmrd but we have not tried these yet.
STIR can handle data from the Siemens mMR Biograph with progress being made for the GE Signa PET/MR. However, STIR currently still relies on some bash scripts for file format conversion, estimation of randoms and scatter etc. Therefore, in the current SIRF release, we do not yet support measured data from any scanner. This will be fixed for version 1.0.
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CCPPETMR website http://www.ccppetmr.ac.uk for links to project overview, meeting notes, design documents etc
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CCPPETMR SIRF Wiki https://github.com/CCPPETMR/SIRF/wiki for detailed instructions and user-contributed content
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CCPPETMR Virtual Machine Wiki https://github.com/CCPPETMR/CCPPETMR_VM/wiki with information on how to use the Virtual Machine that we supply with pre-installed software.
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Inline documentation within MATLAB and Python functions, see below for examples.
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Demo functions to demonstrate SIRF features. After installing SIRF, these will be available in SIRF/examples.
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Our plan for future releases and additional features is available from the Software Documents tab http://www.ccppetmr.ac.uk/softwareframework.html on our website.
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Installation instructions can be found on our Wiki at https://github.com/CCPPETMR/SIRF/wiki/Installation-instructions. Note that on the Virtual machine, this has all has been done for you and you can just use SIRF.
Please note that with the installation set-up, you will normally have two copies of the Matlab/Python module files: the original ones in the SIRF clone and the installed ones. This only matters if you want to debug or modify the files. The installation instructions point Python and Matlab to the “installed” files.
The MR module and the demos create temporary files during operation. They are normally created in the same folder as the input data, but are cleaned up afterwards. Therefore, the data cannot reside in a read-only folder.
SIRF library modules are interfaces to object-oriented C++, which makes it reasonable for them to follow the object-oriented programming paradigm as well. This means that instead of having data containers (arrays, files etc.) and functions that operate on them, we employ objects, which contain data and come with sets of functions, called their methods, that operate on data. Each object contains a special method called constructor, which has the same name as the object class name and must be called to create that object. For example, to create an object of class ImageData that handles MR image data and fill it with data stored in the HDF5 file 'my_image.h5' one needs to do assignment
image = ImageData('my_image.h5');
We note that an MR ImageData object contains not only the voxel values, but also a number of parameters specified by the ISMRMRD format of MR image data. The object data is encapsulated, i.e. is not directly accessible from the user's code (being handled mostly by the underpinning C++ code) and is processed by the object methods. For example, to display the data encapsulated by image, one needs to call its method show():
image.show();
and to copy the data into a Matlab array one uses method as_array():
image_data_array = image.as_array();
Parameters of objects are modified/accessed via set/get methods (mutators and accessors). For example, the value of an objective function handled by object named obj_fun on an image data object image is computed by its method get_value() as
obj_fun_value = obj_fun.get_value(image);
The mutators are also responsible for basic error checking.
Some classes are derived from other classes, which means that they have all the methods of the classes they are derived from: we say that these methods are inherited. For example, class AcquisitionModelUsingRayTracingMatrix is derived from AcquisitionModelUsingMatrix, which in turn is derived from AcquisitionModel, and so it inherits all the methods of the latter two.
Error handling is via exceptions, i.e. functions do not return an error status, but throw an error if something did not work. The user can catch these exceptions if required as illustrated in the demos.
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Types/classes start with capitals, every word is capitalised, no underscores, e.g.
AcquisitionModel. -
Class methods are lower case, underscores between different words, e.g.
get_voxel_size(). -
Methods indicating
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a number of things start with
num, e.g.num_gates. -
the number of an item in a sequence end with
num, e.g.gate_num.
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Distances are expressed in mm.
For arrays in the target language, we use “native” ordering of indices in Python and Matlab. These are unfortunately opposite, so we would write
image_array[z,y,x] # Python
image_array(x,y,z) % Matlab
For images, the meaning of x, y and z is currently acquisition dependent. Geometric information will be added later.
