Elekta Leeds Analysis

Hi James,

We have two leeds image. One is good (taken in March) and the other one is bad (April). We wonder why it could not detect the phantom…

Attached are the images, pdf report, code and error message.

Thank you for your comments.


The bad one has error message during the analysis pylinac v 3.10.0:

%Run test_Leeds.py
Traceback (most recent call last):
File “N:\python\test_Leeds.py”, line 6, in
leeds.analyze(ssd = 1600)
File “C:\Users\physics.FPH\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.9_qbz5n2kfra8p0\LocalCache\local-packages\Python39\site-packages\pylinac\planar_imaging.py”, line 330, in analyze
File “C:\Users\physics.FPH\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.9_qbz5n2kfra8p0\LocalCache\local-packages\Python39\site-packages\pylinac\planar_imaging.py”, line 2234, in _check_inversion
circle = self._circle_profile_for_phantom_angle(start_angle_deg=0)
File “C:\Users\physics.FPH\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.9_qbz5n2kfra8p0\LocalCache\local-packages\Python39\site-packages\pylinac\planar_imaging.py”, line 2219, in _circle_profile_for_phantom_angle
File “C:\Users\physics.FPH\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.9_qbz5n2kfra8p0\LocalCache\local-packages\Python39\site-packages\pylinac\planar_imaging.py”, line 755, in phantom_center
else self._phantom_center_calc()
File “C:\Users\physics.FPH\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.9_qbz5n2kfra8p0\LocalCache\local-packages\Python39\site-packages\pylinac\planar_imaging.py”, line 775, in _phantom_center_calc
return bbox_center(self.phantom_ski_region)
File “D:\env\env1\lib\site-packages\cached_property.py”, line 36, in get
value = obj.dict[self.func.name] = self.func(obj)
File “C:\Users\physics.FPH\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.9_qbz5n2kfra8p0\LocalCache\local-packages\Python39\site-packages\pylinac\planar_imaging.py”, line 251, in phantom_ski_region
raise ValueError(
ValueError: Unable to find the phantom in the image. Potential solutions: check the SSD was passed correctly, check that the phantom isn’t at the edge of the field, check that the phantom is centered along the CAX.

00073420 (2 MB)

00073245 (2 MB)

leeds_00073245.pdf (191 KB)

leeds_python_code.txt (266 Bytes)

error_message.txt (1.96 KB)

Hi all,

May I know whether any other Elekta user having this problem?



I am not an Elekta user but I think the problem might be your ssd. In your code you use an ssd of 1600 which seems to be too large.
The DICOM Tag for the RTImageSID is only 1536, so your phantom should sit somewhere closer to the source than that.

When I run the analysis with an ssd of 1500 I get results. They do not seem perfect (hf MTF not dropping monotonically) but at least you would get results.


Hi Thomas,

Thanks for highlighting that the DICOM RT Image SID was at 1536. With 1536, I am able to get results.

Thanks again!