Joshua M. Wilson

A Systematic Review of the Factors Affecting Accuracy of SUV Measurements

OBJECTIVE There is growing interest in using PET/CT for evaluating early response to therapy in cancer treatment. Although widely available and convenient to use, standardized uptake value (SUV) measurements can be influenced by a variety of biologic and technologic factors. Many of these factors can be addressed with close attention to detail and appropriate quality control. This article will review factors potentially affecting SUV measurements and provide recommendations on ways to minimize when using serial PET to assess early response to therapy. CONCLUSION Scanner and reconstruction parameters can significantly affect SUV measurements. When using serial SUV measurements to assess early response to therapy, imaging should be performed on the same scanner using the same image acquisition and reconstruction protocols. In addition, attention to detail is required for accurate determination of the administered radiopharmaceutical dose.

A methodology for image quality evaluation of advanced CT systems.

PURPOSE This work involved the development of a phantom-based method to quantify the performance of tube current modulation and iterative reconstruction in modern computed tomography (CT) systems. The quantification included resolution, HU accuracy, noise, and noise texture accounting for the impact of contrast, prescribed dose, reconstruction algorithm, and body size. METHODS A 42-cm-long, 22.5-kg polyethylene phantom was designed to model four body sizes. Each size was represented by a uniform section, for the measurement of the noise-power spectrum (NPS), and a feature section containing various rods, for the measurement of HU and the task-based modulation transfer function (TTF). The phantom was scanned on a clinical CT system (GE, 750HD) using a range of tube current modulation settings (NI levels) and reconstruction methods (FBP and ASIR30). An image quality analysis program was developed to process the phantom data to calculate the targeted image quality metrics as a function of contrast, prescribed dose, and body size. RESULTS The phantom fabrication closely followed the design specifications. In terms of tube current modulation, the tube current and resulting image noise varied as a function of phantom size as expected based on the manufacturer specification: From the 16- to 37-cm section, the HU contrast for each rod was inversely related to phantom size, and noise was relatively constant (<5% change). With iterative reconstruction, the TTF exhibited a contrast dependency with better performance for higher contrast objects. At low noise levels, TTFs of iterative reconstruction were better than those of FBP, but at higher noise, that superiority was not maintained at all contrast levels. Relative to FBP, the NPS of iterative reconstruction exhibited an ~30% decrease in magnitude and a 0.1 mm(-1) shift in the peak frequency. CONCLUSIONS Phantom and image quality analysis software were created for assessing CT image quality over a range of contrasts, doses, and body sizes. The testing platform enabled robust NPS, TTF, HU, and pixel noise measurements as a function of body size capable of characterizing the performance of reconstruction algorithms and tube current modulation techniques.

Impact of Dual-Energy Multi–Detector Row CT with Virtual Monochromatic Imaging on Renal Cyst Pseudoenhancement: In Vitro and in Vivo Study

PURPOSE To investigate whether dual-energy multi-detector row computed tomography (CT) with virtual monochromatic imaging can overcome renal cyst pseudoenhancement in a phantom experiment and a clinical study. MATERIALS AND METHODS This retrospective single-center HIPAA-compliant study was approved by the institutional review board, with waiver of informed consent. Four renal compartments inserted into torso phantoms were filled with saline to simulate the unenhanced state and with iodinated solutions to simulate the three levels of renal parenchyma enhancement (140, 180, and 240 HU). Saline-filled spheres simulating renal cysts (15 and 18 mm in diameter) were serially suspended in the renal compartments and imaged with dual-energy and single-energy multi-detector row CT at four different energy levels (80, 100, 120, and 140 kVp). In addition, 28 patients (mean age, 66 years ± 10; mean body mass index, 31.3 kg/m(2) ± 6.2) with 34 intrarenal cysts were included. Virtual monochromatic images were reconstructed in 10-keV increments at energy levels ranging from 40 to 140 keV. Phantom and clinical data were analyzed by using multivariate regression analysis. RESULTS In the phantom experiment, all polychromatic image data sets showed pseudoenhancement (postcontrast attenuation increase >10 HU) in all investigated conditions, with a significant effect on cyst size (P <.001), location (P <.001), and renal background attenuation level (P <.001). Virtual monochromatic images at energy levels ranging from 80 to 140 keV did not show pseudoenhancement, with the minimum attenuation increase (mean, 6.1 HU ± 1.6; range, 1.6-7.7 HU) on 80-keV images. In patients, pseudoenhancement never occurred on virtual monochromatic images at energy levels ranging from 90 to 140 keV. Patient body size had a significant effect (P = .007) on selection of the optimal monochromatic energy level. CONCLUSION Dual-energy multi-detector row CT with reconstruction of virtual monochromatic images at an optimal energy level can overcome renal cyst pseudoenhancement.

