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					Department of Electronics and Communication
 Government Engineering College, Thrissur




               Seminar Report
                   2004

    Dual Energy X-ray Absorptiometry


                Presented by

             Ajesh Kumar A.S.
                  S7 ECE
              Roll No: 01-602
                    ACKNOWLEDGMENT




      I would like to thank everyone who helped to see this seminar to
completion. In particular, I would like to thank my seminar coordinator
Mrs. Muneera.C.R for her moral support and guidance to complete my
seminar on time. Also I would like to thank Mr. C. D. Anil Kumar for
his invaluable help and support.


      I would like to take this opportunity to thank Prof. Indiradevi,
Head of the Department, Electronics & Communication Engineering for
her support and encouragement.


     I express my gratitude to all my friends and classmates for their
support and help in this seminar.
                         CONTENTS




Abstract                                                  4
1. Introduction                                           5
2. Principles of Dual Energy X-ray Absorptiometry         6
   2.1 Principal components of a DEXA system              6
   2.2 Interaction of X-ray photon with physical matter   7
   2.3 Mass attenuation coefficient (U)   and
       mass per unit area (M) of the absorber             9
   2.4 Determination of Mass per unit area
       of a homogeneous absorber                          11
3. DEXA analysis of human body composition                13
   3.1 Components of a human body                         13
   3.2 A model DEXA analysis                              15
4. Actual DEXA calculation                                17
   4.1 The R value                                        17
   4.2 Use of R value                                     18
5. Advantages of DEXA scanning                            23
6. Conclusions                                            24
7. References                                             25
            Dual Energy X-ray Absorptoimetry


                                    ABSTRACT


        The basic principles of Dual Energy X-ray Absorptiometry have been
discussed in this presentation. DEXA is a instrumental technique used to measure
bone mineral density (BMD), which is the widely accepted indicator of bone strength.
DEXA scanner is the most widely used modern electronic machine to diagnose the
disease osteoporosis, the thinning of bones. Human body being a heterogeneous
system, use of a dual energy, rather than single energy, X-ray source is necessary for
scanning. The interaction of the sample with the X-ray beams results in a reduction or
attenuation of the energy of the X-ray beam. The extent to which the photon energy
is attenuated is a function of the initial energy of the X-ray photon, the mass per unit
area (M) of the absorber material and the mass attenuation coefficient (U) of the
absorber. For a given absorber material, U (which is a measure of the degree of
attenuation) is a constant at any given photon energy.
        U increases with the density of the absorber material and decreases with the
energy of the X-ray beam. U can be used to calculate the Mass per unit area (M) of a
homogenous absorber irradiated at a specific incident X-ray energy. The mass of
bone and soft tissue „below‟ this square would represent the mass per unit area of the
absorber, viz., leg.   For instance, if there are 100 grams of bone and soft tissue below
this square, the mass per unit area (M) would be 100 g/cm2. Knowledge of M of the
human body components, especially of bone, is important in determining the
possibility of osteoporosis. The calculations of M of the various components of the
body are discussed in detail.
        From knowledge of mass attenuation coefficient (U) of the absorber and the
energy of the incident X-ray beam (E0) and of the emerging beam (E), we can
calculate M of a homogeneous absorber from the following relationship connecting
these properties.
                         1. INTRODUCTION


      Dual Energy X-ray Absorptiometry (DEXA) is an instrumental
technique used to measure bone mineral density (BMD) that includes the
hip and spine, compared to SXA (Single Energy X-ray Absorptiometry)
that measures only the wrist or heel bone. BMD is the widely accepted
indicator of bone strength.


      DEXA (the whole body scanner) uses low dose x-rays to give us
information on bone content and density. It is currently the most widely
used machine in the clinical setting to diagnose the disease osteoporosis,
the thinning of bones.
      2. PRINCIPLES OF DUAL ENERGY X-RAY
                          ABSORPTIOMETRY


2.1    Principal components of a DEXA system

       A typical DEXA system is shown in the figure.




