Philip Aschoff

MRI-Based Attenuation Correction for PET/MRI: A Novel Approach Combining Pattern Recognition and Atlas Registration

For quantitative PET information, correction of tissue photon attenuation is mandatory. Generally in conventional PET, the attenuation map is obtained from a transmission scan, which uses a rotating radionuclide source, or from the CT scan in a combined PET/CT scanner. In the case of PET/MRI scanners currently under development, insufficient space for the rotating source exists; the attenuation map can be calculated from the MR image instead. This task is challenging because MR intensities correlate with proton densities and tissue-relaxation properties, rather than with attenuation-related mass density. Methods: We used a combination of local pattern recognition and atlas registration, which captures global variation of anatomy, to predict pseudo-CT images from a given MR image. These pseudo-CT images were then used for attenuation correction, as the process would be performed in a PET/CT scanner. Results: For human brain scans, we show on a database of 17 MR/CT image pairs that our method reliably enables estimation of a pseudo-CT image from the MR image alone. On additional datasets of MRI/PET/CT triplets of human brain scans, we compare MRI-based attenuation correction with CT-based correction. Our approach enables PET quantification with a mean error of 3.2% for predefined regions of interest, which we found to be clinically not significant. However, our method is not specific to brain imaging, and we show promising initial results on 1 whole-body animal dataset. Conclusion: This method allows reliable MRI-based attenuation correction for human brain scans. Further work is necessary to validate the method for whole-body imaging.

MRI-Based Attenuation Correction for Whole-Body PET/MRI: Quantitative Evaluation of Segmentation- and Atlas-Based Methods

PET/MRI is an emerging dual-modality imaging technology that requires new approaches to PET attenuation correction (AC). We assessed 2 algorithms for whole-body MRI-based AC (MRAC): a basic MR image segmentation algorithm and a method based on atlas registration and pattern recognition (AT&PR). Methods: Eleven patients each underwent a whole-body PET/CT study and a separate multibed whole-body MRI study. The MR image segmentation algorithm uses a combination of image thresholds, Dixon fat–water segmentation, and component analysis to detect the lungs. MR images are segmented into 5 tissue classes (not including bone), and each class is assigned a default linear attenuation value. The AT&PR algorithm uses a database of previously aligned pairs of MRI/CT image volumes. For each patient, these pairs are registered to the patient MRI volume, and machine-learning techniques are used to predict attenuation values on a continuous scale. MRAC methods are compared via the quantitative analysis of AC PET images using volumes of interest in normal organs and on lesions. We assume the PET/CT values after CT-based AC to be the reference standard. Results: In regions of normal physiologic uptake, the average error of the mean standardized uptake value was 14.1% ± 10.2% and 7.7% ± 8.4% for the segmentation and the AT&PR methods, respectively. Lesion-based errors were 7.5% ± 7.9% for the segmentation method and 5.7% ± 4.7% for the AT&PR method. Conclusion: The MRAC method using AT&PR provided better overall PET quantification accuracy than the basic MR image segmentation approach. This better quantification was due to the significantly reduced volume of errors made regarding volumes of interest within or near bones and the slightly reduced volume of errors made regarding areas outside the lungs.

Histological verification of 11C-choline-positron emission/computed tomography-positive lymph nodes in patients with biochemical failure after treatment for localized prostate cancer.

To evaluate the potential of 11C‐choline‐positron emission tomography (PET)/computed tomography (CT) for planning surgery in patients with prostate cancer and prostate‐specific antigen (PSA) relapse after treatment with curative intent.

Computed tomography, positron emission tomography, positron emission tomography/computed tomography, and magnetic resonance imaging for staging of limited pleural mesothelioma: initial results.

