- This projection provides a profile image of this sesamoid bone at the first MTP joint for evaluation of extend of injury.
Note: A lateral of first digits in dorsification also may be taken to evaluate these sesamoids.
Tecnical Factors:
- IR size-18 x 24cm (8 x 10 inches), crosswise
- Divide in half if combined with another projection
- Detail screen, tabletop
- Digital IR - used lead masking
- 50 to 60 kV range
- mAs 2
Shielding:
- Place leas shield over pelvic area to shield gonads.
Patient Position:
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| Tangential Sesamoids: Lewis Method |
- Take radiograph with patient prone, provide pillow for head and small sponge or folded towel under lower leg for patient comfort.
Part Position:
- Dorsiflex the foot so that the plantar surface of the foot forms about a 15 to 20 degrees angle from vertical.
- Dorsiflex the first digit (great toe) and rest on cassette to maintain position.
- Ensure that long axis of foot is not ratated; place sandbags or other support on both sides of foot to prevent movement.
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| Tangential Sesamoids: Holly Method |
Note: This is an comfortable and often painful position; do not keep patient in this position longer than necessary.
Central Ray:
- CR perpendicular to IR, directed tangentially to posterior aspect of first MTP joint (depending on amount of dorsiflexion of foot, may need to angle CR slightly for true tangential projection)
- Minimum SID of 40 inches (100 cm)
Collimation:
- Collimate closely to area of interest. Include at least the first, second, and third distal metatasals for possible sesamoids but with CR at first MTP joint.
Alternate Projection:
- If patient cannot tolerate the above prone position, this may be taken in a reverse projection with patient supine with the use of a long strip of gauze for patient to hold the toes as shown. CR again would be directed tangential to posterior aspect of first MTP joint. Use support to prevent motion. This however, is not a desirable projection because of the increased OID with accompanying magnification and loss of definition.
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| Tangential Sesamoids: Lewis Method |
Structure Shown:
- Sesamoids should be seen in profile free of superimposition.
Position:
- Borders of posterior margins of first to third distal metatasals are seen in profile, incading correct dorsiflexion of foot.
Collimation and CR:
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| Tangential Sesamoids: A. Holly Method B. Sesamoids with fracture (arrow) |
- A minimum of the first three distal metatasals should be included in collimation field for possible sesamoids, with the center of the four-sided collimation field (CR) at the posterior portion of the MTP joint.
- Centering and angulation are correct if the sesamoids are free of any bony superimposition and open space is demontrated between sesamoids and the metatasal.
Exposure Criteria:
- No motion as evidenced by sharp bony cortical margins and detailed trabeculae.
- Optimal contrast and density will allow visualization of bony cortical margins and trabeculae and soft tissue structure without the sesamoids appearing overexposed.
