Dosimetric StuInvestigation of

E. Lamanna, Member, IEEE, A

Abstract– This work is a status report of thfinalized to the estimation of the dose distributiof a patient undergoing a high resolutionTomography) for the diagnosis of pathologies reear. Geant4 (GEometry ANd Tracking) has beethe head and propagate the particles in the huear has a very complex structure, in particularincludes many very small structures, which arecompact region (a few square centimeters), ahave dimensions below the millimeter. The seand vestibule are part of the Vestibular Compthe feeling of balance, rotation, gravity and liOtoliths, receptors in the vestibule, are small gcarbonate by the maximum diameter of 200 mdifferent approaches have been checked for thethe human head. The first phantom has been covector graphics approach through the geometrby Geant4. The second phantom has been conanatomical division into voxels. In particulavolumetric model definition very close to the dlayers. In this approach the simulation may using DICOM CT images. In this work we will pobtained using the Geant4 toolkit and the suggproceed .

I. INTRODUCTION

THIS document describes the simulatiRITOR, a project within the INFN (Italian Nfor Nuclear Research) to make a new tomogthe diagnosis of pathologies of the inner earhas a very complex structure, is anatomicaouter ear, middle and inner. The latter consisthe semicircular canals and the vestibule. Se

Manuscript received October 15, 2010. This work wItalian INFN (Istituto Nazionale di Fisica Nucleare).

Ernesto Lamanna is with the Medicine Faculty University, 88100 Catanzaro Italy and Gruppo Collegato(corresponding author phone: +3909613694151; fax: +lamanna@unicz.it ).

Antonino Secondo Fiorillo is with the Medicine FaculUniversity, 88100 Catanzaro Italy and Gruppo Collegato(e-mail: nino@unicz.it ).

Alessandro Gallo is with the Medicine Faculty University, 88100 Catanzaro Italy and Gruppo Collegato(e-mail: ales.gallo@gmail.com).

Antonio Narciso was with the Gruppo Collegato INmail: narciso.antonio@unicz.it)

Luca Belmonte was with the Gruppo Collegato INFmail: lucabelmonte@gmail.com)

udy in the Human Head f the Inner Ear Using the

Toolkit A. S. Fiorillo, Member, IEEE , A. Gallo, A. Narciso,

he job in progress ion inside the head n CT (Computed elated to the inner

en used to simulate uman tissues. The r the inner section e located in a very nd many of them emicircular canals plex that provides inear acceleration. groups of calcium micrometers. Two e representation of onstructed using a ric solids provided nstructed using an ar, we achieved a division into axial be customized by present the results

gestions we find to

ion approach in National Institute graphic setup for r. The human ear ally divided into

sts of the cochlea, emicircular canals

was supported by the

of Magna Graecia o INFN Cosenza Italy +390961369 ; e-mail:

lty of Magna Graecia o INFN Cosenza Italy

of Magna Graecia o INFN Cosenza Italy

NFN Cosenza Italy (e-

FN Cosenza Italy (e-

and vestibule are part of the vestibuthe feeling of balance, rotation, gravThe vestibule contains two membrathe Utricle. Receptors in the saotoliths, generate the specific sensaacceleration. An otolith is a tincarbonate incorporated in a protein in the endolymph of the inner ehexagonal shape and very small dmicrometers.

Fig. 1: Elements of th

The inner ear is made up of manyare in a very compact region (a fewmany of them have dimensions bediseases are linked to malformatioelements of the inner ear. Strudimension (below mm) can cause seloss. The characteristics of a tomogdesigned for the ear must provide anhigh frame rate and must guarantee simulations in this work are inteestimate of the dose delivered toexposition. The simulations of the has been implemented to provide photon detection that will be the newsimulates the human head using Gdifferent representations. . In the fgraphics approach while in the secovoxels.

