Systematic evaluation of human soft tissue attenuation correction in whole-body PET/MR: Implications from PET/CT for optimization of MR-based AC in patients with normal lung tissue.

BACKGROUND: Attenuation correction (AC) is an important methodical step in positron emission tomography/magnetic resonance imaging (PET/MRI) to correct for attenuated and scattered PET photons. PURPOSE: The overall quality of magnetic resonance (MR)-based AC in whole-body PET/MRI was evaluated in direct comparison to computed tomography (CT)-based AC serving as reference. The quantitative impact of isolated tissue classes in the MR-AC was systematically investigated to identify potential optimization needs and strategies. METHODS: Data of n = 60 whole-body PET/CT patients with normal lung tissue and without metal implants/prostheses were used to generate six different AC-models based on the CT data for each patient, simulating variations of MR-AC. The original continuous CT-AC (CT-org) is referred to as reference. A pseudo MR-AC (CT-mrac), generated from CT data, with four tissue classes and a bone atlas represents the MR-AC. Relative difference in linear attenuation coefficients (LAC) and standardized uptake values were calculated. From the results two improvements regarding soft tissue AC and lung AC were proposed and evaluated. RESULTS: The overall performance of MR-AC is in good agreement compared to CT-AC. Lungs, heart, and bone tissue were identified as the regions with most deviation to the CT-AC (myocardium -15%, bone tissue -14%, and lungs ±20%). Using single-valued LACs for AC in the lung only provides limited accuracy. For improved soft tissue AC, splitting the combined soft tissue class into muscles and organs each with adapted LAC could reduce the deviations to the CT-AC to < ±1%. For improved lung AC, applying a gradient LAC in the lungs could remarkably reduce over- or undercorrections in PET signal compared to CT-AC (±5%). CONCLUSIONS: The AC is important to ensure best PET image quality and accurate PET quantification for diagnostics and radiotherapy planning. The optimized segment-based AC proposed in this study, which was evaluated on PET/CT data, inherently reduces quantification bias in normal lung tissue and soft tissue compared to the CT-AC reference.

Towards a fast PET/MRI protocol for breast cancer imaging: maintaining diagnostic confidence while reducing PET and MRI acquisition times.

OBJECTIVES: To investigate the diagnostic feasibility of a shortened breast PET/MRI protocol in breast cancer patients. METHODS: Altogether 90 women with newly diagnosed T1tumor-staged (T1ts) and T2tumor-staged (T2ts) breast cancer were included in this retrospective study. All underwent a dedicated comprehensive breast [18F]FDG-PET/MRI. List-mode PET data were retrospectively reconstructed with 20, 15, 10, and 5 min for each patient to simulate the effect of reduced PET acquisition times. The SUVmax/mean of all malign breast lesions was measured. Furthermore, breast PET data reconstructions were analyzed regarding image quality, lesion detectability, signal-to-noise ratio (SNR), and image noise (IN). The simultaneously acquired comprehensive MRI protocol was then shortened by retrospectively removing sequences from the protocol. Differences in malignant breast lesion detectability between the original and the fast breast MRI protocol were evaluated lesion-based. The 20-min PET reconstructions and the original MRI protocol served as reference. RESULTS: In all PET reconstructions, 127 congruent breast lesions could be detected. Group comparison and T1ts vs. T2ts subgroup comparison revealed no significant difference of subjective image quality between 20, 15, 10, and 5 min acquisition times. SNR of qualitative image evaluation revealed no significant difference between different PET acquisition times. A slight but significant increase of IN with decreasing PET acquisition times could be detected. Lesion SUVmax group comparison between all PET acquisition times revealed no significant differences. Lesion-based evaluation revealed no significant difference in breast lesion detectability between original and fast breast MRI protocols. CONCLUSIONS: Breast [18F]FDG-PET/MRI protocols can be shortened from 20 to below 10 min without losing essential diagnostic information. KEY POINTS: • A highly accurate breast cancer evaluation is possible by the shortened breast [18F]FDG-PET/MRI examination protocol. • Significant time saving at breast [18F]FDG-PET/MRI protocol could increase patient satisfaction and patient throughput for breast cancer patients at PET/MRI.

Intra-Individual Comparison of 124I-PET/CT and 124I-PET/MR Hybrid Imaging of Patients with Resected Differentiated Thyroid Carcinoma: Aspects of Attenuation Correction.

