Findings of the AAPM Ad Hoc committee on magnetic resonance imaging in radiation therapy: Unmet needs, opportunities, and recommendations

The past decade has seen the increasing integration of magnetic resonance (MR) imaging into radiation therapy (RT). This growth can be contributed to multiple factors, including hardware and software advances that have allowed the acquisition of high-resolution volumetric data of RT patients in their treatment position (also known as MR simulation) and the development of methods to image and quantify tissue function and response to therapy. More recently, the advent of MR-guided radiation therapy (MRgRT) - achieved through the integration of MR imaging systems and linear accelerators - has further accelerated this trend. As MR imaging in RT techniques and technologies, such as MRgRT, gain regulatory approval worldwide, these systems will begin to propagate beyond tertiary care academic medical centers and into more community-based health systems and hospitals, creating new opportunities to provide advanced treatment options to a broader patient population. Accompanying these opportunities are unique challenges related to their adaptation, adoption, and use including modification of hardware and software to meet the unique and distinct demands of MR imaging in RT, the need for standardization of imaging techniques and protocols, education of the broader RT community (particularly in regards to MR safety) as well as the need to continue and support research, and development in this space. In response to this, an ad hoc committee of the American Association of Physicists in Medicine (AAPM) was formed to identify the unmet needs, roadblocks, and opportunities within this space. The purpose of this document is to report on the major findings and recommendations identified. Importantly, the provided recommendations represent the consensus opinions of the committee's membership, which were submitted in the committee's report to the AAPM Board of Directors. In addition, AAPM ad hoc committee reports differ from AAPM task group reports in that ad hoc committee reports are neither reviewed nor ultimately approved by the committee's parent groups, including at the council and executive committee level. Thus, the recommendations given in this summary should not be construed as being endorsed by or official recommendations from the AAPM.

Multi-site, multi-platform comparison of MRI T1 measurement using the system phantom

Recent innovations in quantitative magnetic resonance imaging (MRI) measurement methods have led to improvements in accuracy, repeatability, and acquisition speed, and have prompted renewed interest to reevaluate the medical value of quantitative T₁. The purpose of this study was to determine the bias and reproducibility of T₁ measurements in a variety of MRI systems with an eye toward assessing the feasibility of applying diagnostic threshold T₁ measurement across multiple clinical sites. We used the International Society of Magnetic Resonance in Medicine/National Institute of Standards and Technology (ISMRM/NIST) system phantom to assess variations of T₁ measurements, using a slow, reference standard inversion recovery sequence and a rapid, commonly-available variable flip angle sequence, across MRI systems at 1.5 tesla (T) (two vendors, with number of MRI systems n = 9) and 3 T (three vendors, n = 18). We compared the T₁ measurements from inversion recovery and variable flip angle scans to ISMRM/NIST phantom reference values using Analysis of Variance (ANOVA) to test for statistical differences between T₁ measurements grouped according to MRI scanner manufacturers and/or static field strengths. The inversion recovery method had minor over- and under-estimations compared to the NMR-measured T₁ values at both 1.5 T and 3 T. Variable flip angle measurements had substantially greater deviations from the NMR-measured T₁ values than the inversion recovery measurements. At 3 T, the measured variable flip angle T₁ for one vendor is significantly different than the other two vendors for most of the samples throughout the clinically relevant range of T₁. There was no consistent pattern of discrepancy between vendors. We suggest establishing rigorous quality control procedures for validating quantitative MRI methods to promote confidence and stability in associated measurement techniques and to enable translation of diagnostic threshold from the research center to the entire clinical community.

A standard system phantom for magnetic resonance imaging

Purpose

A standard MRI system phantom has been designed and fabricated to assess scanner performance, stability, comparability and assess the accuracy of quantitative relaxation time imaging. The phantom is unique in having traceability to the International System of Units, a high level of precision, and monitoring by a national metrology institute. Here, we describe the phantom design, construction, imaging protocols, and measurement of geometric distortion, resolution, slice profile, signal‐to‐noise ratio (SNR), proton‐spin relaxation times, image uniformity and proton density.

Methods

The system phantom, designed by the International Society of Magnetic Resonance in Medicine ad hoc committee on Standards for Quantitative MR, is a 200 mm spherical structure that contains a 57‐element fiducial array; two relaxation time arrays; a proton density/SNR array; resolution and slice‐profile insets. Standard imaging protocols are presented, which provide rapid assessment of geometric distortion, image uniformity, T₁ and T₂ mapping, image resolution, slice profile, and SNR.

Results

Fiducial array analysis gives assessment of intrinsic geometric distortions, which can vary considerably between scanners and correction techniques. This analysis also measures scanner/coil image uniformity, spatial calibration accuracy, and local volume distortion. An advanced resolution analysis gives both scanner and protocol contributions. SNR analysis gives both temporal and spatial contributions.

Conclusions

A standard system phantom is useful for characterization of scanner performance, monitoring a scanner over time, and to compare different scanners. This type of calibration structure is useful for quality assurance, benchmarking quantitative MRI protocols, and to transition MRI from a qualitative imaging technique to a precise metrology with documented accuracy and uncertainty.

Linearity and Bias of Proton Density Fat Fraction as a Quantitative Imaging Biomarker: A Multicenter, Multiplatform, Multivendor Phantom Study

Background Proton density fat fraction (PDFF) estimated by using chemical shift-encoded (CSE) MRI is an accepted imaging biomarker of hepatic steatosis. This work aims to promote standardized use of CSE MRI to estimate PDFF. Purpose To assess the accuracy of CSE MRI methods for estimating PDFF by determining the linearity and range of bias observed in a phantom. Materials and Methods In this prospective study, a commercial phantom with 12 vials of known PDFF values were shipped across nine U.S. centers. The phantom underwent 160 independent MRI examinations on 27 1.5-T and 3.0-T systems from three vendors. Two three-dimensional CSE MRI protocols with minimal T1 bias were included: vendor and standardized. Each vendor's confounder-corrected complex or hybrid magnitude-complex based reconstruction algorithm was used to generate PDFF maps in both protocols. The Siemens reconstruction required a configuration change to correct for water-fat swaps in the phantom. The MRI PDFF values were compared with the known PDFF values by using linear regression with mixed-effects modeling. The 95% CIs were calculated for the regression slope (ie, proportional bias) and intercept (ie, constant bias) and compared with the null hypothesis (slope = 1, intercept = 0). Results Pooled regression slope for estimated PDFF values versus phantom-derived reference PDFF values was 0.97 (95% CI: 0.96, 0.98) in the biologically relevant 0%-47.5% PDFF range. The corresponding pooled intercept was -0.27% (95% CI: -0.50%, -0.05%). Across vendors, slope ranges were 0.86-1.02 (vendor protocols) and 0.97-1.0 (standardized protocol) at 1.5 T and 0.91-1.01 (vendor protocols) and 0.87-1.01 (standardized protocol) at 3.0 T. The intercept ranges (absolute PDFF percentage) were -0.65% to 0.18% (vendor protocols) and -0.69% to -0.17% (standardized protocol) at 1.5 T and -0.48% to 0.10% (vendor protocols) and -0.78% to -0.21% (standardized protocol) at 3.0 T. Conclusion Proton density fat fraction estimation derived from three-dimensional chemical shift-encoded MRI in a commercial phantom was accurate across vendors, imaging centers, and field strengths, with use of the vendors' product acquisition and reconstruction software. © RSNA, 2021 See also the editorial by Dyke in this issue.

