Small target repeatability of 68Ga and 18F: effects of target concentration and imaging time on SUV measurements in clinically relevant phantoms.

Quantification of tumor uptake using PET imaging is important for the evaluation of therapy response. For 18F FDG PET scans, a change in uptake of 25% is commonly considered significant. For scans using novel radiopharmaceuticals, the threshold of significance is unclear. Factors including imaging time, tumor size, activity concentration, and radiopharmaceutical may affect the repeatablity of uptake metrics. This work evaluates the effect of these parameters on the repeatablity of maximum SUV (SUVmax) and mean SUV (SUVmean) in phantoms using 18F and 68Ga. An Esser PET phantom (Data Spectrum, Durham NC) was scanned on a Biograph Horizon PET/CT scanner (Siemens Medical Solutions, Malvern PA) using 18F and 68Ga. Data were acquired for 5 minutes with reconstructions between 0.5-5 minutes. The background activity mimicked clinical scans with target-to-background (T/B) ratios from 1.7-19.8. The SUVmax and SUVmean were measured for 5 slices. The mean, standard deviation, and coefficient of variation (COV) were calculated. The effects of radionuclide, imaging time, activity concentration, and target size on COV were evaluated using multivariate gamma regressions. COV for 68Ga was 40% higher and 54% higher on average than for 18F for SUVmax and SUVmean, respectively. Decreased lesion size, imaging time, and activity concentration were significantly associated with increased COV for both metrics (P < 0.001). COV was substantially reduced at high T/B for 68Ga. At the highest T/B the COV for SUVmax and SUVmean was within the typical range seen for 18F. COV is relatively high for small targets (8 mm) but is dramatically reduced with high radiotracer uptake.

Physical characteristics of 68Ga DOTATATE PET/CT affecting small lesion detectability.

BACKGROUND AND PURPOSE: 68Ga DOTATATE PET/CT protocols are similar to 18F FDG protocols despite differences in physical properties, biodistribution, and tumor uptake. The purpose of this study is to evaluate the impact of scan time (counts), and target activity on signal-to-noise ratio (SNR) in various sized targets, or lesions. To evaluate this, phantom experiments and analysis of clinical 68Ga DOTATATE PET/CT studies were performed. MATERIALS AND METHODS: 68Ga was first compared to 18F in phantom studies to evaluate recovery coefficients and SNR. 68Ga phantom studies were also acquired in list mode, and at varying target activities to evaluate the effects of acquisition time and high target concentrations on SNR in clinically relevant small (8 mm) and larger targets (≥ 12 mm). Clinical studies (n = 50) were analyzed to determine if phantom target concentrations and SNR are present in clinical 68Ga DOTATATE studies at similarly very high tumor activity concentrations (n = 159). RESULTS: In phantoms, recovery coefficient and SUVmax for 68Ga were ~87% of 18F. SNR for 68Ga was ~65% of 18F. For the 68Ga small target (8 mm) at standard T/B = 2.4, increasing scan time from 5 to 15 minutes increased SNR from < 1 to 1.6, and did not result in target identification. Increasing T/B from 2.4 to 10.9, however, dramatically increased SNR from < 1 to 22.3. Increased T/B resulted in clear visibility of the 8 mm target, even for 1-minute scans. In patients, high hepatic tumor SUVmax (27.3±29.6), resulted in high SNR (12.5±9.8). For extrahepatic tumors, high SUVmax (41.6±42.8), resulted in high SNR (43.8±49.9). CONCLUSION: Very high target or T/B, even in small targets, can offset the physical limitations of 68Ga. High target uptake and high T/B are primary factors influencing small lesion detectability.

Practical application of AAPM Report 270 in display quality assurance: A report of Task Group 270.

Published in January 2019, AAPM Report 270 provides an update to the recommendations of the AAPM's "TG18" report. Report 270 provides new definitions of display types, updated testing patterns, and revised performance standards for the modern, flat-panel displays used as part of medical image acquisition and review. The focus of the AAPM report is on consistent image quality and appearance, and how to establish a quality assurance program to achieve those two goals. This work highlights some of the key takeaways of AAPM Report 270 and makes comparisons with existing recommendations from other references. It also provides guidance for establishing a display quality assurance program for different-sized institutions. Finally, it describes future challenges for display quality assurance and what work remains.

Performance characteristics and quality assurance considerations for displays used in interventional radiology and cardiac catheterization facilities.

PURPOSE: While the performance of displays used for the acquisition and primary interpretation of medical images has been well-characterized, notably absent are publications evaluating and discussing the performance of displays used in Interventional Radiology (IR) suites and Cardiac Catheterization (CC) laboratories. The purpose of this work was to evaluate the performance of these displays and to consider the challenges in implementation of display quality assurance practices in this environment. METHODS: Ten large format displays used in IR and CC suites were evaluated. A visual inspection of available test patterns was performed followed by a quantitative evaluation of several performance characteristics including luminance ratio, luminance response function, and luminance uniformity. Additionally, the local ambient lighting conditions were evaluated. RESULTS: Luminance ratios ranged from 243.0 to 1182.1 with a mean value of 500.1 ± 289.2. The maximum deviation between the luminance response function and the DICOM Grayscale Standard Display Function ranged from 11.2% to 38.3% with a mean value of 26.2% ± 10.9%. When evaluating luminance uniformity, the mean maximum luminance deviation was 13.2% ± 3.5%. The mean value of luminance deviation from the median was 7.8% ± 1.0%. Measured values of background illuminance ranged from 29.1 to 310.0 lux with a mean value of 107.6 lux ± 80.4 lux. While no mura or bad pixels were observed during visual inspection, damage including scrapes and scratches as well as smudges was common to most of the displays. CONCLUSION: This work provides much needed data for the characterization of the performance of the large format displays used in IR and CC laboratory suites. These data may be used as a point of comparison when implementing a display QA program.