Objective:To evaluate and compare the role of computed tomography (CT), positron emission tomography (PET), PET/CT, and magnetic resonance imaging (MRI) in the correct staging of patients with limited malignant pleural mesothelioma (MPM). Materials and Methods:Fifty-four patients with an epithelial MPM (34 men and 20 women) were included in this study. Patients were referred to our department for staging in a predicted resectable state (stage II/III). Within 3 days, PET/CT and MRI was performed in all patients. Images were evaluated by 3 specialists in the field of PET/CT and MRI. The subexaminations of PET/CT, PET, and CT were independently evaluated with respect to tumor stage. Subexaminations were compared with each other, with MRI and PET/CT. N-stage was verified by mediastinoscopy. Afterward, consensus reading was performed. In 52 patients, surgery served as gold standard. In 2 patients, follow-up control served as gold standard as an inoperable situation with distant metastases was found. Additionally, interobserver variability (&kgr; value) was calculated. Results:In stage II, accuracy was 0.77 (CT), 0.86 (PET), 0.8 (MRI), 1.0 (PET/CT), and in stage III 0.75, 0.83, 0.9, 1.0. PET/CT was significantly more accurate (P < 0.05) in stages II and III compared with all other techniques. CT and MRI were not able to detect distant metastases in 2 patients, which changed therapy (operable vs. inoperable). Interobserver variability was 0.7, 0.9, 0.8, 1.0 in stage II and 0.9, 0.9, 0.9, 1.0 in stage III. Conclusion:PET/CT makes it possible to stage patients with limited MPM with high accuracy and low interobserver variability.

Positron Emission Tomography/Computed Tomography and Whole-Body Magnetic Resonance Imaging in Staging of Advanced Nonsmall Cell Lung Cancer-Initial Results

Objective:To evaluate and compare positron emission tomography/computed tomography (PET/CT) with whole-body magnetic resonance imaging (wbMRI) in the correct staging of patients with advanced nonsmall cell lung cancer (NSCLC). Materials and Methods:Fifty-two patients with an NSCLC stage IIIa or IIIb (36 males and 16 females) were included in this study. Patients were referred to our department for restaging. Within 1 week PET/CT and wbMRI were performed in all patients. Images were examined independently by 2 experienced physicians from the Department of Nuclear Medicine and Radiology. Afterward, consensus reading was performed. In 22 patients, surgery served as gold standard, whereas in 30 patients, follow-up controls (after 2 months) were performed. Results:The use of wbMRI correctly T-staged all patients. Especially volume interpolated breathhold examination sequence correctly T-staged all tumors. PET/CT did not correctly stage chest wall infiltration in 4 cases [sensitivity 92.3% (P < 0.05 to wbMRI)/specificity 100%], verified by surgery. PET/CT correctly N-staged 51 patients (sensitivity 96.1%/specificity 100%). WbMRI showed a significant tendency to understage N-status [sensitivity 88.5% (P < 0.05)/specificity 96.1%]. Different N-status by PET/CT changed operability in 4 patients. In 2 patients, distant metastases were detected by both techniques. Conclusion:In the correct staging of advanced NSCLC, PET/CT has advantages in N-staging. This is of high relevance for therapy planning. WbMRI especially using volume interpolated breathhold examination sequences, has certain advantages in T-staging.

Kostenüberlegungen zur ganzkörper-MRT und PET-CT im rahmen des onkologischen stagings

