Introduction to imaging: Bone and joint

In the sixth part of our series, John Frank discusses bone and joint imaging

The best way to consider bone and joint imaging is to think of bone disease as "congenital" or "acquired." The suitability of the various imaging techniques depends on the type of disease.

Congenital disease

Congenital bone disease can affect a limb, a single bone, or several bones and includes conditions such as the mucopolysaccharidoses, achondroplasia, and other dwarfisms. The abnormalities may be of little consequence or part of a wider syndrome. The best way to image these conditions is using conventional x rays because the anatomy of the skeleton is affected and changes will indicate a diagnosis. However, you may occasionally need to use computed tomography or magnetic resonance imaging; nuclear medicine imaging and ultrasound have no part to play in congenital bone disease.

Figures 1 to 4 show examples of congenital malformations. Consult specialised radiology and paediatric textbooks to see the huge number of congenital abnormalities with bone manifestations.^{1 2}

Fig 1 (Above) X ray of a child's hands showing small accessory digits attached to the little fingers; of cosmetic importance only


Fig 2 (Above) X ray showing a vertebral developmental defect. Partial fusion leading to a "butterfly" vertebra, which can lead to chronic back pain. Scoliosis centred on the abnormal vertebra which has two pedicles on the right side and one on the left


Fig 3 (Above) X ray of a congenital abnormality known as osteogenesis imperfecta, in which there is abnormal fragility of the bones leading to multiple fractures, often in utero. The tibia has been pinned because of multiple fractures.


Fig 4 (Right) Chest x ray showing small clavicles

In children, care must be taken to differentiate this condition from the multiple fractures seen in non- deliberate injury and abuse. This is such a complex problem that further advice must always be sought immediately where there is a suspicion of deliberate injury.

Bone problems caused by infection

Infection of the bone and soft tissue are called osteomyelitis and cellulitis. The best way to differentiate the two is with a two phase bone scan. In this investigation, a 99-technetium labelled bone agent (a bisphosphonate) is injected. Images of the suspected area are obtained about 5 minutes after injection to get a blood pool image, which shows the distribution of the tracer throughout the soft tissues and the bone. After three hours, you then get a bone image.

In cellulitis, there is increased uptake in the blood pool image but a normal delayed bone image, but in osteomyelitis abnormalities can be seen in both images. This is because in cellulitis the underlying bone is normal, but in osteomyelitis inflammatory reactive change occurs in soft tissue, as well as the infection to the bone (fig 5).

Fig 5 (Left) Area of greatly increased uptake in the mid-left tibia, on both early and late images. The appearances suggest bone infection

Conventional x rays have little part to play in bone infection, as by the time changes are seen, the disease has been present for a long time. Magnetic resonance imaging is helpful, however, and shows altered marrow signal where there is bone infection, and alteration in muscle signal where there is muscle inflammation, such as in this case of myositis of the subclavian muscle (fig 6).

Fig 6 (Left) MRI: Signal from the muscle is generally inhomogeneous and is typical of a diffuse infection. Proton density of muscle alters due to the inflammation, so the signal obtained is irregular

Patients with diabetes have special problems. As they have diminished sensitivity to pain, they may damage a joint and continue using it, leading to the development of a Charcot or neuropathic joint. The x ray of this is abnormal, and it is impossible to tell from a conventional x ray if the joint has any infection, and this is obviously a crucial diagnosis to make (fig 7).

Fig 7 X ray of a diabetic foot shows typical disorganisation of the tarsus, and makes any diagnosis of infection difficult

The best way of showing infection is using a specialised nuclear medicine technique, either labelling the patient's white cells with ⁹⁹Tc, or labelling a monoclonal antibody to white cells. This can then be injected into the patient. The white cells will then gather at the sites of infection (fig 8).

