Reducing femoral flow artefacts in radial magnetic resonance fingerprinting of the prostate using region-optimised virtual coils

High acceleration factors in radial magnetic resonance fingerprinting (MRF) of the prostate lead to strong streak-like artefacts from flow in the femoral blood vessels, possibly concealing important anatomical information. Region-optimised virtual (ROVir) coils is a beamforming-based framework to create virtual coils that maximise signal in a region of interest while minimising signal in a region of interference. In this study, the potential of removing femoral flow streak artefacts in prostate MRF using ROVir coils is demonstrated in silico and in vivo. The ROVir framework was applied to radial MRF k-space data in an automated pipeline designed to maximise prostate signal while minimising signal from the femoral vessels. The method was tested in 15 asymptomatic volunteers at 3 T. The presence of streaks was visually assessed and measurements of whole prostate T1, T2 and signal-to-noise ratio (SNR) with and without streak correction were examined. In addition, a purpose-built simulation framework in which blood flow through the femoral vessels can be turned on and off was used to quantitatively evaluate ROVir's ability to suppress streaks in radial prostate MRF. In vivo it was shown that removing selected ROVir coils visibly reduces streak-like artefacts from the femoral blood flow, without increasing the reconstruction time. On average, 80% of the prostate SNR was retained. A similar reduction of streaks was also observed in silico, while the quantitative accuracy of T1 and T2 mapping was retained. In conclusion, ROVir coils efficiently suppress streaking artefacts from blood flow in radial MRF of the prostate, thereby improving the visual clarity of the images, without significant sacrifices to acquisition time, reconstruction time and accuracy of quantitative values. This is expected to help enable T1 and T2 mapping of prostate cancer in clinically viable times, aiding differentiation between prostate cancer from noncancer and healthy prostate tissue.

Performance of magnetic resonance imaging-based prostate cancer risk calculators and decision strategies in two large European medical centres

OBJECTIVES

To compare the performance of currently available biopsy decision support tools incorporating magnetic resonance imaging (MRI) findings in predicting clinically significant prostate cancer (csPCa).

PATIENTS AND METHODS

We retrospectively included men who underwent prostate MRI and subsequent targeted and/or systematic prostate biopsies in two large European centres. Available decision support tools were identified by a PubMed search. Performance was assessed by calibration, discrimination, decision curve analysis (DCA) and numbers of biopsies avoided vs csPCa cases missed, before and after recalibration, at risk thresholds of 5%-20%.

RESULTS

A total of 940 men were included, 507 (54%) had csPCa. The median (interquartile range) age, prostate-specific antigen (PSA) level, and PSA density (PSAD) were 68 (63-72) years, 9 (7-15) ng/mL, and 0.20 (0.13-0.32) ng/mL2 , respectively. In all, 18 multivariable risk calculators (MRI-RCs) and dichotomous biopsy decision strategies based on MRI findings and PSAD thresholds were assessed. The Van Leeuwen model and the Rotterdam Prostate Cancer Risk Calculator (RPCRC) had the best discriminative ability (area under the receiver operating characteristic curve 0.86) of the MRI-RCs that could be assessed in the whole cohort. DCA showed the highest clinical utility for the Van Leeuwen model, followed by the RPCRC. At the 10% threshold the Van Leeuwen model would avoid 22% of biopsies, missing 1.8% of csPCa, whilst the RPCRC would avoid 20% of biopsies, missing 2.6% of csPCas. These multivariable models outperformed all dichotomous decision strategies based only on MRI-findings and PSAD.

CONCLUSIONS

Even in this high-risk cohort, biopsy decision support tools would avoid many prostate biopsies, whilst missing very few csPCa cases. The Van Leeuwen model had the highest clinical utility, followed by the RPCRC. These multivariable MRI-RCs outperformed and should be favoured over decision strategies based only on MRI and PSAD.

The impact of pre-processing and disease characteristics on reproducibility of T2-weighted MRI radiomics features

PURPOSE

To evaluate the reproducibility of radiomics features derived via different pre-processing settings from paired T2-weighted imaging (T2WI) prostate lesions acquired within a short interval, to select the setting that yields the highest number of reproducible features, and to evaluate the impact of disease characteristics (i.e., clinical variables) on features reproducibility.

MATERIALS AND METHODS

A dataset of 50 patients imaged using T2WI at 2 consecutive examinations was used. The dataset was pre-processed using 48 different settings. A total of 107 radiomics features were extracted from manual delineations of 74 lesions. The inter-scan reproducibility of each feature was measured using the intra-class correlation coefficient (ICC), with ICC values > 0.75 considered good. Statistical differences were assessed using Mann-Whitney U and Kruskal-Wallis tests.

RESULTS

The pre-processing parameters strongly influenced the reproducibility of radiomics features of T2WI prostate lesions. The setting that yielded the highest number of features (25 features) with high reproducibility was the relative discretization with a fixed bin number of 64, no signal intensity normalization, and outlier filtering by excluding outliers. Disease characteristics did not significantly impact the reproducibility of radiomics features.

CONCLUSION

The reproducibility of T2WI radiomics features was significantly influenced by pre-processing parameters, but not by disease characteristics. The selected pre-processing setting yielded 25 reproducible features.

Label-set impact on deep learning-based prostate segmentation on MRI

BACKGROUND

Prostate segmentation is an essential step in computer-aided detection and diagnosis systems for prostate cancer. Deep learning (DL)-based methods provide good performance for prostate gland and zones segmentation, but little is known about the impact of manual segmentation (that is, label) selection on their performance. In this work, we investigated these effects by obtaining two different expert label-sets for the PROSTATEx I challenge training dataset (n = 198) and using them, in addition to an in-house dataset (n = 233), to assess the effect on segmentation performance. The automatic segmentation method we used was nnU-Net.

RESULTS

The selection of training/testing label-set had a significant (p < 0.001) impact on model performance. Furthermore, it was found that model performance was significantly (p < 0.001) higher when the model was trained and tested with the same label-set. Moreover, the results showed that agreement between automatic segmentations was significantly (p < 0.0001) higher than agreement between manual segmentations and that the models were able to outperform the human label-sets used to train them.

CONCLUSIONS

We investigated the impact of label-set selection on the performance of a DL-based prostate segmentation model. We found that the use of different sets of manual prostate gland and zone segmentations has a measurable impact on model performance. Nevertheless, DL-based segmentation appeared to have a greater inter-reader agreement than manual segmentation. More thought should be given to the label-set, with a focus on multicenter manual segmentation and agreement on common procedures.

CRITICAL RELEVANCE STATEMENT

Label-set selection significantly impacts the performance of a deep learning-based prostate segmentation model. Models using different label-set showed higher agreement than manual segmentations.

KEY POINTS

• Label-set selection has a significant impact on the performance of automatic segmentation models. • Deep learning-based models demonstrated true learning rather than simply mimicking the label-set. • Automatic segmentation appears to have a greater inter-reader agreement than manual segmentation.

Pelvic PET/MR attenuation correction in the image space using deep learning

Introduction

The five-class Dixon-based PET/MR attenuation correction (AC) model, which adds bone information to the four-class model by registering major bones from a bone atlas, has been shown to be error-prone. In this study, we introduce a novel method of accounting for bone in pelvic PET/MR AC by directly predicting the errors in the PET image space caused by the lack of bone in four-class Dixon-based attenuation correction.

Methods

A convolutional neural network was trained to predict the four-class AC error map relative to CT-based attenuation correction. Dixon MR images and the four-class attenuation correction µ-map were used as input to the models. CT and PET/MR examinations for 22 patients ([18F]FDG) were used for training and validation, and 17 patients were used for testing (6 [18F]PSMA-1007 and 11 [68Ga]Ga-PSMA-11). A quantitative analysis of PSMA uptake using voxel- and lesion-based error metrics was used to assess performance.

Results

In the voxel-based analysis, the proposed model reduced the median root mean squared percentage error from 12.1% and 8.6% for the four- and five-class Dixon-based AC methods, respectively, to 6.2%. The median absolute percentage error in the maximum standardized uptake value (SUVmax) in bone lesions improved from 20.0% and 7.0% for four- and five-class Dixon-based AC methods to 3.8%.

Conclusion

The proposed method reduces the voxel-based error and SUVmax errors in bone lesions when compared to the four- and five-class Dixon-based AC models.

CROPro: a tool for automated cropping of prostate magnetic resonance images

Purpose

To bypass manual data preprocessing and optimize deep learning performance, we developed and evaluated CROPro, a tool to standardize automated cropping of prostate magnetic resonance (MR) images.

