Dual energy X-ray absorptimetry: Fundamentals, methodology, and

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Radiología. 2012;54(5):410---423
www.elsevier.es/rx
UPDATE IN RADIOLOGY
Dual energy X-ray absorptimetry: Fundamentals, methodology,
and clinical applications夽
R.M. Lorente Ramos ∗ , J. Azpeitia Armán, N. Arévalo Galeano, A. Muñoz Hernández,
J.M. García Gómez, J. Gredilla Molinero
Unidad Central de Radiodiagnóstico de la CAM, Hospital Infanta Leonor, Madrid, Spain
Received 15 March 2011; accepted 27 September 2011
KEYWORDS
Dual-energy X-ray
absorptiometry;
Densitometry;
DXA;
DEXA;
Osteoporosis;
Bone mineral density;
Body composition
PALABRAS CLAVE
Absorciometría con
rayos X de doble
energía;
Densitometría;
DXA;
DEXA;
Osteoporosis;
Densidad mineral
ósea;
Composición corporal
Abstract Dual-energy X-ray absorptiometry (DXA; DEXA) is the technique of choice to diagnose osteoporosis and to monitor the response to treatment. It is also useful for measuring
body composition. In recent years, new applications have been developed, including vertebral
morphometry through the study of the lateral spine, prosthesis integration in orthopedics, and
lipodystrophy in HIV+ patients, although its use in these cases is not well established. DXA
densitometry is accurate and precise. It is essential to optimize each step of the diagnostic
process, taking care to ensure the best acquisition, image analysis, and interpretation of the
results. Thus, to obtain the greatest utility from DXA, radiologists need to know the technique,
its indications, and its pitfalls. This article reviews the fundamentals, modalities, methods, and
clinical applications of DXA.
© 2011 SERAM. Published by Elsevier España, S.L. All rights reserved.
Absorciometría con rayos X de doble energía. Fundamentos, metodología
y aplicaciones clínicas
Resumen La absorciometría con rayos X de doble energía (DXA o DEXA) es la técnica de elección para diagnosticar la osteoporosis y monitorizar la respuesta al tratamiento. Además, es útil
para estudiar la composición corporal. En los últimos años han surgido nuevas aplicaciones como
la morfometría vertebral, estudiando la columna en visión lateral, la integración de prótesis
en ortopedia, o la lipodistrofia en los pacientes con infección por VIH, aunque su utilización en
estos casos no está bien consolidada. En el estudio de la osteoporosis, densitometría es precisa
y exacta. Para ello, es imprescindible optimizar cada etapa del proceso diagnóstico, cuidando la
adquisición, el análisis de imágenes y la interpretación de los resultados. Por ello, para obtener
la máxima utilidad para el clínico y el paciente, el radiólogo debe conocer la técnica, sus
夽 Please cite this article as: Lorente Ramos RM, et al. Absorciometría con rayos X de doble energía. Fundamentos, metodología
y aplicaciones clínicas. Radiología. 2012;54:410---23.
∗ Corresponding author.
E-mail address: [email protected] (R.M.
Lorente Ramos).
2173-5107/$ – see front matter © 2011 SERAM. Published by Elsevier España, S.L. All rights reserved.
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Dual energy X-ray absorptimetry: Fundamentals, methodology, and clinical applications
411
indicaciones y las dificultades. El objetivo de este artículo es revisar la DXA, haciendo hincapié
en sus fundamentos, modalidades, metodología y aplicaciones clínicas.
© 2011 SERAM. Publicado por Elsevier España, S.L. Todos los derechos reservados.
Introduction
Dual X-ray absorptiometry (DXA), also known as densitometry or dual energy X-ray absorptiometry (DEXA), can
distinguish different body structures. Axial bone densitometry of the lumbar spine and hip is the modality most
commonly used in clinical practice. This technique is useful for measuring the bone mineral density (BMD), and from
these data the risk of fracture can be estimated, therapeutic decisions can be taken and the response to treatment
can be assessed.1
On the other hand, DXA, the least known method for
whole body imaging, allows us to assess the total body
composition. This is particularly useful in patients with
weight disorders secondary to endocrine diseases and in
pediatric patients with delayed growth.2 Whole body DXA
can also be useful to assess lipodystrophy associated with
retroviral infections,3 in the monitoring of arthroplasties4
or to determine the cardiovascular risk.5
DXA is still little known among radiologists, who consider this technique to be more typical of other specialties.
