Division of Nuclear Medicine SimSET Collimator Module

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Contents:

1. Overview
   

1. Supported Models
2. Outputs
3. Customization
4. Collimator models - description and configuratio
1. Activating collimator modeling
2. Simple PET
3. Monte Carlo PET
4. UNC SPECT
5. Slat DHCI
1. Parameter type definitions for collimator parameter files
   
   
1. Example collimator parameter files
   

Overview

Supported models

The Collimator Module receives photons from the PHG and tracks photons through the collimator being modeled. Two styles of collimator are simulated, geometric and Monte Carlo. For SPECT, collimators are modeled using geometric transfer functions (footnote 1, Tsui and Gullberg 1990), rather than using Monte Carlo techniques. Parallel, fan, and cone beam collimators can be simulated. For PET, photons are tracked through cylindrical collimators using Monte Carlo techniques. Alternatively, a simple PET set-up using geometrically modelled perfectly absorbing end-plates can be used. For DHCI, photons are tracked through slat collimators using Monte Carlo methods.

The algorithms used for Compton and coherent scattering and those used for selecting SPECT collimator positions will be described in Physics and Algorithms.

Outputs

Collimated photon data is passed from the collimator module to the detector module if detectors are being simulated, to the binning module if binning is being done on-the-fly, or directly to the history file module if one is being created.

WARNING: When using UNC SPECT collimation it is important to remember that the photon weights are affected by this module.

Customization

Each model allows the user to write a User Routine to modify the behavior of the collimator software. This routine is supplied the collimator parameters and the current photon. It is called before the collimator code models the transport through the collimator. The user routine may modify any parameter of the current photon or have it rejected by the collimator module. To use this feature, modify 'dummy' collimator module user routine with user code (ColUsr.c) and re-build SimSET software.

 

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Collimator models - description and configuration

Activating collimator modeling

The collimator module uses a parameter file format to select the collimator model, define collimator parameters, and select simulation options. To activate collimator modeling:

1. Create a Collimator Parameters file. Entries in this file have the following format:

   DATA_TYPE

   parameter_name

   =

   value

   For example,

   REAL

   collimator_depth

   =

   2.0

   
   
   More detailed information on parameter file formats can be found in Appendix VI. Data type definitions for collimator parameter files may be found below.
   
   

2. Set 'collimator_params_file' in the PHG run-time parameters file to this file's pathname.
   
   
3. Create the following entries in the Collimator Parameters file:

Set 'history_file' to the desired name of the new history file. Only photons and detection records accepted by the detector module are written to this history file. It must have a different name from any other history files specified in the simulation (such as from the phg run-time parameters or elsewhere). Leave blank for no history file creation.

Set 'history_params_file' to the name of the customized history parameters file. Leave blank if no history file is being created.

Set 'collimator_type' to desired model:

simple_pet = geometric modeling of infinitely thin, perfectly absorbing end-plates
monte_carlo_pet = simulation of cylindrical PET collimators
unc_spect = geometric modeling of SPECT collimators

 

The remaining collimator parameters depend on the collimator type chosen:

• Simple PET
• Monte-Carlo PET
• UNC SPECT
• Slat DHCI

 

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Simple PET

Model

End-plates are modeled with inner radius equal to the target cylinder radius. The z-positions of the end-plates are automatically set to the minimum and maximum z of the target cylinder.

Figure 1. End-plates for Simple PET collimators

 

Configuration

In the collimator parameters file:

Set 'collimator_depth' to desired depth of the end-plates.

 

[top of page] [example parameter files]

 

Monte Carlo PET

Model

This mode provides a full simulation (including scatter, absorption, and penetration) of photons passing through a collimator. The collimator is defined using a geometry based on Chris Thompson's PETSIM software (footnote 2, Thompson et al, 1992). The collimators are composed of cylinders that can have multiple radial and axial layers; the collimators can be tapered (figure 2).

Figure 2. Axial and transaxial cross-sections through a PET collimator.

The user specifies an unlimited number of axial segments (see figure 3). Each segment is defined by its material, a minimum and maximum axial position, and an inner and outer radius.

 

Figure 3. Axial segments in a Monte-Carlo PET collimator simulation.

 

The axial segments may be split into multiple radial layers (see figure 4), allowing simulation of tapered collimators and other unusual collimator geometries.

