Download comparison of teflon phantom image from pet/ct scanner and monte

JOURNAL of NUCLEAR And Related TECHNOLOGIES, Vol. 6, No. 2, December, 2009.
COMPARISON OF TEFLON PHANTOM IMAGE FROM PET/CT
SCANNER AND MONTE CARLO SIMULATION
Z. Rafidah 1, M.S. Jaafar1, A. Shukri1, M.A.A Khader2 and Abdel Munem E1
1
Medical Physics Research Group, School of Physics, Universiti Sains Malaysia, 11800
Minden, Pulau Pinang, Malaysia.
2
Dept. of Nuclear Medicine, Penang Hospital, Residency Road, 10990 Penang, Malaysia.
ABSTRACT
The objective of this study was to compare the acquired image of teflon, human bone equivalent
material on a Positron Emission Tomography/Computed Tomography (PET/CT) scanner with
Monte Carlo simulation (MCNP). The cylindrical shape teflon phantom with dimensions of 19.5
cm length and 5.0 cm diameter was used for imaging with different settings of kilovolts (kV) and
milliamperes (mA) of PET/CT. In this simulation, the photon flux in each pixel was accumulated
by the Flux Image Radiograph (FIR) tally as flux image detectors and the image was plotted
using Microsoft Office Excel. Results show that MCNP image was comparable with that of CT
image and the obtained MCNP image depends on pixels size of the FIR tally.
ABSTRAK
Objektif kajian ini ialah untuk bandingkan imej yang diperolehi teflon, tulang manusia bahan
setara pada Positron Emission Tomography / Computed Tomography (PELIHARAAN / CT)
pengimbas dengan simulasi Monte Carlo (MCNP). Hantu teflon bentuk silinder dengan dimensi
19.5 cm panjang dan 5.0 cm garis pusat telah digunakan untuk pengimejan dengan
persekitaran yang berbeza kilovoltan (kV) dan milliamperes (mA) PET / CT. Dalam simulasi
ini, fluks foton dalam setiap piksel telah dikumpulkan oleh Flux Image Radiograph (FIR) kira
sebagai pengesan-pengesan imej fluks dan imej dirancang menggunakan Microsoft Office
Excel. Keputusan menunjukkan imej MCNP itu setanding dengan daripada imej CT dan
memperolehi imej MCNP bergantung kepada saiz pixel catatan FIR.
Keywords: PET/CT, MCNP, FIR Tally, teflon phantom.
INTRODUCTION
PET/CT is a dual-imaging technology which provides functional imaging at the molecular level
and anatomical structure of human body. This combined scanner is operated from a single
console with application-specific task cards selected for the CT and PET acquisition. The CT
tomography is usually in the front of the gantry and the PET tomography is situated at the back
of the system. A PET/CT imaging protocol usually involves acquisition of an initial CT scout
scan which takes prescribed time of 10 seconds, followed by a CT and a PET scans. The CT
scout scan serves as an anatomic reference for the PET/CT scan (Sureshbabu and Mawlawi,
2005).
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JOURNAL of NUCLEAR And Related TECHNOLOGIES, Vol. 6, No. 2, December, 2009.
In this work, CT image of teflon phantom is compared to Monte Carlo simulation. This study
was done to evaluate the simulation image from FIR tally with CT image acquired from a
PET/CT scanner. Monte Carlo numerical simulation methods can be described as statistical
methods that use random numbers as a base to perform simulation of any specified situation
(Attix, 1986). The demand of Monte Carlo simulation PET/CT imaging is rising due to
increasing complexity and cost of technical components. The Monte Carlo method is widely
used for solving many scientific problems involving statistical processes and is particularly well
suited for medical physics and biomedical engineering applications due to the stochastic nature
of radiation emission, transport and detection processes (Zaidi and Ay, 2007). Indeed, Monte
Carlo methods are accurate statistical simulation methods, based on first principles that are ideal
to simulate particle transport in complex geometry.
Applications of Monte Carlo techniques in the field of radiological imaging include
performance assessment and optimisation of design geometries and scanning parameters (Ay
and Zaidi, 2005). The general idea of MCNP analysis is to create a model which is more or less
similar to the real system under study and calculate the interaction within the modelled system
based on known probabilities of occurrence using random sampling of probability density
functions (pdf) for each event. The evaluation of the effect of physical, geometrical and other
design parameters on scanner performance and resulting image quality and patient dose could
be achieved through cumbersome experimental measurements using developed test prototypes
(DeMarco et al, 2007) or sophisticated Monte Carlo simulations (Colijn and Beekman, 2004).
MATERIALS AND METHODS
Image from cylindrical shaped teflon phantom was acquired on a Discovery ST PET/CT
scanner (General Electric Medical Systems) using different settings of kV and mA; 140 kV, 120
kV, 100 kV, 80 kV and 120 mA, 100 mA and 80 mA respectively. Practically, increasing the
kV will increases the electron strike speed, which in turn increases the energy of the X-ray
photon beam while increasing the mA increases the number of electrons that become available
to produce X-ray. Higher concentrations of electrons will lead to improve image resolution.
