Nuclear Medicine/Bone Mineral Density
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Bone Mineral Density
- 1 DXA Scan
- 1.1 Indications
- 1.2 How To Read a DXA scan
- 1.3 FRAX Calculation
- 1.4 Reporting Findings
- 1.5 Follow-Up scanning
- 1.6 Abreviations used in this document
- 1.7 References
Established Risk Factors for Osteoporosis
- Age greater than 65
- Gender - At the same age, females are at higher risk than males for an osteoporotic fracture in general. However, males are at higher risk for a hip fracture specifically.
- Weight and Height (Body Mass Index)
- A personal or family history of fracture from something simple like a fall or minor bump (also described as a "fragility fracture")
- Other causes such as certain drugs like steroids or chemotherapy
- Various metabolic conditions that put one at risk for low bone density:
- Type I Diebetes
- Long-standing untreated Hyperthyroidism
- Hypogonadism - high risk for males but normal if a female has had menopause after the age of 45
- Rheumatoid arthritis
- Consuming >3 alcoholic beverages per day
Appropriate ordering of DXA scans
- In any patient, there are certain indications for scanning that do not require further stratification:
- Any personal history of fragility fractures?(like a fracture from a simple bump or low-impact fall) -> Scanning is appropriate.
- Prior scanning showing low bone density? -> Follow up scanning is appropriate.
- Prior scanning normal but secondary causes for potential interval bone loss are significant and/or expected.
- All other patients:
- Low risk factors? -> Follow up visit for reevaluation only, no definite age is known for screening DXA
- High risk factors (as above) or with clinical suspicion of low bone density -> Scanning is appropriate
- Greater than 65 y/o? -> Scanning is appropriate
- Less than 65 y/o?:
- High risk factors (as above) or with clinical suspicion of low bone density -> Scanning is appropriate.
- Low risk factors -> Follow-up reevaluation only.
How To Read a DXA scan
- Attention needs to be given to anything which might alter the BMD, such as overlap of the hip with the ischium or dense foreign bodies. Anything aberrant density that exists within the background ROI or the ROI that draws the total hip or femoral neck will change bone density.
- If a corner of the femoral neck ROI does include part of the ischium, this usually can be manually excluded by the technologist performing the exam.
- Optimal rotation of the hip allows you to just see the lesser trochanter, which helps the computer to draw the ROIs correctly.
- Many hips have prostheses such as a hip replacement. Obviously, this cannot be used and the other hip or the forearm should be used for analysis instead.
- Significant sclerosis or scolioses should be noted and likely elevates the bone density in the spine. When describing these findings, try to not use clinical diagnoses such as "osteoarthritis of the spine," which is a clinical diagnosis and not a radiological finding. The correct term could be "sclerosis." When there is a significant amount of sclerosis, this could be due to compression fracture (see below) or degenerative changes. As patients age, sclerosis may sequentially increase over time which may cause longitudinal measurements to increase or stay the same in cases when the patient's overall bone density is decreasing over time. For these reasons, it is wise to image more than one region of the body.
- Compression fractures need to be described. There is some controversy about whether or not interpreting clinicians should use DXA to screen for fractures, but it is possible with with alternate techniques. Using appropriate scanning technique, this is sometimes called visual fracture analysis (VFA) and can be billed for in some cases. Please note that a compression fracture may be a sole indicator for treatment despite a normal DXA measurement. It is especially important to carefully observe any changes in the angle of the spine or clear changes in vertebral height. One can correlate these changes with a new-onset back pain. Most clinicians order a diagnostic radiograph of the lumbar spine if something is concerning on the DXA.
T vs Z scores
- Used for women
- Postmenopausal - no longer having periods
- Peri-menopausal - periods becoming irregular or becoming more spaced in interval
- Used for men ≥ 50 y/o
- Used for women
- Used for pre-menopausal women, children, and all men < 50 y/o
- See the table at the bottom of this page for WHO classification for postmenopausal osteoporosis
- WHY USE THIS?
- There confusion about when to treat patients pharmacologically in borderline cases.
- FRAX calculation is used to make pharmacologic treatment decisions which can be defined as treatment outside of Calcium and Vitamin D supplementation such as with a bisphosphonate. This definition is often insufficient because of other new types of treatment (i.e. Evista)
- Calculation can be applied to all patients aged 40-90 y/o.
- A known fragility fracture is a sole indication for treatment and should be included as a disclaimer when FRAX risk does not meet the threshold for treatment.
- It is found that simply using a T-score in bone mineral density (BMD) has been shown to be insufficient in assessing fracture risk in patients and is not ideal as a sole indicator in guiding bone-building or bone-retaining pharmacological treatment outside of normal vitamin D and calcium supplementation. Additionally, there is a general feeling in the public and in medicine that if you have "Osteopenia" or "Low bone density" that these are entities that require treatment. However, solely using these diagnoses for treatment decisions is controversial and does not carry the best support in the evidence-based literature.
