Bone Mineral Density

Quantitative CT bone mineral densitometry is a method to accurately measure bone density and strength. It is one of the methods cited by the National Osteoporosis Foundation as useful and safe in the evaluation of osteoporosis. The data are used to measure an important fracture risk factor and determine the necessity, choice and efficacy of therapy. The results are reproducible, allowing measurements to be taken over time showing progression of disease or improvement in bone density due to treatment.

Clinical indications for CT bone mineral densitometry defined by the National Osteoporosis Foundation include:

  1. To assess bone density of peri-menopausal women for initiation of estrogen replacement therapy.
  2. To establish a diagnosis of osteoporosis or assess its severity in the context of general clinical care.
  3. To monitor bone density in patients receiving steroid therapy.
  4. To diagnose low bone density in patients with metabolic disorders such as mild primary hyper-parathyroidism.

FREQUENTLY ASKED QUESTIONS

How is the QCT examination performed?

The QCT examination is performed on a modern CT scanner and takes approximately 10 minutes:

  • A lateral scout scan of the lower thoracic and lumbar spine is taken.
  • Scans are obtained through 4 vertebral bodies between T11 and L4. The average density of trabecular bone is measured in each vertebra and expressed and compared with a bone standard.
  • The scout and axial scans are evaluated quantitatively and any related or incidental findings (vertebral compressions, kidney stones, etc.) are noted in the report. The average bone density of the patient is then:
    • Graphically compared to age and sex matched controls.
    • Related to a fracture risk threshold below which most compression fractures occur.
    • Categorized as to prevalence of fractures in a similar age group and bone density.

Am I at risk for osteoporosis?

In the years following menopause, women will naturally experience bone loss because of a decrease in estrogen. Most will have no symptoms, even while the disease progresses. These factors add to your risk of developing osteoporosis.

  • Age
  • Caucasian or Asian Descent
  • Thin or Small Build
  • Previous Fracture
  • Family History of Osteoporosis
  • Early Menopause
  • Smoking
  • Inactive Lifestyle
  • Certain Medications (including steroids & thyroid hormones)
  • Alcohol Abuse
  • Inadequate Calcium Intake

Why is early diagnosis important?

Prevention of osteoporosis, delaying its onset or lessening its severity will result in decreasing the chance of osteoporotic fractures, which can lead to pain, lack of mobility and even increase the chance of death. There are now treatments to prevent osteoporosis, once it has been diagnosed.

How is QCT different from other methods?

CT bone densitometry and DEXA both measure spinal bone mass, a significant advantage over other methods, such as ultrasound, which measure bone in the peripheral skeleton where bone density is slow to change in response to disease therapy. Spinal bone is a mixture of trabecular (spongy) bone and slowly-changing cortical (compact) bone. DEXA measures the sum of these two compartments but also includes aortic calcification and osteophytes in the calculation of bone mineral in the spine. Only CT bone mineral densitometry isolates the metabolically-active trabecular bone for analysis. New ultrasound methods for the knee or the heel are being used for osteoporosis screening, but unlike CT or DEXA they do not measure bone mass, and cannot be used to measure the spine where most osteoporotic fractures occur.

What about radiation exposure?

All bone density measurements, except ultrasound, use small amounts of radiation to determine the amount of bone present. For a CT bone density study, the exposure is restricted to four 10 mm-thick slices of the abdomen and a lateral digital radiograph. There is no measurable gonadal exposure for the low-dose methods normally used for QCT. The relative radiation exposure to the bone marrow is about the same as 5 mrem to the whole body. For comparison, a chest x-ray would be about 3 mrem whole-body-equivalent, a cross-country plane flight is about 2 mrem, and natural background is about 365 mrem per year.

How should I prepare for the procedure?

  • Bring a copy of the order for the procedure from your referring physician.
  • Wear comfortable, loose-fitting clothing for your CT examination. Metal objects can affect the image, so avoid clothing with zippers and snaps.
  • Take your usual medications.
  • Women should always inform their doctor or x-ray technologist if they are, or could be, pregnant.
  • The maximum weight limit for the CT scanners is 400 lbs.

 

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