                           Figure 1. The DEXA system


       The DEXA scanner consists of the following basic components:


1.     Source of X-rays
2.     The sample space
3.     The detector


       A block diagram of the instrument is shown below as Figure 2. Two separate
beams of X-rays of known energies (E0,1 and E0,2) produced at the source are passed
through a desired absorber material (sample)-usually human body- positioned in the
sample space (DEXA table). The sample interacts with the incident beams altering
their energies to E1 and E2. The detector determines the energies (E1 and E2) of the
emerging beams of X-radiation. A data acquisition and control unit manipulates the
data. The operation of this electronic machine is fully controlled by a computer
system.




                Figure 2. The principal components of DEXA system




       We will understand the working principle of DEXA by first considering the
interaction of a single beam of X-Rays with matter.




2.2    Interaction of X-ray photon with physical matter

       When a beam of X-rays is allowed to pass through an absorber material, the
X-ray photons interact with the electrons of the material in 2 different ways.


a.     The photon knocks the weakly bound outer orbit electron giving up some of
its energy to the electron and gets deflected (scattered) from its path. The scattered
photon has a lower energy and hence a longer wavelength than the incident photon.
This is Compton scattering (Figure 3a).
                             Figure 3a. Compton effect


b.     The photon collides with more tightly bound orbit electron giving up all its
energy to the electron and the photon ceases to exist. This is photoelectric collision
(Figure 3b).




                           Figure 3b. Photoelectric effect
       Both these interactions result in a reduction or attenuation of the energy of the
X-ray beam.     In fact, the incident photon energy is exponentially reduced or
attenuated as it passes through the absorber (see Figure 4).




      Figure 4. Attenuation of X-Ray energy by the interaction of the absorber.


2.3    Mass attenuation coefficient (U) and mass per unit area
(M) of the absorber


       The extent to which the photon energy is attenuated is a function of the initial
energy of the X-ray photon, the mass per unit area (M) of the absorber material and
the mass attenuation coefficient (U) of the absorber. For a given absorber material,
U (which is a measure of the degree of attenuation) is a constant at any given photon
energy. For instance, at an incident photon energy of 40 keV, A for hydrogen is
0.3458 cm2/g; at an incident photon energy of 70 keV, it is 0.3175 cm 2/g. The mass
attenuation coefficients of some absorber elements are given below for two photon
energies for examination:


Table 1 : Mass attenuation coefficients of some elements.
Element             Atomic number     U (cm2/g)
                                      40 keV                 70 keV
H                   1                 0.3458                 0.3175
C                   6                 0.2047                 0.1678
N                   7                 0.2246                 0.1722
O                   8                 0.2533                 0.1788
Na                  11                0.3851                 0.2022
Mg                  15                0.4704                 0.2244
P                   15                0.7784                 0.2839
S                   16                0.9509                 0.3258
Cl                  18                1.1000                 0.3491
K                   19                1.4840                 0.4297
Ca                  20                1.7920                 0.5059




From the table it is clear, that


(a)     U increases with atomic number of the element. In other words, higher atomic
number elements attenuate the X-ray beam to a greater degree than lower atomic
number elements.
(b)     The lower energy beam is always attenuated to a greater degree than the
higher energy beam.


        U can be used to calculate the Mass per unit area (M) of a homogenous
absorber irradiated at a specific incident X-ray energy.


        A homogeneous material is any single material for which U is known at a
specific incident X-ray photon energy. This can be an element, a compound, or a
solution, as given below.
Table 2: Mass attenuation coefficients of some homogeneous absorber materials.
Component              U
                       40 keV                         70 keV
Ca                     1.7920                         0.5059
Water                  0.2636                         0.1942
Protein                0.2363                         0.1831
Fat(Oleic acid)        0.2273                         0.1872
Bone mineral           0.9039                         0.3159


2.4 Determination of Mass per unit area of a homogeneous
absorber

          When X-rays scan a 3-dimensional absorber such as a human being, it
produces a 2-dimensional flat image. Let us consider the X-ray image of a human leg.




