How to pass image variable into function?

I am using the Scripting tool in ImageJ/Fiji to process two images (Gaussian blur) and then run the SIFT algorithm on those images. I am having trouble passing my image variable that has been modified with the Gaussian blur into the run() command for SIFT.
I am getting this error:

TypeError: cannot concatenate 'str' and 'ij.ImagePlus' objects

Here is my code:

from ij import IJ

# Open images
img1 = IJ.openImage('img1.jpg')
img2 = IJ.openImage('img2.jpg')

# Gaussian blur, "Gaussian Blur...", "sigma=5"), "Gaussian Blur...", "sigma=5")

# Sift and feature matching"Extract SIFT Correspondences", "source_image=["+img1+"] target_image=["+img2+"] initial_gaussian_blur=1.60 steps_per_scale_octave=3 minimum_image_size=64 maximum_image_size=1024 feature_descriptor_size=4 feature_descriptor_orientation_bins=8 closest/next_closest_ratio=0.92 filter maximal_alignment_error=25 minimal_inlier_ratio=0.05 minimal_number_of_inliers=7 expected_transformation=Similarity")

I also tried using & to pass in variables:"Extract SIFT Correspondences", "source_image=&img1 target_image=&img2

In this case, the code runs but I get the following message:
Is there any way to pass in img1 and img2 without opening them?

In the source and target image of the SIFT call the image title ist required and not the ImagePlus object.
Simply use the macro recorder to get the right commands or hints.

See also: