Radiographic Procedures · ARRT 2025
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 reserved for a sagging, unsupported waist.
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 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 72 of 200 scored ARRT questions.
What you’ll learn in this chapter
The 34 lessons in this chapter break down as follows. The full lesson content is unlocked when you start a free account.
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 the 8 terms most likely to appear on the ARRT registry from this chapter. Use them as a flashcard pre-quiz.
- Pars Interarticularis
- The portion of each lamina 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
- The most used landmark of the 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 from the 68-question Head, Spine, and Pelvis bank. See the format, the rationale style, and the difficulty before you sign up.
A 45° posterior oblique projection of the lumbar spine demonstrates the Scotty Dog pattern. A defect appearing as a 'collar' on 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, best assessed on lateral imaging.
B, Correct: Correct. The Scotty Dog's neck represents the pars interarticularis. A 'collar' on the neck indicates spondylolysis, a defect or fracture of the pars. This finding is pathognomonic on the 45° lumbar oblique.
C, Incorrect: Disk herniation is not directly visible on plain radiographs and requires MRI for diagnosis.
D, Incorrect: Compression fractures involve loss of vertebral body height and are best demonstrated on lateral views.
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Frequently asked questions
What does the ARRT Radiography Radiographic Procedures category 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 lessons are in the Lumbar Spine, Sacrum & Coccyx on the ARRT chapter?
This chapter contains 34 lessons across 14 sections, plus a knowledge-check quiz at the end. The full lesson content is unlocked with a Premium subscription. The free tier includes 3 complete chapters.
Is this chapter aligned with the ARRT 2025 Content Specifications?
Yes. Every chapter on this site maps directly to the ARRT Radiography Content Specifications effective 2025. This chapter falls under the Radiographic Procedures domain of the official ARRT exam blueprint.