Physics: Based on detection of the amount of x-rays which pass through the body. The more x-rays that are absorbed by a structure the denser (whiter) it will appear on the film. Thus, structures that absorb a lot of x-rays will appear white (e.g. cortical bone) and substances that absorb little or no x-ray will appear light gray to black (fat, air filled bowel)
Tissues
: Air Þ black, Fat Þ deep gray, soft tissue Þ gray, mineralization (calcifications or bone) Þ white
Application
: Provides unsurpassed resolution for depiction of fine osseous detail.Limited for soft tissue evaluation.
at least two orthogonal views are required (views taken at 90
° from each other)
Can evaluate
:
Osseous mineralization : osteosclerosis (too much bone), osteopenia (too little bone)
Osseous integrity: fracture, destruction by tumor
Relationship of bones: detect subluxation or dislocation
Osseous quality: osteomalacia
Computed Tomography
Physics: X-ray absorption also. Tomographic images presented in the axial plane are constructed by a complex mathematical analysis of many x-rays which are sent through a section of the body in many different directions. Images can also be reconstructed for view in the coronal and sagittal planes. 3-D rendering also possible.
Tissues
: Air, Fat, Fluid, Muscle, Calcium. Better soft tissue discrimination. Can differentiate muscle from fluid.
Application
: Can evaluate:
Soft tissue assessment – can visualize abscesses, tumors, foreign bodies, and penetrating injuries
Assessment of bones – evaluate full extent of complex fractures (especially around articular surfaces)
Complex anatomy evaluation – acetabulum
2D, 3D reconstruction
Magnetic Resonance Imaging (MRI)
Physics: This technique reflects the amount of mobile protons in tissue. The patient’s body or portion of the body is placed in a magnetic field and it’s nuclei (hydrogen protons of the tissue) are excited by radiofrequency pulses at angles to the fields axis. Resulting signals from the hydrogen ions are processed through a computer to produce an image. By varying the radiofrequency pulse sequences, the apparent contrast of adjacent tissues and of black and white or color values can be altered. Tissues with very little mobile protons will not have much MR signal (tendon). In contrast, muscle has many mobile protons and gives a strong signal.
Tissues
: Air, fat, fluid, muscle, tendons, ligaments, bone Þ Black (because no proton response; opposite of plain film)
detail of osseous structures limited
Application
: BEST soft tissue characterization
By choosing the appropriate sequence (difference in the signal intensity from each tissue represented on the images on a scale from black to white) the contrast between different tissues can be delineated.
Internal derangement of joints (meniscal tears, ligament and tendon tears)
Ultrasound
Physics: Technique studies tissues by detecting how sound waves interact with various tissues.
Tissues
: Water Þ transmit and do not reflect sound waves (anechoic); fat Þ reflects sound waves (hyperechoic)
Application
: Limited role for osseous and intra-articular structures. Cortex of bone reflects most sound waves therefore unable to evaluate it. Medullary canal and surround structures also cannot be assessed.
Can evaluate:
Abdominal and pelvic organs
Cystic vs. solid masses
Tendon and ligament integrity (limited)
Nuclear Radioactive Bone Scanning
Physics: Delivery of radioactive materials that are tagged on to specific carriers that deliver the radioisotopes to specific sites. In Bone scanning, technetium disphosphonate is used to be incorporated into bone.
Tissues
: bone, heart, lungs, and other organs
Application
: Provides images of the skeleton based on the rate of bone turnover.. Fractures and infections have areas of high turnover which will appear very dark. Gives more physiological than anatomic info. Lacks detailed anatomic info.