In both Matlab and Python, SIRF operates with handles to objects, which affects the meaning of the assignment x = y: instead of creating a separate copy of y stored in x, x simply points to the same underlying data. As the result, any changes in x simultaneously change y.
In order to have a true (i.e. independent) copy of a SIRF object, the user must call the object methods that create copies of them (see below).
At present, the SIRF library provides two Python interface modules sirf.STIR and sirf.Gadgetron for STIR and Gadgetron respectively, and two respective Matlab modules sirf.STIR and sirf.Gadgetron.
We remind that to see the contents of a Python module, the user needs to import it and use Python's help, and in Matlab one needs to use doc. For example,
# Python
import sirf.STIR
help(sirf.STIR)
will show the components of the module sirf.STIR, and similarly
% Matlab
doc sirf.Gadgetron
will show the components of sirf.Gadgetron. In the same way,
# Python
help(sirf.Gadgetron.ImageData)
will provide information on the class ImageData defined in the module sirf.Gadgetron, and
% Matlab
doc sirf.STIR.AcquisitionData
on the sirf.STIR.AcquisitionData class. Regrettably, help and doc show all methods, including some common built-in methods such as __weakref__ method in Python or addlistener method in Matlab. Methods that are not related to SIRF is relatively easy to identify in Python (built-in methods have underscores in names). In Matlab they are difficult to identify, which is why we mark relevant Matlab methods other than constructors with ***SIRF***. Methods not marked this way should be ignored.
In order to understand the functionality of a derived class (see Object-oriented paradigm), you are advised to first get help on the classes it is derived from. In Python, you can see that a class is derived by the presence of "Method resolution order" section in Python help output, which lists all classes it is derived from. You are advised to get help on all these classes except Python's class builtins.object. In Matlab, look at "Superclasses" item in "Class Details", and get help on the classes listed there except Matlab's class handle.
Most classes have a constructor to create an object from a file
image_data = ImageData(filename)
and a method to create a copy of the object
a_copy = image_data.clone()
“Processing” classes normally use the following pattern
recon.set_input(acquisition_data);
recon.setup(image_data);
recon.process();
output_image_data=recon.get_output();
Classes follow a simple hierarchy, where top-level describes the generic functionality, and derived classes add/specify functionality. To see an example, look up Reconstructor and IterativeReconstructor classes in sirf.STIR or sirf.STIR using help or doc. We note that help(sirf.STIR.IterativeReconstructor) and doc sirf.STIR.IterativeReconstructor will show all the functionality of this class, i.e. including that of Reconstructor (and also some built-in functionality common to Python/Matlab classes).
In the rest of the document we give basic information on the SIRF classes, including brief descriptions of the methods that are of interest to the user. Please use the inline help facility discussed above for more information. Descriptions are given for Python modules, which usually contain more functionality.
Reconstructed data are represented by ImageData objects. Currently they represent 3D volumes discretised using voxels.
Measured data (either raw or after some pre-processing) are represented by AcquisitionData objects. These contain everything what is needed to be able to reconstruct the data (including scanner information and geometry).
Class for acquisition data.
AcquisitionData
(PET/MR) Constructor. If no arguments are present, creates an
empty object, otherwise:
PET: Specifies the file containing raw data or
creates new AcquisitionData based on scanner information
that comes either from a template AcquisitionData object
or discerned from the scanner name and parameters given
in the arguments;
MR: Specifies the file containing raw data.
set_storage_scheme
(PET/MR) Specifies whether the intermediate data should be kept in
files or in RAM (see Acquisition data storage scheme
management in Appendix).
get_storage_scheme
(PET/MR) Returns currently used storage scheme.
create_uniform_image
(PET) Returns new compatible ImageData object.
as_array (PET/MR) Returns the object data as an array.
fill (PET/MR) Replaces the object data with user-supplied data.
clone (PET/MR) Returns a copy of this object.
write (PET/MR) Writes the object data to a file.
sort (MR) Sorts the acquisition data.
is_sorted (MR) Returns true if and only if the acquisition data is sorted.
get_info (MR) Returns information on the acquisition data.
process (MR) Processes the acquisition data by a chain of gadgets.
dimensions (MR) Returns the acquisition data dimensions
show (PET/MR) Displays the acquisition data as a set of 2D sinograms (PET)
or xy-slices (MR)
Class for data representing 3D objects.
