assessment of the integrity of structural shielding of

Radiation Protection Dosimetry Advance Access published February 17, 2013 Radiation Protection Dosimetry (2013), pp. 1–9

doi:10.1093/rpd/nct021

ASSESSMENT OF THE INTEGRITY OF STRUCTURAL SHIELDING OF FOUR COMPUTED TOMOGRAPHY FACILITIES IN THE GREATER ACCRA REGION OF GHANA A. Nkansah1,*, C. Schandorf2, M. Boadu1 and J. J. Fletcher2 1 Radiation Protection Institute, PO Box LG 80, Legon, Accra, Ghana 2 Department of Nuclear Safety and Security, Graduate School of Nuclear and Allied Sciences, University of Ghana, PO Box AE 1, Atomic, Accra, Ghana *Corresponding author: [email protected] Received July 20 2012, revised December 11 2012, accepted January 23 2013

INTRODUCTION The structural shielding thicknesses of the walls of four computed tomography (CT) facilities in Ghana were re-evaluated to verify the shielding integrity using the new shielding design methods recommended by the National Council on Radiological Protection and Measurements (NCRP) and to verify whether the repair or reinstallation of electrical outlets, plumbing or air conditioning ducts and so on has impaired the shielding integrity. This work was undertaken to assess and re-evaluate the integrity of structural shielding to validate the protection of the staff and the public due to the changing work load spectrum encountered during the last 5- to 10-y operational states of the CT facilities. Also, CT is an X-ray procedure that generates high-quality crosssectional images of the body and can cause high doses (5–100 mSv)(1). Leakage radiation and scattered radiation are not useful to the patient and worker or even members of the public and hence all these groups of people should be well shielded and protected from the harmful effects of these radiations(2 – 4). Evaluation of the shielding integrity of a CT facility is therefore necessary to uncover problems with: † †

room layout due to changes that occurred after the design phase; un-attenuated primary and secondary radiation paths into control areas and other areas outside the X-ray facility;

† changes in equipment layout, room configuration or adjacent areas that would affect the shielding design; † impaired shielding due to the installation of electrical outlets plumbing or air conditioning ducts and so on.

MATERIALS The equipment that was used in this research work included a calibrated RADOS 120 survey meter with serial number 210148, which measures dose rates as low as 0.05 mSv h21 and as high as 10 Sv h21, a tape measure and a PUNDIT plus ultrasonic pulse velocity (UPV) set used to measure pulse velocities.

METHODS Three main methods were used in this research work, namely dose rate measurements, DLP method for shielding calculations and ultrasonic testing method for the determination of voids and other defects. The following two methods were used to determine the required thickness: † Method 1 involves the usage of derived DLP values from CTDIvol values of the ImPACT scan website in calculating DLP values for the head and body(5).

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The structural shielding thicknesses of the walls of four computed tomography (CT) facilities in Ghana were re-evaluated to verify the shielding integrity using the new shielding design methods recommended by the National Council on Radiological Protection and Measurements (NCRP). The shielding thickness obtained ranged from 120 to 155 mm using default DLP values proposed by the European Commission and 110 to 168 mm using derived DLP values from the four CT manufacturers. These values are within the accepted standard concrete wall thickness ranging from 102 to 152 mm prescribed by the NCRP. The ultrasonic pulse testing of all walls indicated that these are of good quality and free of voids since pulse velocities estimated were within the range of 3.496+ + 0.005 km s21. An average dose equivalent rate estimated for supervised areas is + 0.15 mSv week21, which are within acceptable values. 3.4+ + 0.27 mSv week21 and that for the controlled area is 18.0+

A. NKANSAH ET AL.

†

Method 2 involves the usage of default DLP values by the European Commission (EC) in calculating DLP values for the head and body(6).

Radiation shielding parameters The parameters listed below were considered for the calculation of barrier thicknesses. Allowance was made for possible future changes in any one or all of these parameters, including increases in use and occupancy factors, operating tube voltage and workload, shielding parameters, air kerma rates, distances, as well as modifications in techniques that may require ancillary equipment (2 – 4).

