Tải bản đầy đủ - 0 (trang)
VI. Nuclear Magnetic Resonance Imaging

VI. Nuclear Magnetic Resonance Imaging

Tải bản đầy đủ - 0trang

42



L. A. G. AYLMORE



thickness, and resolution. If the principal source of hydrogen protons is

from water, then the hydrogen proton concentration can be correlated with

water content. By using different methods of generating the image, differences in the nature of the protons can be accentuated, e.g., the mobile

proton density, relaxation time, or magnetic susceptibility. The images are

produced by protons with motional correlation times faster than about

lo-* sec. Computed tomography can be used to reconstruct a slice of the

spatial distribution of water content with resolution as low as 50 X 50 pm

with a slice thickness of 1.25 mm (Johnson et al., 1986; Woods et al., 1989;

Wehrli et al., 1988).

NMR imaging has been successfully used in the nondestructive measurement of water content in plants (Bottomley et al., 1986; Brown et d.,

1986; Omasa et al., 1985; Williamson et al., 1992). Bottomley et al. (1986)

used a spatial resolution of 0.6 mm with an unidentified slice definition to

observe the movement of a dilute solution of CuSO, into and through

roots of Viciu faba. Brown et al. (1986) were able to differentiate anatomical regions of the Pelargonium hortorum roots with a spatial resolution of

0.1 X 0.1 X 1.2 mm. Omasa et al. ( 1985)used NMR imaging with a spatial

resolution of 2 mm to image root seedlings and show changes in water

content in the seedlings, and Williamson et al. (1992) used NMR for

noninvasive histological studies of ripening red raspberry fruits.

However, Anderson and Gantzer (1989), in comparing results obtained

by X-ray CT with those obtained by NMR imaging, were unable to obtain

images of the soil cores with NMR. They attributed their failure to the

limited range in settings of the pulse repetition time and the spin-echo time

on the commercial magnetic resonance imaging (MRI) unit used in the

study. Paetzold et al. (1985, 1987;Paetzold, 1986)used NMR spectroscopy

to measure the water content of bulk soil, but not in specific regions. More

recently, McFall et al. (1991), by using a series of reference tubes (sand

phantoms) filled with acid-washed sand at various water contents to provide a rapid reference calibration curve, were able to monitor water uptake

by loblolly pine seedlings from a fine sand and to compare qualitatively the

relative efficiencies of fine, lateral, and taproots in water uptake.

Naturally occumng soil materials, both organic and inorganic, are generally poor specimens for direct NMR study because line broadening due

to chemical heterogeneity severely reduces resolution and cannot be removed by magic-angle spinning (MAS) or technologically accessible field

strengths (Bleam, 199I). In addition, major difficulties arise in quantitatively measuring water which is physically bound within the soil matrix.

Hence the value of this technique is likely to be limited in its usefulness for

studying soil/plant /water relations.



CAT STUDIES OF WATER MOVEMENT



43



VII. DUAL-ENERGY SCANNING

A major limitation of the use of single-energy X-or pray CAT scanning

systems in studies involving measurements of the spatial distribution of

water content in soils has been the necessary assumption of uniform or

constant bulk density. Because the attenuation is a function of both the

bulk density and the water content of the soil, an accurate determination of

water content in soils is not possible when changes in bulk density occur

during experiments (Petrovic et al., 1982; Hainsworth and Aylmore, 1983;

Anderson et al., 1988; Phogat and Aylmore, 1989). Even in effectively

nonswelling soils the difficulties in obtaining uniformly packed soil columns pose problems in applying Eq. (12) to monitor water contents near

plant roots, because this requires the exact superposition of wet and dry

scans. Any redistribution of soil as a result of wetting will also degrade the

accuracy of water content determination.

To monitor changes in the spatial distribution of bulk density and water

content in situations in which the bulk density of the soil changes due to

swelling, shrinking, or redistribution on addition or removal of water,

independent estimates of attenuation associated with both bulk density

and water content are required. This can only be obtained by the simultaneous use of two sources of different energies.



A. THEORY

OF DUAL-ENERGY

SCANNING

For two pray energies the attenuation equations for Eq. (12) may be

written as

PLanta



=Paps



k - e t b = p&Ps



+ Pwa&



(18)



+pwbev



(19)



where subscript “a” refers to the low-energy radiation and subscript “b”

refers to the high-energy radiation. Thus p-, pwb,p,, and p& are the mass

attenuation coefficients for water and soil solids, respectively.

