2007 Inyournational Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-02-1

PROGRESSES IN THE DEVELOPMENT OF AN ANTHROPOMORPHIC AND ANTHROPOMETRIC EYE PHANTOM FOR DOSIMETRICAL TLD ENDS

Luciana Batista Nogueira and Tarcísio Passos Ribeiro Campos

Programa de Pós-Graduação em Ciências e Técnicas Nucleares, Departamento de Engenharia Nuclear Universidade Federal de Minas Gerais Av. Antônio Carlos, 6627 31270901, Belo Horizonte, MG [email protected]

ABSTRACT

This work addresses the progress in the preparation of a pair of phantom of anthropomorphic and anthropometric eye of radiological equivalence that will help futures studies in radiodosimetry, applied to radiology and radiotherapy, mainly in treatment techniques in head's area and neck, in which the eyes are more susceptible to irradiation. The eye phantom that was projected represents one of an adolescent between 14 and 16 years. The phantom is constituted of equivalent tissues (TE) and can simulate the components of the human eye in the exposition to the radiation. Through the accomplishment of an radiodiagnostic image for computer tomography with axial section, in which the phantom of eye’s object was setting up in another phantom, an anthropomorphic and anthropometric head and neck phantom developed in the research group NRI, it was verified the equivalence of the tissues, comparative to an axial section of a computer tomography of a real eyes. The improvement resumes in the production of the equivalent tissue, due to dehydration factors in function of the time that committed the equivalence anthropometric and anthropomorphic of the same. The present work approaches these improvements as well as the way of positioning of a group of dosimetrics type TLD's, in the vitreous, for dosimetry ends.

1. INTRODUCTION

The eyes present great radiosensibility in your lenses (crystalline), this means, that depending on the dose absorbed by them, these can suffer several permanent damages. If the eyes receive a acute dose of x-ray at 2Gy, detectable opacity will occur in the crystalline lens, a acute dose at 5Gy will already provoke a reduction in the vision [1]. In general, to protect the eyes from the irradiation, mainly when it takes place in a x-ray of the head, for instance, the area is irradiated in a subsequent-previous position so that the eyes receive the smallest possible dose and if necessary a lead protector is used, such that it won't harm the area to be examined [2].

It is defined as a phantom a physical simulator used to reproduce the transport of nuclear particles in the human body [3]. The major macroscopic anatomical structures macroscopic of the human eye are: sclera, choroids, conjunctive, cornea, retina, crystalline, iris, optical nerve [4]. The relevance of building a pair of anthropomorphical and anthropometrical phantoms of the eye with radiological equivalence is addressed as follow: i) inexistence of an product with this end with equivalent tissue (anthropomorphic) and shape (anthropometric); ii) the needs for dosimetric experimentation; iii) and a possible contribution through the radiodosimetric studies .

2. MATERIALS AND METHODS

2.1. Development of eye phantom

In the production of the eye phantom the following equivalent tissue were prepared (TE): membranes (sclera, choroids and retina), crystalline, vitreous humor, optical nerve, and ligaments suspensors. The diameter used for the external eyes ball was of 23mm, while for the membranes a thickness on the range of 2 to 3mm was assumed.

2.1.1. Producing the pieces and assembly of phantom

The TE of the membranes (sclera, choroids and retina) which represents the external part of the eye phantom, is shown in Fig.1(a). Figure 1(b) shows the cristaline. Figure 1(c) shows the optical nervus equivalent material which was placed in the bottom of the eyes ball. The crystalline lens made of TE was glued close to the ligaments suspensors made of an artifitial material with equivalent proproieties , shown in Fig.1(b). On the ligaments holes were made, for occurring the fluid transport of the vitreous humor made of TE. Thus vitrious humor filled all the cave made of the membranes, being this vitreous humor the substance that constitutes 2/3 of the real human eye.

The parts of the membranes were united using the TE vitreous itself in its liquid phase as connected material. Soon after the external part of the eye phantom was made, a varnish based on polyurethane, avoiding the dehydration and the contaction in contact with the air. In order to represent the real human eye, this was covered again by a layer of synthetic enamel (resin alquídica), color ice. And to represent the pupil, its respectve area was covered with a synthetic enamel (resin alquídica, Eucatex) opaque black color. The optical nerve was represented by an rubber made of latex of thickness of 6,0mm, in according to Fig.1(c). The oblique superior muscles, oblique inferior, superior rectum, rectum lateral and straight inferior, straight medial was represented with the same material TE of the ligaments suspensors, a sheet of pink rubber, as shown in Fig.1(d). That rubber was measured in agreement with the dimensions of each muscle and fixed in agreement with the real position in the human eye, identifying the right eye and the left eye.

The eye phantom was introduced in a phantom of head and neck of the group NRI/CNPq (Nucleus of Radiations Ionizants ), for the accomplishment of the imaging, represented in Fig.1E [5].

(a) (b) (c)

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(d) (e)

Figure 1. (a) membranes with T.E-equivalent, separted in two parts; (b) frontal view of the materials T.E of the crystalline lens, close to the ligament suspensor; (c) lateral view of T.E of the optical nerve; (d) lateral view of the phantom of right eye, before be covered by skin, demonstrating the positioning of the ocular muscles; (and) ocular phantoms placed in the bone structure equivalent to the uvea.

2.1.2 Imaging with computerized tomography (TC)

The eye phantom was taken to the Clinica Med Image, Sete Lagoas, MG, for the accomplishment of a computerized tomography of the head and neck. The tomography made possible, through axial cuts accomplished in the area of the eyes, to verify the radiological equivalence to the real eyes, in comparison with an axial cut of tomography computerized of a real human eyes, according to Fig.2(a) and 2(b). Verifying that the sections are not in the same position in the images and that the eye phantom belongs to an adolescent, while the one of the human real eye belongs to an adult.

