MSK Radiology- LECT 2
Foundations of the Musculoskeletal System
The Function of Bones: Bones provide the primary structural framework of the body in a gravitational environment. Without them, the human body would lack rigidity and essentially be "socially just like jelly."
Muscles and Locomotion: Muscles are the drivers of movement, exerting force on the bones to allow for the body to physically move through space.
Connective Tissues: * Tendons: These structures connect muscle to bone. * Ligaments: These structures connect bone to bone and provide stability to joints. * Cartilage: A smooth, gelatinous substance found between joint surfaces. Its primary function is to reduce friction and ensure the smooth gliding of joints.
Osteoarthritis (OA): This condition occurs when the protective cartilage wears down, leading to bone-on-bone friction and pain.
Diagnostic Imaging Modalities and Physics
X-Ray (Radiographs): * Characteristics: The "bread and butter" of MSK imaging. It is low-cost, incredibly fast (taking a few seconds), and provides high resolution for bone details, fractures, and joint spaces. * Resolution Limitations: Poor at detecting soft tissue details and gas (subcutaneous emphysema). * Terminology: * Radiopaque: Appears bright/white. This occurs when an object (like bone cortex or trabeculae) attenuates or blocks X-rays. * Radiolucent: Appears dark. This occurs when X-rays pass through easily (like air in the lungs).
Computed Tomography (CT-Scan): * Characteristics: Provides 3D representations of the body through cross-sectional images. It offers better resolution of soft tissues than X-rays but is primarily used for complex bone detail and finding fractures missed on standard X-rays. Takes a few minutes to perform. * Contrast: Can be used to highlight arteries, veins, or tumors. * Terminology: * Hyperdense: Bright. * Hypodense: Dark. * Isodense: Relative term meaning the same brightness as another structure (e.g., "isodense to muscle").
Magnetic Resonance Imaging (MRI): * Characteristics: Has the highest soft tissue resolution of any modality. Sequences (T1 and T2) are used to visualize ligaments, muscles, meniscus, and marrow detail. It is a slow process, taking approximately minutes or more. * Physics of T1 vs. T2 Sequences: * Fluid Rule: Fluid is bright on T2 and dark on T1. * Fat Rule: Fat is bright on both T1 and T2, though slightly darker on T2. * Gadolinium Contrast: Always used on T1 sequences because fluid on T2 would interfere with identifying the contrast. * Terminology: * Hyperintense: Bright. * Hypointense: Dark.
Ultrasound: * Characteristics: Uses a piezoelectric crystal to generate sound waves through electricity. It is portable (handheld units cost roughly ), good for joint effusions, and includes Doppler flow to detect blood flow (useful for DVT studies or tumors). * Terminology: * Hyperechoic: Bright (bouncing back many echoes). * Hypoechoic: Dark (less echoes). * Anechoic: No echoes; characteristic of pure water or simple cysts.
Nuclear Medicine: * Characteristics: Involves injecting a radioactive tracer. The patient becomes the source of radiation, and the machine reads what is emitted. It is used for physiological imaging and detecting metastasis. * SPECT CT: Combines a CT function with nuclear medicine to overlay 3D physiological data onto anatomical images.
Imaging Resolutions and Measurements
Spatial Resolution: The ability to distinguish between two objects placed closely together. Lower numeric values indicate better resolution. * CT / Echocardiography: High spatial resolution ( to ). * PET / PET CT: Low spatial resolution (approximately ). * Mammography: One of the best for spatial resolution, measured in line pixels per millimeter ().
Contrast Resolution: The ability to distinguish between different densities or colors. MRI and CT are superior; PET/PET CT is also high for radioactive vs. non-radioactive substances.
Temporal Resolution: Differentiation in time (live imaging). * Best: Live fluoroscopy and echocardiography (real-time flow). * Worst: PET/CT or MR (takes time to acquire slices).
