Rationale and Objectives
The impact of varying image acquisition parameters on the precision of measurements using quantitative computed tomography is currently based on studies performed before the advent of helical image acquisition and multidetector-row scanners. The aim of this study was to evaluate helical multidetector-row quantitative computed tomography to determine the factors contributing to the overall precision of measurements on quantitative computed tomography conducted using current vintage computed tomographic (CT) scanners.
Materials and Methods
The effects of CT protocol parameters (x-ray tube voltage and current, pitch, gantry rotation speed, detector configuration, table height, and reconstruction algorithm) and short-term scanner variation were examined on two commercially available quantitative CT (QCT) systems (ie, a combination of reference phantoms and analysis software) using seven multidetector-row CT scanners (available from a single vendor) operated in helical mode. Combined with simulated patient repositioning using three ex vivo spine specimens, precision (coefficient of variation) estimates were made on the basis of three scenarios: “best case,” “routine case,” and “worst case.”
Results
The overall best-case QCT precision was 1.4%, provided that no changes were permitted to the bone mineral density (BMD) scan protocol. Routine-case examination (with a BMD reference phantom in place) that permitted some variation in the x-ray tube current and table speed produced a precision of 1.8%. Without any constraints on the clinical QCT examinations, the worst-case precision was estimated at 3.6%.
Conclusions
Although small in appearance, these errors are for single time points and may increase substantially when monitoring changes through QCT measurements over several time points. This calls for increased caution and attention to detail whenever using helical multidetector-row quantitative computed tomography for the assessment of BMD change.
Dual-energy x-ray absorptiometry (DXA) has become the standard clinical method of screening for osteoporosis ( ). For some patients and under certain circumstances, quantitative computed tomography is an attractive alternative to DXA and is often used to assess osteoporosis and metabolic bone disease ( ). Quantitative computed tomography can be helpful to place mineral mass value in a broader context, as with spinal metastases, fractures, or arthritis ( ). The calibration of computed tomographic (CT) images is not unique to quantitative computed tomography and may be required for other specific quantification tasks, such as the assessment of coronary artery calcification ( ), detection of the progression of emphysema ( ), the characterization of lung nodules ( ), and the development of finite element models for bone strength prediction ( ).
No systematic studies have yet investigated how acquisition parameters can influence the precision of quantitative CT (QCT) measurements conducted using modern helical multidetector-row CT (MDCT) scanners. Several investigators have shown DXA to be superior or equal to single-slice quantitative computed tomography in terms of accuracy and the precision of bone mineral density (BMD) measurements ( ). In addition to the basic technique parameters of x-ray tube voltage, current, and rotation speed, additional parameters that must be controlled for MDCT scanners include overall x-ray beam width, detector configuration (how detector elements are assigned to individual data channels), table speed, and reconstructed image thickness.
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Materials and methods
BMD Phantoms
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QCT Phantom Scan Acquisition
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QCT Phantom Image Data Analysis
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CT Scanner Models
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Helical MDCT Scan Parameter Variation
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Transverse Versus Helical Acquisition Mode (Using Three CT Scanners)
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Protocol Parameters (Using a Single CT Scanner)
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Table Height Variation (Using Three CT Scanners)
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Table 1
Proposed Clinical QCT Protocol on a 16-Channel Helical MDCT Scanner
Parameter Value X-ray tube potential (kVp) 120 X-ray tube current (mA) 230 Pitch (table travel per rotation/beam width) 0.938 Gantry rotation speed (s/rotation) 0.5 Image thickness (mm) 1.25 Display field of view (cm) 36 Detector configuration (channels × channel width [mm]) 16 × 0.625 Reconstruction algorithm Standard
MDCT, multidetector-row computed tomographic; QCT, quantitative computed tomographic.
