Indication

The pelvis and hips may be examined for the following reasons:

Perthes’

A disorder of the upper femoral epiphysis, this manifests as osteonecrosis of the capital femoral epiphysis.² This is where the growing epiphysis shows ischaemic changes. It presents most commonly in children aged 4–9 years, and boys are four times more likely to be affected than girls.³

Slipped upper femoral epiphysis

The epiphysis is displaced, usually medially and posteriorly. The patient often presents with spontaneous pain in the groin. Thirty percent of patients suffering from this are obese, and the condition is possibly related to hormonal imbalance.³

Anteroposterior (AP) pelvis and hips (Fig. 10.2A,B,C)

There are two methods for producing the AP pelvis: the first demonstrates the full pelvis and hip joints, the second is required for hips only. This second projection is usually requested for follow-up after hip replacement surgery and may be referred to as a ‘low centred’ pelvis. However, it is a valid and recognised projection, having been described in texts for many years.⁴^–⁷ Radiographers should not, therefore, assume that this is simply a ‘mis-centred pelvis’ and consider it to be a suboptimal procedure. Field size is as for the pelvis and hips AP, as it is often necessary to include a longer section of the femur to ensure the whole length of hip prosthesis is demonstrated (if relevant).

Figure 10.2 (A) AP position – pelvis and hips. (B) AP pelvis and hips; (C) centring the AP pelvis for hips.

IR is horizontal, employed with antiscatter grid (for adults).

Positioning

• The patient lies supine on the table with their legs extended and their head resting on a pillow

• The median sagittal plane (MSP) is at 90° to the table-top and the anterior superior iliac spines (ASISs) should be equal distance from the table-top

• The arms are raised onto the pillow

• The legs are slightly internally rotated to bring the necks of femora parallel to the table-top

• Gonad protection is applied if appropriate. Exceptions are when examining the female pelvis in trauma, for a first examination of a child, or when there is a non-specific region of pain in the first examination

For this projection both feet are internally rotated slightly during positioning to bring them into the true anatomical position and allow the neck of femur (NOF) to lie parallel to the image receptor (IR).^4,⁷ This facilitates demonstration of the femoral neck with minimal foreshortening, and also clears the greater trochanter from the femoral neck, the lesser trochanter appearing in profile medially. If the feet are excessively internally rotated the lesser trochanter will be obscured and if they are externally rotated, or even if the toes point in an upwards and vertical direction, the NOF will appear foreshortened on the resulting image, with the greater trochanter superimposed over the neck.^4,⁷

In the case of trauma, foot position can provide an indication of a fractured NOF. The patient will present with the affected leg in noticeable external rotation, often with the lateral aspect of the foot in contact with the trolley top and apparent shortening of the leg. No attempt must be made to move or internally rotate this leg.²

A line known as Shenton’s line (Fig. 10.3) follows the curve of the upper border of the obturator foramen and continues to travel inferiorly down the medial border of the femoral neck. This line can be used as a guide to compare the two sides when checking for injury, as a disruption in the normally smooth, curved line indicates subluxation, dislocation or change in femoral neck position as a result of fracture.

Figure 10.3 Shenton’s line.

Overexposure of the greater trochanters can be a problem in this projection, particularly in thin patients who have little soft tissue in this region. This can be resolved by careful consideration of exposure factors: a reduction in mAs will reduce the degree of image blackening, and to cater for this a kVp of at least 70 will reduce the level of subject contrast.^4,⁷ It is suggested that a minimum of 70 kVp be used in all AP pelvis examinations.

Some patients will present with a body shape which is relatively slim over the legs and hip joint but larger over the pelvis and abdomen. This shape appears to occur most frequently in elderly women and can pose a problem in producing an image with a useful range of densities. The use of a kVp even higher than 70 may be useful in these cases, as the central region of the pelvis may appear underexposed when the hips appear correctly exposed (and vice versa) on the image when insufficient kVp is used. Use of an automatic exposure device can create problems in patients with this type of build: use of the outer chambers will result in termination of exposure related to thinner body tissue areas, therefore the central pelvic area will be of low radiographic density; use of the central chamber may mean that the area over the hips is over-blackened.

Beam direction and focus receptor distance (FRD)

Vertical at 90° to the IR

115–120 cm FRD

Note that the FRD suggested is longer than the traditional 100 cm used for the majority of radiographic projections. The buttocks elevate the pelvis, which is a relatively large structure, thereby increasing object–receptor distance (ORD) and magnification of the pelvis. The larger pelvis will potentially be less likely to be included within the perimeter of the IR; to overcome this, increasing the FRD reduces this magnification and improves on image sharpness.

Those patients with a noticeably larger amount of adipose tissue will, in effect, find their pelvis raised even higher above the IR than slimmer patients. Therefore, an FRD of 120 cm is recommended in these patients.

Centring point

For the pelvis and hips: in the midline, midway between the ASIS and the upper border of the symphysis pubis

While centring, it is wise to check that the tops of the iliac crests lie within the upper border of the IR; this will ensure that the maximum amount of anatomy distal to the iliac crests is demonstrated on the image.

For the hips: in the midline, 2.5 cm above the superior border of the symphysis pubis (the upper border of the symphysis pubis is located level with the greater trochanters)

Collimation

For the pelvis and hips: iliac crests, proximal portion of femora, greater and lesser trochanters

The IR may be aligned with the X-ray beam before examining the patient and, as originally suggested by Unett and Royle ⁴ and later by others,⁷ collimation can be adjusted at this point. This avoids the temptation to open the collimators wider than necessary when X-raying a larger than average patient.

For the hips: acetabulae, greater and lesser trochanters, upper third of femur or full length of the prosthesis (if present)

If previous images are available it is recommended that they be viewed to establish the length of any surgical device that may be present in the hip/s to ensure that the IR is positioned correctly for their inclusion.

Criteria for assessing image quality

• Iliac crests and greater and lesser trochanters are demonstrated for full pelvis, acetabulae, trochanters and appropriate amount of femur for the hips only

• Iliac bones, heads and necks of femora and the greater and lesser trochanters and obturator foramina should be symmetrical

• Sharp image demonstrating the range of densities of the bony cortex and trabeculae of the pelvis and its soft tissues, hips and trochanters

Common errors	Possible reasons
Asymmetry of structures	MSP not 90° to table-top (rotated patient). This could be due to muscular atrophy or simply the patient lying awkwardly. Use of radiolucent pads may help correct this in the case of muscular atrophy. The notable features in the pelvis are the obturator foraminae and iliac bones; study of a rotated pelvis image will show a larger obtutator foramen (compared with the other obturator foramen) and narrowed ilium on the side that is raised from the table
Greater trochanters obscured and overlying the NOF	Feet are not internally rotated (this is unavoidable in patients with fractured NOF)
Overexposed image (see section after positioning, for overexposure of the greater trochanters and uneven exposure of hips and pelvis)	If an automatic exposure device (AED) has been used for a patient with hip prostheses the exposure will continue for longer than necessary to try to expose the hips correctly. There are other problems associated with AED use (see comments after the positioning section for the pelvis). Setting a manual exposure is a suitable solution in both events described here