A Study and Optimization of Lumbar Spine X-Ray Imaging

8/2/2019 A Study and Optimization of Lumbar Spine X-Ray Imaging

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A study and optimization of lumbar spine X-ray imaging

systems

1G MCVEY, D.Phil,

2M SANDBORG, PhD,

1D R DANCE, PhD, FIPEM and

2G ALM CARLSSON, PhD, FInstP

1Joint Department of Physics, The Royal Marsden NHS Trust, Fulham Road, London SW3 6JJ, UK and 2Department of

Radiation Physics, Faculty of Health Sciences, Linkoping University, SE581 85 Linkoping, Sweden

Abstract. A Monte Carlo program has been developed that incorporates a voxel phantom of an adult patient in

a model of the complete X-ray imaging system, including the anti-scatter grid and screenfilm receptor. This

allows the realistic estimation of patient dose and the corresponding image (optical density map) for a wide

range of equipment configurations. This paper focuses on the application of the program to lumbar spine

anteroposterior and lateral screenfilm examinations. The program has been applied to study the variation of

physical image quality measures and effective dose for changing system parameters such as tube voltage, grid

design and screenfilm system speed. These variations form the basis for optimization of these system

parameters. In our approach to optimization, the best systems are those that can match (or come close to) the

calculated image quality measure of systems preferred in a recent European clinical trial, but with lower patient

dose. The largest dose savings found were 21% for a 400 speed class system with a grid having a strip density of40 cm21 and a grid ratio of 16. A further dose saving of 13% was possible when a 600 speed class system was

employed. The best systems found from the optimization correspond to those recommended by the European

Commission guidelines on image quality criteria for diagnostic radiographic images.

Lumbar spine radiographs allow clinicians to judge the

configuration and alignment of bones with a high degree

of accuracy. Malalignment or other changes in the shape

of the vertebrae can then be identified and may imply the

presence of a tumour, fracture or infection. Lumbar spine

radiography is a routine examination for lower back pain,

which is very common; 27 patients per 1000 inhabitants in

the UK undergo plain radiography of the lumbar spineeach year [1]. These examinations contribute 4.3% of the

annual collective effective dose for all medical and dental

exposures compared with 0.9% for chest examinations in

the UK [2].Optimization is necessary to balance the requirement for

good image quality with low patient dose. The Commis-

sion of the European Communities (CEC) image quality

criteria [3] describe the presentation of the normal

anatomy in a lumbar spine radiograph. Almen et al [4]

have evaluated the image quality of lumbar spine radio-

graphs using the CEC criteria [3]. These studies showed

that systems using a low tube voltage (70 kV) and a

medium speed class (400) for the screenfilm receptorfulfilled more of the image criteria for the anteroposterior

(AP) projection than those using high tube voltage and

high speed class. The systems using high speed class (600)

and low tube voltage (77 kV) fulfilled more criteria for the

lateral (LAT) projection than those using low speed classand high tube voltage. Vano et al [5] optimized lumbarspine imaging by varying different technical parametersand found the largest dose saving by decreasing the optical

density by changing the settings of the automatic exposurecontrol (AEC). Almen et al did not study the effect ofoptical density as they did not use AEC.

The assessment by Almen et al [4] of clinical imagequality has been complemented by theoretical modelling aspart of the same project. A realistic Monte Carlo model ofthe patient (voxel phantom) and the complete imagingsystem has been developed [6] for this purpose. The modelcan be used to calculate physical measures of imagequality and patient dose. In Sandborg et al [7], thecorrelations between our calculated physical measures ofimage quality and the clinical assessments of image qualityare presented for chest and lumbar spine radiographs.

For the latter, the signal-to-noise ratio (SNR) of trabe-cular structures was found to be a good predictor ofclinical image quality. This paper presents the application

of the Monte Carlo program to the study and optimization

of lumbar spine imaging. The optimization approach issimilar to that used, with the same model, for chestradiography [8] and involves the use of a reference systemknown to be of good image quality. Preliminary results forthis study are outlined in Dance et al [9]. In this paper, ourpreliminary study is considerably extended so that theinfluence of tube voltage, grid design, screenfilm speedand operating optical density are all considered for bothAP and LAT projections.

Methods and materials

Monte Carlo model and voxel phantom

A Monte Carlo computer program has been developedto simulate diagnostic X-ray examinations. It is based on

Received 8 April 2002 and in revised form 30 August 2002, accepted 21October 2002.

Current address for G McVey: North Wales Medical Physics, Glan

Clwyd Hospital, Bodelwyddan, Denbighshire LL18 5UJ, UK.

This work was supported by grants from the Commission ofEuropean Communities (Nos. FI4P CT950005 and FIGM-CT2000-00036). The Swedish authors were supported by grants from the

Swedish Radiation Protection Institute, SSI (Nos. SSI P1018.97 andSSI P1083.98), and the Swedish Foundation for Strategic Research(No. R98:006).