Figure 5. X-ray image of a human leg. The square mark on the image represents an
area of 1 cm2
       This is a flat 2-dimensional image of a real 3-dimensional leg. The image is
made up of many small “picture elements” or “pixels”. Each pixel is uniform and
represents a „snap shot‟ taken during the X-ray scan. Let us now consider a square of


area 1cm2 on the image. The mass of bone and soft tissue „below‟ this square would
represent the mass per unit area of the absorber, viz., leg.   For instance, if there are
100 grams of bone and soft tissue below this square, the mass per unit area (M) would
be 100 g/cm2.
        Knowledge of M of the human body components, especially of bone, is
important in determining the possibility of osteoporosis.


Calculation of M:


       From a knowledge of mass attenuation coefficient (U ) of the absorber and the
energy of the incident X-ray beam (E0) and of the emerging beam (E), we can
calculate M of a homogeneous absorber from the following relationship connecting
these properties.
                              ln ( E0 / E ) = U x M
                              M = ln ( E0 / E ) / U


       For instance, let us consider that we allow a 40 keV X-ray beam to pass
through the absorber bone mineral, whose U value is 0.9039 cm2/g (see Table 2).
Some of the energy will be lost due to Compton scattering and photoelectric effect.
Let the emerging X-ray beam be attenuated to 10 keV. Then, the mass per unit area
of this homogeneous absorber, bone mineral, is given by


                              M = ln (40/10) / 0.9039
                                 = ( ln 4 ) / 0.9039
                                 = 1.534 g /cm2
       Thus using a single X-ray beam we are able to determine the mass of bone
mineral in our sample.
       Unfortunately, a human body is not a homogeneous absorber since there
are several different components in the body, such as fat, lean tissue, and bone.
A single X-ray beam cannot differentiate among these different components. For this
we must utilize a “dual energy X-ray” beam.




         3. DEXA ANALYSIS OF HUMAN BODY
                               COMPOSITION

        While a mono energetic X-ray source is capable of measuring the areal density
of a homogeneous absorber, a dual energy X-ray source is required to determine the
areal densities of up to two components of an absorber. Before we discuss the DEXA
body composition analysis, let us have a look at the various components of human
body.


3.1 Components of a human body

                       Bone mineral
                       Non-bone mineral
                       Glycogen
                       Proteins
                       Water
                       Fat


        The sum of all these make up the body weight. These 6 components can be
conveniently grouped into a 2-component system: Soft tissue mass and Bone
mineral mass. Here soft tissue mass includes all the non-bone mass (items 2 to 6)
made up of lean tissue mass (items 2 to 5) and fat tissue mass (item 6).


        In areas that contain no bone, the soft tissue component can be divided into its
own 2-component model consisting of Fat soft tissue and Lean soft tissue. By
considering the body to be made up of a series of 2-component systems, DEXA can
analyze each 2-component system separately and then combine the results for a
complete body composition analysis.




       Thus, when the dual energy X-ray beams are over a position of the body that
contains no bone, DEXA can analyze the area for the 2 components, fat tissue mass
and lean tissue mass. When the dual energy X-ray beams are over a position of the
body that does contain bone, DEXA can analyse the area for the 2 components, soft
tissue mass (fat and lean combined) and bone mineral mass.       The fat and lean
components of the bone-containing areas can then be deducted by a method that we
shall discuss. This way, the human body can be regarded as consisting of 3
principal components viz., fat mass, lean mass and bone mass (see Figure 6) and
these 3 components can be estimated by a 2-component technique using dual
energy X-rays.
                         Figure 6. Human body composition




3.2 A model of DEXA analysis

       For convenience, we shall reduce the human example into a block of tissue
containg the 3 components we are interested in. The left half of the block represents
an area of tissue containg only soft tissue (fat + lean). The right half represents an
area of tissue that contains both soft tissue and bone (see Figure 7).




                              Figure 7. A block of tissue


       As the dual energy X-ray beams pass through the “soft-tissue only” region, the
mass of the 2 components, fat tissue and lean tissue, can be determined. Similarly, as
the dual energy X-ray beams scan through the “bone + soft-tissue” region, the mass of
its 2 components, bone mineral and soft tissue, can be determined.


