Michael R. Bruesewitz

Optimal Tube Potential for Radiation Dose Reduction in Pediatric CT: Principles, Clinical Implementations, and Pitfalls

In addition to existing strategies for reducing radiation dose in computed tomographic (CT) examinations, such as the use of automatic exposure control, use of the optimal tube potential also may help improve image quality or reduce radiation dose in pediatric CT examinations. The main benefit of the use of a lower tube potential is that it provides improved contrast enhancement, a characteristic that may compensate for the increase in noise that often occurs at lower tube potentials and that may allow radiation dose to be substantially reduced. However, selecting an appropriate tube potential and determining how much to reduce radiation dose depend on the patients size and the diagnostic task being performed. The power limits of the CT scanner and the desired scanning speed also must be considered. The use of a lower tube potential and the amount by which to reduce radiation dose must be carefully evaluated for each type of examination to achieve an optimal tradeoff between contrast, noise, artifacts, and scanning speed.

CT Dosimetry : Comparison of Measurement Techniques and Devices

In x-ray computed tomography (CT), the most common parameter used to estimate and minimize patient dose is the CT dose index (CTDI). The CTDI is a volume-averaged measure that is used in situations where the table is incremented in conjunction with the tube rotation. Variants of the CTDI correct for averaging across the field of view and for adjacent beam overlaps or gaps. CTDI is usually measured with a pencil-shaped ionization chamber, although methods have been developed that use alternative detectors, including an optically stimulated luminescence probe and a solid-state real-time dosimeter. Because the CTDI represents an averaged dose to a homogeneous cylindrical phantom, the measurements are only an approximation of the patient dose. Furthermore, dose from interventional or perfusion CT, in which the table remains stationary between multiple scans, is best evaluated with point dose measurements made with small detectors. CTDI and point dose values are nearly the same for measurement of surface dose from spiral CT. However, for measurement of surface dose from perfusion CT, the dose is overestimated by a factor of two or more with CTDI values in comparison with point dose values. Both CTDI and point dose measurement are valuable for evaluating CT scanner output and estimating patient dose.

The phantom portion of the American College of Radiology (ACR) Computed Tomography (CT) accreditation program: Practical tips, artifact examples, and pitfalls to avoid

The ACR CT accreditation program, begun in 2002, requires the submission of approximately 20 images, several completed data sheets and printouts of three Excel worksheets. The procedure manual is very detailed, yet participants unfamiliar with the program or having minimal CT experience have needed to redo aspects of their submission, or in some cases do not receive accreditation, due to mistakes made by the physicist. This review of the phantom portion of the ACR CT accreditation program supplements the ACR provided instructions with additional photos of phantom setup, region-of-interest (ROI), and image placement on the film sheets, and examples of completed portions of actual (but anonymous) submissions. Common mistakes, as well as uncommon but interesting images, are shown and explanations are given as to what could have been done to avoid the problem. Additionally, a review of CT dose measurement techniques and calculations will enable the physicist to better assist sites where typical exam doses are above the ACR reference values.

Dose and Image Quality Evaluation of a Dedicated Cone-Beam CT System for High-Contrast Neurologic Applications

OBJECTIVE The purpose of our study was to evaluate the dose and image quality performance of a dedicated cone-beam CT (CBCT) scanner in comparison with an MDCT scanner. MATERIALS AND METHODS The conventional dose metric, CT dose index (CTDI), is no longer applicable to CBCT scanners. We propose to use two dose metrics, the volume average dose and the mid plane average dose, to quantify the dose performance in a circular cone-beam scan. Under the condition of equal mid plane average dose, we evaluated the image quality of a CBCT scanner and an MDCT scanner, including high-contrast spatial resolution, low-contrast spatial resolution, noise level, CT number uniformity, and CT number accuracy. RESULTS For the sinus scanning protocol, the CBCT system had comparable high-contrast resolution and inferior low-contrast resolution to those obtained with the MDCT scanner when the doses were matched (mid plane average dose 9.2 mGy). The CT number uniformity and accuracy were worse on the CBCT scanner. The image artifacts caused by beam hardening and scattering were also much more severe on the CBCT system. CONCLUSION With a matched radiation dose, the CBCT system for sinus study has comparable high-contrast resolution and inferior low-contrast resolution relative to the MDCT scanner. Because of the more severe image artifacts on the CBCT system due to the small field of view and the lack of accurate scatter and beam-hardening correction, the utility of the CBCT system for diagnostic tasks related to soft tissue should be carefully assessed.

Spatial resolution and radiation dose of a 64-MDCT scanner compared with published CT urography protocols.

