Ferdinand Seith

MR-based respiratory and cardiac motion correction for PET imaging.

HighlightsPET motion correction from simultaneously acquired MR‐derived motion model.Fast MR acquisition freeing scan time per PET bed for further diagnostic sequences.Clinically feasible setup: streamlined processing in Gadgetron evaluation on a cohort of 36 patients.Publicly available: https://sites.google.com/site/kspaceastronauts. Graphical abstract Figure. No caption available. ABSTRACT Purpose: To develop a motion correction for Positron‐Emission‐Tomography (PET) using simultaneously acquired magnetic‐resonance (MR) images within 90 s. Methods: A 90 s MR acquisition allows the generation of a cardiac and respiratory motion model of the body trunk. Thereafter, further diagnostic MR sequences can be recorded during the PET examination without any limitation. To provide full PET scan time coverage, a sensor fusion approach maps external motion signals (respiratory belt, ECG‐derived respiration signal) to a complete surrogate signal on which the retrospective data binning is performed. A joint Compressed Sensing reconstruction and motion estimation of the subsampled data provides motion‐resolved MR images (respiratory + cardiac). A 1‐POINT DIXON method is applied to these MR images to derive a motion‐resolved attenuation map. The motion model and the attenuation map are fed to the Customizable and Advanced Software for Tomographic Reconstruction (CASToR) PET reconstruction system in which the motion correction is incorporated. All reconstruction steps are performed online on the scanner via Gadgetron to provide a clinically feasible setup for improved general applicability. The method was evaluated on 36 patients with suspected liver or lung metastasis in terms of lesion quantification (SUVmax, SNR, contrast), delineation (FWHM, slope steepness) and diagnostic confidence level (3‐point Likert‐scale). Results: A motion correction could be conducted for all patients, however, only in 30 patients moving lesions could be observed. For the examined 134 malignant lesions, an average improvement in lesion quantification of 22%, delineation of 64% and diagnostic confidence level of 23% was achieved. Conclusion: The proposed method provides a clinically feasible setup for respiratory and cardiac motion correction of PET data by simultaneous short‐term MRI. The acquisition sequence and all reconstruction steps are publicly available to foster multi‐center studies and various motion correction scenarios.

Self‐navigated 4D cartesian imaging of periodic motion in the body trunk using partial k‐space compressed sensing

To enable fast and flexible high‐resolution four‐dimensional (4D) MRI of periodic thoracic/abdominal motion for motion visualization or motion‐corrected imaging.

Diagnosing Lung Nodules on Oncologic MR/PET Imaging: Comparison of Fast T1-Weighted Sequences and Influence of Image Acquisition in Inspiration and Expiration Breath-Hold

Objective First, to investigate the diagnostic performance of fast T1-weighted sequences for lung nodule evaluation in oncologic magnetic resonance (MR)/positron emission tomography (PET). Second, to evaluate the influence of image acquisition in inspiration and expiration breath-hold on diagnostic performance. Materials and Methods The study was approved by the local Institutional Review Board. PET/CT and MR/PET of 44 cancer patients were evaluated by 2 readers. PET/CT included lung computed tomography (CT) scans in inspiration and expiration (CTin, CTex). MR/PET included Dixon sequence for attenuation correction and fast T1-weighted volumetric interpolated breath-hold examination (VIBE) sequences (volume interpolated breath-hold examination acquired in inspiration [VIBEin], volume interpolated breath-hold examination acquired in expiration [VIBEex]). Diagnostic performance was analyzed for lesion-, lobe-, and size-dependence. Diagnostic confidence was evaluated (4-point Likert-scale; 1 = high). Jackknife alternative free-response receiver-operating characteristic (JAFROC) analysis was performed. Results Seventy-six pulmonary lesions were evaluated. Lesion-based detection rates were: CTex, 77.6%; VIBEin, 53.3%; VIBEex, 51.3%; and Dixon, 22.4%. Lobe-based detection rates were: CTex, 89.6%; VIBEin, 58.3%; VIBEex, 60.4%; and Dixon, 31.3%. In contrast to CT, inspiration versus expiration did not alter diagnostic performance in VIBE sequences. Diagnostic confidence was best for VIBEin and CTex and decreased in VIBEex and Dixon (1.2 ± 0.6; 1.2 ± 0.7; 1.5 ± 0.9; 1.7 ± 1.1, respectively). The JAFROC figure-of-merit of Dixon was significantly lower. All patients with malignant lesions were identified by CTex, VIBEin, and VIBEex, while 3 patients were false-negative in Dixon. Conclusion Fast T1-weighted VIBE sequences allow for identification of patients with malignant pulmonary lesions. The Dixon sequence is not recommended for lung nodule evaluation in oncologic MR/PET patients. In contrast to CT, inspiration versus expiratory breath-hold in VIBE sequences was less crucial for lung nodule evaluation but was important for diagnostic confidence.