Moreover, DXA tends to be wrongly considered as a routine and automated technique, unlikely to be optimized
and not requiring a radiological report. This is far from the
truth. DXA, like any other diagnostic modality, requires an
adequate indication, careful methodology and precise interpretation, which is only possible with appropriate training
and interaction between technicians and radiologists.
As a consequence, our aim is to examine the current status of DXA, particularly emphasizing its fundamentals, main
modalities, methodology and clinical applications.
Fundamentals and modalities of dual energy X-ray
absorptiometry
DXA is based on the variable absorption of X-ray by the different body components and uses high and low energy X-ray
photons. Depending on the equipment used, these photons
can be obtained using two mechanisms.6 In some cases, the
generator emits alternating radiation of high (140 kVp) and
low (70---100 kVp) kilovoltage while moving across the surface of the body to be examined. In others, the generator
emits a constant beam while a rare-earth filter separates
high energy (70 keV) from low energy (40 keV) photons.
The available DXA systems include different types of
hardware (filters, collimators, detectors) and software
(analysis algorithms).7 The X-ray source can emit a pencilbeam (pinhole collimator), which is registered by a single
detector, or a fan beam (slit collimator), which is registered
by a multiple detector.8 The latter system reduces the acquisition time and improves image quality.9 At the same time,
the analysis algorithm discriminate bone from soft tissue in
a variable way.10
The main modalities of DXA in clinical practice are axial
bone densitometry with stationary scan table, the modality
of choice to measure the BMD, and whole body densitometry,
used to assess body composition.
Bone densitometry with dual energy X-ray
absorptiometry
Historical perspective
Axial DXA of the lumbar spine and hip is at present the
technique of choice to study osteoporosis,11 although other
imaging techniques are potentially useful to assess and measure the bone structure and study its quality1,7,12 (Table 1).
Plain radiography is useful to assess bone structure,
although it cannot measure BMD. Some authors have tried
to apply digital radiography with dual energy to obtain an
estimated measurement of the BMD.
Quantitative computed tomography (QCT) of the lumbar spine (central QCT) is performed using conventional
computed tomography (CT) systems. QCT of radius or tibia
(peripheral QCT) can be performed using less sophisticated equipment. QCT provides volumetric acquisitions from
which BMD can be estimated. Central QCT has advantages
over DXA, since it allows us to differentiate between cortical
and trabecular bone, assess the geometry of the vertebrae,
and estimate the BMD volumetrically, expressed in g/cm3 .
The disadvantages of central QCT are the radiation dose and
the lack of validated diagnostic criteria.13
High resolution magnetic resonance (MR) imaging may be
used for assessment of the trabecular structure of peripheral bones (calcaneus, distal radius and phalanx).14 The bone
architecture studied using CT or MR, quantified in terms
of scale, shape, anisotropy and connectivity, allows for the
assessment of bone strength without considering the BMD.
Advanced MR techniques such as diffusion, perfusion and
spectroscopy will most likely provide useful additional information in the future.
Quantitative ultrasound (QUS) is used for measuring BMD
in the peripheral skeleton, generally at the calcaneus.
Photonic absorptiometry with iodine-125 (I-125) was initially used to study the peripheral skeleton (radius and
calcaneus). It was subsequently replaced by dual photonic absorptiometry that uses gadolinium-153 and may be
employed to study the axial skeleton (hip, spine and whole
skeleton).15
These modalities were later on substituted by X-ray based
technology, initially by plain X-ray and subsequently by DXA,
which allowed for the measurement of the axial skeleton.
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412
Table 1
R.M. Lorente Ramos et al.
Densitometry modalities.