ImageData (PET/MR) Constructor. Reads data from a file or creates empty object.
initialise (PET) Sets the image size in voxels, voxel sizes and the origin.
fill (PET/MR) Replaces the object data with user-supplied data.
as_array (PET/MR) Returns the object data as an array.
clone (PET/MR) Returns a copy of this object.
read_from_file
(PET/MR) Reads the image data from file.
get_uniform_copy
(PET) Returns a copy of this image filled with a constant value.
add_shape (PET) Adds a shape to the image.
show (PET/MR) Displays the image as a set of 2D xy-slices.
write (PET/MR) Writes the object data to a file.
dimensions (PET) Returns the object data dimensions
voxel_sizes (PET) Returns the voxel sizes
Class for storing coil sensitivity maps.
CoilSensitivityData Constructor. Creates empty object.
calculate Calculates coil sensitivities from the acquisition data
Specified by the argument.
csm_as_array Returns the coil sensitivity map for the slice/repetition
specified by the argument as an array.
PET_image = ImageData('image.hv'); % read image data from a file
PET_image0 = ImageData(); % create empty image object
PET_image0.initialise([128,128,31], [3,3,3.375]); % in Python: (128,128,31) etc.
PET_image0.fill(1.0); % assign value 1.0 at each voxel
PET_image_array = PET_image.as_array(); % copy image data to a Matlab array
MR_image_array = MR_image.as_array(); % copy image data to a Matlab array
MR_acquisition_data = AcquisitionData('mr_raw_data.h5');
cs_data = CoilSensitivityData(); % create empty object
cs_data.calculate(MR_acquisition_data); % calculate coil sensitivities
csm0 = cs_data.csm_as_array(0); % obtain coil sensitivities for slice 0 as array
Class for objects that process ImageData objects.
ImageDataProcessor
(PET/MR) Constructor. Creates new ImageDataProcessor object
(PET: empty, MR: defined by the argument).
set_input (PET/MR) Sets the processor input.
process (PET/MR) Processes image data on input.
get_output (PET/MR) Retrieves the processed image data.
apply (PET) Processes the ImageData argument.
Class for the image processor that zeroes the image outside a cylinder. Inherits the methods of ImageDataProcessor.
set_strictly_less_than_radius Defines the behaviour on the cylinder boundary.
get_strictly_less_than_radius Exposes the behaviour on the cylinder boundary.
Class for objects that process AcquisitionData objects.
AcquisitionDataProcessor
(MR) Constructor. Creates new processor object (a chain of gadgets,
see section Programming chains of Gadgetron gadgets) defined by
the argument.
set_input (MR) Sets the processor input.
process (MR) Processes the image data on input.
get_output (MR) Retrieves the processed image data.
filter = TruncateToCylinderProcessor();
filter.apply(PET_image);
img_proc = ImageDataProcessor({'ExtractGadget'}); % Python: ['ExtractGadget']
img_proc.set_input(MR_image);
img_proc.process();
MR_image_magnitude = img_proc.get_output();
acq_proc.set_input(MR_acquired_data);
acq_proc.process();
preprocessed_data = acq_proc.get_output();
Class for a generic image reconstructor.
set_input (PET/MR) Sets the input (AquisitionData object).
process (PET/MR) Runs the reconstruction.
get_output (PET/MR) Returns the output (ImageData object).
set_output_filename_prefix
(PET/MR) Specifies the naming for the output files.
