An ultrasound of the sacrum is a noninvasive imaging test that uses high-frequency sound waves to view the lower spine and nearby soft tissues. It helps evaluate sacral anatomy, soft‑tissue masses, fluid collections, sacroiliac joint inflammation, or neonatal spinal anomalies not seen on X‑ray. The scan is done with a transducer over the lower back and is safe, painless and radiation‑free.

Ultrasound has a limited role in detecting spinal problems. It cannot image the adult spinal cord or intervertebral discs well because bone blocks sound, so MRI, CT or X‑ray are preferred for most spine disorders. Ultrasound is useful for assessing superficial soft tissues, guiding injections or procedures, evaluating newborn spines before ossification, and detecting fluid collections or superficial nerve issues.

Ultrasound is advised for atypical sacral dimples — located above the gluteal crease, off-midline, large or deep (≈>5 mm), multiple, or with hair tuft, skin discoloration, sinus/drainage, or neurological signs (abnormal tone, leg weakness) or other anomalies. Perform lumbar ultrasound in early infancy (ideally before 6 months); if imaging is inconclusive or the infant is older, MRI may be needed.

Diagnosing sacral pathology starts with history (trauma, pain pattern, red flags) and a focused physical exam: inspection, palpation, gait, and provocative sacroiliac/sacral tests (FABER, Gaenslen, sacral thrust). Perform a neurologic exam for radiculopathy. Imaging (X‑ray, CT for fractures; MRI for marrow/soft tissue/infection; bone scan for occult injury) and labs (CBC, CRP) guide diagnosis and management.

The sacrum is a triangular, fused-bone segment at the base of the spine that connects the lumbar vertebrae to the pelvis. It transmits body weight to the hips and legs through the sacroiliac joints, stabilizes the pelvic girdle, protects sacral nerves and pelvic organs, and provides attachment points for ligaments and muscles essential for posture, balance and walking.

Therapeutic ultrasound has limited, inconsistent evidence for sacroiliac (SI) joint pain and isn’t usually recommended alone. Ultrasound-guided SI joint injections (local anesthetic ± steroid) can improve accuracy, help confirm the pain source, and often give short-term relief. For ongoing SI dysfunction, combine injections with structured exercise, manual therapy and specialist review.