X-ray
QCT
MR
QUS
SPA
DPA
SXA
DXA/DEXA
Plain X-ray
Quantitative computed tomography
Magnetic resonance
Quantitative ultrasound
Single photon absorptiometry
Dual photon absorptiometry
Single X-ray absorptiometry
Dual X-ray absorptiometry/Dual energy X-ray absorptiometry
Equipment
Peripheral DXA performed with portable units (such as
AccuDXA® ) focuses on the study of phalanxes. It is not very
accurate, but its cost is also low.1 AccuDXA can be used to
select patients likely to be assessed with central DXA16 on
stationary table or as a substitute where central DXA is not
available.17
Axial DXA of the lumbar spine and femur (central DXA) is
the preferred technique to measure BMD because of its good
resolution and reliability, rapid acquisition and little radiation. Several devices are commercially available (Lunar,
Hologic, Norland) with different characteristics. This fact
makes it advisable to perform the follow up of each patient
always in the same unit. The accuracy of bone densitometry
performed on a DXA system with stationary table is high,
with a margin of error of 1---2%.18
X-ray
X-ray (CT)
MR
Ultrasound
Radioisotope 125-I
Radioisotope Gd-153
X-ray
X-ray
Foundation (NOF), osteoporosis affects 10 million Americans, but another 34 million are at risk of developing the
condition. It is estimated that approximately 50% of women
over the age of 50 will suffer an osteoporic fracture during their lives.22 In Europe, the International Osteoporosis
Foundation (IOF)23 collects data and promotes initiatives in
every country. In Spain, approximately two million women
have osteoporosis (26.1% prevalence in women ≥50 years).24
In 1994, the WHO introduced the measurement of BMD
with DXA as the reference standard to quantify osteoporosis. Based on a study performed on postmenopausal white
women that revealed a correlation between BMD and risk of
fracture,20 osteoporosis was defined as a ‘‘T-score of −2.5’’.
Other values of reference for potentially useful parameters
were also determined (Table 3).25 Thus, axial DXA became
the reference standard for osteoporosis.
Methodology
Indications
The main use of DXA is the diagnosis of osteoporosis. It
may also predict the risk of fractures, help determine the
treatment, and monitor the response to treatment.1 Its
indications are set out in the official guidelines of the International Society for Clinical Densitometry, which are revised
every two years (Table 2).19,20
Osteoporosis is the ‘‘reduction in bone mass and increase
of bone fragility which in turn increases the risk of fracture’’.21 Osteoporosis is a common, often silent disease
that can lead to an increased risk of fractures, sometimes
atraumatic. Osteoporosis poses a serious problem to public health because of its prevalence and the cost associated
with its comorbidity. According to the National Osteoporosis
Table 2
Indications for bone densitometry.
Females
Males
Both sexes
Older than 65 years
Younger than 65 years (postmenopausal
or perimenopausal)
Older than 70 years
Younger than 70 years with risk factors
of fracture
Unexplained fracture
Diseases or chronic treatments
Any patient to whom possible treatment
is considered and to monitor the
response to treatment
The methodology of bone densitometry with axial DXA
requires optimization and careful execution. The importance of each step to obtain good results must be
highlighted.
Preparation
In order to properly plan the study, detailed patient information is required, and both the medical report provided by
the referring physician as well as the preliminary questionnaire completed in the diagnostic center. The request form
must include the indications for the study, so the areas to be
examined can be determined. Bone diseases that might alter
the shape or density of the bone, such as osteopetrosis or
ankylosing spondylitis, need to be ruled out. Previous fractures or joint replacements that might alter the planning
must also be ruled out. Contraindications to DXA include
pregnancy, recent (<5 days) oral administration of a contrast agent, and a recent (<2 days) isotopic study.26 The
patient does not require any specific preparation except for
the removal of any metal items located on the body area to
be imaged.
Areas of study
For most adult patients, the DXA examination should include
the lumbar spine and proximal femur; the forearm can also
be included when hip or spine cannot be measured.19 In
children and adolescents (younger than 20 years), the measurement is only performed in the lumbar spine.27,28 The
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Dual energy X-ray absorptimetry: Fundamentals, methodology, and clinical applications
Table 3
413
Parameters evaluated in dual energy X-ray absorptiometry.
BMC
BMD
SD
T-score
Z-score
Osteopenia
Osteoporosis
BMI
A/G ratio
Bone mineral content
Bone mineral density
Standard deviation
Difference in number of standard deviations between the mean BMD value of the
patient and the mean of a young adult reference population of the same sex
Difference in number of standard deviations between the mean BMD value of the
patient and the mean of a reference population of the same race, sex and age
T-score between −1 and −2.5
T-score ≤−2.5
Body mass index
Ratio of android and gynoid A/G pelvic fat
final result of the densitometry would be the lowest value
of the two regions studied.
The postero-anterior (PA) scan of the lumbar spine
includes the vertebral bodies of L1---L4, from which a mean
BMD of these four vertebrae is obtained. Vertebrae with
changes due to fracture or focal lesions will be excluded. To
assess the lumbar spine, at least two evaluable vertebrae
are required.