These distances are chosen because it is presumed to be closest to sensitive organs of the person who normally occupy these areas. In this work, 0.5, 1.0 or 2.0 m were chosen, respectively, to represent the distance closest to the sensitive organs(2). Shielding design goal Shielding design goals are used in the design or evaluation of barriers constructed for the protection of occupationally exposed persons and members of the public (Table 1). These are based on the type of area, the occupancy and distance to the occupied area. The shielding design goal used for this research work is 0.02 mGy week21 for supervised areas and 0.1 mGy week21 for controlled areas(2).

Workload (W ) CT workloads were calculated from the knowledge of local situations. Workload can also be expressed in the form of procedures per week(2 – 4). A workload of 250 procedures per week was assumed and used in the shielding calculations for all the CT facilities(7). Distances The distances measured are in metres from either the primary (dpri) or secondary (dsec) radiation source to the occupied area. NCRP Report 147 recommends the following default distances: for areas above the source (storey building or room above source): 0.5 m; – for areas behind the barrier wall: 0.3 m; and – for areas below the source: 1.7 m.

Air kerma rates Air kerma values at designated points behind the secondary barrier were calculated (with inverse square law) using scatter air kerma fractions (for CT shielding) at 1 m measured using the head and body phantoms provided by the Food and Drug Administration (FDA)(8). Default values of DLP The DLP values proposed by the EC (1999) and the default values of DLP (1200 mGy cm for the head and 550 mGy cm for the body) were used for the calculations(8). These values are shown in Table 2. Derived DLP values Another set of DLP values were derived from CTDIvol values provided by various manufacturers. The default scatter fractions kbody and khead per centimetre for the peripheral axis of the FDA(9) body and head phantoms are given as follows: kbody ¼ 3 104 cm1 ;

ð1Þ

khead ¼ 9 105 cm1 :

ð2Þ

–

Since these measured k values include a small tube leakage radiation component, the air kerma calculated from them is denoted as ksec. These CTDIvol

Table 1. The structural shielding design goal adopted for the shielding calculations.

NCRP-49 (1976) NCRP-147 (2004) Effect

Controlled area

Uncontrolled area

50 mGy y21 ¼1 mGy week21 1/2 of 10 mGy y21 limit for new operations¼5 mGy y21 (approximately matches fetal dose limit) ¼ 0.1 mGy week21 Factor of 10 decrease

5 mGy y21 ¼0.1 mGy week21 5 mGy y21 ¼ 0.02 mGy week21

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Factor of 5 decrease


Occupancy (T ) Occupancy is a fraction of time a particular place is occupied by staff, patients or public and has to be conservative. The range of occupancy factors used for this work varies from 1 for all work areas to 1/ 20 for toilets and 1/40 for unattended car parks. T¼1 for administrative offices and receptionist areas, laboratories, pharmacies and other areas fully occupied by an individual, attended waiting rooms, children’s indoor play areas, adjacent X-ray rooms, image viewing areas, nurses’ stations, X-ray control rooms and living quarters(2 – 4).

ASSESSMENT OF STRUCTURAL SHIELDING INTEGRITY

values were taken from values provided by CT manufacturers at the (Image Performance Assessment of Computed Tomography Scanners) ImPACT scan website(10). These values are shown in Table 3.

Total air kerma rates The total secondary unshielded air kerma or work1 , used was calculated using load (mGy week21), ksec equation (3): 1 ¼ ksec

1 1 ½ksec ðbodyÞ þ ksec ðheadÞ ; d2

ð3Þ

KsecðxÞ¼

1 NKsecð0Þ 2 dsec

ð4Þ

;

Table 2. Default DLP values by the EC. Procedures

Scan length, L (cm)

DLP (mGy cm)

20 35 25 20 –

1200 525 625 500 550

Head Chest Abdomen Pelvis Body average (chest, abdomen or pelvis)

Only the DLP values for the head and abdomen in Table 2 were used for the shielding calculation because it is assumed that it reasonably represents the head and body phantoms as far as scattered radiation from a patient is concerned.

