Lumbar Sacrum Coccyx Study Guide

Lumbar Spine, L5-S1, Sacrum & Coccyx Radiographic Positioning — Condensed Study Guide

1. Lumbar-Lumbosacral Vertebrae — AP & PA Projections

Patient Preparation and Importance

Proper patient preparation is crucial for achieving clear radiographic images:

• Clear gas & fecal material: This is necessary because these materials can obstruct bone visibility, thus impacting image quality.
• Empty the urinary bladder: A full bladder can lead to increased scatter radiation, which makes the image appear foggy.

AP vs. PA Projections

Key Differences

• AP Projection (Most Common)
    - Patient Position: Supine (lying on the back), recumbent.
    - Lordotic Curve: Extended legs increase lordosis, which can distort the image. To correct this, flex the hips and knees to ensure the back is flat against the table.
    - Disk Spaces: Lordosis can cause uneven disk spaces but can be rectified by aligning the disk spaces with the x-ray beam to achieve an open and parallel view.
    - Dose: Standard radiation dose.

• PA Projection (Optional)
    - Patient Position: Prone (lying face down).
    - Lordotic Curve Impact: Prone positioning reduces lordosis, thereby flattening the back.
    - Disk Spaces: Disk spaces appear even and aligned.
    - Dose: Lower dose due to a thinner abdomen position (gonads are farther from the beam), though a slightly higher dose may be delivered to the spine marrow.

Technical Setup for AP Lumbar Projection

• IR / CR Plate: 14 × 17 inches (35 × 43 cm), placed lengthwise.
• SID (Source-to-Image Distance): Recommended at 48 inches (122 cm) to minimize distortion and better open disk spaces.
• Patient Position: Mid-sagittal plane (MSP) centered to midline of grid; ensure shoulders and hips are in horizontal alignment.
• Arms: Flex elbows and place hands on upper chest while keeping forearms out of the exposure field.
• Reduce Lordosis: Flex hips and knees, utilizing radiolucent support beneath the lower pelvic area if necessary.
• Central Ray (CR): Perpendicular (⊥) to IR at the level of the iliac crests (L4) for lumbosacral; center CR 1.5 inches (3.8 cm) above iliac crest (L3) for lumbar only.
• Collimation: For lumbosacral, the collimation should be 8 × 17 inches (18 × 43 cm); for lumbar only, 8 × 14 inches (18 × 35 cm).
• Respiration: Suspend at the end of expiration.
• Shield Gonads: Yes, especially for male patients.

Structures Shown & Evaluation Criteria for AP/PA

On the image, the following structures should be visible:

• Lumbar bodies
• Intervertebral disk spaces
• Laminae
• Interpediculate spaces
• Spinous & transverse processes
• May include 1-2 lower thoracic vertebrae, sacrum, coccyx, and pelvic bones.
• Note: The L5-S1 disk space is not well depicted in this view; a lateral projection is required for visualization.

Evaluation Checklist:

• Proper collimation and side marker clear of anatomy.
• Area from lower thoracic vertebrae to sacrum captured.
• X-ray beam adequately collimated to lateral margin of psoas muscles.
• Absence of mid-abdomen artifacts (e.g., from elastic or clothing).
• Proper rotation — spinous processes should be centered on bodies; sacroiliac joints equidistant from the vertebral column.
• Open intervertebral disk spaces visible.
• Bony trabecular detail and surrounding soft tissues should be apparent.

Note for Board Preparation

To assess whether a patient is rotated during an AP lumbar exam, check the alignment of spinous processes between the pedicles. Misalignment will indicate rotation, with one sacroiliac joint appearing wider than the other.

2. Lumbar Spine — Lateral Projection

Patient Position

Use the same body position (recumbent or upright) as in the AP/PA views. The patient should be in an open-backed gown for ease of adjustment.

• If lateral recumbent, place the patient on the affected side, flexing the hips and knees for comfort.
• Align the mid-coronal plane (MCP) of the body to the midline of the grid, ensuring vertical alignment without tilt.

Aligning the Spine to be Horizontal

It is critical to achieve a horizontal spine:

• Place radiolucent support beneath the lower thorax, ensuring that the long axis of the spine is parallel to the table.
• Center the IR at the level of the iliac crest (L4) for lumbosacral; for lumbar only, adjust to 1.5 inches above the crest (L3).
• The CR enters the mid-coronal plane; if the spine cannot be aligned horizontally, angle the CR caudad to be perpendicular to the long axis of the spine.
• Average Angles for CR Adjustment:
    - Males: 5° caudad
    - Females: 8° caudad (due to wider pelvis).

Technical Setup for Lateral Lumbar Projection

• IR: 14 × 17 inches (35 × 43 cm), placed lengthwise.
• Central Ray (CR): Perpendicular to IR at the iliac crest (L4) for lumbosacral or 1.5 inches above for lumbar only.
• Collimation: 8 × 17 inches (18 × 43 cm); for lumbar only, 8 × 14 inches (18 × 35 cm).
• Respiration: Suspend at the end of expiration.
• Prevent Rotation: Superimpose the knees precisely; use a small sponge or cloth between them for stability.

