Imaging Flashcards
When is imaging indicated?
Only when positive findings will influence decision making.
Sensitivity of plain film radiographs
Not sensitive to subtle pathology. Significant change to bone must occur before radiographs will reveal it. False negatives for stress injuries, metabolic bone disease, infections, non-displaced fractures, and neoplasms is high.
What is the standard for acute injuries?
MRI, due to low potential for false negatives
Symptomatic areas taking away attention from other areas
Sometimes the more obvious or more symptomatic areas obscure the need for imaging or other injuries (ankle sprains accompanied by foot fractures)
What are radiographs useful for
Demonstrate bony pathology or relationship of foreign objects to skeletal structures. (Some bony tumors/infections, fractures, avulsion fractures, and late stage stress fractures)
Tissue colors on radiograph
Air absorbs the least radiation and appears most radiolucent (has darkest appearance). Fat, fluids, soft tissue, muscle, bone, and metal will progressively absorb more radiation (appearing more white or radiopaque)
Minimal radiographic examination
At least 2 views of the body part at right angles to each other
When to use more sensitive imaging, even after using x-rays
The sensitivity is not great and if your pretest probability highly suggests the pathology
Ottawa Knee Rules
- Aged 55 or older
- TTP at fibular head
- Isolated TTP of patella
- Inability to flex knee to 90
- Inability to bear weight (4 steps; 2 on each leg, regardless of limping. Immediately and at presentation)
Ottawa Ankle/Foot Rules
- Bony TTP along 6 cm of posterior edge or tip of medial malleolus
- Bony TTP along 6 cm of posterior edge or tip of medial malleolus
- Inability to bear weight immediately and in ER for 4 steps
- Bony TTP at base of 5th
- Bony TTP at navicular bone
Canadian C-Spine Rules
High Risk Factors:
Age > 65, dangerous mechanism (fall >1 m or 5 stairs; axial load to head; MVC > 100 k/h, rollover, or ejection; motorized recreational vehicle; bike collision), paresthesias in extremities
Low Risk Factors (Don’t Need Images):
Simple rear end MVC, sitting while in ER, ambulatory at any time, delayed onset neck pain, absence of midline c-spine TTP
Neck Rotation:
Can they rotate 45 degrees to L and R
Tomography
Tomography is a type of body section radiography that allows for visualization of lesions down to 1 mm or lesions that might be obscured by overlying structures when imaged with plain radiographs.
Conventional tomography and compute tomography (CT scans)
CT Scans Image What?
The most striking difference is between bone and soft tissue. The ability to differentiate types of soft tissue such as tendons and ligaments is limited. Well suited to examine the spine and extremities. Much better than MRI for cortical bone details.
CT vs MRI for spine
CT provides better details for things like spinal osteophytes, spinal fractures (pars or burst). MRI better for disc herniations.
MRI
Images both bone and soft tissue structures with excellent resolution in 3 dimensions
What is signal intensity?
The strength of the radio wave that tissue emits following removal of radio frequency pulse. The strength of the radio wave produces either bright high-signal-intensity or dark-low-signal intensity images. Signal intensity in a specific tissue will depend both on the T1 and T2 relaxation times and the relative concentration of hydrogen ions.
T1 vs T2 relaxation time
T1 or longitudinal relaxation refers to return of protons to equilibrium following application and removal of RF pulse. The T2 or transverse relaxation time describes the associated loss of coherence between individual protons immediately following application of the RF pulse.
Signal Intensity for T1-weighted
Subacute hemorrhage and fat have high signal intensity. Bone has higher fat content and appears bright. Abscesses and cysts have medium appearance, other soft tissues have low intensity. Particularly well suited for details of anatomy (clearly delineate architecture of a variety of soft tissue structures)
Signal Intensity for T2 images?
Fluids have high signal. The “2” in H2O can help to remember fluids and fluid-containing structures appear bright (bursae, inflamed tendons, tumors, abscesses). T2, overall, reveal less detail in soft tissue structures and are less suited for fine anatomic features.
Proton-density-weighted
Combine the properties of T1 and T2 images and produce good anatomic detail with little tissue contrast.