Approach

CROPro enables automatic cropping of MR images regardless of patient health status, image size, prostate volume, or pixel spacing. CROPro can crop foreground pixels from a region of interest (e.g., prostate) with different image sizes, pixel spacing, and sampling strategies. Performance was evaluated in the context of clinically significant prostate cancer (csPCa) classification. Transfer learning was used to train five convolutional neural network (CNN) and five vision transformer (ViT) models using different combinations of cropped image sizes ( 64 × 64 , 128 × 128 , and 256 × 256  pixels2), pixel spacing ( 0.2 × 0.2 , 0.3 × 0.3 , 0.4 × 0.4 , and 0.5 × 0.5    mm 2 ), and sampling strategies (center, random, and stride cropping) over the prostate. T2-weighted MR images ( N = 1475 ) from the online available PI-CAI challenge were used to train ( N = 1033 ), validate ( N = 221 ), and test ( N = 221 ) all models.

Results

Among CNNs, SqueezeNet with stride cropping (image size: 128 × 128 , pixel spacing: 0.2 × 0.2    mm 2 ) achieved the best classification performance ( 0.678 ± 0.006 ). Among ViTs, ViT-H/14 with random cropping (image size: 64 × 64 and pixel spacing: 0.5 × 0.5    mm 2 ) achieved the best performance ( 0.756 ± 0.009 ). Model performance depended on the cropped area, with optimal size generally larger with center cropping ( ∼ 40    cm 2 ) than random/stride cropping ( ∼ 10    cm 2 ).

Conclusion

We found that csPCa classification performance of CNNs and ViTs depends on the cropping settings. We demonstrated that CROPro is well suited to optimize these settings in a standardized manner, which could improve the overall performance of deep learning models.

Artificial intelligence for prostate MRI: open datasets, available applications, and grand challenges

Artificial intelligence (AI) for prostate magnetic resonance imaging (MRI) is starting to play a clinical role for prostate cancer (PCa) patients. AI-assisted reading is feasible, allowing workflow reduction. A total of 3,369 multi-vendor prostate MRI cases are available in open datasets, acquired from 2003 to 2021 in Europe or USA at 3 T (n = 3,018; 89.6%) or 1.5 T (n = 296; 8.8%), 346 cases scanned with endorectal coil (10.3%), 3,023 (89.7%) with phased-array surface coils; 412 collected for anatomical segmentation tasks, 3,096 for PCa detection/classification; for 2,240 cases lesions delineation is available and 56 cases have matching histopathologic images; for 2,620 cases the PSA level is provided; the total size of all open datasets amounts to approximately 253 GB. Of note, quality of annotations provided per dataset highly differ and attention must be paid when using these datasets (e.g., data overlap). Seven grand challenges and commercial applications from eleven vendors are here considered. Few small studies provided prospective validation. More work is needed, in particular validation on large-scale multi-institutional, well-curated public datasets to test general applicability. Moreover, AI needs to be explored for clinical stages other than detection/characterization (e.g., follow-up, prognosis, interventions, and focal treatment).

Reducing prostate biopsies and magnetic resonance imaging with prostate cancer risk stratification

Objectives

Patients and Methods

Results

Conclusion

Divergent Regulation of Decidual Oxidative-Stress Response by NRF2 and KEAP1 in Preeclampsia with and without Fetal Growth Restriction

Utero-placental development in pregnancy depends on direct maternal–fetal interaction in the uterine wall decidua. Abnormal uterine vascular remodeling preceding placental oxidative stress and placental dysfunction are associated with preeclampsia and fetal growth restriction (FGR). Oxidative stress is counteracted by antioxidants and oxidative repair mechanisms regulated by the transcription factor nuclear factor erythroid 2-related factor 2 (NRF2). We aimed to determine the decidual regulation of the oxidative-stress response by NRF2 and its negative regulator Kelch-like ECH-associated protein 1 (KEAP1) in normal pregnancies and preeclamptic pregnancies with and without FGR. Decidual tissue from 145 pregnancies at delivery was assessed for oxidative stress, non-enzymatic antioxidant capacity, cellular NRF2- and KEAP1-protein expression, and NRF2-regulated transcriptional activation. Preeclampsia combined with FGR was associated with an increased oxidative-stress level and NRF2-regulated gene expression in the decidua, while decidual NRF2- and KEAP1-protein expression was unaffected. Although preeclampsia with normal fetal growth also showed increased decidual oxidative stress, NRF2-regulated gene expression was reduced, and KEAP1-protein expression was increased in areas of high trophoblast density. The trophoblast-dependent KEAP1-protein expression in preeclampsia with normal fetal growth indicates control of decidual oxidative stress by maternal–fetal interaction and underscores the importance of discriminating between preeclampsia with and without FGR.

The Reproducibility of Deep Learning-Based Segmentation of the Prostate Gland and Zones on T2-Weighted MR Images

Volume of interest segmentation is an essential step in computer-aided detection and diagnosis (CAD) systems. Deep learning (DL)-based methods provide good performance for prostate segmentation, but little is known about the reproducibility of these methods. In this work, an in-house collected dataset from 244 patients was used to investigate the intra-patient reproducibility of 14 shape features for DL-based segmentation methods of the whole prostate gland (WP), peripheral zone (PZ), and the remaining prostate zones (non-PZ) on T2-weighted (T2W) magnetic resonance (MR) images compared to manual segmentations. The DL-based segmentation was performed using three different convolutional neural networks (CNNs): V-Net, nnU-Net-2D, and nnU-Net-3D. The two-way random, single score intra-class correlation coefficient (ICC) was used to measure the inter-scan reproducibility of each feature for each CNN and the manual segmentation. We found that the reproducibility of the investigated methods is comparable to manual for all CNNs (14/14 features), except for V-Net in PZ (7/14 features). The ICC score for segmentation volume was found to be 0.888, 0.607, 0.819, and 0.903 in PZ; 0.988, 0.967, 0.986, and 0.983 in non-PZ; 0.982, 0.975, 0.973, and 0.984 in WP for manual, V-Net, nnU-Net-2D, and nnU-Net-3D, respectively. The results of this work show the feasibility of embedding DL-based segmentation in CAD systems, based on multiple T2W MR scans of the prostate, which is an important step towards the clinical implementation.

Exploring the diagnostic potential of adding T2 dependence in diffusion-weighted MR imaging of the prostate

Background

Magnetic resonance imaging (MRI) is essential in the detection and staging of prostate cancer. However, improved tools to distinguish between low-risk and high-risk cancer are needed in order to select the appropriate treatment.

Purpose

To investigate the diagnostic potential of signal fractions estimated from a two-component model using combined T2- and diffusion-weighted imaging (T2-DWI).

Material and methods

62 patients with prostate cancer and 14 patients with benign prostatic hyperplasia (BPH) underwent combined T2-DWI (TE = 55 and 73 ms, b-values = 50 and 700 s/mm²) following clinical suspicion of cancer, providing a set of 4 measurements per voxel. Cancer was confirmed in post-MRI biopsy, and regions of interest (ROIs) were delineated based on radiology reporting. Signal fractions of the slow component (SF_slow) of the proposed two-component model were calculated from a model fit with 2 free parameters, and compared to conventional bi- and mono-exponential apparent diffusion coefficient (ADC) models.

Results

All three models showed a significant difference (p<0.0001) between peripheral zone (PZ) tumor and normal tissue ROIs, but not between non-PZ tumor and BPH ROIs. The area under the receiver operating characteristics curve distinguishing tumor from prostate voxels was 0.956, 0.949 and 0.949 for the two-component, bi-exponential and mono-exponential models, respectively. The corresponding Spearman correlation coefficients between tumor values and Gleason Grade Group were fair (0.370, 0.499 and -0.490), but not significant.

Conclusion

Signal fraction estimates from a two-component model based on combined T2-DWI can differentiate between tumor and normal prostate tissue and show potential for prostate cancer diagnosis. The model performed similarly to conventional diffusion models.

Decidual and placental NOD1 is associated with inflammation in normal and preeclamptic pregnancies

INTRODUCTION

Inflammation is a normal physiological process that increases to harmful levels in preeclampsia. It affects the interaction between maternal immune cells and fetal trophoblasts at both sites of the maternal-fetal interface; decidua and placenta. The pattern recognition receptor nucleotide-binding oligomerization domain-containing protein (NOD)1 is expressed at both sites. This study aimed to characterize the cellular expression and functionality of NOD1 at the maternal-fetal interface of normal and preeclamptic pregnancies.

METHODS

Women with normal or preeclamptic pregnancies delivered by caesarean section were included. Decidual (n = 90) and placental (n = 91) samples were analyzed for NOD1 expression by immunohistochemistry and an automated image-based quantification method. Decidual and placental explants were incubated with or without the NOD1-agonist iE-DAP and cytokine responses measured by ELISA.