Broad beam barrier transmission Broad beam transmission, B(x, m), of X rays through a shielding barrier of thickness x of a given material m is defined as the ratio of the air kerma from a broad beam to an occupied area when shielded, k(x), to that in the unshielded condition kð0Þ(2): Bðx; mÞ ¼

Kð0Þ ; KðxÞ

ð5Þ

where k(0) is the unshielded air kerma rate; k(x), shielded air kerma rate. Transmission depends on the energies of the X rays, the thickness and the material of the shielding barrier. The transmission, B, of broad X-ray beams through a variety of shielding materials in medical X-ray imaging applications has been found to be well described by a mathematical model published by Archer et al.(5). This model has the form where x is the thickness of shielding material, and a, b, g are fitting parameters (11): B¼

1þ

b agx b 1=g e : a a

ð6Þ

This equation may be solved for the thickness x as a function of transmission B. g 1 B þ b=a x¼ : ln ag 1 þ b=a For the purpose of this work, the required attenuation or barrier transmission factor B was calculated

Table 3. Derived DLP values from CTDIvol, CT dose index values by the ImPACT scan. Name of the facility A B C D

Model of CT

General Electric, GE BrightSpeed Marconi Medical Systems Philips Mx8000 Dual Siemens AR Star

CTDIvol for head/100 mA s (mGy mA s21)

CTDIvol for head (mGy)

CTDIvol for body/100 mA s (mGy mA s21)

CTDIvol for body (mGy)

0.350

35.0

0.190

19.0

0.301

30.0

0.214

21.4

0.130

13.0

0.660

66.0

0.270

27.0

0.144

14.40

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where d is the distance from the isocentre to the critical point behind the secondary wall. The total air kerma values calculated are shown in Tables 4–7. The inverse square law was used to determine the shielded air kerma rate for the secondary radiation, shown in equation (4):

1 is the total air kerma value behind a where KsecðxÞ barrier of thickness x; dsec, the distance from the source of secondary radiation to the location of maximally exposed individual beyond the secondary barrier (in metres); N, the expected number of patients examined in the room.

Table 4. Shielding parameters used and results of wall thickness calculation for Hospital A. Shielding parameters

Common parameters to both methods

Method 1

Method 2

ksec(x) (mGy) 3.52 3.03 2.10 1.87 4.15 4.53

Bsec 5.71023 6.71023 1.91021 5.41022 1.91021 2.21023

ksec(x)

Bsec

W

P

d (m)

5.71023 6.71023 1.91021 5.41022 1.91021 2.21023

A

dA1 dA2 dB1 dB2 dC dD

2.55 2.75 3.3 3.5 2.35 2.25

(mGy)

7.07 6.08 4.22 3.75 8.33 9.08

B C D

Calculated wall thickness Method 1

Method 2

x1 (mm)

x2 (mm)

120a 118 30 65 30 90

140b 135 52 82 50 108

Existing wall thickness

Standard wall thickness

xE (mm)

xS (mm)

304.80 304.80 304.80 304.80 304.80 304.80

152 152 152 152 152 152

A. NKANSAH ET AL.

Table 5. Shielding parameters used and results of wall thickness calculation for Hospital B. Shielding parameters


Method 1 ksec(x) (mGy) 1.84 1.43 1.53 1.46 4.25 4.43

Method 2 Bsec 1.11022 1.41022 6.51022 6.91022 2.41022 2.31022

ksec(x)

(mGy)

3.45 2.68 2.87 2.74 7.98 8.33

Bsec

W

P

d (m)

5.81023 7.51023 3.51023 3.61023 1.31023 1.21023

A


3.65 4.14 4.40 5.40 2.40 2.15

B C D

Calculated wall thickness Method 1

Method 2

x1 (mm)

x2 (mm)

120a 118 30 65 30 90

140b 135 52 82 50 108

1 1 ksecð0Þ for Method 1 ¼ 24.50 mGy; ksecð0Þ for Method 2 ¼ 46.00 mGy. d, distance in metres; W, name of the wall; P, designated point; Bsec, transmission factor. a The maximum wall thickness using Method 1. b The maximum wall thickness using Method 2.


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1 1 ksecð0Þ for Method 1¼22.90 mGy; ksecð0Þ for Method 2¼46.00 mGy. d, distance in metres; W, name of the wall; P, designated point; Bsec, transmission factor. 1 ksecð0Þ ; unshielded air kerma rate at 1 m from the isocentre (in mGy). ksec(x), total air kerma value behind a barrier of thickness x (in mGy). a The maximum wall thickness using Method 1. b The maximum wall thickness using Method 2.



xE (mm)

xS (mm)

304.80 304.80 304.80 304.80 304.80 304.80

152 152 152 152 152 152

Table 6. Shielding parameters used and results of wall thickness calculation for Hospital C. Shielding parameters