The British Journal of Radiology, 76 (2003), 177188 E 2003 The British Institute of Radiology

DOI: 10.1259/bjr/52734084

177The British Journal of Radiology, March 2003


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our earlier work, which employed a homogeneousphantom in a model of the complete imaging system[10, 11]. For the present model, the program has beenextended by the inclusion of a voxel phantom to providea more accurate model of the patient. The programtransports photons through the patient and the anti-scattergrid to the imaging device. The energy imparted to the

voxel phantom allows the patient dose to be calculatedand the energy imparted to the screen allows the imagequality measures to be calculated. These parameters are

discussed in more detail below.The voxel phantom used is that developed by Zubal et al

[12, 13] and was obtained by segmentation of a series ofCT slices of an adult male. Female organs (breasts, uterusand ovaries) were added by us to facilitate the calcula-tion of effective dose [14]. An extra layer of voxels wasincluded in the phantom to model the couch top. Forthe AP view, the dimensions of the voxel phantomwere 899 mm from the top of the head to the bottomof the pelvis (236 voxels), 356 mm wide (128 voxels)and 214 mm thick (77 voxels). The phantoms length was

adjusted to correspond to the sitting height of theaverage European male [15]. The phantoms width and

thickness were found by the comparison of calculations ofentrance air kerma for the voxel phantom with measure-ments from a patient study [6, 16]. Each voxel in the

phantom belongs to 1 of 55 organs and each organ isassociated with one of four tissue types: average softtissue (1030 kg m23); lung (260 kg m23); average bone(1490 kg m23); or bone spongiosa (1180 kg m23). Tissuecompositions were obtained from the InternationalCommission on Radiation Units and Measurements(ICRU) [17], except for average bone, which was takenfrom Kramer [18].

Figure 1 shows the model of the voxel phantom and thecomponents of the imaging system. The photon spectrumwas obtained from Birch et al [19]. A grid was used as theanti-scatter technique and was specified in terms of stripdensity N, lead strip width d, grid ratio r and the materials

in the interspaces and covers. The model of the imagereceptor included the cassette front, fluorescent screen andfilm characteristic (H and D) curve, measured by Dr FVerdun, Lausanne (personal communication). The Monte

Carlo code calculates the contrast and SNR of anatomicaldetails at different positions in the image to provide aphysical measure of image quality. These parameterswere calculated with a large number of photon historiesso that the uncertainty of their values is less than 3%

(1 standard deviation).

Important contrast details

The important contrast details used for the calculationof image quality were carefully selected to correspond tothe diagnostic requirements described in the CEC imagequality criteria [3] and following discussions with localradiologists in London and Linkoping. Lumbar spineX-ray images help the clinician appraise the presentationof the lumbar spine vertebrae and thus, all the detailschosen represent bony anatomy. For modelling the APprojection, the L1, L3 and L5 transverse processes were

selected as low contrast details with thicknesses of2.0 mm (L1T), 3.5 mm (L3T) and 5.0 mm (L5T), respec-tively. For modelling the LAT projection, the L1, L3 and

L5 spinous processes were selected as low contrast

details with thicknesses of 5.0 mm (L1S), 5.5 mm (L3S)

and 6.0 mm (L5S), respectively. The thicknesses were

obtained from measurements on a skeleton. All bony

processes were simulated as cortical bone (1920 kg m23)

and their contrast was calculated against a background of

soft tissue.

Small high contrast details were also chosen. These werethe trabecular structures on the L1, L3 and L5 vertebrae

in the AP projection, referred to subsequently as L1D,

L3D and L5D, respectively. For the LAT projection, the

trabecular structures were selected to be at an anterior

position on the L1 and L5 vertebrae and at a posterior

position on the L5 vertebra, referred to subsequently as

L1F, L5F and L5B, respectively). All of the trabecular

structures were 1 mm thick. This is quite similar to the

important detail size of 0.3 mm to 0.5 mm given in the

CEC image quality criteria document [3]. Trabecular

structures were simulated as bone marrow cavities

(1030 kg m23) and their contrast was calculated against

a background of cortical bone. The compositions of theanatomical details and tissue backgrounds were taken

from the ICRU [17].

Figure 1. The imaging system included in the Monte Carlomodel of the lumbar spine anterior-posterior projection. The

bony structures in the voxel phantom have been highlighted.

G McVey, M Sandborg, D R Dance and G Alm Carlsson

178 The British Journal of Radiology, March 2003


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Image quality and patient dose parameters

ContrastContrast was calculated in the Monte Carlo program as

the difference in optical density (DOD) beside and behind theimportant details superimposed on the voxel phantom. The

effects of film gradient and imaging system unsharpness weretaken into account in the calculation ofDOD using:

DOD~ log10 (e)|c(ODdet)|Ce|cdfMTF 1

The H and D curve was measured in accordance with theISO-standard [20] by Dr F Verdun, Lausanne (PrivateCommunication, 1998). The film gradient (c) was derivedfrom the H and D curve for the OD beside the detail (ODdet).The quantitycdfMTF is thereduction in contrast causedby thetotal system unsharpness (total modulation transfer func-tion, MTFtot). Image receptor (screenfilm) and geometricunsharpness (focal spot size and magnification) are all takeninto account in the calculation of the MTFtot. The MTFs

of the screenfilm combination were also measured by Dr FVerdun, Lausanne (personal communication). Sandborg

et al [8] describes the calculation of cdfMTF.The object contrast C

ewas found from Monte Carlo

calculations of energy imparted to the fluorescent screenper unit area:

C~E(p1){E(p2)

E(p1)|

1

1z E(s)=E(p1) 2

Here, ep1 and ep2 are the energy imparted to the receptorper unit area by primary photons beside and behind thedetail, respectively, and es is the energy imparted to thereceptor per unit area by scattered photons. The notationE denotes the expectation value. It was assumed that thedetail does not alter the distribution of scattered photons

in the imaging plane.

