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Computed Tomography as a Source of Electron Density
Information for Radiation Treatment Planning
Witold Skrzyn´ski1, Sylwia Zielin´ska-Da˛browska2, Marta Wachowicz2, Wioletta S
´lusarczyk-Kacprzyk1,
Paweł F. Kukołowicz2, Wojciech Bulski1
Purpose: To evaluate the performance of computed tomography (CT) systems of various designs as a source of electron density
(ρel) data for treatment planning of radiation therapy.
Material and Methods: Dependence of CT numbers on relative electron density of tissue-equivalent materials (HU-ρel relation-
ship) was measured for several general-purpose CT systems (single-slice, multislice, wide-bore multislice), for radiotherapy simu-
lators with a single-slice CT and kV CBCT (cone-beam CT) options, as well as for linear accelerators with kV and MV CBCT systems.
Electron density phantoms of four sizes were used. Measurement data were compared with the standard HU-ρel relationships
predefined in two commercial treatment-planning systems (TPS).
Results: The HU-ρel relationships obtained with all of the general-purpose CT scanners operating at voltages close to 120 kV were
very similar to each other and close to those predefined in TPS. Some dependency of HU values on tube voltage was observed
for bone- equivalent materials. For a given tube voltage, differences in results obtained for different phantoms were larger than
those obtained for different CT scanners. For radiotherapy simulators and for kV CBCT systems, the information on ρel was much
less precise because of poor uniformity of images. For MV CBCT, the results were significantly different than for kV systems due
to the differing energy spectrum of the beam.
Conclusion: The HU-ρel relationships predefined in TPS can be used for general-purpose CT systems operating at voltages close
to 120 kV. For nontypical imaging systems (e.g., CBCT), the relationship can be significantly different and, therefore, it should
always be measured and carefully analyzed before using CT data for treatment planning.
Key Words: Radiotherapy · Computed tomography · Electron density
Strahlenther Onkol 2010;186:327–33
DOI 10.1007/s00066-010-2086-5
CT-Systeme als Datenquelle der Elektronendichte in Bestrahlungsplanungssystemen
Ziel: Vergleich verschiedener Computertomographie-(CT-)Systeme zur Bestimmung der Elektronendichte (ρel) für die Bestrah-
lungsplanung.
Material und Methodik: Die Relation des CT-Werts zur Elektronendichte wurde an verschiedenen modernen CT-Scannern
(„single-slice“, „multislice“, „wide-bore multislice“) ermittelt, für die Therapiesimulatoren mit einem „single-slice“-CT und
kV-CBCT-(„cone-beam“-CT-)Optionen sowie für Linearbeschleuniger mit kV- und MV-CBCT-Systemen. Vier unterschiedlich große
Phantome zweier Hersteller wurden zur Messung der Elektronendichte benutzt. Die Messdaten wurden mit den Standardumrech-
nungsformeln zweier marktüblicher Therapieplanungssysteme (TPS) verglichen.
Ergebnisse: Die HU-ρel-Relationen, die in allen modernen CT-Systemen vorhanden sind, waren untereinander sehr ähnlich, eben-
so wie zu den vorgegebenen Relationen in den TPS. Einige Abweichungen der HU-Werte in Abhängigkeit von der Röhrenspannung
wurden bei knochenäquivalentem Material beobachtet. Bei vorgegebener Röhrenspannung wurden bei den verschiedenen Phan-
tomen größere Differenzen gemessen als in den verschiedenen CT-Scannern. Weniger exakt waren die Informationen über ρel mit
den Therapiesimulatoren und KV-CBCT-Systemen aufgrund der mäßigen Uniformität der Bilder. Die Ergebnisse des MV-CBCT unter-
schieden sich aufgrund des unterschiedlichen Energiespektrums der Röntgenstrahlen signifikant von denen der kV-Systeme.
Schlussfolgerung: Die im TPS vorgegebene HU-ρel-Relation kann bei modernen CT-Systemen mit einer Röhrenspannung im
Bereich von 120 kV genutzt werden. Signifikant unterschiedlich dagegen ist die Relation bei nichttypischen Bildsystemen (z.B.
CBCT). Deshalb sollte bei solchen Systemen immer gemessen und sorgfältig analysiert werden, bevor die CT-Daten für die Thera-
pieplanung herangezogen werden.