BACKGROUND: This study evaluates the quantitative differences between 124-iodine (I) positron emission tomography/computed tomography (PET/CT) and PET/magnetic resonance imaging (PET/MR) in patients with resected differentiated thyroid carcinoma (DTC). METHODS: N = 43 124I PET/CT and PET/MR exams were included. CT-based attenuation correction (AC) in PET/CT and MR-based AC in PET/MR with bone atlas were compared concerning bone AC in the head-neck region. AC-map artifacts (e.g., dentures) were noted. Standardized uptake values (SUV) were measured in lesions in each PET data reconstruction. Relative differences in SUVmean were calculated between PET/CT and PET/MR with bone atlas. RESULTS: Overall, n = 111 124I-avid lesions were detected in all PET/CT, while n = 132 lesions were detected in PET/MR. The median in SUVmean for n = 98 congruent lesions measured in PET/CT was 12.3. In PET/MR, the median in SUVmean was 16.6 with bone in MR-based AC. CONCLUSIONS: 124I-PET/CT and 124I-PET/MR hybrid imaging of patients with DTC after thyroidectomy provides overall comparable quantitative results in a clinical setting despite different patient positioning and AC methods. The overall number of detected 124I-avid lesions was higher for PET/MR compared to PET/CT. The measured average SUVmean values for congruent lesions were higher for PET/MR.

Free-breathing 3D Stack of Stars GRE (StarVIBE) sequence for detecting pulmonary nodules in 18F-FDG PET/MRI.

BACKGROUND: The free-breathing T1-weighted 3D Stack of Stars GRE (StarVIBE) MR sequence potentially reduces artifacts in chest MRI. The purpose of this study was to evaluate StarVIBE for the detection of pulmonary nodules in 18F-FDG PET/MRI. MATERIAL AND METHODS: In this retrospective analysis, conducted on a prospective clinical trial cohort, 88 consecutive women with newly diagnosed breast cancer underwent both contrast-enhanced whole-body 18F-FDG PET/MRI and computed tomography (CT). Patients' chests were examined on CT as well as on StarVIBE and conventional T1-weighted VIBE and T2-weighted HASTE MR sequences, with CT serving as the reference standard. Presence, size, and location of all detectable lung nodules were assessed. Wilcoxon test was applied to compare nodule features and Pearson's, and Spearman's correlation coefficients were calculated. RESULTS: Out of 65 lung nodules detected in 36 patients with CT (3.7 ± 1.4 mm), StarVIBE was able to detect 31 (47.7%), VIBE 26 (40%) and HASTE 11 (16.8%), respectively. Overall, CT showed a significantly higher detectability than all MRI sequences combined (65 vs. 36, difference 44.6%, p < 0.001). The VIBE showed a significantly better detection rate than the HASTE (23.1%, p = 0.001). Detection rates between StarVIBE and VIBE did not significantly differ (7.7%, p = 0.27), but the StarVIBE showed a significant advantage detecting centrally located pulmonary nodules (66.7% vs. 16.7%, p = 0.031). There was a strong correlation in nodule size between CT and MRI sequences (HASTE: ρ = 0.80, p = 0.003; VIBE: ρ = 0.77, p < 0.001; StarVIBE: ρ = 0.78, p < 0.001). Mean image quality was rated as good to excellent for CT and MRI sequences. CONCLUSION: The overall lung nodule detection rate of StarVIBE was slightly, but not significantly, higher than conventional T1w VIBE and significantly higher than T2w HASTE. Detectability of centrally located nodules is better with StarVIBE than with VIBE. Nevertheless, all MRI analyses demonstrated considerably lower detection rates for small lung nodules, when compared to CT.

Evaluation of improved CT-based hardware attenuation correction in PET/MRI: Application to a 16-channel RF breast coil.

PURPOSE: The aim of this study was to compare and evaluate three different bilinear conversion curves for attenuation correction (AC) of a 16-channel radiofrequency (RF) coil in positron emission tomography/magnetic resonance (PET/MR) breast cancer imaging. METHODS: The quantitative impact of three different bilinear conversions of computed tomography (CT) data for the AC of a 16-channel RF breast coil was systematically evaluated in phantom measurements and on n = 20 PET/MR patients with breast cancer. PET data were reconstructed four times: (1) no coil AC (C-NAC) serving as a reference, (2) established bilinear conversion by Carney et al., (3) bilinear conversion by Paulus et al., and (4) bilinear conversion by Oehmigen et al. Relative differences in PET data were calculated. RESULTS: Independent of the choice of bilinear conversion, significant gains in PET signal, compared to C-NAC, were measurable in all phantom and patient measurements. Mean relative differences of ca. 10% in SUVmean (i.e., standardized uptake value; maximal relative differences up to 30%) due to the integration of the coil AC were calculated, compared to C-NAC in phantom and patient measurements. Relative difference images depict that the quantitative impact of coil AC is highest in regions close to the RF coil when compared to no AC data. Bilinear conversion by Carney et al. shows a slightly overcorrection (2.9%), whereas the conversion by Paulus et al. provides a slight undercorrection of the PET images (-1.6%) in comparison to the no-coil measurement. The bilinear conversion proposed by Oehmigen et al. provides the most appropriate AC for the breast coil in this phantom experiment (-0.2%). A total of 23 congruent lesions could be detected in all patients. All lesions could be detected in all reconstructions. CONCLUSIONS: For the best possible PET image quality and accurate PET quantification in breast PET/MRI, the AC of MR hardware components is important. The bilinear conversion proposed by Oehmigen et al. provides the most appropriate AC for the breast coil in this study.