Signal-to-noise ratioThe SNR of the ideal observer, SNRI [21], of a small

detail at an optical density ODdet was obtained using:

SNRI(ODdet)~SNRMC(edet)|

ffiffiffiffiffiffiffiffiffiffiA

AMC

s|

SNRDF(ODdet)

3

The SNRMC (edet) was calculated by the Monte Carloprogram. It was obtained from the energy imparted to anarea of the detector AMC with and without the detail being

present assuming that the only noise source is quantummottle and neglecting image unsharpness. The SNR2I wasscaled from the area of the detector element AMC50.25 mm2 to the area of the detail A. The SNRMC due to

quantum noise has been shown to give good agreementwith experiments [22, 23] for details with diameters largerthan or equal to 3 mm. Hence, as many of the details usedin our Monte Carlo model were similar to or smaller than3 mm, the model needs to take into account the effectof image unsharpness. This was implemented using theSNR degradation factor SNRDF, which also accounts for:(i) the different efficiencies with which the signal andquantum noise are transferred through the screen caused

by light emitted from different depths in the screen [24]; (ii)the statistical variations in the transport of light from thescreen to the film [25]; and (iii) the total system noise

including that from the film. These corrections are derivedfollowing the methods of Nishikawa and Yaffe [26]. Amore detailed description of the implementation is givenby Sandborg et al [8].

Calculation of entrance air kermaThe Monte Carlo program calculates air kerma, without

backscatter, at the entrance surface of the phantom, airkerma at the surface of the cassette front and energyimparted to the screen per unit area. The entrance airkerma for a fixed OD can be calculated using thesequantities combined with the H and D curve measured interms of the cassette entrance air kerma. The calculationwas implemented in two parts.

In the first part, the experimental set-up used to measurethe H and D curve was simulated and the air kerma at thesurface of the cassette front and the energy imparted to thescreen per unit area calculated. In this way, the H and Dcurve was expressed as the OD for a given value of theenergy imparted to the screen per unit area.

In the second part, the voxel phantom in the lumbarspine imaging system under investigation was simulated.Ratios of energy imparted to the screen per unit area tothe incident air kerma at the phantom were calculated forapproximately 200 evenly spaced points of interest acrossthe whole image and the median ratio found. The cali-brated H and D curve was used to convert an OD to be

used as a normalization point, for example, the medianOD of a radiograph or set of radiographs, to an energyimparted per unit area. The entrance air kerma was then

calculated by this value of the energy imparted divided bythe median value of the ratio.

Effective doseEffective dose has been used in this work to quantify the

radiation risk. The voxel phantom was segmented intoorgans each with known mass. The Monte Carlo codecalculated the energy imparted to each voxel associatedwith an organ. The organ dose was obtained by dividingthe sum of the energy imparted to all voxels of an organwith the mass of that organ. The effective dose was thenfound by combining the organ doses with the tissueweighting factors according to the InternationalCommission on Radiological Protection [14]. The MonteCarlo code calculates the ratio of the effective dose tothe incident air kerma at the voxel phantom surface. Theeffective dose for a given situation was found from the

product of this ratio and the incident air kerma (seeprevious section).

Validation of the modelThe Monte Carlo program has been validated in two

parts. Firstly, Monte Carlo calculations of OD behindpolymethyl methacrylate (PMMA) phantoms were com-pared with measurements carried out under carefullycontrolled conditions. Good agreement, within 13% wasfound providing that there was detailed knowledge of theimaging system [6, 16]. Secondly, patient images werecollected and the entrance air kerma measured for chestand lumbar spine examinations in both frontal and lateral

projections. The images were digitized and analysed. Mea-surements of contrast were extracted from the digitizedradiographs. For the lumbar spine AP projection, it was

Optimization of lumbar spine X-ray imaging systems



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found that the calculated entrance air kerma was slightlylower than the minimum value in the range of measuredentrance air kermas. This was due to the voxel phantombeing slightly thinner than required. However, as thecalculated entrance air kermas were within the range ofmeasured values for the other projections, it was decidednot to increase the thickness of the voxel phantom for the

lumbar spine AP projection as the calculated value wasstill reasonably representative of the range of calculatedvalues. The program was also successfully validated by

comparing the calculated contrast of important ana-tomical details and the calculated dynamic range of theimage with the range of measured values [6, 16]. The voxelphantom was thus found to be sufficiently representativeof a patient undergoing both chest and lumbar spine X-rayexaminations.

Reference system and optimizationIn order to optimize the parameters used in X-ray

imaging systems, one system had to be identified that

provided good image quality, and this was designated asthe reference system. Thus, we determined a suitablereference system to be the imaging system that producedimages with the highest image quality as judged by anexpert panel of European radiologists in a recent clinicaltrial [4]. These preferred images were thus the referenceimages. Table 1 shows the characteristics of the reference

imaging systems for the AP and LAT views. The refer-ence systems used 72 kV with a 400 speed class screenfilmsystem for the AP view and 77 kV with a 600 speed class

screenfilm system for the LAT view. The preferred systemfrom the clinical trial corresponds to the good radio-graphic practice outlined in the CEC image criteria docu-

ment [3], except that a lower tube voltage was used thansuggested by the guidelines.In our theoretical study, we have investigated what

happens to the image quality and the patient dose if theimaging parameters are varied from their reference values.The range of the parameters studied is also given inTable 1. This study allows a greater understanding of theoptimization results.