Schlüsselwörter: Radiotherapie · Computertomographie · Elektronendichte
Original Article
Strahlentherapie
und Onkologie
1Medical Physics Department, Center of Oncology, Warsaw, Poland,
2Medical Physics Department, Holycross Cancer Center, Kielce, Poland.
Received: September 4, 2009; accepted: March 5, 2010
Published Online: May 17, 2010
327
Strahlenther Onkol 2010 · Nr. 6 © Urban & Vogel
Skrzyn´ski W, et al. CT as a Source of ρel Information for Radiotherapy
Introduction
Calculation of dose distribution within the treated volume is
an essential step of contemporary treatment planning in ra-
diotherapy. Many factors have an influence on the dose distri-
bution, heterogeneity of the patient’s body being one of them.
Data characterizing each patient are therefore needed for the
calculations. X-ray computed tomography (CT) has been used
as a basic source of such data for over 30 years now [17], and
is used as a base for treatment planning even in less common
radiotherapy techniques, such as helical tomotherapy [22] or
radiotherapy with proton beams [5]. Other imaging modalities
are sometimes used as an addition to CT, as they may offer
better visualization of target volume (e.g., magnetic resonance
imaging [18], positron emission tomography [1], or both of
them [26]). Nevertheless, the role of X-ray CT is fundamental,
as it provides information on the attenuation of radiation by
the patient’s tissues in a form of CT numbers, expressed in
Hounsfield units (HU) as in the following equation:
HUtissue = [(μtissue – μwater) / μwater] × 1,000,
where μ is the linear attenuation coefficient of water and of the
tissue. It is known that precise calculation of dose distribution
in radiotherapy can be performed on the basis of knowledge
of the electron density of the tissues [20]. Treatment-planning
systems (TPS) usually convert HU values to ρel (relative elec-
tron density, normalized to water) by means of the predefined
relationship between the two quantities, e.g., one given by
Knöös et al. [11]. In some TPS the relation is fixed, in others
the user is allowed to change it.
It should be remembered that Hounsfield numbers for a
given tissue depend on the quality of the X-ray beam; there-
fore, the values can differ between scanners. Even for a sin-
gle scanner, CT numbers for the same tissue depend on the
kV setting and on beam filtration [3, 4]. As a result, the dose
calculated by TPS can change by as much as 2% if CT scans
are obtained using 80 kV instead of 130 kV while using the
same HU-ρel relationship [7]. The HU-ρel relationships can
be measured with the use of phantoms with tissue-equivalent
materials [3], i.e., materials that have an atomic composition
similar to human tissues [8]. Data obtained with such phan-
toms for the particular CT scanner operating at a particular
kV can then be introduced into the TPS to make the calcula-
tions more precise.
However, different manufacturers of commercial elec-
tron density phantoms use different tissue-equivalent materi-
als. It is known that solid (resin-based) bone-equivalent mate-
rials give systematically lower HU values than water solutions
of CaCl2 of the same electron density [24]. Even for a single
CT unit and a single scanning protocol differences of 0.075 in
ρel can be observed depending on the choice of phantom (all
resin-based), leading to differences of the calculated dose in
the order of 1–2% [6].
HU values observed for a given material also depend on
the dimensions of the phantom [6, 9] and on the positioning of
the phantom, especially the presence and the type of patient
support [4] or location on/off axis [9]. Also, a change in posi-
tion of the tissue-equivalent insert in the phantom can lead
to a difference of up to 80 HU [3]. Such differences may be
attributed to differences in beam quality caused by different
filtration at different depths in the phantom (beam hardening
effect). Many of the described dependencies have also been
confirmed in Monte Carlo simulations [19]. Some uncertainty
of ρel data seems therefore to be unavoidable, especially as
patients also differ between themselves in dimensions and in
the composition of their tissues.