Dual-Energy MDCT in Hypervascular Liver Tumors: Effect of Body Size on Selection of the Optimal Monochromatic Energy Level

OBJECTIVE The purpose of this article is to investigate the effect of body size on the selection of optimal monochromatic energy level for maximizing the conspicuity of hypervascular liver tumors during late hepatic arterial phase using dual-energy MDCT. MATERIALS AND METHODS An anthropomorphic liver phantom in three body sizes and iodine-containing inserts simulating low- and high-contrast hypervascular lesions was imaged with dual- and single-energy MDCT at various energy levels (80, 100, 120, and 140 kVp). Dual-energy MDCT was also performed in 48 patients with 114 hypervascular liver tumors; virtual monochromatic images were reconstructed at energy levels from 40 to 140 keV. The effect of body size and lesion iodine concentration on noise and tumor-to-liver contrast-to-noise ratio was compared among different datasets for phantoms and patients. RESULTS The highest tumor-to-liver contrast-to-noise ratio was noted at 80 kVp for all phantom sizes. On virtual monochromatic images, the minimum noise was noted at 70 keV for small and medium phantoms and at 80 keV for the large phantom. Tumor-to-liver contrast-to-noise ratio was highest at 50 keV for small and medium phantoms and at 60 keV for the large phantom (p<0.0001). Compared with 80-kVp images, an optimal monochromatic energy level yielded a significantly higher (p<0.0001) tumor-to-liver contrast-to-noise ratio for high-contrast lesions in the large body size and for low-contrast lesions in all phantom sizes. In patients, the optimal monochromatic energy level for tumor-to-liver contrast-to-noise ratio increased proportionally along with body size (p<0.0001). CONCLUSION Selection of the optimal monochromatic energy level for maximizing the conspicuity of hypervascular liver tumors is significantly affected by patients body size.

Virtual Monochromatic Images from Dual-Energy Multidetector CT: Variance in CT Numbers from the Same Lesion between Single-Source Projection-based and Dual-Source Image-based Implementations

PURPOSE To determine the variance in virtual monochromatic computed tomography (CT) numbers from the same lesion, comparing the two clinically available dual-energy multidetector CT hardware implementations (single-source projection-based and dual-source image-based), in a phantom-based simulated abdominal environment. MATERIALS AND METHODS This phantom-based study was exempt from institutional review board oversight. Polyethylene terephthalate spheres (15 and 18 mm) with two iodine-to-saline dilutions (0.8 and 1.2 mg of iodine per millilliter) were serially suspended in a cylindrical polypropylene bottle filled with diluted iodinated contrast material. The bottle was placed into a 36-cm-wide torso-shaped water phantom simulating the abdomen of a medium-sized patient. Dual-energy (80/140 kVp) and single-energy (100 and 120 kVp) scans were obtained with single-source and dual-source multidetector CT implementations. Virtual monochromatic images were reconstructed at energy levels of 40-140 keV (in 10-keV increments) in either the projection-space or image-space domain. A multivariate regression analysis approach was used to investigate the effect of energy level, lesion size, lesion iodine content, and implementation type on measured CT numbers. RESULTS There were significant differences in the attenuation values measured in the simulated lesions with the single-source projection-based platform and the dual-source image-based implementation (P < .001 for all comparisons). The magnitude of these differences was greatest at lower monochromatic energy levels and at lower iodine concentrations (average difference at 40 keV: 25.7 HU; average difference at 140 keV: 7 HU). The monochromatic energy level and the lesion iodine concentration had a significant effect on the difference in the measured attenuation values between the two implementations, which indicates that the two imaging platforms respond differently to changes in investigated variables (P < .001 for all comparisons). CONCLUSION There is a statistically significant variance in virtual monochromatic CT numbers from the same lesion examined with single-source projection-based and dual-source image-based implementations. The magnitude of the variance is a function of the selected energy level and the lesion iodine content.

