Dual-energy X-ray absorptiometry overview -
The accuracy of LM and FM values produced by DXA strictly depends on the multi-step process of whole-body scan execution. This multi-step process is composed of three parts, each of them of great importance: subject preparation, subject positioning, and scan post-processing.
Figure 1 shows an example of two properly positioned and analysed DXA BC scan. Proper patient preparation is crucial for maintaining the biological variability of LM and FM measurements as low as possible. It is well known that BC evaluation is influenced by hydration and digestive tract content.
Meals have been associated with an increase of trunk and whole body lean mass values Exercise is another condition that can increase LM values at limbs with a decrease of trunk LM, due to fluid movement from trunk to periphery Body FM seems less affected by meals and exercise, but can vary according to the hydration status.
As a general rule, the good practice would be to measure BC with DXA always in standardized conditions in terms of activities, food intake, hour of the day.
Patient positioning is another critical aspect, as errors in this step lead to unavoidable difficulties in the subsequent analysis process. The first step is to ensure that subjects receiving the DXA remove all external metal that may interfere and attenuate the X-ray beam, thus creating artifacts.
Patients should be positioned in the center of the scan with the use of reference lines on the scan table central line and border limit lines. ISCD official positions suggest the following method for positioning the patient that should be used whenever possible 24 , The upper limbs should lie along the body, with hands palm down without any superimposition with the body.
Feet should stay in a neutral or slightly internal rotated position, with residual space left between the lower limbs. Head should be positioned whit chin in neutral position, face up; pillows should not be used 6. As a general rule, symmetry should be maintained between the left and right side when placing regional lines, in order to include on both sides the same amount of soft tissue and bone.
The position of major lines is similar for both GE Lunar and Hologic densitometers: the upper horizontal line has to be placed just under the jaw, while the pelvis horizontal placement is just above the iliac crests. One last vertical line should be placed between legs to separate them 4 , See Figure 1 for a detailed explanation of DXA analysis.
In addition, Figure 2 shows some examples of inaccuracies in positioning and analysing a whole body DXA scan. Sarcopenia is a disease typically associated with advancing age, characterized by a progressive and generalized loss of skeletal muscle mass and strength 13 , 26 - There is increasing awareness of the medical community for this condition, as it has been estimated a consistent increase of the prevalence of sarcopenia due to advancing age, as well as for its association with several negative outcomes such as physical disability, falls, hospitalization 30 - The disease is costly to healthcare systems, as it has been shown that the cost of care during hospitalization is increased in subject with sarcopenia In , the EWGSOP revised its operational definition of sarcopenia, giving primary importance to muscle strength as the most important parameter for predicting adverse outcomes More practically, according to EWGSOP definition, a diagnosis of sarcopenia is probable when low muscle strength is detected.
When also low physical performance is present, the diagnosis of sarcopenia is considered severe Each of these parameters can be measured: muscle strength and physical performance are typically measured by means of different tests such as grip strength, chair stand, gait speed or the Short Physical Performance Battery; on the contrary, the evaluation of muscle quantity or quality needs the use of imaging or non-imaging techniques, such as DXA, MRI, CT or Bioelectrical impedance analysis BIA 15 , 34 , Among the different imaging techniques available to quantify muscle mass, DXA is currently favoured by clinicians and working groups DXA is capable of determining regional and total body muscle quantity, providing both absolute values of LM as well as LM indices, such as the appendicular skeletal muscle mass ASMM.
Nevertheless, it has to be taken into account that muscle mass correlates to body size. As a consequence, values from DXA are typically adjusted to body parameters such as height 2 , weight, or body mass index BMI In fact, such diagnosis needs the concurrent presence of low muscle strength detected at functional tests.
In addition to this, the medical community is debating about the clinical value of combining the use of ASMMI and femoral neck BMD to predict fracture risk, similarly to what is commonly performed in the Fracture Risk Assessment Tool FRAX ® To date, literature evidence suggests that DXA-derived ASMMI has a limited role in the prediction of incident fractures Despite the well-known link between bone and muscle, fracture is just one outcome resulting from sarcopenia, and ASMMI may have better value for identifying other adverse outcomes such as falls.
