Photon Factory Activity Report 2005 #23 Part B (2006)
Biological Science

12A/2004G391

Distribution of light elements including Ca and P in mammalian cells calculatedusing soft X-ray images obtained by contact microscopy
Atsushi ITO*1, Yohei OHNO1, Takahiro ARIKAWA1, Kunio SHINOHARA2
School of Engineering, Tokai Univ., Hiratsuka, Kanagawa 259-1292, Japan
2
JASRI/SPring-8, Sayo-gun, Hyogo 679-5198, Japan

1Introduction
Soft X-ray microscopy has a unique characteristic toobtain high-resolution maps of light elements thatconstitute biological specimens. In addition to major C, Oand N elements, distribution of minor light elements suchas Ca and P provides useful information about theirparticipation in cellular physiological responses.
For the quantitative mapping of these elements, wehave been developing computer programs to calculate
light elements from a set of soft X-ray images taken at
wavelengths including both sides across absorption edgesof contained elements. Contact X-ray microscopy in
combination with an electronic zooming tube is a useful
tool for this purpose in the applicability to the wide
wavelength region to cover absorption edges of
intracellular light elements. In the computer algorithm,
subtraction of absorption between both sides of theabsorption edges is performed for each pixel to obtain themap of elemental contents. An iteration procedure is also
introduced to find absorption changes at the absorption
edges that are hidden by a large absorption of major lightelements [1]. In our previous algorithm a set of the above
subtracted equations is solved simultaneously. However
this algorithm required a lot of calculation time.
In the present study, we have improved the algorithmto reduce the calculation time drastically. The calculated
results gave the same values as those obtained with theprevious one.
Materials and Methods
Contact X-ray microscopy with an electronic zooming
tube was used to obtain X-ray images of dried humancancer HeLa cells in the wavelength range from 1.5 nm to
10 nm at the BL-12A beamline. The computer algorithm
to calculate elemental maps is as follows:
The absorption (A) for each pixel is expressed asshown in the Eq. (1).
(1)
A = µC ρC x + µO ρO x + µ N ρ N x + µ P ρ P x + ⋅⋅⋅where µC etc. are mass absorption coefficient of carbon
etc., and ρCx etc. denote mass thickness of carbon etc.
Step 1: Suppose that in the absorption spectrum anabsorption jump at the absorption edge is apparent forcarbon. Then the following subtraction equation isobtained:(2)
AC = ACS − ACL = (µ C −CS − µC −CL ) ρ C x
where CS is a wavelength of the shorter side of the CK edge, and CL is a wavelength of the longer side of theC-K edge. µC-CS and µC-CL show mass absorption
coefficients of C at the wavelength of CS and CL. The
first approximated value of carbon content (ρCx) was
obtained from Eq. (2).
Step 2: A-µCρCx is calculated to detect absorption
jumps besides carbon. If we find an absorption jump at
the oxygen absorption edge, another equation is describedin addition to the carbon case. The two equations aresolved, and the second approximated content of carbon is
the sum of the first approximated value and the value
from the second equation. The first approximated value ofoxygen is also calculated.
Then the steps of 1 and 2 are repeated until noabsorption jumps are detected.
Results and Discussion
Fig. 1 shows the distributions of phosphorus andcalcium in HeLa cells. The results show the predominantdistribution of phosphorus in the nuclei, and interestingly
calcium was also present preferentially in the nuclear
region. The distribution and the elemental content wereidentical with those in our previous study. Compared withour previous algorithm, the calculation time was largely
reduced to one fifth.
Phosphorus

Calcium

Fig. 1. Distributions of phosphorus and calcium inHeLa cells.

References[1] Ito A., Matsuda H., Kitajima Y. and Shinohara K., J.
Phys. IV France, 104 (2003) 297-300*aeito@keyaki.cc.u-tokai.ac.jp

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