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HLT-RADI64-07 · Lesson 6 · 30 clock hours · field

Lower Extremity Radiography: Foot, Ankle, Knee, and Hip

Objective. The student will produce diagnostic-quality radiographs of lower extremity structures in all required projections for the ARRT Limited Scope extremity pathway, applying correct patient positioning and technical factor selection.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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Lower Extremity Radiography: Foot, Ankle, Knee, and Hip

Lower extremity radiography covers a wide range of structures, from the small bones of the foot to the large ball-and-socket hip joint. Producing diagnostic-quality images requires accurate patient positioning, appropriate technical factor selection, and consistent attention to patient safety and radiation protection.

General Principles Before You Begin

Always confirm patient identity using at least two identifiers before any exposure. Review the clinical indication so you understand what the radiologist needs to see. Place lead markers (R or L) on the image before exposure — never add them digitally after the fact. Collimate to the region of interest to reduce patient dose and improve image contrast. Use the shortest exposure time practical to minimize motion blur, especially in patients who cannot hold still.

Foot Radiography

The standard foot series typically includes the AP (dorsoplantar), oblique, and lateral projections.

AP (Dorsoplantar): The patient sits or lies with the knee flexed and the plantar surface of the foot flat on the detector. The central ray is angled approximately 10 degrees toward the heel to compensate for the natural arch of the foot. This projection demonstrates the phalanges, metatarsals, and most of the tarsals.

Oblique: Rotate the foot medially about 30 to 45 degrees with the plantar surface still on the detector. This opens the spaces between the metatarsals and demonstrates the lateral tarsal bones with less superimposition.

Lateral: The patient lies on the affected side; the foot is in a true lateral position. The lateral view shows the calcaneus, the ankle mortise in profile, and the arch of the foot.

Weight-bearing foot views are frequently ordered to evaluate flat foot (pes planus) or for surgical planning. For these, the patient stands on the detector with full body weight applied. Use a low-speed, high-detail receptor and reduce scatter by collimating tightly.

Ankle Radiography

The routine ankle series includes AP, mortise (AP oblique with 15–20 degrees internal rotation), and lateral projections.

AP: The leg is extended, the foot is dorsiflexed to 90 degrees when possible, and the central ray is directed to the ankle joint.

Mortise view: Rotate the entire leg and foot 15 to 20 degrees internally so that the intermalleolar line is parallel to the detector. This opens the ankle mortise (the space between the talus and both malleoli) for evaluation of fracture, ligament injury, and joint space symmetry.

Lateral: The affected side is placed down; the medial and lateral malleoli are superimposed. The central ray enters the medial malleolus.

Weight-bearing ankle projections may be ordered for stress or instability evaluation.

Knee Radiography

The routine knee series includes the AP, lateral, and sometimes a PA axial (tunnel) or tangential (sunrise/Merchant) view for specific pathology.

AP: The patient is supine, leg extended, and the central ray is directed to the joint space — typically about 1 cm below the apex of the patella. A slight caudal angle (about 5–7 degrees) may be used to open the joint space in patients with a thick thigh.

Lateral: The patient is on the affected side with the knee flexed approximately 20 to 30 degrees. The femoral condyles should be superimposed. The patella should be visible in profile.

Weight-bearing AP knees are commonly ordered bilaterally for osteoarthritis assessment. The patient stands with equal weight on both feet; bilateral images allow the radiologist to compare joint space narrowing side-to-side.

Hip and Pelvis Radiography

Hip imaging typically includes the AP hip or AP pelvis, and a lateral projection. The cross-table lateral (also called the trauma lateral or horizontal beam lateral) is used when the patient cannot be moved or rotated due to suspected fracture.

AP Pelvis/Hip: The patient is supine. Rotate the feet internally 15 to 20 degrees to bring the femoral necks parallel to the image receptor — this projects the femoral neck in its true length and profile. If internal rotation is contraindicated (suspected hip fracture), do NOT force the leg; image it in the position of comfort and note this on the requisition.

Frog-leg lateral (Lauenstein method): The hip is abducted, knee flexed, and the plantar surface of the foot is placed adjacent to the opposite knee. This is used only when fracture has been ruled out.

Cross-table lateral hip: The patient remains supine. The unaffected leg is elevated out of the field. The detector is placed vertically against the lateral aspect of the hip and the horizontal beam enters medially. This projection avoids moving a potentially fractured hip. Staff must step behind the protective barrier or use a right-angle technique to stay out of the primary beam.

