Imaging and Special Studies
1. #### Radiation safety
1. Should be considered for every fluoroscopic case
2. Increased radiation exposure associated with
1. Imaging of larger body parts
2. Positioning the extremity closer to the x-source
FIG. 1.59 STOP-BANG screening questionnaire for identification of obstructive sleep apnea.
From Shilling AM et al: Anesthesia and perioperative medicine. In Miller MD, Thompson SR, editors:
DeLee and Drez’s orthopaedic sports medicine: principles and practice,
ed 4, Philadelphia, 2014, Saunders, p 365.
3. Use of large C-arm rather than mini C-arm
3. Factors to decrease the amount of radiation exposure
1. Minimizing exposure time
2. Using collimation to manipulate the x-ray beam
3. Use of protective shielding
4. Maximizing the distance between the surgeon and the radiation beam
5. Utilizing mini C-arm whenever feasible (associated with minimal radiation exposure)
6. Surgeon control of the C-arm
2. #### Nuclear medicine
1.
Bone scan (
Table 1.37)
1. Technetium Tc 99m phosphate complexes
1. Reflect increased blood flow and metabolism (infection, trauma, neoplasia)
2. Absorbed onto hydroxyapatite crystals in bone
2. Whole-body views and more detailed (pinhole) views possible
3. Uses
1. Subtle or stress fractures
2. Avascular necrosis
1. Hypoperfused early
2. Increased uptake in reparative phase
3. Osteomyelitis
1. Also in conjunction with gallium citrate Ga 67 or indium In 111 scan
4. THA and TKA loosening
1. Especially femoral components
2. In conjunction with gallium scan to rule out infection
5. Phase studies
6. Three-phase (or even four-phase) studies
1. Helpful for reflex sympathetic dystrophy and osteomyelitis
2. First phase (blood flow, immediate)
1. Blood flow through the arterial system
3. Second phase (blood pool, 30 minutes)
2. Gallium scan
1. Equilibrium of tracer throughout the intravascular volume
1. Third phase (delayed, 4 hours)
1. Displays sites of tracer accumulation
2. Localizes in sites of inflammation and neoplasia
1. Exudation of labeled serum proteins
3. Delayed imaging required (24–48 hours or more)
4. Less dependent on vascular flow than technetium
1. May identify foci otherwise missed
5. Difficulty differentiating cellulitis from osteomyelitis
1. Indium scan
1. Labeled WBCs (leukocytes)
2. Uses
3. Collect in areas of inflammation
4. Do not collect in areas of neoplasia
5. Acute infections (e.g., osteomyelitis)
6. Possibly total joint arthroplasty (TJA) infections
Table 1.37 Nuclear Medicine Studies Study Uses Comments --- Bone scan | Subtle fractures Avascular
necrosis Osteomyelitis Total joint
loosening Osteochondritis
| Three-phase scan useful for osteomyelitis, reflex sympathetic dystrophy, acute scaphoid fractures
Gallium
| Inflammation Neoplasms
| Localizes in sites of inflammation Requires prolonged uptake
Indium (In 111)
| Acute
infections Possible
arthroplasty infections
| Labeled WBC uptake in areas of infection
1. Radiolabeled monoclonal antibodies
1. May identify primary malignancies and metastatic disease
2. DVT/PE scan
1. Radioactive iodine
1. Labels fibrinogen in clot on scanning
2. Inaccurate near surgical wounds
3. Single-photon emission computed tomography (SPECT)
1. Scintigraphy with CT to evaluate overlapping structures
1. Femoral head osteonecrosis
2. Patellofemoral syndrome
3. Spondylolysis
1. #### Arthrography
1. Commonly used in association with advanced imaging (CT or MRI)
2. Improves sensitivity of intraarticular soft tissue pathology (labral tear in shoulder, triangular fibrocartilage complex [TFCC], and intercarpal ligament tears in wrist)
3. Also frequently used in pediatric population
4. Hip
1. Aspiration for infection
2. #### MRI
1. Following reduction of developmental dysplasia of the hip (DDH)
2. Assessing deformity in Legg-Calvé-Perthes disease
1. Excellent for evaluating soft tissues and bone marrow
1. Study of choice for evaluating knee ligamentous/meniscal injuries and shoulder cuff injuries
2. Ineffective in evaluating trabecular bone and cortical bone
1. These tissues have virtually no hydrogen nuclei.
3. Used to evaluate osteonecrosis, neoplasms, infection, and trauma
4. Contraindications
1. Pacemakers
2. Cerebral aneurysm clips
3. Shrapnel or hardware, in certain locations
5.
