Manual CT 7800TX

Iran. J. Radiat. Res., 2006; 3 (4): 183-1189

Conventional and spiral CT dose indices in Yazdgeneral hospitals, Iran

F. Bouzarjomehri1*, M. H. Zare2, D. Shahbazi2

1 Department of Medical Physics, Shahid Sadoghi University of Medical Sciences, Yazd, Iran2 Department of Medical Physics, Isfahan University of Medical Sciences, Isfahan, Iran

INTRODUCTION

CT provides high quality X-ray imagingwith substantial benefits in health care.Clinical application of this technique hascontinued to increase. So that CTexamination accounts for approximately 35-40% of the annual collective dose frommedical X-rays whilst representing only 5%of their total number now (1). It is importantto assess patient doses in different protocol ofCT examination and ways to reduce patientdoes without considerable defection in imagequality. There are different methods todescribe (2, 3) and measure(4, 5) radiation doesin CT. European Guidelines (EG) on qualitycriteria for CT were published by theEuropean Commission(6) in which two doesdescriptors, weighted Computed TomographyDose Index (CTDIW) and Dose-Lengthproduct (DLP) were proposed as ReferenceDose Levels (RDLS). CTDIw is derived fromthe principle dosimetric quality computedtomography dose index (CTDI)(2), which is theintegral along a line parallel to single slice,divided by the nominal slice thickness T.Related equation is defined by :

(1)

N is the number of acquired section perscan. (Also referred to as the number of datachannels used during acquisition). CTDI canbe measured free-in-air (CTDIair) or inphantom (CTDIW). CTDIair measured atisocentre of gantry and CTDIW measured incenter of head and body phantom (CTDIC),*Corresponding author:Dr. Fatollah Bouzarjomehri, Department of MedicalPhysics, Shahid Sadoghi University of Medical Sciences,Yazd, Iran Fax: +98 351 7244078E-mail: [email protected]

Background: While the benefits of ComputedTomography (CT) are well known in accurate diagnosis,those benefits are not risk free. CT is a device withhigher patient dose in comparison with otherconventional radiological procedures. Is the reductionof exposures by requiring optimization of CTprocedures [a principle concern in radiologicalprotection]? In this study, the radiation dose ofconventional and spiral CT were investigated andcompared with European Commission Reference DoseLevels (EC RDLs) Materials and Methods: Thedosimetric quantities proposed in the EuropeanGuidelines (EG) for CT are Weighted ComputedTomography Dose Index (CTDIW) for a single slice foraxial scanning or per rotation for helical scanning andDose-Length Product (DLP) for a completeexamination. The patient-related data were collectedfor brain, neck, chest, abdomen and pelvisexaminations in each scanner. For each type ofexamination, 10 typical patients were randomlyincluded. CTDI with an active length of 10cm wasmeasured in two CT scanners by using UNFORS (Mult-O-Meter 601) in head and body phantom (PMMA) with16 cm and 32 cm in diameter; respectively. Meanvalues of CTDIW, DLP and Effective Dose(ED) wereestimated for those examinations. Results: CTDIWhad a range of 15.8-24.7 mGy for brain, 16.1-30.6mGyfor neck, 6.8-9.2 mGy for chest, 6.8-9.8 mGy forabdomen and pelvis. DLP had a range of 246.4-397.7mGy.cm for brain, 104.6-262.2 mGy.cm for neck, 135-248.4 mGy.cm for chest, 187-298.9 mGy.cm forabdomen and 197.2-319.4 mGy.cm for pelvis. Themean values of effective dose were 0.70 mSv forbrain, 1 mSv for neck, 3.2 mSv for chest, 3.3 mSv forabdomen and 5.1 mSv for pelvis. Conclusion: Theobtained results in this study have shown that CTDIWand DLP are lower than EC RDLs and other studies, inother words, the performance of all scanners has beensatisfactory as far as CTDIw and DLP are concerned.The CTDIW and DLP in the conventional CT are higherthan the spiral CT values. With regard to ALARAprinciple, for the establishment of reference doselevels, the radiation dose with spiral CT scannersshould be taken into account. Iran. J. Radiat. Res.,2006; 3 (4): 183-189

Keywords: CTDIw, patient dose, CT, DLP, RDLs.