for CT e Geant4

, L. Belmonte

ular complex that provides vity and linear acceleration. anous sacs the Saccule and ccule and utricle, called

ations of gravity and linear ny formation of calcium matrix, which is contained ar. They have a roughly dimensions from 3 to 30

e inner ear

y very small structures that w square centimeters), and elow the millimeter. Many ons, broken, expansion of uctures from very small erious illnesses and hearing graphic device, specifically n high spatial resolution, an a low delivered dose). The

ended to give a realistic o the patient in the CT entire acquisition process guidance on the chain of

w tomographic system. We Geant4 [1] tools in two first we have used vector ond we divide the head in

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2010 IEEE Nuclear Science Symposium Conference Record N64-2

II. METHODS The Monte Carlo simulations helps in two

selection of the hardware configuration of approach; b) the estimation of the dose patient. Two approaches are considered for tup. The first, the fan-beam configuration, is The beam is collimated only in one directionis made along a line.

Fig. 2: fan-beam set-up - System with collimation

photons detection along a line .

The second approach, the cone-beam, is showbeam is collimated in two directions and contained in the optical cone. The readout vertical plane using a XY detector.

Fig. 3: Cone-beam set-up – The photons are producecone and are read using a planar detect

The selection of one of the two approache

of the set-up parameters (the beam geometrysource-head and head-detector distances, ththe X ray detector) must take into accoresolution (50 micrometers) and an acceptabto the patient.

The Monte Carlo simulation is useful in thcomputation time required for the generationof the photons is reasonable, considering thesamples required to perform a tomography athe computers available today.

The representation of the human head is ofin the evaluation of computation time. For thifocused our preliminary efforts to study thethe digital human head to be taken.

o sections: a) the the tomographic delivered to the the hardware set-shown in Fig. 2.

n and the readout

n in one direction and

wn in Fig. 3. The the photons are is made along a

ed inside one optical tor.

es and the tuning y and energy, the he exposure time, ount the needed

ble dose delivered

hese studies if the n and the tracking e large number of and the power of

f great importance is reason we have e configuration of

III. MODE

During our study different approarepresent in detail the anatomical reinside the head. All of them havGeant4. In particular, starting from the code: the MIRD (Medical application [2,3] and the DICOM ap

The first model considers the volrepresent the head. In Fig.4 the headsub-volumes. In particular how the implemented is shown. The morphothe assigned tissue, for each volume

Fig. 4: Human head implemente

The simulation with this represe

satisfactory. This phantom is usefudelivered dose. It allows fast simustatistics. However several probleanatomical variations of the order of

TABLE I. MATERIALS USED IN T

Name Color External head Pink Skull Orange Brain Yellow Eyes Green External ear Green Middle ear ossicles Blue Inner ear coclea Violet

The second model starts from

defining the custom volumes (boxeThe phantom is reconstructed usreversing the procedure used in the p

EL

aches have been used to to egion of interest (inner ear) ve been developed using two examples included in

Internal Radiation Dose) pplication [4,5]. lumes defined in Geant4 to d is shown with the defined

inner ear region has been ological characteristics and e, are specified in Tab. I.

d using Geant4 volumes

entation has not been fully ul for global studies of the ulations and produces high ems arise by introducing f 50-200 micrometers.

THE VECTOR MODEL

Tissue Soft tissue Skeleton Soft tissue Soft tissue Soft tissue Skeleton Water

DICOM axial images for es) that make up the head. sing each axial slice by production of images.

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2010 IEEE Nuclear Science Symposium Conference Record N64-2

The CT images, in DICOM format, has been used considering the following steps:

• The Hounsfield CT numbers are converted to density; • The density is compared to the ICRU report 46 density

range to extract the tissue type and the chemical composition;

• The voxel are defined selecting a size connected to the image resolution and a preselected compression factor.

One slices reconstructed in Gean4 is shown in Fig. 6. The

colors represents the different tissues.

Fig. 6: Reconstructed volume in Geant4

The main advantages of this approach is the customization

performed by using directly the DICOM patients imagesfiles. In Tab. II the attribution of the material related to the

density values are shown. The conversion has been selected using the values published in [6] and [7].

