PLAIN RADIOGRAPHIC FILMS AND DIGITAL RADIOGRAPHY

In a modern medical imaging department the plain radiograph taken on X-ray film is now almost obsolete. Plain radiographic images are mostly obtained with digital equipment using a much smaller X-ray dose compared to conventional film. The resulting image is displayed on a computer screen where it can be manipulated to alter the image brightness and contrast, magnify areas of interest, and measure the size of abnormalities. Whether obtained with a digital system or on ‘hard copy’ X-ray film, plain radiographs continue to be the commonest imaging modality used in orthopaedics. They are usually the only diagnostic images required for most cases of trauma and the great majority of degenerative disorders. Fracture diagnosis and management is satisfactorily guided by plain films, as is planning joint replacement surgery for arthritic disease in the hip and knee. The use of plain films has diminished in other areas of orthopaedic practice, where for many years they were the standard imaging investigation. An example would be patients with back pain, where it is now recognised that the plain film has little to offer in evaluation and MRI is the investigation of choice.

COMPUTERISED TOMOGRAPHY (CT)

CT is a technique that utilises X-rays, but acquires multiple thin slices axially through the area of interest. With the advances in digital technology modern CT scanners can now acquire this data very rapidly. Current models in use can obtain 16 slices simultaneously, while new machines will be able to acquire 64 and even 256 slices at a time. The result is a stack of very thin radiographic images, usually about 1 mm in thickness. These can be manipulated by the CT computer to generate images in any plane with excellent detail. Typically these images are reformatted into coronal and sagittal planes so that images are similar to looking at AP and lateral radiographs (Fig. 2.1). These images can be acquired in only a few seconds and the reconstructions are also available within a similar short time. Modern scanners can also rapidly reconstruct a three-dimensional model of the scanned area (Fig. 2.2). The commonest role of CT in orthopaedics is assisting in the management of trauma patients with complex fractures. While the fracture is usually easily visualised on plain radiographs, the exact position, number and size of the fracture fragments may be more difficult to identify correctly. This is where CT and its multiplanar capability is of most use in orthopaedic patient management. Typical areas where CT is of most value are in the assessment of fractures in complex anatomical sites, such as the spine, pelvis, tibial plateau and calcaneus. Fracture treatment planning is greatly aided by the multiple views that can be obtained of these complex injuries and in selected cases three-dimensional reconstructions can also add useful information.

Fig. 2.1 Reconstructed CT images acquired in axial plane but reformatted in the coronal and sagittal planes. With current scanners the quality and detail of the images is as good as on the original axial scans. A A sagittal reconstruction of the cervical spine. B A coronal reconstruction of a hip.

Fig. 2.2 CT scan which has been reconstructed to give a three-dimensional image. On computer screen this can be rotated on real time to allow full evaluation of the bony structures in detail.

Non-traumatic conditions where CT can be useful are the evaluation of bone- and cartilage-based tumours. Although MRI is the principal method used for the anatomical staging of these tumours, CT may add further useful information regarding the extent of the ossifying or calcified portions of these tumours which an MR scan can underestimate (Fig. 2.3). Another example is osteoid osteoma, where a CT scan provides the diagnostic finding of a very small central sclerotic nidus (Fig. 2.4). A CT scan is also valuable for the full staging of bone and soft-tissue sarcomas, where it is used routinely to detect the presence of lung metastases (Fig. 2.5).

Fig. 2.3 A MRI scan of a very large mass arising from the pelvis and occupying a large part of the abdomen. B A coronal reformatted CT of the same lesion. This shows the bony origin and the bone content of the tumour which is not well seen on the MRI scan.

Fig. 2.4 An axial CT through an osteoid osteoma. The central sclerotic nidus is identified (arrow) and is best seen on CT.

Fig. 2.5 CT of lungs in a patient with an osteosarcoma of the femur demonstrating pulmonary metastases.

MAGNETIC RESONANCE IMAGING (MRI)

MRI is a new technology now commonly used in the evaluation of orthopaedic patients without the use of ionising radiation and with no known harmful effects. MR scanning can produce images in any plane and the tissue contrast can be altered to highlight particular anatomical structures and areas of pathology.

When a patient is placed into the core of a very powerful magnet the protons (hydrogen atoms) of the body align themselves to the magnetic field. If a radiofrequency pulse is then applied the protons will be tipped slightly from the line of the field and for a short time will oscillate (or resonate – hence the name). These resonating protons emit low-power radio waves which can be detected by magnetic coils within the scanner and can then be used to form a three-dimensional image. By altering how the resonance is performed and how the radio waves are captured, it is possible to formulate the images into slices in any plane. It is also possible to alter the characteristics of the image in a large variety of ways. The most common types used in orthopaedic imaging are called T1 and T2 weighted images. Images where signal from fat is suppressed, termed fat saturation, are also useful.