single1CTDI D (z)dz(mGy)NT

F. Bouzarjomehri, D. Shahbazi, M. H. Zare

and peripheral of head and body phantom(CTDIP)(7), in which;

(2)

The European Commission (EC) havesuggested the use of a normalized dose index,nCTDIW, which takes into non-uniformitiesof CTDI values measured at center or theperiphery of these phantom(8):

(3)

C is the radiographic exposure (mAs).The weighted CT dose index (CTDIW),

which is the first of the two reference dosequantities proposed by the EC for a singleslice in serial scanning or per rotation inhelical scanning is then simply:CTDIW = nCTDIW . C (mGy) (4)

In practice, measurements are carried outusing a pencil shaped ionization chamberwith 100mm active length (CTDI100) or usingthermo luminescent dosimeter (TLDS) orusing Unfors Mult-O-Meter 601 with 100mmactive length. The second quality is the Dose-Length Product (DLP) that characterized thetotal radiation from a complete examination,and is estimated by the following formula:

For axial scanning:

(5)

For helical scanning

(6)

Where I, represent each serial or helicalscan sequence forming part of anexamination; T is the slice thickness (cm); N,the number of slices; C, the radiographicexposure; A, the tube current (mA) and t, istotal acquisition time. CTDIW and DLPvalues for a specific examination usingdifferent protocols or scanners will provideinformation on relative performance (9). Inorder to estimate the radiation riskassociated with CT examination, it isnecessary to estimate effective dose (ED)which is the sum of the products of organdoses and corresponding weighting factors(10).

Shrimpton et al. calculated E from CTDImeasurements using Monte Carlo conversioncoefficients (11, 12). Another way for measuringED is using an anthropomorphic physicalphantom, that dose are measured in thelocation of organ or tissue of interest usuallyby using thermo luminescent dosimeter(TLDS) and then ED can be calculated. As apractical alternative, EC(13) give region-specific normalized coefficients (EDLP) toestimate the risk of CT examination protocol.Effective dose is derived from values of DLPwith following equation: E = EDLP . DLP (mSv) (7)

Where EDLP is in mSv.mGy-1cm-1 and DLPis in mGy.cm unit. General Levels fordifferent regions of patient (Brain, Neck,Chest, Abdomen and Pelvis) are given intable 1. However, these dose values are basedon the result of older survey data from late1980s (14). The technical improvement in CT,in particular use of the spiral technique, hasoffered new possibilities in both diagnosisand dose reduction (14). The tube current timeproduct for spiral CT usually cannot be set ashigh as for conventional CT due to the limitedtube heat capacity; therefore, the radiationdose should be effectively lower for spiralthan for conventional CTs (14). The results ofolder survey that were based on investigationof dose for conventional CT may not berepresentative of the present situation. Toour knowledge, there are no measureddosimetry data with PMMA phantom in Iran.The purpose of this study was to evaluateroutine examination protocols utilized in

Type ofexamination

CTDIW(mGyair)

DLP(mGy.cm)

EDLP(mSv.mGy-11cm-11)

Brain 60 1024 0.0023Necka 60 1024 0.0054Chest 30 650 0.0170

Abdomen 35 780 0.0150Pelvis 35 570 0.0190

Table 1. Proposed European Commission reference Levels andregion specific normalized effective doses for some routine CT

examination (14).

CTDIW: weighted CT Dose index; DLP: dose-length product; EDLP:region specific normalized effective dose. a: No specific reference value for neck is yet available, but forcomparison brain values are used.

W C P1 2CTDI CTDI CTDI (mGy)3 3

1W C P

1 1 2nCTDI ( CTDI CTDI )mGymAsC 3 3

Wi

DLP nCTDI .T.N.C(mGy,cm)

Wi

DLP nCTDI .T.A.t(mGy,cm)

184 Iran. J. Radiat. Res.; Vol. 3, No. 4, March 2006

Iran. J. Radiat. Res.; Vol. 3, No. 4, March 2006 185

Conventional and spiral CT dose indices

Yazd city, and to compare results withEuropean Commission Reference Dose Levels(EC RDLS).

