Eye Lens Dose Reduction in Head CT Using Bismuth Shielding: Application in CT Facility in Cameroon

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AASCIT Journal of Health 2018; 5(1): 11-15 http://www.aascit.

Bangassi 1, 2, 3, *, Odette Ngano Samba 1, 3, Hubert Thierens 2, Klaus Bacher 2 1 Centre for Atomic Molecular Physics and Quantum Optics, Department of Physics, University of Douala, Douala, Cameroon

1 AASCIT Journal of Health 2018; 5(1): ISSN: (Print); ISSN: (Online) Eye Lens Dose Reduction in Head CT Using Bismuth Shielding: Application in CT Facility in Cameroon Thierry Narcisse Kouagou Bangassi 1, 2, 3, *, Odette Ngano Samba 1, 3, Hubert Thierens 2, Klaus Bacher 2 1 Centre for Atomic Molecular Physics and Quantum Optics, Department of Physics, University of Douala, Douala, Cameroon 2 Department of Basic Medical Sciences, University of Ghent, Ghent, Belgium 3 General Hospital of Yaounde, Yaounde, Cameroon Keywords Eye Lens, X-Rays, Bismuth Shielding, CT-Scan, Dose Reduction, Radiation-Induced Cataract Received: July 9, 2017 Accepted: January 2, 2018 Published: January 11, address thierbk@yahoo.fr (T. N. K. Bangassi), nosambacm@yahoo.fr (O. N. Samba), hubert.thierens@ugent.be (H. Thierens), klaus.bacher@ugent.be (K. Bacher) * Corresponding author Citation Thierry Narcisse Kouagou Bangassi, Odette Ngano Samba, Hubert Thierens, Klaus Bacher. Eye Lens Dose Reduction in Head CT Using Bismuth Shielding: Application in CT Facility in Cameroon. AASCIT Journal of Health. Vol. 5, No. 1, 2018, pp Abstract The purpose of this study is to compare the radiation dose received by the patients during head Computed tomography (CT) scan examination in a developing country like Cameroon (General Hospital of Yaounde (GHY)) with a developed country like Belgium (UZ Ghent). We also evaluate the ability of thin overlying bismuth shielding to reduce the x-ray dose to the eye lens during CT. With in-plane shielding, the x-ray beam is partially blocked to reduce the dose to the underlying tissue while allowing a sufficient amount of x-rays to pass in order to generate a diagnostic quality CT image. Bismuth garments are mainly used for breast, thyroid and eye-lens shielding in CT to protect the organs at risk from direct exposure. This work has shown that patients in Yaounde receive a higher lens dose than those of Ghent, related to the use of CT equipped with innovation dose reduction techniques. With the bismuth shielding we obtained a dose reduction of about 38% at the General Hospital of Yaounde. 1. Introduction Since their development, CT devices have constantly been improved technically. Some important improvements comprise the shift from a pencil beam to a fan-shaped x- ray beam, the increasing number of detectors on one array, the helical scanning technique and exposing different detectors simultaneously leading to the currently use of multi-slice CT scanners. All these advances have led to a serious decrease in scanning time making CT exams very user-friendly for both the patient and the radiologist. In view of this and the high diagnostic value, the number of CT scans has increased largely over the years. Today CT is the biggest medical contributor of collective dose to the population. Furthermore, medical facilities imaging are scarce in developing countries. Unlike developed countries, developing countries in general and African countries south of the Sahara in particular do not have access to sufficient number and powerful CT scanners reducing the dose level received by patients during CT scan. For example, the General

12 Thierry Narcisse Kouagou Bangassi et al.

system receiving all patients in the sub region of Central Africa with a population of about 40 million inhabitants.

These indisputable benefits for patients should however not detract our attention from the potential risks associated with the use of ionizing radiation.