Method 1

Method 2

ksec(x) (mGy)

ksec(x)

1.441021 2.671023 1.111023 1.141021 6.681023 8.981022 5.061022

(mGy)

7.98 6.13 2.56 6.31 3.07 1.59 2.83

Method 1

Method 2



Bsec

W

P

d (m)

x1 (mm)

x2 (mm)

xE (mm)

xS (mm)

2.01022 1.61022 7.81023 1.61023 3.31021 1.31022 7.11023

A B

dA dB1 dB2 dC dD1 dD2 dD3

2.40 2.74 4.24 2.70 3.87 5.37 4.03

40 140 168a 50 118 52 68

90 100 115 155b 80 107 120

228.60 228.60 228.60 228.60 228.60 228.60 228.60

152 152 152 152 152 152 152

C D

Table 7. Shielding parameters used and results of wall of thickness calculation for Hospital D. Shielding parameters Method 1 ksec(x) (mGy) 0.72 2.25 2.38 2.04 1.19

Method 2 Bsec 12.71022 8.881023 4.191022 4.89 1022 8.371022

ksec(x)

(mGy)

1.44 4.52 4.79 4.10 2.40

1.41022 4.41023 2.11022 2.41022 4.21022


Wall thickness Method 1

Method 2



W

P

d (m)

x1 (mm)

x2 (mm)

xE (mm)

xS (mm)

A B C D E

dA dB dC dD dE

5.65 3.19 3.10 3.35 4.38

80 110a 72 68 55

100 145b 90 85 72

203.2 203.2 203.2 203.2 203.2

152 152 152 152 152

1 1 for Method 1¼22.90 mGy; ksecð0Þ for Method 2¼46.00 mGy. ksecð0Þ d, distance in metres; W, name of the wall; P, designated point; Bsec, transmission factor. a The maximum wall thickness using Method 1. b The maximum wall thickness using Method 2.


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1 1 ksecð0Þ for method 1¼64.20 mGy; ksecð0Þ for Method 2¼46.00 mGy. d, distance in metres; W, name of the wall; P, designated point; Bsec, transmission factor. a The maximum wall thickness using Method 1. b The maximum wall thickness using Method 2.


11.10 8.55 3.57 8.81 4.29 2.30 3.95

Bsec

Wall thickness

A. NKANSAH ET AL.

using the following equation(2, 3):

Bsec ðxÞ ¼

P ; 1 T KsecðxÞ

extrapolated from the CT transmission curve provided by Simpkin(11). ð7Þ

1 where KsecðxÞ is the total air kerma value behind a barrier of thickness x; P, weekly design dose (mGy week21); T, occupancy. All the barrier transmission factors and required wall thicknesses have been calculated and shown in Tables 4–7. The appropriate wall thicknesses were

Shielding calculations The assumptions made in NCRP Report 147, 2004, was used and DLP values in Tables 2 and 3 were used for the shielding calculations. RESULTS AND DISCUSSIONS It was observed from Figures 1–4 that each barrier had different levels of shielding as a result of

Figure 2. Comparison of calculated wall thickness and existing wall thickness with standard wall thickness for Hospital B.

Page 6 of 9


Figure 1. Comparison of calculated wall thickness and existing wall thickness with standard wall thickness for Hospital A.


differences in distances, air kerma, DLP values, transmission factors and occupancies behind those boundaries. Therefore, a pragmatic approach to radiation shielding was considered, i.e. specifying a consistent level of shielding for all barriers in all the CT rooms rather than specifying different levels of shielding in each barrier(7). This would result in a more prudent and possibly more cost-effective

approach to shielding. The maximum or highest wall thickness calculated was therefore chosen as the required thickness; this is because it is better to overprotect than to under-protect. Comparing Methods 1 and 2 shows that using the latter (i.e. default DLP values) in designing the required wall thickness gives the highest wall thickness and therefore provides better protection.

Figure 4. Comparison of calculated wall thickness and existing wall thickness with standard wall thickness for Hospital D.

Page 7 of 9


Figure 3. Comparison of calculated wall thickness and existing wall thickness with standard wall thickness for Hospital C.

