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A Study and Optimization of Lumbar Spine X-Ray Imaging
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Transcript of 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
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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.
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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
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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
N, strip density; r, grid ratio; d, lead strip width.
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
N, strip density; r, grid ratio; d, lead strip width.
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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.
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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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