Martin Hüllner

Contrast-Enhanced PET/MR Imaging Versus Contrast-Enhanced PET/CT in Head and Neck Cancer: How Much MR Information Is Needed?

Considering PET/MR imaging as a whole-body staging tool, scan time restrictions in a single body area are mandatory for the cost-effective clinical operation of an integrated multimodality scanner setting. It has to be considered that 18F-FDG already acts as a contrast agent and that under certain circumstances MR contrast may not yield additional clinically relevant information. The concept of the present study was to understand which portions of the imaging information enhance the sensitivity and specificity of the hybrid examination and which portions are redundant. Methods: One hundred fifty consecutive patients referred for primary staging or restaging of head and neck cancer underwent sequential whole-body 18F-FDG PET with CT-based attenuation correction, contrast-enhanced (ce) CT, and conventional diagnostic MR imaging of the head and neck in a trimodality PET/CT–MR system. Assessed were image quality, lesion conspicuity, diagnostic confidence, and the benefit of additional coronal and sagittal imaging planes in cePET/CT, PET/MR imaging with only T2-weighted fat-suppressed images (T2w PET/MR imaging), and cePET/MR imaging. Results: In 85 patients with at least 1 PET-positive lesion, 162 lesions were evaluated. Similar robustness was found for CT and MR image quality. T2w PET/MR imaging performed similarly to (metastatic lymph nodes) or better than (primary tumors) cePET/CT in the morphologic characterization of PET-positive lesions and permitted the diagnosis of necrotic or cystic lymph node metastasis without application of intravenous contrast medium. CePET/MR imaging yielded a higher diagnostic confidence for accurate lesion conspicuity (especially in the nasopharynx and in the larynx), infiltration of adjacent structures, and perineural spread. Conclusion: The results of the present study provide evidence that PET/MR imaging can serve as a legitimate alternative to PET/CT in the clinical workup of patients with head and neck cancers. Intravenous MR contrast medium may be applied only if the exact tumor extent or infiltration of crucial structures is of concern (i.e., preoperatively) or if perineural spread is anticipated. In early assessment of the response to therapy, in follow-up examinations, or in a whole-body protocol for non–head and neck tumors, T2w PET/MR imaging may be sufficient for coverage of the head and neck. The additional MR scanning time may instead be used for advanced MR techniques to increase the specificity of the hybrid imaging examination.

Anatomic Evaluation of 3-Dimensional Ultrashort-Echo-Time Bone Maps for PET/MR Attenuation Correction

Ultrashort-echo-time (UTE) sequences have been proposed in the past for MR-based attenuation correction of PET data, because of their ability to image cortical bone. In the present work we assessed the limitations of dual-echo UTE imaging for bone segmentation in head and neck imaging. Sequentially acquired MR and PET/CT clinical data were used for this purpose. Methods: Twenty patients referred for a clinical oncology examination were scanned using a trimodality setup. Among the MR sequences, a dual-echo UTE acquisition of the head was acquired and used to create tissue R2 maps. The different undesired structures present in these maps were identified by an experienced radiologist. Global and local measurements of the overlap between R2-based and CT-based bone masks were computed. Results: UTE R2 maps displayed a nonfunctional relation with CT data. The obtained bone masks showed acceptable overlap with the corresponding CT data, in the case of the skull itself (e.g., 47% mismatch for the parietal region), with decreased performance in the base of the skull and in the neck (e.g., 78% for the maxillary region). Unwanted structures were detected, both anatomic (e.g., sternocleidomastoid, temporal, and masseter muscles) and artifactual (e.g., dental implants and air–tissue interfaces). Conclusion: It is indeed possible to estimate the anatomic location of bone tissue using UTE sequences. However, using pure parametric maps for attenuation correction may lead to bias close to certain anatomic structures and areas of high magnetic field inhomogeneity. More sophisticated approaches are necessary to compensate for these effects.