       The composition of the soft tissue over the bone is nearly the same as the
composition of the soft tissue in the no-bone area. For instance, if the total soft
tissue mass of the “soft tissue only” area is 10g and it contains 2g fat (known from
scan), then we have the following results:




No-Bone area
    Fat       = 2 x 100 /10 = 20
  Lean      = 100 – 20 = 80


       This composition of the soft tissue in the “no-bone” area is assumed to be the
composition of the soft tissue in the “bone” area also. Thus,




Bone area
   Fat mass = 2g
  Fat        = 2 x 100 /10 = 20
  Lean     = 100 – 20 = 80




       If the total mass of the soft tissue in the bone area is 5g (known from the scan),
then the fat mass of this area can now be calculated as




Bone area
    fat       = 20
   soft tissue mass (from scan) = 5g
 fat mass = 5 x 20/100 = 1g
 lean mass = 5 – 1 = 4g
  (Bone mass is also known from the scan)
       We have thus understood how 3 components of the body can be determined
using a technique that can only measure 2 components at one time. We shall now
attempt to understand how DEXA actually measures bone, fat and lean mass.




                 4. ACTUAL DEXA CALCULATION


4.1 The R-value

       To understand this, we need to define a new term, namely R-value. R-value is
simply the ratio of the low-energy attenuation coefficient to the high-energy
attenuation coefficient. Let us return to Tables 1 and 2 to calculate the R values of
some homogeneous absorbers. We get a column of R-values for these absorbers in
relation to the 40 and 70 keV X-rays (see Tables 3 and 4 generated from Tables 1 and
2).




Table 3 : R values of some elements.
Element           Atomic number      U (cm2/g)                          R-value
                                     40 keV           70 keV
H                 1                  0.3458           0.3175            1.0891
C                 6                  0.2047           0.1678            1.2199
N                 7                  0.2246           0.1722            1.3043
O                 8                  0.2533           0.1788            1.4167
Na                11                 0.3851           0.2022            1.9045
Mg                15                 0.4704           0.2244            2.0963
P                 15                 0.7784           0.2839            2.7418
S                 16                 0.9509           0.3258            2.9187
Cl                18                 1.1000           0.3491            3.1600
K                   19                 1.4840            0.4297             3.4536
Ca                  20                 1.7920            0.5059             3.5422




Table 4: R-values of some homogeneous absorber materials.
Component                U                                        R-value
                         40 keV              70 keV
Ca                       1.7920              0.5059               3.5422
Water                    0.2636              0.1942               1.3574
Protein                  0.2363              0.1831               1.2906
Fat(Oleic acid)          0.2273              0.1872               1.2136
Bone mineral             0.9039              0.3159               2.8613


          For absorbers composed of more than one component, the R-value is a
function of mass attenuation coefficient of each component as well as the mass
fraction of each component.


4.2 Use of R-value


          Using R-value, we can determine the mass fraction of each component in a 2-
component system, if we also know the mass attenuation coefficients of each
component. In fact, the R-value for soft tissue (made up of fat and lean) is linearly
related to the amount of fat in the tissue. It decreases with increase in the fat content
(see Figure ).


          Let us now consider a DEXA scanning experiment using a low-energy X-ray
beam of energy 40 keV and a high-energy X-ray beam of energy 70 keV.                 The
sample used is the block of tissue considered earlier. The DEXA detectors measure
the energies of the attenuated X-ray beams emerging from the sample.
          Let us first scan through the “soft tissue (ST) only” area. The energy of the 40
keV beam has been attenuated to 0.358 keV and that of 70 keV to 2.291 keV. Now
scan the bone (B) area. The energy of the 40 keV beam has been attenuated to 0.080
keV and that of 70 keV to 1.960 keV. The data collected may be represented as
shown below:



40                                                   70
     E0 = 40 keV                                          E0 = 70 keV
40                                                   70
     EST = 0.358 keV                                      EST = 2.291 keV
40                                                    70
     EB = 0.080 keV                                        EB = 1.960 keV


          We have now collected all the necessary DEXA data to determine the
composition of our tissue block. We need to know the mass attenuation coefficients
and R-values for fat tissue (F), lean tissue (L) and bone (B). These are known from
experiments and are given below.