Class for PET reconstruction algorithms that use Ordered Subsets technique whereby the acquisition data is split into subsets, and the objective function and its gradient are represented as the sums of components corresponding to subsets. Typically, one iteration of such algorithm would deal with one subset, and is therefore referred to as sub-iteration. Inherits the methods of Reconstructor.
set_num_subsets Sets the number of subsets,
get_num_subsets Returns the number of subsets.
set_num_subiterations Sets the number of subiterations.
get_num_subiterations Returns the number of subiterations.
get_subiterations_num Returns the current subiteration number.
set_save_interval Specifies how often to save image estimates.
set_objective_function Specifies the objective function.
set_up Prepares the reconstructor for use.
set_current_estimate Sets the current image estimate.
get_current_estimate Returns the current image estimate.
update_current_estimate Updates the current image estimate.
set_current_subset_num Specifies the current subset number.
get_subset_sensitivity Returns sensitivity image for the current subset.
reconstruct Reconstructs using the argument as initial image.
process Reconstructs using current image estimate as initial.
update Updates using the argument as current image estimate.
Class for reconstructor objects using Ordered Subsets Maximum A Posteriori One Step Late reconstruction algorithm, see http://stir.sourceforge.net/documentation/doxy/html/classstir_1_1OSMAPOSLReconstruction.html. Inherits the methods of IterativeReconstructor.
OSMAPOSLReconstructor Constructor. Creates new OSMAPOSL reconstructor object.
Class for reconstructor objects using Ordered Subsets Separable Paraboloidal Surrogate reconstruction algorithm, see http://stir.sourceforge.net/documentation/doxy/html/classstir_1_1OSSPSReconstruction.html. Inherits the methods of IterativeReconstructor.
OSSPSReconstructor Constructor. Creates new OSSPS reconstructor object.
Class for a reconstructor from fully sampled Cartesian raw data. Inherits the methods of Reconstructor.
FullySampledReconstructor Constructor. Creates new reconstructor object.
Class for a reconstructor from undersampled Cartesian raw data. Inherits the methods of Reconstructor.
CartesianGRAPPAReconstructor Constructor. Creates new reconstructor object.
SIRF is capable of performing rigid, affine and non-rigid registrations. Resampling functionality is also available. Initially, this has provided through the wrapping of NiftyReg (although future releases may incorporate other packages).
Below examples are given for rigid/affine and non-rigid registrations, as well as resampling. More complete examples for both Matlab and python can be found in the examples folder.
set_parameter_file Set the parameter file
set_parameter Set a parameter
set_reference_image Set the reference image
set_floating_image Set the floating image
set_reference_mask Set the mask of the reference image
set_floating_mask Set the mask of the floating image
process Start the registration process
get_output Get the registered image
get_transformation_matrix_forward Get the forward transformation matrix
get_transformation_matrix_inverse Get the inverse transformation matrix
get_deformation_field_forward Get the forward deformation field
get_deformation_field_inverse Get the inverse deformation field
get_displacement_field_forward Get the forward displacement field
get_displacement_field_inverse Get the inverse displacement field
reg = NiftyAladinSym()
reg.set_reference_image(ref)
reg.set_floating_image(flo)
reg.set_parameter_file(par_file)
reg.set_parameter('SetPerformRigid','1')
reg.set_parameter('SetPerformAffine','0')
reg.process()
output = reg.get_output()
set_parameter_file Set the parameter file
set_parameter Set a parameter
set_reference_image Set the reference image
set_floating_image Set the floating image
set_reference_mask Set the mask of the reference image
set_floating_mask Set the mask of the floating image
process Start the registration process
get_output Get the registered image
get_deformation_field_forward Get the forward deformation field
get_deformation_field_inverse Get the inverse deformation field
get_displacement_field_forward Get the forward displacement field
get_displacement_field_inverse Get the inverse displacement field
set_initial_affine_transformation Set the initial affine transformation
reg = NiftyF3dSym()
reg.set_reference_image(ref)
reg.set_floating_image(flo)
reg.set_parameter_file(par_file)
reg.set_parameter('SetPerformRigid','1')
reg.set_parameter('SetPerformAffine','0')
reg.process()
output = reg.get_output()
set_reference_image Set the reference image
set_floating_image Set the floating
process Start the registration process
get_output Get the registered image
add_transformation Add transformation (any type)
set_interpolation_type Set interpolation type
res = NiftyResample()
res.set_reference_image(ref)
res.set_floating_image(flo)
res.set_interpolation_type(1)
res.add_transformation(trans1)
res.add_transformation(trans2)
res.process()
output = res.get_output()
Class for converting raw data from listmode format into sinograms,
i.e. histogrammed data in the format of PET AcquisitionData.