Clinical Evaluation of Zero-Echo-Time MR Imaging for the Segmentation of the Skull

MR-based attenuation correction is instrumental for integrated PET/MR imaging. It is generally achieved by segmenting MR images into a set of tissue classes with known attenuation properties (e.g., air, lung, bone, fat, soft tissue). Bone identification with MR imaging is, however, quite challenging, because of the low proton density and fast decay time of bone tissue. The clinical evaluation of a novel, recently published method for zero-echo-time (ZTE)–based MR bone depiction and segmentation in the head is presented here. Methods: A new paradigm for MR imaging bone segmentation, based on proton density–weighted ZTE imaging, was disclosed earlier in 2014. In this study, we reviewed the bone maps obtained with this method on 15 clinical datasets acquired with a PET/CT/MR trimodality setup. The CT scans acquired for PET attenuation-correction purposes were used as reference for the evaluation. Quantitative measurements based on the Jaccard distance between ZTE and CT bone masks and qualitative scoring of anatomic accuracy by an experienced radiologist and nuclear medicine physician were performed. Results: The average Jaccard distance between ZTE and CT bone masks evaluated over the entire head was 52% ± 6% (range, 38%–63%). When only the cranium was considered, the distance was 39% ± 4% (range, 32%–49%). These results surpass previously reported attempts with dual-echo ultrashort echo time, for which the Jaccard distance was in the 47%–79% range (parietal and nasal regions, respectively). Anatomically, the calvaria is consistently well segmented, with frequent but isolated voxel misclassifications. Air cavity walls and bone/fluid interfaces with high anatomic detail, such as the inner ear, remain a challenge. Conclusion: This is the first, to our knowledge, clinical evaluation of skull bone identification based on a ZTE sequence. The results suggest that proton density–weighted ZTE imaging is an efficient means of obtaining high-resolution maps of bone tissue with sufficient anatomic accuracy for, for example, PET attenuation correction.

Characterization of tumor heterogeneity using dynamic contrast enhanced CT and FDG-PET in non-small cell lung cancer

PURPOSE Dynamic contrast-enhanced CT (DCE-CT) quantifies vasculature properties of tumors, whereas static FDG-PET/CT defines metabolic activity. Both imaging modalities are capable of showing intra-tumor heterogeneity. We investigated differences in vasculature properties within primary non-small cell lung cancer (NSCLC) tumors measured by DCE-CT and metabolic activity from FDG-PET/CT. METHODS Thirty three NSCLC patients were analyzed prior to treatment. FDG-PET/CT and DCE-CT were co-registered. The tumor was delineated and metabolic activity was segmented on the FDG-PET/CT in two regions: low (<50% maximum SUV) and high (≥50% maximum SUV) metabolic uptake. Blood flow, blood volume and permeability were calculated using a maximum slope, deconvolution algorithm and a Patlak model. Correlations were assessed between perfusion parameters for the regions of interest. RESULTS DCE-CT provided additional information on vasculature and tumor heterogeneity that was not correlated to metabolic tumor activity. There was no significant difference between low and high metabolic active regions for any of the DCE-CT parameters. Furthermore, only moderate correlations between maximum SUV and DCE-CT parameters were observed. CONCLUSIONS No direct correlation was observed between FDG-uptake and parameters extracted from DCE-CT. DCE-CT may provide complementary information to the characterization of primary NSCLC tumors over FDG-PET/CT imaging.

Clinical Evaluation of PET Image Quality as a Function of Acquisition Time in a New TOF-PET/MRI Compared to TOF-PET/CT—Initial Results