2 12 Thierry Narcisse Kouagou Bangassi et al.: Eye Lens Dose Reduction in Head CT Using Bismuth Shielding: Application in CT Facility in Cameroon Hospital of Yaounde (GHY) radiology department at Yaounde in Cameroon has a Hitachi Eclos CT system receiving all patients in the sub region of Central Africa with a population of about 40 million inhabitants. Most of the medical and surgical decisions made in contemporary health care depend upon imaging. CT scan is an indispensable tool for the diagnosis of many diseases, and monitoring of treatment. These indisputable benefits for patients should however not detract our attention from the potential risks associated with the use of ionizing radiation. Recently, some non-cancer diseases were found to be possibly induced by radiation exposure. In some populations exposed to radiation, an increase is found in the frequency of non-cancer diseases like cataract, heart diseases, stroke. Especially radiation-induced cataract is a point of attention. It's a patient s deleterious effect on the transparency of the eye lens. In any head CT examination, the eye - even without being the object of interest - is irradiated and should therefore be especially protected. CT examinations involve larger radiation doses than the more common, conventional x-ray imaging procedures. Many authors have reported that a small but increased incidence of cancer occurs in patients who are exposed to levels of radiation equal to that of CT [2, 6]. During CT of the brain the eye receives approximately 50mGy, of radiation [13, 15]. The lens is particularly radiosensitive. CT exams are known as high-dose x-ray exposures, so the implementation of dose-reducing measures is important to ensure that risks related to CT x-ray exposure are low as possible while still providing the necessary diagnostic quality. The ICRP has reviewed recent epidemiological evidence and has issued a statement after its meeting on 21st April According to this statement, the threshold absorbed dose for the lens of the eye is considered to be 500mGy with respect to cataract. Different studies pointed to the nonexistence or a very low (< 100mGy) dose threshold for cataract. Because uncertainties exist regarding the threshold level dose required to avoid damage to the lens of the eye, it is prudent to minimize the dose to the eye. Therefore controlling radiation exposure to the eye is important, especially in patients with visual impairment, cataracts, young or sensitive eyes, and in patients who require multiple scans. One possibility for dose reduction is the use ofa bismuth shield. The radiation dose to specific organs depends on a number of factors, of which the most important are the use of tube potential (kv) and tube current (ma), filtration, the exposure time, the distance, the size of the patient and the specific design of the x-ray device [3]. Protocols should be tailored to the type of examination and the size of patient. Our goal in this study is first to compare dose received by patients during head computed tomography (CT) at the GHY with this received to the patients at UZ Ghent. Secondly, we investigate the possibility of reducing the x-ray dose to the eye lens during head diagnostic computed tomography (CT) without decreasing image quality in the organs of interest using bismuth shielding. 2. Materials and Method 2.1. Patients At the GHY, measurements of absorbed dose to the patient s eye were performed on 30 adult patients (15 with shield and 15 without shield) undergoing head CT examinations. Many of these patients have a cerebrovascular accident or brain tumor. The Responsible of GHYgave ethical approvements before conducting studies on patients. This study was carried out on 30 patients (12 males and 18 females) consisting of patients young adults (18-74 yrs) who visited or referred to Radiology Department of General Hospital of Yaounde. The patients undergone through cranial CT scan for various ailments. The Table 1 shows the data of patients and their medical conditions. The dose was measured using preannealed TLD Phantom An anthropomorphic Rando phantom (Phantom Laboratory, Salem, NY, USA) representing the skull and trunk of a 175cm tall male weighting 73,5kg was used for this study. The phantom is composed of synthetic materials representing the density and x-ray attenuation of soft tissues and includes a human skeleton. An image of Rando phantom is shown in figure 1. Figure 1. An image of Rando phantom with eye bismuth shield Bismuth Shielding The use of bismuth shielding to provide protection from xrays in CT is reportedfor a wide range of organs. Bismuth shielding has higher density and mass attenuation coefficients compared tolead shielding which shows that

