Procedures · Head, spine and pelvis2 min read5-question check
Thoracic and lumbar spine
Flex the knees, place the anode at the head for the T-spine, and read the Scotty dog to check your oblique.
Patient and part
For the AP thoracic spine the patient is supine, knees flexed, not rotated. Place the head at the anode end so the anode heel effect puts less intensity over the thin upper thorax. The lateral thoracic spine is a left lateral with arms raised to 90 degrees and the spine parallel to the table. The AP lumbar spine is supine with knees flexed, which flattens the lumbar curve and opens the disk spaces. The lateral lumbar spine adds waist support. For the zygapophyseal joints, rotate 45 degrees into RPO and LPO positions.
Central ray
All of these use 40 inches and a perpendicular CR. Center the thoracic spine to T7, about 3 to 4 inches below the jugular notch. Center a lumbar spine on a large IR at the iliac crest, about L4 to L5; obliques center to L3. For the lateral L5 to S1 spot, center about 1.5 inches below the iliac crest and 2 inches posterior to the ASIS. The lateral thoracic spine often uses a breathing technique: low mA and a long exposure time while the patient breathes quietly, blurring the ribs and lung markings.
What a good image shows
A good AP shows spinous processes centered between the pedicles. A good lateral shows open intervertebral foramina and superimposed posterior vertebral borders. On a 45 degree oblique, each vertebra looks like a Scotty dog: the eye is the pedicle, the ear the superior articular process, the nose the transverse process, the front leg the inferior articular process and the neck the pars interarticularis. Posterior obliques show the downside joints, so an LPO shows the left. Anterior obliques show the upside joints.
Fixing it
If the Scotty dog's eye sits far forward, toward the anterior vertebral body, the patient is rotated too little; if it sits far back, toward the posterior body, rotation is too much. Closed disk spaces on a lateral lumbar spine often mean the spine sagged; add waist support or angle slightly caudad.
Clinical Example
Your lateral L5 to S1 spot shows a closed joint space on a patient with wide hips and no waist support. You place a sponge under the waist so the spine is horizontal and repeat. The L5 to S1 space is now open.
Common Mistakes
- Leaving the legs straight for the AP lumbar spine, which increases lordosis and narrows disk spaces.
- Putting the feet at the anode end for the AP thoracic spine.
- Expecting an RPO to show the left upside joints; it shows the right downside joints.
- Using a short exposure time for a breathing technique lateral.
Why It Matters
Good positioning prevents repeats and shows pars defects clearly. Registry items often show a Scotty dog and ask which part is which or which side the oblique demonstrates.
Key Relationships
- T-spine: CR to T7, head at anode end
- Lumbar on large IR: CR at iliac crest
- Posterior oblique: downside zygapophyseal joints
- Scotty dog eye = pedicle, neck = pars interarticularis
- Breathing technique: low mA, long time, quiet breathing