AP PROJECTION: FOOT

DORSOPLANTAR PROJECTION

Pathology Demonstrated:

• Location and extend of fracture and fragment alignments, joint space abnormalities soft tissue effussions, and location of opaque foreign bodies are demonstrated.

Technical Factors:

• IR size - 24 x 30 cm (10 x 12 inches), lenghtwise
• Divided in half for AP and oblique
• Detail screen, tabletop
• Digital IR - use lead masking
• 60+- 5 kV range; or 70 to 75 kV and reduced mAs for incresed exposure latitude for more uniform density of phalanges and tarsals
• Technique and dose @ 70 kV
• mAs 2

Shielding:

• Place lead shield over pelvic area to shield gonads.

Patient Position:

• Take radiograph with patient supine; give pillow to patient for head; flex knee and place plantar (sole) of affected foot flat on cassette (IR).

AP foot projection

Part Position:

• Extend (plantar flex) foot but maintain plantar surface resting flat and firmly on cassette (IR).
• Align and center long axis of foot to CR and to long axis of portion of IR being exposed. (use sandbags if necessary to prevent cassette from slipping on tabletop.)
• If immobilization is needed, flex opposite knee also and rest against affected knee for support.

Central Ray:

• Angle CR 10 degree posteriorly (toward heel) with CR perpendicular to metatarsal (see note)
• Direct CR to base of third metatasal.
• Minimum SID is 40 inches (100 cm).

Collimation:

• Collimate to outer margins of foot on four sides.

CR and DR:

• Close collimation and lead masking are important over unused portions of image plate to prevent fogging from scatter radiation to the hypersensitivity image plate of receptor.

Note:

• A high arch requires a greater angle (15 degree) and a low arch nearer 5 degree to be perpendicular to the matatasals. For foreign body, CR should be perpendicular to IR with no CR angle.

Radiographic Criteria:

AP foot perpendicular CR

Structure Shown:

• Entire foot should be demonstrated, including all phalanges and metatarsals and the navicular, cuneiforms, and cuboids.

Position:

• Long axis of foot should be aligned to long axis of portion of IR being exposed.
• No rotation as evidenced by nearly equal distance between second through fifth metatarsals.
• Bases of first and second metatarsals generally are separated, but bases of second to fifth metatasals appear overlapped.
• Intertarsal joint space between first and secod cunieforms should be demonstrated.

Collimation and CR:

• Center of four-sided collimation field (CR) should be at the base of the third metatarsal with collimation borders, including the soft tissue surrounding the foot.
• The MTP joints generally should appear open.
• IP joints, however, may appear partially closed because of divergent rays.

Exposure Criteria:

• Optimal density and contrast with no motion should visualize sharp borders and trabecular markings of distal phalanges and tarsals distal to talus. (see higher kV technique for more uniform densities between phalanges and tarsals.)
• Sesamoid bones (if present) should be seen through head of first metatarsal.