Characteristic image quality of a third generation dual-source MDCT scanner: Noise, resolution, and detectability

PURPOSE The purpose of this work was to assess the inherent image quality characteristics of a new multidetector computed tomography system in terms of noise, resolution, and detectability index as a function of image acquisition and reconstruction for a range of clinically relevant settings. METHODS A multisized image quality phantom (37, 30, 23, 18.5, and 12 cm physical diameter) was imaged on a SOMATOM Force scanner (Siemens Medical Solutions) under variable dose, kVp, and tube current modulation settings. Images were reconstructed with filtered back projection (FBP) and with advanced modeled iterative reconstruction (ADMIRE) with iterative strengths of 3, 4, and 5. Image quality was assessed in terms of the noise power spectrum (NPS), task transfer function (TTF), and detectability index for a range of detection tasks (contrasts of approximately 45, 90, 300, -900, and 1000 HU, and 2-20 mm diameter) based on a non-prewhitening matched filter model observer with eye filter. RESULTS Image noise magnitude decreased with decreasing phantom size, increasing dose, and increasing ADMIRE strength, offering up to 64% noise reduction relative to FBP. Noise texture in terms of the NPS was similar between FBP and ADMIRE (<5% shift in peak frequency). The resolution, based on the TTF, improved with increased ADMIRE strength by an average of 15% in the TTF 50% frequency for ADMIRE-5. The detectability index increased with increasing dose and ADMIRE strength by an average of 55%, 90%, and 163% for ADMIRE 3, 4, and 5, respectively. Assessing the impact of mA modulation for a fixed average dose over the length of the phantom, detectability was up to 49% lower in smaller phantom sections and up to 26% higher in larger phantom sections for the modulated scan compared to a fixed tube current scan. Overall, the detectability exhibited less variability with phantom size for modulated scans compared to fixed tube current scans. CONCLUSIONS Image quality increased with increasing dose and decreasing phantom size. The CT system exhibited nonlinear noise and resolution properties, especially at very low-doses, large phantom sizes, and for low-contrast objects. Objective image quality metrics generally increased with increasing dose and ADMIRE strength, and with decreasing phantom size. The ADMIRE algorithm could offer comparable image quality at reduced doses or improved image quality at the same dose. The use of tube current modulation resulted in more consistent image quality with changing phantom size.

Attenuation artifacts and time-of-flight PET

Attenuation artifacts in PET are seen when attenuation correction (AC) is not performed and when AC is performed but is based on an incorrect attenuation map. PET attenuation artifacts are generally more profound than the straightforward differences in photon attenuation through surrounding tissue from one source location to another. Effects such as the apparent radioactivity in gas pockets in the body or concave contours of the body surface, distortions, and a pronounced body contour are all in addition to the expected nonuniformities due to depth in the body. We have investigated the effects of time-of-flight (TOF) reconstruction on PET artifacts. Uniformity without AC (NAC) was investigated in a large tapering F-18-fillled phantom that increases from a small end to the size of a large patient. Throughout the length of the phantom, the NAC images with TOF were more uniform that the non-TOF images. A whole-body phantom with oval cross section was imaged with uniform F-18 background and a 5.5 cm air-filled sphere at one section, and a 5.5 cm hot (8x background) sphere at another section. In NAC images, the air-filled lesion was artificially hotter than background with non-TOF, but comparable to background with TOF. When this section was corrected with a uniform AC (as if the gas pocket had moved before transmission scan), the non-TOF image showed the sphere to be artificially hot (hotter than background) whereas the TOF images reduced the artifact. In the section with the hot sphere, the NAC distortions typical near the bladder (higher counts anterior and posterior to bladder; depleted areas lateral to bladder) were greatly diminished with TOF images. While AC is necessary for quantitation in PET, the use of TOF and NAC may be useful in providing more interpretable images in some situations were an attenuation map is impossible to obtain, or introduces errors of its own. TOF may also lessen some artifacts in corrected images.

Dual-Energy Multi–Detector Row CT with Virtual Monochromatic Imaging for Improving Patient-to-Patient Uniformity of Aortic Enhancement during CT Angiography: An in Vitro and in Vivo Study