RESULTS

NOD1 was markedly expressed by maternal cells in the decidua and by fetal trophoblasts in both decidua and placenta, with trophoblasts showing the highest NOD1 expression. Preeclampsia with normal fetal growth was associated with a trophoblast-dependent increase in decidual NOD1 expression density. Compared to normal pregnancies, preeclampsia demonstrated stronger correlation between decidual and placental NOD1 expression levels. Increased production of interleukin (IL)-6 or IL-8 after in vitro explant stimulation confirmed NOD1 functionality.

DISCUSSION

These findings suggest that NOD1 contributes to inflammation at the maternal-fetal interface in normal pregnancies and preeclampsia and indicate a role in direct maternal-fetal communication. The strong expression of NOD1 by all trophoblast types highlights the importance of combined assessment of decidua and placenta for overall understanding of pathophysiological processes at the maternal-fetal interface.

Detection of Recurrent Prostate Cancer With 18F-Fluciclovine PET/MRI

Objective

Simultaneous PET/MRI combines soft-tissue contrast of MRI with high molecular sensitivity of PET in one session. The aim of this prospective study was to evaluate detection rates of recurrent prostate cancer by ¹⁸F-fluciclovine PET/MRI.

Methods

Patients with biochemical recurrence (BCR) or persistently detectable prostate specific antigen (PSA), were examined with simultaneous ¹⁸F-fluciclovine PET/MRI. Multiparametric MRI (mpMRI) and PET/MRI were scored on a 3-point scale (1-negative, 2-equivocal, 3-recurrence/metastasis) and detection rates (number of patients with suspicious findings divided by total number of patients) were reported. Detection rates were further stratified based on PSA level, PSA doubling time (PSAdt), primary treatment and inclusion criteria (PSA persistence, European Association of Urology (EAU) Low-Risk BCR and EAU High-Risk BCR). A detailed investigation of lesions with discrepancy between mpMRI and PET/MRI scores was performed to evaluate the incremental value of PET/MRI to mpMRI. The impact of the added PET acquisition on further follow-up and treatment was evaluated retrospectively.

Results

Among patients eligible for analysis (n=84), 54 lesions were detected in 38 patients by either mpMRI or PET/MRI. Detection rates were 41.7% for mpMRI and 39.3% for PET/MRI (score 2 and 3 considered positive). There were no significant differences in detection rates for mpMRI versus PET/MRI. Disease detection rates were higher in patients with PSA≥1ng/mL than in patients with lower PSA levels but did not differ between patients with PSAdt above versus below 6 months. Detection rates in patients with primary radiation therapy were higher than in patients with primary surgery. Patients categorized as EAU Low-Risk BCR had a detection rate of 0% both for mpMRI and PET/MRI. For 15 lesions (27.8% of all lesions) there was a discrepancy between mpMRI score and PET/MRI score. Of these, 10 lesions scored as 2-equivocal by mpMRI were changed to a more definite score (n=4 score 1 and n=6 score 3) based on the added PET acquisition. Furthermore, for 4 of 10 patients with discrepancy between mpMRI and PET/MRI scores, the added PET acquisition had affected the treatment choice.

Conclusion

Combined ¹⁸F-fluciclovine PET/MRI can detect lesions suspicious for recurrent prostate cancer in patients with a range of PSA levels. Combined PET/MRI may be useful to select patients for appropriate treatment, but is of limited use at low PSA values or in patients classified as EAU Low-Risk BCR, and the clinical value of ¹⁸F-fluciclovine PET/MRI in this study was too low to justify routine clinical use.

Cholesterol Crystals and NLRP3 Mediated Inflammation in the Uterine Wall Decidua in Normal and Preeclamptic Pregnancies

Preeclampsia is a hypertensive and inflammatory pregnancy disorder associated with cholesterol accumulation and inflammation at the maternal-fetal interface. Preeclampsia can be complicated with fetal growth restriction (FGR) and shares risk factors and pathophysiological mechanisms with cardiovascular disease. Cholesterol crystal mediated NLRP3 inflammasome activation is central to cardiovascular disease and the pathway has been implicated in placental inflammation in preeclampsia. Direct maternal-fetal interaction occurs both in the uterine wall decidua and at the placental surface and these aligned sites constitute the maternal-fetal interface. This study aimed to investigate cholesterol crystal accumulation and NLRP3 inflammasome expression by maternal and fetal cells in the uterine wall decidua of normal and preeclamptic pregnancies. Pregnant women with normal (n = 43) and preeclamptic pregnancies with (n = 28) and without (n = 19) FGR were included at delivery. Cholesterol crystals were imaged in decidual tissue by both second harmonic generation microscopy and polarization filter reflected light microscopy. Quantitative expression analysis of NLRP3, IL-1β and cell markers was performed by immunohistochemistry and automated image processing. Functional NLRP3 activation was assessed in cultured decidual explants. Cholesterol crystals were identified in decidual tissue, both in the tissue stroma and near uterine vessels. The cholesterol crystals in decidua varied between pregnancies in distribution and cluster size. Decidual expression of the inflammasome components NLRP3 and IL-1β was located to fetal trophoblasts and maternal leukocytes and was strongest in areas of proximity between these cell types. Pathway functionality was confirmed by cholesterol crystal activation of IL-1β in cultured decidual explants. Preeclampsia without FGR was associated with increased trophoblast dependent NLRP3 and IL-1β expression, particularly in the decidual areas of trophoblast and leukocyte proximity. Our findings suggest that decidual accumulation of cholesterol crystals may activate the NLRP3 inflammasome and contribute to decidual inflammation and that this pathway is strengthened in areas with close maternal-fetal interaction in preeclampsia without FGR.

A Quality Control System for Automated Prostate Segmentation on T2-Weighted MRI

Computer-aided detection and diagnosis (CAD) systems have the potential to improve robustness and efficiency compared to traditional radiological reading of magnetic resonance imaging (MRI). Fully automated segmentation of the prostate is a crucial step of CAD for prostate cancer, but visual inspection is still required to detect poorly segmented cases. The aim of this work was therefore to establish a fully automated quality control (QC) system for prostate segmentation based on T2-weighted MRI. Four different deep learning-based segmentation methods were used to segment the prostate for 585 patients. First order, shape and textural radiomics features were extracted from the segmented prostate masks. A reference quality score (QS) was calculated for each automated segmentation in comparison to a manual segmentation. A least absolute shrinkage and selection operator (LASSO) was trained and optimized on a randomly assigned training dataset (N = 1756, 439 cases from each segmentation method) to build a generalizable linear regression model based on the radiomics features that best estimated the reference QS. Subsequently, the model was used to estimate the QSs for an independent testing dataset (N = 584, 146 cases from each segmentation method). The mean ± standard deviation absolute error between the estimated and reference QSs was 5.47 ± 6.33 on a scale from 0 to 100. In addition, we found a strong correlation between the estimated and reference QSs (rho = 0.70). In conclusion, we developed an automated QC system that may be helpful for evaluating the quality of automated prostate segmentations.

Automated reference tissue normalization of T2-weighted MR images of the prostate using object recognition

Objectives

To develop and evaluate an automated method for prostate T2-weighted (T2W) image normalization using dual-reference (fat and muscle) tissue.

Materials and methods

Transverse T2W images from the publicly available PROMISE12 (N = 80) and PROSTATEx (N = 202) challenge datasets, and an in-house collected dataset (N = 60) were used. Aggregate channel features object detectors were trained to detect reference fat and muscle tissue regions, which were processed and utilized to normalize the 3D images by linear scaling. Mean prostate pseudo T2 values after normalization were compared to literature values. Inter-patient histogram intersections of voxel intensities in the prostate were compared between our approach, the original images, and other commonly used normalization methods. Healthy vs. malignant tissue classification performance was compared before and after normalization.

Results

The prostate pseudo T2 values of the three tested datasets (mean ± standard deviation = 78.49 ± 9.42, 79.69 ± 6.34 and 79.29 ± 6.30 ms) corresponded well to T2 values from literature (80 ± 34 ms). Our normalization approach resulted in significantly higher (p < 0.001) inter-patient histogram intersections (median = 0.746) than the original images (median = 0.417) and most other normalization methods. Healthy vs. malignant classification also improved significantly (p < 0.001) in peripheral (AUC 0.826 vs. 0.769) and transition (AUC 0.743 vs. 0.678) zones.

Conclusion

An automated dual-reference tissue normalization of T2W images could help improve the quantitative assessment of prostate cancer.

Electronic supplementary material

The online version of this article (10.1007/s10334-020-00871-3) contains supplementary material, which is available to authorized users.