Basic principles of MRI (Tables
1.38
through
1.41)
1. Radiofrequency pulses on tissues in a magnetic field
2. Images in any desired plan
3. Nuclei with odd numbers of protons/neutrons (with a normally random spin) aligned parallel to a magnetic field
Table 1.38 Magnetic Resonance Imaging Terminology Term Explanation --- T1 | Time constant of exponential growth of magnetism; T1 signal measures how rapidly a tissue gains magnetism T2 | Time constant of exponential decay of signal after an excitation pulse; a tissue with a long T2 signal (such as that with a high water content) maintains its signal (is bright on T2-weighted image) T2 ∗ | Similar to T2 but includes the effects of magnetic field homogeneity TR | Time to repetition; the time between successive excitation pulses; short TR
<80ms, long TR >80 ms
TE
| Time to echo; the time that an echo is formed by the refocusing pulse; short TE <1000ms, long TE >1000
NEX
| Number of excitations; higher NEX results in decreased noise with better images
FOV
| Field of view
Spin- echo
| A commonly used pulse sequence
FSE
| Fast spin-echo; a type of pulse sequence
GRE
| Gradient-recalled echo; a type of pulse sequence
1. Field strength: 0.5–15 T (1 T = 10,000 G)
1. #### 3.0 T has nine times greater proton energy than 1.5T
2. Nuclear magnetic moments of these particles are deflected by radiofrequency pulses; deflection results in an image.
3. The use of surface coils decreases the signal-to-noise ratio.
1. Body coils are used for large joints
2. Smaller coils are available
4. Sequences developed to demonstrate the differences in T1 and T2 relaxation between tissues
1. Dark on T1- and bright on T2-weighted images
1. Water
2. Cerebrospinal fluid
3. Acute hemorrhage
4. Soft tissue tumors
5. Tissues showing similar intensity on both T1- and T2-weighted images:
1. Dark: cortical bone, rapidly flowing blood, fibrous tissue
2. Gray: muscle and hyaline cartilage
3. Bright: fatty tissue, nerves, slowly flowing (venous) blood, bone marrow
Table 1.39 Signal Intensities on Magnetic Resonance Imaging Tissue Appearance on T1- Appearance on T2-Weighted Image Weighted Image --- Cortical bone | Dark
| Dark Osteomyelitis | Dark
| Bright Ligaments | Dark
| Dark Fibrocartilage | Dark
| Dark Hyaline cartilage | Gray
| Gray Meniscus | Dark
| Dark Meniscal tear | Bright
| Gray Yellow bone marrow (fatty-appendicular) | Bright
| Gray Red bone marrow (hematopoietic-axial) | Gray
| Gray Marrow edema | Dark
| Bright Fat | Bright
| Gray Normal fluid | Dark
| Bright Abnormal fluid (pus) | Gray
| Bright Acute blood collection | Gray
| Dark Chronic blood collection | Bright
| Bright Muscle | Gray
| Gray Tendon | Dark
| Dark Intervertebral disc (central) | Gray
| Bright Intervertebral disc (peripheral) | Dark
| Gray
Modified from Brinker MR, Miller MD:
Fundamentals of orthopaedics,
Philadelphia, 1999, Saunders, p 24.
6. T1-weighted images best for demonstrating anatomic structure
7. T2-weighted images best for contrasting normal and abnormal tissues
8. Magic angle phenomenon:
1. Tendon or ligament tissue oriented near 55 degrees to the field produces bright T1-weighted
images.
2. False appearance of pathologic process
3. Most common in shoulder, ankle, knee
9. Techniques for identifying contrast between fluid and nonfluid elements (e.g., bone, fat)
1. STIR imaging
2. Fat-suppressed T2-weighted imaging
10. Specific applications
1. Osteonecrosis
1. Highest sensitivity and specificity for early detection
1. Detects early marrow necrosis
2. Detects ingrowth of vascularized mesenchymal tissue
2. Specificity of 98% and high reliability for estimating age and extent of disease
3. Diseased marrow dark on T1-weighted images
1. Allows direct assessment of overlying cartilage
2. Infection and trauma
1. Excellent sensitivity to increased free water
2. Shows areas of infection and fresh hemorrhage
1. Dark on T1-weighted images, bright on T2-weighted images
3. Excellent (accurate and sensitive) for occult fractures
1. Particularly in hip in elderly patients
3. Neoplasms
1. MRI has many applications in the study of primary and metastatic bone tumors.
2. Primary tumors are well demonstrated.
1. Particularly tumors in
11. Spine
soft tissue (extraosseous and marrow)