In their editorial, Harvey concludes that the use of DXA-derived lean mass indices still does not provide additional risk information when BMD is also considered; further studies are warranted to understand the role of possible use of ASMMI in the FRAX.
The possible introduction of newer DXA based tools for evaluating bone strain together with body composition may even provide additional information for fracture risk prediction A possible DXA drawback is related to the fact that densitometers of different brands may not give the same results 36 , A practical suggestion is to perform the follow-up scan always with the same densitometer, to remove such source of variability.
Several adipose indices can be obtained with whole body DXA, some of them being similar to lean indices. These values help in providing additional data about whole body and regional FM distribution. FMI is calculated as the total body fat mass with height 2 adjustment.
The use of FMI has been advocated to diagnose obesity, similarly to what done by BMI. The rationale of using FMI is based on the assumption that BMI, despite its ease of use, is not a direct adiposity measurement parameter On the contrary, despite the use of FMI which is a direct measurement of adiposity has its rationale, there is an ongoing debate about what cut-off point should be used to better diagnose obesity.
Figure 3 compares BMI thresholds to a list of FMI cut-off points proposed for defining obesity categories both for male and female subjects using FMI, according to the National Health and Nutrition Examination Survey NHANES reference values Nevertheless, it is still unclear which associations exist between such thresholds and obesity related negative outcomes.
Regarding obesity, several studies clearly showed that DXA measurements of FM including FMI are strongly correlated with negative cardiovascular and metabolic outcomes, independent of BMI 44 , A recent study from Vasan et al.
evaluated the associations between DXA values and conventional anthropometry measurements of fat waist and hip circumference , together with cardiovascular disease CVD risk markers The result of this study confirmed that conventional anthropometry underestimated the associations of regional adiposity subcutaneous and visceral fat with diabetes and CVD risk markers.
Another parameter that can be used for evaluating the CVD risk is the AG fat mass ratio, which is analogue to the more commonly used anthropomorphic measurement of waist-to-hip ratio DXA machines obtain this value as the ratio between the android and gynoid region of interest ROI.
Android ROI is typically defined as the region between the last thoracic rib and the upper part of iliac wings. Gynoid ROI is located below the android ROI, and includes the gluteo-femoral region with an upper horizontal line placed caudally to the pelvis line, and a lower horizontal line identified by measuring twice the height of the android ROI 4.
It has been shown that an increase in the android fat distribution with values of AG ratio greater than 1 is associated with conditions such as dyslipidemia and insulin resistance, as well as other cardiovascular risk factors such as impaired glucose tolerance, hypercholesterolemia, hypertriglyceridemia, and hypertension 47 - Unfortunately, as for FMI, despite the evidence of a strong correlation between the increasing amount of android fat and CVD risk, there is still no consensus on the possible cut-off points to be used for defining a specific high-risk patient.
As a matter of fact, DXA is currently the only technique which is capable of identifying all these conditions at the same time, by evaluating the presence of low ASMMI together with adipose indices such as FMI and BMD with DXA performed at lumbar spine and femur.
Figure 4 compares two subjects with reduced muscle mass according to ASMMI values, but with different percentage of fat mass as evaluated with FMI. In fact, the association of lipodystrophy with agents such as zidovudine and stavudine has been extensively reported, being confirmed in a systematic review of randomized controlled trials Nevertheless, the ISCD Reporting guideline of clearly specified that a consensus about the values to be accepted has never been reached Recently in a study by Alikhani et al.
evaluated the prevalence of lipodystrophy in HIV patients and the association to cumulative exposure of newer antiretroviral drugs agents Surprisingly, lipodystrophy resulted still very common in HIV infected patients, being correlated with the duration of some new antiretroviral drugs such as raltegravir.
Such results keep open the possibility to still use DXA for the evaluation of suspected lipodystrophy, and probably suggests additional investigations are required to understand the utility of such adipose indices. One of the most recent developments for whole body DXA is the possibility to evaluate visceral adipose tissue VAT.
This can be done with both GE and Hologic densitometers, thanks to new software which are called CoreScan TM for GE-Lunar and InnerCore TM for Hologic 4. This software firstly estimates the amount of subcutaneous fat SAT in the android ROI of DXA scan, by detecting the fat located on both sides of the abdominal cavity.