PURPOSE To determine whether virtual monochromatic imaging from a dual-energy acquisition can improve patient-to-patient uniformity of aortic enhancement during multi-detector row computed tomographic (CT) angiography. MATERIALS AND METHODS This retrospective single-center HIPAA-compliant study was approved by the institutional review board, with a waiver of informed consent. A proprietary tapered hollow phantom that contained a bone-mimicking insert and a hollow tube insert that mimicked the aorta was used. The aortic insert was filled with different iodine dilutions to mimic various degrees of enhancement. The phantom was imaged with both dual-energy and single-energy multi-detector row CT at four energy levels (80, 100, 120, and 140 kVp). Dual-energy multi-detector row CT was also performed in 62 patients (38 men; mean age, 60 years ± 12.7 [standard deviation]). For both the phantom and the patients, virtual monochromatic images were reconstructed from 40 to 140 keV, at 20-keV increments. The relationship between aortic attenuation and effective diameter was assessed by using a statistical model. RESULTS For all polychromatic data sets, the mean aortic attenuation decreased proportionally to the effective diameter of the phantom (slope, ≥3.0 HU/cm). For virtual monochromatic data sets ranging from 80 to 140 keV, the regression slopes of aortic attenuation as a function of the phantoms effective diameter were negligible (slope, <1.0 HU/cm) for all iodine-to-water dilutions. In patients, the slope of the regression lines was also negligible (-0.69 < slope < 0.16) for virtual monochromatic data sets ranging from 100 to 140 keV. CONCLUSION Within an energy range of 100-140 keV, virtual monochromatic images improve patient-to-patient uniformity of aortic enhancement compared with conventional polychromatic acquisitions.

Multisphere phantom and analysis algorithm for PET image quality assessment

PET image quality measurements of lesion detectability frequently use a small, radioactive sphere in a larger phantom. The typical analysis of a small single sphere in background has several shortcomings as a measure for detectability and quantitation: the measurement has low statistical power; the region of interest (ROI) is susceptible to large pixel-to-pixel fluctuations; only a single point in the axial and transaxial field of view is analyzed; background noise measurements in regions away from the signal sphere may bias the detectability measurement and user-placed ROIs can cause inconsistent measurements. For a more robust measurement and repeatable analysis of small lesion detectability in PET images, a multisphere phantom and analysis algorithm were developed. The multisphere phantom consists of a collection of 50 1.0-cm spheres, mounting rods and a gridded plate. A PET/CT study is presented where 29 spheres with a 4:1 sphere-to-background radioactivity ratio were acquired for multiple frame durations and reconstructed. An analysis algorithm was implemented and applied to the acquired PET/CT that detects the contrast-enhanced spheres in a CT, places ROIs on the spheres and their respective proximal background, applies the ROIs to the PET and performs quantitation. Results are presented that show the impact of increasing number of signal spheres and of different background ROI placement methods on the image quality measurement. Increasing the number of spheres reduced the variability in the image quality measurements, but only up to a point, beyond which increasing the number of spheres did not considerably reduce the variability. A phantom with numerous spherical inserts increases several measurement aspects: the flexibility of sphere placement during setup, the number of radioactivity concentrations that can be used during a single study and the statistical power of measurements. Additionally, an automated algorithm that localizes spheres, places ROIs and performs quantitation will increase reliability and reproducibility of image quality assessment, in addition to simplifying the analysis.

TOF-PET small-lesion image quality measured over a range of phantom sizes

The purpose of this study was to parameterize TOF image quality improvement over non-TOF PET using small lesions in a tapering phantom that represents a range of body sizes. A set of 46 1.0-cm spherical inserts was divided into 6 groups and positioned in a fillable, tapered phantom that represents a range of body dimensions (an oval cross section from 38.5 cm × 49.5 cm to 6.8 cm × 17.8 cm with a length of 51.1 cm). The hot spheres were positioned in a uniform warm background (8:1 radioactivity concentration) and were distributed in 6 regions of different cross-sectional areas: 6 spheres at 306 cm2, and 8 spheres at 426, 574, 754, 964, and 1187 cm2. The PET/CT study was acquired on a Discovery 690 PET/CT (GE Healthcare), which uses LYSO crystals (4.25 × 6.3 × 25 mm3) in three rings of 6 × 9 blocks. Six frames, with a different set of spheres centered for each frame, were each acquired. Images were reconstructed with and without TOF information using 3D OSEM with 8 subsets and 1–20 iterations. The 50-cm field of view (FOV) was reconstructed to a 128×128 matrix with attenuation, normalization, decay, scatter, singles-based random corrections, and z-axis smoothing, but no post-filter was applied. An automated algorithm was used to locate and place regions of interest on the hot spheres in the CT, and then apply the ROIs to the PET images to measure signal and background variability (image noise). When both TOF and non-TOF images are near convergence, the ratio of contrast recovery for TOF to non-TOF is ~1.0 for small body sizes, but the CRC increase ranges from 10%–25% for body sizes with diameters >30.0 cm. When comparing the small lesion signal-to-noise ratios between TOF and non-TOF images, the square of the SNR ratios for TOF to non-TOF was a function of body size and timing resolution with an offset of -1.36.