Simulation of tracer dose reduction in 18F-FDG-Positron emission tomography / magnetic resonance imaging (PET/MRI): Effects on oncologic reading, image quality and artifacts

The aim of our study was to evaluate the effect of stepwise-reduced doses on objective and subjective image parameters and on oncologic readings in whole-body 18F-FDG PET/MRI. Methods: We retrospectively simulated the stepwise reduction of 18F-FDG doses of 19 patients (mean age ± SD, 50.9 ± 11.7 y; mean body mass index ± SD, 22.8 ± 3.2 kg/m2) who received a whole-body PET/MRI examination from 3 to 0.5 MBq/kg of body weight (kgBW) in intervals of 0.25. Objective imaging parameters were assessed by measuring the SUV and coefficient of variation in different regions (aorta, liver, spleen, kidney, small bowel, lumbar vertebra, psoas muscle, urinary bladder) as well as the noise-equivalent counting rates in each bed position. Subjective image quality was evaluated with a masked reading of each simulated PET compared with the dose of 2 MBq/kgBW. Oncologic reading was performed first according to PERCIST in each dose and second by defining malignant lesions in doses of 2 MBq/kgBW and the maximum dose image (gold standard). The diagnostic confidence of each lesion was measured using a Likert scale. Results: With decreasing doses, regions in the mid abdomen showed a stronger decrease of SUVmean and noise-equivalent counting rates than regions in the upper abdomen (SUVmean, −45% and −15% on average in the small bowel and the liver, respectively). The coefficient of variation showed a nonlinear increase, pronounced below 1.5 MBq/kgBW. Subjective image quality was stable over a range between 1.25 and 2.75 MBq/kgBW compared with 2 MBq/kgBW. However, large photopenic areas in the mid abdomen were observed in 2 patients. In the PERCIST reading, target lesions were above the liver threshold with a stable SUVpeak in all cases down to 2 MBq/kgBW. Eighty-six of 90 lesions were identified correctly with a dose of 2 MBq/kgBW; Likert scores did not differ significantly. Conclusion: A reduction of doses in 18F-FDG PET/MRI might be possible down to 2 MBq/kgBW in oncologic whole-body examinations. The image quality in the mid abdomen seems to be more affected by lower doses than in the upper abdomen, and in single cases large photopenic areas can occur. Therefore, we do not recommend reducing doses below 3 MBq/kgBW in adults at this time.

Simultaneous multislice diffusion-weighted imaging in whole-body positron emission tomography/magnetic resonance imaging for multiparametric examination in oncological patients

ObjectivesThe aim of this study was to compare the diagnostic performance of simultaneous multislice diffusion-weighted imaging (DWI-SMS) with that of standard DWI (DWI-STD) in whole-body 3-T PET/MRI examination protocols in oncological patients.MethodsIn a phantom study, we evaluated the apparent diffusion coefficients (ADC) from the two techniques. In ten volunteers, we assessed ADC values in different organs. In 20 oncological patients, we evaluated subjective image quality (Likert scale, 5 indicating excellent) and artefacts in different body regions. We also rated the conspicuity and acquired the ADC values of PET-positive tumorous lesions.ResultsThe scan time for the whole-body DWI-SMS examinations was 40% shorter than the scan time for the DWI-STD examinations (84 s vs. 140 s per table position). The phantom and volunteer studies showed lower ADC values from DWI-SMS in the liver and muscle (psoas muscle 1.4 vs. 1.3). In patients, DWI-SMS provided poorer subjective image quality in the thoracoabdominal region (3.0 vs. 3.8, p = 0.02) and overall more artefacts (138 vs. 105). No significant differences regarding conspicuity and ADC values of lesions were found.ConclusionsDWI-SMS seems to provide reliable conspicuity and ADC values of tumorous lesions similar to those provided by DWI-STD. Therefore, although providing poorer image quality in certain regions, DWI-SMS can clearly reduce PET/MRI scan times in oncological patients.Key points• DWI-SMS can reduce PET/MRI scan times in oncological patients.• DWI-SMS provides reliable ADC values and good lesion conspicuity similar to those provided by DWI-STD.• DWI-SMS may provide poorer image quality in regions with low signal.