OBJECTIVE The objective of our study was to compare the spatial resolution and effective dose from 64-MDCT with several published CT urography protocols. MATERIALS AND METHODS A phantom containing 1-, 2-, or 4-mm cylindric channels to simulate ureters with 0.25- to 3-mm plugs to simulate ureteral filling defects or ureteral diverticula was imaged using eight helical CT urography protocols. Computed radiography (CR) was also performed. Coronal maximum-intensity-projection images were created and, with the CR image, were evaluated independently by two genitourinary radiologists. Spatial resolution was evaluated by scoring each abnormality as present, visible; or as absent, not visible. Effective dose estimates for 11 CT urography protocols, including the radiographs obtained in the CT urography protocol, were calculated using published Monte Carlo organ dose coefficients. RESULTS All ureteral abnormalities detected on CR were detected on the highest-spatial-resolution reconstruction using the evaluated 64-MDCT system. The smallest filling defect identified by both was 0.25 mm. Three 0.25-mm filling defects were not detected using the evaluated 16-MDCT system. The 4-MDCT system protocols showed the poorest performance. The range of effective doses for the evaluated CT urography protocols was 20.1-66.3 mSv. The number of phases, anatomic coverage per phase, and scanning parameters all contributed to this variation in dose. CONCLUSION The evaluated 64-MDCT system showed detection accuracy identical to that of CR. Limiting anatomic coverage for specific phases and combining phases can reduce dose for multiphase protocols by up to a factor of 2 relative to early (circa 2000) 4-MDCT.

Methods for Clinical Evaluation of Noise Reduction Techniques in Abdominopelvic CT

Most noise reduction methods involve nonlinear processes, and objective evaluation of image quality can be challenging, since image noise cannot be fully characterized on the sole basis of the noise level at computed tomography (CT). Noise spatial correlation (or noise texture) is closely related to the detection and characterization of low-contrast objects and may be quantified by analyzing the noise power spectrum. High-contrast spatial resolution can be measured using the modulation transfer function and section sensitivity profile and is generally unaffected by noise reduction. Detectability of low-contrast lesions can be evaluated subjectively at varying dose levels using phantoms containing low-contrast objects. Clinical applications with inherent high-contrast abnormalities (eg, CT for renal calculi, CT enterography) permit larger dose reductions with denoising techniques. In low-contrast tasks such as detection of metastases in solid organs, dose reduction is substantially more limited by loss of lesion conspicuity due to loss of low-contrast spatial resolution and coarsening of noise texture. Existing noise reduction strategies for dose reduction have a substantial impact on lowering the radiation dose at CT. To preserve the diagnostic benefit of CT examination, thoughtful utilization of these strategies must be based on the inherent lesion-to-background contrast and the anatomy of interest. The authors provide an overview of existing noise reduction strategies for low-dose abdominopelvic CT, including analytic reconstruction, image and projection space denoising, and iterative reconstruction; review qualitative and quantitative tools for evaluating these strategies; and discuss the strengths and limitations of individual noise reduction methods.

The calibration of experimental self-developing Gafchromic® HXR film for the measurement of radiation dose in computed tomography

A prototype, self-developing Gafchromic HXR film has sensitivity an order of magnitude larger than that of the commercially available Gafchromic XR film used in interventional radiological applications. The higher sensitivity of the HXR film allows the possibility of acquisition of high-resolution calibrated dose profiles within the diagnostic range of exposure levels, below 10 R (87.7 mGy). We employed a commercially available, optical flatbed scanner for digitization of the film and image analysis software to determine the response of the HXR films to ionizing radiation. Spatial uniformity and temporal repeatability of the flatbed scanner were determined and used in optimization of the digitization protocol. The HXR film postexposure density growth and sensitivity to ambient light were determined using multiple scans of two simultaneously exposed sheets, one stored in light-tight conditions and the other continuously illuminated with white light. A calibrated step wedge of the HXR film was obtained by simultaneous irradiation of a portion of a film strip and a calibrated ionization chamber using a radiographic x-ray tube with beam characteristics matched to a typical CT scanner (8 mm Al HVL, 120 kVp). Repeated digitization of the calibration film was used to determine the precision of the film response measurements. The precision, as measured by the standard deviation of multiple measurements, was better than 1% over the full dynamic range of film response. This precision was measured using exposures ranging from 0.5 to 12 R (4.4 to 105.3 mGy). This exposure range is highly relevant to x-ray computed tomography. Preliminary radiation dose profiles demonstrate the utility of this technique.

Selection of appropriate computed tomographic image reconstruction algorithms for a quantitative multicenter trial of diffuse lung disease.

