Original

Dual-energy bone-mineral Richard

Research

x-ray absorptiometry for total-body and soft-tissue composition1’2

B Mazess,

Howard

S Barden,

Joseph

P Bisek,

and

Bone mineral density (BMD) and soft-tissue total body and major subregions were measured with dual-energy x-ray absorptiometry (DEXA). Total body scans were made in 12 young adults (6 male, 6 female) on five occasions at both a medium speed (20 mm) and a fast speed ( 10 mm). There were no significant differences in mean results or in precision errors between the two speeds. The precision errors (1 SD) for total body BMD, percent fat in soft tissue (% Fat), fat mass, and lean tissue mass were < 0.0 1 g/cm2, 1.4%, 1 .0 kg, and 0.8 kg, respectively. These results corresponded to a relative error of0.8% for total body BMD and 1.5% for lean body mass. Regional BMD and soft-tissue values (arms, legs, ABSTRACT

composition

ofthe

trunk)

determined

were

with

slightly

higher

precision

errors.

was 5.8 ± 0.5% oflean tissue mass (r p < 0.001). DEXA provides precise composition analysis a low radiation exposure ( 3 g/cm2) but no significant

relationship

between

R value

and

a minimal amount of soft bone (< 0.05 g/cm2). The actual

composition

=

ucts,

=

Wilmington,

alcohol. and can

DE),

Alcohol be used

and

and plastics to simulate

a mixture

cients at the energies used, polyoxymethylene is 40% fat. This ibrate

the

percent

of water

and

isopropyl

have attenuation similar to fat it. Based on attenuation coeffiwater is equivalent to 5% fat and

fat in soft

tissue

‘

relationship (% Fat)

was

used

to cal-

for determinations

in vivo. A more detailed relationship is shown for polystyrene in Figure 3. We also examined the relationship between R value and tissue thickness (Fig 4). There was a slight increase in R value (0.01) over the range from 15 to 25 cm; however, the difference between lard and water was constant with thickness. Consequently there is some influence ofthickness on fat determination; subjects with thicker tissues potentially appear to have

slightly

inflated

R values

and

less

fat.

This

thickness

de-

pendence is readily compensated because the subject thickness is measured. Temporal variations in the composition calibration also can be corrected. We made 230 measurements on an 8-cm thick polyoxymethylene phantom over a l-y period. The

100 80

1

w

Intrascanner (n=34)

Interscanner (n=37)

z LU

60 F(I, -

I . 164 ± 0.007 (0.58) 0.499 ± 0.006 ( 1 .27) 0.801 ± 0.005 (0.58) 0.587 ±0.004(0.72) 0.582 ± 0.012 (2.00) 0.679 ± 0.008 (1.18) 0.497 ± 0.007 ( 1.48) 0.772 ± 0.003 (0.40) 1± SD: % CV in parentheses.

Each skeleton

1.459

±

0.499

±

0.026 (1.78) 0.0 1 1 (2.20)

0.672

±

0.009(1.34)

0.619 0.5 17

±

0.008(1.29) 0.013(2.1)

0.721

± 0.014(1.94)

±

0.575 ± 0.010(1.74) 0.73 1 ±0.007(0.96)

had very-low

density.

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110

-I

0

a-

*

VALUE

is shown

Precision ofBMD (g/cm2) in vitro ofone skeleton (34 determinations) by use ofone DEXA scanner and ofanother skeleton (37 determinations) by use of37 different DEXA scanners

Head Arms Legs Trunk Ribs Pelvis Spine Total

1.40

FIG 2. The relationship between % Fat and R value for lard, water, and 2 1% alcohol at a thickness of 20 cm. The regression was % Fat -518*Rvalue + 725(r> 0.99,SEE 1.5% Fat).

in Figure 2 from measurements on water, lard (99.5% fat), Delrim (polyoxymethylene, an acetyl resin; DuPont Polymer Prod-

TABLE

1.35

20

0 1.20

1.25

1.30

1.35

1.40

R VALUE FIG 3. The relationship between R value and polystyrene tion in a 25-cm thick water bath. The regression was % Fat value + 802.5 (r > 0.99, SEE 0.9 % Fat).

concentra-

=-579.9

*

R

1 108

MAZESS

ET

AL

spine, and ribs), and head. The BMD and soft-tissue composition were determined for each of these areas. In four of the above cases and in 1 1 other cases, measurements of BMD and

I .5

1.4

soft-tissue

WATER DELRIN

1.3

composition

made

by use

of DPA

with

‘53Gd

‘

DP4) as well as with DEXA. The results from DPA and DEXA were compared in these 1 5 cases. The measurements were approved by the University of Wisconsin Committee for

#{149}a

II

Protection

1.2

were

(Lunar

.

LARD

.

.

ofHuman

Subjects.

.

#{149}.#{149}#{149}

Results .

.

I

.

Table 2 shows the mean ble 3 gives the corresponding

1.1 0

20

10

THICKNESS FIG 4. The change and lard.

30

at the

(cm)

ofR value with tissue thickness

fast speed

for water, Delrin,

average R value was 1.3 1 ± 0.002; the SD was equivalent to a variation of ± 1% fat. The % Fat times the measured soft-tissue mass gave the fat mass; the remainder was the lean tissue mass (LTM). The LTM was used rather than lean body mass because we desired to separately assess the skeletal contribution to composition.

Measurements were made on 12 normal young adult subjects (6 male, 6 female) on five occasions by use ofboth the fast and medium speeds. The 10 scans were made over a period of from 5 to 7 d with an average of two scans per day. The SD and percent CV among measurements on each individual were calculated as indices of precision (16). The total body scans were divided into several regions: arms, legs, trunk (pelvis,

and regional

BMC

Men and women

Fast

Both

and BMD (n

by DEXA

at the two

relation

between

was

very

high

and

tissue

speeds,

higher

than

both

speeds and

regional was

the results

=

composition.

The

ing results from the two averaged 50 g and 0.01 tively. For the fast speed cm2 or 1.3% and 0.5%, values for composition; values whereas Table 6