MR-based truncation correction using an advanced HUGE method to improve attenuation correction in PET/MR imaging of obese patients.

PURPOSE: Truncation artifacts in the periphery of the magnetic resonance (MR) field-of-view (FOV) and thus, in the MR-based attenuation correction (AC) map, may hamper accurate positron emission tomography (PET) quantification in whole-body PET/MR, which is especially problematic in patients with obesity with overall large body dimensions. Therefore, an advanced truncation correction (TC) method to extend the conventional MR FOV is needed. METHODS: The extent of MR-based AC-map truncations in obese patients was determined in a dataset including n  =  10 patients that underwent whole-body PET/MR exams. Patient inclusion criteria were defined as BMI > 30 kg/m2 and body weight > 100 kg. Truncations in PET/MR patients with obesity were quantified comparing the MR-based AC-map volume to segmented non-AC PET data, serving as the reference body volume without truncations to demonstrate the need of improved TC. The new method implemented in this study, termed "advanced HUGE," was modified and extended from the original HUGE method by Blumhagen et al. in order to provide improved TC across the entire axial MR FOV and to unlock new clinical applications of PET/MR. Advanced HUGE was then systematically tested in PET/MR NEMA phantom measurements. Relative differences between computed tomography (CT) AC PET data of the phantom setup (reference) and MR-based Dixon AC, respectively Dixon + advanced HUGE AC, were calculated. The applicability of the method for advanced TC was then demonstrated in first MR-based measurements in healthy volunteers. RESULTS: It was found that the MR-based AC maps of obese patients often reveal truncations in the anterior-posterior direction. Especially, the abdominal region could benefit from improved TC, where maximal relative differences in the AC-map volume up to -17% were calculated. Applying advanced HUGE to improve the MR-based AC in PET/MR, PET quantification errors in the large-volume phantom setup could be considerably reduced from average -18.6% (Dixon AC) to 4.6% compared to the CT AC reference. Volunteer measurements demonstrate that formerly missing AC-map volume in the Dixon-VIBE AC-map could be added due to advanced HUGE in the anterior-posterior direction and thus, potentially improves AC in PET/MR. CONCLUSIONS: The advanced HUGE method for truncation correction considerably reduces truncations in the anterior-posterior direction demonstrated in phantom measurements and healthy volunteers and thus, further improves MR-based AC in PET/MR imaging.

CAD-based hardware attenuation correction in PET/MRI: First methodical investigations and clinical application of a 16-channel RF breast coil.

PURPOSE: Aim of this study was to evaluate the use of computer-aided design (CAD) models for attenuation correction (AC) of hardware components in positron emission tomography/magnetic resonance (PET/MR) imaging. METHODS: The technical feasibility and quantitative impact of CAD-AC compared to computer tomography (CT)-based AC (reference) was investigated on a modular phantom consisting of 19 different material samples (plastics and metals arranged around a cylindrical emission phantom) typically used in phantoms, patient tables, and radiofrequency (RF) coils in PET/MR. The clinical applicability of the CAD-AC method was then evaluated on a 16-channel RF breast coil in a PET/MR patient study. The RF breast coil in this study was specifically designed PET compatible. Using this RF breast coil, the impact on clinical PET/MR breast imaging was systematically evaluated in breast phantom measurements and, furthermore, in n = 10 PET/MR patients with breast cancer. PET data were reconstructed three times: (1) no AC (NAC), (2) established CT-AC, and (3) CAD-AC. For both phantom measurements, a scan without attenuating hardware components (material probes or RF breast coil) was acquired serving as reference. Relative differences in PET data were calculated for all experiments. RESULTS: In all phantom and patient measurements, significant gains in PET signal compared to NAC data were measurable with CT and CAD-AC. In initial phantom experiments, mean relative differences of -0.2% for CT-AC and 0.2% for CAD-AC were calculated compared to reference measurements without the material probes. The application to a RF breast coil depicts that CAD-AC results in significant gains compared to NAC data (10%) and a slight underestimation in PET signal of -1.3% in comparison to the no-coil reference measurement. In the patient study, a total of 15 congruent lesions in all 10 patients with a mean relative difference of 14% (CT and CAD-AC) in standardized uptake value compared to NAC data could be detected. CONCLUSIONS: To ensure best possible PET image quality and accurate PET quantification in PET/MR imaging, the AC of hardware components such as phantoms and RF coils is important. In initial phantom experiments and in clinical application to an RF breast coil, it was found that CAD-based AC results in significant gains in PET signal compared to NAC data and provides comparably good results to the established method of CT-based AC.