Validated imaging biomarkers as decision-making tools in clinical trials and routine practice: current status and recommendations from the EIBALL* subcommittee of the European Society of Radiology (ESR)

Observer-driven pattern recognition is the standard for interpretation of medical images. To achieve global parity in interpretation, semi-quantitative scoring systems have been developed based on observer assessments; these are widely used in scoring coronary artery disease, the arthritides and neurological conditions and for indicating the likelihood of malignancy. However, in an era of machine learning and artificial intelligence, it is increasingly desirable that we extract quantitative biomarkers from medical images that inform on disease detection, characterisation, monitoring and assessment of response to treatment. Quantitation has the potential to provide objective decision-support tools in the management pathway of patients. Despite this, the quantitative potential of imaging remains under-exploited because of variability of the measurement, lack of harmonised systems for data acquisition and analysis, and crucially, a paucity of evidence on how such quantitation potentially affects clinical decision-making and patient outcome. This article reviews the current evidence for the use of semi-quantitative and quantitative biomarkers in clinical settings at various stages of the disease pathway including diagnosis, staging and prognosis, as well as predicting and detecting treatment response. It critically appraises current practice and sets out recommendations for using imaging objectively to drive patient management decisions.

ACR guidance document on MR safe practices: Updates and critical information 2019

The need for a guidance document on MR safe practices arose from a growing awareness of the MR environment's potential risks and adverse event reports involving patients, equipment, and personnel. Initially published in 2002, the American College of Radiology White Paper on MR Safety established de facto industry standards for safe and responsible practices in clinical and research MR environments. The most recent version addresses new sources of risk of adverse events, increases awareness of dynamic MR environments, and recommends that those responsible for MR medical director safety undergo annual MR safety training. With regular updates to these guidelines, the latest MR safety concerns can be accounted for to ensure a safer MR environment where dangers are minimized. Level of Evidence: 1 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2020;51:331-338.

Recommendations towards standards for quantitative MRI (qMRI) and outstanding needs

LEVEL OF EVIDENCE

5 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2019.

Development and evaluation of an arterial spin-labeling digital reference object for quality control and comparison of data analysis applications

Longitudinal assessment of quantitative imaging biomarkers (QIBs) requires a comprehensive quality control (QC) program to minimize bias and variance in measurement results. In addition, the availability of data analysis software from multiple vendors emphasizes the need for a means of quantitatively comparing the computed QIB measures produced by the applications. The purpose of this work is to describe a digital reference object (DRO) that has been developed for the evaluation of arterial spin-labeling (ASL) measurement results. The ASL DRO is a synthetic data set consisting of 10  ×  10 voxel square blocks with a range of ASL control image signal-to-noise ratio (SNR_Control), blood flow (BF), and proton density (PD) image SNR values (SNR_Control:1-100, BF:10-210 ml/100 g min^-1, SNR_PD:10-100). A pseudo-continuous ASL sequence was simulated with acquisition parameters and modeled signal intensities defined according to those typically associated with clinically-acquired ASL images. ASL parameters were estimated using the commercially-available nordicICE software package (NordicNeuroLab, Inc, Milwaukee, WI). Percent bias measures and Bland-Altman analyses demonstrated decreased bias and variance with increasing SNR_Control and BF values. Excellent agreement with reference values was seen for all BF values above an SNR_Control of 5 (concordance correlation coefficient greater than 0.92 for all SNR_PD values). The ASL DRO developed in this work allows for the evaluation of software bias and variance across physiologically-meaningful BF and SNR_Control values. Such studies are essential to the transition of quantitative ASL-based BF measurements into widespread clinical research applications, and ultimately, routine clinical care.

Quantitative imaging biomarkers alliance (QIBA) recommendations for improved precision of DWI and DCE-MRI derived biomarkers in multicenter oncology trials

Physiological properties of tumors can be measured both in vivo and noninvasively by diffusion-weighted imaging and dynamic contrast-enhanced magnetic resonance imaging. Although these techniques have been used for more than two decades to study tumor diffusion, perfusion, and/or permeability, the methods and studies on how to reduce measurement error and bias in the derived imaging metrics is still lacking in the literature. This is of paramount importance because the objective is to translate these quantitative imaging biomarkers (QIBs) into clinical trials, and ultimately in clinical practice. Standardization of the image acquisition using appropriate phantoms is the first step from a technical performance standpoint. The next step is to assess whether the imaging metrics have clinical value and meet the requirements for being a QIB as defined by the Radiological Society of North America's Quantitative Imaging Biomarkers Alliance (QIBA). The goal and mission of QIBA and the National Cancer Institute Quantitative Imaging Network (QIN) initiatives are to provide technical performance standards (QIBA profiles) and QIN tools for producing reliable QIBs for use in the clinical imaging community. Some of QIBA's development of quantitative diffusion-weighted imaging and dynamic contrast-enhanced QIB profiles has been hampered by the lack of literature for repeatability and reproducibility of the derived QIBs. The available research on this topic is scant and is not in sync with improvements or upgrades in MRI technology over the years. This review focuses on the need for QIBs in oncology applications and emphasizes the importance of the assessment of their reproducibility and repeatability. Level of Evidence: 5 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019;49:e101-e121.

Opportunities and challenges to utilization of quantitative imaging: Report of the AAPM practical big data workshop

BACKGROUND

This article is a summary of the quantitative imaging subgroup of the 2017 AAPM Practical Big Data Workshop (PBDW-2017) on progress and challenges in big data applied to cancer treatment and research supplemented by a draft white paper following an American Association of Physicists in Medicine FOREM meeting on Imaging Genomics in 2014.

AIMS

The goal of PBDW-2017 was to close the gap between theoretical vision and practical experience with encountering and solving challenges in curating and analyzing data.

CONCLUSIONS

Recommendations based on the meetings are summarized.

Comparing the Performances of Magnetic Resonance Imaging Size vs Pharmacokinetic Parameters to Predict Response to Neoadjuvant Chemotherapy and Survival in Patients With Breast Cancer

PURPOSE

To compare the value of dynamic contrast-enhanced magnetic resonance imaging-pharmacokinetic (PK) parameters vs tumor volume in predicting breast cancer neoadjuvant chemotherapy response (NACR) and patient survival.

SUBJECTS AND METHODS

Sixty-six patients with locally advanced breast cancer who underwent breast MRI monitoring of NACR were retrospectively analyzed. We compared baseline transfer constant (K^trans), reflux rate contrast (k_ep), and extracellular extravascular volume fraction (v_e) with the same parameters obtained at early postchemotherapy MRI, and examined model-independent changes in time-intensity curves (maximum slope, contrast enhancement ratio, and IAUC90). Tumor size changes (tumor volume, single dimension, and Response Evaluation Criteria in Solid Tumors [RECIST]) were also analyzed. The Spearman correlation test was used to assess the association between size and PK parameters, and regression analysis to assess the association with 5-year disease-free survival.