A. NKANSAH ET AL. Table 10. Dose rates behind the secondary barriers of Hospital C. W

Name of the location

P

d (m)

xo (mSv h21)

x (mSv h21)

A

Reception Waiting area Control console Corridor 1 Corridor2 Corridor 3


3.65 4.14 4.40 5.40 0.11 2.15

0.05 0.08 0.10 0.05 0.11 0.05

0.09 0.08 0.45 0.09 0.11 0.083

B C D

Type of exam: head/brain (kVp: 120; mA s: 200); xo ¼ background dose rate; x ¼ actual dose rate. d, distance in metres; W, name of the wall; P, designated point. For the Radiation Protection Board of Ghana, the maximum dose rate for members of the public is 0.5 mSv h21 and the maximum dose rate for controlled areas is 7.5 mSv h21.

Table 11. Dose rates behind the secondary barriers of Hospital D. Table 8. Dose rates behind the secondary barriers of Hospital A. W

A B C D


P

Main entrance Waiting area Switch room Control console Walkway Injection room


d (m)

xo (mSv h21)

x (mSv h21)

2.55 2.75 4.20 5.20 2.35 2.25

0.05 0.05 0.10 0.10 0.05 0.05

0.19 0.05 0.06 0.45 0.05 0.07

Type of exam: head/brain (kVp: 120; mA s: 200); xo ¼background dose rate; x¼actual dose rate. d, distance in metres; W, name of the wall; P, designated point.

W


P

d (m)

xo (mSv h21)

x (mSv h21)

A B

Waiting area General X-ray room Corridor Changing room Control console

dA dB

2.40 3.19

0.05 0.05

0.06 0.11

dC dD dE

3.10 3.35 4.38

0.05 0.05 0.05

0.06 0.11 0.44

C D E

Type of exam: head/brain (kVp: 110; mA s: 350); xo ¼ background dose rate; x ¼ actual dose rate. d, distance in metres; W, name of the wall; P, designated point.

Table 12. Summary of UPV testing results. Table 9. Dose rates behind the secondary barriers of Hospital B. W

A B C D


P

Switch room Corridor Reception MRI room Control console Waiting area

dA dB1 dB2 dC dD1 dD2

d (m)

xo (mSv h21)

x (mSv h21)

2.40 2.74 4.24 2.70 3.87 5.37

0.05 0.05 0.05 0.05 0.05 0.05

0.06 0.10 0.09 0.08 0.45 0.10

Type of exam: head/brain (kVp: 120; mA s: 350); xo ¼ background dose rate; x ¼ actual dose rate. d, distance in metres; W, name of the wall; P, designated point.

CT facilities

Transit time (ms)

Pathlength (m)

Pulse velocity (km s21)

Standard deviation

A B C D Average

116.29 107.27 122.82 122.65 117.23

0.407 0.375 0.429 0.437 0.412

3.494 3.495 3.496 3.498 3.496

0.009 0.003 0.004 0.005 0.0053

suitable selection of constituent materials is suggested to improve on the quality of sandcrete blocks. In Ghana and most African countries, sandcrete is mostly used for construction because it is considerably cheaper but other properties of the sandcrete

Page 8 of 9


From Tables 4–7 and Figures 1–4, it can be observed that the existing wall thicknesses of all four hospitals are thicker than the calculated shielding design. This is because the required concrete wall is stronger (compressive strength) and more solid and therefore would last longer when compared with the existing concrete wall, which has more coarse aggregates and therefore the thickness of the latter needs to be increased to compensate for the stronger and solid concrete wall. Therefore, if concrete is used as a shielding material, then it should be solid or if hollow blocks are used then the voids needs to be filled with grout or mortar and reinforcement such as iron rods should be added. However, the attenuation of radiation through concrete is dependent on the crushing weight, thickness and density of the material and not the compressive strength of the material(12, 13). Concrete may have a higher density due to the presence of less coarse aggregates when compared with the sandcrete wall but proper curing and


soil such as the mix ratio, crushing weight, density and relative density need to be verified in consultation with the architect and structural engineer to determine its appropriateness as a shielding material(12). For the facilities, the average density of the concrete used was 2.3103 kg m23.