3 AASCIT Journal of Health 2018; 5(1): bismuth may be used as substitute for lead in order toimprove radiation-shielding properties. Shieldsfor in-plane shielding are available under the name AT tenu Rad radio protective garments (F & L Medical Products Co., Vandergrift, PA, USA). They consist of thin, flexible latexsheets containing bismuth. In addition, the bismuth radioprotective is elastic andmodelable to the body's surface. It is implemented in the UZ Gent CT department and in GHY. The reduction in absorbed dose achieved with radioprotective bismuth latex garments was determined by measurements CT Measurements with Fixed Tube Current In GHY, measurements with fixed tube current were performedon a standard clinical CT system (Hitachi Eclos 16 bars) in spiral mode. The absorbeddose was measured on the patients undergoing head CT examinations. In the otherhand, in the UZ Ghent, the same measurements were performed on a Siemens SOMATOM Definition Flash but on the Rando phantom. The parameters were chosen as standard values of head CT examinations used in thesetwo hospitals. In GHYthe helical craniofacial (120kVp; 175mAs; 2.5mm thick slices) and at UZ Gent (120kVp; 165mAs; 3mm thick slices) protocols wereperformed Dosimetry A single batch of thermoluminescent dosimeters (TLDs) was used for thisstudy to measure radiation dose. For both the phantom and clinical studies, TLDswere measured manually and annealed to remove anyresidual dose allowing their reuse. The TLD detector does not significantly attenuate x-ray [10], it is ideal for in-vivo radiation dose measurement. The Pre-annealing was carried out using the external oven at the TLD laboratory of Department of Basic Medical Sciences, University of Ghent. Annealing was carried out at a temperature of 400 C for 1 hour and allowed to cool for 24 hours before used. After the initial annealing the chips were calibrated and coded for easy identification during exposure and reading. After preparing the TLD chips of dimension 3.2 mm square and thickness 0.15 mm they were placed on the eyebrow of the patient where the team enters into the patient. Due to the size and composition of the TLD chip, it does not affect the radiograph produced. The consent of the patients were obtained from the family members or guardians. Exposed TLD (chips) were returned to the Department of Basic Medical Sciences, University of Ghent for reading and recording using TLD reader model Results Table 1. The data of Patients and their medical conditions. Various quantities are used to describe the radiation dose delivered by CT scanning, the most relevant being absorbed dose to organs and tissues, effective dose, and volume CT dose index (or CTDIVol). The quantities used in this study to describe the radiation dose delivered byct are the absorbed dose and the volume CT dose index. Table 1 shows patient identification number, age, sex and the medical condition of the patient during the CT scan. The table shows that the age of the patient range between 18 and 74 years. Other patients include 13 young adult and 17 adults. Patients Age (Yr) Sex Medical conditions 1 18 Female Road Traffic Accident 2 19 Male Intra Cellebrallession probably neoplastic 3 23 Female Paranasal Sinuses (Recurrent Nasal discharge) 4 23 Male Right lower Jaw swelling of 6yrs 5 23 Female Loss of Consciousness 20hrs from Road Traffic Accident, Severe open Head Injury 6 23 Female Road Traffic Accident 7 24 Female Right Hemi Facial hypertrophy since Birth being prepared for reconstruction 8 24 Male Paranasal Sinuses (Nasal Tumor) 9 29 Female Road Traffic Accident Male Road Traffic Accident Female Multiple Cranial Nerve, Intracranial Space occupying lesion Male Moderate Head injury following Road Traffic Accident Female Headache, Sleeplessness associated with blurring of vision Male Moderate Head injury following Road Traffic Accident Female Paranasal Sinuses Female Road Traffic Accident Male Road Traffic Accident Female Headache Male Hemispheric Stroke probably hemorrlegic Male Hemispheric Stroke probably hemorrlegic Female Sudden loss of Consciousness while working in the Farm, Haemorrhage CVD Male Hemispheric CVD Female Headache, Neck Pain and loss of Consciousness Female Brain-stem stroke with cross hemiparesis Male Hemispheric Stroke probably hemorrlegic Male Brain-stem stroke with cross hemiparesis Female Brain-stem stroke with cross hemiparesis

Falls in a known hypertensive with sub-optimal B.

4 14 Thierry Narcisse Kouagou Bangassi et al.: Eye Lens Dose Reduction in Head CT Using Bismuth Shielding: Application in CT Facility in Cameroon Patients Age (Yr) Sex Medical conditions Female Left Hemispheric Female Recurrent Falls in a known hypertensive with sub-optimal B. P control Female Convulsion, loss of Consciousness Table 2 shows the volume CT dose index and the average of absorbed dose induced by the head CT examination. For each measurement four scans were done, each exposure was repeated four times. The errorbars represent the standard deviation of the absorbed dose values. Table 2. The received dose and the volume CT dose index during head CT examinations. Scan description UZ Gent (Phantom Rando) GH Yaounde (Patients) CTDI Vol (mgy) Dose to the eye (mgy) CTDI Vol (mgy) Dose to the eye (mgy) Fixed tube current with no eye shielding ± ±1.08 Fixed tube current with bismuth eye shielding ± ±1.27 Figure 2 shows the comparison of the absorbed dose to the eye received by the patientsat the GHY and at UZ Gent with both techniques namely fixed tube current and fixed tube current with bismuth shielding. Figure 2. Comparison with absorbed dose received during head CT examinations at UZ Ghent andyaounde General Hospital. 4. Discussion The lack of data regarding thorough artifact evaluation following eye-lens protection, and the risk for cataract formation reported at occupational doses of the level of 100mGy were the arguments to start this study. In addition, the use of the CT technology is increasing steadily. Therefore, the use and the risks have not been well characterized [23]. Our focus was restricted to the eye lens because cataracts caused by cumulative dose to the eye require specified attention, even with low-dose head CT, especially in patients with cerebrovascular accident or brain tumor who are likely to undergo multiple CT examinations. In addition, many studies [O'CLOC, Chodick] have suggested that any threshold for cataract is several times lower than that upon which current permissible exposure limits are based. The results obtained in this work indicate that doses received by patients at the GHY are relatively high compared to those of UZ Gent. This is related to the dose reduction methods used by the CT scanner at UZ Gent such as tube currentmodulation Using the sameprotocol (Fixed tube current) the dose to the eyereceived at UZ Gent is 22% lowercompared to dose received at the GHY. With the bismuth shield, the dose received at UZ Gent is still lower than that received at the GHY. But the difference is reduced to 13.6%. On the other hand at the GHY the bismuth shield reduces thepatient dose up to 38%. Bismuth shields are easy to use and have been shown to reduce dose to anteriororgans in CT scanning. However, there are several disadvantages associated with theuse of bismuth shields, especially when used with automatic exposure control or tubecurrent modulation. It increasesthe dose tosurrounding tissueunprotected. Indeveloping countries like Cameroon, CT scanning systems with automatic exposure control or tube current modulation are not available. Bismuth shieldingis an interesting dose reduction technique for scanners without automatic exposure controlsuch as Hitachi Eclos at the GHY. This is a pioneering work in Cameroon. Our results are original concerning radiationprotection of superficial radiosensitive organs such as lens during CT examinations. Further studiesshouldbe conducted to determine local reference dose of CT examinations in GHY. We note that the dose received by patients at the GHY is relatively high if bismuth shield is not used. Using bismuth shield in each head CT examination may reduce the risk for radiation induced cataract. The results obtained in UZ Gent are in perfect agreement with literaturedata [20, 21, 22]. 5. Conclusion In developing countries and Cameroon particularly, currently the only dose reduction possibility to the patients' eyes duringthe head CT examination is the use of bismuth shielding. Bismuth shieldingallows reducing eye lens dose received by the patients by up to 38% and does not affect image quality. It is important to control radiation exposure to the eye becauseaccumulated data indicate that cataracts are seen at doses lower than the ICRP dose threshold.