RESULTS

Higher v_e values at baseline were associated with greater decreases in tumor size (P = 0.008). Changes in K^trans and IAUC90 were the strongest predictors of NACR. Changes in IAUC90 (P = 0.04) and RECIST (P = 0.003) were independently associated with pathologic response. The only parameter significantly associated with 5-year survival was change in RECIST (P = 0.001). However, there was a trend toward statistical significance for changes in v_e and K^trans, with greater changes associated with longer survival.

CONCLUSION

Changes in PK and dynamic contrast-enhanced magnetic resonance imaging kinetic parameters may have a role in predicting NACR in breast tumors. Although changes in K^trans and IAUC90 are helpful in predicting NACR, they do not show significant association with survival. Early RECIST size change measured by MRI remains the strongest predictor of overall patient survival.

Imaging biomarker roadmap for cancer studies

Imaging biomarkers (IBs) are integral to the routine management of patients with cancer. IBs used daily in oncology include clinical TNM stage, objective response and left ventricular ejection fraction. Other CT, MRI, PET and ultrasonography biomarkers are used extensively in cancer research and drug development. New IBs need to be established either as useful tools for testing research hypotheses in clinical trials and research studies, or as clinical decision-making tools for use in healthcare, by crossing 'translational gaps' through validation and qualification. Important differences exist between IBs and biospecimen-derived biomarkers and, therefore, the development of IBs requires a tailored 'roadmap'. Recognizing this need, Cancer Research UK (CRUK) and the European Organisation for Research and Treatment of Cancer (EORTC) assembled experts to review, debate and summarize the challenges of IB validation and qualification. This consensus group has produced 14 key recommendations for accelerating the clinical translation of IBs, which highlight the role of parallel (rather than sequential) tracks of technical (assay) validation, biological/clinical validation and assessment of cost-effectiveness; the need for IB standardization and accreditation systems; the need to continually revisit IB precision; an alternative framework for biological/clinical validation of IBs; and the essential requirements for multicentre studies to qualify IBs for clinical use.

Why women bypass front-line health facility services in pursuit of obstetric care provided elsewhere: a case study in three rural districts of Tanzania

OBJECTIVES

In the Tanzanian health system, women are expected to first visit their nearest front-line health facility (FLF) for delivery. However, women frequently bypass these FLF. Our study estimates the extent of bypassing for childbirth and assesses factors associated with this behaviour.

METHODS

Data describing the experiences of 597 women who recently delivered at a facility and the EmONC service capability at 107 health facilities were collected in 2011. Women who did not deliver at their nearest FLF were considered 'bypassers'. Factors associated with bypassing were assessed using multivariate logistic regression models. Three sets of analyses were conducted: among 597 women who delivered at the first facility they visited, among 521 women with no previous complications, and among 407 women not primigravida and without previous complications.

RESULTS

More than 75.4% of women bypassed. In the fully adjusted model of all 597 women those who had experienced complications were more likely to bypass for delivery [OR = 6.31 (2.36, 16.86)]. In the fully adjusted model excluding women with previous complications, primigravida women were more likely to bypass [OR = 3.70 (1.71, 8.01)]. Fully adjusted models for each set of analysis found that, for each additional emergency obstetric and newborn care signal function (EmONC SF) available at the nearest FLF, women's odds of bypassing almost halved.

CONCLUSIONS

Bypassing is highly associated with EmONC SF score at nearest FLF, controlling for individual and community-level factors.

Dependence of DCE-MRI biomarker values on analysis algorithm

Background

Dynamic contrast-enhanced MRI (DCE-MRI) biomarkers have proven utility in tumors in evaluating microvascular perfusion and permeability, but it is unclear whether measurements made in different centers are comparable due to methodological differences.

Purpose

To evaluate how commonly utilized analytical methods for DCE-MRI biomarkers affect both the absolute parameter values and repeatability.

Materials and Methods

DCE-MRI was performed on three consecutive days in twelve rats bearing C6 xenografts. Endothelial transfer constant (K^trans), extracellular extravascular space volume fraction (v_e), and contrast agent reflux rate constant (k_ep) measures were computed using: 2-parameter (“Tofts” or “standard Kety”) vs. 3-parameter (“General Kinetic” or “extended Kety”) compartmental models (including blood plasma volume fraction (v_p) with 3-parameter models); individual- vs. population-based vascular input functions (VIFs); and pixel-by-pixel vs. whole tumor-ROI. Variability was evaluated by within-subject coefficient of variation (wCV) and variance components analyses.

Results

DCE-MRI absolute parameter values and wCVs varied widely by analytical method. Absolute parameter values ranged, as follows, median K^trans, 0.09–0.18 min^-1; k_ep, 0.51–0.92 min^-1; v_e, 0.17–0.23; and v_p, 0.02–0.04. wCVs also varied widely by analytical method, as follows: mean K^trans, 32.9–61.9%; k_ep, 11.6–41.9%; v_e, 16.1–54.9%; and v_p, 53.9–77.2%. K^trans and k_ep values were lower with 3- than 2-parameter modeling (p<0.0001); k_ep and v_p were lower with pixel- than whole-ROI analyses (p<0.0006). wCVs were significantly smaller for v_e, and larger for k_ep, with individual- than population-based VIFs.

Conclusions

DCE-MRI parameter values and repeatability can vary widely by analytical methodology. Absolute values of DCE-MRI biomarkers are unlikely to be comparable between different studies unless analyses are carefully standardized.

Dynamic contrast-enhanced perfusion processing for neuroradiologists: model-dependent analysis may not be necessary for determining recurrent high-grade glioma versus treatment effect

BACKGROUND AND PURPOSE

Dynamic contrast-enhanced perfusion MR imaging has proved useful in determining whether a contrast-enhancing lesion is secondary to recurrent glial tumor or is treatment-related. In this article, we explore the best method for dynamic contrast-enhanced data analysis.

MATERIALS AND METHODS

We retrospectively reviewed 24 patients who met the following conditions: 1) had at least an initial treatment of a glioma, 2) underwent a half-dose contrast agent (0.05-mmol/kg) diagnostic-quality dynamic contrast-enhanced perfusion study for an enhancing lesion, and 3) had a diagnosis by pathology within 30 days of imaging. The dynamic contrast-enhanced data were processed by using model-dependent analysis (nordicICE) using a 2-compartment model and model-independent signal intensity with time. Multiple methods of determining the vascular input function and numerous perfusion parameters were tested in comparison with a pathologic diagnosis.

RESULTS

The best accuracy (88%) with good correlation compared with pathology (P = .005) was obtained by using a novel, model-independent signal-intensity measurement derived from a brief integration beginning after the initial washout and by using the vascular input function from the superior sagittal sinus for normalization. Modeled parameters, such as mean endothelial transfer constant > 0.05 minutes(-1), correlated (P = .002) but did not reach a diagnostic accuracy equivalent to the model-independent parameter.

CONCLUSIONS

A novel model-independent dynamic contrast-enhanced analysis method showed diagnostic equivalency to more complex model-dependent methods. Having a brief integration after the first pass of contrast may diminish the effects of partial volume macroscopic vessels and slow progressive enhancement characteristic of necrosis. The simple modeling is technique- and observer-dependent but is less time-consuming.

