16.2 Thoracic & Lumbar Spine, Sacrum & Coccyx
Key Takeaways
- The lateral thoracic spine uses the breathing technique (a 3-4 second exposure) to blur ribs and lung markings and demonstrate the intervertebral foramina.
- Lumbar posterior obliques at 45 degrees show the zygapophyseal joints nearest the IR, forming the 'Scotty dog'; a lucency across the neck (pars) indicates spondylolysis.
- The AP axial sacrum angles the CR 15 degrees cephalad, and the AP axial coccyx angles the CR 10 degrees caudad.
- The AP and lateral lumbar spine are centered at L4, at the level of the iliac crest, with knees flexed to reduce the lordosis.
- Scoliosis surveys use an erect PA projection to reduce breast, thyroid, and gonadal dose by as much as 90 percent.
Thoracic and Lumbar Anatomy
The thoracic spine has 12 vertebrae (T1-T12), each articulating with a pair of ribs. Its zygapophyseal (apophyseal) joints sit 70-75 degrees to the MSP, so they need a steep 70-degree oblique, while its intervertebral foramina open 90 degrees to the MSP and are therefore demonstrated on the lateral. The lumbar spine has five large vertebrae (L1-L5). Its zygapophyseal joints lie 30-50 degrees to the MSP (about 45 degrees) and are shown on oblique projections; its intervertebral foramina open at 90 degrees to the MSP and appear on the lateral.
Surface landmarks guide centering: the jugular notch = T2-3, the sternal angle = T4-5, the inferior angle of the scapula = T7, the xiphoid process = T9-10, the inferior costal margin = L2-3, the iliac crest = L4-5, and the ASIS = S1-2.
Thoracic Spine
The AP thoracic projection centers the CR perpendicular to T7 (about 3-4 inches below the jugular notch), with the knees flexed to reduce the kyphotic sag. Exploit the anode heel effect by placing the cathode (higher-intensity) end of the tube over the thicker lower thorax (abdomen) and the anode end over the thinner upper thorax for even density along the spine. The lateral thoracic projection demonstrates the bodies, disk spaces, and intervertebral foramina. Use the breathing technique (also called orthostatic breathing or autotomography): a long exposure of 3-4 seconds with shallow, continuous breathing blurs the overlying ribs and pulmonary markings while the stationary spine stays sharp. If the spine sags below horizontal, support the waist or angle the CR 10-15 degrees cephalad.
Lumbar Spine
The AP (or PA) lumbar projection centers the CR at L4, at the level of the iliac crest, with the knees flexed to flatten the lumbar lordosis and open the disk spaces. Posterior oblique projections (RPO and LPO) rotated 45 degrees demonstrate the zygapophyseal joints on the side nearest the IR, producing the classic 'Scotty dog'. In the Scotty dog the pedicle is the eye, the transverse process is the nose, the pars interarticularis is the neck, the superior articular process is the ear, and the inferior articular process is the front leg; a collar-like lucency across the neck signals spondylolysis.
The lateral lumbar projection is centered at L4 (iliac crest) and shows the bodies, disk spaces, spinous processes, and intervertebral foramina. A dedicated L5-S1 spot lateral collimates tightly to the lumbosacral junction, centering about 1.5 inches inferior to the iliac crest and 2 inches posterior to the ASIS. Angle the CR 5-8 degrees caudad (males) or 8-10 degrees (females) only if the waist is unsupported and the spine does not lie horizontal.
Sacrum and Coccyx
Because the sacrum and coccyx curve, each AP axial projection uses an angled CR. The AP axial sacrum angles the CR 15 degrees cephalad to open and elongate the sacral segments and project them clear of the pubis. The AP axial coccyx angles the CR 10 degrees caudad, centering about 2 inches superior to the pubic symphysis. Empty the bladder and, when possible, the bowel beforehand. A single lateral projection with tight collimation and increased technique demonstrates both the sacrum and coccyx in profile.
Scoliosis Series
A scoliosis survey evaluates lateral spinal curvature on a long (14 x 36 inch) receptor. Image the patient erect in the PA rather than AP projection: the PA orientation moves the breasts, thyroid, and gonads away from the entrance beam and can reduce breast dose by as much as 90 percent. The Ferguson method adds a second erect image with a block under the foot on the convex side to distinguish the primary (structural) curve from a compensatory curve. Use gonadal and breast shielding and the fastest appropriate imaging system.
Evaluation Criteria and Common Errors
On an AP lumbar spine, the spinous processes should lie midline and the sacroiliac (SI) joints appear equidistant from the spine, confirming no rotation; the L1-L4 disk spaces open when the knees are flexed. A correctly rotated lumbar oblique shows the zygapophyseal joint of interest open with the pedicle (the Scotty dog's eye) centered over the middle of the vertebral body: if the pedicle projects too far anteriorly the patient is under-rotated (less than 45 degrees), and if it projects too far posteriorly the patient is over-rotated. On the lateral, the intervertebral foramina should be open and the posterior margins of the vertebral bodies superimposed. Do not confuse the thoracic and lumbar oblique requirements: the thoracic zygapophyseal joints need roughly 70-degree obliques, whereas the lumbar zygapophyseal joints need 45 degrees. Finally, verify the AP axial sacrum shows the segments open and not foreshortened, which confirms the 15-degree cephalad angle was correct; a perpendicular central ray would foreshorten the curved sacrum.
| Projection | Central ray | Demonstrates | Notes |
|---|---|---|---|
| AP thoracic | Perpendicular to T7 | Bodies, disk spaces | Heel effect: cathode toward abdomen |
| Lateral thoracic | Perpendicular to T7 | Intervertebral foramina | Breathing technique, 3-4 s |
| AP lumbar | Perpendicular to L4 (iliac crest) | Bodies, disk spaces | Knees flexed |
| Posterior oblique lumbar (RPO/LPO) | Perpendicular, 45 deg | Near-side zygapophyseal joints (Scotty dog) | Neck lucency = spondylolysis |
| L5-S1 spot lateral | 5-8 deg caudad if unsupported | Lumbosacral junction | 1.5 in below crest, 2 in posterior to ASIS |
| AP axial sacrum | 15 deg cephalad | Sacral segments | Empty bladder |
| AP axial coccyx | 10 deg caudad | Coccyx | Center 2 in above symphysis |
| Scoliosis (PA erect) | Perpendicular, 72-inch SID | Full-spine curvature | PA reduces breast dose |
For the AP axial projection of the sacrum, the central ray is directed:
On a 45-degree posterior oblique lumbar spine, the 'Scotty dog' is visualized. A lucency across the dog's neck indicates spondylolysis. Which anatomic structure forms the neck?
Why is a scoliosis survey preferentially performed as an erect PA rather than an AP projection?