A good quality image may be one that fulfils itsdiagnostic purpose but may not always be an image withthe highest possible contrast or SNR [27]. In ouroptimization scheme, it was decided to use the bestsystems from the clinical trials as the reference systems andthe images they produce as the reference images. It was

assumed that an image for which the contrast or SNR

were 10% lower than those in the reference image wouldstill fulfil its diagnostic purpose. Values of SNRI and DODwere calculated for each detail for a specified scatter-

rejection technique, speed class, OD and film type for tubevoltages between 60 kV and 110 kV in the AP view andbetween 70 kV and 110 kV in the LAT view. The tubevoltages required to give 0.9 of the appropriate SNRI andDOD value for each detail were then deduced. The detailrequiring the lowest tube voltage is referred to as thelimiting detail. This tube voltage is the highest employablethat ensures all details fulfil the criterion of the associatedimage quality measure being greater than or equal to 0.9of that for the reference system. The effective dose iscalculated for this limiting tube voltage and compared

with the values for the reference system. The procedure isthen repeated for different imaging systems and the system

resulting in the lowest effective dose is the optimum.

Results

Effect of varying image system parameters on patient

dose and image quality

Tube voltageFigure 2 shows the results for the AP projection of

varying the tube voltage between 60 kV and 110 kV on (a)

the effective dose, (b) the contrast of the L5 transverseprocess and (c) the SNR of a trabecular structure on theL1 vertebra. The reference system gives an incident airkerma without backscatter of 0.88 mGy and an effectivedose of 0.12 mSv. The calculated incident air kerma iswithin the range of entrance surface doses given in Hartet al [28]. The calculated effective dose is lower than wouldbe expected for example, from the effective doses given in

Table 1. The parameters for the anteroposterior (AP) and lateral reference imaging systems. The range of imaging system parametersis also given

AP imaging systems Lateral imaging systems

Parameters Range Reference system Range Reference system

Tube voltage 60110 kV 72 kV 70110 kV 77 kV

Filtration 4.7 mmAl 4.7 mmAl 4.7 mmAl 4.7 mmAl

Focal spot size 0.9 mm 0.9 mm 0.9 mm 0.9 mm

Focusfilm distance 1.46 m 1.46 m 1.46 m 1.46 m

Grid ratio 816 10 816 10

Strip density 4070 cm21 52 cm21 4070 cm21 52 cm21

Strip width 2040 mm 36 mm 2040 mm 36 mm

Cover material Aluminium and carbon fibre Aluminium Aluminium and carbon fibre Aluminium

Interspace material Aluminium and carbon fibre Carbon fibre Aluminium and carbon fibre Carbon fibre

Speed class 320, 400 and 600 400 320, 400 and 600 600

Screen material Gd2O2S Gd2O2S Gd2O2S Gd2O2S

Screen types Lanex Medium/Regular, Lanex

Regular Plus and Lanex Fast

Lanex Regular Plus Lanex Medium/Regular, Lanex

Regular Plus and Lanex Fast

Lanex Fast

Film type aKodak TML aKodak TML aKodak TML aKodak TML

Median OD 0.23.0 1.36 0.23.0 1.36

OD, optical density. aEastman Kodak Campany, Rochester, NY.




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Hughes [29], owing to the voxel phantom thickness being

slightly thinner than is required. However, this will notaffect the results as they are quoted relative to thereference system values in this paper. The effective dosedecreases by 73% between 60 kV and 110 kV. The threetransverse processes show approximately the same varia-tion of contrast with tube voltage. The same applies to theSNR for the three trabecular structures. For example, thecontrast of the L5 transverse process decreases by 54%between 60 kV and 110 kV, with a similar decrease inSNR of 58% for the L5 trabecular structure.

Figure 2 also shows the results for the LAT projectionof varying the tube voltage between 70 kV and 110 kV on

(d) the effective dose, (e) the contrast of the L3 spinousprocess and (f) the SNR of a trabecular structure on thefront of the L1 vertebra. The reference system gives anincident air kerma of 2.57 mGy and an effective dose of0.14 mSv. Again, the calculated incident air kermacompares well with Hart et al [28] and the calculated

effective dose is lower than expected [29] due to thethickness of the voxel phantom. The effective dosedecreases by 59% between 70 kV and 110 kV, which is a

smaller decrease than for the AP view owing to the smallervoltage range. The SNR and contrast show a similarvariation with tube voltage. There is a 47% decrease inthe SNR of a trabecular structure on the L5 vertebra

and a 43% decrease in the contrast of a L5 spinous pro-cess between 70 kV and 110 kV. The variation of thecontrast and SNR is less for the LAT projection than

the AP projection as a smaller range of tube voltages

was studied.

Grid designFigure 3 shows the results for the AP projection of

increasing the grid ratio (r5816) for three grids: (1) stripdensity N540 cm21, strip width d540 mm, aluminiumcovers and interspaces; (2) the same parameters exceptwith carbon fibre covers and interspaces; and (3)N570 cm21, d520 mm, carbon fibre covers and inter-spaces. The figure shows the variation of (a) effective dose,(b) contrast of the L3 transverse process and (c) the SNR

of the trabecular structure on the L3 vertebra. The resultsare shown relative to the reference system, which has agrid constructed with N552 cm21, r510, d536 mm withaluminium covers and carbon fibre interspaces. The effec-

tive dose increases for increasing grid ratio for all grids,for example, increasing by 34% for the aluminium gridbetween r58 and r516. The carbon fibre grids give thelowest effective dose. For the carbon fibre grid withN540 cm21, the effective dose is lower by 11% (r58)compared with the mixed material grid, and lower by 13%(r58) compared with the aluminium grid. There is afurther dose reduction by increasing the strip density anddecreasing the strip width. The effective dose for the

N570 cm21

, d520 mm grid is lower by 19% (r58) than forthe N540 cm21 carbon fibre grid.