Quantitative imaging biomarkers: a review of statistical methods for computer algorithm comparisons

Quantitative biomarkers from medical images are becoming important tools for clinical diagnosis, staging, monitoring, treatment planning, and development of new therapies. While there is a rich history of the development of quantitative imaging biomarker (QIB) techniques, little attention has been paid to the validation and comparison of the computer algorithms that implement the QIB measurements. In this paper we provide a framework for QIB algorithm comparisons. We first review and compare various study designs, including designs with the true value (e.g. phantoms, digital reference images, and zero-change studies), designs with a reference standard (e.g. studies testing equivalence with a reference standard), and designs without a reference standard (e.g. agreement studies and studies of algorithm precision). The statistical methods for comparing QIB algorithms are then presented for various study types using both aggregate and disaggregate approaches. We propose a series of steps for establishing the performance of a QIB algorithm, identify limitations in the current statistical literature, and suggest future directions for research.

Prequit fMRI responses to pleasant cues and cigarette-related cues predict smoking cessation outcome

INTRODUCTION

The reasons that some smokers find it harder to quit than others are unclear. Understanding how individual differences predict smoking cessation outcomes may allow the development of more successful personalized treatments for nicotine dependence. Theoretical models suggest that drug users might be characterized by increased sensitivity to drug cues and by reduced sensitivity to nondrug-related natural rewards. We hypothesized that baseline differences in brain sensitivity to natural rewards and cigarette-related cues would predict the outcome of a smoking cessation attempt.

METHODS

Using functional magnetic resonance imaging, we recorded prequit brain responses to neutral, emotional (pleasant and unpleasant), and cigarette-related cues from 55 smokers interested in quitting. We then assessed smoking abstinence, mood, and nicotine withdrawal symptoms during the course of a smoking cessation attempt.

RESULTS

Using cluster analysis, we identified 2 groups of smokers who differed in their baseline responses to pleasant cues and cigarette-related cues in the posterior visual association areas, the dorsal striatum, and the medial and dorsolateral prefrontal cortex. Smokers who showed lower prequit levels of brain reactivity to pleasant stimuli than to cigarette-related cues were less likely to be abstinent 6 months after their quit attempt, and they had higher levels of negative affect during the course of the quit attempt.

CONCLUSIONS

Smokers with blunted brain responses to pleasant stimuli, relative to cigarette-related stimuli, had more difficulty quitting smoking. For these individuals, the lack of alternative forms of reinforcement when nicotine deprived might be an important factor underlying relapse. Normalizing these pathological neuroadaptations may help them achieve abstinence.

Meta-analysis of the technical performance of an imaging procedure: guidelines and statistical methodology

Medical imaging serves many roles in patient care and the drug approval process, including assessing treatment response and guiding treatment decisions. These roles often involve a quantitative imaging biomarker, an objectively measured characteristic of the underlying anatomic structure or biochemical process derived from medical images. Before a quantitative imaging biomarker is accepted for use in such roles, the imaging procedure to acquire it must undergo evaluation of its technical performance, which entails assessment of performance metrics such as repeatability and reproducibility of the quantitative imaging biomarker. Ideally, this evaluation will involve quantitative summaries of results from multiple studies to overcome limitations due to the typically small sample sizes of technical performance studies and/or to include a broader range of clinical settings and patient populations. This paper is a review of meta-analysis procedures for such an evaluation, including identification of suitable studies, statistical methodology to evaluate and summarize the performance metrics, and complete and transparent reporting of the results. This review addresses challenges typical of meta-analyses of technical performance, particularly small study sizes, which often causes violations of assumptions underlying standard meta-analysis techniques. Alternative approaches to address these difficulties are also presented; simulation studies indicate that they outperform standard techniques when some studies are small. The meta-analysis procedures presented are also applied to actual [18F]-fluorodeoxyglucose positron emission tomography (FDG-PET) test-retest repeatability data for illustrative purposes.

Multiparametric fat-water separation method for fast chemical-shift imaging guidance of thermal therapies

PURPOSE

A k-means-based classification algorithm is investigated to assess suitability for rapidly separating and classifying fat/water spectral peaks from a fast chemical shift imaging technique for magnetic resonance temperature imaging. Algorithm testing is performed in simulated mathematical phantoms and agar gel phantoms containing mixed fat/water regions.

METHODS

Proton resonance frequencies (PRFs), apparent spin-spin relaxation (T2*) times, and T1-weighted (T1-W) amplitude values were calculated for each voxel using a single-peak autoregressive moving average (ARMA) signal model. These parameters were then used as criteria for k-means sorting, with the results used to determine PRF ranges of each chemical species cluster for further classification. To detect the presence of secondary chemical species, spectral parameters were recalculated when needed using a two-peak ARMA signal model during the subsequent classification steps. Mathematical phantom simulations involved the modulation of signal-to-noise ratios (SNR), maximum PRF shift (MPS) values, analysis window sizes, and frequency expansion factor sizes in order to characterize the algorithm performance across a variety of conditions. In agar, images were collected on a 1.5T clinical MR scanner using acquisition parameters close to simulation, and algorithm performance was assessed by comparing classification results to manually segmented maps of the fat/water regions.

RESULTS

Performance was characterized quantitatively using the Dice Similarity Coefficient (DSC), sensitivity, and specificity. The simulated mathematical phantom experiments demonstrated good fat/water separation depending on conditions, specifically high SNR, moderate MPS value, small analysis window size, and low but nonzero frequency expansion factor size. Physical phantom results demonstrated good identification for both water (0.997 ± 0.001, 0.999 ± 0.001, and 0.986 ± 0.001 for DSC, sensitivity, and specificity, respectively) and fat (0.763 ± 0.006, 0.980 ± 0.004, and 0.941 ± 0.002 for DSC, sensitivity, and specificity, respectively). Temperature uncertainties, based on PRF uncertainties from a 5 × 5-voxel ROI, were 0.342 and 0.351°C for pure and mixed fat/water regions, respectively. Algorithm speed was tested using 25 × 25-voxel and whole image ROIs containing both fat and water, resulting in average processing times per acquisition of 2.00 ± 0.07 s and 146 ± 1 s, respectively, using uncompiled MATLAB scripts running on a shared CPU server with eight Intel Xeon(TM) E5640 quad-core processors (2.66 GHz, 12 MB cache) and 12 GB RAM.

CONCLUSIONS

Results from both the mathematical and physical phantom suggest the k-means-based classification algorithm could be useful for rapid, dynamic imaging in an ROI for thermal interventions. Successful separation of fat/water information would aid in reducing errors from the nontemperature sensitive fat PRF, as well as potentially facilitate using fat as an internal reference for PRF shift thermometry when appropriate. Additionally, the T1-W or R2* signals may be used for monitoring temperature in surrounding adipose tissue.