- FRAX is a means to add clinical risk factors to the measurement of bone density, as a means for making treatment decisions. A study of approximately 60,000 patients (40-90 yr old men and women) was used to stratify the risk of fracture as it applied to the BMD of the femoral neck and/or various indicators in the patient's past medical history. This data was used to form a calculation tool to help guide treatment, called FRAX (Fracture Risk Assessment Tool). This tool can be found online and can give very helpful specified information with variables including race and nationality.
- A calculated 10 year FRAX risk of fracture of at least 3% at the hip, and at least 20% for a "major osteoporotic fracture," would indicate that the patient would benefit from pharmacological treatment.
- It must be noted that FRAX is not intended to be used as a sole determinant in "treat vs not treat." The consensus appears to be that a variety of clear indications for treatment exist, that may include:
- History of vertebral or hip fracture that is felt to be caused by low BMD.
- A diagnosis of primary osteoporosis - This diagnosis is made only if other treatable, non-primary causes of osteoporosis have been ruled out and the patient has a T-score that is less than or equal to -2.5 (also signifying -2.5 standard deviations from the mean).
- It must be remembered that the first consideration for treatment in patients with secondary osteoporosis (caused by hyperparathyroidism or corticosteroid use, for example) should be focused on approach to correct the secondary cause. However, it is yet reasonable to pharmacologically treat the patient if the clinical scenario indicates it. This must be approached on an individual, clinical basis.
- It is also reasonable to initiate pharmacologic treatment if any of the above criteria are not completely satisfied, based upon additional risk factors that may or may have not been included in the WHO fracture study. These decisions are usually up to the patient's primary care provider.
Interpreting FRAX calculation
- Using a DXA measurement of the femoral neck, FRAX is calculated with at least the following information:
- Current Smoking
- Consumption of >3 alcoholic beverages a day
- Rheumatoid Arthritis
- Glucocorticoid use such as Prednisone, at any time of life - 5mg/day for equal to or more than 3 months (not necessarily consecutive).
- Personal history of fracture not caused by a minor incident such as a low-impact fall. For example, high impact fractures.
- Family history of fracture not caused by a minor incident such as a low-impact fall.
- FRAX cannot be calculated for any patient less than 40 y/o or above 90 y/o.
- If the patient's weight is greater than 125Kg the patient's FRAX can be calculated, but is assumed to be a maximum weight of 125Kg for calculation.
- This is key! For example, African-Americans have been found to have less risk of fracture at the same BMD.
- FRAX can be calculated without the DXA scan BMD of the femoral neck; however, it may be more desirable to apply an objective measurement.
- In order to calculate FRAX without the DXA, personal risk factors and any and all causes of secondary osteoporosis must be comprehensively inquired in order to correctly stratify the patient's fracture risk. This can be done in the primary care setting without the need for any equipment other than internet access. This particular information is important to the interpretation of the DXA scan because with the use of a femoral neck BMD, it is no longer necessary to consider causes of secondary osteoporosis other than those listed above.
A brief outline of the findings should include:
- A declaration of the body part(s) studied using a definable type of equipment and technique.
- Any visual deformities of the body parts of interest or issues precluding diagnosis.
- The densities of the areas of interest and associated T or Z scores.
- The change in bone densities since last study and/or since the highest/lowest density the patient has had in the past.
- FRAX calculation (if indicated)
- According to the ISCD Follow-up is appropriate when the expected change in BMD is ≥ the LSC
- Using the LSC alone as an indication of when to call something "significantly changed" is erroneous information and does not account for the expected change in BMD for a certain ROI with a certain type of therapy.
- The Monitoring Time Interval (MTI) can be calculated using the LSC:
- When the MTI is ≤ 1.0 the measured change has exceeded the expected change for a given institution, treatment, and patient. Therefore, a repeat BMD at or after this time will result in clinically relevant information.
- LSC is calculated per the standards laid out by the ISCD (see iscd.org).
- Expected change per year is dependent on therapy type and where you measure.
- A simple method is to scan one year after initiating or changing therapy, and then at less frequent intervals once a pattern is evident.
Abreviations used in this document
- BMD - Bone Mineral Density
- DXA - also known as DEXA; bone density scan
- FRAX - Fracture Risk Assessment Tool
- ISCD - International Society for Clinical Densitometry
- LSC - Least Significant Change (Usually at the 95% confidence interval)
- ROI - Region of interest
- VFA - Visual Fracture Analysis
- ^ PMID 19426925 - "2008 Santa Fe Bone Symposium: update on osteoporosis." Lewiecki EM, Baim S, Bilezikian JP, Eastell R, LeBoff MS, Miller PD. (J Clin Densitom. 2009 Apr-Jun;12(2):135-57.)
- WHO publication - Kanis JA, on behalf of the World Health Organisation Scientific Group. Assessment of osteoporosis at the primary health care level. WHO Collaborating Centre for Metabolic Bone Diseases, University of Sheffield 2007 (available on request from the WHO Collaborating Centre or the IOF).
- ^ Online FRAX calculation tool
- ^ iscd.org - International Society of Clinical Densitometry
|Low Bone Mass ("Osteopenia," "Low Bone Density")||> -2.5 and < -1.0|
|Severe ("Established") Osteoporosis||≤ -2.5 for young-adult women, with a history of fragility fracture|