The study of the femur can indistinctly be performed on
the right or left hip, although it is useful to become used
to studying always the same side. Hips with changes due
to fracture, focal lesions or replacements will be excluded.
The total proximal femur or the femoral neck are used,
whichever is lowest.
The study of the non-dominant forearm is performed
when hip or spine cannot be measured (in order to have
a second measurable region), to obese patients (to bypass
technical difficulties), and to patients suffering from hyperparathyroidism (since forearm bones change before the axial
skeleton).
Patient positioning
Optimization of the patient’s position on the DXA table is
essential. Incorrect positioning is one of the main causes of
errors in the estimation of BMD.29 In the PA study of the lumbar spine, the patient lies supine on the table with the legs
flexed over a support pad that reduces the lumbar lordosis
and approaches the spine to the table (Fig. 1).
For the study of the hip, the patient lies supine with legs
slightly in abduction in order to maintain the femoral axis
straight and in internal rotation (15---30◦ ), in a way that the
lesser trochanter is not visible on the image (Fig. 2).
In the study of the forearm, the patient is seated, side
against the table with the arm resting on the tabletop with
the palm of the hand downwards patients sit beside the
table and with their forearm resting on it, with the hand
in pronation and strapped (Fig. 3).
The field of view must include 1---2 cm above and below
the area to be imaged. The bone must be straight and centered.
Acquisition
For most patients, the DXA includes PA projections of the
lumbar spine and proximal femur. The lateral spine is
not used in a standard osteoporosis study, but it is used
in vertebral morphometry. In those devices with mobile
Figure 1 Dual energy X-ray absorptiometry. PA study of the
lumbar spine. Patient’s positioning.
scanning arm, measurements can be done with the patient
lying supine, whereas in systems with stationary arm measurements can only be done in lateral decubitus.
Regarding the scanning time, the first densitometers with
pencil-beam scans took around 5 min per scan, but now the
images can be obtained in less than one minute. According
Figure 2 Dual energy X-ray absorptiometry of the hip.
Patient’s positioning.
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414
R.M. Lorente Ramos et al.
DXA uses low radiation doses and generally most of these
devices do not require lead shielding of the room or special
protection measures for the technicians. In the DXA units
using pencil-beam scans, the equivalent surface dose for
spine and hip examinations is approximately 20---100 ␮Sv per
examination, and the equivalent effective dose is 1---5 ␮Sv
per examination.31 For fan beam DXA, the dose is slightly
higher, approximately 56 ␮Sv for the hip, 59 ␮Sv for the
spine, and 75 ␮Sv for whole body. The disperse radiation that
the technician would receive being 1 m away from the table
using a fan beam unit, performing 4 hip studies and 4 spine
ones per hour would be of around 4 ␮Sv.32
Figure 3 Dual energy X-ray absorptiometry of the forearm.
Patient’s positioning.
to the examination guidelines of the SERAM (Spanish Society
of Medical Radiology),30 the time of room occupancy for a
spine or hip densitometry is 8 min, 10 min for the whole body,
and the radiologist reporting time is 5 min.
Analysis
Once the image is acquired, the assessment is carried
out by selecting different regions of interest (ROI). Inadequate placement of the ROIs is another important source
of errors.33 Although the equipment automatically proposes
specific areas, both the technician and the radiologist must
validate them and, given the case, rectify them.
In the study of the lumbar spine, the ROIs are placed
on the L1---L4 vertebral bodies (Fig. 4). It must be remembered that D12 is usually the last vertebra connected to a
rib (although this is not always the case), and that L3 usually
has the longest transverse process.
In the study of the hip, the ROI must be placed at the
femoral neck, avoiding superimposition of the ischiopubic
ramus and the greater trochanter (Fig. 5). The system measures automatically the inclination of the femoral axis and
the rest of the ROIs.
In the forearm, the area of analysis is set at the distal
radius, with the line of reference at the urnal styloid process
(Fig. 6).
Screening of the causes of error
The radiologist must try to detect all those artifacts that
might be possible causes of error in the acquisition, analysis
and interpretation of the study (Table 4). Once the possible
sources of error are identified, they must be excluded when
performing the analysis and subsequent interpretation.