There is a contrast and SNR advantage to using the

Figure 2. The effect of tube voltage on (a) effective dose, (b) optical density (OD) difference of the L5 transverse process and (c)signal-to-noise ratio (SNR) of a trabecular structure on the L1 vertebra for the anteroposterior projection. The effect of tube voltage

on (d) effective dose, (e) OD difference of the L3 spinous process and (f) SNR of a trabecular structure on the front of the L1

vertebra for the lateral projection.




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carbon fibre grids (r.8) rather than the mixed material oraluminium grids. There is a 13% increase in the L3

transverse process contrast and a 14% increase in the SNR

of the L3 trabecular structure for a grid with N540 cm21,

r516. The contrast and SNR advantage is less for

increasing strip density and decreasing strip width. There

is only a 5% increase in the L3 transverse process contrast

and a 4% increase in the L3 trabecular structure SNR for

a grid with N570 cm21, d520 mm, r516. The loss of

contrast and SNR for reducing the lead strip width is only

slightly compensated for by increasing the strip density.Figure 3 also shows the results for the LAT projection

for increasing grid ratio for the three grids mentioned

above. The results show the variation of (d) effective dose,(e) the contrast of the L3 spinous process and (f) the SNR

of the trabecular structure at an anterior position on the

L5 vertebra. The dose reductions obtained with a carbon

fibre grid are less for the LAT view than the AP view

owing to the higher tube voltage. The effective dose for

the N540 cm21 grid (r58) is 6% less than the reference

system. By increasing the strip density and decreasing the

strip width the dose is decreased by a further 14%.The contrast and SNR advantages from using carbon

fibre grids in the LAT view are generally the same or

smaller than for the AP view. The contrasts obtained using

the N540 cm21, r516 and N570 cm21, d520 mm, r516

grids are 8% and 5% greater than for the reference system.The corresponding increases in SNR for these grids

compared with the reference system are 14% and 3%,

respectively. For grids with low grid ratios where there isless contrast or SNR than the reference system, the tubevoltage does not need to be decreased significantly,especially if carbon fibre grids are used since the loss ofcontrast and SNR is small. For carbon fibre grids withhigh grid ratios, the tube voltage may be increased without

losing contrast or SNR and therefore, such a system mayhave a significantly reduced dose.

Screenfilm speedFigure 4 shows the results for the AP projection of

varying the speed class between 320 and 600 (all usingTML film) on (a) the effective dose, (b) the contrast of the

L3 transverse process and (c) the SNR of the trabecularstructure on the L3 vertebra. The results are shown at both72 kV and 90 kV. The effective dose decreases by 42% asthe speed class increases from 320 to 600 for both 72 kV

and 90 kV X-rays. At 72 kV, the contrast of the L3transverse process is near its maximum value for the 400speed class system. The contrast decreases by 10% and 3%when the 400 speed class system is replaced by a 320 and600 speed class systems, respectively. This is due todifferences in the shape of the H and D curves for thedifferent screenfilm combinations. At 72 kV, the SNR ofthe trabecular structure varies by a greater amount thanthe contrast. The SNR decreases by 19% for increasing the

speed class from 320 to 600. Similar variations of contrastand SNR are observed at 90 kV. If a 600 speed classsystem is used instead of a 400 speed class system, the tube

Figure 3. The effect of two grids with strip density N540 cm21 with aluminium and carbon fibre covers and interspaces and a thirdgrid with a strip density N570 cm21, strip width d520 mm with carbon fibre covers and interspaces on (a) effective dose, (b) optical

density (OD) difference of the L3 transverse process and (c) signal-to-noise ratio (SNR) of the trabecular structure on the L3 verte-

bra for the anteroposterior projection. The effect of the same grids on (d) effective dose, (e) OD difference of the L3 spinous process

and (f) SNR of the trabecular structure at an anterior position on the L5 vertebra for the lateral projection.




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voltage has to be decreased slightly to regain the loss of

contrast but significantly more to regain the loss of SNR.However, large dose reductions are still possible due tothe greater sensitivity of the system, despite significantlylower tube voltages being required to maintain contrastand SNR.

Figure 4 also shows the results for the LAT projectionof varying the speed class between 320 and 600 on (d)effective dose, (e) contrast of the L3 spinous process and(f) the SNR of the trabecular structure at an anteriorposition on the L5 vertebra. The effective dose decreases

by 43% with increasing speed class from 320 to 600 forboth 77 kV and 95 kV. At 77 kV, the contrast of the L3spinous process is lower by 10% and the SNR of the

trabecular structure is higher by 23% for the 320 speed

class system. There are similar results at 95 kV.