Structures Shown & Evaluation Criteria for Lateral Projection

Visible structures should include:

• Lumbar bodies
• Intervertebral disk spaces
• Spinous processes
• L5-S1 junction profile image of intervertebral foramina (L1–L4); L5 foramina are not visible in this view and oblique views are necessary.

Evaluation Checklist:

• Lumbosacral junction (lower thoracic vertebrae to coccyx) or lumbar only to proximal sacrum is correctly visualized.
• Alignment of vertebrae down the center of the image.
• Confirmation of no rotation, wherein posterior margins of bodies are superimposed, and ilia nearly superimposed.
• Spinous processes should appear in profile.
• Open disk spaces and intervertebral foramina (L1–L4) should be clearly visible.

3. L5-S1 Lumbosacral Junction — Lateral Projection

Need for a Separate Spot View

The L5-S1 junction is located deep in the pelvis and often appears too dense in the full lateral view. A tight-collimated "spot" lateral projection offers a clearer visualization of this joint.

Setup for L5-S1 Lateral Projection

• IR: 10 × 12 inches (24 × 30 cm), placed lengthwise.
• Patient Position: Lateral recumbent with flexed hips and knees, placing a support between knees for comfort.
• Pillow: Adjust to ensure the mid-sagittal plane (MSP) of the head aligns with the spine.
• MCP Alignment: This should line up with the hips and shoulders, being perpendicular to the IR.
• Arms: Flex elbows with the dependent arm at a right angle to the body.
• Spine Alignment: Use radiolucent support below the lower thorax to maintain a horizontal spine (preferred method).

Two Methods for Central Ray Placement

1. Standard (ASIS) Method: Locate the elevated anterior superior iliac spine (ASIS) and center on a coronal plane 2 inches (5 cm) posterior to ASIS and 1.5 inches (3.8 cm) inferior to the iliac crest. Then, center the IR to this CR.
2. Francis Method (Interiliac Line): Identify both iliac crests and draw an imaginary line between them. Angle the CR to be parallel with this line, especially when the spine is not horizontal.

• If the spine isn't aligned horizontally, angle the CR caudad: 5° for males and 8° for females.

Collimation and Evaluation Criteria for L5-S1 Projection

• Collimation: Should be 6 × 8 inches (15 × 20 cm); due to high scatter from this projection, close collimation is essential.
• Respiration: Suspend during exposure.
• Structures Shown: L5-S1 junction along with lower 1-2 lumbar vertebrae and the upper sacrum should be visible.

Evaluation Criteria:

• The L5-S1 joint should be centered in the image.
• The lumbosacral intervertebral disk space must appear open.
• Crests of the ilia should be closely superimposed (when the beam is not angled).

Mnemonic for CR Angle: "ASIS is your BFF" to locate the CR entry point for L5-S1 lateral. Find ASIS → go 2 inches behind it and 1.5 inches below the crest.

4. Lumbar Zygapophyseal Joints — AP Oblique (RPO/LPO)

Overview

The zygapophyseal (facet) joints of the lumbar spine are angled between 30° and 60° to the mid-sagittal plane (MSP) in most patients. This angle is variable both individually and along the spine from top to bottom (cephalad to caudad). Both sides should be imaged for comparison.

Position and Rotation

Key Factors

• Starting Position: Begin supine (AP) or upright; oblique projections are generally taken right after the AP.
• Rotation: Rotate the patient approximately 45° from the supine position. Ensure that the side of interest is closest to the image receptor (IR).
• Body Alignment: The patient’s long axis should be parallel to the table, and the spine should be centered to the midline of the grid.
• Support: Provide support under the elevated shoulder, hip, and knee to reduce patient motion.
• Arms: Positioned comfortably to avoid creating artifacts during imaging.
• Respiration: Suspend at the end of expiration.
• Shield Gonads: Always protect gonads.

Analyzing Joint Visuals

In the AP oblique view, the joints rendered are those on the side closest to the IR (the "down" side); for example:

• Right Posterior Oblique (RPO): Shows right zygapophyseal joints.
• Left Posterior Oblique (LPO): Shows left zygapophyseal joints.

Central Ray and Collimation

CR Entry Points

• For lumbar region: The central ray is perpendicular, entering 2 inches (5 cm) medial to the elevated ASIS and 1-1.5 inches (2.5-3.8 cm) above the iliac crest (L3).
• For L5-S1 zygapophyseal joint: The CR is perpendicular, entering 2 inches (5 cm) medial to the elevated ASIS, midway between the iliac crest and ASIS.