40
     UF = 0.23 cm2/g                                  70
                                                           UF = 0.19 cm2/g      RF = 1.211
40
     UL = 0.27 cm2/g                                  70
                                                           UL = 0.19 cm2/g      RL = 1.421
40                 2                                  70                2
     UB = 1.00 cm /g                                       UB = 0.32 cm /g      RB = 3.125


          We also need to know the mass attenuation coefficients and R-value for soft
tissue (ST). These values will vary from subject to subject. (Recall the variation of
soft tissue R value with the amount of fat in the subject.) So we have to determine
them from our experimental results by the following procedure.


Calculation of RST:


We know,
                 ln ( E0 / E ) = U x M


Thus for 40 keV X-ray we have
                ln ( 40E0 / 40E ) =      40
                                              U x M


Similarly, for 40 keV X-ray we have
                ln ( 70E0 / 70E ) =      70
                                              U x M


       Note that M, the mass per unit area of the tissue will not change with the
energy of the radiation. Applying these equations for calculating the R value of the
soft tissue, we obtain


                ln ( 40E0 / 40EST )          40
                                                  UST x MST         40
                                                                         UST
                ----------------- = ----------------- =            ------      = RST
                ln ( 70E0 / 70EST )          70
                                                  UST x MST         70
                                                                         UST


       Thus, substituting the known values on the LHS, we can cal culate the value of
RST.


                           ln ( 40E0 / 40EST )                ln ( 40 / 0.358 )
                RST =       -----------------         =       ------------------
                           ln ( 70E0 / 70EST )                ln ( 70 / 2.291 )
                        = 1.379


       We can now calculate the  Lean content of the soft tissue from the equation


 Lean = [(RST – RF) / (RL – RF)] x 100 = [(1.379-1.211) / (1.421-1.211)] x 100
          = [0.168/ 0.21] x 100
          =     80
 Fat        = 20


                 40             70
Calculation of        UST and        UST :

                40
                     UST = (Lean fraction) x 40UL + (Fat fraction) x 40UF

                 70
                      UST = (Lean fraction) x 70UL + (Fat fraction) x 70UF
          Substituting our experimental data, we obtain
                 40
                      UST = 0.262 cm2/g
                 70
                      UST = 0.19 cm2/g




The summary of data so far developed
From DEXA scan
40                                                       70
     E0 = 40 keV                                              E0 = 70 keV
40                                                       70
     EST = 0.358 keV                                          EST = 2.291 keV
40                                                       70
     EB = 0.080 keV                                           EB = 1.960 keV




Experimentally known and calculated values
40
     UF = 0.23 cm2/g                                    70
                                                             UF = 0.19 cm2/g     RF = 1.211
40
     UL = 0.27 cm2/g                                    70
                                                             UL = 0.19 cm2/g     RL = 1.421
40
     UB = 1.00 cm2/g                                    70
                                                             UB = 0.32 cm2/g     RB = 3.125
40
     UST = 0.262 cm2/g                                   70
                                                              UST = 0.19 cm2/g   RST = 1.379


Calculation of areal densities of the components :
Bone mineral density (MB):
It is calculated using the equation,


                           ln ( 40E0/ 40EB ) - RST x ln ( 70E0 / 70EB )
                 MB = ------------------------------------------------
                           40          70
                                UB -        UB x RST
On substituting the valures we obtain
MB = 2.30 g / cm2
Soft tissue density (MST), Lean tissue density (ML) and Fat tissue density (MF) over
the bone:
This is calculated using the expression
                            ln ( 40E0/ 40EB ) - RB x ln ( 70E0 / 70EB )
                 MST = ------------------------------------------------
                         40           70
                              UST -        UST x RB
                      = 14.95 g/ cm2
       We now assume that the composition of this soft tissue in the bone area is
approximately equal to that of the adjacent soft tissue in the no-bone area:




