Back to Basics

Simple solutions for the lumbar vertebrae

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Figure 1.) Radiograph of the AP lumbar spine: 1. transverse process, 2. spinous process (Olive Peart, MS, RT(R)(M))

Figure 2.) Sagging of the lateral lumbar spine: Tube angulation is used to project the central ray parallel through the joint spaces.

Figure 3.) Radiograph of the lateral lumbar spine: 1. intervertebral foramina, 2. spinous process, 3. body of L4, 4. intervertebral disc space, 5. joint space between L5/S1

Figure 4.) The Scottie dog

Figure 5.) Radiograph of the oblique lumbar spine: 1. inferior articular process (forefoot), 2. lamina (body), 3. superior articular process (ear), 4. pedicle (eye), 5. transverse process (nose), 6. pars interarticularis (neck)

When it's time to go back to the basics, one of the best places to start is with anatomy. The lumbar vertebrae are the largest of the spinal vertebrae. The main and biggest part of each vertebra is called the body. From either side of the body the pedicle extends posteriorly with the transverse process extending laterally at the junction between the pedicle and the lamina. The part of the lamina between the superior and inferior articular process is called the pars interarticularis. The spinous process extends posterior.

The lumbar spine is imaged in the anterioposterior (AP) or posterioanterior (PA) projection, plus lateral and oblique. In addition, a lateral coned projection of the L5/S1 junction is always imaged. Although not always requested, the patient should receive bowel preparation prior to imaging the lumbar vertebrae because of the location of the vertebrae behind the intestinal tract. Proper internal preparation will remove fecal material or gases from obscuring areas of the anatomy. Gonadal shielding can be used for both male and female patients on all lumbar spine projections.

AP or PA Lumbar

Positioning for the AP lumbar begins with the patient supine on the X-ray table. Because of the thickness of the back, the lumbar spine is always imaged using a grid. The patient should be centered with the midsagittal plane in the midline of the table and centered to the grid. Hands and arms should be moved from the collimated field. The central ray is directed to the middle at the level of L3, about 1.5 inches above the crest. In the AP position, with the patient's feet extended, the lumbar spine curves up (concave, facing the tabletop). This generally exaggerates the lumbar curvatures and results in distortion of the intervertebral disc spaces. To reduce the lordotic curvature the patient should slightly flex the knees or a pillow support can be placed under the knees.

The PA lumbar actually places the intervertebral disc space so that the divergent beam runs parallel to the joint space. This position also compresses the abdominal contents, reducing radiation dose to the patient.

The radiograph should demonstrate the lumbar bodies, intervertebral disc spaces and transverse process clearly. The last intervertebral disc space at the L5/S1 junction will not be seen because of the spine curvature. The spinal processes are demonstrated in the midline. The sacroiliac joint should be equidistant from the vertebral column (Figure 1).

Lateral Lumbar

From the AP or PA recumbent position the patient is rotated to the affected side with the hips and knees flexed and superimposed. The patient should be aligned so that the midcoronal plane is centered in the midline of the tabletop and centered to the grid. Both arms can be flexed to the prayer position, to remove them from the field of view and to ensure patient stability.

The patient's spine should be horizontal and the central ray directed perpendicular to the spine at the L3 level. If the spine cannot be positioned horizontally, either because a wide hip or a wide shoulder is causing sagging in the midline, there are two possible alternatives. If the patient has a wide hip, a 5 degree to 8 degree caudal angulation will direct the central ray perpendicular to the spine. Generally, the smaller degree is necessary for men and the larger degree for women. The reverse is necessary for a patient with wide shoulders, i.e. cephalic tube angulation.

The alternative is to build up the area of sag using radiolucent sponge supports to ensure that the spine is horizontal (Figure 2).

Because of the high kVp necessary when imaging the lateral lumbar, placing a sheet of lead rubber on the table behind the patient will help reduce the amount of scattered radiation that reaches the film and will improve contrast.

The radiograph should demonstrate the area from the lower thoracic to the mid-sacrum region. The vertebral disc spaces should be open and the spinous process clearly seen (Figure 3).

Oblique Lumbar

The oblique lumbar can be imaged in either the AP or PA oblique position. Radiographs are generally obtained of both sides, therefore, if imaging AP the patient will be positioned for both the right posterior oblique (RPO) and the left posterior oblique (LPO). When imaging PA, the right anterior oblique (RAO) and the left anterior oblique (LAO) positions are utilized.

From the supine, the patient is rotated 45 degrees to the affected side. The AP oblique will demonstrate the zygapophyseal joints closest to the image receptor. The long axis of the patient should remain parallel to the tabletop. When rotating the patient, take care to maintain an equal rotation of both the shoulder and the hip joints.

The central ray is directed 2 inches medial to the raised anterior superior iliac spine (ASIS) at approximately 1.5 inches above the crest (at the level of L3).

The radiograph should show the open zygapophyseal joints closest to the image receptor (IR) on the AP and the joints farthest from the IR on the PA. A true oblique will also demonstrate parts of the "Scottie dog," anatomy that appears on the film as a small dog. The superior articular process is the ear, the transverse process the nose, the pedicle the eyes, the pars interarticularis the neck, the lamina the body and the inferior articular process the forefoot (Figures 4 and 5).

Coned L5/S1 Junction

The L5/S1 junction is often examined in the lateral projection and included in the lumbar spine study. The patient should lie laterally with the hips and knees flexed with arms and hands raised from the area of interest. Because of the high kVp necessary for penetration, close collimation and the use of a lead sheet placed behind the patient will help reduce scatter and improve image quality.

To demonstrate the joint, the central ray should be directed 2 inches posterior to the ASIS and 1.5 inches inferior to the iliac crest. The central ray must run directly through to the joint. If the spine is not horizontal, patient position must be corrected before exposure or alternatively, a 5 to 8 degree caudal tube angulation can be used. The radiograph should demonstrate an open lumbar sacral junction (Figure 6).

— Olive Peart, MS, RT(R)(M), is the clinical instructor of the radiology program at Stamford Hospital in Connecticut, as well as an editorial consultant for and regular contributor to RT Image. Comments and questions can be directed to editorial@rt-image.com.