PurposeThe purpose of this study was to compare only the performance of the PET component between a TOF-PET/CT (henceforth noted as PET/CT) scanner and an integrated TOF-PET/MRI (henceforth noted as PET/MRI) scanner concerning image quality parameters and quantification in terms of standardized uptake value (SUV) as a function of acquisition time (a surrogate of dose). The CT and MR image quality were not assessed, and that is beyond the scope of this study.ProceduresFive brain and five whole-body patients were included in the study. The PET/CT scan was used as a reference and the PET/MRI acquisition time was consecutively adjusted, taking into account the decay between the scans in order to expose both systems to the same amount of the emitted signal. The acquisition times were then retrospectively reduced to assess the performance of the PET/MRI for lower count rates. Image quality, image sharpness, artifacts, and noise were evaluated. SUV measurements were taken in the liver and in the white matter to compare quantification.ResultsQuantitative evaluation showed strong correlation between PET/CT and PET/MRI brain SUVs. Liver correlation was good, however, with lower uptake estimation in PET/MRI, partially justified by bio-redistribution. The clinical evaluation showed that PET/MRI offers higher image quality and sharpness with lower levels of noise and artifacts compared to PET/CT with reduced acquisition times for whole-body scans while for brain scans there is no significant difference.ConclusionThe TOF-PET/MRI showed higher image quality compared to TOF-PET/CT as tested with reduced imaging times. However, this result accounts mainly for body imaging, while no significant differences were found in brain imaging.

Cluster-based segmentation of dual-echo ultra-short echo time images for PET/MR bone localization

BackgroundMagnetic resonance (MR)-based attenuation correction is a critical component of integrated positron emission tomography (PET)/MR scanners. It is generally achieved by segmenting MR images into tissue classes with known attenuation properties (e.g., bone, fat, soft tissue, lung, air). Ultra-short echo time (UTE) have been proposed in the past to locate bone tissue. In this study, tri-modality computed tomography data was used to develop an improved algorithm for the localization of bone in the head and neck.MethodsTwenty patients were scanned using a tri-modality setup. A UTE acquisition with 22-cm transaxial and 24-cm axial field of view was acquired, with a resolution of 1.5 × 1.5 × 2.0 mm3. The sequence consisted of two echoes (30 μs, 1.7 ms) with a flip angle of 10° and 125-kHz bandwidth. The CT images of all patients were classified by thresholding and used to compute maps of the posterior probability of each tissue class, given a pair of UTE echo values. The Jaccard distance was used to compare with CT the bone masks obtained when using this information to segment the UTE datasets.ResultsThe results show the desired bony structures as a cluster pattern in the space of dual-echo measurements. The clusters obtained for the tissue classes are strongly overlapped, indicating that the MR data will not, regardless of the chosen space partition, be able to completely differentiate the bony and soft structures.The classification obtained by maximizing the posterior probability compared well to previously published methods, providing a more intuitive and robust choice of the final classification threshold. The distance between MR- and CT-based bone masks was 59% on average (0% being a perfect match), compared to 76% and 69% for two previously published methods.ConclusionsThe study of tri-modality datasets shows that improved bone tissue classification can be achieved by estimating maps of the posterior probability of voxels belonging to a particular tissue class, given a measured pair of UTE echoes.

Effect of Time-of-Flight Information on PET/MR Reconstruction Artifacts: Comparison of Free-breathing versus Breath-hold MR-based Attenuation Correction.

Purpose To evaluate the magnitude and anatomic extent of the artifacts introduced on positron emission tomographic (PET)/magnetic resonance (MR) images by respiratory state mismatch in the attenuation map. Materials and Methods The method was tested on 14 patients referred for an oncologic examination who underwent PET/MR imaging. The acquisition included standard PET and MR series for each patient, and an additional attenuation correction series was acquired by using breath hold. PET data were reconstructed with and without time-of-flight (TOF) information, first by using the standard free-breathing attenuation map and then again by using the additional breath-hold map. Two-tailed paired t testing and linear regression with 0 intercept was performed on TOF versus non-TOF and free-breathing versus breath-hold data for all detected lesions. Results Fluorodeoxyglucose-avid lesions were found in eight of the 14 patients included in the study. The uptake differences (maximum standardized uptake values) between PET reconstructions with free-breathing versus breath-hold attenuation ranged, for non-TOF reconstructions, from -18% to 26%. The corresponding TOF reconstructions yielded differences from -15% to 18%. Conclusion TOF information was shown to reduce the artifacts caused at PET/MR by respiratory mismatch between emission and attenuation data.