Tumor Targeting by αvβ3-Integrin-Specific Lipid Nanoparticles Occurs via Phagocyte Hitchhiking

Although the first nanomedicine was clinically approved more than two decades ago, nanoparticles' (NP) in vivo behavior is complex and the immune system's role in their application remains elusive. At present, only passive-targeting nanoformulations have been clinically approved, while more complicated active-targeting strategies typically fail to advance from the early clinical phase stage. This absence of clinical translation is, among others, due to the very limited understanding for in vivo targeting mechanisms. Dynamic in vivo phenomena such as NPs' real-time targeting kinetics and phagocytes' contribution to active NP targeting remain largely unexplored. To better understand in vivo targeting, monitoring NP accumulation and distribution at complementary levels of spatial and temporal resolution is imperative. Here, we integrate in vivo positron emission tomography/computed tomography imaging with intravital microscopy and flow cytometric analyses to study α_vβ₃-integrin-targeted cyclic arginine-glycine-aspartate decorated liposomes and oil-in-water nanoemulsions in tumor mouse models. We observed that ligand-mediated accumulation in cancerous lesions is multifaceted and identified "NP hitchhiking" with phagocytes to contribute considerably to this intricate process. We anticipate that this understanding can facilitate rational improvement of nanomedicine applications and that immune cell-NP interactions can be harnessed to develop clinically viable nanomedicine-based immunotherapies.

TLR3 expression by maternal and fetal cells at the maternal-fetal interface in normal and preeclamptic pregnancies

Inflammation and oxidative stress at the maternal-fetal interface characterize the placental dysfunction that underlies the pregnancy disorder preeclampsia. Specialized fetal trophoblasts directly interact with leukocytes at both sites of the maternal-fetal interface; the uterine wall decidua; and the placenta. TLR3 has been implicated in the harmful inflammation at the maternal-fetal interface in preeclampsia, but the cellular involvement in the decidua and placenta has not been determined. This study aimed to characterize and quantify cell-specific TLR3 expression and function at the maternal-fetal interface in normal and preeclamptic pregnancies. TLR3 expression was assessed by immunohistochemistry and quantified by a novel image-based and cell-specific quantitation method. TLR3 was expressed at the maternal-fetal interface by all decidual and placental trophoblast types and by maternal and fetal leukocytes. Placental, but not decidual, TLR3 expression was significantly higher in preeclampsia compared to normal pregnancies. This increase was attributed to placental intravillous tissue and associated with both moderate and severe placental dysfunction. TLR3 pathway functionality in the decidua and placenta was confirmed by TLR3 ligand-induced cytokine response, but the TLR3 expression levels did not correlate between the two sites. In conclusion, functional TLR3 was broadly expressed by maternal and fetal cells at both sites of the maternal-fetal interface and the placental intravillous expression was increased in preeclampsia. This suggests TLR3-mediated inflammatory involvement with local regulation at both sites of the maternal-fetal interface in normal and preeclamptic pregnancies.

Relative Enhanced Diffusivity in Prostate Cancer: Protocol Optimization and Diagnostic Potential

BACKGROUND

Relative enhanced diffusivity (RED) is a potential biomarker for indirectly measuring perfusion in tissue using diffusion-weighted magnetic resonance imaging (MRI) with 3 b values.

PURPOSE

To optimize the RED MRI protocol for the prostate, and to investigate its potential for prostate cancer (PCa) diagnosis.

STUDY TYPE

Prospective.

POPULATION

Ten asymptomatic healthy volunteers and 35 patients with clinical suspicion of PCa.

SEQUENCE

3T T₂ - and diffusion-weighted MRI with b values: b = 0, 50, [100], 150, [200], 250, [300], 400, 800 s/mm² (values in brackets were only used for patients).

ASSESSMENT

Monte Carlo simulations were performed to assess noise sensitivity of RED as a function of intermediate b value. Volunteers were scanned 3 times to assess repeatability of RED. Patient data were used to investigate RED's potential for discriminating between biopsy-confirmed cancer and healthy tissue, and between true and false positive radiological findings.

STATISTICAL TESTS

Within-subject coefficient of variation (WCV) to assess repeatability and receiver-operating characteristic curve analysis and logistic regression to assess diagnostic performance of RED.

RESULTS

The repeatability was acceptable (WCV = 0.2-0.3) for all intermediate b values tested, apart from b = 50 s/mm² (WCV = 0.3-0.4). The simulated RED values agreed well with the experimental data, showing that an intermediate b value between 150-250 s/mm² minimizes noise sensitivity in both peripheral zone (PZ) and transition zone (TZ). RED calculated with the b values 0, 150 and 800 s/mm² was significantly higher in tumors than in healthy tissue in both PZ (P < 0.001, area under the curve [AUC] = 0.85) and PZ + TZ (P < 0.001, AUC = 0.84). RED was shown to aid apparent diffusion coefficient (ADC) in differentiating between false-positive findings and true-positive PCa in the PZ (AUC; RED = 0.71, ADC = 0.74, RED+ADC = 0.77).

DATA CONCLUSION

RED is a repeatable biomarker that may have value for prostate cancer diagnosis. An intermediate b value in the range of 150-250 s/mm² minimizes the influence of noise and maximizes repeatability.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2020;51:1900-1910.

Simultaneous 18F-fluciclovine Positron Emission Tomography and Magnetic Resonance Spectroscopic Imaging of Prostate Cancer

Purpose: To investigate the associations of metabolite levels derived from magnetic resonance spectroscopic imaging (MRSI) and ¹⁸F-fluciclovine positron emission tomography (PET) with prostate tissue characteristics.

Methods: In a cohort of 19 high-risk prostate cancer patients that underwent simultaneous PET/MRI, we evaluated the diagnostic performance of MRSI and PET for discrimination of aggressive cancer lesions from healthy tissue and benign lesions. Data analysis comprised calculations of correlations of mean standardized uptake values (SUV_mean), maximum SUV (SUV_max), and the MRSI-derived ratio of (total choline + spermine + creatine) to citrate (CSC/C). Whole-mount histopathology was used as gold standard.

Results: The results showed a moderate significant correlation between both SUVmean and SUVmax with CSC/C ratio.

Conclusions: We demonstrated that the simultaneous acquisition of ¹⁸F-fluciclovine PET and MRSI with an integrated PET/MRI system is feasible and a combination of these imaging modalities has potential to improve the diagnostic sensitivity and specificity of prostate cancer lesions.

The Effect of Including Bone in Dixon-Based Attenuation Correction for 18F-Fluciclovine PET/MRI of Prostate Cancer

The objective of this study was to evaluate the effect of including bone in Dixon-based attenuation correction for ¹⁸F-fluciclovine PET/MRI of primary and recurrent prostate cancer. Methods: ¹⁸F-fluciclovine PET data from 2 PET/MRI studies-one for staging of high-risk prostate cancer (28 patients) and one for diagnosis of recurrent prostate cancer (81 patients)-were reconstructed with a 4-compartment (reference) and 5-compartment attenuation map. In the latter, continuous linear attenuation coefficients for bone were included by coregistration with an atlas. The SUV_max and mean 50% isocontour SUV (SUV_iso) of primary, locally recurrent, and metastatic lesions were compared between the 2 reconstruction methods using linear mixed-effects models. In addition, mean SUVs were obtained from bone marrow in the third lumbar vertebra (L3) to investigate the effect of including bone attenuation on lesion-to-bone marrow SUV ratios (SUVR_max and SUVR_iso; recurrence study only). The 5-compartment attenuation maps were visually compared with the in-phase Dixon MR images for evaluation of bone registration errors near the lesions. P values of less than 0.05 were considered significant. Results: Sixty-two lesions from 39 patients were evaluated. Bone registration errors were found near 19 (31%) of these lesions. In the remaining 8 primary prostate tumors, 7 locally recurrent lesions, and 28 lymph node metastases without bone registration errors, use of the 5-compartment attenuation map was associated with small but significant increases in SUV_max (2.5%; 95% confidence interval [CI], 2.0%-3.0%; P < 0.001) and SUV_iso (2.5%; 95% CI, 1.9%-3.0%; P < 0.001), but not SUVR_max (0.2%; 95% CI, -0.5%-0.9%; P = 0.604) and SUVR_iso (0.2%; 95% CI -0.6%-1.0%; P = 0.581), in comparison to the 4-compartment attenuation map. Conclusion: The investigated method for atlas-based inclusion of bone in ¹⁸F-fluciclovine PET/MRI attenuation correction has only a small effect on the SUVs of soft-tissue prostate cancer lesions, and no effect on their lesion-to-bone marrow SUVRs when using signal from L3 as a reference. The attenuation maps should always be checked for registration artifacts for lesions in or close to the bones.

