Nuclear Medicine and Imaging Modalities Review

Methyl Diphosphate (MDP) Bone Scan

  • Type of bisphosphonate.
  • Binds and is taken up by bones, making it suitable for bone scans.
  • Typical bone scan: Three phases.
  • Phases:
    • Can be done with just one phase (the bone uptake phase) by skipping the first two phases.
    • Typically done in three phases.
    • Can also be done in four phases.

White Blood Cell Label Scans

  • Uses:
    • Indium-labeled white blood cells.
    • HMPAO-labeled white blood cells.
  • Specificity and Sensitivity:
    • Similar specificity and sensitivity to Indium.
      *Nuclear medicine generally has high sensitivity.
  • Specificity is lower because increased uptake indicates a problem but not the cause.

Sensitivity vs. Specificity

  • Sensitivity is high in nuclear medicine and MRI.
  • Sensitivity >> Specificity
  • Specificity is lower in both.
  • White blood cell label increases specificity in differentiating trauma vs. infection or Charcot vs. OM.

Ultrasound

  • Non-invasive (in podiatry).

  • Very good for detecting foreign bodies.

    • First step: X-ray to identify radiodense or radiolucent foreign bodies.

  • Patient reliability:

    • Patient descriptions of the foreign body may be unreliable.
    • Always obtain an X-ray as a baseline.
      If the foreign body is not visible on X-ray, use ultrasound because it often provides a better, faster, and more definitive answer than MRI.

  • Visibility:

    • Small foreign bodies near certain tissues might blend in on MRI.

  • Artifacts:

    • Ultrasound can reveal edema or artifacts around the foreign body.
    • Dense foreign bodies (metal, glass, ceramic) show a comet tail or reverberation artifact.

MRI Sequences: T1 vs. T2

  • T1: Most specific sequence, not most sensitive.
  • T2: Most sensitive sequence because it is fluid-weighted.
  • Bone marrow edema:
    • Mainstay of diagnosing OM but not the most specific.
    • Edema on T2 does not always indicate OM (could be reactive bone marrow edema).
  • T2 is fluid-sensitive, meaning it easily detects fluid.
  • Usage:
    • T2 is used for a survey to locate potential problem areas due to its high sensitivity.
    • Findings are confirmed with T1 sequences.
      Unlikely for true OM to be negative on T1; a normal T1 should raise suspicion.

STIR Sequence

  • T2-weighted sequence with fat suppression.
  • Fluid looks bright; fat looks dark.
  • Highly similar to T2 fat sat, making differentiation difficult.
  • The fat appears more homogenously dark on STIR making it easier to read.

Gadolinium Contrast

  • STIR is never used with Gadolinium.

  • Gadolinium contrast is used with T1 sequences.

    • Compare pre-contrast and post-contrast T1 images.

  • Evaluation will show how tissues/pathologies are affected by contrast administration.

Purpose of Gadolinium Contrast

  • Differentiates viable from nonviable soft tissue or bone.
  • Assists in planning debridement or amputation.
  • Helps assess the extent of nonviable tissue, impacting treatment decisions.

Clinical Implications of Viable vs. Nonviable Tissue

  • The amount of nonviable tissue significantly affects treatment.
  • It may determine the difference between amputation and non-amputation treatment.

Cuboid Bone

  • In rocker bottom foot deformity:
    • The cuboid often becomes the most inferior bone.
  • This can lead to potential ulcerations and subsequent infections.

MRI Interpretation: T1 vs. T2

  • False statement: "Good definition of the margin of bone on T1 which disappears on T2 or T1 fat sat with contrast."
  • Correct statement: Poor definition of the margin of bone on T1, which reappears on T2 or T1 fat sat with contrast."

Sanders-Freckberg Classification

  • Relates to Charcot arthropathy.
  • Should not be confused with the Sanders classification for calcaneal fractures.
  • MPJ = 1.
  • Lisfranc joint = 2.

Muir's Angle

  • Always described as normal or increased.
  • Direction: Can be increased with the talar bisection below or above the first metatarsal.
  • If talar bisection is below the first metatarsal: Indicates a pes planus type of presentation.
  • If talar bisection is above the first metatarsal: Indicates a cavus or cavovarus presentation.

X-ray Findings in Charcot

  • Decreased calcaneal inclination.
  • Decreased cuboid height.
  • Potential for new ulceration due to cuboid position.

Distinguishing Gas in X-rays

  • Linear appearance from skin surface towards the bone.
  • Does not look like it travels across fascial planes.
  • Clinical correlation is essential to confirm the presence of gas.
  • Advanced imaging may be necessary if gas is suspected.

MRI Interpretation: Gas

  • On MRI, air or gas has a low signal.
  • Similar to tendons, ligaments, cortical bone, and fracture lines.
  • Gas has very decreased density on CT (appearing black).

Soft Tissue Emphysema

  • True statement: The arrow points to soft tissue emphysema but MRI is not the best imaging modality to assess it.
  • Best imaging modality: CT scan.

