There are three methods for examining osteoporosis. What are the differences between them?

Author: Cheng Xiaoguang, Chief Physician, Beijing Jishuitan Hospital

Reviewer: Gao Xianshu, Chief Physician, Peking University First Hospital

According to the survey data of the Chinese Center for Disease Control and Prevention, the incidence of osteoporosis in my country is very high, about 6% in men and 32% in women. In addition, there are some people who do not meet the osteoporosis standard, but their bone mass has been significantly reduced.

1. What are the examination methods for osteoporosis?

The earliest method for detecting osteoporosis is to use isotopes to measure the forearm wrist joint. We call it single photon, or SPA detection method. Because this method is relatively old, it was mainly used for community screening at the beginning, and is currently rarely used in large medical institutions.

Another type is various types of ultrasound, which uses ultrasound to measure the heel, called calcaneal ultrasound. But in fact, ultrasound does not measure bone density, but the attenuation of ultrasound after passing through the bone. This attenuation has a certain correlation with bone density, which is why it can be used to detect osteoporosis.

We classify the above two methods as limb bone density measurement. This measurement method is currently mainly used for screening, such as in communities and maternal and child hospitals. These devices have a good effect, but cannot be used for diagnosis.

In terms of bone density, the most commonly used, or the best used in hospitals and academic fields is the dual-energy X-ray machine, abbreviated as DXA, which uses the principle of radiation to perform examinations.

During the examination, the patient lies on the bed and radiation passes through the parts to be examined, mainly the lumbar spine and hip joints. The detector receives the radiation and we know the bone density. Then we diagnose whether there is osteoporosis or whether the bone density is within the normal range based on the results.

There is another method that is very good from a technical point of view, called quantitative CT bone density measurement, abbreviated as QCT. It uses CT images to analyze and measure the bone density, and then makes a diagnosis based on the corresponding diagnostic criteria.

Figure 1 Original copyright image, no permission to reprint

Generally speaking, dual-energy X-ray is a flat projection, and its measurement results are affected by obesity or thinness. However, CT is a cross-sectional image, which is three-dimensional. Therefore, obesity or thinness will affect the measurement results of DXA bone density, but will not affect QCT.

Also, as people get older, many will experience vascular hyperplasia, calcification, and bone hyperplasia. These will have no effect on QCT but will have an effect on DXA. These are all situations that require attention, because these lesions will affect the judgment of the patient and will also have a certain impact on the subsequent monitoring of the efficacy of the treatment.

2. What are the criteria for DXA diagnosis of osteoporosis?

Dual-energy X-ray can measure the lumbar spine and hip joints, and a bone density value will be obtained after the measurement. When we evaluate whether a person's bone density is normal, dual-energy X-ray uses a value called T value.

The T value is to compare the bone density of a person with that of the normal population to see what range the bone density is in. It is to subtract the average bone density of the normal population from the person's own bone density value, and then divide it by the standard deviation of this population, so the unit of the T value is the number of standard deviations.

If the T value is greater than -1.0 standard deviation, the bone density is normal. If the bone density is between -1 and -2.5, it is called low bone mass. If the bone density is less than -2.5, we diagnose osteoporosis.

It is important to note that the T value is only a diagnostic standard for elderly men and postmenopausal women. If it is a young person, we do not use the T value for diagnosis. For young people, we have a concept of Z value, which is a comparison of people of the same age and gender. For example, if you are 80 years old, you should compare yourself with 80-year-olds, and if you are 70 years old, you should compare yourself with 70-year-olds. It is to compare your bone density with people of the same age.

Figure 2 Original copyright image, no permission to reprint

Z-score is generally not used for diagnosis. If the Z-score is low, it means that your bone density is lower than that of people of the same age, and it also means that there may be problems with your bones. Therefore, it is generally recommended that if the Z-score is lower than -2.0 standard deviations, you should go for further examination to see if your bones have secondary osteoporosis problems.

3. What are the criteria for QCT diagnosis of osteoporosis?

Under normal circumstances, the average bone density of the lumbar spine in young people is around 165 mg/cm3.

If the bone density of the lumbar spine is lower than 80 mg/cm3, it is osteoporosis; if it is between 80-120 mg/cm3, it is low bone mass; if it is above 120 mg/cm3, it is normal bone density.

So this is easier to understand, it is an absolute value.