PA Spine Bone Density
Densitometric reference: L1-L4
BMD (g/m²)
1.42
Normal
1.30
YA T-Score
2
1
1.18
0
L1
1.06
-1
-2
L2
0.94 Osteopenia
0.82
0.70
-4
L3
-3
Osteoporosis
0.58
20 30 40 50
Figure 4
70
80
90
-5
100
Age (years)
1
L4
60
Region
L1
L2
L3
L4
L1-L4
L2-L4
BMD
(g/m²)
0.987
0.930
1.053
0.961
0.983
0.982
2
Young-Adult
(%)
87
78
88
80
83
82
Young-Adult
T-Score
-1.2
-2.2
-1.2
-2.0
-1.6
-1.8
2
Dual energy X-ray absorptiometry image of a PA lumbar spine. The study includes the L1---L4 vertebral bodies.
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Dual energy X-ray absorptimetry: Fundamentals, methodology, and clinical applications
Left Femur Bone Density
415
Densitometric reference: Neck
BMD (g/m²)
1.22
Normal
1.10
Femoral
Neck
YA T-Score
2
1
0.98
0
0.86
-1
0.74 Osteopenia
0.62
-2
0.50
-4
-3
Osteoporosis
0.38
20
30
40
50
60
70
80
90
-5
100
Age (years)
1
BMD
(g/m²)
Total Femur
Region
Neck
Ward’s
Troc.
Diaphysis
Total
0.992
0.811
0.873
1.331
1.095
Young-Adult
(%) T-score
101
0.1
89
-0.8
111
0.8
110
0.8
2
Adj. to age
(%) Z-score
112
0.9
110
0.6
118
1.2
117
1.3
3
Figure 5 Dual energy X-ray absorptiometry of the left hip. The lesser trochanter must not be visualized. The area of analysis
(ROI) is placed on the femoral neck.
UD Cubitus
UD Radius
33%
cubitus
33% cubitus 33% radius
Figure 6
Table 4
Technique
Artifacts
Region
1
BMD
(g/m²)
UD radius 0.164
UD cubitus 0.131
33% radius 0.378
33% cubitus 0.419
0.152
Both UD
Both 33% 0.398
Total radius 0.276
Total cubitus 0.267
0.272
Both total
0.164
0.131
0.378
0.419
0.152
0.398
0.276
0.267
0.272
2
Young adult
(%) T-score
35
43
40
-
-6.7
-5.7
-6.7
-
3
Adj. to age
(%) Z-score
52
64
58
-
-3.4
-2.4
-3.3
-
Dual energy X-ray absorptiometry of the forearm. The line of reference is placed at the urnal styloid.
Sources of error in densitometry.
Patient’s positioning
Movement
Region of interest (ROI) placement
Foreign bodies
Bone diseases
Surgical material
Calcifications
Contrast media
Spondyloarthrosis
Fractures
Lytic or sclerotic lesions
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416
R.M. Lorente Ramos et al.
Figure 7 On the left, dual energy X-ray absorptiometry of the spine. Dense image superimposed on the L3 vertebral body (arrow).
On the right, the plain abdominal radiograph shows a calcified mesenteric lymph node (arrow).
islets, Paget’s disease, or hemangiomas. In cases of unknown
suspected bone lesion, complementary radiological studies
are required. It must be remembered that in patients with
severe osteoporosis, the DXA scan might simulate a lytic
lesion (Fig. 11).
First, the correct positioning of the patient and absence
of movement during the scanning must be checked. Artifacts
caused by superimposition of dense structures (including
surgical material, calcifications or contrast agents) must be
ruled out (Fig. 7).34,35 Bone lesions36 may also alter the analysis and must be mentioned in the report.
The condition that most frequently distorts the analysis is spondyloarthrosis, which is associated with bone
proliferation (osteophytes, endplate sclerosis) and morphologic changes (Fig. 8). Vertebral fractures (Fig. 9), lytic or
sclerotic lesions (Fig. 10), metastases, lymphomas, bone
Parameters evaluated in bone densitometry
Following the acquisition and analysis of DXA, the system
calculates various parameters37 (Table 3). The BMC is the
quantity of calcium estimated by the energy absorbed by
it in a specific region. The BMD, much more relevant, is
PA Spine Bone Density
Densitometric reference: L1-L4
BMD (g/m²)
1.54
Normal
1.42
YAT-Score
3
2
1
1.30
L1
L2
1.18
0
1.06
-1
0.94 Osteopenia
0.82
-2
-3
-4
0.70
L3
Osteoporosis
0.58
20 30 40 50
60
70
80
Age (years)
1
L4
-5
90 100
2
Region
BMD
(g/m²)
Young adult
(%) T-score
L1
L2
L3
L4
L1-L4
L2-L4
0.883
0.991
1.326
1.217
1.122
1.184
78
83
111
101
95
99
-2.1
-1.7
1.1
0.1
-0.5
-0.1
3
Adj. to age
(%) Z-score
80
85
113
104
97
101
-1.8
-1.5
1.3
0.4
-0.3
0.1
Figure 8 Dual energy X-ray absorptiometry image of spine. The increase of density in the radiography (arrow) corresponds to
an L3 and L4 sclerosis. In the analysis, the bone mineral density (BMD) in L3 and L4 is higher than in L1 and L2. Exclusion of both
vertebrae shows a T-score L1---L2 = −1.9 (L1---L4 = −0.5).