Comparison of pre- and post-contrast-enhanced attenuation correction using a CAIPI-accelerated T1-weighted Dixon 3D-VIBE sequence in 68Ga-DOTATOC PET/MRI.

OBJECTIVES: To investigate the influence of contrast agent administration on attenuation correction (AC) based on a CAIPIRINHA (CAIPI)-accelerated T1-weighted Dixon 3D-VIBE sequence in 68Ga-DOTATOC PET/MRI. MATERIAL AND METHODS: Fifty-one patients with neuroendocrine tumors underwent whole-body 68Ga-DOTATOC PET/MRI for tumor staging. Two PET reconstructions were performed using AC-maps that were created using a high-resolution CAIPI-accelerated Dixon-VIBE sequence with an additional bone atlas and truncation correction using the HUGE (B0 homogenization using gradient enhancement) method before and after application of Gadolinium (Gd)-based contrast agent. Standardized uptake values (SUVs) of 21 volumes of interest (VOIs) were compared between in both PET data sets per patient. A student's t-test for paired samples was performed to test for potential differences between both AC-maps and both reconstructed PET data sets. Bonferroni correction was performed to prevent α-error accumulation, p < 0.0024 was considered to indicate statistical significance. RESULTS: Significant quantitative differences between SUVmax were found in the perirenal fat (19.65 ±â€¯48.03 %, p < 0.0001), in the axillary fat (17.46 ±â€¯63.67 %, p < 0.0001) and in the dorsal subcutaneous fat on level of lumbar vertebral body L4 (10.26 ±â€¯25.29 %, p < 0.0001). Significant differences were also evident in the lungs apical (5.80 ±â€¯10.53 %, p < 0.0001), dorsal at the level of the pulmonary trunk (15.04 ±â€¯19.09 %, p < 0.0001) and dorsal in the basal lung (51.27 ±â€¯147.61 %, p < 0.0001). CONCLUSION: The administration of (Gd)-contrast agents in this study has shown a considerable influence on the AC-maps in PET/MRI and, consequently impacted quantification in the reconstructed PET data. Therefore, dedicated PET/MRI staging protocols have to be adjusted so that AC-map acquisition is performed prior to contrast agent administration.

Evaluation of improved attenuation correction in whole-body PET/MR on patients with bone metastasis using various radiotracers.

PURPOSE: This study evaluates the quantitative effect of improved MR-based attenuation correction (AC), including bone segmentation and the HUGE method for truncation correction in PET/MR whole-body hybrid imaging specifically of oncologic patients with bone metastasis and using various radiotracers. METHODS: Twenty-three patients that underwent altogether 28 whole-body PET/MR examinations with findings of bone metastasis were included in this study. Different radiotracers (18F-FDG, 68Ga-PSMA, 68Ga-DOTATOC, 124I-MIBG) were injected according to appropriate clinical indications. Each of the 28 whole-body PET datasets was reconstructed three times using AC with (1) standard four-compartment µ-maps (background air, lung, muscle, and soft tissue), (2) five-compartment µ-maps (adding bone), and (3) six-compartment µ-maps (adding bone and HUGE truncation correction). The SUVmax of each detected bone lesion was measured in each reconstruction to evaluate the quantitative impact of improved MR-based AC. Relative difference images between four- and six-compartment µ-maps were calculated. MR-based HUGE truncation correction was compared with the PET-based MLAA truncation correction method in all patients. RESULTS: Overall, 69 bone lesions were detected and evaluated. The mean increase in relative difference over all 69 lesions in SUVmax was 5.4 ± 6.4% when comparing the improved six-compartment AC with the standard four-compartment AC. Maximal relative difference of 28.4% was measured in one lesion. Truncation correction with HUGE worked robust and resulted in realistic body contouring in all 28 exams and for all 4 different radiotracers. Truncation correction with MLAA revealed overestimations of arm tissue volume in all PET/MR exams with 18F-FDG radiotracer and failed in all other exams with radiotracers 68Ga-PSMA, 68Ga-DOTATOC, and 124I- MIBG due to limitations in body contour detection. CONCLUSION: Improved MR-based AC, including bone segmentation and HUGE truncation correction in whole-body PET/MR on patients with bone lesions and using various radiotracers, is important to ensure best possible diagnostic image quality and accurate PET quantification. The HUGE method for truncation correction based on MR worked robust and results in realistic body contouring, independent of the radiotracers used.

