Procedures · Head, Spine and Pelvis

Thoracic Spine Positioning Series

Compare the AP and lateral projections of the routine thoracic spine series.

Thoracic spine

Left

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The routine thoracic spine series

The projections this region is examined with. Set them up yourself in Practice positioning.

AP thoracic spine

Anteroposterior projection · central ray 90° to the image receptor

Demonstrates: The twelve thoracic vertebral bodies and their intervertebral joint spaces, the spinous and transverse processes, the posterior ribs and the costovertebral articulations, on a field running from C7 to L1. No rotation is proved by the sternoclavicular joints lying equidistant from the spine.
Part position: Supine, with the arms at the side and the head on the table or a thin pillow. Align the midsagittal plane to the central ray and to the midline of the table. Flex the hips and knees to reduce the thoracic curvature, and check that neither the thorax nor the pelvis is rotated.
Central ray: 90° to the image receptor, centered to the midsagittal plane at the level of T7 — 3 to 4 inches (8 to 10 cm) below the jugular notch, or 1 to 2 inches (2.5 to 5 cm) below the sternal angle. T7 also lies at the midpoint of the jugular notch and the xiphoid process, so the centring is the same as for an AP chest.
Why it is positioned this way: The thoracic spine is the least uniform part of the whole column to expose: the upper vertebrae are small and surrounded by air-filled lung, while the lower ones are large and lie beneath the dense tissues at the diaphragm. A uniform beam therefore over-penetrates the top and under-penetrates the bottom, which is why this one projection recruits the tube itself — the anode heel effect — to grade its own intensity, and why the exposure is made on expiration rather than inspiration.

Also covered by this projection

Putting the anode heel effect to workThe beam is not equally intense from one end of the tube to the other — intensity falls toward the anode, because x-rays emitted toward that side are absorbed in the heel of the target itself. Place the patient so the CATHODE, the more intense end, lies over the thick thoracolumbar region and the anode over the thin upper spine, and the tube grades the exposure to match the anatomy. A wedge compensating filter does the same job the other way round: its thick part goes over the THIN upper vertebrae, absorbing beam where the patient does not.

Why expirationThe exposure is suspended on expiration, which reduces the volume of air in the thorax and gives the vertebrae a more uniform background — the deliberate opposite of chest radiography, where full inspiration is wanted to expand the very lung fields this projection is trying to quiet.

Taken erectThe same projection can be made erect, seated or standing with the arms at the side and the weight evenly distributed on both feet, when the patient cannot tolerate lying supine.

Lateral thoracic spine

Left lateral position · central ray 90° to the image receptor

Demonstrates: The thoracic vertebral bodies and their intervertebral joint spaces in profile, and the intervertebral foramina — which open at 90° to the midsagittal plane, so the lateral is the projection that shows them. T1 to T3 are not well demonstrated, because the shoulders lie over them; when the upper vertebrae are of interest, a cervicothoracic (swimmer’s) lateral is added. No rotation is proved by the posterior borders of the vertebral bodies lying superimposed, with less than half an inch (1.25 cm) of separation between the posterior ribs.
Part position: Lateral recumbent on the left side, with the head on a pillow and the knees flexed with a support between them. Align the posterior half of the thorax — the strip between the midcoronal plane and the posterior surface — to the central ray and the midline of the table. Raise the arms to 90° from the trunk with the elbows flexed, place a radiolucent support under the waist so the long axis of the spine lies near parallel to the table, and check that neither the shoulders nor the pelvis has rotated.
Central ray: 90° to the image receptor and to the long axis of the thoracic spine, centered to T7 at the level of the inferior angle of the scapula — 3 to 4 inches (8 to 10 cm) below the jugular notch, or 7 to 8 inches (18 to 20 cm) below the vertebra prominens.
Why it is positioned this way: From the side, the ribs and the lungs lie over every thoracic vertebra, and this projection removes them with TIME rather than geometry: the orthostatic (breathing) technique — a deliberately long exposure of at least 2 to 3 seconds at a low mA while the patient breathes quietly. The moving ribs and pulmonary markings blur out of the image while the stationary vertebrae stay sharp; automatic exposure control is not used, because the timer rather than the detector has to own a 3-second exposure. A patient who cannot breathe evenly and stay still is instead exposed on suspended full inspiration.

Also covered by this projection

Swimmer’s lateral for the upper vertebraeThe cervicothoracic (swimmer’s) lateral picks up C5 to T3, which the shoulders bury on this projection. The arm nearest the receptor is raised with the forearm resting on the head; the far arm stays down and rotates slightly posterior, so one shoulder is lifted clear and the other dropped. The central ray stays 90° to the image receptor, centered to T1 — about 1 inch (2.5 cm) above the jugular notch anteriorly, at the level of the vertebra prominens posteriorly — with 3° to 5° caudad when the shoulders will not separate, at 60 to 72 inches (150 to 180 cm) SID.

The 70° oblique, and the rule that flipsThe thoracic zygapophyseal joints sit at 70° to 75° to the midsagittal plane, so demonstrating them takes a much steeper oblique than the lumbar 45°: the patient rotates 70° from supine, only 20° short of a true lateral. And the laterality rule REVERSES here — an AP oblique (RPO or LPO) demonstrates the joints FARTHEST from the receptor, so an RPO shows the LEFT joints, where the same RPO one region down shows the right. A PA oblique (RAO or LAO) shows the joints closest to the receptor. Both sides are imaged either way, with the same rotation used bilaterally.

When the waist sagsThe waist support is what keeps the long axis of the spine parallel to the receptor; without enough of it the column sags toward the table and the disc spaces close against a straight beam. If the waist cannot be supported — broad shoulders make the sag worse — angle the central ray 10° to 15° cephalad instead. The rule is the line, not the number: the ray should meet the long axis of the spine at 90°, however that is achieved.

Scatter at this thicknessThe lateral thoracic spine is thick enough to generate serious scatter. Collimate closely, and lay a lead mat on the tabletop behind the patient so scatter from the trunk does not reach the receptor — precautions that matter more with a digital detector, not less, because a digital image of a badly scattered exposure still looks viewable instead of failing visibly.

Procedures lessonVertebral column