X-ray Physics and Radiation Safety

  • Changing kVp affects the contrast.
    Changing mAs does not affect the contrast.
  • As kVp increases, contrast decreases.
  • High kVp allows more penetration.
  • High mAs is not safer. It exposes the patient to more radiation.
    • MAMA: Milliamperes. ss: Seconds
      Principle: High kVp and low mAs is the safest.
  • ALARA principle: Review for boards; emphasizes minimizing radiation exposure.

Medial Oblique View

  • Standard method involves pronating the foot.
  • The beam is aimed at the fourth metatarsal base.
    If a lateral is required, supinate the foot, therefore the beam will be aimed at the first metatarsal base

Radiographic Positioning

  • Fifth Metatarsal is on Lateral.
  • Second Metatarsal is on DP.

Harris Beath View

  • Used to look at talocalcaneal collision, not calcaneonavicular.
    Calcaneonavicular is best seen on medial oblique.

Oblique View for Ankle

  • Named after the side of the foot closer to the plate.
    True Statement: A patient is standing, the beam is over here, they have their leg internally rotated- This is a medio oblique. So you supinate the foot and bring against the plate.

Calcaneal Axial View

  • Done at 45 degrees.
  • If using lead, place it on the patient’s back.
  • Used intraoperatively during calcaneal fracture ORIF to assess frontal plane rotation and compare to the leg. This frontal comparison is important when rebuilding facets as the lateral can be misleading.
  • Goal is to avoid leaving the patient in varus: Thou shalt not varus.

Ultrasound Terminology

  • Dense foreign bodies:
    *Hyperechoic
  • Fluid:
    *Hypoechoic or anechoic.

Nerve Appearance on Ultrasound

*Honeycomb appearance on short axis.
*On long axis, can be mistaken for a tendon.

Ultrasound-Guided Injections

*It’s the transducer to needle orientation that’s important.

  • Out-of-plane: Needle is perpendicular to the transducer.
  • In-plane: Needle is parallel to the transducer.

Lauge Hansen Classification

  • Four subtypes: SAD, SER, PAB, PER.

Supination-Adduction (SAD)

  • Two stages.
  • Starts laterally, goes medially.
  • Weber A.
  • Vertical medial malleolus fracture.

External Rotation (ER)

  • Example: Spiral or long oblique fibular fracture.

Three-Phase Bone Scan

  • Review
    *Performed as a three-phase system, if there is poor blood flow a fourth phase will be needed.

FDG PET Scan

  • Nuclear medicine test using a glucose analog.
  • High sensitivity and specificity for OM, especially chronic.
  • Acute OM diagnosis is easier due to inflammation.
    FDG PET has one of the best sensitivity and specificity to diagnose chronic OM.

Bone Ossicles: Os Peroneum

  • Located near the base of the fifth metatarsal.
  • Found in the peroneus longus tendon.
    Always remember to differentiate that from an Os Vesaliannum and a fifth met fracture.

Apophysis vs. Epiphysis

*Both secondary growth centers forming after birth and ossifying as the child grows.
*Apophysis is the tendon insertion site.
*Epiphysis transforms into a joint surface.

Os Trigonum

  • Large
    *Posterior impingement: Patients with large os trigonum are predisposed to posterior ankle impingement which will visually show
    *Edema, synovitis, inflammation.
    *Patient foot position.
    *Hard time putting foot in plantar flexion.

Chronic Osteomyelitis: Cloaca, Sequestrum, Involucrum

  • Cloaca: Sinus tract, radiolucent (defect).
  • Sequestrum: Dead bone, same density as other bone until remodeled, then will have a loosened rim around it.
  • Involucrum: New bone formation, will be dense.
    After inflammation which creates a radio-lucent presence the bone around will become demineralized.

Brodie's Abscess

*Appears radiolucent and will have sclerotic sides that the body is trying to contain.
*The body trying to maintain control will also denote it as chronic.

Osteomyelitis

  • Common radiographic findings rarefaction, periosteal reaction, and osteolysisare.
  • Rarefaction is a localized decreased bone density.
  • Compare to prior X-rays, zoom in closely.
  • One of the first things that is seen so comparing prior is important.
    *Cortical indistinctness is also a common finding.

The Gull Sign

  • Appears as 2 sides to an MRI, on the left is a T1, on the right is a t1 fat suppressed with contrast.
    *Necrosis of bone is present, hypointensity, contrast enhancement does not occur in dead bone.
    *Therefore, we are unable to make a good call on the T1 since blood flow is number 1 thing that must be met.
    *There must clearly be an improvement that can be seen on the bones marrow, even though perfection is not able to be met within the T1 the edema is clear.

Soft Tissue Abscess on MRI

*There would be a lack of clear T1 in the image.
*An abscess is an encapsulated, isolated spot: not cellulitis.
*The antibiotics are unable to enter within the infected, isolated spot so, as the I&D is what the person would like.

MRI vs. CT Contrast

False Statement: When it looks gas, and edema, we have the MRI where we do this through an iodine contrast. It’s the other way around. When it comes to gas, edema, bone, we want what to diagnose it correctly?
*CT- iodine.
*MRI- gadolinium.