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DOI: 10.1148/radiol.2442060606
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(Radiology 2007;244:524-531.)
© RSNA, 2007

Musculoskeletal Imaging

Multidetector CT in Patients Suspected of Having Lumbar Disk Herniation: Comparison of Standard-Dose and Simulated Low-Dose Techniques1

Pascale Bohy, MD, Viviane de Maertelaer, PhD, Aymeric Roquigny, MD, Caroline Keyzer, MD, Denis Tack, MD, PhD, and Pierre Alain Gevenois, MD, PhD

1 From the Department of Radiology, Hôpital Erasme, Université Libre de Bruxelles, Route de Lennik 808, 1070 Brussels, Belgium (P.B., P.A.G.); Department of Radiology, Réseau Hospitalier de Médecine Sociale, Baudour, Belgium (A.R., D.T.); Department of Radiology, Centre Hospitalier Universitaire de Charleroi, Charleroi, Belgium (C.K.); and Statistical Unit, Institut de Recherche Interdisciplinaire en Biologie Humaine et Moléculaire, Université Libre de Bruxelles, Brussels, Belgium (V.d.M.). Received April 5, 2006; revision requested June 5; revision received July 4; accepted August 2; final version accepted December 7. Address correspondence to P.A.G. (e-mail: Pierre.alain.gevenois{at}ulb.ac.be).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Purpose: To compare standard-dose and simulated low-dose multidetector computed tomography (CT) in patients suspected of having lumbar disk herniation.

Materials and Methods: The institutional review board approved the research protocol with a waiver of patient informed consent. Sixty consecutive patients underwent multidetector CT with four detector rows at 1 mm collimation at 140 kVp, with tube current–time product adapted to body mass index (BMI): 200 (BMI< 22 kg/m2), 300 (BMI ≥22 to <30 kg/m2), and 400 effective mAs (BMI ≥30 kg/m2). Simulated doses at 65%, 50%, 35%, and 20% of the dose were used for acquisition. During two separate sessions, three independent radiologists coded each of three caudal disks as normal, bulging, or herniated and graded canal and foramen compromise. Median numbers of discrepancies between the standard and reduced doses were compared with Friedman and Wilcoxon tests. Agreements within and between readers were evaluated through {kappa} statistics.

Results: Dose reduction had no effect on a reader's ability to identify bulging disks (P = .128) and left and right foramen compromises (P = .413 and .665, respectively). However, for normal disks (P = .002), herniated disks (P = .004), and canal compromise (P = .002), dose reduction did have a significant effect. For normal disks and canal compromise, a reduction dose effect was not detected at 65% (P = .121 and .250, respectively) but appeared at 50% (P = .004 and .008, respectively). For herniation, a dose reduction effect was detected at 35% (P = .031). Agreements within and between readers ranged from poor to excellent and tended to decrease with dose reduction.

Conclusion: For patients suspected of having lumbar disk herniation, tube charge settings could be reduced to 65% of the standard dose adapted to the BMI.

© RSNA, 2007


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Sciatica affects up to 40% of people living in Western countries (1). It is a major cause of work-related disability and has a high cost to society. The prevalence of sciatica in the working population with disabling symptoms for at least 1 day per year is approximately 20% (2,3), and its recurrence rate is 10% in men and 15% in women (4).

As lumbar disk herniation is the most common cause of sciatica (5), cross-sectional imaging, either with computed tomography (CT) or magnetic resonance (MR) imaging, is routinely performed to rule out or localize herniation before surgical intervention. Both imaging techniques have a similar diagnostic performance, with a sensitivity of 88%–94% and a specificity of 57%–88% (6–9).

Despite requiring ionizing radiation, CT is frequently the first technique utilized as it is more accessible and less expensive than MR. As sciatica may affect young individuals (most patients are aged 20–50 years) (10), with possibly frequent recurrences, radiation dose should be as low as possible. The purpose of our study, therefore, was to compare standard-dose and simulated low-dose multi–detector row CT in patients suspected of having lumbar disk herniation.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Patients
The institutional review board approved our research protocol and agreed to a waiver of patient informed consent since this study was based on manipulating existing data. There was neither additional interaction with nor additional radiation exposure to patients.