Voxelwise computed diffusion-weighted imaging for the detection of cytotoxic oedema in brain imaging: a pilot study

Aim To evaluate voxelwise computed diffusion-weighted imaging (vcDWI) for the detection of cytotoxic oedema in brain imaging and to quantify the benefit of lesion contrast in comparison to standard b = 1000 s/mm2 by the example of acute ischaemic stroke. Materials and methods A retrospective evaluation of 66 patients (63 ± 15.9 years) suspected for acute ischaemic stroke who received diffusion-weighted magnetic resonance imaging and fluid-attenuated inversion recovery sequence. A neuroradiologist evaluated all examinations for acute ischaemic stroke based on diffusion-weighted imaging, the apparent diffusion coefficient and fluid-attenuated inversion recovery (reference standard) and 6 weeks later the vcDWI in a randomised manner. Time of analysis was noted. Signal intensities were acquired in lesions, in healthy tissue as well as in the cerebrospinal fluid. Contrast ratios and coefficients of variation were computed. Results A total of 218 lesions was found in 46/66 patients. vcDWI identified all patients and lesions correctly. The median evaluation time was 36 seconds (4–126 s) for the vcDWI and 44 seconds (9–186 s; P < 0.001) for the diffusion-weighted imaging/apparent diffusion coefficient reading. The contrast ratio in vcDWI (mean value 2.57, range 1.73–4.11) was higher than in b = 1000 s/mm2 (2.33, 0.83–3.85, P = 0.03) and the apparent diffusion coefficient map (1.83, 1.00–3.00, P < 0.001), respectively. Coefficients of variation in lesions and tissue did not differ significantly between vcDWI and b = 1000 s/mm2 (P = 0.81/P = 0.26). The signal intensity of cerebrospinal fluid was lower in vcDWI than in b = 1000 mm2/s (0.08 and 34.8, P < 0.001). Conclusion It could be shown that vcDWI has the potential to accelerate the detection of diffusion-restricted lesions in neuroimaging by improving the contrast ratios and reducing the T2 shine-through effect in comparison to standard diffusion-weighted imaging in brain imaging.

Computed diffusion weighted imaging (cDWI) and voxelwise-computed diffusion weighted imaging (vcDWI) for oncologic liver imaging: A pilot study

Objective Aim of the study was to evaluate the influence of the selection of measured b-values on the precision of cDWI in the upper abdomen as well as on the lesion contrast of PET-positive liver metastases in cDWI and vcDWI. Methods We performed a retrospective analysis of 10 patients (4 m, 63.5 ± 12.9 y/o) with PET-positive liver metastases examined in 3 T-PET/MRI with b = 100,600,800,1000 and 1500s/mm2. cDWI (cb1000/cb1500) and vcDWI were computed based on following combinations: i) b = 100/600 s/mm2, ii) b = 100/800 s/mm2, iii) b = 100/1000s/mm2, iv) b = 100/600/1000s/mm2 v) all measured b-values. Mean signal intensity (SI) and standard deviation (SD) in the liver, spleen, kidney, bone marrow and in liver lesions were acquired. The coefficient of variation (CV = SD/SI), the differences of SI between measured and calculated high b-value images and the lesion contrast (SI lesion/liver) were computed. Results With increasing upper measured b-values, the CV in cDWI and vcDWI decreased (CV in the liver in cb1500: 0.42 with b100/600 s/mm2 and 0.28 with b100/b1000s/mm2) while the differences of measured and calculated b-value images decreased (in the liver in cb1500: 30.7% with b = 100/600 s/mm2, 19.7% with b100/b1000s/mm2). In diffusion-restricted lesions, lesion contrast was at least 1.6 in cb1000 and 1.4 in cb1500, respectively, with an upper measured b-value of b = 800 s/mm2 and 2.1 for vcDWI with an upper measured b-value of b = 1000s/mm2. Overall, the lesion contrast was superior in cb1500 and vcDWI compared to cb1000 (15% and 11%, respectively). Conclusion Measuring higher upper b-values seems to lead to more precise computed high b-value images and a decrease of CV. vcDWI provides a comparable lesion contrast to b = 1500s/mm2 and offers additionally the reduction of T2 shine-through effects. For vcDWI, measuring b = 1000s/mm2 as upper b-value seems to be necessary to guarantee good lesion visibility in the liver based on our preliminary results.