MATERIALS AND METHODS

This survey was performed on two CTscanners which are operating in 2 radiologydepartments of two general hospitals in Yazd.In one center, Shimadzu 7800 TX (Shimadzu,Tokyo, Japan) (A), and in the other washelical and conventional scanners Shimadzu3000 TX (Shimadzu, Tokyo, Japan) (B) inwere applied. It is necessary to mention thatscanner A operated both conventional andhelical. The examinations were categorizedas follows: 1) brain, 2) neck, 3) chest, 4)abdomen and 5) pelvis. For eachexamination, data concerning examinationparameters, such as kVp, mAs, number ofslices, slice thickness and slice increment for10 typical patients were recorded in CT clinicwhich performed the related examination.For conventional CT slice increment, and forspiral CT, pitch factor P is recorded. In total,150 patients (100 patients for conventionaland 50 patients for helical) were included inthis study. It was found that for eachexamination, each CT center used constantkVp, mAs and slice thickness and only theslices frequency varied slightly from patientto patient. We determined actual slicesfrequency or average scan length from thedata related to 10 patients for eachexamination, and for each clinic. All themeasurements were carried out with a pencilprobe solid state dosimeter (Unfors Mult-O-Meter 601, Sweden), with active Length of100mm, and a diameter of 9mm. Formeasurement of CTDIair (free-in-air) probe ofdosimeter was placed on the axis of rotationof each scanner, and then with selection ofappropriate parameter for differentexamination, the measurement wasperformed. For measurement of CTDI inphantom, a head and body phantom wereused. Phantoms were cylindrical solidPerspex (PMMA) used according to UnitedStates Food and Drug Administration (15). Aphantom with diameter of 16cm and lengthof 14cm for head and a diameter of 32cm andlength of 14cm for body were made by sevenPMMA circular slabs (with 2-centimeterthickness). The circular slabs were adherent

together. Head and body phantom had five13mm diameter holes (the size of holes iswith consider to diameter of dosimeter probe)were drilled parallel to its long axis, one atthe axial central and four around theperimeter, 1cm apart from the edge. Holeswere positioned at 12, 3, 6, 9 O'clock (16).Measurement in the conventional CT wasperformed in whole holes of phantoms, andmeasurement in helical (Spiral) CT wasperformed in the center and the hole thatpositioned at the 12 O'clock according tocurrent practice in ACR (American College ofRadiology). All the used holes must have beenfilled with a cylindrical solid Perspex rod.The head phantom for head and Neck and thebody phantom for chest, abdomen and pelvisexaminations were used. The head phantomwas placed on the head holder and the bodyphantom was placed on the patient table ofthe CT scanner. CTDIW, DLP and effectivedose were then calculated according toEuropean Commission (13) to checkcompliance with dose criteria and comparedwith other studies. The dosimeter wascalibrated by the seller company. The overallaccuracy of Mult-O-Meter for measurementof dose was estimated to be ±5%. The mean ofCTDIW was calculated for each ofexaminations from three measurements inthe head and body phantoms

RESULTS

The examination protocol details areshown in tables 2 and 3. Table 2 presents thedifferent examination protocols used in twoscanners presented in this study for thehead, neck, chest, abdomen and pelvis intypical patient with fixed kV, mAs, T, sliceincrement, I and mean scan length, L at eachscanner, in the conventional CT. Table 3presents the different examination protocolsused in CT scanner A, for brain, neck, chest,abdomen and pelvis in typical patient withfixed kV, mAs, T, pitch, factor, P and meanscan length L, in the spiral CT. The accuracyof kVp and mAs set up of CT scanner werechecked up by Mult-O-Meter in mode of kVpand mAs measurement. The accuracy was in±2%.

All examinations were performed with aconstant tube voltage (120 kV). The slicethickness for all examination protocol, in

spiral and conventional CT was 10 mm,except the neck examination which was 5mm. The variable parameter betweenscanners and the examinations was mAs,were the lowest value (130 mAs) was used inthe brain examination of the spiral CT, andthe highest value (240 mAs) in the neckexamination of the conventional CT. It wasfound that the scanner B had used constanttube current-time product (180 mAs) for theall examinations. The pitch factor value wasequal to 1.5 in the spiral CT, and the valuesof slice increment (I) was equal to the slicethickness (packing factor =1) to have a seriesof contiguous slices in all examinations. Inother words, according to clinic requirement,it was not necessary to have overlappingslices because of patient dose decrease. TheCTDI measurements in air are shown intable 4. In order to compares the scanners theresults are normalized by the tube current-exposure time product (mAs).