This estimate of SAT is then subtracted from the total FM in the android ROI, thus providing the final amount of VAT.
Figure 5 explains the working principle beyond VAT estimation by DXA. DXA VAT measurements have several advantages over CT, first and foremost related to lower radiation dose provided to patients. The clinical importance of VAT is widely recognized, as it has been showed that VAT is a better predictor of mortality than SAT or AG ratio Thus, it is possible that VAT will replace the AG ratio as a risk factor for the assessment of CVD risk with DXA It is important to consider that even subjects with normal values of BMI may have an increased amount of VAT accumulation, thus being at higher risk than that estimated by only conventional anthropometry measurements Hip fractures are associated with increased short-term mortality and high morbidity [ 3 ].
Hip, vertebral, and radius fractures increase the risk of future fractures. A decrease in bone mineral and modification of bone structure is a common part of aging. The densitometric definition of osteoporosis was first established by consensus of a panel convened by the World Health Organization WHO in During a DXA examination, a patient is irradiated with an X-ray beam of two different energies, which enables bone attenuation to be separated from soft tissue attenuation.
It is extremely important that facilities performing DXA examinations recognize that the main output of this test is quantitative. A quality-control program must be implemented and followed to assure clinicians and patients that the information obtained falls within accepted ranges for precision and accuracy.
Depending on specific factors, related to the clinical status of the patient or technical aspects of the test itself, interpretation of DXA examinations ranges from straightforward to complicated. The DXA information includes the BMD, T -score, and Z -score.
The T -score enables categorization of patients into one of three diagnoses: normal, low bone mineral density, or osteoporosis. We describe below an approach to evaluation of these factors and production of clinically relevant DXA reports.
The approach includes division of examinations into straightforward, mildly complex, and complex. In this approach, initial screening examinations are regarded as straightforward, follow-up examinations as mildly complex, and examinations that require recalculation of data as complex.
The examination should not be scheduled close to a nuclear medicine scan where emitted radiation could interfere with accurate BMD measurement. To provide the most clinically useful DXA interpretation, the context for the patient must be known. If information is not available from an electronic medical record, a history sheet or questionnaire should be completed by the patient and confirmed by discussion with the technician or the physician.
Because estimation of fracture risk is most commonly performed by use of the FRAX ® calculator, a history sheet should include, at least, the questions used by FRAX ® , i. The FRAX ® calculator is applicable for patients from age 40—90 years.
Many DXA scanners manufactured after incorporate the FRAX ® calculator in their software. A DXA examination most commonly includes a scan of the lumbar spine and one hip.
The lumbar spine is usually scanned first Fig. The patient is then repositioned and the hip is scanned Fig. As the scan is being acquired the technician should view the scan image to confirm patient positioning is correct. The scan takes approximately 10 min for an uncomplicated ambulatory patient.
If one of the regions scanned contains artifacts, e. a hip prosthesis, a scan of the forearm should be performed Fig. The radius provides information about cortical bone and should be the primary region of interest for patients with hyperparathyroidism [ 6 ].
DXA of the lumbar spine. The lower extremities are elevated and supported with the knees flexed. This positioning reduces the normal lordosis of the spine and enables more consistent positioning of the regions of interest.
The spine should be aligned with the center of the table. The table has a centerline drawn on the support surface visible in Fig. DXA of the hip. The patient is positioned supine on the scanning table with the lower extremities in internal rotation, to obtain a more en-face projection of the femoral neck.
All manufacturers provide a positioning aid to help maintain this position. DXA of the forearm. The patient sits in a chair next to the scanning table with the forearm positioned parallel to the centerline.
The output from the DXA examination includes images of the body part scanned, quantitative data from the scanned area, including the bone mineral content BMC , BMD, scanned area, T -scores, and Z -scores, and a graph of where the patient fits within the reference population.
The interpreting physician should check the scan for proper patient positioning and region of interest ROI placement. The spine should be aligned with the long axis of the scanner.
The scan should include the inferior portion of T12 and the superior portion of L5. The DXA image shows the scanned regions from which the BMD measurement has been obtained Fig.
There should be no focal areas of increased or reduced density within the regions of interest. Normal positioning. Initial DXA for pretreatment assessment of a year-old woman with newly diagnosed breast carcinoma.