Reproducibility and comparison of DCE-MRI and DCE-CT perfusion parameters in a rat tumor model

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and computed tomography (DCE-CT) provide independent measures of biomarkers related to tumor perfusion. We compared the reproducibilities and absolute values of DCE-MRI and DCE-CT biomarkers in the same tumors in an animal model, to investigate the physiologic validity of both approaches. DCE-MRI and DCE-CT were each performed sequentially on three consecutive days in each of twelve rats bearing C6 glioma xenografts. DCE-MRI yielded endothelial transfer constant (K(trans)), extracellular, extravascular space volume fraction (v(e)), and contrast agent reflux rate constant (k(ep)); and DCE-CT, blood flow (BF), blood volume (BV), mean transit time (MTT), and permeability-surface area product (PS) using Tofts and deconvolution physiological models, with 6.6 and 0.4 seconds temporal resolutions, respectively. Variability in DCE-CT and DCE-MRI were evaluated by variance components analysis. Intra-rat coefficients of variation for DCE-CT parameters BF, BV, MTT and PS were 25%, 22%, 18% and 23%; and for DCE-MRI parameters K(trans), k(ep) and v(e) were 23%, 16% and 20%, respectively. Mean (±SD) BF, BV, MTT and PS were: 44.6 (±13.7) ml min(-1) 100 g(-1), 5.7 (±1.5) ml 100 g(-1), 10.8 (±2.3) seconds, and 14.6 (±4.7) ml min(-1) 100 g(-1), respectively. Mean (±SD) K(trans), k(ep) and v(e) were: 0.21 (±0.05) min(-1), 0.68 (±0.14) min(-1), and 0.29 (±0.06), respectively. Permeability estimates from DCE-MRI (K(trans)) were 44% higher than from DCE-CT (PS), despite application of appropriate corrections. DCE-MRI and DCE-CT biomarkers of tumor perfusion have similar reproducibilities suggesting that they may have comparable utility, but their derived parameter values are not equivalent.

Applications of imaging technology in radiation research

Imaging research and advances in systems engineering have enabled the transition of medical imaging from a means for accomplishing traditional anatomic visualization (i.e., orthopedic planar film X ray) to a means for noninvasively assessing a variety of functional measures. Perfusion imaging is one of the major highlights in functional imaging. In this work, various methods for measuring perfusion using widely-available commercial imaging modalities and contrast agents, specifically X ray and MR (magnetic resonance), will be described. The first section reviews general methods used for perfusion imaging, and the second section provides modality-specific information, focusing on the contrast mechanisms used to calculate perfusion-related parameters. The goal of these descriptions is to illustrate how perfusion imaging can be applied to radiation biology research.

Do brain responses to emotional images and cigarette cues differ? An fMRI study in smokers

Chronic smoking is thought to cause changes in brain reward systems that result in overvaluation of cigarette-related stimuli and undervaluation of natural rewards. We tested the hypotheses that, in smokers, brain circuits involved in emotional processing: (i) would be more active during exposure to cigarette-related than neutral pictures; and (ii) would be less active to pleasant compared with cigarette-related pictures, suggesting a devaluation of intrinsically pleasant stimuli. We obtained whole-brain blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging data from 35 smokers during the presentation of pleasant (erotica and romance), unpleasant (mutilations and sad), neutral, and cigarette-related pictures. Whole-brain analyses showed significantly larger BOLD responses during presentation of cigarette-related pictures relative to neutral ones within the secondary visual areas, the cingulate gyrus, the frontal gyrus, the dorsal striatum, and the left insula. BOLD responses to erotic pictures exceeded responses to cigarette-related pictures in all clusters except the insula. Within the left insula we observed larger BOLD responses to cigarette-related pictures than to all other picture categories. By including intrinsically pleasant and unpleasant pictures in addition to neutral ones, we were able to conclude that the presentation of cigarette-related pictures activates brain areas supporting emotional processes, but we did not find evidence of overall reduced activation of the brain reward systems in the presence of intrinsically pleasant stimuli.

Phase II study of aflibercept in recurrent malignant glioma: a North American Brain Tumor Consortium study

PURPOSE

Antivascular endothelial growth factor (anti-VEGF) therapy is a promising treatment approach for patients with recurrent glioblastoma. This single-arm phase II study evaluated the efficacy of aflibercept (VEGF Trap), a recombinantly produced fusion protein that scavenges both VEGF and placental growth factor in patients with recurrent malignant glioma.

PATIENTS AND METHODS

Forty-two patients with glioblastoma and 16 patients with anaplastic glioma who had received concurrent radiation and temozolomide and adjuvant temozolomide were enrolled at first relapse. Aflibercept 4 mg/kg was administered intravenously on day 1 of every 2-week cycle.

RESULTS

The 6-month progression-free survival rate was 7.7% for the glioblastoma cohort and 25% for patients with anaplastic glioma. Overall radiographic response rate was 24% (18% for glioblastoma and 44% for anaplastic glioma). The median progression-free survival was 24 weeks for patients with anaplastic glioma (95% CI, 5 to 31 weeks) and 12 weeks for patients with glioblastoma (95% CI, 8 to 16 weeks). A total of 14 patients (25%) were removed from the study for toxicity, on average less than 2 months from treatment initiation. The main treatment-related National Cancer Institute Common Terminology Criteria grades 3 and 4 adverse events (38 total) included fatigue, hypertension, and lymphopenia. Two grade 4 CNS ischemias and one grade 4 systemic hemorrhage were reported. Aflibercept rapidly decreases permeability on dynamic contrast enhanced magnetic resonance imaging, and molecular analysis of baseline tumor tissue identified tumor-associated markers of response and resistance.

CONCLUSION

Aflibercept monotherapy has moderate toxicity and minimal evidence of single-agent activity in unselected patients with recurrent malignant glioma.

Randomized double-blind placebo-controlled trial of bevacizumab therapy for radiation necrosis of the central nervous system

PURPOSE

To conduct a controlled trial of bevacizumab for the treatment of symptomatic radiation necrosis of the brain.

METHODS AND MATERIALS

A total of 14 patients were entered into a placebo-controlled randomized double-blind study of bevacizumab for the treatment of central nervous system radiation necrosis. All patients were required to have radiographic or biopsy proof of central nervous system radiation necrosis and progressive neurologic symptoms or signs. Eligible patients had undergone irradiation for head-and-neck carcinoma, meningioma, or low- to mid-grade glioma. Patients were randomized to receive intravenous saline or bevacizumab at 3-week intervals. The magnetic resonance imaging findings 3 weeks after the second treatment and clinical signs and symptoms defined the response or progression.

RESULTS

The volumes of necrosis estimated on T(2)-weighted fluid-attenuated inversion recovery and T(1)-weighted gadolinium-enhanced magnetic resonance imaging scans demonstrated that although no patient receiving placebo responded (0 of 7), all bevacizumab-treated patients did so (5 of 5 randomized and 7 of 7 crossover) with decreases in T(2)-weighted fluid-attenuated inversion recovery and T(1)-weighted gadolinium-enhanced volumes and a decrease in endothelial transfer constant. All bevacizumab-treated patients-and none of the placebo-treated patients-showed improvement in neurologic symptoms or signs. At a median of 10 months after the last dose of bevacizumab in patients receiving all four study doses, only 2 patients had experienced a recurrence of magnetic resonance imaging changes consistent with progressive radiation necrosis; one patient received a single additional dose of bevacizumab and the other patient received two doses.

CONCLUSION

The Class I evidence of bevacizumab efficacy from the present study in the treatment of central nervous system radiation necrosis justifies consideration of this treatment option for people with radiation necrosis secondary to the treatment of head-and-neck cancer and brain cancer.

