Musculoskeletal ImagingComputed tomography (CT)
a planar, transaxial imaging method providing excellent contrast
resolution, which can be used to evaluate the musculoskeletal system. Computed tomography (
CT scanning) can often define alterations in soft tissue and bone that are undetectable with conventional radiography because of its cross-sectional display, excellent contrast
resolution, and ability to measure specific attenuation values.
CT scanning is also capable of making quantitative measurements of bone mineral content.
CT images are produced in computers, which allows refomation of transaxial data in the
coronal or
sagittal plane and three-dimensional analysis of image data.
For a general description, see computed tomography CT and CT generation.
The use of CT numbers, or Hounsfield units provides an indication of the nature of the tissues being imaged (Table 1). For example, a lesion with an attenuation value close to that of water is likely to be a cyst. The measurement of attenuation values of intraosseous lesions may be somewhat difficult, however, especially in narrow bones in which the contribution of the cortex may prohibit accurate assessment. For imaging bones, a near-maximum window width (1,000 - 2,000 HU) and a relatively high window level (200 250 HU) allow good visualization; for soft tissues, a window width of 400 - 600 HU and a window level of 0 - 100 HU are generally acceptable.
Computed tomography (CT), Table 1. CT numbers for various tissues
| Tissue | CT number (HU) |
|---|
| Bone | 1000 |
| Liver | 40 - 60 |
| White matter (brain) | 46 |
| Grey matter (brain) | 43 |
| Blood | 40 |
| Muscle | 10 - 40 |
| Kidney | 30 |
| Cerebrospinal fluid | 15 |
| Water | 0 |
| Fat | -50 -100 |
| Air | -1000 |
The administration of a radiopaque contrast material or injection of air into a joint can be a useful adjunct to CT of the musculoskeletal system, as in the assessment of the vascularity of a soft tissue or osseous lesion, in the identification of recurrent herniated intervertebral disc, or in the evaluation of the glenoid labrum, patellar cartilage, synovial plicae and cruciate ligaments of the knee.
Three-dimensional display helps in the imaging of regions of complex anatomy in the musculoskeletal system, such as the face, pelvis, spine, shoulder, wrist, knee, midfoot and hindfoot. Such displays facilitate surgical planning and even allow rehearsal surgery of complex reconstructive procedures.
CT can be used to evaluate many anatomic regions of the body and a variety of musculoskeletal disorders. In particular, CT scanning is of value of assessment of trauma (fractures, dislocations, cartilaginous and ligamentous injuries), infection (osteomyelitis, septic arthritis), neoplasms, joint disease, neuromuscular disease, vascular lesions (aneurysms, arterial entrapment syndromes), congenital or metabolic disease, and low back pain.
In the spine, CT has largely replaced conventional tomography as the technique of choice after routine radiography in the evaluation of complex fractures and dislocations. Other lesions, such as dislocations of the sternoclavicular and glenohumeral joints, may be extremely difficult to diagnose by conventional imaging techniques amd require CT scanning. In musculoskeletal neoplasms, CT has not replaced conventional radiographic techniques, but it has become more important in the surgical planning for treatment of primary bone neoplasms.
DR