DCM memory allocation error

I have a dicom file containing 353 images (706 MB) that I want to
convert in PGM files. I use the command:
$ convert input.dcm ouput%03i.pgm

After a while, I received the following error message:
convert(22562) malloc: *** mmap(size=524288) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
convert: Memory allocation failed
`/Users/Alzathar/Biomech/project/STAC/acquisition/090418/dicom/090418EM1_NW1_55kV_BH05.dcm'
@ dcm.c/ReadDCMImage/3380.
convert: missing an image filename
`/Users/Alzathar/Biomech/project/STAC/acquisition/090418/dicom/test/test%03i.pgm'
@ convert.c/ConvertImageCommand/2774.

I'm using ImageMagick  6.5.3 on Mac OS X 10.5.7 (Leopard). The problem
is not a hardware problem as I can open this Dicom with ITK.