1. Used in evaluating skip lesions and spinal metastases
1. Nuclear medicine study remains the procedure of choice for seeking metastatic foci in bone.
2. Demonstrates benign bony tumors
1. Typically bright on T1-weighted images and dark on T2-weighted images
3. Demonstrates malignant bony lesions
1. Often bright on T2-weighted images
4. Differential diagnosis is best made on the basis of plain radiographs.
1. Disc disease is well demonstrated on T2-weighted images.
1. Degenerated discs lose water.
Table 1.40 Magnetic Resonance Imaging of Bone Marrow Disorders Disorder Pathologic Examples MRI Features Changes --- Reconversion | Yellow →
red
| Anemia,
metastasis
| ↓ T1-
weighte intensit
Marrow infiltration
| |
Tumor, infection
| ↓ T1-
weighte intensit
Myeloid depletion
| |
Anemia,
chemotherapy
| ↓ T1-
weighte intensit
Marrow edema
| |
Trauma, complex regional pain syndrome
| ↓ T1-
weighte intensit
↑ T2-
weighte intensit
Marrow ischemia
| |
Osteonecrosis
| ↓ T1-
weighte intensit
↑, Increased; ↓, decreased.
Table 1.41 Magnetic Resonance Imaging Changes of Meniscal Disease Disease Characteristics Group --- I | Globular areas of hyperintense signal II | Linear hyperintense signal III | Linear hyperintense signal that communicates with the meniscal surface (tears) IV
| Vertical longitudinal tear/truncation
2. Appear dark on T2-weighted images
3. Extent of herniation of discs is also well shown.
4. Recurrent disc herniation is best diagnosed with gadolinium MRI scan.
1. Differentiation from scar
2. Bone marrow disorders
3. T1-weighted image
1. Scar: decreased signal
2. Free fragment: increased signal
3. Extruded disc: decreased signal
4. T2-weighted image
1. Scar: increased signal
2. Free fragment: increased signal
3. Extruded disc: decreased signal
5. MRI is most sensitive for diagnosing early discitis.
1. Decreased signal on T1-weighted images, increased signal on T2-weighted images
1. #### Other imaging studies
1.
Best demonstrated by MRI (poor specificity) (see
Table 1.40)
1. Computed tomography
1. Demonstrates details of bony anatomy better than any other study
2. Hounsfield units used to identify tissue types
1. −100 HU = air
2. −100–0 HU = fat
3. 0 HU = water
4. 100 HU = soft tissue
5. 1000 HU = bone
3. In spine, shows herniated nucleus pulposus better than myelography alone
1. CT may be helpful differentiating disc herniation from scar
2. IV contrast material is taken up in scar tissue but not in disc tissue.
4. Frequently used with contrast material
1. Arthrographic CT, myelographic CT
5. CT digital radiography (CT scanography)
1. Accurate demonstration of leg length discrepancy with minimal radiation exposure
1. Particularly when joint contractures exist (lateral scanography)
6. Images distorted by metal implants
2. Ultrasonography: uses continue to expand
1. Shoulder: evaluation of rotator cuff tears
2. Hip
3. Knee
1. Diagnosis and follow-up of DDH
2. Dynamic examination of femoroacetabular impingement
3. Determination of articular cartilage thickness
4. Identification of intraarticular fluid
1. Soft tissue masses
2. Hematoma
3. Tendon rupture
4. Abscesses
5. Foreign body location
6. Intraspinal disorders in infants
7. Intraarticular injections
3. Myelography
1. More invasive than MRI but shows excellent detail
2. Useful in patients with contraindications to MRI
3. Useful in failed back (surgery) syndrome
4. Can be used with other studies such as CT
4. Discography
1. Use controversial
2. Helpful for evaluating symptomatic disc degeneration
3. Pathologic discs: reproduction of pain with injection and characteristic changes on discograms
4. Commonly used with CT
5. Measurement of bone density (noninvasive)
1. Single-photon absorptiometry
1. Cortical bone density is inversely proportional to quantity of photons passing through it.
2. Radioisotope iodide 125 (125I) emits a single energy beam of photons.
1. 125I passes through bone.
2. A sodium iodide scintillation counter detects the transmitted photons.
3. Denser bone attenuates the photon beam.
1. Fewer photons reach the scintillation counter.
4. Best used in the appendicular skeleton
1. Radius: diaphysis or distal metaphysis
5. Findings are unreliable in the axial skeleton
1. Soft tissue depth alters the beam.
2. Dual-photon absorptiometry
1. Also an isotope-based method
2. Allows for measurement of the axial skeleton and the femoral neck
1. Accounts for soft tissue attenuation
3. Quantitative CT
1. Preferred for measurement of trabecular bone density
1. Trabecular bone is at greatest risk for early metabolic changes
2. Simultaneous scanning of phantoms of known density
4. DEXA