FUNDING This work was partly supported by the Radiation Protection Institute and the Non-destructive Testing Lab of the Ghana Atomic Energy Commission. REFERENCES

Comparison of dose rate measurements with acceptable levels

CONCLUSION The structural shielding thicknesses for the entire selected CT facilities are adequate to protect those who are occupationally exposed as well as members of the public. These values are within the accepted standard concrete wall thickness ranging from 102 to 152 mm prescribed by the NCRP(2). Total dose equivalent rates estimated per week were within the acceptable levels of 100 mSv week21 for the occupationally exposed and 20 mSv week21 for members of the public(1, 14). The average pulse velocity obtained for the points on the tested concrete walls was 3.496+0.005 km s21 as shown in Table 12, which indicates that the concrete used was of good quality and free of voids(12).

ACKNOWLEDGEMENTS The authors acknowledge the staff of the Radiation Protection Institute for their technical support and the heads of the four selected X-ray facilities in the Greater Accra Region of Ghana.

Page 9 of 9


Radiation survey results shown in Tables 8 –11 show the total dose rates measured (same as weekly dose equivalent) for the supervised areas: 3.4+0.026, 3.6+0.026, 3.4+ 0.027 and 3.4+0.027 mSv week21 for Hospitals A–D, respectively. Total dose rates measured (same as weekly dose equivalent) for the controlled areas are 18.0+0.15, 18.0+0.15, 18.0+0.15, 17.6+0.14 mSv week21 for Hospitals A–D, respectively. Comparing the weekly dose equivalent levels in the controlled and supervised areas with the ICRP dose reference level of 100 mSv week21 for occupationally exposed workers and 20 mSv week21 for members of the public, the measured levels were within the acceptable reference levels. The UPV testing results for facilities A, B, C and D are 3.494+0.009, 3.495+0.003, 3.496+0.004 and 3.498+0.005 km s21, respectively, as shown in Table 12.

1. International Commission on Radiological Protection. General principles for the radiation protection of workers. Publication No. 75. Pergamon Press (1997). 2. National Council on Radiation Protection and Measurements. Structural shielding design for medical of X-rays imaging facilities, recommendations of the National Council on Radiation Protection and Measurements. NCRP Report No. 147. NCRP (2004). 3. Sutton, D. G. and Williams, J. R. Radiation shielding for diagnostic X-rays: report of a Joint British Institute of Radiology and Institute of Physicists in Medicine. BIR/IPEM Working Party (2000). 4. International Atomic Energy Agency. Training material on radiation protection in diagnostic and interventional radiology, part 12.1: shielding and X-ray room design. IAEA (2000). 5. Archer, B. R., Fewell, T. R., Conway, B. J. and Quinn, P. W. Attenuation properties of diagnostic x-ray shielding materials. Med. Phys. 21(19), 1499–1507 (1994). 6. European Union Guidelines on radiation dose to patients. Retrieved in January 2010 from EU website http://www .drs.dk/guidelines/ct/quality/page02.htm (2009). 7. Melissa, C. M. Diagnostic X-ray Shielding, Multi-slice CT Scanners, using NCRP147 Methodology. Therapy Physics (2006). 8. International Electrochemical Commission. Medical electrical equipment: particular requirements for the safety of X-ray equipment for computed tomography. Retrieved on 10 December 2009 from IEC website www.iec.ch/tctools/dashbd-e.thtm (2002). 9. US Food and Drug Administration. (2001). Reducing the radiation risk from computed tomography for pediatric and small adult patients. Retrieved 15 December 2009 from http://www.fda.gov/cdrh/safety/110201-ct.html. 10. Image Performance Assessment of Computed Tomography Scanners. Updated CTDI Tables, United Kingdom’s CT scanner evaluation centre. http://www .impactscan.org (accessed December 2009) (2004). 11. Simpkin, D. J. The expanding role of medical physics in diagnostic radiology. New concepts for Radiation Shielding of Medical Diagnostic X-ray Facilities. American Association of Physicists in Medicine, AAPM Monograph (1997). 12. Abdullahi, M. Compressive strength of sandcrete blocks in Bosso and Shiroro areas of Minna, Niger State, Nigeria. AU J. Tech. 9(2): 126–132 (2005). 13. Kalender, W. A., Schmidt, B., Zankl, M. and Schmidt, M. A PC program for estimating organ dose and effective dose values in computed tomography. Eur. J. Radiol 9:555– 562 (1999b). 14. International Commission on Radiological Protection. Recommendations of the International Commission on Radiological Protection. Managing patient dose in computed tomography. A report of the International Commission on Radiological Protection. Ann. ICRP 30(4):7–45 (2000).