Optical densityFigure 5 shows the results for the AP projection of

varying the value of the median OD between 0.2 and 3.0using the Lanex Regular (Eastman Kodak Campany,Rochester, NY) screen with TML film on (a) effectivedose, (b) the contrast of the L1, L3 and L5 transverseprocesses and (c) the SNR of the trabecular structures onthe L1, L3 and L5 vertebrae. The effective dose increaseslinearly with OD between 0.4 and 1.6. There is a rapidincrease in effective dose above an OD of 1.6 due tothe shape of the TML H and D curve. The effec-

tive dose is 22% greater at a median OD of 1.6 than atthe median OD of 1.36 used in the reference system. Thetransverse processes have a maximum contrast at different

median ODs due to the differing OD beside each

anatomical detail and, therefore, their position onthe H and D curve. The L1, L3 and L5 transverse

processes have maximum contrasts at ODs of 1.6, 1.4

and 1.2, respectively. The contrast of the L3 process

at an OD of 1.6 is very similar to that at 1.36. The

trabecular structures also have a maximum SNR atdifferent median ODs. The details on the L1, L3 and

L5 vertebrae have maximum SNRs at ODs of 2.6, 2.4

and 2.2, respectively. The maximum SNR values are

47%, 33% and 20% greater than the SNR values for the

L1, L3 and L5 trabecular structures using the reference

system.Figure 5 also shows the results for the LAT projection

of varying the median OD between 0.4 and 3.0 on (d)

effective dose, (e) the contrast of the L1, L3 and L5spinous processes and (f) the SNR of the trabecular

structures on the L1 and L5 vertebrae. The effective dose

shows the same variation as for the AP projection with a

23% increase at a median OD of 1.6 compared with the

effective dose at a median OD of 1.36. The maximum

contrast values occur at an OD of 1.0 for the L1 and L3processes and at an OD of 1.8 for the L5 process. These

maximum contrasts are at most 8% greater than the

contrast of the details obtained with the reference system.

The maximum SNR values occur at an OD of 2.0 for the

details on the anterior position of the L1 and L5 vertebra

and at an OD of 2.6 for the detail on the posterior posi-

tion on the L5 vertebra. The maximum SNR values are38%, 12% and 16% greater, respectively, than the SNR

values for the posterior positioned detail on the L5

Figure 4. The effect of screenfilm speed class on (a) effective dose, (b) optical density (OD) difference of the L3 tranverse processand (c) signal-to-noise ratio (SNR) of the trabecular structure on the L3 vertebra for the anteroposterior projection. The effect of

screenfilm sensitivity class on (d) effective dose, (e) OD difference of the L3 spinous process and (f) SNR of the trabecular structure

at an anterior position on the L5 vertebra for the lateral projection.




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vertebra and the anterior positioned details on the L5 andL1 vertebrae using the reference imaging system.

Results of optimization

Scatter rejection techniqueTable 2 shows the tube voltages for the six important

details which give 0.9 of the contrast and SNR values for

the lumbar spine AP reference system. These results are for

an imaging system using a grid with N540 cm21 and r58

and a Lanex Regular/TML screenfilm system (400 speed

class). The table shows that there are differences in the

voltage required for each detail. The lowest tube voltage is

found for the L1 trabecular structure. The imaging

requirement for this detail then limits the tube voltage

to be less than or equal to this value so that the image

quality criterion is met for all six details. The L1 trabecular

structure was found to be the limiting detail for this grid,

and for some of the other grids investigated (Table 3), as in

these cases the SNR for this detail had the largest response

to tube voltage. Thus for the grid under investigation, the

largest dose saving that can be achieved is 18%.Table 3 shows the optimization of different grid designs

using the Lanex Regular/TML screenfilm system (400speed class) for the AP projection. The highest tube

voltages that satisfy the image quality criterion and the

corresponding effective doses calculated with these systems

are compared with the reference system, which is also 400

speed class. All scatter-rejection techniques produce a dose

saving compared with the reference system except for the

grid with N570 cm21, r58 and d536 mm. The largest

dose saving is for a grid with N570 cm21, r516 and

d520 mm, which gives 22% lower effective dose than the

reference system. These dose reductions are partly owing

to the lower attenuation of the carbon fibre covers and

interspace of the grids studied compared with the mixed

material grid used in the reference system. The large dosereductions obtained for a large grid ratio are also owing

to the fact that the tube voltage has to be increased

Figure 5. The effect of median optical density (OD) on (a) effective dose, (b) OD difference and (c) signal-to-noise ratio (SNR)for the anteroposterior projection. The effect of median OD on (d) effective dose, (e) OD difference and (f) SNR for the lateral

projection.

Table 2. The tube voltages which produce 10% lower contrast(DOD, difference in optical density) or signal-to-noise ratio

(SNR) for the six anatomical details (see Important Contrast

Details section) than obtained with the lumbar spine anterior-

posterior reference system using an imaging system with a

N540 cm21, r58, d540 mm grid and a Lanex Regular/TML

screenfilm system (400 speed class). The corresponding effec-

tive dose relative to the value for the reference imaging system

(Eastman Kodak Campany, Rochester, NY) is also given. The

detail which limits the optimization, i.e. the one which requires

the lowest tube voltage, is written in bold italics

Detail Image quality

measure

Tube

voltage (kV)

Relative

effective dose

L5T DOD 78.3 0.76

L3T DOD 77.1 0.78

L1T DOD 77.0 0.78

L5D SNR 76.4 0.80

L3D SNR 75.8 0.81L1D SNR 75.5 0.82




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substantially in order to reduce the contrast and SNR toexactly match the image quality criterion. The oppositewas found for the chest AP projection [8] where gridswith a low grid ratio were found to be optimal. This wasdue to the increase in effective dose with increasing tube

voltage above 110 kV.Table 4 shows the optimization of different grid designs

using the Lanex Regular/TML screenfilm system for theLAT projection. It was found that there are no dosesavings for these grids compared with the reference system.This is due to the reference system using the more sensi-

tive Lanex Fast screenfilm system (600 speed class).Therefore, a compromise for a 400 speed class imagingsystem would be to use a N540 cm21 and r58 grid in boththe AP and LAT projections. This provides a smalloverall dose saving of 5% compared with the respectiveAP and LAT reference imaging systems.