                Lean of ST (No-bone area) = 80
                  Fat of ST (No-bone area) = 20
So,
                Lean of ST (Bone area)               = 80
                Lean of Fat (Bone area)              = 20
               ML (Bone area)              = 14.95 x 0.80 = 11.96 g/ cm2
                  MF (Bone area)            = 14.95 x 0.20 = 2.99 g/ cm2
Soft tissue density (MST), Lean tissue density (ML) and Fat tissue density (MF) in the
no- bone area:
       Finally, we calculate the areal densities of the Lean and Fat tissue fractions of
the “ST only” area using the same above formulas.
               ML (No-Bone area)             = 18 x 0.80 = 14.4 g/ cm2
               MF (No-Bone area)             = 18 x 0.20 = 3.6 g/ cm2
       If we sum all the areal densities that we have calculated, we would obtain an
accurate measurement of the total weight of our tissue block:
               MB                            = 2.3 g / cm2
               ML (Bone area)               = 11.96 g / cm2
               MF (Bone area)                    = 2.99 g / cm2
               ML (No-Bone area)             = 14.4 g / cm2
               MF (No-Bone area)             =   3.6 g / cm2
               ------------------------------------------------
               Total tissue weight           = 35.25 g / cm2
       Evidently, these are tedious calculations to do by hand. We have just shown
the calculations with respect to one tissue. In reality, a very large number of tissues
are to be scanned and then the results are to be consolidated.              A computer is
absolutely necessary to achieve this. In fact the technician operates the DEXA
apparatus from a PC. After completing the scan, the data are analysed using the PC as
well. It is possible to divide the scanned image into various regions of interest (ROI‟s)
by properly positioning the cut lines. The DEXA software then analyzes each of the
ROI‟s and generates a report of the composition of each of the ROI‟s , as well as the
whole body analysis.




         5. ADVANTAGES OF DEXA SCANNING

       Dual Energy X-ray Absortiometry, or DEXA scanning, is currently the most
widely used method to measure bone mineral density. For the test, a patient lies down
on an examining table, and the scanner rapidly directs x-ray energy from two
different sources towards the bone being examined. The X-ray source and the
detector move in a coordinated rectilinear pattern over the patient. The mineral
density of the patient‟s bone weakens, or prolongs the transmission of these two
sources of x-ray energy through a filter onto a counter in a degree related to the
amount of bone mass present. The greater the bone mineral density, the greater the
signal picked up by the photon counter. The use of the two different x-ray energy
sources rather than more traditional radioisotope studies (such that would be used for
a bone scan) greatly improves the precision and accuracy of the measurements. The
DEXA images of hip and spine are shown below.
          DEXA scanning has become the most widely used method for measuring bone
mineral density for several reasons. When compared with radiographic absortiometry
or single energy x-ray absortiometry, DEXA scanning more precisely documents
small changes in bone mass and is also more flexible since it can be used to examine
both the spine and the extremities. A scan of the spine, hip or the total body requires
only one, two or four minutes respectively. Qualitative computed tomography (QCT)
is the only technique that can directly measure bone density and volume but can
distinguish trabecular from cortical bone. DEXA scanning is less expensive, exposes
the patient to less radiation and is more sensitive and accurate at measuring subtle
changes in bone density over time or in response to drug therapy than is QCT.




                            6. CONCLUSIONS


          DEXA is the most commonly used modern technique to determine the bone
density and hence the bone strength. The DEXA results help to predict the patient‟s
risk factors for osteoporosis. It is a fast, accurate, and less expensive technique. It
exposes the patient to fewer amounts of radiations. So the risk is reduced to a great
extend.
          Studies using DEXA scanning have shown that women with osteoporosis have
substantially lower bone density measurements than normal, age-matched women.
Bone mineral density is widely accepted as a good indicator of bone strength. Thus
low values can be compared against standard bone density measurements and help
predict a patient‟s risk for fracture based upon the DEXA scan measurements.
                    7. REFERENCES


1. Pietrobelli, A., et al., Am J. Physiol., 271: E941 – E951
  (1996)


2. Phoenix 5‟s Prostate Cancer Glossary, 2002


3. Genant HK et al. Review Noninvasive assessment of bone
  mineral and structure, J Bone Miner Res. 11, 707-730, 1996.

				
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