Consecutive patients who had been referred to the radiology department of a community hospital (Réseau Hospitalier de Médecine Sociale, Baudour, Belgium) between May and September 2004 and who met the following criteria were included: those more than 18 years old presenting with low back pain and sciatic nerve radiation in whom a CT examination was requested by the physician to evaluate for lumbar disk herniation. Patients with prior lumbar surgery, trauma, known anatomic variants of the lumbosacral spine, and pregnancy were not eligible. Five patients with anatomic variants of the lumbosacral spine detected at CT were excluded. The study group consisted of 60 patients (31 women, 29 men; mean age, 47 years; range, 23–80 years). For women, the mean age was 49 years (range, 25–80 years); for men, it was 47 years (range, 23–68 years). Mean body mass index (BMI, calculated as the patient's weight, in kilograms, divided by the square of his or her height, in meters) (11), available in the medical chart was 27.2 kg/m2 ± 5.2 (standard deviation) (range, 18.9–38.6 kg/m2). Patients were categorized into three groups depending on BMI: group 1 included eight patients (six women, two men) with a BMI less than 22 kg/m2; group 2 included 37 patients (18 women, 19 men) with a BMI between 22 and 30 kg/m2; and group 3 included 15 patients (seven women, eight men) with a BMI of 30 kg/m2 or greater.

CT Examination
CT examination was performed with a commercially available four–detector row scanner (Somatom Volume Zoom; Siemens Medical Solutions, Forchheim, Germany). Patients were examined in supine position with both arms lying overhead. A lateral 26-cm scout view was obtained at 140 kVp and 100 mA, followed by a standard-dose CT acquisition in the craniocaudal direction from the pedicles of the third lumbar vertebra to the laminae of the first sacral vertebra. Tube voltage was set at 140 kVp and the collimation was set at 4 mm (1 mm x four detector rows). At the time of this study, no software for automated exposure control was available on our CT unit. In our clinical practice of lumbar spine CT, we take into account differences in dose requirements according to the patient's body habitus (12).

In order to maintain an approximately constant image noise among patients, we adapted the tube current–time product as recommended by Boone et al (12) to the following: 200 effective mAs for group 1, 300 effective mAs for group 2, and 400 effective mAs for group 3. Table feed was 3.0 mm per 0.75-second scanner rotation. These parameters result in a pitch factor of 0.75:1. As defined by Manesh et al (13), effective tube charge corresponds to tube charge divided by the pitch; the pitch is defined by Silverman et al (14) as the ratio between the table feed per rotation and the x-ray beam width. To determine these dose levels, we have considered a standardized body as represented by the Monte Carlo model (weight, 70 kg; height, 1.70 m; BMI, 24.2 kg/m2). The standard dose corresponded to a volume (CTDIvol) of 40 mGy and a weighted CTDI (CTDIw) of 30 mGy (where CTDIw is defined as CTDIvol divided by the pitch). This standard dose was very close to the recommended reference level for body CT (15).

Simulated Low-Dose Techniques
We used a computer-assisted method as described by Sennst et al (16) (Syngo Platform; Siemens Medical Solutions) to generate images simulating reduced doses at 65%, 50%, 35%, and 20% of the tube charge presets used for acquisition. This process involved superimposing computer-calculated noise on the original scan raw data. The amount of noise was increased stepwise, where the magnitude of increase was proportional to the square root of the measured x-ray attenuation for each detector channel (17). A total of 300 raw data sets (60 original and 240 simulated data sets) were reconstructed with 2-mm-thick sections at 1-mm increments and a soft-tissue algorithm (Table).