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Dual energy X-ray absorptimetry: Fundamentals, methodology, and clinical applications
417
BMD (g/m²)
1.48
[L1]
[L2]
YAT-Score
2
1.36
1
1.24
0
1.12
-1
1.00
-2
0.88
-3
0.76
-4
L3
0.64
20
30
40
Region
L1
L2
L3
L4
L3-L4
BMD
(g/m²)
0.884
1.054
0.876
0.844
0.858
BMC
(g)
9.20
11.12
10.28
11.58
21.87
L4
50
60
70
80
Age (years)
Densitometry
Area
(m²)
10.23
10.55
11.74
13.73
25.47
YA
YA
(Z) T-score
77
-2.2
85
-1.5
71
-3.0
68
-3.3
69
-32
-5
100
90
AM
AM
(Z) Z-score
83
-1.5
91
-0.8
76
-2.3
73
-2.6
74
-2.5
Figure 9 Above, dual energy X-ray absorptiometry of spine. Increased density of the L2 vertebral body (arrow) that corresponds
to a fracture (arrow) on the radiograph below. Both images show a calcification (arrowheads) that is not superimposed to the spine
(not affecting the analysis).
Densitometric reference: L2-L4
PA Spine Bone Density
YAT-score
2
BMD (g/m²)
1.44
Normal
1.32
R
1
1.20
0
1.08
-1
0.96 Osteopenia
0.84
-2
-3
-4
0.72
(L1)
Osteoporosis
0.60
20 30 40 50
60
70
80
90
-5
100
Age (years)
L2
L3
L4
Region
L1
L2
L3
L4
L2-L4
1
BMD
(g/m²)
8.822
0.751
0.851
0.920
0.847
Young-Adult
(%) T-score
73
-2.6
63
-3.7
71
-2.9
77
-2.3
71
-2.9
2
Adj. to age
(%) Z-score
93
-0.5
79
-1.7
90
-0.8
97
-0.2
69
-0.9
3
Figure 10 Dual energy X-ray absorptiometry and AP radiograph of spine (right). Increased density in the pedicle of right L1 (arrow)
corresponding in the radiography to a dense pedicle. The vertebra must be excluded from the analysis.
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418
R.M. Lorente Ramos et al.
Figure 12 Whole body dual energy X-ray absorptiometry.
Patient’s positioning.
Figure 11 Dual energy X-ray absorptiometry of spine in a
patient with severe osteoporosis. The image simulates a defect
of L3 and L4 (arrows), not confirmed by the radiographies.
the mean quantity of mineral per unit area. It is calculated
dividing the BMC by unit area (g/cm2 ). The values used for
diagnosis (T- and Z-score) are obtained by comparison with
the reference database.
The T-score is the value used to diagnose osteoporosis
in postmenopausal women and in men aged 50 and over.
T-score is the number of standard deviations the patient’s
BMD above or below the mean for the young adult reference
population of the same sex. A T-score >−1.0 is considered
normal, osteopenia when the T-score is between −1 and
−2.5, and osteoporosis is a T-score <−2.5.