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X-Ray Tube Temperature (Using a Single CT Scanner)
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Short-Term Scanner-to-Scanner Variation (Using Seven CT Scanners)
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Simulated Patient Repositioning Determination (Using a Single CT Scanner)
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Statistical Analysis
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Results
Transverse Versus Helical Acquisition Mode (Using Three MDCT Scanners)
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Protocol Parameters (Using a Single CT Scanner)
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Table Height Variation (Using Three CT Scanners)
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X-Ray Tube Temperature (Using a Single CT Scanner)
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Short-term Scanner-to-Scanner Variation (Using Seven CT Scanners)
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Table 2
Short-Term QCT Precision
Scanner QCT System LS1 LS2 LS3 LS4 LS5 LSP QX/i All Scanners Combined Image Analysis Mean ± SD (mg/cm 3 ) 98.5 ± 0.57 98.1 ± 0.63 98.2 ± 0.57 98.3 ± 0.60 98.3 ± 0.56 95.1 ± 0.48 94.7 ± 0.91 97.5 ± 1.57 CV (%) 0.58 0.64 0.58 0.61 0.57 0.51 0.96 1.56 Mindways Mean ± SD (mg/cm 3 ) 200.2 ± 1.15 199.0 ± 1.06 199.3 ± 1.06 201.1 ± 1.16 198.8 ± 0.93 199.9 ± 0.76 199.0 ± 1.12 199.6 ± 1.24 CV (%) 0.57 0.53 0.53 0.58 0.47 0.38 0.56 0.62
CV, coefficient of variation; LS1 to LS5, LightSpeed 16-channel scanners; LSP, LightSpeed Plus four-channel scanner; QX/i, LightSpeed QX/i four-channel scanner; QCT, quantitative computed tomographic; SD, standard deviation.
For short-term QCT precision, measurements were made on seven separate scanners over a 6-month period, using both Image Analysis and Mindways systems. The Mindways system was able to adapt to individual scanners and demonstrated an equivalent variation among all scanners (CV, 0.47%–0.58%); however, the Image Analysis measurements resulted in more variation among the seven scanners (CV, 0.51%–0.96%).
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Simulated Patient Repositioning Determination (Using a Single CT Scanner)
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Table 3
BMD Assessed for Three Ex Vivo Spine Segments, Each Scanned 10 Times After Intentional Repositioning, using the Proposed Clinical Protocol ( Table 1 )
Spine 1 Spine 2 Spine 3 Mean ± SD BMD (mg/cm 3 ) (center vertebra) 228.0 ± 2.1 141.9 ± 1.7 83.8 ± 0.9 CV (%) 0.9% 1.2% 1.1%
BMD, bone mineral density; CV, coefficient of variation; SD, standard deviation.
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Multislice Helical QCT Precision for Three Scenarios
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Table 4
Overall Helical MDCT Precision Results for QCT Measurements
Scenario Sources of Variability Incorporated Overall Precision (CV) † Least Significant Change (%) ‡ Best case Short-term variation of a single helical MDCT scanner (CV, 0.62%), patient repositioning (CV, 1.23%) 1.4% 3.9% Routine case ⁎ Best-case scenario plus table speed and product of tube current and rotation time (CV, 1.2%), reconstruction algorithm (CV, 0.08%) 1.8% 5.0% Worst case Routine-case scenario plus table height (CV, 2.3%), tube heat (CV, 1.0%), and 140 kVp (CV, 1.8%) 3.6% 10.0%
CV, coefficient of variation; MDCT, multidetector-row computed tomographic; QCT, quantitative computed tomographic.
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Discussion
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Table 5
Overall Recommendations for QCT Examinations Performed on Helical MDCT Scanners
Perform each QCT exam on the same scanner for an individual patient. Use acquisition parameters on replacement CT scanners to best match previous QCT technique. Do not use 140 kVp for QCT measurements. Do not permit variation for pitch, detector configuration, table speed, or gantry rotation speed for QCT exams. Table height used for baseline QCT exam should be used for all subsequent QCT exams. Do not perform QCT exams with a CT scanner that has been idle. Warm up x-ray tube when necessary. Minimize patient repositioning variability as much as possible. Implement a rigorous QA program according to QCT system vendor specifications.
CT, computed tomographic; MDCT, multidetector-row computed tomographic; QA, quality assurance; QCT, quantitative computed tomographic.
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