In the posteroanterior view of the lumbar spine the lines through the disc spaces and the lines outlining the lateral margins of the vertebra demarcate the ROI. This patient has five lumbar vertebrae. The figure includes labels showing numbering of the vertebrae. There is modest variability in the number of non-rib-bearing vertebrae and a transitional vertebra may be present at the lumbosacral junction.
The remainder of the population has other combinations of ribs and non rib-bearing vertebrae. For the purpose of vertebral numbering, a line drawn from the highest point of one iliac crest to the other iliac crest traverses the L4—L5 intervertebral disc space.
If a lumbar spine radiograph is available, it may aid accurate numbering. The provider responsible for interpretation should check to ensure numbering of the vertebrae is correct.
Especially when a scan is being repeated, numbering of the vertebrae should be consistent. The interpreting physician should check the scan for proper patient positioning and ROI placement. The femur should be aligned with the long axis of the scanner and the lower extremity should be internally rotated Fig.
The regions of interest ROI should be placed in accordance with the manufacturer recommendations. Irrespective of the manufacturer, the femoral neck region of interest may not include any ischium or greater trochanter. There should be no artifacts within the measured regions of interest. Same patient as Fig.
The hip is in internal rotation to obtain a more en-face projection of the femoral neck. With internal rotation, the lesser trochanter is small or invisible white arrowhead. Markup of the ROI varies with manufacturer. In this image, the total hip is indicated by the yellow outline of the bone margins, the superior margin of the femoral neck ROI, and the subtrochanteric horizontal line.
The arrows indicate the femoral neck ROI. The femoral neck ROI varies with manufacturer but should not include the ilium or ischium Color figure online.
The forearm must be aligned with the long axis of the scanner. The ROIs should be placed in accordance with the manufacturer recommendations Fig. Scans with patient motion, previous fractures, orthopedic hardware, or other artifacts cannot be interpreted.
DXA of the forearm from another patient, a year-old woman. Note that the forearm is aligned with the long axis of the scanner. The table displays each vertebra individually and then combinations of vertebrae. The BMD should gradually increase from L1 to L4. For some patients, L4 will be less dense than L3.
Nonetheless, each vertebra should be within one standard deviation of the next. If they are not, interpretation is not straightforward and the reader should refer to the section on complex interpretations. Table of BMD, T -scores, and Z -scores.
The DXA software calculates the BMC in the entire ROI and divides by the area of the ROI to calculate the BMD. Note that the information is presented for individual vertebrae and for the L1—L4 combination a.
The vertebral information may also be presented for other combinations of fewer than four vertebrae b. In the most straightforward cases, the area included in the outlines of the four lumbar vertebrae is the region where the BMD is determined. A T -score and Z -score are provided for the BMD at each location.
The WHO definitions of normal, osteopenia, and osteoporosis are based on the DXA measurement of BMD [ 4 ]. To standardize among manufacturers and populations, the concepts of T -score and Z -score were introduced.
Although T -scores are not strictly the same as standard deviations, for clinical purposes, they can be assumed to function the same way. The reference population for the T -score consists of young asymptomatic subjects.
The Z -score functions similarly to the T -score but compares the patient with a database of people of the same age. In the US, the International Society for Clinical Densitometry ISCD recommends using the Caucasian database for all patients [ 9 ].
A T -score and Z -score are provided for the BMD at each of these locations. The bone density graph. This graph shows the bone densities of the reference population with which the patient is being compared central light green region.
The bone densities for normal, osteopenia, and osteoporosis are superimposed on the graph. A graph is also provided for the hip not shown Color figure online. For diagnostic categorization, the region with the lowest T -score is identified from among the lumbar spine, hip, or radius. For the spine, the L1—L4 region is used unless focal artifacts require exclusion of individual vertebra.
A single vertebra should not be used. For the hip, the lower T -score between the femoral neck and total hip is used. The most commonly used method for assessment of fracture risk is the FRAX ® tool developed for the World Health Organization.