The Z-score is used in premenopausal women, in men
younger than 50 years, and in children and adolescents (up
to 20 years). Z score is the number of standard deviations the
patient’s BMD above or below the mean for the reference
population of the same race, sex and age. A Z-score <−2
World Health Organization BMI Classification
BMI=16.5 (kg/m2)
13
18.5
25
Underweight
35
Normal
50
30
Overweight
68
35
Obese
82
95
Weight (kg) for height=165.0 cm
Composition
Region
Figure 13
Total mass
Tissue
Centile
(kg)
(% fat)
Region Tissue
(%fat)
(g)
Fat
(g)
Lean BMC
(g)
(g)
Total left
16.6
-
22.5
Right arm
8.2
-
2.2
7.695 1.673 6.021 427
14.6 10.236 1.547 8.689 380
15.8 21.347 1.543 17.804 1.146
7.6 2.065 169 1.896 151
Right leg
21.7
-
8.3
20.5
Right torso
15.1
-
10.0
14.6
Total right
Left leg
21.7
-
8.1
Left torso
15.1
-
10.6
16.5
-
23.0
Arms
8.2
-
4.5
Legs
21.7
-
16.4
Torso
15.1
-
Android
10.8
-
2.5
Gynoid
Total
28.0
16.6
-
7.0
45.5
-
20.6
20.6
7.843 1.703 6.140 442
9.667 1.463 8.203 360
15.6 21.703 3.583 18.120 1.276
7.6 4.229 345 3.884 304
20.6 15.537 3.376 12.161 870
14.6 19.902 3.010 16.892 740
50
10.6 2.432 263 2.168
27.1 6.822 1.910 4.911 225
15.7 43.049 7.125 35.924 2.422
Fat lib
6.4
9.0
18.9
2.0
6.5
8.5
19.3
4.1
13.0
17.6
2.2
5.1
38.3
Whole body dual energy X-ray absorptiometry. Underweight patient.
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Dual energy X-ray absorptimetry: Fundamentals, methodology, and clinical applications
419
standard deviations is defined as ‘‘below the expected range
for age’’.38
Reporting
The report should be tailored to the particular characteristics of each center, but in general it must
comply with some minimum requirements. In addition to
the patient’s personal details, the report must include
the date of examination, the type of DXA system used,
as well as the protocol followed. It must also specify if
any region of analysis has been excluded and the reasons, and the possible presence of artifacts or suspected
lesions.
The report must also include the scans obtained for each
region of study, with a graph showing the patient’s data and
then related back to the reference curve, the numerical values of the BMD, T- and Z-scores, and finally, a diagnostic
category based on WHO criteria. The value considered most
useful to diagnose a particular patient must be specified.
In some cases, an estimation of the risk of fracture is also
included. In general, the risk of fracture doubles for each
standard deviation reduction in BMD. The study’s conclusion
should be global, taking into account the lowest value of the
areas studied. In patients with previous studies, any possible
changes should be reported.
Estimation of the body composition using dual
energy X-ray absorptiometry
Several techniques have been used to assess the composition
and quantification of the body fat, mostly anthropometric
methods such as the waist circumference, the waist-to-hip
ratio, and the skin fold thickness.39 Among the imaging studies, CT stands out to measure visceral fat, but with some
drawbacks such as radiation, among others. Bioimpedance
techniques and isotope dilution have also been
used.40
Whole body DXA allows for an easy and rapid measurement of body composition. It provides an estimate of
body fat, and when needed, it may also estimate the BMD
of the whole body. DXA is very accurate, with a margin of
error of 2---6% for body composition.18 Unlike anthropometric methods, DXA has advantage of providing regional and
whole-body measurements. DXA is increasingly being used;
for many clinicians it is a common tool and for some authors
it is the reference standard.
Indications
Whole body DXA is used in eating disorders, especially those
where there might be a hormonal disorder or cardiovascular
risk factors. It is also used in gastrointestinal diseases, hepatobiliary disorders, advanced renal failure, in endocrinal
diseases, bone disorders such as Paget’s disease or osteopetrosis, lung diseases and various chronic treatments.18 DXA
may also help design the diet in patients with malnutrition and in the follow-up of patients with eating
disorders.
Methodology
Patient’s positioning. The patient lies supine, centered on
the table with the arms along sides of the body, hands
Figure 14 Whole body dual energy X-ray absorptiometry.
Obese patient.
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420
R.M. Lorente Ramos et al.
facing the legs without touching them and the thumbs up
(Fig. 12). If the patient is wider than the width of the
DXA table, a half-body scanning is performed (including
the neck and head, one whole side with their corresponding arm and leg). In this case, the patient lies supine
but is positioned off the center line of the table to
ensure that one side is completely included in the scan
field.
Analysis. As in BMD studies, the correct positioning and
absence of motion artifacts must be verified. Following the
acquisition, two images of the whole body are obtained,
one of bones and other of soft tissues (Figs. 13 and 14).