It is suggested that one universal HU-ρel relationship can
be used for all CT scanners operating at typical voltages (120–
140 kV), leading to dose calculation errors not greater than
1% [24]. These errors are comparable to those caused, e.g., by
use of uncorrected contrast-enhanced CT scans, which leads to
change in dose calculation of 1% on average (3% maximum)
in the lung [2], or of 0.67% on average (1.8% maximum) in the
brain [28]. Larger errors in dose calculations can, however, be
expected if applying standard predefined HU-ρel relationships
to data obtained with nontypical CT scanners, e.g., cone-beam
CT (CBCT) systems installed onto radiotherapy linear ac-
celerators or radiotherapy simulators. It is known, that such
systems are more prone to inaccuracies of HU values than
conventional CT systems because of the higher effect of X-ray
scatter associated with cone-beam geometry [21]. The use of
kV CBCT data for dose calculation could introduce errors of
3%, partly because of nonuniformity of CBCT images [12, 23].
In the case of MV CBCT, acceptable accuracy of calculated
doses can be obtained, if the images are uniformity-corrected
and if density calibration is done [15, 25].
The aim of this study was to evaluate the performance of
CT systems of various designs as a source of ρel data for treat-
ment planning of radiation therapy, and to compare the stan-
dard HU-ρel relationships predefined in TPS with the actual
relationships obtained by measurements.
Material and Methods
All the CT systems included in the study are listed in Table 1.
Three of them are general-purpose CT scanners and are rou-
tinely used as a source of electron density data, the other four
are designed as radiotherapy verification tools. The systems
present a wide range of designs, from single-slice X-ray CT to
megavoltage CBCT.
Electron density phantoms of four sizes made by two
manufacturers were used in the measurements as listed in
Table 2. For each phantom, several tissue-equivalent inserts
were available, covering a wide range of tissue densities
and compositions (lungs, soft tissues, bones). RMI 465 is a
typical electron density phantom. RMI 463 was designed for
general quality assurance and can accommodate only three
328 Strahlenther Onkol 2010 · Nr. 6
Skrzyn´ski W, et al. CT as a Source of ρel Information for Radiotherapy
tissue-equivalent inserts simultaneously. The phantom has
some internal structures designed for assessment of image
quality, however, they are not located very close to the inserts
and they do not cause artifacts in the image. We assumed that
the presence of the structures does not significantly influ-
ence HU readings of tissue-equivalent inserts. The CIRS 062
electron density phantom consists of inner and outer sec-
tions. The inner section alone can be used to simulate a pa-
tient’s head. When used together, the two sections simulate a
patient’s torso.
The relationships measured for different CT units were
compared with each other and with default relationships
implemented in two commercial TPS, namely Oncentra Mas-
terPlan [16] and Varian Cadplan [27]. Criteria proposed by
ESTRO [13] were adopted, i.e., it was assumed that the values
of ρel calculated by TPS should not differ from the known true
values by more than 0.05 for ρel < 1.5 and by more than 0.1 for
ρel > 1.5. Larger differences were treated as significant. The
influence of scan parameters (e.g., kV) and of the choice of
phantom on the results was also evaluated.
Results
General-Purpose CT Scanners
Figure 1 presents results obtained with RMI 465 phantom for
three general-purpose X-ray CT scanners operating at various
kV settings. For ρel between 0 and 1 (air, lungs, soft tissues),
all datasets did not seem to differ, while for ρel > 1 (bones), the
results were dependent on kV setting, and for a given kV, they
were different between scanners.
For each scanner operating at the most common voltage
setting (120 kV), the dependence of the results on the choice
of other parameters was also investigated with RMI 465. The
parameters included tube current, slice width, imaging mode
(axial/spiral), pitch in spiral mode, position of the inserts with-
in the phantom, and positioning of the phantom in the gantry
(i.e., on-axis or few centimeters off-axis). The range of HU
values obtained for each material for the GE HiSpeed unit
was generally smaller than 20 HU, only for high-density bones
(ρel > 1.4) it reached 50 HU. For both Siemens units, larger
differences were observed, reaching 50–60 HU for lung tissue
and 100–170 HU for high-density bones.
Figure 2 presents the dependence of the results obtained
for the GE HiSpeed scanner on the choice of phantom and
field of view (FOV; all the other parameters remaining the
same). Similar measurements were also done for the Siemens
Somatom Sensation Open unit and similar dependencies were
Table 1. List of computed tomography (CT) systems used in the study.
Systems marked with “+” in the TP (treatment planning) column are
routinely used as a source of electron density data for radiation treat-
ment planning.
Tabelle 1. Liste der in dieser Studie verwendeten Computertomo-
graphie-(CT-)Systeme. Die in der Spalte TP (Therapieplanung) mit „+“
markierten Systeme dienen in der Routine als Datenquelle für die
Elektronendichte an Bestrahlungsplanungssystemen.