The value of 18F-FDG-PET/CT imaging in oral cavity cancer patients following surgical reconstruction

Follow‐up of patients with oral cavity squamous cell carcinoma (OCSCC) after tumor resection and reconstruction with tissue transfer is challenging. We compared contrast‐enhanced computed tomography (ceCT), 18F‐fluorodeoxyglucose‐positron emission tomography combined with noncontrast enhanced CT (18F‐FDG‐PET/CT), and 18F‐FDG‐PET combined with ceCT (18F‐FDG‐PET/ceCT) to determine the accuracy for detection of residual/recurrent disease after flap reconstruction for OCSCC.

Clinical evaluation of a block sequential regularized expectation maximization reconstruction algorithm in 18F-FDG PET/CT studies

Purpose To investigate the clinical performance of a block sequential regularized expectation maximization (BSREM) penalized likelihood reconstruction algorithm in oncologic PET/computed tomography (CT) studies. Methods A total of 410 reconstructions of 41 fluorine-18 fluorodeoxyglucose-PET/CT studies of 41 patients with a total of 2010 lesions were analyzed by two experienced nuclear medicine physicians. Images were reconstructed with BSREM (with four different &bgr; values) or ordered subset expectation maximization (OSEM) algorithm with/without time-of-flight (TOF/non-TOF) corrections. OSEM reconstruction postfiltering was 4.0 mm full-width at half-maximum; BSREM did not use postfiltering. Evaluation of general image quality was performed with a five-point scale using maximum intensity projections. Artifacts (category 1), image sharpness (category 2), noise (category 3), and lesion detectability (category 4) were analyzed using a four-point scale. Size and maximum standardized uptake value (SUVmax) of lesions were measured by a third reader not involved in the image evaluation. Results BSREM-TOF reconstructions showed the best results in all categories, independent of different body compartments. In all categories, BSREM non-TOF reconstructions were significantly better than OSEM non-TOF reconstructions (P<0.001). In almost all categories, BSREM non-TOF reconstruction was comparable to or better than the OSEM-TOF algorithm (P<0.001 for general image quality, image sharpness, noise, and P=1.0 for artifact). Only in lesion detectability was OSEM-TOF significantly better than BSREM non-TOF (P<0.001). Both BSREM-TOF and BSREM non-TOF showed a decreasing SUVmax with increasing &bgr; values (P<0.001) and TOF reconstructions showed a significantly higher SUVmax than non-TOF reconstructions (P<0.001). Conclusion The BSREM reconstruction algorithm showed a relevant improvement compared with OSEM reconstruction in PET/CT studies in all evaluated categories. BSREM might be used in clinical routine in conjunction with TOF to achieve better/higher image quality and lesion detectability or in PET/CT-systems without TOF-capability for enhancement of overall image quality as well.

Benefit of 18F-fluorocholine PET imaging in parathyroid surgery

ObjectivesTo assess the additional diagnostic value of 18F-fluorocholine PET imaging in preoperative localization of pathologic parathyroid glands in clinically manifest hyperparathyroidism in case of negative or conflicting ultrasound and scintigraphy results.MethodsA retrospective, single-institution study of 26 patients diagnosed with hyperparathyroidism. In cases where ultrasound and scintigraphy failed to detect the location of an adenoma in order to allow a focused surgical approach, an additional 18F-fluorocholine PET scan was performed and its results were compared with the intraoperative findings.ResultsA total of 26 patients underwent 18F-fluorocholine PET/CT (n = 11) or PET/MRI (n = 15). Adenomas were detected in 25 patients (96.2%). All patients underwent surgery, and the location predicted by PET hybrid imaging was confirmed intraoperatively by frozen section and adequate parathyroid hormone drop after removal. None of the patients needed revision surgery during follow-up.ConclusionsThese results demonstrate that 18F-fluorocholine PET imaging is a highly accurate method to detect parathyroid adenomas even in case of previous localization failure by other imaging examinations.Key Points• With18F-fluorocholine PET imaging, parathyroid adenomas could be detected in 96.2%.• 18F-fluorocholine imaging is a highly accurate method to detect parathyroid adenomas.• We encourage its use, where ultrasound fails to detect an adenoma.