It has 2 main functions:
process()can be used to read prompts and/or delayed coincidences to produce a single PETAcquisitionData. Two variables decide what is done with 3 possible cases:store_prompts=true,store_delayeds=false: only prompts are storedstore_prompts=false,store_delayeds=true: only delayeds are storedstore_prompts=true,store_delayeds=true: prompts-delayeds are stored Clearly, enabling thestore_delayedsoption only makes sense if the data was acquired accordingly.
estimate_randoms()can be used to get a relatively noiseless estimate of the random coincidences.
ListmodeToSinograms Constructor. Takes an optional text string argument with
the name of a STIR parameter file defining the conversion options.
If no argument is given, default settings apply except
for the names of input raw data file, template file and
output filename prefix, which must be set by the user by
calling respective methods.
set_input Specifies the input raw data file.
set_output_prefix Specifies the prefix for the output file(s), which will
be appended by `_g1f1d0b0.hs`.
set_template Specifies the file containing acquisition data to be
used as a source of information about the scanner.
set_time_interval Specifies the scanning time sub-interval to be converted
(an empty interval indicates that all raw data must be converted)
flag_on Turns on (i.e. assigns value true to) a conversion flag.
flag_off Turns off (i.e. assigns value false to) a conversion flag.
set_up Sets up the converter.
process Performs the conversion.
get_output Returns AcquisitionData object containing converted data.
estimate_randoms Estimates randoms. (Currently via a Maximum Likelihood estimate
of the singles, based on the delayed coincidences).
lm2sino = ListmodeToSinograms()
lm2sino.set_input(list_file)
lm2sino.set_output_prefix(sino_file)
lm2sino.set_template(tmpl_file)
lm2sino.set_time_interval(0, 10)
lm2sino.flag_on('store_prompts')
lm2sino.set_up()
lm2sino.process()
acq_data = lm2sino.get_output()
randoms_acq_data = lm2sino.estimate_randoms()
Class for the acquisition process modelling. Main component is the forward projection operation F that for a given image data x estimates the data y = F(x) to be acquired by the scanner (simulated acquisition data). The transpose B of the Frechet derivative of F is referred to as backprojection (if F is linear, e.g. a matrix, then B is the transpose of F).
For PET, F(x) is the right-hand side of the following equation:
(F) y = S(G x + a) + b
where
G is ray tracing matrix, (conceptually) a matrix whose columns correspond to the image voxels and rows to pairs of scanner's detectors (bins), each column simulating the impact of this voxel's radiation on the data acquired by the bins (this matrix is never actually computed);
a and b are additive and background terms representing the effects of accidental coincidences and scattering;
S is acquisition sensitivity model representing detector sensitivities and attenuation.
Accordingly, the backprojection B is the right-hand side of
(B) x = G' S y
where G' is the transpose of G.
AcquisitionModel (PET/MR) Constructor. Creates an acquisition model
(PET: empty, MR: empty or based on the image and
acquisition data templates specified by the
arguments).
forward (PET/MR) Returns F(x) for the image data x specified
by the argument.
backward (PET/MR) Returns B(y) for the acquisition data y specified
by the argument.
set_up (PET/MR) Sets up the model based on acquisition and image data
templates provided by the arguments.
set_additive_term (PET) Sets term a in (F).
set_acquisition_sensitivity
(PET) Defines AcquisitionSensitivityModel S (see below).
set_coil_sensitivity_maps
(MR) Sets coil sensitivity maps to be used.
MR_model = AcquisitionModel(acq_template, image_template);
MR_model.set_coil_sensitivity_maps(cs_data);
sim_data = MR_model.forward(MR_image);
Class for the PET acquisition process model that uses (implicitly) a sparse matrix for G in (F). This class inherits the methods of PET AcquisitionModel class, with forward projection defined by (F) and backprojection by (B).