Name Details TP
GE HiSpeed DX/i General-purpose single-slice CT, third ge-
neration
+
Siemens Somatom
AR
General-purpose multislice CT, third gene-
ration
+
Siemens Somatom
Sensation Open
General purpose multislice CT, third genera-
tion (large gantry bore – ∅ 82 cm)
+
Varian Ximatron kV CT as an option of radiotherapy simulator
(single slice, part of image intensifier used
as a detector)
–
Nucletron Simulix
Evolution
kV CBCT with flat-panel detector, as an opti-
on of radiotherapy simulator
–
Varian OBI kV CBCT with flat-panel detector, installed
on linear accelerator
–
Siemens MVision MV CBCT with flat-panel detector, installed
on linear accelerator
–
Table 2. List of electron density phantoms used in the study.
Tabelle 2. Übersicht der Phantome, an denen die Elektronendichte in
dieser Studie gemessen wurde.
Phantom Dimensions Description
RMI 465 ∅ 33 cm
(body)
ρel phantom, 16 inserts (∅ 2.8 cm each):
tissue-equivalent inserts, water, four
solid-water rods in various locations (for
uniformity check), titanium
RMI 463 ∅ 16 cm
(head)
Quality assurance phantom, can simulta-
neously accommodate up to three inserts
from RMI 465
CIRS 062
(inner
section)
∅ 18 cm
(head)
ρel phantom, eight tissue-equivalent in-
serts (∅ 3.05 cm each) and water
CIRS 062
(both
sections)
∅ 33 × 27 cm
(body, ellip-
tic)
ρel phantom, consists of inner and outer
sections, each section with identical set
of eight tissue-equivalent inserts
Figure 1. Results of measurements with RMI 465 phantom for three
general-purpose X-ray CT scanners (GE HiSpeed, Siemens Somatom
AR, Siemens Somatom Sensation Open) operating at three selected
kV settings.
Abbildung 1. Messergebnisse mit drei Routine-CT-Scannern (GE
HiSpeed, Siemens Somatom AR, Siemens Somatom Sensation Open)
am RMI-465-Phantom bei drei vorgewählten kV-Stufen.
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Skrzyn´ski W, et al. CT as a Source of ρel Information for Radiotherapy
observed. For the GE unit, the HU values were dependent on
the FOV itself, even for the same phantom and tube voltage.
This can be explained as, for that particular scanner, different
beam filtration is automatically chosen for large FOV rather
than for a small one, resulting in different beam quality.
Table 3 presents data on the variability of the HU values
for three general-purpose CT scanners. The largest source of
the variability is kV setting. Should this be kept constant at
120 kV, the largest remaining source of the variability is the
choice of phantom and of the FOV. Differences caused by
change of other parameters, or even by change of the scanner,
seem to be less significant.
Radiotherapy Simulator with Image Intensifier
Measurements on a Varian Ximatron radiotherapy simula-
tor with CT option were done for RMI 465 phantom only
(Figure 3). Significant nonuniformity was visible in the image
as a white ring, and CT numbers for four identical solid-water
inserts placed in different positions of the phantom ranged
from –208 HU to +13 HU (for general-purpose CT scanners
the values would be identical within a few HU).
kV CBCT
Measurements on two kV CBCT systems were performed
with CIRS 062 phantom (Figure 4). For Varian OBI and inner
section of the phantom (“head”), the results were almost in
agreement with the relationships predefined in TPS, signifi-
cant differences were observed only for lung-equivalent mate-
rials. Some dependency on dimension of FOV was observed,
which could be explained as an effect of differences in acquisi-
tion geometry and beam quality – for small FOV (Ø 25 cm
and smaller) a full-fan acquisition was used, while for larger
FOV half-fan geometry and a different bow-tie filter was used.
For both sections of CIRS 062 phantom placed together (“tor-
so”), the results were significantly different. HU values for
two identical tissue-equivalent inserts simultaneously placed
in outer and inner sections of the phantom differed by more
than 300 HU.
Another kV CBCT system included in the study was Nu-
cletron Simulix Evolution radiotherapy simulator. The results
did not agree with the predefined relationships and were also
dependent on the dimension of the phantom. Nonuniformity
of the images of the CIRS torso phantom was visible, similarly
as described for Varian OBI.
