Spine & pelvis

Spine & pelvis

Exam category: Procedures → Head, Spine and Pelvis
Why it matters: The spine is a projection-logic exam, not a memorization exam. Each region has one geometric fact that tells you which view opens which structure — the cervical foramina angle, the thoracic facet angle, the lumbar "Scottie dog." Learn those five or six angles and the whole spine falls out of them. The registry leans hard on the two places students reliably flip: cervical-vs-lumbar obliques, and the trauma-safe alternatives for the hip.

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The big picture (plain English)

Every part of the spine has joints and openings that only show up when the beam lines up with them. The bony landmarks don't move, so positioning is just "what angle opens this structure?" In the neck, the openings (intervertebral foramina) sit at 45°, so you oblique 45° to see them. In the thoracic spine, the facet joints sit at 70–75°, so you oblique steeper. In the lumbar spine, a 45° oblique turns the vertebra into a little dog, and reading that dog is reading the anatomy. Get the angle, get the view.

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1. Cervical spine — the projection map

The whole cervical series comes from one idea: the foramina and the joints point in different directions, so different views open them.

That first row is the one students get wrong. On the obliques, which side of foramina you see depends on whether you enter from the front or the back:

Anterior obliques (RAO/LAO) are preferred because they put the thyroid and breasts on the exit side of the beam, lowering their dose. Both sides are taken for comparison.

2. Cervical special views

• AP axial: CR 15–20° cephalad. The lower cervical bodies are slightly lipped, so the straight-on beam closes their disk spaces; the cephalad angle opens them.
• AP open-mouth (odontoid): the only projection that shows the dens (odontoid process) and the C1–C2 (atlantoaxial) joints. The mouth is held wide open with the beam directed to its midpoint; the upper incisors and the base of the skull are lined up so neither covers the dens.
• Lateral: the single most important film clinically — it must show all seven cervical vertebrae plus the C7–T1 junction. Shot at an increased SID of 60–72 inches (150–180 cm), commonly 72 inches (180 cm), because the spine sits far from the IR (large OID from the shoulders), and the longer distance limits the resulting magnification. That same increased SID applies to the cervical obliques and the swimmer's lateral, for the same reason. (Cervical spine minimum SID is 40 inches / 100 cm.)
• Swimmer's (cervicothoracic / Twining) lateral: used when the shoulders superimpose C7–T1 on a standard lateral. The arm nearest the IR goes up (forearm resting on the head), the far arm goes down and slightly posterior, so one shoulder is raised and the other dropped out of the way. CR is directed to T1.

3. Thoracic spine

Routine is AP and lateral. Two positioning facts carry most of the exam weight:

• AP: center the CR to T7 (roughly the middle of the sternum). Because the thoracic spine is thin and air-surrounded up top but thick and dense at the bottom, density is uneven — use the anode heel effect (anode/less-intense end toward the thinner upper spine) or, better, a wedge compensating filter with its thick part over the thin upper vertebrae.
• Lateral: use the orthostatic (breathing) technique — a deliberately long exposure (a minimum of about 2–3 seconds at low mA) while the patient breathes quietly. The moving ribs and lung markings blur out, leaving the stationary vertebrae sharp. If the patient can't cooperate, suspend on full inspiration instead.

The thoracic zygapophyseal joints sit at 70–75° to the midsagittal plane, so they need a steeper oblique than the cervical or lumbar spine. Upper thoracic vertebrae (T1–T3) that don't show on a lateral are picked up with the swimmer's view.

4. The Scottie dog (oblique lumbar spine, 45°)

A 45° oblique lumbar spine doesn't look like a spine anymore — it looks like a little dog. Learning to read that dog is learning oblique lumbar anatomy.

5. Lumbar spine routine + the L5-S1 problem

Routine: AP (or PA), oblique (anterior or posterior), lateral, plus a dedicated lateral L5-S1 spot. The L5-S1 junction needs its own centered image because the lumbosacral angle means a standard lateral lumbar spine doesn't center well on that specific joint space.

6. Scoliosis series

Scoliosis follow-up means a young patient imaged erect (weight-bearing, so the spine's real curve under gravity shows) and imaged repeatedly over years — which is why dose discipline dominates this series.

• Shoot PA, not AP. In a PA projection the breasts sit on the exit side of the beam, away from the entrance dose. Over dozens of follow-up films on an adolescent, that swing in breast dose is the whole reason PA is the standard.
• Ferguson method: two erect PA images. The first is a standard erect PA; for the second, a 3–4 inch block is placed under the foot (standing) or hip (sitting) on the convex side of the curve. Comparing the two separates a primary/structural curve (won't change) from a compensatory curve (straightens out when the pelvis is leveled).
• Compensating filters even out the density between the thin upper spine and thick lower spine on a single long (14 × 36 inch) image.

7. Hip and pelvis — the trauma-safe pattern

Same logic as Neer for the trauma shoulder: Clements-Nakayama exists specifically because frog-leg requires movement a trauma patient may not be able to safely do.

8. Sacrum, coccyx, and SI joints

Sacrum and coccyx get both combined (one lateral image of both) and separate (individual AP axial) projections — the axial CR angle differs by structure, since the sacrum and coccyx curve in different planes.

Sacroiliac (SI) joints are imaged two ways. An AP axial projection angles the CR 30–35° cephalad (≈30° for males, 35° for females) to open the joints through the lumbosacral curve. Posterior obliques (RPO/LPO) put one joint in profile — rotate 25–30° with the side of interest elevated (up); the joint demonstrated is the one farthest from the IR, so an RPO opens the left SI joint and an LPO opens the right — the same "posterior oblique → farthest from the IR" rule as the cervical foramina back in section 1.