Improving the CT (140 kVp) to PET (511 keV) conversion in PET/MR hardware component attenuation correction.

PURPOSE: Today, attenuation correction (AC) of positron emission tomography/magnetic resonance (PET/MR) hardware components is performed by using an established method from PET/CT hybrid imaging. As shown in previous studies, the established mathematical conversion from computed tomography (CT) to PET attenuation coefficients may, however, lead to incorrect results in PET quantification when applied to AC of hardware components in PET/MR. The purpose of this study is to systematically investigate the attenuating properties of various materials and electronic components frequently used in the context of PET/MR hybrid imaging. The study, thus, aims at improving hardware component attenuation correction in PET/MR. MATERIALS AND METHODS: Overall, 38 different material samples were collected; a modular phantom was used to for CT, PET, and PET/MR scanning of all samples. Computed tomography-scans were acquired with a tube voltage of 140 kVp to determine Hounsfield Units (HU). PET transmission scans were performed with 511 keV to determine linear attenuation coefficients (LAC) of all materials. The attenuation coefficients were plotted to obtain a HU to LAC correlation graph, which was then compared to two established conversions from literature. Hardware attenuation maps of the different materials were created and applied to PET data reconstruction following a phantom validation experiment. From these measurements, PET difference maps were calculated to validate and compare all three conversion methods. RESULTS: For each material, the HU and corresponding LAC could be determined and a bi-linear HU to LAC conversion graph was derived. The corresponding equation was y = 1.64 ∗ 10 - 5 × HU + 1000 + 8.3 ∗ 10 - 2 . While the two established conversions lead to a mean quantification PET bias of 4.69% ± 0.27% and -2.84% ± 0.72% in a phantom experiment, PET difference measurements revealed only 0.5 % bias in PET quantification when applying the new conversion resulting from this study. CONCLUSIONS: An optimized method for the conversion of CT to PET attenuation coefficients has been derived by systematic measurement of 38 different materials. In contrast to established methods, the new conversion also considers highly attenuating materials, thus improving attenuation correction of hardware components in PET/MR hybrid imaging.

Improving 68Ga-PSMA PET/MRI of the Prostate with Unrenormalized Absolute Scatter Correction.

A limitation of using 68Ga-labeled prostate-specific membrane antigen (68Ga-PSMA) for detection and staging of prostate cancer is a frequently observed halo artifact around the urinary bladder caused by inaccurate scatter correction (SC) of PET data. The aim of this study was to investigate the impact of unrenormalized absolute SC on 68Ga-PSMA PET quantification in PET/MRI of the prostate in 100 patients. Methods: The PET data of 100 patients were reconstructed twice using standard SC and improved unrenormalized SC. The visual presence of the halo artifact was rated in each PET data reconstruction using 5 grades (0, no halo artifact; 4, severe halo artifact). The number of visible lesions in the pelvis was recorded. SUVmean and SUVmax were measured in the lesions, in the bladder, in the gluteus maximus, and within the halo margin. Furthermore, the signal-to-noise-ratio and image noise were measured in all PET data. Relative differences between standard and unrenormalized SC were calculated. Results: With standard SC, the average grade in the presence of the halo artifact was 2 (moderate halo artifact), whereas for unrenormalized SC, the average grade was 0.9 (slight halo artifact). The same number of congruent lesions (n = 74) was detected for both PET data reconstructions. Relative changes in PET signal-to-noise-ratio and image noise were not statistically significant (P > 0.05). The mean (±SD) increase in SUVmean using unrenormalized SC was 23.0% ± 9.2% in the gluteus maximus, 7.1% ± 4.5% in the bladder, 325.4% ± 748.5% in the halo margin, and 12.4% ± 16.8% in all 74 detected lesions. The mean increase using unrenormalized SC in SUVmean was 17.5% for lesions inside the halo margin (38 lesions) and 6.9% for lesions outside the halo margin (36 lesions). Conclusion: For PET/MRI of prostate cancer using 68Ga-PSMA, a proper SC is important to ensure the best possible diagnostic quality and PET quantification. Unrenormalized absolute SC significantly reduces the halo artifact around the bladder and improves PET/MRI of the prostate.