MV CBCT
Figure 5 presents results for Siemens MVision MV CBCT
and RMI 463 phantom. The relationship differed from those
obtained for general-purpose CT scanners, especially for ma-
terials of high electron density. Some dependence on choice
parameters (MU, reconstruction kernel) was observed.
Figure 2. Results of measurements with all available phantoms for GE
HiSpeed scanner operating at 120 kV. Relationships implemented in
two TPS shown for comparison.
Abbildung 2. Messergebnisse mit allen verfügbaren Phantomen des
GE-HiSpeed-Scanners bei 120 kV. Die Relationen wurden in zwei TPS
implementiert und zum Vergleich dargestellt.
Figure 3. Results of measurements with RMI 465 phantom for Varian
Ximatron radiotherapy simulator operating as single-slice CT at 120 kV.
Relationships implemented in two TPS shown for comparison.
Abbildung 3. Messergebnisse mit dem RMI-465-Phantom des Radio-
therapiesimulators Varian Ximatron als Einzelschicht-CT bei 120 kV.
Die Relationen wurden in zwei TPS implementiert und zum Vergleich
dargestellt.
Table 3. Variability of Hounsfield (HU) values for three general-pur-
pose computed tomography (CT) scanners.
Tabelle 3. Variabilität der HU-Werte (Hounsfield-Einheiten) für drei
Allzweck-Computertomographie-(CT-)Scanner.
Altered
parameter Constant parameters Range of observed HU
values (maximum–mi-
nimum)
Lung Soft
tissue Bone
kV CT scanner, phantom 46 44 377
Phantom CT scanner, 120 kV 35 51 257
Protocol CT scanner, 120 kV, phantom 47 39 120
Scanner 80 kV, phantom 72 42 225
Scanner 140 kV, phantom 54 11 63
Scanner 120 kV, phantom 46 23 23
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Skrzyn´ski W, et al. CT as a Source of ρel Information for Radiotherapy
Handling of High-Density Materials (Titanium)
Despite some visible artifacts, small-diameter (ca. 1 cm) ti-
tanium insert in the RMI 465 phantom did not significantly
disturb the HU values for tissue-equivalent materials on gen-
eral-purpose scanners. The result could possibly be different
for a larger volume of high-density material (e.g., phantom
simulating hip prosthesis with metal alloys). For MV CBCT,
the titanium did not introduce any artifacts. It is worth noting
that for the GE HiSpeed scanner, the HU value obtained for
titanium was 4,000, which is basically the maximum HU value
used in that CT unit.
Discussion
Table 4 presents maximum differences between real values
of ρel (as given by the manufacturers of the phantoms) and
ρel calculated from the measured CT numbers with use of un-
modified Cadplan calibration curve. The criteria suggested
by ESTRO [13] were generally fulfilled for general-purpose
CT scanners operating at 120 kV or 140 kV and TPS using
predefined relationships. Only for some specific combina-
tions of scanner, phantom, and parameters, the differences
for materials of high density (bone with ρel of 1.47) reached
0.08. This was outside the tolerance, as the differences should
not exceed 0.05. Anyway, it should be remembered that for
slightly higher ρel (≥ 1.5), higher differences are allowed
(0.10). It should also be noted, that for the same scanners and
the same voltages (but different phantom, or different set-
tings) the differences were lower than 0.05. All the results ob-
tained for general-purpose CT scanners operating at 120 kV
or 140 kV fall within tolerance levels proposed by Kilby et al.
[10], which are based on calculation of impact of inaccuracies
on dose calculations. The results suggest that relationships
implemented in TPS can generally be used for general-pur-
pose CT systems operating at voltages close to 120 kV. Data
measured with electron density phantom can, of course, be
used to modify the relationship (provided that the TPS al-
lows it), however, some uncertainty of ρel data will remain as
there is no obvious way to eliminate dependencies on size and
shape of the phantom (or patient).
The results for the radiotherapy simulator were not
very different from those obtained for general-purpose CT
scanners, however, the information on ρel was much less pre-
cise because of the decidedly worse uniformity of images. Also
for two kV CBCT systems, very serious nonuniformity of im-
ages was observed for large phantoms (and patients, e.g., in
pelvic cases). All three systems could probably be used as a
Table 4. Maximum differences between electron density calculated
from the measured computed tomography (CT) numbers (with use
of unmodified Cadplan calibration curve) and true values of electron
density.