Figure 4. Using Multiple Layers to create Tapered Collimators in a Monte-Carlo PET simulation.

 

Configuration

In the collimator parameters file:

The number of layers are specified first, then the segments are specified for each layer. The innermost layer (radially) is specified first, and the segments are specified from lowest (axially) to highest. Layers must be contiguous radially and segments must be contiguous axially (i.e. the air between collimator sections must also be specified).

Set 'NUM_ELEMENTS_IN_LIST layers_list' to the desired number of radial layers.

For each layer (starting with the innermost layer)

Set 'NUM_ELEMENTS_IN_LIST segment_list' to the desired number of segments in the current layer.

For each segment (starting with the lowest axial layer)

 For a segment with rectangular cross section:

• Set 'NUM_ELEMENTS_IN_LIST segment' to 6.
• Set 'seg_type' to "parallel".
• Set 'material' to the index of the desired material in the PHG attenuation table.
• Set 'inner_radius' to the desired inner radius of the segment.
• Set 'outer_radius' to the desired outer radius of the segment.
  Note: within a given layer all segments must have the same inner and outer radius. Collimators with staggered inner and outer radii must be simulated using additional layers.
• Set 'inner_min_z' to the desired lower z boundary of the segment.
• Set 'inner_max_z' to the desired upper z boundary of the segment.
  Note: for rectangular cross section segments the software automatically sets 'outer_min_z' and 'outer_max_z' to the 'inner_min_z' and 'inner_max_z' values.

For a segment with trapezoidal cross section:

• Set 'NUM_ELEMENTS_IN_LIST segment' to 8.
• Set 'seg_type' to "tapered".
• Set 'material' to the index of the desired material in the PHG attenuation table.
• Set 'inner_radius' to the desired inner radius of the segment.
• Set 'outer_radius' to the desired outer radius of the segment.
  Note: within a given layer all segments must have the same inner and outer radius. Collimators with staggered inner and outer radii must be simulated using additional layers.
• Set 'inner_min_z' to the desired lower z boundary of the segment at its inner radial edge.
• Set 'outer_min_z' to the desired lower z boundary of the segment at its outer radial edge.
• Set 'inner_max_z' to the desired upper z boundary of the segment at its inner radial edge.
• Set 'outer_max_z' to the desired upper z boundary of the segment at its outer radial edge.

 

[top of page] [example parameter files]

 

UNC SPECT

Model

Much of the software in this module was written at the University of North Carolina by Drs. Eric Frey and Dave Lewis.

The collimator is not modeled using Monte-Carlo photon tracking but rather by using a geometric transfer function (footnote 3, Tsui and Gullberg, 1990). Three types of hole geometries are supported: parallel hole, fan beam, and cone beam. The hole apertures at the collimator front face are circular and identical across the face. For parallel hole collimation the hole is a right circular cylinder perpendicular to the collimator face; for fan-beam the holes are tapered, slanted transaxially to focus on a focal line; for cone-beam the holes are tapered, slanted both axially and transaxially to focus on a focal point (see figure 5).

 

 

Figure 5. Fan-beam and Cone-beam collimation in a UNC SPECT simulation

 

The collimator holes are modeled in a hexagonal pattern. The collimator response is calculated as if the pattern were moved randomly about the collimator face. Thus, the holes do not correspond to fixed positions on the collimator face and can not, for example, give rise to a "collimator pattern" in the simulated data. The density of the holes is based on the user specified hole radius and septal thickness (see figure 6). Note that septal_thickness is defined to be HALF the smallest distance between the holes. For each photon the software computes the probabilty that the photon would pass through a hole without hitting the collimator. Photons for which this probability is non-zero are accepted, their weights multiplied by this probability. Septal penetration and scatter within the collimator are not modeled.

Figure 6. Effective front face of collimator in a UNC SPECT simulation

 

The simulated collimator positions are determined from the parameters 'start_angle', 'stop_angle', and 'num_views'. The first collimator position is centered on start_angle, and the last collimator position is centered at (stop_angle - one bin).

A new feature under implementation is the parameter 'first_angle_offset'. This specifies whether the first collimator position is centered on start_angle or is offset by a half-bin (see figure 7). In this case the last collimator position is centered at (stop_angle - one bin) or (stop_angle - one half bin) respectively.

Figure 7. Camera positions in a UNC SPECT simulation.
NOTE: first_angle_offset is not yet supported (16 Feb 1999).