Fig 8 (Top and centre left) Leukoscan used to detect infection

In this case, whether there was any infection in the diabetic foot was resolved by doing both nuclear medicine scans. The leukoscan used a labelled monoclonal antibody to human white cells, and the labelled white cell scan uses the patient's own white cells which are reinjected after labelling. Neither showed any abnormal uptake in the tarsus, so doctors excluded osteomyelitis. The uptake at the heel on the labelled white cell scan was due to a superficial ulcer which was infected.

Another problem requiring a diagnosis is the question of loosening or infection of an orthopaedic prosthesis. A conventional x ray will show the prosthesis (fig 9).

Fig 9 (Top right, bottom left) Images showing bilateral total knee replacements, but the left shows abnormally increased uptake around the tibial part of the prosthesis on both blood pool and delayed images, indicating infection

To show infection or loosening of a prosthesis, doctors can use nuclear medicine scan. In the first instance, a conventional two phase bone scan is used. In this, the area under question is imaged some five minutes after injection of the ⁹⁹Tc bone agent to get a blood pool image and then again at three hours, to show the bone. Where there is infection or loosening, the scans will be abnormal. Infection will show abnormalities on both scans, but in cases of loosening, there will be a localised area of abnormal uptake in the delayed image. Other sites of infection may be more difficult to diagnose, especially where there is other known disease, especially malignancy.

Degenerative conditions

Degenerative bone disease is common, and is most usually imaged with conventional x rays (figs 10 and 11), although degenerative change in soft tissue is best shown using ultrasound (fig 12).

Fig 10 (Bottom centre) X ray showing serious degenerative disease in both hips, with marked loss of the joint spaces


Fig 11 (Bottom right) X ray showing not only serious osteoarthritis in the right knee, but a total knee replacement on the left, obviously for osteoarthritis as well12


Fig 12 Ultrasound image of a patient with calcific tendinitis of the shoulder, the rotator cuff syndrome. The calcified lesion in the tendon of the shoulder is shown as a white crescent (arrowed) which casts an acoustic shadow below it

Metabolic bone disease

Metabolic bone disease is best shown using x rays.³ The commonest metabolic bone disease is osteoporosis, and this is best shown using a dual energy x ray absorption scan, which scans the lumbar spine and pelvis on a special machine, and gives a value for bone density, compared with a standard matched for age, sex, and ethnic origin (fig 13). This is a reproducible test, and can be used serially to monitor treatment for osteoporosis. The radiation to the patient from this test is much less than using computed tomography to determine bone mineral density.

Fig 13 Normal range and values are printed out and the white square in the blue lines shows the values for the particular patient

A common form of metabolic bone disease in the United Kingdom is Paget's disease. In this disease, there is extensive vascularity of the bone with increased marrow fibrosis and intense cellular activity. This accounts for the overgrowth of the bone and enlargement shown in x ray images (figs 14 and 15).

Fig 14 X ray clearly showing the thickened cortex and trabecular architecture typical of Paget's disease of the bone. Although this looks very dense, it may be metabolically active, and this is best shown on a bone scan


Fig 15 Bone scan showing the increased uptake in the cortex, exactly mirroring the x ray

In other rarer forms of metabolic bone disease, such as rickets, hyperparathyroid bone disease, acromegaly, and so on, there are well marked skeletal changes, and a standard radiology textbook should be consulted.¹²

Neoplastic bone disease

Neoplastic bone disease may be either primary or secondary. Secondary malignant bone disease is much more common than primary, and is best imaged using either nuclear medicine or magnetic resonance imaging. Although magnetic resonance imaging is more sensitive than bone scanning and does not use ionising radiation, it is more expensive and takes much longer. For this reason, bone scanning is the primary investigation in the United Kingdom.

Most bone scans in metastatic disease look fairly similar and simply tell us that there is widespread disease. They do not usually indicate the primary.