AcquisitionModelUsingRayTracingMatrix
Constructor. Creates an acquisition model.
acq_model = AcquisitionModelUsingRayTracingMatrix();
acq_model.set_up(acq_template, image_template)
sim_data = acq_model.forward(image);
Class for a part of AcquisitionModel that accounts for bin efficiencies and attenuation.
Provides methods for for applying S factor in (F) and (B) or its inverse.
AcquisitionSensitivityModel
Constructor. Creates a new object of this class
- from an ECAT8 file or
- from an attenuation image (ImageData object) or
- from bin efficiencies (AcquisitionData object) or
- by chaining two objects of this class.
In the last case, the normalisation n is the product
of the two objects' normalisations.
set_up Sets up the object.
normalise Applies the inverse of S to the AcquisitionData argument.
unnormalise Applies S to the AcquisitionData argument.
forward Returns the argument multiplied by S. The argument
is not changed.
invert Returns the argument multiplied by the inverse of S.
The argument is not changed.
# obtain an acquisition data template
template = AcquisitionData(temp_file)
# create acquisition sensitivity model from ECAT8 normalization data
asm = AcquisitionSensitivityModel(norm_file)
# create acquisition sensitivity model from attenuation image
attn_image = ImageData(attn_file)
am = AcquisitionModelUsingRayTracingMatrix()
am.set_up(template, attn_image)
asm = AcquisitionSensitivityModel(attn_image, am)
# create acquisition sensitivity model from bin efficiencies
asm = AcquisitionSensitivityModel(bin_eff)
Class for objective functions maximized by iterative Ordered Subsets reconstruction algorithms. At present we use Poisson logarithmic likelihood function with linear model for mean and a specific arrangement of the acquisition data. To make our interface more user-friendly, we provide a convenience function make_PoissonLogLikelihood that creates objects of this class (instead of the usual constructor) based on the acquisition data to be used.
The user have an option of adding a penalty term (referred to as prior) to the objective function. At present, we have just one particular kind of prior implemented, the quadratic prior described in the next section.
ObjectiveFunction Constructor. Creates a new empty object.
set_prior Specifies the prior.
set_num_subsets Specifies the number of subsets.
set_up Prepares this object for use.
get_value Returns the value of the objective function.
get_gradient Returns the gradient of the objective function.
get_subset_gradient
Returns the component of the gradient for the specified subset.
get_backprojection_of_acquisition_ratio
Returns the backprojection of the ratio of measured to estimated
acquisition data.
set_acquisition_model
Specifies the acquisition model to be used.
set_acquisition_data
Specifies the acquisition data to be used.
Class for a penalty term to be added to the objective function.
Prior Constructor. Creates a new empty object.
set_penalisation_factor Specifies the prior's scaling factor.
get_gradient Returns the prior gradient.
preprocess_acquisition_data (MR) Preprocesses the MR acquisition data.
make_Poisson_loglikelihood (PET) Returns Poisson objective function.
SIRF offers the users two options in handling acquisition data generated by SIRF scripts. The default one keeps all acquisition data generated by the script in scratch files deleted after the script terminates. An alternative one stores acquisition data in memory. The user can specify the storage scheme to be employed by calling a static method set_storage_scheme of AcquisitionData class:
AcquisitionData.set_storage_scheme(scheme)
where scheme is either "memory" or "file". To see which scheme is currently used, use
scheme = AcquisitionData.get_storage_scheme()
A particular setting of storage scheme by a Matlab script or a Python script run from Spyder is persistent: any script run afterwards will use the same storage scheme unless a different storage scheme is explicitly set by set_storage_scheme or Matlab/Spyder is re-started.
Gadgetron is an MR reconstruction framework which was designed to process a datastream, i.e. rather than waiting for a complete 3D k-space to be acquired, each readout (frequency encoding line) is processed immediately (if possible - e.g. Fourier transform along phase encoding can only be applied once all phase encoding lines have been acquired). The reconstruction is performed by a chain of gadgets, i.e. pieces of code implementing specific tasks. The chain of gadgets runs on the server, which can be just a command line window, or it can be another computer or a VM. In order to set up the chain, the server needs to receive an xml text describing it from the client, which again can be another command line window on the same or another computer. The first gadget in the chain then starts waiting for acquisition data to arrive from the client in chunks of certain size. Having processed a chunk of data, the first gadget passes the result to the second and starts processing the next chunk and so on. The last gadget sends the reconstructed images back to the client.