5 AASCIT Journal of Health 2018; 5(1): Acknowledgements UZ Gent Hospital and General Hospital of Yaoundé are acknowledged for allowing us to make measurements in their areas. One of us (TNKB) has received a grant from the Belgian DGCD through the project PIC-MAC-1245 and partial support from the ICTP through the OEA-AC-71. The authors would like to thank these institutions. References [1] Ainsbury EA, Bouffler SD, Dorr W, Graw J, Murhead CR, Edwards AA, Cooper J. Radiation Cataractogenesis: A Review of Recent Studies. Radiation research 2009, 172 (1): 1-9. [2] Brenner DJ, Elliston CD, and Berdon WE. Estimated risks of radiation- induced fatal cancer from pediatric CT AJR 2001, 176: [3] Brenner DJ, Hall EJ. Computed tomography an increasing source of radiation exposure, New Engl J Med, 2007; 357: [4] Frame PW. A history of radiation detection instrumentation Health Phys, 2004, 87 (2): [5] Geleijns J, SalvadoArtells M, Veldkamp WJH, Lopez Tortosa M, Calzado Cantera A. Quantitative assessment of selective in-plane shielding of tissues in computed tomography through evaluation of absorbed dose and image quality. Eur Radiol 2006; 16 (10): [6] Hopper K. D, King SH, Lobell ME, TenHave TR, and Weaver J. S. The breast: in-plane x-ray protection during diagnostic thoracic CT - shielding with bismuth radioprotective garments. Radiology 1997, 205: [7] ICRP, 1984, Nonstochastic effects of irradiation. ICRP Publication 41. Ann. ICRP 14 (3). [8] ICRP, 1990, ICRP Publication 58. Ann. ICRP 20 (4). [9] ICRP, 1991a Recommandations of the International Commission on Radiological Protection. ICRP Publication 60. Ann. ICRP 21 (1-3). [10] ICRP, 2000, ICRP Publication 85. Ann. ICRP 30 (2). [11] ICRP, 2007, Recommandations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4). [12] Kalra MK, Maher MM, Toth TL, Schmidt B, Westerman BL, Morgan HT, and Saini S. Techniques and applications of automatic tube current modulation for CT. Radiology 2004; 233: [13] Moström U, Ytterbergh C, and Bergström K. Eye lens dose in cranial computed tomography with reference to the technical development of ct scanners Acta Radiol Diagn (Stockh) 1986, 27: [14] S. Jr Mukundan, PI. Wang, DP. Frush, et al. MOSFET dosimetry for radiation dose assessment of bismuth shielding of the eye in children. AJR Am J Roentgenol 2007; 188 (6): [15] Nishizawa K, Maruyama T, Takayama M, Okada M., Hachiya J, and Furuya. Y. Determinations of organ doses and effective dose equivalents from computed tomographic examination. BrJ Radiol 1991, 64: [16] Raissaki M, Perisinakis K, Damilakis J, Gourtsoyiannis N. Reduction of eye lens radiation dose by orbital bismuth shielding in pediatric patients undergoing CT of the head: A Monte Carlo study. Pediatr Radiol 2010; 40 (1): [17] Yu C and Luxton G. TLD dose measurement: A simplified accurate technique for the dose range from 0.5 cgy to 1000 cgy Med. Phys; 1999, 26 (6):

Paediatric Dose Reduction and Image Quality

Paediatric Dose Reduction and Image Quality Alan Whiteside The majority of this work was undertaken as part of MSc Thesis of Helen Dixon. Introduction Paediatric CT protocols result in a higher effective