Impact of improved attenuation correction on 18F-FDG PET/MR hybrid imaging of the heart.

PURPOSE: The aim of this study was to evaluate and quantify the effect of improved attenuation correction (AC) including bone segmentation and truncation correction on 18F-Fluordesoxyglucose cardiac positron emission tomography/magnetic resonance (PET/MR) imaging. METHODS: PET data of 32 cardiac PET/MR datasets were reconstructed with three different AC-maps (1. Dixon-VIBE only, 2. HUGE truncation correction and bone segmentation, 3. MLAA). The Dixon-VIBE AC-maps served as reference of reconstructed PET data. 17-segment short-axis polar plots of the left ventricle were analyzed regarding the impact of each of the three AC methods on PET quantification in cardiac PET/MR imaging. Non-AC PET images were segmented to specify the amount of truncation in the Dixon-VIBE AC-map serving as a reference. All AC-maps were evaluated for artifacts. RESULTS: Using HUGE + bone AC results in a homogeneous gain of ca. 6% and for MLAA 8% of PET signal distribution across the myocardium of the left ventricle over all patients compared to Dixon-VIBE AC only. Maximal relative differences up to 18% were observed in segment 17 (apex). The body volume truncation of -12.7 ± 7.1% compared to the segmented non-AC PET images using the Dixon-VIBE AC method was reduced to -1.9 ± 3.9% using HUGE and 7.8 ± 8.3% using MLAA. In each patient, a systematic overestimation in AC-map volume was observed when applying MLAA. Quantitative impact of artifacts showed regional differences up to 6% within single segments of the myocardium. CONCLUSIONS: Improved AC including bone segmentation and truncation correction in cardiac PET/MR imaging is important to ensure best possible diagnostic quality and PET quantification. The results exhibited an overestimation of AC-map volume using MLAA, while HUGE resulted in a more realistic body contouring. Incorporation of bone segmentation into the Dixon-VIBE AC-map resulted in homogeneous gain in PET signal distribution across the myocardium. The majority of observed AC-map artifacts did not significantly affect the quantitative assessment of the myocardium.

Towards fast whole-body PET/MR: Investigation of PET image quality versus reduced PET acquisition times.

PURPOSE: The trend towards faster acquisition protocols in whole-body positron emission tomography/magnetic resonance (PET/MR) arises the question of whether short PET data acquisition protocols in a whole-body multi-station context allow for reduced PET acquisition times while providing adequate PET image quality and accurate quantification parameters. The study goal is to investigate how reducing PET acquisition times affects PET image quality and quantification in whole-body PET/MR in patients with oncologic findings. METHODS: Fifty-one patients with different oncologic findings underwent a clinical whole-body 18F-Fluorodeoxyglucose PET/MR examination. PET data was reconstructed with 4, 3, 2, and 1 min/bed time intervals for each patient to simulate the effect of reduced PET acquisition times. The 4-minute PET reconstructions served as reference standard. All whole-body PET data sets were analyzed regarding image quality, lesion detectability, PET quantification and standardized uptake values. RESULTS: A total of 91 lesions were detected in the 4-minute PET reconstructions. The same number of congruent lesions was also noticed in the 3 and 2 minutes-per-bed (mpb) reconstructed images. A total of 2 lesions in 2 patients was not detected in the 1 minute PET data reconstructions due to poor image quality. Image noise in the blood pool increased from 22.2% (4 mpb) to 42.1% (1 mpb). Signal-to-noise ratio declined with shorter timeframes from 13.1 (4 mpb) to 9.3 (1 mpb). SUVmean and SUVmax showed no significant changes between 4 and 1 mpb reconstructed timeframes. CONCLUSIONS: Reconstruction of PET data with different time intervals has shown that 2 minutes acquisition time per bed position instead of 4 minutes is sufficient to provide accurate lesion detection and adequate image quality in a clinical setting, despite the trends to lower image quality with shorter PET acquisition times. This provides latitude for potential reduction of PET acquisition times in fast PET/MR whole-body examinations.

A dual-tuned 13 C/1 H head coil for PET/MR hybrid neuroimaging: Development, attenuation correction, and first evaluation.