Tabelle 4. Maximale Unterschiede zwischen Elektronendichte, be-
rechnet aus den gemessenen Computertomographie-(CT-)Zahlen (un-
ter Nutzung der unmodifizierten Cadplan-Eichkurve) und den wahren
Werten der Elektronendichte.
System Maximum error of calculated ρel
Lung Soft tissue Bone
General-purpose CT, 80 kV 0.04 0.03 0.13
General-purpose CT, 120/140 kV 0.04 0.03 0.08
Varian Ximatron 0.07 0.19 0.08
Nucletron Simulix Evolution 0.17 0.14 0.30
Varian OBI 0.13 0.30 0.43
Siemens MVision 0.16 0.02 0.37
Figure 4. Results of measurements with CIRS 062 phantom on two
kV CBCT systems: Varian OBI (125 kV) and Nucletron Simulix Evolution
(100 kV). Relationships implemented in two TPS shown for compari-
son.
Abbildung 4. Messergebnisse mit dem CIRS-062-Phantom an zwei
kV-CBCT-Systemen: Varian OBI (125 kV) und Nucletron Simulix Evolu-
tion (100 kV). Die Relationen wurden in zwei TPS implementiert und
zum Vergleich dargestellt.
Figure 5. Results of measurements with RMI 463 phantom for Sie-
mens MVision MV CBCT system operating at 6 MV. Relationships im-
plemented in two TPS shown for comparison.
Abbildung 5. Messergebnisse mit dem RMI-463-Phantom des
MV-CBCT-Systems Siemens MVision bei 6 MV. Die Relationen wurden
in zwei TPS implementiert und zum Vergleich dargestellt.
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Skrzyn´ski W, et al. CT as a Source of ρel Information for Radiotherapy
source for ρel data. However, in all cases some work should be
done first to correct the nonuniformity of the images.
For MV CBCT, the results were significantly different
than for kV systems due to the different energy spectrum of
the beam. MV CT numbers could be potentially a good source
of data for treatment planning, as they are closely correlated
with ρel and represent attenuation of the therapeutic beam
[25]. Smooth handling of high-density materials (such as tita-
nium) is another advantage. On the other hand, image fusion
with other imaging modalities could be necessary to obtain
anatomic data for treatment planning, because of limited vi-
sualization of soft tissue in MV CBCT images (as compared
to kV CT). However, it is known that image quality can be
substantially improved by correction of uniformity and by op-
timization of system settings [14].
Unfortunately, it was not possible to measure the HU-ρel
relation for each CT scanner for a full range of selectable pa-
rameters and with use of all phantoms. The most extensive
measurements were performed for general-purpose CT scan-
ners. The results showed that the predefined relationships can
be used for those scanners, even if some variability of the re-
sults on choice of phantoms was observed for bone-equivalent
materials. For nontypical CT scanners (radiotherapy simula-
tors, CBCT systems), less extensive evaluations were done, in
some cases with only one phantom, or with only one set of
acquisition parameters. Nevertheless, this was enough to show
that inaccuracies of ρel values are obviously larger than those
observed for general-purpose CT scanners, and that they oc-
cur in the whole range of electron densities.
Conclusion
The results obtained for two TPS showed that the HU-ρel re-
lationships predefined in TPS can be used for general-purpose
CT systems operating at voltages close to 120 kV. We, how ever,
advise to check the accuracy of ρel values calculated by TPS
for the particular CT scanner and scanning protocol. For non-
typical imaging systems (e.g., CBCT), the HU-ρel relationship
can differ significantly from the predefined ones and, there-
fore, it should be measured and carefully analyzed before using
CT data for treatment planning. Some uncertainty of ρel data is
always unavoidable, as even for general-purpose CT scanners,
the HU values for a given tissue can differ depending on the
dimensions of the scanned object, either phantom or patient.
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Address for Correspondence
Witold Skrzyn´ski
Medical Physics Department
Center of Oncology
Roentgena 5
02-781 Warsaw
Poland
Phone/Fax (+48/22) 6449182
e-mail: w.skrzynski@zfm.coi.pl
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