Sacrum

Trauma relating to the sacrum may well be associated with other injury to the pelvic ring and imaging of the pelvis is likely to be required in addition to examination of the sacrum. In cases of severe trauma it is inadvisable to undertake a lateral projection of the sacral area in the position described in this section; a horizontal beam approach would be the method of choice. However, as serious pelvis trauma will probably be assessed by initial pelvis images which will be supplemented by computed tomography (CT) examination, a lateral sacrum image is unlikely to be required.

The sacrum may be a site for metastatic spread of malignancy and plain images of the region would demonstrate such lesions if the secondary tumour has eroded at least 40% of the bone. Magnetic resonance imaging (MRI) is most appropriate for assessing bone metastases.¹ Today it is rare to find radiography requested for assessment of this area, and there is no longer specific reference to assessment of the sacrum in current Royal College of Radiologists (RCR) guidelines in the UK.

The lumbar curve varies with each individual patient and causes variation in the angles created between the sacrum and lumbar vertebrae. As a result it is suggested that the cranial angulation required to strike the sacrum at 90° in the anteroposterior (AP) position will vary from 10° to 25°, according to the individual patient’s build. The most efficient strategy for making a decision on appropriate angulation is to undertake the lateral projection initially and use it to assess the required angle before proceeding with the AP projection. This also applies to the coccyx.

If the examination request outlines that information on the coccyx is required, the lateral projection of the coccyx can be included on the lateral sacrum projection, to reduce the number of exposures. For this reason, the coccyx is also referred to in the description of the lateral projection.

For all projections of the sacrum and coccyx, the image receptor (IR) is horizontal and antiscatter grid is used

Lateral sacrum (Fig. 14.1A,B)

Positioning

• The patient lies on their side, with hips and knees flexed to maintain stability and the feet placed together to prevent the patient from rolling forwards or backwards. The arms are flexed at the elbow and raised to rest on the pillows for comfort and to clear them from the area of interest

• Lead rubber is applied over the raised side, diagonally from anterior superior iliac spine (ASIS) to the femoral head, to cover the anterior portion of the pelvis and to protect the gonads

• The palm of the radiographer’s hand is used to palpate the posterior aspect of the sacrum and ensure that its transverse axis is perpendicular to the table-top

• The long axis of the sacrum is parallel to the table-top; this should be checked with the area at the radiographer’s eye level for accuracy. If patient build affects the relationship of the sacrum to the table-top, a compensating cranial or caudal central ray can be used (see beam direction, below)

Figure 14.1 Lateral sacrum.

Beam direction and focus receptor distance (FRD)

Vertical, directed at 90° to the long axis of the sacrum once this has been assessed

100 cm FRD

Centring

Midway between the posterior superior iliac spines (PSISs) and sacrococcygeal junction

As the coccyx is more difficult to palpate than the sacrum, the level of its first segment may be difficult to locate. An alternative method to palpation uses the relationship of sacrococcygeal junction, which lies approximately level with the midpoint of the upper border of the symphysis pubis; palpation of the symphysis pubis anteriorly will allow the radiographer to estimate the level of the first coccygeal segment posteriorly.

Collimation

Lumbosacral junction, sacral promontory, soft tissues overlying the sacrum posteriorly, coccyx

Criteria for assessing image quality

• Lumbosacral junction, sacral promontory, soft tissues overlying the sacrum posteriorly and coccyx are demonstrated. Omission of the coccyx from the field may be acceptable if demonstration of the coccyx is not required specifically for the examination

• Joint space at the lumbosacral junction is demonstrated

• Ala of sacrum superimposed*

• Sharp image demonstrating bony trabeculae. Adequate penetration to demonstrate detail of sacrum and less dense coccyx on the one image

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