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  		Effective Milliampere-Seconds for Three Patient Groups

 
Validation of Simulated Low-Dose Techniques
We have tested the simulated low-dose techniques on an Alderson anthropomorphic phantom (RSD, Long Beach, Calif) representing a standard patient (weight, 70 kg; height, 1.7 m; BMI, 24.2 kg/m2) by comparing the amount of noise on scans obtained with actual low-dose techniques with those of simulated low-dose techniques at the five effective tube charge levels used in group 2 (field of view, 12 cm; matrix, 512 x 512) (Table). The amount of noise was measured on sections obtained with constant z coordinate at the level of an intervertebral disk. On these sections, circular regions of interest were drawn by a radiologist with 17 years of experience reading CT images (D.T.) with constant x- and y coordinates on the intervertebral disk, dural sac, bone marrow, and cortical bone. Depending on the size of these structures, the surface area of regions of interest ranged from 20 to 400 mm2. For each structure, the amount of noise was given by standard deviation of the mean attenuation value within the region of interest. Five measurements were obtained at each dose and on each structure.

In order to detect a possible dose effect on noise, standard deviation was submitted to an analysis of variance procedure with two repeated factors, radiation dose, at five levels (100%, 65%, 50%, 35%, and 20%), and acquired versus simulated scans, at two levels, plus the interaction between factors. Statistical significance for all tests was set at a P value of less than .05. The statistical software used was SPSS for Windows (version 13.0; SPSS, Chicago, Ill). Analysis of variance showed that no significant difference in noise was detectable between acquired and simulated images (P = .121–.921) but that significant differences were detectable between dose levels for all four investigated phantom tissues (P = <.001–.002) for both acquired and simulated CT images. This phantom study demonstrated that simulated low-dose CT techniques increase the amount of noise in a way not different from that of actual low-dose acquisitions.

Image Analysis
Patient information and simulated tube charge presets were erased from images that were randomly renumbered by using random tables from Fisher and Yates (18). Images were sent to a picture archiving and communication system (TM High End; Telemis, Louvain-la-Neuve, Belgium) and loaded on a clinical workstation with three-dimensional functionalities comprising two 19-inch (48-cm) screens with 1.3-megapixel resolution. Image analysis was performed by three independent readers, in two independent, separate reading sessions. To eliminate possible memorization by a reader of images from one session to another, we separated reading sessions by at least 2 months. Reader 1 (D.T.) was a radiologist with 17 years experience reading CT, reader 2 (A.R.) was a 1st-year radiology resident, and reader 3 (C.K.) was a radiologist with 7 years experience reading CT. None of these readers were involved in selecting patients or in conducting the CT examinations. Readers were blinded to all patient identifiers and to the rate of simulated dose reduction applied. They only knew that the patients were complaining of low back pain with sciatic nerve radiation, but they were unaware of where the pain occurred.

Using workstation functionalities, readers were asked to generate multiplanar reformation images of 2-mm thickness with 1-mm increment in the plane of the three caudal lumbar disks (L3-4, L4-5, and L5-S1) and in the sagittal plane in order to score each disk as normal, bulging, or herniated. Canal compromise and foramen compromise on each side were also recorded.

Before the first reading session, readers were provided with definitions of each condition according to the Combined Task Forces of the North American Spine Society, the American Society of Spine Radiology, and American Society of Neuroradiology (19,20). A disk was coded normal when there was no disk extension beyond the intervertebral spaces in the transverse plane. Bulging was defined as circumferential displacement of disk material beyond the outer edges of the vertebral ring apophyses, exclusive of osteophytic formations. The displacement was considered circumferential when more than 50% of the disk circumference was involved.

Herniation was defined in the transverse plane as localized displacement of disk material beyond the outer edges of the vertebral ring apophyses, exclusive of osteophytic formations. The displacement was considered localized when less than 50% of the disk circumference was involved (Fig 1).


