An alignment fixture for taking X-ray images of a patient, such as an image of the patient's hip region preparatory to hip arthroplasty. The alignment fixture comprises multiple support legs for resting on the patient at known positions, such as upon known bony prominences, in combination with aiming marks used for anatomically aligning the patent's pelvis under an X-ray beam. A ruled grid plate on the fixture is adjustably positioned between an image beam source and the patient, as a function of patient thickness in the pelvic region, so that a ruled grid overlays the anatomical region of interest and exhibits an apparent parallax image magnification corresponding with the parallax image magnification of a centerline of the anatomical region, such as the hip joint. X-ray images taken by use of the alignment fixture can be viewed and/or scaled to select a properly sized prosthesis for the specific patient.