Technical Factor Selection

Lower extremity exams span a wide range of tissue thickness. Small foot bones require lower kVp and mAs than the hip and pelvis, which are among the highest-dose projections in general radiography. Always use a grid for hip and pelvis projections because part thickness exceeds the threshold where scatter significantly degrades contrast (typically above approximately 10 cm). Use non-grid technique for the foot and ankle in most adults.

Radiation Protection

Gonadal shielding should be provided when it will not obscure the area of clinical interest and when the patient is of reproductive age — follow your facility's policy and current ACR/ARRT guidance. Collimate precisely. For cross-table lateral hip, ensure all personnel are outside the primary beam. Document dose indicators and repeat exposures in the logbook.

Image Evaluation

After each exposure, evaluate for positioning accuracy, collimation, exposure indicators, and absence of motion. If a repeat is necessary, identify the cause, correct it, and obtain supervisor approval before re-exposing. Document all repeats.

Key terms8

Ankle mortiseThe U-shaped socket formed by the distal tibia and fibula that receives the dome of the talus; best visualized with 15–20 degrees of internal rotation in the mortise projection.
Weight-bearing projectionA radiographic view taken while the patient bears full body weight on the part being imaged, used to assess structural alignment under load (e.g., flat foot, knee joint space narrowing).
Cross-table lateral hipA horizontal-beam lateral projection of the hip performed with the patient supine and the detector placed vertically at the lateral hip, used when the patient cannot be rotated due to possible fracture.
Internal rotation (hip/ankle)Rotating the limb medially so the toes point inward; in hip radiography this brings the femoral neck parallel to the detector; in ankle radiography it opens the mortise joint space.
GridA device placed between the patient and the image receptor that absorbs scatter radiation and improves image contrast; required when body part thickness exceeds approximately 10 cm.
Frog-leg lateral (Lauenstein)A lateral hip projection obtained by abducting the hip and flexing the knee, placing the foot near the opposite knee; contraindicated when hip fracture is suspected.
CollimationRestricting the x-ray beam to the area of clinical interest using adjustable lead shutters, which reduces patient dose and scatter radiation and improves image contrast.
Lead markerA radiopaque right (R) or left (L) identifier placed on the image receptor before exposure to permanently identify the patient's anatomical side; required by professional and legal standards.
In class: Perform a minimum of 15 lower extremity exams across at least three anatomical regions, with emphasis on weight-bearing techniques where clinically indicated, and document each exam in the logbook. · Review AP pelvis and hip images with the supervising technologist, identifying internal and external rotation landmarks and their clinical significance for diagnosing hip pathology. · Practice cross-table lateral hip positioning on a positioning phantom or mannequin under supervision before applying to patients, noting radiation protection measures for the patient and staff.
Materials: Radiographic table and digital receptor, positioning sponges, grid cassettes for hip/pelvis, lead markers, positioning mannequin or phantom (if available), Bontrager's Handbook, image evaluation rubric, clinical logbook
Check: Formal competency checkoff for one lower extremity region; 8/10 or above on image evaluation rubric; supervisor documents in the competency log.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Patient Positioning and Procedures
1. A patient presents for a routine foot series. For the AP (dorsoplantar) projection, what central ray angulation is typically used, and in which direction?
Patient Positioning and Procedures
2. A physician orders weight-bearing AP knee radiographs bilaterally for a patient with suspected osteoarthritis. What is the primary clinical benefit of this technique compared to a supine AP knee?
Patient Positioning and Procedures
3. When performing an AP hip projection on a patient with no fracture, the technologist rotates the patient's foot internally 15–20 degrees. What is the purpose of this maneuver?
Patient Safety and Care
4. A patient arrives in a wheelchair after a fall with severe hip pain. You suspect a hip fracture. Which action is MOST appropriate?
Radiation Physics and Exposure Factors
5. For which of the following projections is a grid MOST necessary?
Radiation Safety and Protection
6. During a cross-table lateral hip projection, where should the radiographer stand to minimize their occupational radiation exposure?
Patient Positioning and Procedures
7. The mortise ankle view is obtained with what foot/leg positioning?
Image Evaluation and Quality
8. After exposing a lateral knee radiograph, you review the image and notice the femoral condyles are significantly separated rather than superimposed. Which positioning error most likely caused this?
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