Setting Rmagick resolution when reading in an image?

Dear all,

I am having problems when reading in an image (PS/PDF/TIF):
I need to read in images of these formats, cut them in two and
export them. Unfortunately, using

im=Magick::Image.read(filename).first

produces only images with resolutions of 72x72, which is
too blurry (I need 600x600).
I have tried

im.density=“600x600”,

and

im.x_resolution=600

im.y_resolution=600

after reading in, and resampling

im.resample(600,600),

but without success. Is there a way to tell Rmagick what the resolution
is when reading in a file ?

Thank you very much!

Best regards,

Axel