The CTDIW and DLP were calculated foreach examination. The mean results areshown in table 5 for the conventional and the

spiral CT. CTDLw was calculated for each ofexaminations by average of threemeasurements in head and body phantom.EG of CTDIW and DLP are also shown intable 5. CTDLW and DLP for each of theexaminations protocol investigated in thisstudy were lower than the EG.

DISCUSSION

The protocols utilized in CT centers of Yazdgeneral hospitals have CTDIW and DLP


scanner examination kVp mAs T(mm) I(mm) L(cm)A Brain 120 195 10 10 16.1A Neck 120 240 5 5 8.8A Chest 120 140 10 10 27A Abdomen 120 150 10 10 30.5A Pelvis 120 150 10 10 32.6B Brain 120 180 10 10 15.6B Neck 120 180 5 5 6.5B Chest 120 180 10 10 25B Abdomen 120 180 10 10 27.5B Pelvis 120 180 10 10 29

Table 2. Details of examination protocols, including kVp, mAs, slice thickness, T, slice increment I, and mean scan length L in the conventional CT.

A: SCT-7800TX, Shimadzu CT scanner.B: 3000 TX, Shimadzu CT scanner.

scanner Examination* kVp mAs T (mm) p L (cm)A Neck 120 240 5 1.5 8.8A Chest 120 130 10 1.5 27A Abdomen 120 140 10 1.5 30.5A Pelvis 120 140 10 1.5 32.6

Table 3. Details of examination protocols including kVp, mAs, slice thickness T, and pitch factor, P, and mean scan length L in the spiral CT.L in the conventional CT.

A: SCT-7800TX, Shimadzu CT scanner.*: In the scanner A center, spiral technique of brain was not used.

Scanner Slice thickness(mm)

CTDIair

(mGy.mAs-11)A 10 0.197A 5 0.199B 10 0.141B 5 0.148

Table 4. Normalized computed tomography dose index free-in-air (CTDIair).

A: SCT-7800TX, Shimadzu CT scanner;B: 3000 TX, Shimadzu CT scanner.


values which are lower than EG dose criteria.This is encouraging, since the most impor-tant aspect of radiation protection is to havethe amount of dose absorbed by the patientsas low as reasonably achievable, providedthat, this dose does not affect image qualityand accurate diagnosis. One possible methodof dose reduction is mAs reduction in exami-nation protocols, especially for patients whoare thinner than the standard sized patients.

As the table 6 shows the values of weight-ed CT dose index in this study are comparedwith those of Hidajat et al.(14), Scheck etal.(17), Smith et al.(18), Shrimpton et al.(19) andTsapaki et al.(7). The values obtained in thisstudy in the most circumstances, were lowerthan the other studies, because of the lowermAs used and lesser frequency scannerincluded in our study. Only the value of neck

examination (spiral) was similar to the otherstudies using mAs. Table 7 shows the valuesof DLP and they are lower than the otherstudies as a resource of using shorter scanlength the present. The effective dose is calculated for the examination protocolsincluded in this study, with regard to the conversion factor (13). Table 8 presents themean effective dose values of the examina-tions included in this study and others. It isfound that then obtained values are lowerthan the studies.

Mayo et al. (22) presented a study regardingthe minimum tube current required for goodimage quality with the least radiation doseon CT chest examination.

Results of the research of Tsapaki et al. (7)

also indicated that the lowest mAs can beused without affecting diagnosis, despite the


Examination Parameter Scanner A Scanner B Scanner A1 EG

BrainCTDIW 24.7 15.8 - 60

DLP 397.8 246.6 - 1024

NeckCTDIW 30.6 16.1 26.8 60

DLP 269.6 104.8 157.2 1024

ChestCTDIW 9.2 6.8 7.8 30

DLP 248.4 170 135 650

AbdomenCTDIW 9.8 6.8 8.9 35

DLP 300.7 187 180 780

PelvisCTDIW 9.8 6.8 8 35

DLP 321.4 197.2 290.1 570

Table 5. The mean weighted computed tomography dose index (CTDIW, mGy) and the dose-length (DLP, mGy.cm) results werecompared with European Guidelines (EG).L in the conventional CT.

A: SCT-7800TX, Shimadzu CT scanner; B: 3000 TX, Shimadzu CT scanner.A1 is scanner A that is operated in helical.

Examination Thisstudy

Hidajat(14)*et al.

