Procedures · ARRT Radiography
Lumbar Spine, Sacrum & Coccyx on the ARRT
The 45 degree lumbar obliques and the Scottie dog, sacrum 15 cephalad vs coccyx 10 caudad, SI joint obliques, and the scoliosis series for the ARRT Radiography exam.
Overview
Lumbar Spine, Sacrum and Coccyx is a chapter of angles and directions. The lumbar intervertebral foramina sit at 90 degrees to the midsagittal plane and open on a true lateral, while the zygapophyseal joints open 30 to 50 degrees from it and need a 45 degree oblique. Posterior obliques show the downside joints (RPO gives the right), and a correct oblique projects each vertebra as the Scottie dog, with the pedicle as its eye and the pars interarticularis as its neck.
That neck is the point. Spondylolysis is a defect of the pars, the Scottie dog wearing a collar, most common at L4 or L5; spondylolisthesis is the forward slip that can follow, most common at L5-S1. The AP lumbar centers at the iliac crest (L4-L5) on a 35 x 43 cm receptor or at L3 on the smaller one, and the lateral shows the L1-L4 foramina, with a 5 to 8 degree caudad angle when the spine cannot be brought level with the table, as with a wide pelvis and narrow thorax even after the waist is supported. The lateral L5-S1 spot centers 1.5 inches (4 cm) below the iliac crest and 2 inches (5 cm) posterior to the ASIS, with the CR perpendicular when the waist is well supported, or 5 to 8 degrees caudad with less support (Bontrager; Merrill's prints 5 for men and 8 for women, or parallel to the interiliac line).
The sacrum and coccyx take opposite tube angles from the same centering point 2 inches above the symphysis pubis: 15 degrees cephalad for the backward-leaning sacrum, 10 degrees caudad for the forward-curving coccyx, and the bladder is emptied first. One combined lateral covers both bones and lowers gonadal dose. The sacroiliac joints flip the oblique rule: 25 to 30 degrees of rotation with the side of interest elevated demonstrates the upside joint, so an RPO shows the left SI joint.
The scoliosis series is preferably taken PA, cutting breast dose by roughly 90 percent, and the Ferguson method repeats the erect PA with a 3 to 4 inch block under the foot on the convex side to separate the primary curve from the compensatory one. Procedures is worth 66 of 200 scored ARRT questions to 28 February 2027, and 72 from 1 March 2027.
What you’ll learn in this chapter
The 20 study slides and 14 quiz questions in this chapter break down as follows. 3 chapters are free in full with a free account. The others come with a paid plan, and paid plans open soon.
Lumbar Spine, Sacrum & Coccyx
- Lumbar Spine, Sacrum and Coccyx
Anatomy
- The Lumbar Vertebra
- Foramina at 90, Joints at 45
- Quiz
- Sacrum and Coccyx
Scottie dog
- The Scottie Dog
- Quiz
Landmarks
- Topographic Landmarks
Technique
- Technique and Patient Prep
Lumbar spine
- AP (or PA) Lumbar Spine
- Quiz
- Lumbar Obliques
- Quiz
- Lateral Lumbar Spine
- L5-S1
- Quiz
Mobility studies
- Flexion and Extension
Sacrum & coccyx
- AP Axial Sacrum and Coccyx
- Quiz
- Lateral Sacrum and Coccyx
SI joints
- Sacroiliac Joints
- Quiz
Scoliosis
- Scoliosis Series
- Quiz
Summary
- Every Angle in One Place
Image critique
- Critiquing a Lumbar Film
- Quiz
Clinical indications
- What You Are Looking For
Recap
- What to Carry Out of This Chapter
- Quiz
- Quiz
- Quiz
- Quiz
- Quiz
Key terms in this chapter
These are 8 key terms from this chapter. Use them as a flashcard pre-quiz.
- Pars Interarticularis
- The part of the lamina that lies between the superior and inferior articular processes, demonstrated on the 45 degree oblique. A separation in it is spondylolysis.
- Scottie Dog
- The shape a correctly rotated 45 degree oblique gives each lumbar vertebra: eye is the pedicle, ear the superior articular process, neck the pars, front leg the inferior articular process.
- Spondylolisthesis
- Forward movement of one vertebra on another, most common at L5-S1, often following a pars defect. The lateral and the lateral L5-S1 spot projection show it.
