Improving elbow radiography - the importance of positioning and exposure
Whilst the increasing accessibility of Computerised Tomography (CT) has reduced the use of plain film radiography in some practices, a well-positioned and correctly exposed set of elbow radiographs can provide important information and still holds an important role in the diagnosis of elbow joint pathology.
There are three standard projections: flexed mediolateral, extended mediolateral and cranio-caudal. Elbow anatomy is complex and failure to position the patient correctly or to perform a minimum of two orthogonal projections can lead to missed pathology or misdiagnosis.
This article provides a refresher of elbow positioning during acquisition and assessment of images to check correct positioning has been achieved.
Extended Mediolateral view
For all mediolateral projections the patient should be in lateral recumbency with the elbow to be imaged against the table and the contralateral forelimb pulled back and secured in retraction.
The limb should be extended so that the elbow is cranial to the thorax and the angle between the humerus and radius/ulna is 110˚, avoiding overextension. The radius and ulna should be parallel to the imaging plate, placing padding under the carpus is sometimes required to achieve this.
Centre the X-ray beam on the palpable medial epicondyle of the humerus and collimate the beam to include the distal 1/3rd of the humerus and proximal 1/3rd of the radius/ulna.
Flexed Mediolateral view
This projection allows improved visualization of the anconeal process of the ulna compared with the extended mediolateral projection.
The forelimb should be extended so that the elbow is positioned cranial to the thorax. In short-legged dogs it may be necessary to place a pad behind the elbow to push the leg cranially. The elbow should be positioned so that the angle between the humerus and radius/ulna is 45°, avoiding over flexion.
Centre on the medial epicondyle of the humerus and collimate the beam to include the distal 1/3rd of the humerus and proximal 1/3rd of the radius/ulna.
Position the dog in sternal recumbency with the limb to be imaged extended cranially. Large and deep-chested dogs may need to be positioned in a V-trough. Ensure that the condyles of the humerus are equidistant from the imaging cassette or table. The contralateral limb should be in a flexed, neutral position with the head resting on top so that the head is not included within the radiograph. This can be tricky to achieve in dogs with short necks.
It can be helpful to use a radiolucent tie to prevent the elbow from rotating, a small piece of cotton wool under the elbow may also help.
Centre the beam at the level of the humeral condyles and collimate to include the distal 1/3rd of the humerus and proximal 1/3rd of the radius/ulna.
As with all orthopaedic radiography it is vital to ensure that the exposure factors used are correct. The most common error is to use too high a kV value and too low an mAs value. Whilst digital systems cope better with suboptimal exposure values than film systems, there is a risk of ‘burning out’ subtle lesions for example osteochondosis dissecans (OCD) lesions, small bone fragments or mild periarticular new bone formation.
Using a lower kV improves visualization of subtle changes and the soft tissues, but it is vital to increase the mAs to compensate. Quantum mottle is caused by too low an mAs value and reflects a lack of photons reaching the detector. This will lead to a reduction in the amount of information available to create the radiograph and loss of trabecular detail.
On this CR image there are a number of issues, the most significant of which is the excessively high kV which has caused ‘burn out’ of the olecranon process of the ulna. There is some quantum mottling of the condyles of the humerus and poor trabecular pattern which indicates that the mAs is slightly too low.
To improve this image the kV should be reduced by around 10-15kV and the mAs should be increased.
Incorrect extension/flexion of laterals
If the degree of flexion or extension is not accurate further issues may result:
- If the elbow is over-flexed this can result in the proximal ulna rotating and obscuring the margins of the medial coronoid process, thereby preventing assessment of this area.
- If the elbow is over-extended this can result in poor visualization of the anconeal process
- If the images were acquired for a BVA elbow scoring assessment, incorrect positioning may result in rejection of the images.
- It is important that on all mediolateral projections the elbow is positioned cranial to the thorax to prevent superimposition of soft tissue over the area of interest.
In this image the medial and lateral condyles of the humerus are not superimposed; there appears to be a ‘double joint edge’(arrows). This is likely to be a result of over-flexion which has caused rotation.
If the beam is not centered over the medial epicondyle of the humerus the divergence of the beam can lead to a lack of superimposition of the condyles of the humerus (obliquity), potential elongation of the joint and subtle changes being obscured in the resultant image. Poor centering can also cause exposure issues because the overlying soft tissue can effectively double the thickness of tissue in the collimated area.
In this image quantum mottle can be seen due to a lack on x-ray photons reaching the DR detector as a result of too low an mAs value. In addition the limb is not sufficiently protracted and there is superimposition of soft tissue from the sternal anatomy over the region of interest.
The area of collimation should only include the area of interest and essential markers. Positioning aids should not be included within the radiographed area, as well as increasing beam scatter which can negatively impact on image quality, the radiograph will look messy!
What to look out for when assessing elbow radiographs
Flexed Lateral Elbow
When assessing a flexed lateral elbow image, the key indicators of good positioning are:
- The humeral condyles are superimposed resulting in a single bony margin and a clearly visible joint space.
- The Anconeal process is clearly visible.
- The outline of the medial coronoid process of the elbow is clearly visible.
- The Elbow joint is not overlaying the soft tissue of the thorax.
When assessing a cranio-caudal elbow image, the key indicators of good positioning are:
- The Olecranon should be superimposed over the supratochlear foramen.
- Epicondyles (medial and lateral) are aligned and straight.
- Medial coronoid process is clearly visible.
- Appropriate radiographic exposure of all anatomy. If the leg is inadequately extended the humerus can appear under exposed and foreshortened.
Elbow radiography is a very useful imaging modality, if performed correctly. Computerised tomography (CT) is indicated if there is a suspicion of a fragmented coronoid process of the ulna, incomplete ossification of the humeral condyles and in young, active dogs without obvious radiological finding where further diagnostic imaging is indicated for diagnosis.
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