Multi institutional quantitative phantom study of yttrium-90 PET in PET/MRI: the MR-QUEST study

Background

Yttrium-90 (⁹⁰Y) radioembolization involves the intra-arterial delivery of radioactive microspheres to treat hepatic malignancies. Though this therapy involves careful pre-treatment planning and imaging, little is known about the precise location of the microspheres once they are administered. Recently, there has been growing interest post-radioembolization imaging using positron-emission tomography (PET) for quantitative dosimetry and identifying lesions that may benefit from additional salvage therapy. In this study, we aim to measure the inter-center variability of ⁹⁰Y PET measurements as measured on PET/MRI in preparation for a multi-institutional prospective phase I/II clinical trial.

Eight institutions participated in this study and followed a standardized phantom filling and imaging protocol. The NEMA NU2-2012 body phantom was filled with 3 GBq of ⁹⁰Y chloride solution. The phantom was imaged for 30 min in listmode on a Siemens Biograph mMR non-TOF PET/MRI scanner at five time points across 10 days (0.3–3.0 GBq). Raw PET data were sent to a central site for image reconstruction and data analysis. Images were reconstructed with optimal parameters determined from a previous study. Volumes of interest (VOIs) matching the known sphere diameters were drawn on the vendor-provided attenuation map and propagated to the PET images. Recovery coefficients (RCs) and coefficient of variation of the RCs (COV) were calculated from these VOIs for each sphere size and activity level.

Results

Mean RCs ranged from 14.5 to 75.4%, with the lowest mean RC coming from the smallest sphere (10 mm) on the last day of imaging (0.16 MBq/ml) and the highest mean RC coming from the largest sphere (37 mm) on the first day of imaging (2.16 MBq/ml). The smaller spheres tended to exhibit higher COVs. In contrast, the larger spheres tended to exhibit lower COVs. COVs from the 37 mm sphere were < 25.3% in all scans. For scans with ≥ 0.60 MBq/ml, COVs were ≤ 25% in spheres ≥ 22 mm. However, for all other spheres sizes and activity levels, COVs were usually > 25%.

Conclusions

Post-radioembolization dosimetry of lesions or other VOIs ≥ 22 mm in diameter can be consistently obtained (< 25% variability) at a multi-institutional level using PET/MRI for any clinically significant activity for ⁹⁰Y radioembolization.

Geometric distortion correction in prostate diffusion-weighted MRI and its effect on quantitative apparent diffusion coefficient analysis

PURPOSE

To evaluate the effect of correction for B₀ inhomogeneity-induced geometric distortion in echo-planar diffusion-weighted imaging on quantitative apparent diffusion coefficient (ADC) analysis in multiparametric prostate MRI.

METHODS

Geometric distortion correction was performed in echo-planar diffusion-weighted images (b = 0, 50, 400, 800 s/mm² ) of 28 patients, using two b₀ scans with opposing phase-encoding polarities. Histology-matched tumor and healthy tissue volumes of interest delineated on T₂ -weighted images were mapped to the nondistortion-corrected and distortion-corrected data sets by resampling with and without spatial coregistration. The ADC values were calculated on the volume and voxel level. The effect of distortion correction on ADC quantification and tissue classification was evaluated using linear-mixed models and logistic regression, respectively.

RESULTS

Without coregistration, the absolute differences in tumor ADC (range: 0.0002-0.189 mm² /s×10^-3 (volume level); 0.014-0.493 mm² /s×10^-3 (voxel level)) between the nondistortion-corrected and distortion-corrected were significantly associated (P < 0.05) with distortion distance (mean: 1.4 ± 1.3 mm; range: 0.3-5.3 mm). No significant associations were found upon coregistration; however, in patients with high rectal gas residue, distortion correction resulted in improved spatial representation and significantly better classification of healthy versus tumor voxels (P < 0.05).

CONCLUSIONS

Geometric distortion correction in DWI could improve quantitative ADC analysis in multiparametric prostate MRI. Magn Reson Med 79:2524-2532, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

T2-weighted MRI-derived textural features reflect prostate cancer aggressiveness: preliminary results

PURPOSE

To evaluate the diagnostic relevance of T2-weighted (T2W) MRI-derived textural features relative to quantitative physiological parameters derived from diffusion-weighted (DW) and dynamic contrast-enhanced (DCE) MRI in Gleason score (GS) 3+4 and 4+3 prostate cancers.

MATERIALS AND METHODS

3T multiparametric-MRI was performed on 23 prostate cancer patients prior to prostatectomy. Textural features [angular second moment (ASM), contrast, correlation, entropy], apparent diffusion coefficient (ADC), and DCE pharmacokinetic parameters (K^trans and V_e) were calculated from index tumours delineated on the T2W, DW, and DCE images, respectively. The association between the textural features and prostatectomy GS and the MRI-derived parameters, and the utility of the parameters in differentiating between GS 3+4 and 4+3 prostate cancers were assessed statistically.

RESULTS

ASM and entropy correlated significantly (p < 0.05) with both GS and median ADC. Contrast correlated moderately with median ADC. The textural features correlated insignificantly with K^trans and V_e. GS 4+3 cancers had significantly lower ASM and higher entropy than 3+4 cancers, but insignificant differences in median ADC, K^trans, and V_e. The combined texture-MRI parameters yielded higher classification accuracy (91%) than the individual parameter sets.

CONCLUSION

T2W MRI-derived textural features could serve as potential diagnostic markers, sensitive to the pathological differences in prostate cancers.

KEY POINTS

• T2W MRI-derived textural features correlate significantly with Gleason score and ADC. • T2W MRI-derived textural features differentiate Gleason score 3+4 from 4+3 cancers. • T2W image textural features could augment tumour characterization.

Multimodality calibration for simultaneous fluoroscopic and nuclear imaging

Background

Simultaneous real-time fluoroscopic and nuclear imaging could benefit image-guided (oncological) procedures. To this end, a hybrid modality is currently being developed by our group, by combining a c-arm with a gamma camera and a four-pinhole collimator. Accurate determination of the system parameters that describe the position of the x-ray tube, x-ray detector, gamma camera, and collimators is crucial to optimize image quality. The purpose of this study was to develop a calibration method that estimates the system parameters used for reconstruction.

A multimodality phantom consisting of five point sources was created. First, nuclear and fluoroscopic images of the phantom were acquired at several distances from the image intensifier. The system parameters were acquired using physical measurement, and multimodality images of the phantom were reconstructed. The resolution and co-registration error of the point sources were determined as a measure of image quality. Next, the system parameters were estimated using a calibration method, which adjusted the parameters in the reconstruction algorithm, until the resolution and co-registration were optimized. For evaluation, multimodality images of a second set of phantom acquisitions were reconstructed using calibrated parameter sets. Subsequently, the resolution and co-registration error of the point sources were determined as a measure of image quality. This procedure was performed five times for different noise simulations. In addition, simultaneously acquired fluoroscopic and nuclear images of two moving syringes were obtained with parameter sets from before and after calibration.

Results

The mean FWHM was significantly lower after calibration than before calibration for 21 out of 25 point sources. The mean co-registration error was significantly lower after calibration than before calibration for all point sources. The simultaneously acquired fluoroscopic and nuclear images showed improved co-registration after calibration as compared with before calibration.

Conclusions

A calibration method was presented that improves the resolution and co-registration of simultaneously acquired hybrid fluoroscopic and nuclear images by estimating the geometric parameter set as compared with a parameter set acquired by direct physical measurement.

Electronic supplementary material

The online version of this article (doi:10.1186/s40658-016-0156-1) contains supplementary material, which is available to authorized users.

Reduced functional connectivity within the primary motor cortex of patients with brachial plexus injury

This study aims at the effects of traumatic brachial plexus lesion with root avulsions (BPA) upon the organization of the primary motor cortex (M1). Nine right-handed patients with a right BPA in whom an intercostal to musculocutaneous (ICN-MC) nerve transfer was performed had post-operative resting state fMRI scanning. The analysis of empirical functional correlations between neighboring voxels revealed faster correlation decay as a function of distance in the M1 region corresponding to the arm in BPA patients as compared to the control group. No differences between the two groups were found in the face area. We also investigated whether such larger decay in patients could be attributed to a gray matter diminution in M1. Structural imaging analysis showed no difference in gray matter density between groups. Our findings suggest that the faster decay in neighboring functional correlations without significant gray matter diminution in BPA patients could be related to a reduced activity in intrinsic horizontal connections in M1 responsible for upper limb motor synergies.

Quantitative Comparison of 124I PET/CT and 131I SPECT/CT Detectability

Radioiodine therapy with (131)I is used for treatment of suspected recurrence of differentiated thyroid carcinoma. Pretherapeutic (124)I PET/CT with a low activity (~1% of (131)I activity) can be performed to determine whether uptake of (131)I, and thereby the desired therapeutic effect, may be expected. However, false-negative (124)I PET/CT results as compared with posttherapeutic (131)I SPECT/CT have been reported by several groups. The purpose of this study was to investigate whether the reported discrepancies may be ascribed to a difference in lesion detectability between (124)I PET/CT and (131)I SPECT/CT and, hence, whether the administered (124)I activity is sufficient to achieve equal detectability.