- AP Axial Sacrum
- Taken with the CR 15 degrees cephalad, centered 2 inches above the symphysis pubis, because the sacrum leans backward. The bladder is emptied before the examination.
- AP Axial Coccyx
- Taken with the CR 10 degrees caudad from the same centering point 2 inches above the symphysis, matching the coccyx's forward curve. A greater curvature takes up to 15 degrees.
- Sacroiliac Joint Obliques
- 25 to 30 degree posterior obliques with the side of interest elevated. The upside joint, the one farthest from the receptor, is demonstrated, so an RPO shows the left SI joint.
- Ferguson Method
- A scoliosis study pairing a standard erect PA with a second image taken with a 3 to 4 inch block under the foot or hip on the convex side, separating the primary curve from the compensatory curve.
- Iliac Crest
- A key landmark in this chapter, at the L4-L5 level. The large-receptor AP and lateral lumbar both center here.
Sample practice question: Head, Spine, and Pelvis
One free sample question from the Head, Spine, and Pelvis subcategory, like those in its 256-question bank. Several chapters share each sample, so it may not match this chapter's topic. See the format, the rationale style, and the difficulty before you sign up.
A 45° posterior oblique projection of the lumbar spine demonstrates the Scottie dog pattern. A defect at the dog's neck represents which of the following pathologies?
Show answer and rationale
A, Incorrect: Spondylolisthesis is forward slippage of one vertebra on another. Bontrager lists AP and lateral lumbar spine and CT for it.
B, Correct: Correct. The Scottie dog's neck is the pars interarticularis. A defect there is spondylolysis.
C, Incorrect: This is a disk problem, for which Bontrager lists AP and lateral lumbar spine, CT and MRI.
D, Incorrect: Compression fractures involve loss of vertebral body height, not a pars defect.
Hands-on
Practice the positioning
Open the Positioning Lab to drill body position, central ray, anatomy, and common errors for each projection in this chapter.
- Sacrum AP Axial Axial view of sacrum. CR 15° cephalad. SID 40".
- Sacrum Lateral True lateral sacrum. SID 40". CR perpendicular to IR.
- Coccyx AP Axial Axial view of coccyx. CR 10° caudad. SID 40".
- Coccyx Lateral True lateral coccyx. SID 40". CR perpendicular to IR.
- SI Joints AP Axial AP axial bilateral SI joints. CR 30-35° cephalad. SID 40".
- SI Joints AP Oblique (RPO) AP oblique SI joints, RPO. SID 40". CR perpendicular to IR.
- AP Lumbar Supine, knees flexed, CR perpendicular to the iliac crest (L4-5) on a large IR. Grid required.
- Lateral Lumbar Lateral recumbent, knees flexed, CR perpendicular to the iliac crest (L4-5) on a large IR.
- AP Oblique Lumbar Zygapophyseal Joints 45° oblique, RPO or LPO, CR to L3. Facet joints.
- L5-S1 Lumbosacral Junction Lateral recumbent, knees flexed. CR 1.5 in below the iliac crest and 2 in posterior to the ASIS; perpendicular with waist support, otherwise 5 to 8 degrees caudad.
Read 3 chapters free.
The free tier unlocks 3 complete chapters, 50 practice questions, and 1 sample timed exam. No credit card required.
Frequently asked questions
What does the Lumbar Spine, Sacrum & Coccyx on the ARRT chapter cover?
Lumbar Spine, Sacrum and Coccyx is a chapter of angles and directions. The lumbar intervertebral foramina sit at 90 degrees to the midsagittal plane and open on a true lateral, while the zygapophyseal joints open 30 to 50 degrees from it and need a 45 degree oblique. Posterior obliques show the downside joints (RPO gives the right), and a correct oblique projects each vertebra as the Scottie dog, with the pedicle as its eye and the pars interarticularis as its neck.
How many study slides are in the Lumbar Spine, Sacrum & Coccyx on the ARRT chapter?
This chapter has 20 study slides and 14 quiz questions across 14 sections. The full chapter comes with a paid plan, and paid plans open soon. The free tier includes 3 complete chapters.
Is this chapter aligned with the current ARRT Content Specifications?
Every chapter on this site is mapped to the ARRT Radiography Content Specifications, but our line-by-line check against ARRT's outline isn't finished, so keep ARRT's outline beside it. Nothing here follows your exam date, so check which version applies to you. This chapter falls under the Procedures domain of the official ARRT exam blueprint.