Major osteoporotic fracture includes fractures involving the proximal femur, spine, proximal humerus, or distal radius. In the United States, the National Osteoporosis Foundation and the ISCD recommend limiting the use of the FRAX ® tool to patients over age 50, who are not receiving pharmacologic therapy, who have not had hip or spine fracture, and who have DXA-measured BMD in the osteopenic range Fig.
Sixty-seven-year-old woman with previous fracture of the distal radius. This is an ideal patient for FRAX ®. Additional items may be added to the DXA report to assist in management of the patient.
These include a suggestion to evaluate patients for causes of secondary osteoporosis or osteomalacia, a recommendation for treatment initiation, and a recommendation for the next follow up DXA. Inclusion of these recommendations depends on the provider referral base.
For premenopausal women and men younger than 50 years, the WHO criteria should not be used. For these groups diagnosis is based on the Z -score. It is important to recognize that a clinical diagnosis of osteoporosis may be made for patients with fragility fractures, irrespective of their BMD measurement.
Patients frequently undergo repeated examination to monitor decreases in BMD or to evaluate the effects of treatment. When this occurs, a DXA examination becomes somewhat more complex. Physiologically, bone turnover is relatively slow and changes in BMD over time may be small.
To detect these small changes, the normal variance of the test must be known. The provider of DXA services should develop a quality-assurance program that can demonstrate that the BMD measurements fall within accepted ranges. There are two parts of such a program. The first involves in-vitro testing with a phantom of known densities, which ensures the scanner is operating within established limits.
This testing should be performed on a weekly basis and a log of the results maintained. The second type of assessment is an in-vivo evaluation of the combined performance of the scanner and the technician. Because the output of the scanner depends on the training and diligence of the technician, each DXA facility should determine its precision error PE and calculate its least significant change LSC.
According to the ISCD, the minimum acceptable precision is 1. For facilities with multiple technicians, the PE may be calculated using combined information from the group.
The approach for determining precision error has been standardized by the ISCD [ 10 , 11 ]. DXA scanners from different manufacturers vary in the method of BMD measurement.
Also, different DXA scanners from the same manufacturer may vary slightly in how BMD is measured. Because of this variance, it is not possible to make straightforward comparisons for patients examined at different facilities. Ideally, follow-up testing should be performed on the same machine with the same technician.
Listings of the assumed quality of information have been suggested. In descending order of information quality, these include: same machine, same technician; same machine, different technician; different machine within the same enterprise with machines cross calibrated , same technician; different machine within the same enterprise with machines cross calibrated , different technician.
For follow-up reports, the item of interest is the actual BMD and not the T -score. If a previous report is available for an examination performed at a different facility, the interpreting provider for the current examination should state that any degree of BMD change between the two facilities is uncertain.
When monitoring patients using DXA, it is very important to check the DXA images for consistent patient positioning and scan analysis.
For more absorptioemtry Dual-energy X-ray absorptiometry overview PLOS Dual-eneergy Areas, Dual-energy X-ray absorptiometry overview here. Dual-energy X-ray absorptiometry Ovsrview is rapidly becoming more Dual-enerty Dual-energy X-ray absorptiometry overview popular as Metformin and cholesterol technique to monitor body composition. The reliability of DXA has been overvied extensively using a number of different methodological approaches. This study sets up to investigate the accuracy of measuring the parameters of body composition BC by means of the whole-body and the segmental DXA method analysis with the typical error of measurement TEM that allows for expressing the error in the units of measure. The research was implemented in a group of 63 participants, all of whom were university students. Thirty-eight males Dual-energy Dual-energy X-ray absorptiometry overview absorptiometry DXAor DEXA [1] absodptiometry a means of measuring bone mineral density BMD using absorltiometry imaging. Two X-ray beams, overvies different Lower cholesterol to improve heart health levels Dual-energy X-ray absorptiometry overview, are aimed at absorptuometry Dual-energy X-ray absorptiometry overview Duaal-energy. When soft tissue absorption is subtracted out, the bone mineral density BMD can be determined from the absorption of each beam by bone. Dual-energy X-ray absorptiometry is the most widely used and most thoroughly studied bone density measurement technology. The DXA scan is typically used to diagnose and follow osteoporosisas contrasted to the nuclear bone scanwhich is sensitive to certain metabolic diseases of bones in which bones are attempting to heal from infections, fractures, or tumors.
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