How to deal with high signal intensity of fat in certain MRI sequences (differentiate fat from fluid)
Fast spin echo (FSE) T2 produce particularly bright fat images. Fat suppression or saturation produces a dull appearance to fat for better contrast with bright fluids. Inversion recovery (STIR) can also reduce signal from fat while increasing signal from fluid.
Spin echo pulse sequences
Inclue T1, T2, and proton-density weighted images. FSE proton density MRI’s reveal anatomic detail, FSE T2 weight MRI’s with fat saturation are well suited to reveal marrow pathology.
Contraindications to MRI
Implants or ferromagnetic metal that could become dislodged (cerebral aneurysm clips, pacemakers, shrapnel, unstable orthopedic hardware, undetected metal slivers a machinist may have lodge in eye).
Best MRI sequence for articular cartilage
FSE or gradient echo
Adding intra-articular contrast
Increases the sensitivity to diagnose RTC tears, labra lesions, or articular cartilage injuries
Gold standard for stress fractures
MRI (STIR and T2 weighted MRI sequences)
Bone Scan
Also called scintigraphy. You inject a radiopharmeceuitical substance into the patient and then you scan the patient. Areas of bone that are most metabolically active (tumors, infections, healing fractures) will show the most uptake and appear dark on scan.
Ligaments, tendons, and menisci on MRI
They are all low in signal in traditional MRI pulse sequences (dark). Degeneration or tearing manifests as increased signal intensity in affected areas.
Best imaging for menisci
Proton density sequences used to evaluate menisci (due to their high sensitivity in detecting fibrocartilage signal).
Best imaging for detecting acute injury
A fluid-sensitive sequence (fat saturated T2 or inversion recovery) allows visualization of marrow and soft tissue edema
3.0T magnets vs 1.5T magnets
3.0 have substantially more signal to noise and 1.5 allow for higher resolution protocols.
Marrow Edema with ACL tear
Nearly always have associated marrow edema in posterior margin of lateral tibial plateau and/or lateral femoral condyle.
Radiograph views of knee and what they show?
Routine Views
AP view: joint space
Lateral view with partial flexion: patella and joint effusions (seen superior to patella and anterior to femur)
Ancillary Views
Sunrise/Merchant View: relationship of patella to anterior femur
Tunnel view: tibial spines and femoral condyles
Pittsburgh Knee Rules
Blunt trauma or a fall as mechanism of injury PLUS either of the following:
- Younger than 12 or older than 50
- Inability to walk 4 weight bearing steps in the ER
Meniscus Lesion on MRI
The menisci should be black, you’ll see a small while marking on it.
Findings with OA on X-Ray
- Joint space narrowing
- Sclerotic edges (white)
- Bone spurs / osteophytes
- Subchondral cysts
How to differentiate fabella vs loose body?
Fabella is smooth and teardrop shaped. Loose body will be rough or irregular edges.
Identifying bipartite patella versus fracture?
Bipartite patella is usually at the top of the patella and smooth, can be at different locations though.
Segond Fracture
Cortical avulsion fracture of the proximal, lateral tibia just posterior to the insertion of the ITB from pulling of the ITB and lateral knee joint capsule resulting from excessive IR and varus stress (different mechanism that usual ACL tear).
75-100% associated with ACL tear and 66-75% associated meniscal tears. MRI is then indicated.
Appearance of Soft Tissue on MRI?
Ligaments, tendons, and menisci are uniformly low in signal (dark) on traditional MRI pulse sequences.
In the knee, what MRI sequences work best for different tissues?
- Proton density primarily for menisci (high sensitivity detecting fibrocartilage)
- Fluid-Sensitive sequence for acute edema to visualize marrow and soft tissue edema (fat-saturated T2 or inversion revovery sequence)
Views for the ankle and foot on radiograph
Ankle
1. AP
2. Lateral
3. Internal oblique or Mortise
Foot
1. AP
2. Lateral
3. Oblique
These are different views
Imaging for high risk head injury?
In the high risk population, immediate CT scans are appropriate and necessary
Cervical x-rays with head trauma?
Only needed if cervical trauma is reported or suspected.