Anatomy and Specific Fractures
Upper Extremity: * Shoulder: Includes the clavicle, acromion process (part of the scapula), coracoid process (anterior protrusion), and glenoid (shoulder joint socket). * Humerus: Has an anatomic neck and a surgical neck. The bicipital groove lies between the greater and lesser tubercles. * Carpal Bones (Wrist): 1. Scaphoid: Most lateral carpal bone. 2. Lunate: Moon-shaped/crescent on lateral view. 3. Triquetrum. 4. Pisiform. 5. Hamate: Identified by its "hook." 6. Capitate: Central bone. 7. Trapezoid. 8. Trapezium: Base of the thumb (think "trapeze gymnast" using thumbs).
Salter-Harris Growth Plate Classifications (Pediatric): * Type I (S): Straight across the physis (growth plate). * Type II (A): Above; the fracture goes through the physis and the diaphysis/shaft. * Type III (L): Lower; the fracture goes through the physis and the epiphysis. * Type IV (T): Through; the fracture passes through all three: diaphysis, physis, and epiphysis. * Type V (ER): Erased/Crushed; a compression injury of the physis (worst prognosis).
Eponymous Fractures: * Colles Fracture: Distal radius fracture with posterior/dorsal displacement ("dinner fork" deformity). * Smith Fracture: Distal radius fracture with anterior/volar displacement ("reverse Colles"). * Barton Fracture: A fracture involving the articular surface of the distal radius. * Jones Fracture: Fracture of the metatarsal, specifically to above the base (tubercle). Known for poor healing and often requiring surgery. * Segond Fracture: A small fleck of bone off the lateral tibial plateau; highly indicative of an ACL injury.
Bone Pathologies and Tumors
Bone Lesion Categories: * Lytic: Dark spots (bone destruction). * Sclerotic: Bright spots (excessive bone formation).
Tumor Types: * Giant Cell Tumor: Most common lytic lesion that crosses the growth plate; expansile; can metastasize to the lungs. * Osteosarcoma: Malignant; features a "sunburst" pattern or Codman's triangle (pushing periosteum away from cortex). * Ewing Sarcoma: Malignant; features a permeative or "eaten away" look. * Osteoid Osteoma: Benign; classic vignette is "night pain relieved by NSAIDs." * Enchondroma: Cartilaginous neoplasm, common in hands and feet.
Paget's Disease: A state of rapid bone turnover creating thick but weak architectural bone. High risk for malignant transformation and fractures.
Osteomyelitis: Infection of the bone. MRI is the most sensitive imaging study. A clinical rule: if an ulcer "probes to bone," it is osteomyelitis until proven otherwise.
Arthritis Comparison
Rheumatoid Arthritis (RA): * Inflammatory and erosive. * Characterized by "ulnar drift" and periarticular erosions (takes a "bite" out of bone adjacent to the joint). * Systemic/Symmetric.
Osteoarthritis (OA): * Wear-and-tear injury. * Characterized by loss of joint space (medial/superior in the knee), sclerosis (brightening of bone), and osteophytes (bone spur formation).
Non-Accidental Trauma (NAT) - Pediatrics
Definition: Physical abuse markers in pediatric patients.
Red Flags: * Multiple fractures of varying ages (different stages of healing). * Posterior rib fractures (from squeezing). * Metaphyseal corner fractures ("bucket handle" fractures). * Brain bleeds in infants beyond days old (associated with Shaking Baby Syndrome).
Questions & Discussion
Question: What is the difference between T1 and T2 sequences regarding brightness?
Response: The primary way to tell them apart is fluid. Fluid is bright on T2 and dark on T1. While fat is bright on both, T2 is specifically a fluid-sensitive sequence.
Question: Are torus (buckling) fractures exclusive to kids?
Response: Yes, usually, because adult bones are too calcified and hard to bend. They fracture completely. Pediatric bones can deform plastically.
Question: If you're worried about metastasis, do you prefer a PET scan or Nuclear Medicine?
Response: You can combine both with a SPECT CT. It overlays the CT image with nuclear medicine color spots to give the best "lay of the land" for radiologists and oncologists.