 

The collimator position for a photon is randomly selected from those positions at which the photon can pass through the collimator (will be documented in Physics and Algorithms).

 

Configuration

In the collimator parameter file:

Set 'collimator_type' to "unc_spect"

• Set 'hole_geometry' to one of:
  "parallel" for parallel holes
  "fan_beam" for fan beam holes
  "cone_beam" for cone beam holes.
• Set 'focal_length' to non-zero value if using fan or cone beam holes.
• Set 'radius_of_rotation' to inside radius of collimator rotation (see figure 8).
• Set 'thickness' to the desired depth of the collimator (see figure 8).
• Set 'hole_radius' to specify the hole radius (see figure 8).
• Set 'septal_thickness' to HALF the desired distance between holes.
• Set 'min_z' to the desired minimum axial extent of the collimator (zero is the center).
• Set 'max_z' to the desired maximum axial extent of the collimator.
• Set 'start_angle' to the first rotation angle, specify in degrees.
• Set 'stop_angle' to the last rotation angle, specify in degrees.
• Set 'num_views' to the number of discrete sampling positions between start_angle and stop_angle.
  Note: if doing azimuthal angle binning, this value must match the number of angles specified in the binning parameters. Likewise it must match the number of angles specified in the detector module if detection is being modeled.
• Set 'first_angle_offset' to true or false (see figure 7).

Figure 8. Parameters for UNC SPECT collimators

 

[top of page] [example parameter files]

Slat - DHCI

Model

The slat collimator module is a monte carlo simulation of transport through a "slat" collimator geometry (Figure 9). In order to use slat collimators you must use a DHCI detector. As you will see below in the parameter section, defining the over-all dimensions of the collimator are actually done in the detector module. Only collimator specific parameters are specified in this module.

Configuration

In the collimator parameter file:

Set 'collimator_type' to "slat"

• Set 'width' to axial width of slats, (i.e. the thickness of the slats):
  
• Set 'gap' to the distance between slats.
• Set 'depth' to the radial depth of the slats.
• Set 'inner_radius' to the "face" of the collimator, the radial distance from the center of the FOV.
• Set 'material' to specify what the slats are made of, this is from the list of attenuation materials available.

Note that the axial and transaxial dimensions are defined by the detector. The sum of the gap and width must be an integral factor of the axial extent of the detector module.

The collimator is in a slat-gap-slat-gap.... configuration. So the gap begins at the "min z" value defined in the detector module.

Figure 9. Parameters for Slat-DHCI collimators

 

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Parameter type definitions for collimator parameter files

 

Parameter name	Units	Data type
history_file		STR Allows the creation of a history file of photons processed by the collimator module (see history files page)
history_params_file		STR Configures history file creation for photons processed by the collimator module (see history files page)
collimator_type		If text, then ENUM, else INT. Options shown below: ENUM INT simple_pet 1 monte_carlo_pet 2 simple_spect 3 unc_spect 4 slat 5
collimator_depth	cm	REAL
layers_list		NUM_ELEMENTS_IN_LIST
segment_list		NUM_ELEMENTS_IN_LIST
segment		NUM_ELEMENTS_IN_LIST
seg_type		If text, then ENUM, else INT. Options shown below: ENUM INT parallel 1 tapered 2
material		INT
inner_radius	cm	REAL
outer_radius	cm	REAL
inner_min_z	cm	REAL
inner_max_z	cm	REAL
hole_geometry		If text, then ENUM, else INT. Options shown below: ENUM INT parallel 1 fan_beam 2 cone_beam 3
focal_length	cm	REAL
radius_of_rotation	cm	REAL
thickness	cm	REAL
hole_radius	cm	REAL
septal_thickness	cm	REAL
min_z	cm	REAL
max_z	cm	REAL
start_angle	degrees	REAL
stop_angle	degrees	REAL
num_views		INT
first_angle_offset	degrees	REAL

Example Collimator Parameter files 

1. Generic contains samples of all collimator parameters
2. Simple PVI (3D PET) collimator parameter file (geometric collimation)
3. Monte-Carlo PVI collimator parameter file
4. Monte-Carlo 2D PET collimator parameter file
5. UNC SPECT collimator parameter file
6. Slat DHCI collimator parameter file

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Last revised by:	Robert Harrison
Revision date:	14 January 2014