The phantom is positioned horizontally, on the patient table. Before starting the PET/CT scan,
the alignment of the phantom was checked by verifying that the laser lights align with cross line
marks on the phantom. The patient table was moved up and down, to and fro, to have the edge
of the phantom exactly at the isocenter. Scanning is started once the phantom is properly
centered in the PET/CT scanner.
The overall scanning time involved 10 seconds scouting and 6 minutes scanning. The image
was displayed in Centricity DICOM Viewer and measurements were performed to check the
dimensions of the phantom image. The viewer can display a series of four image slices in a
single display.
MONTE CARLO SIMULATION
The teflon phantom with dimensions 19.5 cm length and 5 cm diameter as shown in Figure 1(a),
has been simulated with MCNP5 code. MCNP is a general purpose, three dimensional general
geometry time-dependent, Monte Carlo–N-particle code that is used to calculate coupled
neutron-photon-electron transport. The code allows for generalised 3D object geometry and
materials content. This paper focuses mainly on Monte Carlo simulation of the phantom from
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JOURNAL of NUCLEAR And Related TECHNOLOGIES, Vol. 6, No. 2, December, 2009.
CT scanner. The specification of the computer that had been used for this simulation is Dell
Inspiron 5300 with 3 GHz processor.
The FIR planar radiograph flux image tally was used to construct a 2-D image. This study is
performed at 40 keV to get the preliminary result from FIR tally. The position of the source,
phantom, patient table and detector were maintained at the same distance as that in the real
practical situation as shown in Figure 1(b). The universe in the input file was divided into two
parts, air and void. The source, phantom and the patient table were placed in the air while the
detector was placed in the void in order to construct the image. The particles were initiated from
a single cross plane, and were given a direction perpendicular to the imaging plane.
The size of the image acquired was 24 x 22 cm2 with a pixel size of about 1 x 1 mm2. No
shielding was simulated here in order to simplify the simulation and decrease the run-time to its
minimum value. In the simulation, the radiation source covers the whole phantom with X-ray
energy of 40 keV and the total simulation time was 5 x 105 s.
Source
Air
Phantom
Patient table
Void
FIR detector
(a)
(b)
Figure 1. The positioning of the phantom (a) in PET/CT scanner and (b) in Monte Carlo
simulation.
RESULTS AND DISCUSSION
The obtained CT image of the phantom is a grey scale image on a black background. Figure 2
shows the CT image from PET/CT scanner. The CT images of the phantom are checked for
geometric scaling error in the scanner by measuring the dimension of the phantom in x, y and z
directions. The measured dimension in x direction was found to be + 0.2% offset for 5 cm
diameter and 0% offset for the 19.5 cm length of the phantom. From these images, we can see
that the dimensions of the images are approximately the same as the real dimensions of the
phantom.
The image analysis part of the Monte Carlo calculation was time consuming as it required
manual selection of the parameters of the input file and its simulation. The simulation for
obtaining the required image takes approximately 6 days. Figure 3 shows the projection profile
of transmitted photon generated by the MCNP file. The different colours of the image show the
relative flux on the phantom. It is shown that the MCNP file code can successfully simulate the
required CT image.
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Although the tally was a two dimensional planar tally, the image obtained from the Monte Carlo
simulation gave good information about the shape of the phantom. The length of the real
phantom is 19.5 cm while the bottom of the image shown is 17.4 cm, an increased of up to 23.2
cm. The diameter of the real phantom is 5 cm while the diameter of the image started from 4.4
cm at the bottom of the image and increased up to 13.2 cm at the top part.
Figure 2. CT image of teflon phantom from PET/CT scanner
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Figure 3. The MCNP5 simulation image of the phantom
CONCLUSION
The use of the Monte Carlo method to simulate radiation transport has become the most
accurate means of simulating medical imaging systems with the aim of optimising
instrumentation design or improving the accuracy of quantitative analysis for solving specific
problem. This phantom imaging study supports the best scan choice for PET/CT imaging and
demonstrates the effective CT photon energy. The dimensions of the MCNP simulated image is
approximately the same as that of real dimensions of CT grey scale image of the phantom.
Thus, the MCNP file code can successfully simulate the required CT image. The simulation
techniques obtained from FIR tally provides valuable information in the future of medical
imaging and further work will be embarked using Flux Image Cylinder (FIC) tally to acquire a
better 3-D image.
ACKNOWLEDGMENTS
The author would like to thank all the members of Biophysics and Medical Physics Research
Group, School of Physics, USM, Dato’ Dr Mohamed Ali Abdul Khader and all the staff at
Nuclear Medicine Department in Penang Hospital for their co-operation, encouragement and
support.
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Attix F. H. (1986), Introduction to Radiological Physics and Radiation Dosimetry, Canada, pp
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