Conventional x ray imaging has little place in surveying for metastatic bone disease (fig 16), with one exception--multiple myeloma. Myeloma is an osteoclastic disease, and so does not show up on a bone scan, as it does not create any reaction within the bone. For this reason, doctors need to do a conventional radiological skeletal survey when looking for myeloma.

Fig 16 Bone scanning is widely used to look for metastases from commonly occurring primaries such as breast, bronchus, and prostate. This example shows a bone scan in a patient with prostatic cancer

In primary bone neoplasms, the age of the patient and the site of the disease are important pointers to the diagnosis. Although conventional x ray images are often obtained, the best form of imaging is magnetic resonance imaging (figs 17 and 18).

Fig 17 A bone scan of a child with osteosarcoma

Fig 18 MRI showing deposits in the cervical vertebrae (shown as black within the marrow) are compressing the spinal cord

A child presented with a hot painful knee. The upper tibia is irregularly dense with bone destruction and the bone scan shows that the whole area is metabolically extremely active. From the age and site, the diagnosis of osteosarcoma is supported, but a biopsy is needed for confirmation. Remember to send material to microbiology for culture and sensitivity at the time of biopsy to rule out acute osteomyelitis, which in a child can look similar radiologically. Repeated sub-acute trauma can show the same appearances (see fig 17).

Magnetic resonance imaging is also used in secondary malignant bone disease, especially where there are neurological complications as surgical intervention may be needed, and the exquisite detail on a magnetic resonance image is invaluable to the surgeon.

Trauma

The easiest way of showing a fracture in a bone is using conventional x rays (fig 19).

Fig 19 The x ray shows images of a distal radius and ulna, with a Colles fracture. The importance of x raying the suspected site in two planes and at right angles to each other is vital, as the fracture may be missed

Apart from conventional x rays, fractures can also be shown using magnetic resonance imaging (fig 20).

Fig 20 The two images (left) show a wedge fracture of L1: conventional x ray image (left) and corresponding magnetic resonance image (right)20a

Apart from direct trauma resulting in fractures, as above, there are other forms of trauma such as barotrauma (from increased pressure such as deep diving), which can lead to bone infarcts. These are best shown using magnetic resonance imaging (figs 21, 22, and 23).

Fig 21 Infarcts are clearly seen as irregular areas of altered bone signal (here white) within the medulla of the bone. Other alterations of blood flow to the bone may lead to avascular necrosis


Fig 22 These two images show the x ray appearances with flattening of the femoral head, and the nuclear medicine bone scan of the left hip showing a "cold" femoral head caused by lack of blood flow


Fig 23 Avascular necrosis of the medial femoral condyle is also common and is best shown with magnetic resonance imaging. This shows the altered bone signal with some bone bruising, shown as an irregular low signal (dark) area within the normal bone signal (white)

A further important use of all imaging techniques is in sports related injuries, and these are quite common, ranging from pulled and torn muscles through to serious fractures.

Many conditions can affect bones and joints, and doctors can investigate these in many ways. A logical approach will help, and remember to ask definite questions so that you get the appropriate answer. Asking advice from the imaging department will help in difficult or rare conditions.⁴

John Frank consultant in nuclear medicine and radiology, Charing Cross Hospital, London W6 8RF
Email: jfrank@hhnt.org

I thank Adam Mitchell for help with several of the magnetic resonance images.

studentBMJ 2004;12:89-132 March ISSN 0966-6494

1. Murray RO, Jacobson HG. Radiology of skeletal disorders. London: Churchill Livingstone, 1977.
2. Grainger RG, Allison DJ, Adam A, Dixon AK. Grainger and Allison's diagnostic radiology: a textbook of medical imaging. London: Churchill Livingstone, 2001.
3. Carpenter CCJ, Griggs RC, Loscalzo J, Andreoli TE, eds. CECIL: essentials of medicine. London: Saunders, 2000.
4. Royal College of Radiologists. Making the best use of a department of clinical radiology. 5th ed. London: RCoR, 2003.