The standard way of using Gadgetron is to run gadgetron_ismrmrd_client from a command line (with Gadgetron running in another terminal window), providing the name of the raw data file (in HDF5 format) and the name of the xml file containing the description of the gadget chain via command-line options. SIRF offers an equivalent alternative whereby the data and the gadget chain are defined in a Python or Matlab script. The gadget chain is defined by creating a Reconstructor object and providing the list of gadgets descriptions as an argument:
my_recon = Reconstructor(my_gadget_list);
Here my_gadget_list is a list of strings in Python or a cell array of strings in Matlab, each string describing a gadget in the following format:
[label:]gadget_name[(property1=value1[,property2=value2,...])]
where the names of the gadget and its properties are same as in Gadgetron xml files, and an optional label can be used to change the labelled gadget properties at any time by using set_gadget_property method:
my_recon.set_gadget_property(label, property, value);
The following example of a gadget chain definition is taken from demo script fully_sampled_recon_single_chain.py:
recon = Reconstructor(['RemoveROOversamplingGadget', \
'AcquisitionAccumulateTriggerGadget(trigger_dimension=repetition)', \
'BucketToBufferGadget(split_slices=true, verbose=false)', \
'SimpleReconGadget', 'ImageArraySplitGadget', 'ex:ExtractGadget'])
# ExtractGadget defines which type of image should be returned:
# none 0
# magnitude 1
# real 2
# imag 4
# phase 8
# max 16
# in this example '5' returns both magnitude and imag
recon.set_gadget_property('ex', 'extract_mask', 5)
The input data is defined by creating an AcquisitionData object and passing it to the reconstruction object via its method set_input:
acq_data = AcquisitionData(input_file_name);
my_recon.set_input(acq_data);
and the reconstruction is performed by calling the method process, and the reconstructed images are returned as an ImageData object by the method get_output:
my_recon.process();
image_data = my_recon.get_output();
While the way to use Gadgetron just described is the most efficient performance-wise, some users may like to get more involved in the reconstruction process. SIRF offers such users an opportunity to split a standard reconstruction chain into sub-chains and process intermediate data. For example, the chain defined above can be split into an acquisition processing chain that removes oversampling, a shortened reconstruction chain and an image processing chain:
acq_proc = AcquisitionDataProcessor(['RemoveROOversamplingGadget'])
acq_proc.set_input(acq_data)
acq_proc.process()
preprocessed_data = acq_proc.get_output()
# do something with preprocessed data here
recon = Reconstructor\
(['AcquisitionAccumulateTriggerGadget(trigger_dimension=repetition)', \
'BucketToBufferGadget(split_slices=true, verbose=false)',
'SimpleReconGadget', 'ImageArraySplitGadget'])
recon.set_input(preprocessed_data)
recon.process()
complex_image_data = recon.get_output()
# do something with the complex image data here
img_proc = ImageDataProcessor(['ExtractGadget(extract_mask=1)'])
img_proc.set_input(complex_image_data)
img_proc.process()
real_image_data = img_proc.get_output()
This section provides a concise description of Gadgetron gadgets that can be used by current SIRF release scripts in a way described in the previous section. For further information consult Gadgetron documentation.