(a) (b)

Figure 2. (a) a CT section with axial cut of the eye phantom; (b) a CT section with an axial cut of the real human eye.

2.2 Improvements in Eye Phantom

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2.2.1 Incorporation of TE vitreous humor of collagen and water deionizing

Since the material TE-vitreous humor used in the production of the eye phantom presented high contraction with the time, which causes the decrease of the size of the same, another material was proposed as TE in order to reduce the contraction. The proposed material was constituted of water deionizing and human collagen. The water deionizing was heated up and added of the collagen to your complete dilution, that happened after the ebullition of the mixture. The TE was added to the membranes of the eye phantom, taking quicky cooling for temperature ajustment, Fig.3(a) and 3(b), it presents sections of the final products,

(a) (b)

Figure 3. (a) frontal view of T.E of the membranes, separted in two parts; (b) view of the TE vitreous humor, after filling out the membranes.

2.2.2 Placing the TLD dosimeters

In the internal part of the eye phantom, a dosimetric group of TLDs was placed. Those dosimeters are,of the type MCP-N (LIF: Mg, Ass, P). These same ones were encapsulated with a transparent plastic for not entering in direct contact with the TE vitreous humor. Two dosimetrics are shown in Fig.4(a). Their installation was done in the center of the phantom, as shown in Fig.4(b). Later, the eyes phantom was closed, as shown in Fig.4(c).

The separated parts were glue using the same material TE of the vitreous humor. The positioning of these two dosimetrics is shown in Fig.4(a). Two holes were made inside the eye phantom with a simple cut wit a bistoury number 15. TLD's were manipulated with a tweezers, enclosed on plastic bags. The eye phantom was closed, finning through the unification of the two separated parts, with TE of the membranes (sclera, choroids and retina) around the boards. The final product is shown in Fig.4(c).

(a) (b) (c)

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Figure 4. (a) dosimeters type TLs, encapsulated with protecting bag plastic; (b) eye phantom with dosimeters TL positioned in its interior; (c) view of the eye phantom, after the closing of the same.

2.2.3 Unpermeabilization

The unpermeabilization of the eye phantom was done first by a varnish base on polyuretane., However, after time it was degraded, presenting cracks and flaws, causing the dehydration of the phantom, suffering contraction and reduction of the volume of the same. That decrease committed the esthetic of the phantom, causing the descamation and drying of the color ink of the eye phantom. Then, it was proposed the use of a resin ortofitálica (ortocenterpol 603 S/UV of Fiber Center Ind Ltda), translucent, to make the phantom water-proof . The process was performed by a painting in cooling temperature to retard the quickly hardness. The formation of a fine layer of crystalline resin, as shown in Fig.5(a), was observed, interrupting the dehydration process. This was verified after comparing the dimension of it at the time of fixing the resine and a ,monday later . A same phantom, as control, was done without any protection, which means non material for unpermeabilization , and it was observed that the piece with non-waterproof retracted in the time, reducing its volume, in according to Fig.5(b), while the waterproof piece maintained the original volume in the time. The observed time was of 4 weeks.

(a) (b)

Figure 5. (a) Phantom with protetive resin ortophytalic, with formation of a fine crystalline layer; (b) Distinct dimension in comparison among phantom without and with layer of resin ortophyitalic.

3. RESULTS

The progresses of the construction of a pair of eyes phantom with anthropomorphism and anthropometrism and radiological equivalence, for TL dosimetry ends, was finalized. The autrhos observed that the substitution of TE vitreous humor for collagen and water deionizing was positive for decreasing the contract phenomenon (dehydration) of the eye phantom. The placement of a set of encapsulated TL dosimeter inside the phantom was performed. The TLDs were put in a plastic bag,, avoiding humidity and contamination. Finally, the unpermeabilization with orthophytalic resin of the eye phantom was quite satisfactory compared with the non waterproof piece. However it was observed that the orthophytalic resin, in a liquid phase caused and small disproportion of a side of the eyes, accumulating on the base of the phantom, altering your spherical geometry slightly.

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4. CONCLUSION

The progresses accomplished in the development of the eye phantom were significant, mainly in the aspect of introducing internal dosimeter inside the eye ball. The phantom is ready to be tested in radiology protocols and ocular radiotherapy.

The synthesis of TE combatable to the structures of the membranes, vidrio, crystalline has been satisfactorily performed; however the optic nervous and muscle must be improved. the real human eye. The production of a eyes phantom is justified due tomaterial the lack of an equivalent piece for dosimetric experimentation in the radiological and radiotherapy fields..

ACKNOWLEDGMENTS

The authors are thankful the Service of Clinic Med Image, Sete Lagoas, MG, due to the kindness in performed the the computer tomography images of the phantom. We are thankful to CNPq for concession of a scholarship.

REFERENCES

1. Entrances 453, Guidelines of Protetion Radiological In Radiodiagnostic Medical and Odontologic , System of Sanitary Surveillance, Brazil, (1998). 2. E.J. Hall, Radiobiology goes the Radiologist, 4ºed., (1993). 3. ICRU 44 - Commission International on Radiation Units, tissue substitutes in radiaton dosimetry and measurement, betherda , MD, (1989). 4. L.J. Didio, Agreement of Anatomy Applied Systemic, 2nd ed., São Paulo,Brasil (2002). 5. L. Thompson; Development of a Phantom Anthropomorphic and Anthropometric of Head and Infant-juvenile Neck and of a phantom computational for study radiodosimetric in larynx cancer and pharynx . Dissertation of Master's degree. Department of Nuclear Engineering, (2004).

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