Spiral CT

The advent of spiral CT that resulted from advances in slip ring technology and faster scanning techniques in the 1980s has allowed for the acquisition of thin-slice CT with the capability of 3-plane reformatting.

• Current generation: The newest generation of CT scanners have 64 rows of detectors that rotate in a rapid fashion around the body, allowing the acquisition of multiple slices in a single revolution, making large patient coverage possible in a single breath hold.
• CT angiography: The use of this technology following the administration of a bolus of contrast is known CTA. This permits depiction of the cervical vessels, particularly the vertebral artery as it runs through the transverse foramen close to potential vertebral body pathologies, particularly bone tumors (4). CTA can be of use in surgical planning and in evaluating the potential for preoperative embolization.
• Dual-energy techniques: Acquiring CT images using two X-ray sources at different energy levels allows the subtraction of bone from the image to reveal only the vascular structures.

Coronal CT showing osteoblastoma in C2 vertebral body: The vertebral artery (arrow) is of normal caliber and exits normally from the foramen transversarium of C2 but is displaced anterolaterally by tumor.

3-D reconstruction of CTA: The osteoblastoma (long arrow) can be seen separate from the vertebral artery (short arrows) that runs anterior to the tumor.

• Dynamic CT scanning: This refers to imaging done with the spine oriented in different positions such as flexed and extended. An example woudl be when  it is used to investigate children with torticollis in whom AARF is suspected. Scanning of the craniocervical junction to C3 is performed in 3 positions: the presenting position, the neutral position, and looking away from the presenting position, provided that the patient remains comfortable (5). In a normal individual, most of the rotation of the neck occurs at C1/2 and can be up to 38 degrees in either direction.

CT images of normal physiologic rotation of C1 on C2: Rotation of C1 on C2 is approximately 70 degrees in normal state is shown at two levels (top and bottom row).  In the left column the patient is looking to the right, in center looking straight ahead while in the right the patient is looking to the left.

• No C1-C2 rotation with rotary subluxation: The diagnosis of AARF is made when the head is rotated with respect to the neck in the neutral position and the relationship of the lateral mass of C1 and C2 does not change with rotation of the neck.

CT images of atlantoaxial rotatory fixation: This child with torticollis can only rotate his head a few degrees to the left because C1 and C2 are fixed on the right, consistent with the diagnosis of atlantoaxial rotatory fixation (AARF).