METHODS

Phantom measurements were performed using the National Electrical Manufacturers Association 2007 image-quality phantom. As a measure of detectability, the contrast-to-noise ratio was calculated. The (124)I activity was expressed as the percentage of (131)I activity required to achieve the same contrast-to-noise ratio. This metric was defined as the detectability equivalence percentage (DEP).

RESULTS

Because lower DEPs were obtained for smaller spheres, a relatively low (124)I activity was sufficient to achieve similar lesion detectability between (124)I PET/CT and (131)I SPECT/CT. DEP was 1.5%, 1.9%, 1.9%, 4.4%, 9.0%, and 16.2% for spheres with diameters of 10, 13, 17, 18, 25, and 37 mm, respectively, for attenuation- and scatter-corrected SPECT versus point-spread function (PSF) model-based and time-of-flight (TOF) PET. For no-PSF no-TOF PET, DEP was 3.6%, 2.1%, 3.5%, 7.8%, 15.1%, and 23.3%, respectively.

CONCLUSION

A relatively low (124)I activity of 74 MBq (~1% of (131)I activity) is sufficient to achieve similar lesion detectability between (124)I PSF TOF PET/CT and (131)I SPECT/CT for small spheres (≤10 mm), since the reported DEPs are close to 1%. False-negative (124)I PET/CT results as compared with posttherapeutic (131)I SPECT/CT may be ascribed to differences in detectability for large lesions (>10 mm) and for no-PSF no-TOF PET, since DEPs are greater than 1%. On the basis of DEPs of 3.5% for lesion diameters of up to 17 mm on no-PSF no-TOF PET, (124)I activities as high as 170 MBq may be warranted to obtain equal detectability.

Toward Simultaneous Real-Time Fluoroscopic and Nuclear Imaging in the Intervention Room

PURPOSE

To investigate the technical feasibility of hybrid simultaneous fluoroscopic and nuclear imaging.

MATERIALS AND METHODS

An x-ray tube, an x-ray detector, and a gamma camera were positioned in one line, enabling imaging of the same field of view. Since a straightforward combination of these elements would block the lines of view, a gamma camera setup was developed to be able to view around the x-ray tube. A prototype was built by using a mobile C-arm and a gamma camera with a four-pinhole collimator. By using the prototype, test images were acquired and sensitivity, resolution, and coregistration error were analyzed.

RESULTS

Nuclear images (two frames per second) were acquired simultaneously with fluoroscopic images. Depending on the distance from point source to detector, the system resolution was 1.5-1.9-cm full width at half maximum, the sensitivity was (0.6-1.5) × 10(-5) counts per decay, and the coregistration error was -0.13 to 0.15 cm. With good spatial and temporal alignment of both modalities throughout the field of view, fluoroscopic images can be shown in grayscale and corresponding nuclear images in color overlay.

CONCLUSION

Measurements obtained with the hybrid imaging prototype device that combines simultaneous fluoroscopic and nuclear imaging of the same field of view have demonstrated the feasibility of real-time simultaneous hybrid imaging in the intervention room.

A parallel-cone collimator for high-energy SPECT

In SPECT using high-energy photon-emitting isotopes, such as (131)I, parallel-hole collimators with thick septa are required to limit septal penetration, at the cost of sensitivity and resolution. This study investigated a parallel-hole collimator with cone-shaped holes, which was designed to limit collimator penetration while preserving resolution and sensitivity. The objective was to demonstrate that a single-slice prototype of the parallel-cone (PC) collimator was capable of improving the image quality of high-energy SPECT.

METHODS

The image quality of the PC collimator was quantitatively compared with that of clinically used low-energy high-resolution (LEHR; for (99m)Tc) and high-energy general-purpose (HEGP; for (131)I and (18)F) parallel-hole collimators. First, Monte Carlo simulations of single and double point sources were performed to assess sensitivity and resolution by comparing point-spread functions (PSFs). Second, a prototype PC collimator was used in an experimental phantom study to assess and compare contrast recovery coefficients and image noise.

RESULTS

Monte Carlo simulations showed reduced broadening of the PSF due to collimator penetration for the PC collimator as compared with the HEGP collimator (e.g., 0.9 vs. 1.4 cm in full width at half maximum for (131)I). Simulated double point sources placed 2 cm apart were separately detectable for the PC collimator, whereas this was not the case for (131)I and (18)F at distances from the collimator face of 10 cm or more for the HEGP collimator. The sensitivity, measured over the simulated profiles as the total amount of counts per decay, was found to be higher for the LEHR and HEGP collimators than for the PC collimator (e.g., 3.1 × 10(-5) vs. 2.9 × 10(-5) counts per decay for (131)I). However, at equal noise level, phantom measurements showed that contrast recovery coefficients were similar for the PC and LEHR collimators for (99m)Tc but that the PC collimator significantly improved the contrast recovery coefficients as compared with the HEGP collimator for (131)I and (18)F.

CONCLUSION

High-energy SPECT imaging with a single-slice prototype of the proposed PC collimator has shown the potential for significantly improved image quality in comparison with standard parallel-hole collimators.

Quantitative Monte Carlo-based holmium-166 SPECT reconstruction

PURPOSE

Quantitative imaging of the radionuclide distribution is of increasing interest for microsphere radioembolization (RE) of liver malignancies, to aid treatment planning and dosimetry. For this purpose, holmium-166 ((166)Ho) microspheres have been developed, which can be visualized with a gamma camera. The objective of this work is to develop and evaluate a new reconstruction method for quantitative (166)Ho SPECT, including Monte Carlo-based modeling of photon contributions from the full energy spectrum.

METHODS

A fast Monte Carlo (MC) simulator was developed for simulation of (166)Ho projection images and incorporated in a statistical reconstruction algorithm (SPECT-fMC). Photon scatter and attenuation for all photons sampled from the full (166)Ho energy spectrum were modeled during reconstruction by Monte Carlo simulations. The energy- and distance-dependent collimator-detector response was modeled using precalculated convolution kernels. Phantom experiments were performed to quantitatively evaluate image contrast, image noise, count errors, and activity recovery coefficients (ARCs) of SPECT-fMC in comparison with those of an energy window-based method for correction of down-scattered high-energy photons (SPECT-DSW) and a previously presented hybrid method that combines MC simulation of photopeak scatter with energy window-based estimation of down-scattered high-energy contributions (SPECT-ppMC+DSW). Additionally, the impact of SPECT-fMC on whole-body recovered activities (A(est)) and estimated radiation absorbed doses was evaluated using clinical SPECT data of six (166)Ho RE patients.

RESULTS

At the same noise level, SPECT-fMC images showed substantially higher contrast than SPECT-DSW and SPECT-ppMC+DSW in spheres ≥ 17 mm in diameter. The count error was reduced from 29% (SPECT-DSW) and 25% (SPECT-ppMC+DSW) to 12% (SPECT-fMC). ARCs in five spherical volumes of 1.96-106.21 ml were improved from 32%-63% (SPECT-DSW) and 50%-80% (SPECT-ppMC+DSW) to 76%-103% (SPECT-fMC). Furthermore, SPECT-fMC recovered whole-body activities were most accurate (A(est) = 1.06 × A - 5.90 MBq, R(2) = 0.97) and SPECT-fMC tumor absorbed doses were significantly higher than with SPECT-DSW (p = 0.031) and SPECT-ppMC+DSW (p = 0.031).

CONCLUSIONS

The quantitative accuracy of (166)Ho SPECT is improved by Monte Carlo-based modeling of the image degrading factors. Consequently, the proposed reconstruction method enables accurate estimation of the radiation absorbed dose in clinical practice.

(⁹⁹m)Tc-MAA overestimates the absorbed dose to the lungs in radioembolization: a quantitative evaluation in patients treated with ¹⁶⁶Ho-microspheres

PURPOSE

Radiation pneumonitis is a rare but serious complication of radioembolic therapy of liver tumours. Estimation of the mean absorbed dose to the lungs based on pretreatment diagnostic (99m)Tc-macroaggregated albumin ((99m)Tc-MAA) imaging should prevent this, with administered activities adjusted accordingly. The accuracy of (99m)Tc-MAA-based lung absorbed dose estimates was evaluated and compared to absorbed dose estimates based on pretreatment diagnostic (166)Ho-microsphere imaging and to the actual lung absorbed doses after (166)Ho radioembolization.