The system places the ROIs automatically. The technician
revises the ROIs and, if necessary, modifies them, although
it is advisable to manipulate them as little as possible.
The changes performed on an image are automatically
introduced on the other. The ROIs correspond to anatomic
regions: the head up under the chin; arms separated from
the body with the cut lines passing through the armpits; forearms separated from the body, legs separated from the arms
and the medial cut lines located between both legs; the limits of the spine are adjacent to it, on both sides; pelvis: the
upper cut line located immediately above the pelvis and cut
lines through the femoral neck without touching the pelvis.
World Health Organization BMI Classification
BMI = 57.0
13
18.5
Underweight
36
51
25
Normal
30
Overweight
69
60
Obese
83
165
Weight (kg) for height = 166.0 cm
Figure 15 Whole body dual energy X-ray absorptiometry. The patient is wider than the width of the table so the system performs
half-body scanning (left). The left side is completely included in the scan field, and a contralateral estimation is performed.
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Dual energy X-ray absorptimetry: Fundamentals, methodology, and clinical applications
421
In patients wider than the width of the scan field where only
a half-body is scanned, the ROI is placed as explained and
the system provides a total estimation (Fig. 15).
Interpretation. The system provides various parameters
(Table 3) such as the body mass index (BMI), quantification
of body fat, distribution of the abdominal fat, or values
obtained from these data such as the android/gynoid ratio
(A/G ratio) in the abdominal fat.
The BMI is an anthropometric indicator designed for adult
males and non-pregnant females, but it does not distinguish
between fat and muscle and therefore cannot be assessed
in athletes. The BMI is a number calculated from a person’s
weight and height and is measured in kg/m2 . The distribution of body tissues is expressed as percentages for the
whole body and per regions: percentage of fat (fat mass),
of soft parts and muscle (lean mass) and of body (BMD).
Several studies have calculated curves that can be used as
a reference for different populations.41---44
In addition to the composition by anatomic regions, it
calculates the distribution of fat in predefined regions in the
pelvic area: android (central, the lower limit is the pelvis
and the lateral limit is the arms) and gynoid (hip and thighs,
the lateral limits are the outer part of the legs) (Fig. 16).
The android/gynoid (A/G) ratio of the fat is the relation
between the fat percentage of the android and the gynoid
region. The excess of abdominal fat (android) is associated
with a number of cardiovascular risk factors.45
The determination of the A/G ratio using DXA is an easy
and useful tool to assess the distribution of pelvic fat. This
ratio may have a role in evaluating the cardiovascular risk
in overweight or underweight patients.5
BMD values of the whole body are useful to estimate mineralization, but not to diagnose osteoporosis (it is necessary
to study the spine and hip in order to compare the results
with the reference curves to reach a diagnosis).
Other applications of dual energy X-ray
absorptiometry
Over the time, other potential clinical applications using
DXA are being proposed. DXA systems now offer the possibility of creating personalized analysis areas to perform
composition measurements. There are also studies in the
field of orthopedics that investigate implant integration by
assessing the regional mineralization.4 Moreover, its use
in lipodystrophy and lipoatrophy is being studied, especially
in HIV patients.3
Conclusion
Figure 16 Android (continuous line) and gynoid (dotted line)
areas of fat.
DXA is a fast and reliable technique with little radiation
exposure. It is the technique of choice in the diagnosis and
monitoring of osteoporosis since it objectively measures the
most relevant parameters. Moreover, it is useful to measure the whole body composition and its distribution by
regions. There are other less common applications in potential expansion. Knowledge of the technique, its indications,
methodology and applications are key issues to optimize its
results and streamline its use.
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422
R.M. Lorente Ramos et al.
Authorship
1
2
3
4
5
6
7
8
9
10
Responsible for the integrity of the study: RMLR.
Conception of the study: RMLR.
Design of the study: RMLR and JAA.
Acquisition of data: RMLR, JAA and NAG.
Analysis and interpretation of data: RMLR, AMH, JMGG
and JGM.
Statistical analysis: RMLR
Bibliographic search: RMLR and JAA.
Drafting of the paper: RMLR.
Critical review with intellectually relevant contributions: RMLR, JAA, NAG, AMH, JMGG and JGM.
Approval of the final version: RMLR, JAA, NAG, AMH,
JMGG and JGM.
Conflicts of interest
17.
18.
19.
20.
21.
22.
The authors declare not having any conflicts of interest.
23.
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