Figure 1A
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  		Figure 1a: Transverse multidetector CT images of L5-S1 disk herniation in 34-year-old man with right-sided sciatica and 24.5 kg/m2 BMI. Images obtained at (a) standard dose and at simulated (b) 65%, (c) 50%, (d) 35%, and (e) 20% of the standard dose. Note dorsal deviation of right nerve root (arrow) caused by herniated disk material (arrowhead).

 

Figure 1B
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  		Figure 1b: Transverse multidetector CT images of L5-S1 disk herniation in 34-year-old man with right-sided sciatica and 24.5 kg/m2 BMI. Images obtained at (a) standard dose and at simulated (b) 65%, (c) 50%, (d) 35%, and (e) 20% of the standard dose. Note dorsal deviation of right nerve root (arrow) caused by herniated disk material (arrowhead).

 

Figure 1C
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  		Figure 1c: Transverse multidetector CT images of L5-S1 disk herniation in 34-year-old man with right-sided sciatica and 24.5 kg/m2 BMI. Images obtained at (a) standard dose and at simulated (b) 65%, (c) 50%, (d) 35%, and (e) 20% of the standard dose. Note dorsal deviation of right nerve root (arrow) caused by herniated disk material (arrowhead).

 

Figure 1D
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  		Figure 1d: Transverse multidetector CT images of L5-S1 disk herniation in 34-year-old man with right-sided sciatica and 24.5 kg/m2 BMI. Images obtained at (a) standard dose and at simulated (b) 65%, (c) 50%, (d) 35%, and (e) 20% of the standard dose. Note dorsal deviation of right nerve root (arrow) caused by herniated disk material (arrowhead).

 

Figure 1E
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  		Figure 1e: Transverse multidetector CT images of L5-S1 disk herniation in 34-year-old man with right-sided sciatica and 24.5 kg/m2 BMI. Images obtained at (a) standard dose and at simulated (b) 65%, (c) 50%, (d) 35%, and (e) 20% of the standard dose. Note dorsal deviation of right nerve root (arrow) caused by herniated disk material (arrowhead).

 
If present, canal compromise was graded in three categories according to severity and was evaluated by comparing the size of disk displacement at the site of the more severe compromise with the size of the spinal canal. When present, compromise of less than one-third, between one- and two-thirds, and larger than two-thirds of the spinal canal was respectively graded 1, 2, and 3. The same grading system was used for the foramen compromise on each side.

Two weeks before the first reading session, readers were familiarized with the definitions in a training session using standard-dose lumbar CT images obtained in 15 patients who were not included in our study sample.

Statistical Analysis
For each coded disk condition (normal, bulging, and herniated), agreements within and between readers were analyzed by calculating Cohen {kappa} statistics (21). Consistency (agreements) between readers was assessed for both reading sessions. The null hypothesis of no agreement between two readers was tested, and the associated P values were calculated (22). All {kappa} values were interpreted as proposed in the literature (23): A {kappa} value lower than 0.20 indicated poor agreement; 0.21–0.40, fair agreement; 0.41–0.60, moderate agreement; 0.61–0.80, good agreement; and 0.81–1.00, excellent agreement.

Because a definite diagnosis could not be obtained with an independent standard of reference and because standard-dose multidetector CT is not an a priori reference standard, the present study was based on comparing discrepancies among and between readers with discrepancies between the radiation doses. Investigating the effect of dose reductions incurs two variables: the direct effect on the scan itself of a reduced dose and the "human variable " of the reader. To devise a reference point, we used the results from one session to interpret results from other reading sessions. In addition, the more images a reader reviews, the more adept he or she becomes in correctly interpreting low-dose simulated images. In clinical practice, however, there is one reading only. To account for this, we investigated discrepancies between simulated reduced doses during the first reading session and the standard dose of the second reading session; the latter was considered to be our reference reading.

For each coded item (normal, bulging, or herniated disk), the Friedman test was performed to compare median numbers of discrepancies between the standard dose at the second reading session with all reduced doses at the first reading session. In the case of statistically significant discrepancies, the Wilcoxon test for paired data was then performed to detect which doses were statistically different.