PURPOSE: This study aims to develop, implement, and evaluate a dual-tuned 13 C/1 H head coil for integrated positron emission tomography/magnetic resonance (PET/MR) neuroimaging. The radiofrequency (RF) head coil is designed for optimized MR imaging performance and PET transparency and attenuation correction (AC) is applied for accurate PET quantification. MATERIAL AND METHODS: A dual-tuned 13 C/1 H RF head coil featuring a 16-rung birdcage was designed to be used for integrated PET/MR hybrid imaging. While the open birdcage design can be considered inherently PET transparent, all further electronic RF components were placed as far as possible outside of the field-of-view (FOV) of the PET detectors. The RF coil features a rigid geometry and thin-walled casing. Attenuation correction of the RF head coil is performed by generating and applying a dedicated 3D CT-based template attenuation map (µmap). Attenuation correction was systematically evaluated in phantom experiments using a large-volume cylindrical emission phantom filled with 18-F-Fluordesoxyglucose (FDG) radiotracer. The PET/MR imaging performance and PET attenuation correction were then evaluated in a patient study including six patients. RESULTS: The dual-tuned RF head coil causes a mean relative attenuation difference of 8.8% across the volume of the cylindrical phantom, while the local relative differences range between 1% and 25%. Applying attenuation correction, the relative difference between the two measurements with and without RF coil is reduced to mean value of 0.5%, with local differences of ±3.6%. The quantitative results of the phantom measurements were corroborated by patient PET/MR measurements. Patient scans using the RF head coil show a decrease of PET signal of 5.17% ± 0.81% when compared to the setup without RF head coil in place, which served as a reference scan. When applying attenuation correction of the RF coil in the patient measurements, the mean difference to a measurement without RF coil was reduced to -0.87% ± 0.65%. CONCLUSION: A dual-tuned 13 C/1 H RF head coil was designed and evaluated regarding its potential use in integrated PET/MR hybrid imaging. Attenuation correction was successfully applied. In conclusion, the RF head coil was successfully integrated into PET/MR hybrid imaging and can now be used for 13 C/1 H multinuclear hybrid neuroimaging in future studies.

Impact of improved attenuation correction featuring a bone atlas and truncation correction on PET quantification in whole-body PET/MR.

PURPOSE: Recent studies have shown an excellent correlation between PET/MR and PET/CT hybrid imaging in detecting lesions. However, a systematic underestimation of PET quantification in PET/MR has been observed. This is attributable to two methodological challenges of MR-based attenuation correction (AC): (1) lack of bone information, and (2) truncation of the MR-based AC maps (µmaps) along the patient arms. The aim of this study was to evaluate the impact of improved AC featuring a bone atlas and truncation correction on PET quantification in whole-body PET/MR. METHODS: The MR-based Dixon method provides four-compartment µmaps (background air, lungs, fat, soft tissue) which served as a reference for PET/MR AC in this study. A model-based bone atlas provided bone tissue as a fifth compartment, while the HUGE method provided truncation correction. The study population comprised 51 patients with oncological diseases, all of whom underwent a whole-body PET/MR examination. Each whole-body PET dataset was reconstructed four times using standard four-compartment µmaps, five-compartment µmaps, four-compartment µmaps + HUGE, and five-compartment µmaps + HUGE. The SUVmax for each lesion was measured to assess the impact of each µmap on PET quantification. RESULTS: All four µmaps in each patient provided robust results for reconstruction of the AC PET data. Overall, SUVmax was quantified in 99 tumours and lesions. Compared to the reference four-compartment µmap, the mean SUVmax of all 99 lesions increased by 1.4 ± 2.5% when bone was added, by 2.1 ± 3.5% when HUGE was added, and by 4.4 ± 5.7% when bone + HUGE was added. Larger quantification bias of up to 35% was found for single lesions when bone and truncation correction were added to the µmaps, depending on their individual location in the body. CONCLUSION: The novel AC method, featuring a bone model and truncation correction, improved PET quantification in whole-body PET/MR imaging. Short reconstruction times, straightforward reconstruction workflow, and robust AC quality justify further routine clinical application of this method.

MR-based truncation and attenuation correction in integrated PET/MR hybrid imaging using HUGE with continuous table motion.