SUV-quantification of physiological lung tissue in an integrated PET/MR-system: Impact of lung density and bone tissue

Purpose The aim of the study was to investigate the influence of lung density changes as well as bone proximity on the attenuation correction of lung standardized uptake values (SUVs). Methods and materials 15 patients with mostly oncologic diseases were examined in 18F-FDG-PET/CT and subsequently in a fully integrated PET/MR scanner. From each PET dataset acquired in PET/MR, four different PET reconstructions were computed using different attenuation maps (μ-maps): i) CT-based μ-map (gold standard); ii) CT-based μ-map in which the linear attenuation coefficients (LAC) of the lung tissue was replaced by the lung LAC from the MR-based segmentation method; iii) based on reconstruction ii), the LAC of bone structures was additionally replaced with the LAC from the MR-based segmentation method; iv) the vendor-provided MR-based μ-map (segmentation-based method). Those steps were performed using MATLAB. CT Hounsfield units (HU) and SUVmean was acquired in different levels and regions of the lung. Relative differences between the differently corrected PETs were computed. Results Compared to the gold standard, reconstruction ii), iii) and iv) led to a relative underestimation of SUV in the posterior regions of -9.0%, -13.4% and -14.0%, respectively. Anterior and middle regions were less affected with an overestimation of about 6–8% in reconstructions ii)–iv). Conclusion It could be shown that both, differences in lung density and the vicinity of bone tissue in the μ-map may have an influence on SUV, mostly affecting the posterior lung regions.

Clinical Robustness of Accelerated and Optimized Abdominal Diffusion-Weighted Imaging

Objectives The aim of this study was to assess the robustness of an accelerated and optimized diffusion-weighted sequence in clinical routine abdominal imaging using the simultaneous multislice (SMS) technique for scan time reduction and 3-dimensional (3D) diagonal diffusion mode to optimize image quality. Materials and Methods One hundred fifty consecutive patients received clinically indicated magnetic resonance imaging for abdominal imaging including an optimized SMS diffusion-weighted sequence (DWIOPT: diffusion mode 3D diagonal; SMS factor 2; scan time 1:44 minutes). A subgroup of 41 patients additionally received a standard diffusion-weighted sequence as reference (DWISTD: diffusion mode 4-scan trace; scan time 2:35 minutes). Qualitative and quantitative image parameters of DWISTD and DWIOPT were assessed and compared interindividually within the subgroup using dedicated statistics. Results In all patients, image quality ratings in DWIOPT were rated very high (overall image quality, 4.6 [4–5]; contour sharpness of right/left hepatic lobe, 4.6 [4–5]/4.4 [4–5]; and lesion conspicuity, 4.5 [4.5–5]). Interindividually, DWIOPT proved superior to DWISTD in comparison of overall image quality (4.6 [4.6–4.7] vs 4.2 [4.1–4.2]; P = 0.025) and contour sharpness of the right/left hepatic lobe (4.6 [4.5–4.7]/4.3 [4.0–4.3] vs 4.3 [4.1–43]/4.0[3.0–4.0]; each P = 0.045); lesion conspicuity was comparable in DWIOPT and DWISTD (4.0 [4.8–5] vs 4.4 [4–5]; P = 0.461), and apparent diffusion coefficient (ADC) values showed no statistically significant difference (ADCOPT vs ADCSTD: right hepatic lobe, P = 0.084; kidney, P = 0.445). Interreader agreement was substantial with a kappa value of 0.78 (P < 0.001). Conclusions Diffusion-weighted imaging of the abdomen can be considerably accelerated and optimized integrating the SMS technique and a 3D diagonal diffusion mode. In a large patient cohort, this approach proved of superior image quality while maintaining similar ADC values compared with standard DWI. This technique seems applicable for daily clinical routine.

PET/MR in Oncology

Over the last years, it has been shown that novel imaging technologies rapidly change the working environment of radiologists, nuclear medicine physicians, and oncologists. Positron emission tomography/magnetic resonance imaging (PET/MRI) as a new hybrid modality in clinical environment combines functional and molecular imaging and provides multiparametric tissue information simultaneously. Both, PET and MRI are evolving continuously and have separately become an essential part of oncologic imaging. Thus, it is expected that the combination will lead to further insights and improvements in initial diagnoses, therapy monitoring, and the assessment of therapy response. In this work-in-progress review article, we give a short technical introduction to PET/MRI, summarize the current state in research of PET and MRI in selected oncologic diseases and give an overview of the progress that has been achieved so far with integrated PET/MRI-systems, underlined by the experiences in our own department.