Before performing the same tests for further analysis of canal and foramen compromises, we divided all 60 patients into two categories, no compromise and compromise (regardless of severity).

P values less than .05 were considered statistically significant except in the case of multiple comparisons, where the level was adapted due to a Bonferroni adjustment. The statistical software package used was SPSS for Windows (release 12.0; SPSS, Chicago, Ill).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
A total of 5400 disks were analyzed in this study (60 patients x three disks x five dose settings x three readers x two reading sessions).

Agreements within and between Readers
Agreements within and between readers both tended to decrease with a decreasing dose (Figs 2–7).


Figure 2
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  		Figure 2: Graph shows agreements within readers for normal disk at the five doses: {blacktriangleup} = disk L3-4, bullet = disk L4-5, {blacksquare} = disk L5-S1. {kappa} values for agreement: reader 1 (black), reader 2 (white), and reader 3 (gray).

 

Figure 3
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  		Figure 3: Graph shows agreements within readers for bulging disk. See Figure 2 for definition of keys.

 

Figure 4
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  		Figure 4: Graph shows agreements within readers for herniated disk. See Figure 2 for definition of keys.

 

Figure 5
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  		Figure 5: Graph shows agreements between readers for normal disk at the five dose settings. {blacktriangleup} = disk L3-4, bullet = disk L4-5, {blacksquare} = disk L5-S1. {kappa} values for agreement: between readers 1 and 2 (black), between readers 2 and 3 (white), and between readers 1 and 3 (gray). Solid lines = first reading session, dashed lines = second reading session.

 

Figure 6
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  		Figure 6: Graph shows agreements between readers for bulging disk at the five doses. See Figure 5 for definition of keys.

 

Figure 7
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  		Figure 7: Graph shows agreements between readers for herniated disk at the five doses. See Figure 5 for definition of keys.

 
Dose Effect in Coding Normal, Bulging, and Herniated Disks
Differences in discrepancies between all simulated reduced doses at the first reading session and standard dose at the second reading session reached statistical significance for normal (P = .002) and herniated (P = .004) disks, but not for bulging (P = .128) disk. Statistically significant discrepancies appeared at 50% of the standard dose (P = .004) for normal disk and at 35% of the standard dose (P = .031) for herniated disk. Taking into account a Bonferroni adjustment for four comparisons, the level of statistical significance was lowered to .0125. This entailed no statistically significant difference in discrepancies between any reduced dose and the standard dose.

Dose Effect in Coding Canal and Foramen Compromises
Discrepancies between all simulated reduced doses at the first reading session and standard dose at the second reading session reached statistical significance for canal compromise (P = .002), but not for foramen compromises (left, P = .413; right, P = .665). For canal compromise, significant discrepancies appeared at 50% of the standard dose (P = .008).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 References
 
Our study shows that a reduced dose equivalent to 65% of the standard dose does not impair reader identification of any of the imaging findings evaluated. Further dose reduction begins to affect a reader's ability to consistently identify these findings. Indeed, an effect already began to appear at 50% and 35% of normal dose for normal and herniated disk identification, respectively. For bulging disk, no effect appears, even at 20% of the standard dose.

The effect of a dose reduction also differs according to lateral or medial displacement of the disk material. In fact, for foramen compromise, no dose effect is observed on either left or right side. However, for canal compromise, a dose effect does appear at 50% of the standard dose. These results are not surprising, as CT has proved less effective for the evaluation of the intrathecal nerve root than for the evaluation of the foramen nerve root (24). As compared with other conditions in which considerable dose reduction can be applied (eg, pulmonary nodule screening, virtual colonoscopy, renal colic, acute appendicitis, chronic sinusitis) (25–29), our study reveals a clear effect due to dose reduction. This observation could be explained by the huge x-ray absorption by vertebral bones. Despite a high natural contrast between disk material and surrounding fat, this contrast decreases proportionally with the dose reduction.