Scheck(17)

et al.Smith(18)

et al.Shrimpton(19)

et al.Tsapaki(7)

et al.Brain conventional 20.3 18.2-82.6 51.1±9.8 60±12 50±14.6 27-52

Neck conventional 23.3 15.8-61.6 NA NA NA NAspiral 26.8 15.7-52.5 30.7±9.2 NA NA NA

Chest conventional 8 18.8-40.3 NA NA NA 14-27spiral 7.8 7.41-39.5 12.9±5.5 NA NA NA

Abdomen conventional 8.3 18.8-47.5 NA NA 20.3±7.6 14-27spiral 8.9 11.9-26.4 15.1±4.6 NA NA NA

Pelvis conventional 8.3 23.7-47.5 NA NA 25.6±8.41 14-25.3spiral 8.9 12.6-25.3 NA NA NA NA

Table 6. The weighted CT dose index in this study and the other studies.

Note. Data are the mean and their unit is mGy. *Numbers in parentheses are references. NA=not available



fact that the images may be noisier. In thisstudy, the tube current-time product (mAs) inthe most examination of conventional CT washigher than the spiral CT. In addition oflower tube-current time product, the increasein pitch of spiral CT leads to a dose-lengthproduct that is lesser than those values forconventional CT. Reducing the extent of thescan as much as possible without missing anyvital anatomical regions, could be the firststep to lower DLP and ED.

The RDLS act as the parameter to identifyrelatively poor or inadequate use of the tech-nique. The exposure setting and the extent ofscan should be further investigated for lowerdose without affecting image quality. Theweighted CT dose index values in spiral CTscanner should be measured.

The users of conventional CT scannershould change their examination parametersto get weighted CT dose indexes similar tothose of spiral CT scanners. In other words,

for the establishment of reference dose levels,the radiation dose with spiral CT scannersshould be taken into account.

ACKNOWLEDGMENT

The authors wish to thank the personals ofSadooghi and Rahnemoon Hospitals for theirvaluable assistance in data collection, espe-cially Mr. Shaigh, Salimian and Kafi

REFERENCES

1. Shrimpton PC, Edyvean S (1998) CT scanner dosimetry.Br J Radiol, 71: 1-3.

2. Spokas JJ (1982) Dose descriptors for computedtomography. Med Phys, 9: 288-92.

3. Shope TB, Gagne RM, Johnson GC (1981) A method ofdescribing the doses delivered by transmission X-ray

Examination, DLP This study Shrimpton et al. Tsapaki et al. Hidajat et al.Brain conventional 322.2 882±332 430-758 218-899

Neck conventional 187.2 NA NA 143-946Spiral 157.2 NA NA 164-600

Chest conventional 209.2 517±243 348-807 405-1031Spiral 140.4 NA NA 134-714

Abdomen conventional 243.9 597±281 278-582 284-1185Spiral 180 NA NA 119-543

Pelvis conventional 259.3 443±233 306-592 504-2018Spiral 290.1 NA NA 168-486

Table 7. The mean Dose-Length Product in the present study compared with those of Shrimpton et al.(19),Tsapaki et al.(7), Hidajat et al.(14).

Note: data are the mean and their unit is mGy.cm NA=not available.

Examination This study Tsapakiet al.

Clarcket al.

Polettiet al.

Shrimptonet al.

Hatziioannet al.

Brain 0.7 1.4 1.6 1.8 1.8 1.6Neck 1 NA NA NA NA NAChest 3.2 10.9 7.6 8.9 8.3 6.8

Abdomen 3.3 7.1 7 9.7 7.2 7Pelvis 5.1 9.3 7.6 6.9 7.3 6.4

Table 8. The mean values of effective dose (mSv) compared with those of Tsapaki et al. (7), Clark et al.(20) ,Polettiet et al.(21), Shrimpton et al.(19),Hatziioann et al.(8).

Note: unit is mSv.Numbers in parentheses are references. NA: not available



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Quality Criteria for Computed Tomography, Report EUR16262. Brussels: EC, 15: 49.

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describing the doses delivered by transmission x-raycomputed tomography. Med. Phys., 8: 488-495.

17. Scheck RJ, Coppenrath EM, Kellner MW, et al. (1998).Radiation dose and image quality in spiral computedtomography: multicentre evaluation at six institutions. BrJ Radiol, 71: 734-744.

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