Below internal<N> refers to Gadgetron data objects to which SIRF does not provide interface at present. We emphasize that splitting Gadgetron chains into sub-chains in a way described in the previous section only makes sense if the input of the first gadget and the output of the last gadget of each sub-chain are either AcquisitionData or ImageData.
| input | output | parameters |
|---|---|---|
| AcquisitionData | AcquisitionData | none |
Removes the oversampling along the readout direction.
| input | output | parameters |
|---|---|---|
| AcquisitionData | AcquisitionData | none |
Ensures that the noise between different receiver coils is not correlated and that each receiver coils has a similar noise level.
| input | output | parameters |
|---|---|---|
| AcquisitionData | AcquisitionData | none |
Pads each readout with zeros to compensate for partial echo acquisitions.
| input | output | parameters | default values |
|---|---|---|---|
| AcquisitionData | internal1 | trigger_dimension | "repetition" |
| sorting_dimension | "slice" |
Collects lines of k-space until a certain trigger condition is encountered, i.e., when there is enough data to reconstruct an image.
| input | output | parameters | default values |
|---|---|---|---|
| internal1 | internal2 | N_dimension | "" |
| S_dimension | "" | ||
| split_slices | "true" | ||
| ignore_segment | "true" | ||
| verbose | "true" |
Inserts the collected data into a buffer more suitable for the reconstruction processing.
| input | output | parameters |
|---|---|---|
| internal2 | internal3 | none |
Performs simple fast Fourier transforms to transform acquired k-space data to image space.
| input | output | parameters | default values |
|---|---|---|---|
| internal2 | internal4 | debug_folder | "" |
| perform_timing | "true" | ||
| verbose | "true" | ||
| average_all_ref_N | "true" | ||
| average_all_ref_S | "true" | ||
| prepare_ref_always | "true" |
Selects the reference data used to calculate the GRAPPA kernel
| input | output | parameters | default values |
|---|---|---|---|
| internal4 | internal5 | debug_folder | "" |
| perform_timing | "true" | ||
| verbose | "true" | ||
| image_series | "0" | ||
| coil_map_algorithm | "Inati" | ||
| send_out_gfactor | "true" | ||
| downstream_coil_compression | "true" | ||
| downstream_coil_compression_thres | "0.01" | ||
| downstream_coil_compression_num_modesKept | "0" |
Performs GRAPPA kernel calibration, calculate coil sensitivity maps and carry out unfolding.
| input | output | parameters | default values |
|---|---|---|---|
| internal5 | internal6 | debug_folder | "" |
| perform_timing | "false" | ||
| verbose | "false" |
Adjusts Field Of View and image resolution according to the parameters given for the reconstructed image in the file header.
| input | output | parameters | default values |
|---|---|---|---|
| internal6 | internal3 | perform_timing | "false" |
| verbose | "false" | ||
| min_intensity_value | "64" | ||
| max_intensity_value | "4095" | ||
| scalingFactor | "10.0" | ||
| scalingFactor_dedicated | "100.0" | ||
| use_constant_scalingFactor | "true" | ||
| auto_scaling_only_once | "true" |
Applies scaling to image, g-factor map, SNR map and/or SNR standard deviation map.
| input | output | parameters |
|---|---|---|
| internal3 | ImageData | none |
Splits array of images (7D: [X, Y, Z, CHA, N, S, LOC]) into individual images (4D: [X, Y, Z, CHA])
| input | output | parameters | default values |
|---|---|---|---|
| ImageData | ImageData | extract_mask | "1" |
Extracts a certain type of image data from the reconstructed image stream, i.e. extract_mask = 1 yields images' magnitudes, extract_mask = 2 yields real parts of images.
| input | output | parameters |
|---|---|---|
| ImageData | ImageData | none |
Depending on the image type, the magnitude, real, imaginary or phase of the complex image is returned.
| input | output | parameters | default values |
|---|---|---|---|
| ImageData | ImageData | min_intensity | "0" |
| max_intensity | "32767" | ||
| intensity_offset | "0" |
Scales and transforms images from float to short.
A chain of Gadgetron gadgets
AcquisitionAccumulateTriggerGadget
BucketToBufferGadget
SimpleReconGadget
ImageArraySplitGadget
for fully sampled reconstruction.
| input | output | parameters | default values |
|---|---|---|---|
| AcquisitionData | ImageData | N_dimension | "" |
| S_dimension | "" | ||
| sorting_dimension | "slice" | ||
| trigger_dimension | "repetition" | ||
| split_slices | "true" |