9. Where to center — and how you know it's right

The sections above cover which view opens which structure. This is the other half the registry tests: the centering point, and the finding on the finished image that proves the position was right.

Rotating the limbs for the pelvis and hip

For both the AP pelvis and the AP hip, the lower limbs are internally rotated about 15–20°. The reason is geometric, not comfort: the femoral neck is anteverted, so internal rotation swings it parallel to the IR and shows it in true length without foreshortening. Leave the feet in the neutral anatomic position and the neck foreshortens.

How you know it's right

SID: routine spine work is taken at 40 inches (100 cm). The increased 60–72 inches is reserved for the projections with a large OID — the lateral cervical, the cervical obliques, and the swimmer's — for the reason given in section 2.

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Registry must-know facts

• Routine CR centering: AP open-mouth → through the center of the open mouth; AP and lateral thoracic → T7; AP axial sacrum → 15° cephalad, 2 inches above the symphysis; AP axial coccyx → 10° caudad, same point; AP axial SI joints → 30–35° cephalad (30° M / 35° F), 2 inches below the ASIS; AP pelvis → midway between ASIS and symphysis; AP hip → 3 inches below the ASIS at the femoral neck.
• Sacrum 15° cephalad vs coccyx 10° caudad — opposite directions, identical centering point.
• AP pelvis and AP hip: internally rotate the limbs 15–20° to place the femoral neck parallel to the IR and show it in true length. Correct when the lesser trochanters are not visible and the necks are not foreshortened.
• AP lumbar with no rotation: SI joints equidistant from the spinous processes, spinous processes in the midline, transverse processes equal in length. Wedge-shaped bodies with asymmetric disc spaces means the patient was rotated.
• Routine spine SID is 40 inches; the 60–72 inch SID is for the large-OID cervical projections only.
• Cervical obliques → intervertebral foramina (45° + CR angle); cervical lateral → zygapophyseal joints (C2–C7 are 90°); AP open-mouth → dens + C1–C2 joints.
• Anterior cervical obliques (RAO/LAO) show foramina on the side closest to the IR (CR caudad); posterior obliques (RPO/LPO) show foramina farthest from the IR (CR cephalad). RAO/LAO are preferred for lower thyroid dose.
• Cervical AP axial uses a 15–20° cephalad CR to open the lower disk spaces.
• Lateral cervical spine is shot at 60–72 inches (150–180 cm) — the long end of that range is used erect, to offset the large OID from the shoulders — and must show all seven vertebrae and C7–T1.
• Swimmer's (Twining) lateral demonstrates C7–T1 (the cervicothoracic junction) when the shoulders obscure it — arm up on the IR side, arm down on the far side.
• Thoracic AP centers to T7; the lateral thoracic uses a breathing (orthostatic) technique — a long exposure (a minimum of about 2–3 seconds at low mA) to blur overlying ribs and lung. Thoracic zygapophyseal joints are at 70–75°.
• Scoliosis series is shot PA to reduce breast dose; the Ferguson method adds a second erect image with a block under the convex side to distinguish a primary from a compensatory curve.
• Scottie dog: nose=transverse process, eye=pedicle, neck=pars interarticularis, ear=superior articular process, front leg=inferior articular process. A "broken neck"/collar = spondylolysis (pars defect); spondylolisthesis is the forward slippage of one vertebra on another (most common L5-S1, also L4-L5).
• Lumbar routine includes a dedicated lateral L5-S1 spot, separate from the standard lateral.
• Frog-leg requires patient movement; Clements-Nakayama is the trauma-safe alternative — same logic as Neer for the shoulder. Judet = posterior oblique(s) for acetabular/pelvic-ring fracture.
• SI joints: AP axial with CR 30–35° cephalad (≈30° M / 35° F); posterior obliques (RPO/LPO) at 25–30° with the side of interest elevated show the joint farthest from the IR (upside) — RPO → left, LPO → right.

Common trap

The cervical and lumbar obliques are mirror images of each other, and the registry counts on you mixing them up. In the neck, the oblique opens the foramina and the lateral opens the joints. In the low back, the oblique (Scottie dog) opens the joints and the lateral opens the foramina. Same word "oblique," opposite structure. The second trap is direction on the cervical obliques: an anterior oblique shows the foramina closest to the IR, a posterior oblique shows the ones farthest away — self-consistent with the CR angle (caudad for anterior, cephalad for posterior), and easy to invert under pressure.

Worked/clinical example

A 14-year-old returns for her third scoliosis check this year. You shoot her erect PA on a 14 × 36 inch IR with a compensating filter — PA (not AP) because she'll have many more of these films and the breast-dose difference adds up. Her curve measures the same as last visit, so the physician orders the Ferguson second image: you place a 3-inch block under the foot on the convex side. The curve partially straightens on that image, telling you part of it is a flexible compensatory curve rather than a fixed structural one.

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Source: Bontrager, Textbook of Radiographic Positioning and Related Anatomy, 10th ed., Ch. 7 (femur & pelvic girdle), Ch. 8 (cervical & thoracic spine), Ch. 9 (lumbar spine, sacrum, coccyx, scoliosis). The centering points, CR angles and evaluation criteria in section 9 come from those chapters' per-projection "CR" and "Evaluation Criteria" entries rather than summarized from memory; the SI-joint 30°/35° male-female split was re-verified against the dedicated AP axial SI-joint page (it is stated there, not carried over from the AP axial L5-S1 page, which uses a different 30°/35° pairing).