Quantitative imaging test approval and biomarker qualification: interrelated but distinct activities

Quantitative imaging biomarkers could speed the development of new treatments for unmet medical needs and improve routine clinical care. However, it is not clear how the various regulatory and nonregulatory (eg, reimbursement) processes (often referred to as pathways) relate, nor is it clear which data need to be collected to support these different pathways most efficiently, given the time- and cost-intensive nature of doing so. The purpose of this article is to describe current thinking regarding these pathways emerging from diverse stakeholders interested and active in the definition, validation, and qualification of quantitative imaging biomarkers and to propose processes to facilitate the development and use of quantitative imaging biomarkers. A flexible framework is described that may be adapted for each imaging application, providing mechanisms that can be used to develop, assess, and evaluate relevant biomarkers. From this framework, processes can be mapped that would be applicable to both imaging product development and to quantitative imaging biomarker development aimed at increasing the effectiveness and availability of quantitative imaging.

SUPPLEMENTAL MATERIAL

http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.10100800/-/DC1.

Pharmacokinetics and magnetic resonance imaging of biodegradable macromolecular blood-pool contrast agent PG-Gd in non-human primates: a pilot study

The purpose of this study was to evaluate poly(L-glutamic acid)-benzyl-DTPA-Gd (PG-Gd), a new biodegradable macromolecular magnetic resonance imaging contrast agent, for its pharmacokinetics and MRI enhancement in nonhuman primates. Studies were performed in rhesus monkeys at intravenous doses of 0.01, 0.02 and 0.08 mmol Gd/kg. T(1)-weighted MR images were acquired at 1.5 T using fast spoiled gradient recalled echo and fast spin echo imaging protocols. The small-molecule contrast agent Magnevist was used as a control. PG-Gd in the monkey showed a bi-exponential disposition. The initial blood concentrations within 2 h of PG-Gd administration were much higher than those for Magnevist. The high blood concentration of PG-Gd was consistent with the MR imaging data, which showed prolonged circulation of PG-Gd in the blood pool. Enhancement of blood vessels and organs with a high blood perfusion (heart, liver, and kidney) was clearly visualized at 2 h after contrast injection at the three doses used. A greater than proportional increase of the area under the blood concentration-time curve was observed when the administered single dose was increased from 0.01 to 0.08 mmol/kg. By 2 days after PG-Gd injection, the contrast agent was mostly cleared from all major organs, including kidney. The mean residence time was 15 h at the 0.08 mmol/kg dose. A similar pharmacokinetic profile was observed in mice, with a mean residence time of 5.4 h and a volume of distribution at steady-state of 85.5 ml/kg, indicating that the drug was mainly distributed in the blood compartment. Based on this pilot study, further investigations on the potential systemic toxicity of PG-Gd in both rodents and large animals are warranted before testing this agent in humans.

Quantitative imaging to assess tumor response to therapy: common themes of measurement, truth data, and error sources

RATIONALE

Early detection of tumor response to therapy is a key goal. Finding measurement algorithms capable of early detection of tumor response could individualize therapy treatment as well as reduce the cost of bringing new drugs to market. On an individual basis, the urgency arises from the desire to prevent continued treatment of the patient with a high-cost and/or high-risk regimen with no demonstrated individual benefit and rapidly switch the patient to an alternative efficacious therapy for that patient. In the context of bringing new drugs to market, such algorithms could demonstrate efficacy in much smaller populations, which would allow phase 3 trials to achieve statistically significant decisions with fewer subjects in shorter trials.

MATERIALS AND METHODS

This consensus-based article describes multiple, image modality-independent means to assess the relative performance of algorithms for measuring tumor change in response to therapy. In this setting, we describe specifically the example of measurement of tumor volume change from anatomic imaging as well as provide an overview of other promising generic analytic methods that can be used to assess change in heterogeneous tumors. To support assessment of the relative performance of algorithms for measuring small tumor change, data sources of truth are required.

RESULTS

Very short interval clinical imaging examinations and phantom scans provide known truth for comparative evaluation of algorithms.

CONCLUSIONS

For a given category of measurement methods, the algorithm that has the smallest measurement noise and least bias on average will perform best in early detection of true tumor change.

Inflammatory breast cancer: defining breast magnetic resonance imaging features

Abstract #4008

PURPOSE: &#x2028; Inflammatory breast carcinoma (IBC) is an aggressive cancer with radiological and clinical features of a heterogeneous disease. The clinical presentation of IBC is characterized by the sudden onset of breast erythema and edema, often without an associated palpable breast mass. Mammography is associated with variable, not descriptive findings and it is often not diagnostic. We planned to address the role of Magnetic Resonance Imaging (MRI) of the breast in women with newly diagnosed IBC.&#x2028; MATERIALS AND METHODS:&#x2028; We performed a retrospective analysis of newly diagnosed IBC cases evaluated at the University of Texas. M D Anderson Cancer Center between December 2003 and February 2008. Baseline breast MRI exams were performed on 1.5-Tesla (T) or 3.0-T GE scanners and included dynamic three-dimensional T1-weighted fast spoiled gradient echo sequences with parallel imaging after a bolus injection of gadolinium based IV contrast. Exams were reviewed in batches and the findings rated in accordance with the ACR BI-RADS MRI Lexicon. The following parameters were evaluated; presence and characteristics of breast masses and skin description. All patients had concomitant mammograms and ultrasound exams.&#x2028; RESULTS: &#x2028; Eighty women with a clinical diagnosis of IBC were included in the study with a median age of 52 years, (range, 25 to 78). MRI detected a primary breast lesion in 80 of 82 symptomatic breasts (98%), compared to only 53 of 78 (68%) with mammogram (p &lt; 0.0001). The most common MRI morphologic characteristic of the lesions was irregular margins (72%) with heterogeneous internal enhancement (82%). Qualitative evaluation of the enhancement pattern demonstrated wash-out in 85% of the cases.&#x2028; MRI detected skin thickening in 90% of the patients (average skin thickness of 8 mm [range, 4-17 mm]), compared to 69% with mammogram. Heterogeneous skin enhancement (82%) and nodular or irregular skin lesions (43%) were identified only with MRI. These skin lesions demonstrated a progressive enhancing pattern, in contrast to the rapid wash-out pattern expected for malignant breast lesion.&#x2028; CONCLUSIONS: &#x2028; This study establishes breast MRI as the most sensitive and accurate imaging modality in assessing the breast of IBC patients. The most common radiological features are represented by the presence of a primary breast lesion and global skin thickening with heterogeneous skin enhancement. The combination of morphology and enhancing pattern may provide information to understand the biology of this disease and accurately monitor residual disease during neoadjuvant therapies.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 4008.

Improving fat-suppressed T2-weighted imaging of the head and neck with 2 fast spin-echo dixon techniques: initial experiences

Two modified fast spin-echo (FSE) techniques (a 2-point and a single-scan triple-echo Dixon) were used for T2-weighted imaging of the head and neck in 7 patients along with conventional FSE with fat saturation. Both Dixon techniques provided consistent and more uniform fat suppression (FS) than conventional FSE. The 2-point Dixon technique was noted to be more susceptible to motion artifacts. The triple-echo Dixon technique offered the best scan time efficiency and overall image quality.