Screenfilm speedTable 5 shows the results of the optimization of scatter-

rejection technique using the 600 speed class system for the

AP projection. The highest tube voltages for the 600 speedclass system are on average 5 kV less than the highest tubevoltages for the 400 speed class system (Table 3). The tubevoltage is lower than for the 400 speed class system inorder to recover the reduction in SNR for the faster

system (Figure 4). Overall, the use of the faster screenfilmsystem results in greater dose savings than the 400 speedclass systems. For example, for the N540 cm21 and r516grid, the effective dose is 34% lower than for the referencesystem.

There is a similar effect on the LAT projection using a

faster screenfilm system. The highest tube voltage whichmeets the imaging requirements for the 600 speed class isabout 4 kV lower than the highest tube voltage for the 400speed class. The largest dose reductions are for the gridswith N540 cm21 with the effective dose values between12% and 15% smaller than the effective dose produced bythe reference imaging system.

Optical densityFigure 6 shows the optimization of the median OD,

ODmed in the AP projection. The median OD was variedbetween 80% and 150% of the reference system value of1.36. The system studied used a grid with N540 cm21 and

r512 and a Lanex Regular/TML screenfilm system.Figure 6a shows the variation of the highest tube voltagethat fulfills the image quality requirement as a functionof ODmed. The corresponding limiting detail and image

quality parameter type are shown for each data point. Thehighest tube voltage increases with increasing ODmed untila maximum value of 81 kV is reached at an ODmed of 1.36and then decreases. Below an ODmed of 1.36, the contrastand SNR of each detail all increase with increasing ODmed.The limiting detail is the trabecular structure on the L1vertebra as its SNR has the largest response with ODmed.Above an ODmed of 1.36, the contrasts of the L5 and L3transverse processes decrease with increasing ODmed.

Therefore, the tube voltage has to be decreased in orderto recover the reduced contrast to meet the required imagequality criterion. The L5 transverse process is the limiting

Table 4. The best tube voltages and the corresponding valuesof the relative effective dose for each grid studied with a Lanex

Regular/TML screenfilm system (Eastman Kodak Campany,

Rochester, NY) (400 speed class) for the lateral projection. Thelimiting detail (see Important Contrast Details section) and the

image quality measure (difference in optical density (DOD) or

signal-to-noise ratio (SNR)) are also given. The system which

gives the lowest patient dose is written in bold italics

Scatter rejection

technique

N (cm21), r, d (mm)

Detail Image

quality

measure

Best tube

voltage

(kV)

Relative

effective

dose

40,08,40 L1S DOD 84 1.09

40,12,40 L3S DOD 90 1.12

40,16,40 L1S DOD 90 1.22

70,08,36 L5B SNR 82 1.18

70,12,36 L3S DOD 87 1.24

70,16,36 L1S DOD 90 1.26

N, strip density; r, grid ratio; d, lead strip width.

Table 5. The best tube voltages and the corresponding values ofthe relative effective dose for each grid studied with a Lanex

Fast/TML screenfilm system (Eastman Kodak Campany,

Rochester, NY) (600 speed class) for the anterior-posterior projec-

tion. The limiting detail (see Important Contrast Details section)

and the image quality measure (difference in optical density

(DOD) or signal-to-noise ratio (SNR)) are also given. The system

which gives the lowest patient dose is written in bold italics

Scatter rejection

technique

N (cm21), r, d (mm)

Detail Image

quality

measure

Best tube

voltage

(kV)

Relative

effective

dose

40,08,40 L1D SNR 71 0.74

40,12,40 L3D SNR 76 0.67

40,16,40 L5D SNR 79 0.66

70,08,36 L3D SNR 68 0.89

70,12,36 L3D SNR 74 0.78

70,16,36 L5D SNR 77 0.74

70,08,20 L1D SNR 65 0.83

70,12,20 L1D SNR 70 0.73

70,16,20 L3D SNR 74 0.67


Table 3. The best tube voltages and the corresponding values ofthe relative effective dose for each grid studied with a Lanex

Regular/TML screenfilm system (Eastman Kodak Campany,

Rochester, NY) (400 speed class) for the anterior-posterior projec-

tion. The limiting detail (see Important Contrast Details section)

and the image quality measure (difference in optical density

(DOD) or signal-to-noise ratio (SNR)) are also given. The systems

which give the lowest patient dose are written in bold italics

Scatter rejection

technique

N (cm21), r, d (mm)

Detail Image

quality

measure

Best tube

voltage

(kV)

Relative

effective

dose

40,08,40 L1D SNR 76 0.82

40,12,40 L1D SNR 81 0.80

40,16,40 L5T DOD 85 0.79

70,08,36 L3D SNR 72 1.02

70,12,36 L1D SNR 79 0.89

70,16,36 L5D SNR 82 0.86

70,08,20 L1D SNR 69 0.92

70,12,20 L1D SNR 57 0.82

70,16,20 L1D SNR 79 0.78





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detail as its contrast has the largest decrease withincreasing ODmed. Figure 6b shows that there are dosesavings below an ODmed of 1.50 with the effective dosebeing 25% lower than the value for the reference system at

an ODmed of 1.09.There is a similar variation with ODmed for the lateral

projection. The system used is a 40 cm21, r512 grid with aLanex Regular/TML screenfilm system. Figure 6c showsthat the highest tube voltage reaches a maximum value of90 kV at an ODmed of 1.22. Figure 6d shows that there is aminimum dose at an ODmed of 1.09 with the effective dosebeing 7% lower than reference system value. The highesttube voltage is lower at an ODmed of 1.09 than at 1.36 in

order to recover the lower SNR as ODmed decreases(Figure 5). There is a small dose saving due to using thecarbon fibre grid rather than the mixed material grid.