METHODS

This prospective clinical study included 14 patients with chemorefractory, unresectable liver metastases treated with (166)Ho radioembolization. (99m)Tc-MAA-based and (166)Ho-microsphere-based estimation of lung absorbed doses was performed on pretreatment diagnostic planar scintigraphic and SPECT/CT images. The clinical analysis was preceded by an anthropomorphic torso phantom study with simulated lung shunt fractions of 0 to 30 % to determine the accuracy of the image-based lung absorbed dose estimates after (166)Ho radioembolization.

RESULTS

In the phantom study, (166)Ho SPECT/CT-based lung absorbed dose estimates were more accurate (absolute error range 0.1 to -4.4 Gy) than (166)Ho planar scintigraphy-based lung absorbed dose estimates (absolute error range 9.5 to 12.1 Gy). Clinically, the actual median lung absorbed dose was 0.02 Gy (range 0.0 to 0.7 Gy) based on posttreatment (166)Ho-microsphere SPECT/CT imaging. Lung absorbed doses estimated on the basis of pretreatment diagnostic (166)Ho-microsphere SPECT/CT imaging (median 0.02 Gy, range 0.0 to 0.4 Gy) were significantly better predictors of the actual lung absorbed doses than doses estimated on the basis of (166)Ho-microsphere planar scintigraphy (median 10.4 Gy, range 4.0 to 17.3 Gy; p < 0.001), (99m)Tc-MAA SPECT/CT imaging (median 2.5 Gy, range 1.2 to 12.3 Gy; p < 0.001), and (99m)Tc-MAA planar scintigraphy (median 5.5 Gy, range 2.3 to 18.2 Gy; p < 0.001).

CONCLUSION

In clinical practice, lung absorbed doses are significantly overestimated by pretreatment diagnostic (99m)Tc-MAA imaging. Pretreatment diagnostic (166)Ho-microsphere SPECT/CT imaging accurately predicts lung absorbed doses after (166)Ho radioembolization.

In vivo dosimetry based on SPECT and MR imaging of 166Ho-microspheres for treatment of liver malignancies

(166)Ho-poly(l-lactic acid) microspheres allow for quantitative imaging with MR imaging or SPECT for microsphere biodistribution assessment after radioembolization. The purpose of this study was to evaluate SPECT- and MR imaging-based dosimetry in the first patients treated with (166)Ho radioembolization.

METHODS

Fifteen patients with unresectable, chemorefractory liver metastases of any origin were enrolled in this phase 1 study and were treated with (166)Ho radioembolization according to a dose escalation protocol (20-80 Gy). The contours of all liver segments and all discernible tumors were manually delineated on T2-weighted posttreatment MR images and registered to the posttreatment SPECT images (n = 9) or SPECT/CT images (n = 6) and MR imaging-based R2* maps (n = 14). Dosimetry was based on SPECT (n = 15) and MR imaging (n = 9) for all volumes of interest, tumor-to-nontumor (T/N) activity concentration ratios were calculated, and correlation and agreement of MR imaging- and SPECT-based measurements were evaluated.

RESULTS

The median overall T/N ratio was 1.4 based on SPECT (range, 0.9-2.8) and 1.4 based on MR imaging (range, 1.1-3.1). In 6 of 15 patients (40%), all tumors had received an activity concentration equal to or higher than the normal liver (T/N ratio ≥ 1). Analysis of SPECT and MR imaging measurements for dose to liver segments yielded a high correlation (R(2) = 0.91) and a moderate agreement (mean bias, 3.7 Gy; 95% limits of agreement, -11.2 to 18.7).

CONCLUSION

With the use of (166)Ho-microspheres, in vivo dosimetry is feasible on the basis of both SPECT and MR imaging, which enables personalized treatment by selective targeting of inadequately treated tumors.

Quantitative Monte Carlo-based 90Y SPECT reconstruction

The evaluation of radiation absorbed doses in tumorous and healthy tissues is of increasing interest for (90)Y microsphere radioembolization of liver malignancies. The objectives of this work were to introduce and validate a new reconstruction method for quantitative (90)Y bremsstrahlung SPECT to improve posttreatment dosimetry.

METHODS

A fast Monte Carlo simulator was adapted for (90)Y and incorporated into a statistical reconstruction algorithm (SPECT-MC). Photon scatter and attenuation for all photons sampled from the full (90)Y energy spectrum were modeled during reconstruction by Monte Carlo simulations. The energy- and distance-dependent collimator-detector response was modeled with precalculated convolution kernels. The National Electrical Manufacturers Association 2007/International Electrotechnical Commission 2008 image quality phantom was used to quantitatively evaluate the performance of SPECT-MC in comparison with those of state-of-the-art clinical SPECT reconstruction and PET. The liver radiation absorbed doses estimated by SPECT, PET, and SPECT-MC were evaluated in 5 patients consecutively treated with radioembolization.

RESULTS

In comparison with state-of-the-art clinical (90)Y SPECT reconstruction, SPECT-MC substantially improved image contrast (e.g., from 25% to 88% for the 37-mm sphere) and decreased the mean residual count error in the lung insert (from 73% to 15%) at the cost of higher image noise. Image noise and the mean count error were lower for SPECT-MC than for PET. Image contrast was higher in the larger spheres (diameter of ≥28 mm) but lower in the smaller spheres (≤22 mm) for SPECT-MC than for PET. In the clinical study, mean absorbed dose estimates in liver regions with high absorbed doses were consistently higher for SPECT-MC than for SPECT (P = 0.0625) and consistently higher for SPECT-MC than for PET (P = 0.0625).

CONCLUSION

The quantitative accuracy of (90)Y bremsstrahlung SPECT is substantially improved by Monte Carlo-based modeling of the image-degrading factors. Consequently, (90)Y bremsstrahlung SPECT may be used as an alternative to (90)Y PET.

99mTc-macroaggregated albumin poorly predicts the intrahepatic distribution of 90Y resin microspheres in hepatic radioembolization

In hepatic (90)Y radioembolization, pretreatment (99m)Tc-macroaggregated albumin ((99m)Tc-MAA) nuclear imaging is used for lung shunt analysis, evaluation of extrahepatic deposition, and sometimes for treatment planning, using a partition model. A high level of agreement between pretreatment (99m)Tc-MAA distribution and final (90)Y-microsphere distribution is assumed. The aim of this study was to investigate the value of pretreatment (99m)Tc-MAA SPECT to predict intrahepatic posttreatment (90)Y-microsphere distribution.

METHODS

Volumes of interest (VOIs) were delineated on pretreatment contrast-enhanced CT or MR images according to Couinaud liver segmentation. All VOIs were registered to the (99m)Tc-MAA SPECT and (90)Y SPECT images. The (99m)Tc-MAA SPECT and (90)Y SPECT activity counts were normalized to the total administered activity of (90)Y. For each VOI, this practice resulted in a predictive amount of (90)Y (MBq/cm(3)) based on (99m)Tc-MAA SPECT in comparison with an actual amount of (90)Y based on (90)Y SPECT. Bland-Altman analysis was used to investigate the agreement of the activity distribution between (99m)Tc-MAA SPECT and (90)Y SPECT.

RESULTS

A total of 39 procedures (225 VOIs) in 31 patients were included for analysis. The overall mean difference between pretreatment and posttreatment distribution of activity concentration for all segments was -0.022 MBq/cm(3) with 95% limits of agreement of -0.581 to 0.537 MBq/cm(3) (-28.9 to 26.7 Gy absorbed dose). A difference of >10%, >20%, and >30% of the mean activity per milliliter was found in, respectively, 153 (68%), 97 (43%), and 72 (32%) of the 225 segments. In every (99m)Tc-MAA procedure, at least 1 segment showed an under- or overestimation of >10%. The position of the catheter tip during administrations, as well as the tumor load of the liver segments, significantly influenced the disagreement.

CONCLUSION

In current clinical practice, (99m)Tc-MAA distribution does not accurately predict final (90)Y activity distribution. Awareness of the importance of catheter positioning and adherence to specific recommendations may lead to optimization of individualized treatment planning based on pretreatment imaging.

Intratumoral administration of holmium-166 acetylacetonate microspheres: antitumor efficacy and feasibility of multimodality imaging in renal cancer

Purpose

The increasing incidence of small renal tumors in an aging population with comorbidities has stimulated the development of minimally invasive treatments. This study aimed to assess the efficacy and demonstrate feasibility of multimodality imaging of intratumoral administration of holmium-166 microspheres (¹⁶⁶HoAcAcMS). This new technique locally ablates renal tumors through high-energy beta particles, while the gamma rays allow for nuclear imaging and the paramagnetism of holmium allows for MRI.

Methods

¹⁶⁶HoAcAcMS were administered intratumorally in orthotopic renal tumors (Balb/C mice). Post administration CT, SPECT and MRI was performed. At several time points (2 h, 1, 2, 3, 7 and 14 days) after MS administration, tumors were measured and histologically analyzed. Holmium accumulation in organs was measured using inductively coupled plasma mass spectrometry.