Phase II trial of irinotecan and thalidomide in adults with recurrent glioblastoma multiforme

This phase II study aimed at determining the efficacy and safety of irinotecan combined with thalidomide in adults with recurrent glioblastoma multiforme (GBM) not taking enzyme-inducing anticonvulsants (EIACs). Adult patients (> or =18 years) with recurrent GBM with up to three relapses following surgery and radiation therapy were eligible for this trial. The primary end point was rate of progression-free survival at 6 months (PFS-6); secondary end points were response rate, overall survival, and toxicity. Patients were treated in 6-week cycles with 125 mg/m(2) irinotecan weekly for 4 weeks followed by 2 weeks off treatment and 100 mg of thalidomide daily increased as tolerated to 400 mg/day. Of 32 evaluable patients, 8 (25%) were alive and progression free at 6 months. The median PFS was 13 weeks. One patient experienced a complete response, one a partial response, and 19 stable disease. Median overall survival time from entry into the study was 36 weeks, and the 1-year survival rate was 34%. Adverse events (grade 3 or 4) included diarrhea, abdominal cramps, lymphopenia, neutropenia, and fatigue. Two of the four deaths that occurred were possibly due to treatment-related toxicity. The combination of irinotecan, a cytotoxic agent, and thalidomide, an antiangiogenic agent, shows promising activity against recurrent GBM in patients not receiving EIACs and warrants further study. The results also provide support for similar strategies using combination therapies with newer targeted antiangiogenic agents to generate effective therapies against malignant gliomas.

Comparison of single- and dual-tracer pharmacokinetic modeling of dynamic contrast-enhanced MRI data using low, medium, and high molecular weight contrast agents

Pharmacokinetic parameters corresponding to perfused microvascular volume determined from dynamic contrast-enhanced (DCE) MRI data were compared to immunohistochemical measures of microvascular density (MVD) and perfused microvascular density. DCE MRI data from human mammary tumors (MDA-MB-435) implanted in nude mice using low (Gd-DTPA, MW approximately equal 0.6 kDa), medium (Gadomer-17, MW(eff) approximately equal 35 kDa), and high (PG-Gd-DTPA, MW approximately equal 220 kDa) molecular weight contrast agents were analyzed with single- and dual-tracer pharmacokinetic models. MVD values were determined by two manual counting methods, "hot spot" and summed region of interest (SROI). Pharmacokinetic parameters determined using the single-tracer model (Gd-DTPA [n = 15] and Gadomer-17 [n = 13]) did not correlate with MVD measures using either manual counting method. For dual-tracer studies (Gadomer-17/Gd-DTPA [n = 15] and PG-Gd-DTPA/Gd-DTPA [n = 13]), pharmacokinetic parameters demonstrated a statistically significant correlation with MVD determined by the SROI method, but not the "hot spot" method. Ten mice successfully underwent intravital FITC-labeled lectin perfusion with the hemisphere of highest lectin labeling correlating with pharmacokinetic parameter values in 9 of 10 tumors (single-tracer Gd-DTPA [n = 2], single-tracer Gadomer-17 [n = 3], and dual-tracer Gadomer-17/Gd-DTPA [n = 5]). This study demonstrates that dual-tracer DCE MRI studies yield pharmacokinetic parameters that correlate with immunohistochemical measures of MVD.

Magnetic resonance imaging of therapy-induced necrosis using gadolinium-chelated polyglutamic acids

PURPOSE

Necrosis is the most common morphologic alteration found in tumors and surrounding normal tissues after radiation therapy or chemotherapy. Accurate measurement of necrosis may provide an early indication of treatment efficacy or associated toxicity. The purpose of this report is to evaluate the selective accumulation of polymeric paramagnetic magnetic resonance (MR) contrast agents--gadolinium p-aminobenzyl-diethylenetriaminepentaacetic acid-poly(glutamic acid) (L-PG-DTPA-Gd and D-PG-DTPA-Gd)--in necrotic tissue.

METHODS AND MATERIALS

Two different solid tumor models, human Colo-205 xenograft and syngeneic murine OCA-1 ovarian tumors, were used in this study. Necrotic response was induced by treatment with poly(L-glutamic acid)-paclitaxel conjugate (PG-TXL). T(1)-weighted spin-echo images were obtained immediately and up to 4 days after contrast injection and compared with corresponding histologic specimens. Two low-molecular-weight contrast agents, DTPA-Gd and oligomeric(L-glutamic acid)-DTPA-Gd, were used as nonspecific controls.

RESULTS

Initially, there was minimal tumor enhancement after injection of either L-PG-DTPA-Gd or D-PG-DTPA-Gd, but rapid enhancement after injection of low-molecular-weight agents. However, polymeric contrast agents, but not low-molecular-weight contrast agents, caused sustained enhancement in regions of tumor necrosis in both tumors treated with PG-TXL and untreated tumors. These data indicate that high molecular weight, rather than in vivo biodegradation, is necessary for the specific localization of polymeric MR contrast agents to necrotic tissue. Moreover, biotinylated L-PG-DTPA-Gd colocalized with macrophages in the tumor necrotic areas, suggesting that selective accumulation of L- and D-PG-DTPA-Gd in necrotic tissue was mediated through residing macrophages.

CONCLUSIONS

Our data suggest that MR imaging with PG-DTPA-Gd may be a useful technique for noninvasive characterization of treatment-induced necrosis.

Book Review

Imaging Systems for Medical Diagnostics: Fundamentals, Technical Solutions, Applications for Systems Applying Ionizing Radiation, Nuclear Magnetic Resonance and Ultrasound, Arnulf Oppelt, Editor, Publicis Corporate Publishing, John Wiley and Sons Distributor, ISBN 3‐9578‐226‐2 (cloth) $145

intraoperative neuronavigation using diffusion tensor MR tractography for the resection of a deep tumor adjacent to the corticospinal tract

OBJECTIVE AND IMPORTANCE

Delineation of cerebral white matter tracts using MR tractography adds essential information for planning intracranial surgery. Integrating tractography with intraoperative neuronavigation may reduce the likelihood of new neurological deficits after surgery done to remove tumors adjacent to the projection fibers of eloquent cortex. We report the utility of such integration for the resection of deep (paraventricular) tumors.

CLINICAL PRESENTATION

A 67-year-old male with malignant melanoma underwent stereotactic radiosurgery for a single metastasis within the paraventricular white matter of the right frontal lobe near the corticospinal tract. The lesion doubled in size within 12 months of radiotherapy. Surgical extirpation was performed aided by intraoperative neuronavigation.

TECHNIQUE

MR images of the brain including MR tractography and post-contrast T1-weighted sequences were acquired and imported into a neuronavigational workstation. Asymmetric fusion of contrast-enhanced images and tractography was employed to assist in preservation of the integrity of critical white matter tracts during the surgical procedure.

CONCLUSION

Inclusion of tractography in standard imaging protocols for neuronavigational systems may increase the safety of neurosurgical intervention near white matter tracts, including deep areas adjacent to the ventricles.

Comparison of Monte Carlo calculations around a Fletcher Suit Delclos ovoid with radiochromic film and normoxic polymer gel dosimetry

The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33 microGy m2 h(-1) Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within +/- 2% or 2 mm for 98% of points compared with RCF measurements and to within +/- 3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.

A phase I surrogate endpoint study of SU6668 in patients with solid tumors

PURPOSE

To evaluate the biologic effects of SU6668 in patients with solid tumors using comprehensive measures of pharmacokinetics (PK), functional imaging, and tissue correlative studies.

