Densitometry is a technique for studying the structure of bone tissue based on layer-by-layer X-ray scanning of the bone area under study with a narrow beam of radiation. With the help of CT densitometry, it is possible to obtain a spatial image of the structure of the cortical and spongy layer, as well as accurately measure bone density. Based on quantitative values, a conclusion is made about bone mineral density (norm, osteopenia, osteoporosis) and the risk of fractures. Indications may include old age (over 65-70 years), the presence of risk factors for osteoporosis, frequent fractures in the anamnesis, long-term use of corticosteroids, diseases with impaired calcium metabolism.
As a rule, CT densitometry is performed at the final stage of diagnosis, after screening examination – peripheral bone densitometry, during which a decrease in bone density in the area of the calcaneus or hand bones is detected. Unlike peripheral bone densitometry, which allows examining only the bones of the distal extremities, CT densitometry is used to assess the condition of the spine and hip joints (including the hip neck), that is, bone structures most susceptible to fractures in osteoporosis. At the same time, CT is mainly used to assess the density of trabecular bone tissue, which is transformed faster in osteoporosis and responds faster to drug therapy.
CT-densitometry is prescribed at the stage of clarifying the diagnosis, is performed to accurately determine the severity of osteoporosis and to assess the progression of this pathology in dynamics (both during treatment and during observation in the absence of therapeutic measures). Indications for densitometry are a decrease in bone density according to screening results and conditions accompanied by an increased risk of osteoporosis.
These conditions include prolonged use of hormonal drugs (thyroid hormones, glucocorticoids), the pre- and postmenopausal period in women, age over 60 in men, some rheumatic diseases (for example, late stages of rheumatoid arthritis) and endocrine disorders (hypothyroidism, hyperthyroidism, diabetes mellitus, Itsenko-Cushing’s disease or syndrome, hypogonadism). The list of indications also includes curvature of the vertebral column (scoliosis, kyphosis), degenerative-dystrophic lesions of the spine, the presence of repeated fractures, aching back pain of unclear etiology and progressive posture disorders over the age of 40, a condition after drug therapy of previously diagnosed osteoporosis.
Pregnancy is an absolute contraindication, if it is necessary to assess bone density, this study is replaced by a safer ultrasound. Children’s age is considered a relative contraindication to CT, with congenital, acquired and juvenile osteoporosis, the procedure is prescribed only if there are good reasons (the impossibility of conducting or uninformative methods that do not involve the use of X-rays). The examination is not performed in the presence of motor anxiety of any genesis (due to neurological diseases, intense pain syndrome or mental disorders) and dimensions (weight, waist size) exceeding the technical capabilities of the equipment.
The procedure is prescribed no earlier than 2 days after the radioisotope study and no earlier than 5 days after diagnostic procedures using contrast agents. A few days before the CT scan, it is recommended to exclude foods rich in calcium from the diet, to refuse to take calcium- and phosphorus-containing drugs. It is necessary to submit all medical documentation related to the pathology of bones and joints: the results of previously performed radiographs and densitometry, the conclusions of a traumatologist, orthopedist, vertebrologist, etc.
Methodology of conducting
Before CT-densitometry, you should take off all metal objects and change into clothes without metal parts. The study is carried out in a supine position. During the diagnostic procedure, it is necessary to remain motionless. The examination does not cause any unpleasant sensations, its duration is 1-5 minutes. During this time, the medical staff is in the next room, if necessary, the patient can contact a doctor or a laboratory assistant using the built-in two-way communication system. After the diagnosis is completed, the specialist performs special calculations, prepares a conclusion and gives it to the patient.
Interpretation of results
During CT densitometry, the diagnostician determines two indicators: T-score and Z-score. In the process of determining these values, the specialist takes into account the heterogeneity of bone tissue in different areas. The T-score is the result of comparing the bone density of the subject with the reference indicator – the normal bone density of a healthy person aged 30 years. If the T-score calculated according to CT densitometry is 1 or higher, this is regarded as the norm. A decrease in this indicator from -1 to -2.5 indicates the presence of osteopenia, accompanied by a moderate probability of fractures. With a T-score of less than -2.5, osteoporosis is diagnosed with a high risk of fractures.
The Z-score is calculated based on a comparison of the patient’s bone density with the average bone density of people of the same age group. In the presence of a pronounced deviation from the norm, it is necessary to clarify the cause of violations. The conclusion is prepared within a few hours after densitometry or the next day. With the results of the study, it is necessary to contact an orthopedist, a vertebrologist, an endocrinologist or another specialist (depending on the underlying disease and the causes of diagnosis). The doctor can make a treatment plan or conduct an extended examination to clarify the diagnosis, including prescribing a bone biopsy, hormone tests, etc.
The advantage of dual-energy X-ray is considered to be high accuracy in determining bone loss (about 2%), the advantages of CT densitometry are high image quality, the possibility of layer-by-layer study of the studied bone structures, separation of signals from cortical and trabecular bone tissue, the creation of volumetric models of the spine and hip joints. Another advantage of the tomographic technique is the reduction of the duration of the procedure (from 5-20 to 1-5 minutes), which is especially important in the presence of diseases that prevent prolonged immobility.
Both methods involve the use of X-ray radiation, therefore, they have a number of contraindications associated with radiation exposure to the body. Ultrasound bone densitometry is significantly inferior in accuracy to dual-energy X-ray examination and CT densitometry, but at the same time, along with density, it allows you to assess the elasticity and stiffness of bones, it is completely safe for the body.