Eye Lens Dose Reduction in Head CT Using Bismuth Shielding:...

AASCIT Journal of Health

2018; 5(1): 11-15

http://www.aascit.org/journal/health

ISSN: 2381-1277 (Print); ISSN: 2381-1285 (Online)

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, 2018

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

Thierry Narcisse Kouagou Bangassi1, 2, 3, *

, Odette Ngano Samba1, 3

,

Hubert Thierens2, Klaus Bacher

2

1Centre for Atomic Molecular Physics and Quantum Optics, Department of Physics, University of

Douala, Douala, Cameroon 2Department of Basic Medical Sciences, University of Ghent, Ghent, Belgium 3General Hospital of Yaounde, Yaounde, Cameroon

Email address [email protected] (T. N. K. Bangassi), [email protected] (O. N. Samba),

[email protected] (H. Thierens), [email protected] (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. 11-15.

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.: 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

2011. 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 non-

existence 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.

2.2. 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.

2.3. Bismuth Shielding

The use of bismuth shielding to provide protection from x-

rays in CT is reportedfor a wide range of organs. Bismuth

shielding has higher density and mass attenuation

coefficients compared tolead shielding which shows that

AASCIT Journal of Health 2018; 5(1): 11-15 13

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.

2.4. 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.

2.5. 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 3500.

3. Results

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.

Table 1. The data of Patients and their medical conditions.

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


7 24 Female Right Hemi Facial hypertrophy since Birth being prepared for reconstruction

8 24 Male Paranasal Sinuses (Nasal Tumor)


10 36 Male Road Traffic Accident

11 40 Female Multiple Cranial Nerve, Intracranial Space occupying lesion

12 40 Male Moderate Head injury following Road Traffic Accident

13 41 Female Headache, Sleeplessness associated with blurring of vision

14 49 Male Moderate Head injury following Road Traffic Accident

15 51 Female Paranasal Sinuses


17 52 Male Road Traffic Accident

18 52 Female Headache

19 52 Male Hemispheric Stroke probably hemorrlegic


21 58 Female Sudden loss of Consciousness while working in the Farm, Haemorrhage CVD

22 61 Male Hemispheric CVD

23 68 Female Headache, Neck Pain and loss of Consciousness

24 70 Female Brain-stem stroke with cross hemiparesis


26 70 Male Brain-stem stroke with cross hemiparesis

27 70 Female Brain-stem stroke with cross hemiparesis

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

28 71 Female Left Hemispheric

29 73 Female Recurrent Falls in a known hypertensive with sub-optimal B. P control

30 74 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)

CTDIVol (mGy) Dose to the eye (mGy) CTDIVol (mGy) Dose to the eye (mGy)

Fixed tube current with no eye shielding 38.2 26.11±1.04 37.1 34.03±1.08

Fixed tube current with bismuth eye shielding 38.2 18.30±1.46 37.1 21.18±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.

AASCIT Journal of Health 2018; 5(1): 11-15 15

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.

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