Equations (1 8) and ( 19) can be solved simultaneously to give

P s = [ ( ~ w b k v a a ) - ( & v a ~ w e t b ) l / [ ( & b ~ t s a )- (/?sb/&3)1



(21)

- (/4vbptsa)1

Thus, ps and 8, for an individual pixel can be calculated by scanning the

wet soil column with both radiation sources at a fixed position.

ev



= [(p&&vcta)- (papwetb)l/[(p&pwa)



(20)



44



L. A. G . AYLMORE



B. CHOICE

OF SOURCES

Although the average energy of X-radiation can be adjusted in some

commercially available medical systems, the range is usually relatively

narrow and the polychromatic nature of the beams would greatly complicate the application of Eqs. (20) and (21). A combination of I3’Cs and

”‘Am sources has most commonly been used in conventional dual-source

y scanning because of the 10-fold difference in pray energy (662 and 59.6

keV, respectively) and the long half-lives involved (Corey et al., 1971;

Gardner et al., 1972: Nofziger and Swartzendruber, 1974). However, both

the time required to complete successfully a CAT scan and the precision

obtained depend on the transmission intensity and hence count rate. y-Ray

sources generally emit much smaller photon fluxes than do X-ray sources.

Whereas high-strength 137Cssources are readily available, the beam

strength obtainable from “‘Am has a practical limit because of self-absorption (Miller, 1955). Scan times required with this source are not sufficiently rapid to follow the rapid changes in soil 6,, which may be associated with, for example, water extraction by plant roots or water

infiltration and redistribution in the soil profile. Clearly the minimum

scanning time, commensurate with adequate precision, will be generally

desirable and will be particularly necessary in dual-source CAT scanning.

Hainsworth and Aylmore (1988) suggested the use of 169Ybas an alternative source to

because this provides a similar energy level (63.1

keV) and attenuation coefficient for water (0.176 cm-I), but emits much

higher photon outputs (more than 20 times) at high activities (e.g., 1.0 to

2.0 Ci). The major disadvantage of the la9Ybsource is its relatively short

half-life of 3 1 days, resulting in a working life of approximately 2 months.



C. APPLICATION

OF CAT TO DUAL-ENERGY

SCANNING

The feasibility of applying the CAT scanning procedure to dual-source y

attenuation to enable simultaneous measurements of the spatial distributions of bulk density (p,) and water content (0,) in swelling soils was

examined by Phogat et al. ( 1991). The values of mean water content for

slices of both swelling and nonswelling soils determined by the dual-source

(137Csand 169Yb)y CAT scanning technique showed excellent agreement

(R2= 0.99 1) with the values obtained gravimetrically (but expressed volumetrically) for water contents from 0 to 0.55 cm3/cm3 (Fig. 21). The

differencesin water content fell within the range k0.024 cm3/cm3.Similar

agreement (R2= 0.995) was obtained between mean bulk density values

obtained by the two methods, with a variation in the two determinations of



45



CAT STUDIES OF WATER MOVEMENT

0.6



a



.o



h



0.5



-



5 .

5 0.4



0





0



Dual-York

Dual-Kulin

Cs-York

Cs-Kulin



v



c



C



al



c



u



0.3



5



g



c



0.2



l-



a



g



0.1



0.0



0.0



0.1



0.2



0.3



0.4



0.5



0.6



Volumetric water content (cm3/crn3)

Figure 21. Relationships between volumetric water content and water content determined by dual- and singleenergy y computerized axial tomography scanning for soils from

Kulin and York, Western Australia. (After Phogat et al., 1991.)



approximately f0.015 cm3/cm3. Use of a single source (I3’Cs) markedly

underestimated soil water content particularly in a structurally unstable

soil (Fig. 2 1). The results for the single 13’Cs source scanning of Kulin and

York soils in Figs. 14B and 21 illustrate the effects of bulk density changes

during wetting on the attenuation coefficient measurement. The kaolinitedominated Kulin soil exhibits some swelling, leading to a lower estimate of

the mass attenuation coefficient for water (Fig. 14B). The smectite-dominated York soil swells substantially on wetting, reducing the effective bulk

density and leading to a marked underestimation of water content by the

single y technique.

