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How do i simulate a map from a patient obtained by CT.
The simulation is based on images which means that you need to define index 14 and
15 to -1. You also need to specify the name of the files in the main menu by the
numbers 12,13. SIMIND does not know about the format besides that it has to be an
INTEGER*2 file for both the phantom and the source images so matrix size is defines
by index 78,79 and 81,82.
The phantom file needs to be scaled to density values * 1000 [voxels with water needs
will have values equal to 1000]. The density file [option 13 in main menu] needs to
have the extension *.dmi and the source file [option 14] needs an extension of *.smi.
The number of phantom images is defined by index 34 and the start image in the file by
index 33 [usually equal to 1 but you don't need to start from the beginning of the data
set].
To scale the image to real physical dimensions a pixel size for the phantom image need
to be define by Index 31. This scales the phantom in y,z direction. The length of the
phantom in cm [slice thickness] is defined by the half-length of the phantom [index 5]
divided by the number of images [index 34]. The corresponding pixel size for the
activity maps is in current version of the program only given by the external switch /PX:
Could you explain a little bit how SIMIND to decide the #particles/proj? I tried to
change the parameter #26, which is number of photon history, but seems it has
no effect, the output file always says a number of 767,555.
When you are using images as a source index 26 is not used in order to maintain a
proper sampling of the desired number of decays described by the images. Instead the
value of index 26 is replaced by the sum of pixels in your images [which is equal to
767555]. You can increase this number by multiplying with a factor given by the switch
/NN. For example, /NN=10 give a simulation by 7 675 550 histories.
Could you explain more about the geometry?
If you picture a real gamma camera patient measurement the patient axis from feet to head is
align to the x-axis and the direction from the mid of the patient to the feet corresponds to
negative x-values [SIMIND phantoms are always centered in the origin of the coordinate
system]. Please review the image in section ‘CHANGE’ . If you view the patient from the feet a
positive SIMIND Y-value goes from the midline to the right and a negative y-value to the left. A
positive z-value goes towards the camera [assuming a camera angle of 0 that is the anterior
position] and a negative z-value goes away from the camera. When using standard phantoms
[index 14,15 is 1,2,3,4,5..] the source defined by index 2,3,4 is centered around the origin. You
can move the source around by index 16,17,18 and this is useful when simulated for example a
point source on different depths by changing index 18.
When using voxel-based phantoms from images these are only relative number distributions –
for example 64x64x64 or 128x128x128. You have therefore first to scale to a physical dimension.
The scaling is made by two values – the pixel size [y,z direction] if transaxial images is assumed
and the slice thickness [x direction]. For the phantom – index 31 defines the pixel size for the
phantom images assuming square pixels and index 5 defines the total axial length of the
phantom = number of ct-slices [index 34] times the slice thickness]. Thus, SIMIND calculates
internally the slice thickness. Index 6,7 is not in use for image-based phantoms if orientation
index 32=0. For known phantoms, such as the Zubal phantoms [index 14 = -2,-3,-4] and the
NCAT phantoms the source maps corresponds to the phantom maps to the pixel size for the
source maps will be the same and also the slice thickness. In these cases, to be safe, you can
set also index 2 equal to the value of index 5.
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