Discussion

Our work on the optimization of the scatter rejection

technique has shown that the tube voltage could be

decreased or increased in order to produce a dose reduc-

tion depending on the grid design. Vano et al [5] increased

the tube voltage from 60 kV to 90 kV to produce a dose

reduction of 35% whilst maintaining image quality for the

lumbar spine AP examination. However, Almen et al [4]

have shown that increasing the tube voltage from 70 kV

to 90 kV significantly alters the image quality of AP

films, whereas increasing from 77 kV to 95 kV does not

significantly alter the image quality of LAT films. In our

optimization studies, the tube voltages that fulfilled theimage quality criterion were less than 85 kV for the AP

films and 90 kV for the LAT films.

Figure 6. The optimization of median optical density with (a) the highest tube voltage consistent with the requirement to obtain atleast 90% of the image quality of the reference system for all details considered and (b) the corresponding values of the effective

dose relative to the reference system values for the anterior-posterior view. The optimization of median optical density with (c) the

highest tube voltage and (d) the corresponding effective dose for the lateral view.




11/12

Further evidence that our work closely corresponds tothat of Almen et al is given by Sandborg et al [30].Sandborg showed that the physical parameters such ascontrast and SNR could be used to predict the order thatthe imaging systems were ranked by the European radiol-

ogists [4]. For example, in the AP projection Almen et alfound significant differences in image quality for changing

tube voltage, but not for changing speed class. This canalso be demonstrated from our study of changing theimage parameters and observing their effect on calculatedimage quality. By increasing the tube voltage from 70 kV

to 90 kV, a large decrease of 28% was observed in thecalculated contrast and SNR whereas only a smalldecrease of 10% was seen in the SNR for increasing thespeed class from 400 to 600. It is therefore reassuringthat the work in this paper is consistent with changes inimage quality observed clinically.

Conclusions

The results of varying the different imaging parametersshows how straightforward it is to have high image qualityand high patient dose, e.g. low tube voltage and to have

low image quality and low patient dose, e.g. high tubevoltage. Conversely, it is difficult to balance high imagequality and low patient dose. The optimization of radio-graphic imaging involves several different parameters.Therefore, it is very useful that a Monte Carlo model canbe used to point out imaging systems that give low patientdose whilst still maintaining the same image quality asreference systems. These systems are worth investigation infuture, more time-consuming, clinical trials.

For 400 speed class systems using grids in the APprojection, a dose reduction of between 8% and 22% canbe achieved. A further dose reduction of 13% is possiblewith a 600 speed class system using a grid. Table 6 shows

the imaging system configuration that produced the largestdose reduction in our work. Dose reductions of a similarsize can be obtained for a grid with a high grid ratio(r516), a high strip density (N570 cm

21) and a small leadstrip width (d520 mm).

For 400 speed class systems using grids in the LATprojection, only a small dose reduction of 7% could beachieved by reducing the operating OD from 1.36 to 1.09.The largest dose reduction of 15% was obtained using the

600 speed class screenfilm system shown in Table 6. Thescope for large dose reductions in the LAT projection wasrestricted as a 600 speed class screenfilm system was used

as the reference system. For both AP and LAT projec-tions, the dose advantage of using carbon fibre compo-nents has been shown throughout this work as thereference system used a grid constructed from aluminiumand carbon fibre.

Our work clearly shows that the largest dose reductionsare for 600 speed class systems. However, in a recent

review [28] of patient doses from screenfilm imaging inthe UK for the year 2000, the National RadiologicalProtection Board (NRPB) shows that there are signifi-cantly fewer 600 speed class systems in use compared with

400 speed class systems. The review [28] also shows thecontinuing trend for lower dose per lumbar spine radio-graph of 37% in the period from 1984 to 1995 and 18% inthe period from 1995 to 2000. The NRPB state that this isdue to the increasing use of faster screenfilm combina-tions. Therefore, our work highlights that there are stillpotential optimizations to be made in lumbar spineradiography. It is also reassuring to know that the systems

found by the optimizations are similar to those recom-mended by the CEC guidelines [3] as given in Table 6.

Acknowledgment

Dr F R Verdun (Lausanne, Switzerland) is thanked forsupplying measured H and D curves, modulation transferfunction and noise power spectra of the screenfilmcombinations used in this work. Alexandr Malusek isacknowledged for the image of the voxel phantom inFigure 1.

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Parameters AP imaging systems Lateral imaging systems

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Filtration 3.0 mmAl 4.7 mmAl 3.0 mmAl 4.7 mmAlFocal spot size ,1.3 mm 0.9 mm ,1.3 mm 0.9 mm

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A Study and Optimization of Lumbar Spine X-Ray Imaging