However, the primary objective of the CAT technique is not to measure

average bulk density and water content but to reveal the spatial distributions of these quantities. The results of Phogat et al. (1991) demonstrated

that, using dual-source y CAT scanning, it is possible to measure the spatial

distributions of 8, and ps in soils simultaneously and nondestructively with

a satisfactory level of precision. Unfortunately, despite the enhanced outsource, the accumulation of statistical errors arising

put from the 169Yb

from the random nature of radioactive emissions still necessitated excessively large counting times to provide acceptable accuracy. The height of



L. A. G. AYLMORE



46



13.0



’37cs



16’Yb



Dual source



Gamma radiation source



Figure 22. Reconstructed half-slice three-dimensional computerized axial tomography

scanning images showing the spatial distribution of water content in slices of a uniform field

of water scanned with I3’Cs and I69krb at different counting times and calculated for both

single I3’Cs and l69krb sources as well as for the dual-y-sourceCAT scanning procedure. (Afier

Phogat et aL, 199 I .)



the surface in Fig. 22 represents 8, for each pixel in a slice through a

uniform field of water, scanned with 137Csand la9Yband calculated using

Eq. (20) with counting times of 0.1,2.0, and 13 sec. These images illustrate

that as the counting time is increased, the variation in pixel 8, decreases

and, as a result, the surface of the images becomes more uniform for all

three methods. The higher photon output of the la9Yband the fact that the

average attenuation from la9Ybis greater than that from 137Csreduces the

relative error (the absolute error in attenuation being roughly the same)

and explains the greater uniformity of the Yb scans compared to the Cs

scans. At a counting time of 13 sec, 13’Cs gave a standard deviation for 8,

for pixels of 0.016 cm3/cm3whereas the corresponding value for the la9Yb

scan was f0.007 cm3/cm3. When the scan data of both the sources were

used in Eq. (21), it yielded a standard deviation for 8, for pixels of 0.073

cm3/cm3. This variation in pixel 8, using both sources is high in spite of

the very low values for scans of the individual sources (137Csand la9Yb).

This multiplicative propagation of errors is most evident when, due to



CAT STUDIES OF WATER MOVEMENT



47



random emissions, a particular pixel, estimated to have a low attenuation

using I3’Cs, is estimated to have a high attenuation using 16Vb, or vice

versa. As a result, small variations in the Cs and Yb scans can give rise to

large and unacceptable variations when Eqs. (20) and (21) are applied.

Some 169 sec for an individual ray sum and, hence 112 hr to complete a

dual-source scan were required to achieve average standard deviation

values of pixel water content of the order of 0.025 cm3/cm3for a uniform

field of water. Such large count times limit the equipment used to the study

of steady-state or only slowly changing systems.



VIII. RECENT AND FUTURE DEVELOPMENTS

Current medical X-ray CT systems are designed for low-radiation dosage

and are high speed to freeze the motion, resulting in only moderate spatial

and contrast resolution. In contrast, industrial CT systems designed

for object size/weight /density flexibility and high sensitivity, and not

restricted by object motion or radiation dose level, have recently been

developed [e.g., at Advanced Research and Applications Corporation

(ARACOR), Sunnyvale, California, and Surrey Medical Imaging Systems

(SMIS), Surrey, England]. These include systems capable of inspecting

objects up to 2.4 m in diameter by 5 m long and weighing up to 49,500 kg.

In contrast, inspection requirements in the advanced materials, electronics,

and printed circuit-board industries have also resulted in the development

of X-ray CT systems that provide 25-pm spatial resolution for such objects

(ARACOR). Similar work is in progress at EMBRAPA (Brazil) to improve

resolution capabilities to 1 pm (S. Crestana, personal communication,

1991). Future developments may also include CT systems using backscatter radiation from the object or emission computed tomography (where the

decay of in situ radioactive isotopes produces prays) to form the image,

thus facilitating the measurements in inaccessible regions.

Improved image and data analysis software for the y CAT scanning

system at the University of Western Australia (Schuller and Aylmore,

1993) allows two- and three-dimensional visualization and quantitative

analysis of scan data not offered on typical medical scanners. This software

permits a three-dimensional image to be constructed from multiple contiguous scans, which can then be viewed from any angle and distance, sectioned and sliced, or stripped away by subtractive imaging using 32 colors

or shades to represent variable attenuation ranges. Figure 23 illustrates the

way in which both higher attenuating compacted soil layers or less atten-



48



L. A. G. AYLMORE



Figure 23. Three-dimensional image reconstructions obtained by subtractive imaging.

(A) Three highdensity layers in soil core. Note curvature at edges caused by entry of coring

tube into profile. (B) Lupine root in soil column.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

VI. Nuclear Magnetic Resonance Imaging

Tải bản đầy đủ ngay(0 tr)

×