EXPERIMENTAL DESIGN

Eligible patients with tumors accessible for core needle biopsy were treated with SU6668 at doses of 200 or 400 mg/m(2)/day. Functional computed tomography (CT) scan and dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) were performed at baseline and repeated 4 weeks and 12 weeks after treatment for analysis of tumor angiogenesis. The PK was analyzed using a high-performance liquid chromatography assay. Tumor specimens obtained via core needle biopsy at baseline and 4 weeks later were analyzed for the biologic effects of SU6668.

RESULTS

Six of a total of seven patients received treatment for at least 3 months and underwent comprehensive correlative studies, including PK, imaging, and tissue biopsy. Functional CT showed that five of six patients had decreased blood flow in tumors in response to treatment, and DCE-MRI results indicated significant change of area under the signal intensity vs. time curve (AUC) and/or maximum slope (maximum rate of signal intensity change) in two of four patients evaluated with this technique. PK studies showed that the mean apparent oral clearance (Cl(oral)) measured on day 1 was 6.3 +/- 2.7 L/hr/m(2), yielding a mean AUC of 16.6 +/- 4.3 mg/L.hr. By day 22, the Cl(oral) was 40% more than that observed on day 1.

CONCLUSION

It is feasible to evaluate the biologic effects of antiangiogenic agents using comprehensive surrogate measures.

Synthesis and characterization of poly(L-glutamic acid) gadolinium chelate: a new biodegradable MRI contrast agent

Most currently evaluated macromolecular contrast agents for magnetic resonance imaging (MRI) are not biodegradable. The goal of this study is to synthesize and characterize poly(l-glutamic acid) (PG) gadolinium chelates as biodegradable blood-pool MRI contrast agents. Two PG chelates of gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) were synthesized through the use of difunctional and monofunctional DTPA precursors. The conjugates were characterized with regard to molecular weight and molecular weight distribution, gadolinium content, relaxivity, and degradability. Distributions of the polymeric MRI contrast agents in various organs were determined by intravenous injection of (111)In-labeled polymers into mice bearing murine breast tumors. MRI scans were performed at 1.5 T in mice after bolus injection of the polymeric chelates. PG-Hex-DTPA-Gd, obtained from aminohexyl-substituted PG and DTPA-dianhydride, was partially cross-linked and was undegradable in the presence of cathepsin B. On the other hand, PG-Bz-DTPA-Gd synthesized directly from PG and monofunctional p-aminobenzyl-DTPA(acetic acid-tert-butyl ester) was a linear polymer and was degradable. The relaxivities of the polymers at 1.5 T were 3-8 times as great as that of Gd-DTPA. Both polymers had high blood concentrations and were primarily accumulated in the kidney. However, PG-Bz-DTPA-Gd was gradually cleared from the body and had significantly less retention in the blood, the spleen, and the kidney. MRI with PG-Bz-DTPA-Gd in mice showed enhanced vascular contrast at up to 2 h after the contrast agent injection. The ability of PG-Bz-DTPA-Gd to be degraded and cleared from the body makes it a favorable macromolecular MRI contrast agent.

Experience in implementing continuous arterial spin labeling on a commercial MR scanner

Continuous arterial spin labeling (CASL) is a technique for performing quantitative perfusion measurements without the need for exogenous contrast agent administration. This technique has seen limited use in the clinic due to problems of poor sensitivity and the potential for artifacts. In addition, CASL requires the application of long‐duration radiofrequency pulses and the acquisition of a large number of images, which can cause difficulties when implemented on commercial MR scanners. This work details our experience in implementing CASL on a commercial MR scanner for the measurement of cerebral blood flow, including pitfalls regarding hardware, radiofrequency energy deposition, and practical application in human subjects. Results of studies to determine the optimal acquisition procedures are also presented.

PACS number: 87.61.‐c

Input parameter sensitivity analysis and comparison of quantification models for continuous arterial spin labeling

The regional cerebral blood flow (rCBF) values determined using continuous arterial spin labeling (CASL) are subject to several sources of variability, including natural physiologic variations, sensitivity to the input parameters, and the use of different quantification models. To date, a thorough analysis of the impact of input parameters and the choice of quantification model has not been performed. These sources of variability were investigated through computer simulations using bootstrap techniques on actual CASL data. Coefficients of variation for representative single voxels were 6.7% for gray matter and 29% for white matter, and for eight-voxel regions of interest they were 4.5% for gray matter and 23% for white matter. Comparison of nine CASL quantification models showed differences in gray matter rCBF values of up to 42%. An analysis of the sensitivity of the rCBF to input parameters for each of the nine quantification models demonstrated that accurate quantification of the inversion efficiency, tissue and arterial blood longitudinal relaxation times, and transit times were critical in calculating precise rCBF values. The large potential variations in rCBF and the effect of the choice of quantification model suggest that interpreting absolute rCBF values in CASL studies can be challenging and requires great care.

Real-time motion detection of functional MRI data

The objective of this work was to implement a motion‐detection algorithm on a commercial real‐time functional magnetic resonance imaging (fMRI) processing package for neurosurgical planning applications. A real‐time motion detection module was implemented on a commercial real‐time processing package. Simulated functional data sets with introduced translational, in‐plane rotational, and through‐plane rotational motion were created. The coefficient of variation (COV) of the center of intensity was used as a motion quantification metric. Coefficients of variation were calculated before and after image registration to determine the effectiveness of the motion correction; the limits of correctability were also determined. The motion‐detection module allowed for real‐time quantification of the motion in an fMRI experiment. Along with knowledge of the limits of correctability, this enables determination of whether an experiment needs to be reacquired while the patient is in the scanner. This study establishes the feasibility of using real‐time motion detection for presurgical planning fMRI and establishes the limits of correctable motion.

PACS number: 87.61.‐c

Assessment of geometrical accuracy of magnetic resonance images for radiation therapy of lung cancers

The purpose of this research was to investigate the geometrical accuracy of magnetic resonance (MR) images used in the radiation therapy treatment planning for lung cancer. In this study, the capability of MR imaging to acquire dynamic two-dimensional images was explored to access the motion of lung tumors. Due to a number of factors, including the use of a large field-of-view for the thorax, MR images are particularly subject to geometrical distortions caused by the inhomogeneity and gradient nonlinearity of the magnetic field. To quantify such distortions, we constructed a phantom, which approximated the dimensions of the upper thorax and included two air cavities. Evenly spaced vials containing contrast agent could be held in three directions with their cross-sections in the coronal, sagittal, and axial planes, respectively, within the air cavities. MR images of the phantom were acquired using fast spin echo (FSE) and fast gradient echo (fGRE) sequences. The positions of the vials according to their centers of mass were measured from the MR images and registered to the corresponding computed tomography images for comparison. Results showed the fGRE sequence exhibited no errors >2.0 mm in the sagittal and coronal planes, whereas the FSE sequence produced images with errors between 2.0 and 4.0 mm along the phantom's perimeter in the axial plane. On the basis of these results, the fGRE sequence was considered to be clinically acceptable in acquiring images in all sagittal and coronal planes tested. However, the spatial accuracy in periphery of the axial FSE images exceeded the acceptable criteria for the acquisition parameters used in this study.