CT Scan Technique of Ankle Joint:

Patient Positioning:

Method 1:

The patient lies supine, feet first in the middle of the scanner table with the legs extended. Both joints may be scanned for comparison or the unaffected limb may be positioned out of the scan field by flexion of the knee joint. The legs are internally rotated ro ensure that that malleoli, of both ankle joints, are equidistant from the scanner table. Sandbags should be placed against the knees and positioning straps around the feet to minimise the risk of patient movement. Positioning is aided by transaxial, coronal and sagittal alignment lights, ensureing that the median sagittal plane is perpendicular and the coronal plane parallel to the scanner table. The scan plane is now perpendicular to the long axis of the body. The  patient is moved into the scanner until the scan reference point is at the level of the malleoli and the table height adjusted until the coronal light is at the level of the lateral malleolus.

 

 

 

 

 

 

Method 2:

The patient sits on scanner table with both knees flexed and the plantar aspect of the feet resting on the table top. Both joints may be imaged for comparison. The patient is moved into the scanner until the scan reference point is at the level of the malleoli and the table heightadjusted until the coronal alignment light is at the level of the lateral malleolus.

Imaging Procedure:

A lateral scan projection radiograph is taken including the distal tibia upto the distal phalanges. From this image either 3 or 5 mm contiguous slices are prescribed through the suspected area of abnormality. The images may be acquired with a volume acquisition using a 5 mm slice thickness and 5 mm table increments, but with a 3 mm reconstruction index to give overlapping sections. To improve spatial resolution the images may be reconstructed either prospectively or retrospectively using a small display field of view. A bone reconstruction algorithm is normally selected to enhance bony structures while still allowing soft tissues to be demonstrated adequately. Further reformatted images in either the coronal or sagittal plane may be useful.

Indications:

1. To determine precisely the extent of a complicated fracture (stress fractures) with edema and dislocations
2. To evaluate for fusion of joints (tarsal coalition)
3. To quantitate the degree and location of arthritis at a joint
4. For foreign body localization
5. Ankle sprain following injury (the detailed views of the ankle ligaments at the site of injury)
6. Sinus tarsi syndrome and impingement syndrome
7. Osteochondritis dissecans
8. Tendon disease (Achilles pain)
9. Diabetic foot

Ankle/Distal Tibia Protocol:

The ankle/distal tibia protocol is centered on the ankle joint. This protocol is used for scanning fractures of the distal tibia or of the talar dome (e.g. osteochondral lesions). Using a midsagittal reference image, straight axial images are created in a plane parallel to the bottom of the foot. Then, using an axial reference image through the top of the ankle mortise, mortise coronal and mortise sagittal images are created parallel and perpendicular to an imaginary line through the anterior cortex of the medial and lateral malleoli. For distal tibial fractures, reformatted images (3 mm thick at 3 mm intervals) yields crisp images that do not appear noisy. However, for osteochondral lesions of the talar dome, mortise coronal amd mortise sagittal images be reformated at 1 mm, yields images of higher resolution.

Hind foot/Mid foot Protocol:

The hind foot/Mid foot protocol is centered on the Chopart joint and is used to evaluate hind foot fractures (e.g. calcaneus, talar body) and the subtalar joint (e.g. tardal coalitions). Using an axial reference image. straight sagittal images are reformatted along a plane parallel to the long axis of the foot. The other three planes are reformatted off a midsagittal image. Straight axial images are reformatted in a plane to the bottom of the foot. Oblique coronal and oblique axial images are reformatted in planes both perpendicular and parallel to the posterior facet of the subtalar joint.

Forefoot/Midfoot Protocol:

The forefoot/midfoot protocol is used primarily to assess the alignment of the Lisfranc joint and the integrity of the adjacent bones. Reformatted images are created in three planes relative to the first metatarsal shaft. A sagittal reference image that best delineates the entire length of the first metatarsal is selected. Long axis and short axis planes are reformatteed both parallel and perpendicular to the sagittal length of the first metatarsal. The third plane, sagittal to the first metatarsal is best obtained off an axial reference image that has been obliqued to include the entire length of the first metatarsal.

Navicular Protocol:

The navicular protocol is used to assess the healing of a known navicular faitue fracture . Because these navicular fatigue fractures tend to be incomplete hairline cracks,, the resolution is increased by creating thin (1 mm) reformatted images in a small (6 cm ) FOV. Oblique coronal and oblique axial images are reformatted off a sagittal reference image, and oblique sagittal images are reformatted off an axial reference    image.