Is lead shielding of patients necessary during fluoroscopic procedures? A study based on kyphoplasty.

OBJECTIVE: To determine the benefits, risks, and limitations associated with wrapping a patient with lead shielding during fluoroscopy-guided kyphoplasty procedures as a way to reduce operator radiation exposure. MATERIALS AND METHODS: An anthropomorphic phantom was used to mimic a patient undergoing a kyphoplasty procedure under fluoroscopic guidance. Radiation measurements of the air kerma rate (AKR) were made at several locations and under various experimental conditions. First, AKR was measured at various angles along the horizontal plane of the phantom and at varying distances from the phantom, both with and without a lead apron wrapped around the lower portion of the phantom (referred to here as phantom shielding). Second, the effect of an operator's apron was simulated by suspending a lead apron between the phantom and the measurement device. AKR was measured for the four shielding conditions-phantom shielding only, operator apron only, both phantom shielding and operator apron, and no shielding. Third, AKR measurements were made at various heights and with varying C-arm angle. RESULTS: At all locations, the phantom shielding provided no substantial protection beyond that provided by an operator's own lead apron. Phantom shielding did not reduce AKR at a height comparable to that of an operator's head. CONCLUSIONS: Previous reports of using patient shielding to reduce operator exposure fail to consider the role of an operator's own lead apron in radiation protection. For an operator wearing appropriate personal lead apparel, patient shielding provides no substantial reduction in operator dose.

The effects of patient positioning when interpreting CT dose metrics: A phantom study.

PURPOSE: Review of dose metrics as part of the routine evaluation of CT protocols has become commonplace and is required by the Joint Commission and the American College of Radiology for accreditation. Most CT quality assurance programs include a review of CTDIvol and/or SSDE, both of which are affected by changes in mAs and kV. mAs, and sometimes kV, are largely determined by the Tube Current Modulation (TCM) functions of the scanner. TCM, in turn, relies on localizer scans to provide an accurate estimate of patient size. When patient size estimates are inaccurate, TCM and SSDE calculations are affected, leading to errors in both. It is important that those who are involved in reviewing CT dose indices recognize these effects to properly direct quality improvement initiatives. METHODS: An anthropomophic phantom was scanned on four clinical CT scanners using AP and PA localizers and the institution's routine abdomen protocol. Scans were repeated with the phantom at various heights relative to scanner isocenter. For each height, the projected phantom width, as shown by the localizer scans, was measured and normalized by the width of the helical scan. After each localizer scan, the TCM algorithm determined the mAs to be used for the helical scan. The scanner-reported average CTDIvol was recorded for each helical scan, and the SSDE was calculated from the projected phantom size and the scanner-reported CTDIvol at each phantom height. Last, the phantom was augmented with a lipid-gel bolus material to simulate different body mass distributions and investigate the effect of differing body habitus on projected phantom size. The results were considered in the context of optimizing dose in CT imaging, with particular attention paid to the effect on dose to breast tissue. RESULTS: Vertical mis-positioning of the phantom within the scanner led to errors in estimated phantom size of up to a factor of 1.5. These effects were more severe when localizers were acquired in the PA orientation compared with the AP orientation. Minification effects were more pronounced for AP localizers. As a consequence of inaccuracies in estimated phantom size, TCM resulted in changes in CTDIvol and SSDE of as much as a factor of 4.4 and 2.7, respectively. The effect was more pronounced when the TCM function used data from the PA, rather than the AP, localizer. CONCLUSIONS: Proper patient positioning plays a large role in the function of TCM, and hence CTDIvol and SSDE. In addition, body mass distribution may affect how patients ought to be positioned within the scanner. Understanding these effects is critical in optimizing CT scanning practices.

Imaging acquisition display performance: an evaluation and discussion of performance metrics and procedures.

When The Joint Commission updated its Requirements for Diagnostic Imaging Services for hospitals and ambulatory care facilities on July 1, 2015, among the new requirements was an annual performance evaluation for acquisition workstation displays. The purpose of this work was to evaluate a large cohort of acquisition displays used in a clinical environment and compare the results with existing performance standards provided by the American College of Radiology (ACR) and the American Association of Physicists in Medicine (AAPM). Measurements of the minimum luminance, maximum luminance, and luminance uniformity, were performed on 42 acquisition displays across multiple imaging modalities. The mean values, standard deviations, and ranges were calculated for these metrics. Additionally, visual evaluations of contrast, spatial resolution, and distortion were performed using either the Society of Motion Pictures and Television Engineers test pattern or the TG-18-QC test pattern. Finally, an evaluation of local nonuniformities was performed using either a uniform white display or the TG-18-UN80 test pattern. Displays tested were flat panel, liquid crystal displays that ranged from less than 1 to up to 10 years of use and had been built by a wide variety of manufacturers. The mean values for Lmin and Lmax for the displays tested were 0.28 ± 0.13 cd/m2 and 135.07 ± 33.35 cd/m2, respectively. The mean maximum luminance deviation for both ultrasound and non-ultrasound displays was 12.61% ± 4.85% and 14.47% ± 5.36%, respectively. Visual evaluation of display performance varied depending on several factors including brightness and contrast settings and the test pattern used for image quality assessment. This work provides a snapshot of the performance of 42 acquisition displays across several imaging modalities in clinical use at a large medical center. Comparison with existing performance standards reveals that changes in display technology and the move from cathode ray tube displays to flat panel displays may have rendered some of the tests inappropriate for modern use.