Collimation Details

• General Collimation: 9 × 12 inches (23 × 30 cm) on collimator for 10 × 12 inches IR.
• Lateral Collimation: 9 × 14 inches (23 × 35 cm) for 14 × 17 inches IR and 8 × 10 inches (18 × 24 cm) for L5-S1 zygap joint only.

Evaluation Criteria for Obliques

Evaluation Checklist:

• Should visualize lower thoracic vertebrae through to the sacrum.
• Zygapophyseal joints closest to the IR must appear open and uniformly visible through the vertebral bodies.
• Pedicle Check:
      - Pedicle positioned anterior on the vertebral body indicates insufficient rotation.
      - If posterior, the patient is rotated too much.
• Ensure the vertebral column runs parallel with the tabletop to keep essential disk spaces open (T12-L1 and L1-L2).

The "Scottie Dog" Mnemonic:

• Elements of the "Scottie Dog":
    - Ear = Superior Articular Process
    - Nose = Transverse Process
    - Eye = Pedicle
    - Neck = Pars Interarticularis
    - Body = Lamina
    - Front Leg = Inferior Articular Process
    - Tail = Opposite Superior Articular Process.
  If the mnemonic appears appropriately, this suggests correct oblique positioning.

5. Sacrum & Coccyx — AP/PA Axial & Lateral Projections

General Preparation for Both Projections

To achieve optimal images:

• Bowel contents need clearance as they interfere;
• The urinary bladder must be emptied before examination.
• IR Size: 10 × 12 inches (24 × 30 cm) lengthwise for both sacrum and coccyx.

AP/PA Axial Projections

Sacrum & Coccyx Specifics

Sacrum

• Patient Position: Either supine or prone, ensuring MSP is centered with ASIS equidistant from the grid.
• Arms: Flex elbows and place them bilaterally symmetric.

Coccyx

• Similar positioning to sacrum (supine or prone) with MSP centered.

Central Ray Angles for AP/PA

• For SUPINE (AP) Sacrum:
    - Angle: 15° cephalad (towards the head).
    - Centering: 2 inches (5 cm) superior to the pubic symphysis.
• For SUPINE (AP) Coccyx:
    - Angle: 10° caudad (towards the feet).
    - Centering: 2 inches (5 cm) superior to pubic symphysis.
• For PRONE (PA) Sacrum:
    - Angle: 15° caudad, centering on the clearly visible sacral curve.
• For PRONE (PA) Coccyx:
    - Angle: 10° cephalad, centered to palpable coccyx.

Gonad Shielding

Gonads should be shielded for males only as ovaries cannot be shielded due to their exposure field location.

Respiration and Collimation

• Respiration: Suspend during exposure.
• Collimation Sizes: 10 × 12 inches (24 × 30 cm) for sacrum; 8 × 10 inches (18 × 24 cm) for coccyx.

Evaluation Criteria for Axial Projections

For AP Axial Sacrum

• Centered sacrum, visible in its entirety.
• Straightened sacral curvature, free from foreshortening.
• No overlapping pubic bones over the sacrum.
• No rotation with symmetric alae (sacral wings).

For AP Axial Coccyx

• Centered coccyx visible in its entirety.
• Coccygeal segments should not be obscured by pubic bones.
• Alignment should show that the distal segment is in line with the pubic symphysis.

Radiation Protection Note : Due to the proximity of reproductive organs to the exposure area, maintaining close collimation is essential to minimize radiation exposure to the patient.

Lateral Projection — Sacrum & Coccyx

Setup Details

• IR: 10 × 12 inches (24 × 30 cm) lengthwise.
• Patient Position: Lateral recumbent with flexed hips and knees in a comfortable position. Superimpose knees and use padding for support if necessary to maintain horizontal alignment.
• Spine Alignment: Ensure the interiliac plane is perpendicular to the IR with pelvis and shoulders correctly adjusted to achieve a true lateral view.
• Central Ray:
    - For Sacrum: Perpendicular (⊥), directed to ASIS, at 3.5 inches (9 cm) posterior.
    - For Coccyx: Perpendicular (⊥), directed towards a point 3.5 inches (9 cm) posterior to ASIS, adjusted as per pelvic curvature.
• Collimation Sizes: Sacrum: 10 × 12 inches (24 × 30 cm); Coccyx: 6 × 8 inches (15 × 20 cm).
• Shield Gonads: Yes for both.
• Respiration: Suspend during imaging.

Evaluation Criteria for Lateral Projection

Both sacrum and coccyx should be entirely visible. Additionally, evidence of a lead rubber absorber placed behind the sacrum should be observed as it absorbs backscatter, improving image quality. Confirm that:

• The posterior margins of ischia and ilia are closely superimposed indicating no rotation is present.

Lead Rubber Usage: For lateral sacrum/coccyx projections, place a sheet of lead rubber on the table behind the patient to reduce scatter radiation. This reduces the chance of AEC shutting off too early, which can lead to underexposure.

6. Quick-Reference Board Review Master CR Angle Chart