Results

¹⁶⁶HoAcAcMS were successfully administered to tumor bearing mice. A striking near-complete tumor-control was observed in ¹⁶⁶HoAcAcMS treated mice (0.10±0.01 cm³ vs. 4.15±0.3 cm³ for control tumors). Focal necrosis and inflammation was present from 24 h following treatment. Renal parenchyma outside the radiated region showed no histological alterations. Post administration CT, MRI and SPECT imaging revealed clear deposits of ¹⁶⁶HoAcAcMS in the kidney.

Conclusions

Intratumorally administered ¹⁶⁶HoAcAcMS has great potential as a new local treatment of renal tumors for surgically unfit patients. In addition to strong cancer control, it provides powerful multimodality imaging opportunities.

MRI-based biodistribution assessment of holmium-166 poly(L-lactic acid) microspheres after radioembolisation

Objectives

To demonstrate the feasibility of MRI-based assessment of the intrahepatic Ho-PLLA-MS biodistribution after radioembolisation in order to estimate the absorbed radiation dose.

Methods

Fifteen patients were treated with holmium-166 (¹⁶⁶Ho) poly(L-lactic acid)-loaded microspheres (Ho-PLLA-MS, mean 484 mg; range 408–593 mg) in a phase I study. Multi-echo gradient-echo MR images were acquired from which R₂^* maps were constructed. The amount of Ho-PLLA-MS in the liver was determined by using the relaxivity r₂^* of the Ho-PLLA-MS and compared with the administered amount. Quantitative single photon emission computed tomography (SPECT) was used for comparison with MRI regarding the whole liver absorbed radiation dose.

Results

R₂^* maps visualised the deposition of Ho-PLLA-MS with great detail. The mean total amount of Ho-PLLA-MS detected in the liver based on MRI was 431 mg (range 236–666 mg) or 89 ± 19 % of the delivered amount (correlation coefficient r = 0.7; P < 0.01). A good correlation was found between the whole liver mean absorbed radiation dose as assessed by MRI and SPECT (correlation coefficient r = 0.927; P < 0.001).

Conclusion

MRI-based dosimetry for holmium-166 radioembolisation is feasible. Biodistribution is visualised with great detail and quantitative measurements are possible.

Key Points

• Radioembolisation is increasingly used for treating unresectable primary or metastatic liver tumours.

• MRI-based intrahepatic microsphere biodistribution assessment is feasible after holmium-166 radioembolisation.

• MRI enables quantification of holmium-166 microspheres in liver in a short imaging time.

• MRI can estimate the whole liver absorbed radiation dose following holmium-166 radioembolisation.

Quantitative evaluation of scintillation camera imaging characteristics of isotopes used in liver radioembolization

Background

Scintillation camera imaging is used for treatment planning and post-treatment dosimetry in liver radioembolization (RE). In yttrium-90 (90Y) RE, scintigraphic images of technetium-99m (99mTc) are used for treatment planning, while 90Y Bremsstrahlung images are used for post-treatment dosimetry. In holmium-166 (166Ho) RE, scintigraphic images of 166Ho can be used for both treatment planning and post-treatment dosimetry. The aim of this study is to quantitatively evaluate and compare the imaging characteristics of these three isotopes, in order that imaging protocols can be optimized and RE studies with varying isotopes can be compared.

Methodology/Principal Findings

Phantom experiments were performed in line with NEMA guidelines to assess the spatial resolution, sensitivity, count rate linearity, and contrast recovery of 99mTc, 90Y and 166Ho. In addition, Monte Carlo simulations were performed to obtain detailed information about the history of detected photons. The results showed that the use of a broad energy window and the high-energy collimator gave optimal combination of sensitivity, spatial resolution, and primary photon fraction for 90Y Bremsstrahlung imaging, although differences with the medium-energy collimator were small. For 166Ho, the high-energy collimator also slightly outperformed the medium-energy collimator. In comparison with 99mTc, the image quality of both 90Y and 166Ho is degraded by a lower spatial resolution, a lower sensitivity, and larger scatter and collimator penetration fractions.

Conclusions/Significance

The quantitative evaluation of the scintillation camera characteristics presented in this study helps to optimize acquisition parameters and supports future analysis of clinical comparisons between RE studies.

The influence of self-absorption on PET and PET/CT shielding requirements

PURPOSE

The high energy (511 keV) annihilation photons used in positron emission tomography (PET) imaging generally require a substantial amount of lead to protect personnel and the general public from ionizing radiation. A cost-effective design of the PET facility that ensures radiation does not exceed formal dose limits requires accurate estimation of the necessary PET shielding. The American Association of Physicists in Medicine (AAPM) Task Group 108 recently published broad beam transmission factors based on Monte Carlo calculations of 511 keV photons. In this work, an extension to the AAPM model is presented, based on Monte Carlo simulations including the effects of self-absorption on the photon energy spectrum.

METHODS

Monte Carlo calculations were performed using MCNPX. The photon energy spectrum after self-absorption was computed by simulating a normal 18FDG activity distribution in an anthropomorphic phantom. This spectrum was used to calculate the dose rate transmission factors for various wall thicknesses of lead, concrete, and iron. The method was validated by measurement and corresponding simulation of the transmission factors of an 18FDG source in air and in PMMA. Furthermore, a method to generate 3D area dose rate maps of PET facilities incorporating the calculated transmission tables is presented and applied to several shielding situations.

RESULTS

The calculated self-absorption correction factor and the broad beam transmission factors resulting from Monte Carlo simulations of a monoenergetic point source emitting 511 keV photons were in excellent agreement with the results of the AAPM publication (0.66 vs 0.64 and R2 = 0.999, respectively). However, when all radiation physics, i.e., also the effect of self-absorption on the photon energy spectrum, is included in the Monte Carlo calculations, a substantial reduction in required shielding material was found. For example, including all radiation physics leads to 13.3 mm of lead required to obtain a typical transmission factor of 0.1, instead of 16.0 mm of lead when the AAPM data including only the self-absorption correction factor are used. These findings were confirmed by the experimental measurements. The transmission factors produced in this work can be applied in the same manner as those estimated by AAPM to allow for a cost-effective design of PET and PET/CT facilities without violating radiation safety regulations.

CONCLUSIONS

Taking into account the effect of self-absorption on the photon energy spectrum results in more accurate and cost-effective shielding requirement estimations.

Holmium-166 radioembolization for the treatment of patients with liver metastases: design of the phase I HEPAR trial

BACKGROUND

Intra-arterial radioembolization with yttrium-90 microspheres ( 90Y-RE) is an increasingly used therapy for patients with unresectable liver malignancies. Over the last decade, radioactive holmium-166 poly(L-lactic acid) microspheres ( 166Ho-PLLA-MS) have been developed as a possible alternative to 90Y-RE. Next to high-energy beta-radiation, 166Ho also emits gamma-radiation, which allows for imaging by gamma scintigraphy. In addition, Ho is a highly paramagnetic element and can therefore be visualized by MRI. These imaging modalities are useful for assessment of the biodistribution, and allow dosimetry through quantitative analysis of the scintigraphic and MR images. Previous studies have demonstrated the safety of 166Ho-PLLA-MS radioembolization ( 166Ho-RE) in animals. The aim of this phase I trial is to assess the safety and toxicity profile of 166Ho-RE in patients with liver metastases.

METHODS

The HEPAR study (Holmium Embolization Particles for Arterial Radiotherapy) is a non-randomized, open label, safety study. We aim to include 15 to 24 patients with liver metastases of any origin, who have chemotherapy-refractory disease and who are not amenable to surgical resection. Prior to treatment, in addition to the standard technetium-99m labelled macroaggregated albumin ( 99mTc-MAA) dose, a low radioactive safety dose of 60-mg 166Ho-PLLA-MS will be administered. Patients are treated in 4 cohorts of 3-6 patients, according to a standard dose escalation protocol (20 Gy, 40 Gy, 60 Gy, and 80 Gy, respectively). The primary objective will be to establish the maximum tolerated radiation dose of 166Ho-PLLA-MS. Secondary objectives are to assess tumour response, biodistribution, performance status, quality of life, and to compare the 166Ho-PLLA-MS safety dose and the 99mTc-MAA dose distributions with respect to the ability to accurately predict microsphere distribution.

DISCUSSION

This will be the first clinical study on 166Ho-RE. Based on preclinical studies, it is expected that 166Ho-RE has a safety and toxicity profile comparable to that of 90Y-RE. The biochemical and radionuclide characteristics of 166Ho-PLLA-MS that enable accurate dosimetry calculations and biodistribution assessment may however improve the overall safety of the procedure.