Ch15: MSK Imaging — Modalities & Basic Bone Signs（骨骼肌肉影像基础）

# Ch15: MSK Imaging — Modalities & Basic Bone Signs（骨骼肌肉影像基础）

Preparatory Mindset

Musculoskeletal imaging starts with plain radiographs — usually the indicated primary imaging modality (widely available, reproducible, patient-friendly, inexpensive). Remember two maxims: "one view is no view" (always obtain at least two views) and image the area in question, not the vicinity (consider the pathology, use standard protocols, add supplemental views when needed). Nuclear scintigraphy (bone scan), ultrasound, CT and MRI play specific roles.

Core Concepts

Imaging modalities for MSK

• Plain radiographs: primary modality; 2 views standard; supplemental views (oblique, flexion, delayed films) when indicated (e.g. radial head, scaphoid, C2 arch fractures).
• Nuclear scintigraphy (bone scan): most common = bone scan with Tc-99m bound to phosphate; IV injection + ~3-hour delay + delayed static imaging; very sensitive for skeletal pathology (osteomyelitis, metastases — not myeloma, occult fracture), mildly sensitive for soft tissue; usually nonspecific as an isolated test; "normal is usually normal."
• Ultrasound: tendons, muscles, nerves, ligaments, soft tissue masses; subperiosteal pus collections in osteomyelitis before plain-film changes.
• CT: better assessment of fractures, extent of soft tissue involvement; more comfortable/safer for the severely injured.
• MRI: dark line across fat in the marrow; increased signal = hemorrhage/edema; high sensitivity for neoplastic, inflammatory and traumatic conditions of bone and soft tissue; T1 for fluid collections and fatty marrow abnormalities, T2 for marrow and soft tissue lesions.

Anatomy of long bone

• Ossification center, epiphysis, epiphyseal line, metaphysis, diaphysis, cortex, cancellous bone, periosteum, soft tissue.

Four main groups of bone disease (MSK2 framework)

1. Solitary lytic or sclerotic lesions — bone tumor (malignant primary/secondary or benign), osteomyelitis, bone cyst, fibrous dysplasia, other non-neoplastic defects, uncertain nature (Langerhans' histiocytosis, osteoid osteoma).
2. Multiple focal lesions — several discrete lytic/sclerotic lesions or periosteal reactions in one or more bones (metastases, myeloma, lymphoma/leukemia, multiple periosteal reactions, non-accidental injury, widespread infection, congenital syphilis, neonatal IV-catheter infection, hypertrophic pulmonary osteoarthropathy).
3. Generalized lesions — all bones show diffuse increase or decrease in bone density (osteoporosis, osteomalacia, hyperparathyroidism, myeloma; sclerotic: osteopetrosis, myelosclerosis).
4. Alter the trabecular pattern or change shape — Paget's disease.

Basic abnormalities (lab)

• Osteoporosis: decreased bone density with well-defined cortex; commonest postmenopausal and senile; changes best seen in the spine (partial vertebral collapse, widened disc spaces); causes: idiopathic (juvenile/postmenopausal/senile), Cushing's, steroid therapy, disuse.
• Osteomalacia/rickets: poor mineralization of osteoid; before epiphyseal closure = rickets, adults = osteomalacia. Rickets best seen at knees/wrists/ankles — widened irregularly mineralized, frayed/cupped metaphyses. Osteomalacia: loss of density, thinning trabeculae/cortex, Looser's zones (pseudofractures) — horizontal lucent bands with sclerotic margins.
• Hyperparathyroidism: generalized loss of bone density; subperiosteal bone resorption (hallmark); soft tissue/vascular calcification and chondrocalcinosis; brown tumors (osteitis fibrosa cystica).
• Renal osteodystrophy: osteomalacia/rickets + hyperparathyroidism + sclerosis (rugger-jersey spine — sclerotic bands across upper/lower vertebral body ends).
• Bone destruction (aggressive): loss of normal bone architecture — malignant tumor or infection.
• Periosteal reaction: localized, adjacent to lytic/sclerotic lesion — osteomyelitis, malignant bone tumor (Ewing's, osteosarcoma), occasionally metastasis (neuroblastoma), Langerhans' histiocytosis. Nontraumatic = usually aggressive; traumatic = no underlying bone destruction.
• Osteonecrosis: bone infarction — caisson disease, sickle cell, radiation; irregular calcification in the medulla; flattening/sclerosis of humeral/femoral heads.
• Bone infarction: no abnormality acutely; healed → irregular medullary calcification.

Features to note when deciding the nature of a localized bone lesion (MSK2)

1. Zone of transition — ill-defined suggests aggressive (infection); well-defined suggests neoplasm.
2. Adjacent cortex — destruction = aggressive (malignant tumor or osteomyelitis).
3. Expansion — intact well-formed cortex = slow growing (enchondroma, fibrous dysplasia).
4. Periosteal reaction — osteomyelitis, malignant bone tumor, neuroblastoma metastasis, Langerhans' histiocytosis.
5. Calcific densities within the lesion — popcorn = cartilage tumor; diffuse ill-defined = osteoid formation (osteosarcoma).
6. Soft tissue swelling.
7. Site — e.g. osteomyelitis = metaphyseal; giant cell tumor = subarticular.

High-Yield Points

• Key Point: Plain films first; "one view is no view."
• Key Point: Bone scan = sensitive for osteomyelitis, metastases (not myeloma), occult fracture; nonspecific.
• Key Point: MRI best for marrow/soft tissue; CT for fracture extent; US for tendons/soft tissue.
• Key Point: Ill-defined zone of transition + cortical destruction = aggressive (malignant/infection).
• Key Point: Subperiosteal bone resorption = hyperparathyroidism hallmark; Looser's zones = osteomalacia.
• Key Point: Rugger-jersey spine = renal osteodystrophy.
• Key Point: Rickets = frayed cupped metaphyses at knees/wrists/ankles.

Topic Summary

MSK imaging: plain radiographs are the primary modality (two views mandatory); bone scan is sensitive but nonspecific (osteomyelitis, mets, occult fracture); MRI excels at marrow/soft tissue; CT at fracture extent; US at tendons/soft tissue masses. Bone disease groups: solitary, multiple, generalized, and trabecular-pattern changes. Master the aggressive-lesion features (ill-defined zone of transition, cortical destruction, periosteal reaction) and the metabolic disease signatures (subperiosteal resorption = hyperparathyroidism, Looser's zones = osteomalacia, rugger-jersey spine = renal osteodystrophy).