In reading lumbar spine CT images, it is as important to report normal as well as herniated disks. Therefore, we have considered a reduced dose acceptable only if it had no substantial effect on the reader's ability to correctly identify disk status, regardless of the condition. The acceptable dose reduction corresponds to 65% of the standard dose (130 effective mAs in patients with BMI < 22 kg/m2, 195 effective mAs in those with BMI ≥ 22 kg/m2 and < 30 kg/m2, and 260 effective mAs in those with BMI ≥ 30 kg/m2).

Our study also shows that in reading CT images of the lumbar spine (taking into account all readers, conditions, and doses) agreements within and between readers range from poor to excellent despite the use of standardized nomenclature. These results confirm previously reported data of inter- and intraobserver variability studies in CT and MR image interpretation (30–33). Despite efforts to minimize subjectivity and subsequent variability within and between readers by using precise definitions, studies using such nomenclature have reported only moderate interobserver agreement, even between experienced radiologists (30–33).

Moreover, our results indicate that dose reduction is associated with moderately decreased agreement between readers. At the lowest dose (20% of the standard dose), agreements between readers range from fair to moderate, from moderate to good, and from poor to good, respectively, for herniated, normal, and bulging disks. This moderately decreased agreement between readers affects all pairs of readers to a comparable extent, suggesting that it depends on the dose reduction rather than the reader's experience.

Our study had limitations. First, we did not evaluate the influence of dose reduction on bone structures. As our validation study showed slight increase in noise for bone structures, one can expect few differences in the evaluation of bone structures and facet joint diseases with low tube charge settings. In addition, alterations in bone structures often affect elderly patients, a population for which the risk from radiation is low because of reduced life expectancy and for which low-dose CT is not recommended.

Second, we have adapted acquisition parameters according to patient BMI by dividing them into three groups. We were, therefore, unable to investigate a possible effect of BMI. Since effective doses and subsequent risks related to irradiation are higher in thin patients than in obese ones, it appeared unethical to use identical tube charge settings for all patients, independent of their BMI (12,34). In order to standardize these risks as well as the image quality, we increased the tube current–time product setting stepwise with the BMI. If available, an automatic tube current modulation would have adapted to individual BMI (rather than to group BMI categories).

Third, we had no independent method of reference. Surgery is inappropriate for ethical reasons, and follow-up studies could also be inappropriate, as pain related to herniation may be self-limiting—recovery being reported within 2 months in up to 90% of patients after conservative treatment (35,36). Consequently, since a definite diagnosis cannot be independently obtained, we were unable to calculate and compare sensitivity, specificity, and predictive values of standard- versus low-dose multidetector CT.

Finally, our study included multiple comparisons, where some could have been significant by chance. Nevertheless, we decided to make no adjustment for multiple comparisons. Indeed, such adjustment might have decreased the threshold of statistical significance and then led us to not detecting any difference between readings at standard and lower doses. In order to avoid suggesting dramatic dose reduction with the subsequent risk of diagnostic errors, we preferred to make no such adjustments.

In conclusion, our results suggest that in multidetector CT of patients suspected of having disk herniation, the radiation dose could be reasonably reduced to 65% of the standard dose, adapted for patient BMI (130 effective mAs if BMI < 22 kg/m2, 195 effective mAs if BMI ≥22–<30 kg/m2, and 260 effective mAs if BMI > 30 kg/m2).


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
References
 


   ACKNOWLEDGMENTS
 
The authors thank A. Van Muylem, PhD, for figure preparation.


   FOOTNOTES
 

Abbreviations: BMI = body mass index

Authors stated no financial relationship to disclose.

Author contributions:Guarantors of integrity of entire study, P.B., P.A.G.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, P.B., A.R., D.T.; clinical studies, P.B., A.R., C.K., D.T., P.A.G.; statistical analysis, V.d.M.; and manuscript editing, P.B., V.d.M., C.K., D.T., P.A.G.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 

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