TABLE II. MATERIALS AND DENSITY IN VOXEL MODEL

Density range (mg/cm3) Material [0. - 0.919] Air [0.919 – 0.979] Adipose Tissue [0.979 - 1.034] Water [1.034 - 1.044] Brain [1.044 - 1.054] Muscle [1.054 - 1.064] Blood [1.064 - 1.074] Eye Lens [1.074 - 1.094] Skin [1.094 - 1.140] Cartilage [1.140 - 1.610] Skull [1.610 – 2.000] Tooth

The density attributed to each voxel is the average density

of the starting DICOM pixels collected for the definition of the volume. The number of pixels collected in each voxel is preselected through the choice of a compression factor.

IV. TEST SIMULATIONS The voxel approach is the better solution from the point of

view of the details which can be introduced. However the computation time may increase in such way that the solution

cannot be accepted. For this reason we have tested different sizes of the voxel by using two different computers :

- Intel Pentium 4 - 3.00 GHz Single CPU - 1 GB RAM; - Workstation - Xeon E5530 2 CPU - 8 GB RAM. The setup has been selected simulating photons in a cone-

beam configuration, with an opening angle of 28 degree and a fixed energy of 100 keV. The source has been positioned at 200 mm from the head. More sample of photons have been produced to extrapolate the results up to the number of photons required to reduce the statistical uncertainties to 1% in the region of the otolith. Around 109 events in one area of 1 cm2 which covers the inner ear section are needed.

In Fig. 7 the computation time as a function of the number of tracked photons is shown using two pixel size, 0.3 and 1.0 mm. On the top using the Pentium 4 cpu and on the bottom using the Xeon E5530 cpu.

Fig. 7: Simulation times depending on the number of events. Times related

to CPU Pentium 4 (up). Times related to CPU Xeon E5530 (down).

The change in workstation with a different CPU provides a factor 2 of gain but an even bigger factor (10) is obtained using a different granularity.

V. CONCLUSIONS The Monte Carlo simulation plays a key role in the design

and construction of a biomedical device, mainly for two

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reasons: to reduce the time and cost of construction and to make a preliminary study of the device to prevent large dose delivered to the patient during the exposure for an high resolution CT.

The main objective of this study was to design and make a digital head with the realistic reproduction of the inner ear.

The characteristics of the phantoms available in Geant4 and elsewhere indicate that such approach cannot be used to produce large statistical sample of data. The promising approach is the combination of a high-granularity representation of the region investigated and a rough description of the body. In our case the head can be defined using the vector approach or the structure in large voxels, while the inner ear must be defined with voxel of size 50 microns.

REFERENCES [1] GEANT4 Physics Reference Manual, (Version: geant4 9.3 - Dec, 2009)

[Online]. Available: http://geant4.cern.ch/support/index.shtml. [2] Implementation of Japanese Male and Female Tomographic Phantoms

to Multi-particle Monte Carlo Code for Ionizing Radiation Dosimetry – Choonsik LEE, Tomoaki NAGAOKA, Jai-Ki LEE – May 19, 2006.

[3] Object oriented design of anthropomorphic phantoms and Geant4-based implementations – Rosana de Souza e Silva, Marcia Begalli – Institute of Physics State University of Rio de Janeiro, Brazil.

[4] The DICOM application has been developed by: Louis

Archambault,*Luc Beaulieu, +Vincent Hubert-Tremblay. *Centre Hospitalier Universitaire de Quebec (CHUQ), + Université Laval, Québec (QC) Canada.

[5] DICOM standards commitee, (2006) DICOM - part 3: Information

object definitions. 1300N, 17th Street, Rosslyn, Virginia 22209, USA; National Electrical Manufactures Association.

[6] Schneider W, Bortfeld T, Schlegel W. Correlation between CT numbers

and tissue parameters needed for Monte Carlo simulations of clinical dose distributions. Phys Med Biol. 2000;45(2):459-78.

[7] The calibration of CT Hounsfield units for radiotherapy treatment

plannig – Uwe Schneider, Eros Pedroni, Antony Lomax – Phys. Med. Biol. 41 (1996) 111-124.

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