PURPOSE: The objective of this study was to introduce and evaluate a method for MR-based attenuation and truncation correction in phantom and patient measurements to improve PET quantification in PET/MR hybrid imaging. METHODS: The fully MR-based method HUGE (B0 Homogenization using gradient enhancement) provides field-of-view extension in MR imaging, which can be used for truncation correction and improved PET quantification in PET/MR hybrid imaging. The HUGE method in this recent implementation is combined with continuously moving table data acquisition to provide a seamless nontruncated whole-body data set of the outer patient contours to complete the established standard MR-based Dixon-VIBE data for attenuation correction. The method was systematically evaluated in NEMA standard phantom experiments to investigate the impact of HUGE truncation correction on PET quantification. The method was then applied to 24 oncologic patients in whole-body PET/MR hybrid imaging. The impact of MR-based truncation correction with HUGE on PET data was compared to the impact of the established PET-based MLAA algorithm for contour detection. RESULTS: In phantom and in all patient measurements, the standard Dixon-VIBE attenuation correction data show geometric distortions and signal truncations at the edges of the MR imaging field-of-view. In contrast, the Dixon-VIBE-based attenuation correction data additionally extended by applying HUGE shows significantly less distortion and truncations and due to the continuously moving table acquisition robustly provides smooth outer contours of the patient arms. In the investigated patient cases, MLAA frequently showed an overestimation of arm volume and associated artifacts limiting contour detection. When applying HUGE, an average relative increase in SUVmean in patients' lesion of 4.2% and for MLAA of 4.6% were measured, when compared to standard Dixon-VIBE only. In specific lesions maximal differences in SUVmean up to 13% for HUGE and 14% for MLAA were measured. Quantification in truncated regions showed maximal differences up to 40% for both, MLAA and HUGE. Average differences in those regions in SUVmean for HUGE are 13.3% and 14.6% for MLAA. In a patient with I-124 radiotracer PET-based MLAA contour detection completely failed in this specific case, whereas HUGE as MR-based approach provided accurate truncation correction. CONCLUSIONS: The HUGE method for truncation correction combined with continuous table movement extends the lateral MR field-of-view and effectively reduces truncations along the outer contours of the patient's arms in whole-body PET/MR imaging. HUGE as a fully MR-based approach is independent of the choice of radiotracer, thus also offering robust truncation correction in patients that are not injected with Fluordesoxyglucose (FDG) as radiotracer. Therefore, this method improves the standard Dixon MR-based attenuation correction and PET image quantification in whole-body PET/MR imaging applications.

Integrated PET/MR breast cancer imaging: Attenuation correction and implementation of a 16-channel RF coil.

PURPOSE: This study aims to develop, implement, and evaluate a 16-channel radiofrequency (RF) coil for integrated positron emission tomography/magnetic resonance (PET/MR) imaging of breast cancer. The RF coil is designed for optimized MR imaging performance and PET transparency and attenuation correction (AC) is applied for accurate PET quantification. METHODS: A 16-channel breast array RF coil was designed for integrated PET/MR hybrid imaging of breast cancer lesions. The RF coil features a lightweight rigid design and is positioned with a spacer at a defined position on the patient table of an integrated PET/MR system. Attenuation correction is performed by generating and applying a dedicated 3D CT-based template attenuation map. Reposition accuracy of the RF coil on the system patient table while using the positioning frame was tested in repeated measurements using MR-visible markers. The MR, PET, and PET/MR imaging performances were systematically evaluated using modular breast phantoms. Attenuation correction of the RF coil was evaluated with difference measurements of the active breast phantoms filled with radiotracer in the PET detector with and without the RF coil in place, serving as a standard of reference measurement. The overall PET/MR imaging performance and PET quantification accuracy of the new 16-channel RF coil and its AC were then evaluated in first clinical examinations on ten patients with local breast cancer. RESULTS: The RF breast array coil provides excellent signal-to-noise ratio and signal homogeneity across the volume of the breast phantoms in MR imaging and visualizes small structures in the phantoms down to 0.4 mm in plane. Difference measurements with PET revealed a global loss and thus attenuation of counts by 13% (mean value across the whole phantom volume) when the RF coil is placed in the PET detector. Local attenuation ranging from 0% in the middle of the phantoms up to 24% was detected in the peripheral regions of the phantoms at positions closer to attenuating hardware structures of the RF coil. The position accuracy of the RF coil on the patient table when using the positioning frame was determined well below 1 mm for all three spatial dimensions. This ensures perfect position match between the RF coil and its three-dimensional attenuation template during the PET data reconstruction process. When applying the CT-based AC of the RF coil, the global attenuation bias was mostly compensated to ±0.5% across the entire breast imaging volume. The patient study revealed high quality MR, PET, and combined PET/MR imaging of breast cancer. Quantitative activity measurements in all 11 breast cancer lesions of the ten patients resulted in increased mean difference values of SUVmax 11.8% (minimum 3.2%; maximum 23.2%) between nonAC images and images when AC of the RF breast coil was applied. This supports the quantitative results of the phantom study as well as successful attenuation correction of the RF coil. CONCLUSIONS: A 16-channel breast RF coil was designed for optimized MR imaging performance and PET transparency and was successfully integrated with its dedicated attenuation correction template into a whole-body PET/MR system. Systematic PET/MR imaging evaluation with phantoms and an initial study on patients with breast cancer provided excellent MR and PET image quality and accurate PET quantification.