Objective: To determine the appropriate computed tomographic (CT) image reconstruction algorithms for a quantitative multicenter trial of diffuse lung disease. Methods: Phantom images were reconstructed using relevant reconstruction algorithms from 2 CT manufacturers to measure mean CT numbers and image noise. High-contrast spatial resolution and edge response function were determined for each algorithm. Clinical images of patients with diffuse lung disease were evaluated by a thoracic radiologist in terms of image quality and disease extent. Results: The CT numbers were accurate for most reconstruction algorithms for both manufacturers, although some algorithms with strong midfrequency enhancement altered CT numbers. The Bone (GE) and B46f (Siemens) algorithms provided the higher spatial resolution deemed clinically necessary for imaging diffuse lung disease while preserving CT number accuracy. The extent of diffuse lung disease was strongly dependent on the reconstruction algorithm. Conclusions: A moderately sharp reconstruction algorithm (Bone/B46f) was selected for the evaluation of diffuse lung disease.

Novel anthropomorphic hip phantom corrects systemic interscanner differences in proximal femoral vBMD.

Quantitative computed tomography (QCT) is increasingly used in osteoporosis studies to assess volumetric bone mineral density (vBMD), bone quality and strength. However, QCT is confronted by technical issues in the clinical research setting, such as potentially confounding effects of body size on vBMD measurements and lack of standard approaches to scanner cross-calibration, which affects measurements of vBMD in multicenter settings. In this study, we addressed systematic inter-scanner differences and subject-dependent body size errors using a novel anthropomorphic hip phantom, containing a calibration hip to estimate correction equations, and a contralateral test hip to assess the quality of the correction. We scanned this phantom on four different scanners and we applied phantom-derived corrections to in vivo images of 16 postmenopausal women scanned on two scanners. From the phantom study, we found that vBMD decreased with increasing phantom size in three of four scanners and that inter-scanner variations increased with increasing phantom size. In the in vivo study, we observed that inter-scanner corrections reduced systematic inter-scanner mean vBMD differences but that the inter-scanner precision error was still larger than expected from known intra-scanner precision measurements. In conclusion, inter-scanner corrections and body size influence should be considered when measuring vBMD from QCT images.

Optimization of CT image reconstruction algorithms for the lung tissue research consortium (LTRC)

To create a repository of clinical data, CT images and tissue samples and to more clearly understand the pathogenetic features of pulmonary fibrosis and emphysema, the National Heart, Lung, and Blood Institute (NHLBI) launched a cooperative effort known as the Lung Tissue Resource Consortium (LTRC). The CT images for the LTRC effort must contain accurate CT numbers in order to characterize tissues, and must have high-spatial resolution to show fine anatomic structures. This study was performed to optimize the CT image reconstruction algorithms to achieve these criteria. Quantitative analyses of phantom and clinical images were conducted. The ACR CT accreditation phantom containing five regions of distinct CT attenuations (CT numbers of approximately -1000 HU, -80 HU, 0 HU, 130 HU and 900 HU), and a high-contrast spatial resolution test pattern, was scanned using CT systems from two manufacturers (General Electric (GE) Healthcare and Siemens Medical Solutions). Phantom images were reconstructed using all relevant reconstruction algorithms. Mean CT numbers and image noise (standard deviation) were measured and compared for the five materials. Clinical high-resolution chest CT images acquired on a GE CT system for a patient with diffuse lung disease were reconstructed using BONE and STANDARD algorithms and evaluated by a thoracic radiologist in terms of image quality and disease extent. The clinical BONE images were processed with a 3 x 3 x 3 median filter to simulate a thicker slice reconstructed in smoother algorithms, which have traditionally been proven to provide an accurate estimation of emphysema extent in the lungs. Using a threshold technique, the volume of emphysema (defined as the percentage of lung voxels having a CT number lower than -950 HU) was computed for the STANDARD, BONE, and BONE filtered. The CT numbers measured in the ACR CT Phantom images were accurate for all reconstruction kernels for both manufacturers. As expected, visual evaluation of the spatial resolution bar patterns demonstrated that the BONE (GE) and B46f (Siemens) showed higher spatial resolution compared to the STANDARD (GE) or B30f (Siemens) reconstruction algorithms typically used for routine body CT imaging. Only the sharper images were deemed clinically acceptable for the evaluation of diffuse lung disease (e.g. emphysema). Quantitative analyses of the extent of emphysema in patient data showed the percent volumes above the -950 HU threshold as 9.4% for the BONE reconstruction, 5.9% for the STANDARD reconstruction, and 4.7% for the BONE filtered images. Contrary to the practice of using standard resolution CT images for the quantitation of diffuse lung disease, these data demonstrate that a single sharp reconstruction (BONE/B46f) should be used for both the qualitative and quantitative evaluation of diffuse lung disease. The sharper reconstruction images, which are required for diagnostic interpretation, provide accurate CT numbers over the range of -1000 to +900 HU and preserve the fidelity of small structures in the reconstructed images. A filtered version of the sharper images can be accurately substituted for images reconstructed with smoother kernels for comparison to previously published results.