PURPOSE The aim of this study was to evaluate and discuss economic aspects of whole-body MRI and PET/CT in oncologic staging. Considerations from the perspective of the health care system, the radiologist, and the patients are presented. MATERIALS AND METHODS Costs of both whole-body techniques are compared with the conventional radiologic diagnostic recommendations of the AWFM (Arbeitsgemeinschaft Wissenschaftlich Medizinischer Fachgesellschaften) in oncologic staging of the five most frequent tumor entities. Temporal and monetary aspects are calculated. Invasive, endoscopic, and endosonographic techniques are regarded as essential and cannot be replaced by other techniques. Thus only the minimal potential for cost reduction is quantified. RESULTS In the German system there is no cipher to correctly balance whole-body MRI and PET/CT. Using the frequently applied ciphers 5700-5730 and 5378, 5489 (factor 1.0) total costs were 440.45 euros, and adding the cipher for additional series 545.37 euros (60 min examination time) for whole-body MRI and 774.74 euros (879.66 euros) (60/90 min examination time) for whole-body PET/CT. Using the common factor 1.8 costs were 981.66 and 1583.38 euros. On the basis of a simple full cost analysis total costs of whole-body PET/CT were higher than of whole-body MRI by a factor of about 2.0 (about 1123 vs 575 euros). There were substantial monetary and temporal differences between tumor entities. In extended bronchial carcinoma 375.32 euros and 55 min can be saved using whole-body MRI in comparison to conventional recommended techniques and using whole-body PET/CT 88.14 euros and 45 min. In tumor entities of lower stages with thus less essential radiologic diagnostics the potential for cost reduction is substantially lower. CONCLUSION Whole-body imaging techniques make it possible to reduce the number of necessary separate radiologic examinations and thus time in oncologic staging. A substantial reduction of health care costs seems to be possible in many tumor entities but differences between different tumor entities are decisive.

PET/CT with 18F-FLT: Does It Improve the Therapeutic Management of Metastatic Germ Cell Tumors?

Because 18F-FDG PET alone has only limited value in metastatic germ cell tumors (GCTs), we investigated the addition of 3′-deoxy-3′-18F-fluorothymidine (FLT) to 18F-FDG for early response monitoring and prediction of the histology of residual tumor masses in patients with metastatic GCT. Methods: Eleven patients with metastatic GCT were examined with both 18F-FDG PET/CT and 18F-FLT PET/CT before chemotherapy, after the first cycle of chemotherapy (early response), and 3 wk after completion of chemotherapy. In 1 patient with negative 18F-FLT PET/CT results before chemotherapy, no further 18F-FLT scanning was performed. PET images were analyzed visually and, using standardized uptake values (SUVs), semiquantitatively. The results were compared with the findings of CT and tumor marker levels and validated by histopathologic examination of resected residual masses, including Ki-67 immunostaining (7 patients), or by clinicoradiologic follow-up for at least 6 mo (4 patients). A responder was defined as a patient showing the presence of necrosis, a complete remission, or a marker-negative partial remission within a minimum progression-free interval of 6 mo. Early treatment response was judged according to the criteria of the European Organization for Research and Treatment of Cancer. Results: Before chemotherapy, reference lesions showed increased 18F-FDG uptake (mean SUV, 8.8; range, 2.9–15.0) in all patients and moderate 18F-FLT uptake (mean SUV, 3.7; range, 1.7–9.7) in 10 of 11 patients. After 1 cycle of chemotherapy, mean SUV decreased in responders and nonresponders by 64% and 60%, respectively, for 18F-FDG (P = 0.8) and by 58% and 48%, respectively, for 18F-FLT (P = 0.5). After the end of chemotherapy, mean SUV decreased in responders and nonresponders by 85% and 73%, respectively, for 18F-FDG (P = 0.1) and by 68% and 65%, respectively, for 18F-FLT (P = 0.8). The results of early and final PET were inconsistent in 6 of 11 patients for 18F-FDG and in 4 of 10 patients for 18F-FLT. Both patients with teratoma had false-negative results on both 18F-FDG and 18F-FLT. The sensitivity, specificity, positive predictive value, and negative predictive value for detection of viable tumor after 1 cycle of chemotherapy were 60%, 33%, 43%, and 50%, respectively, for 18F-FDG and 60%, 80%, 75%, and 67%, respectively, for 18F-FLT PET/CT. The respective values after the end of chemotherapy were 20%, 100%, 100%, and 60% for 18F-FDG and 0%, 100%, 0%, and 50% for 18F-FLT PET/CT. Conclusion: PET-negative residual masses after chemotherapy of metastatic GCT still require resection, since the low negative predictive value of 18F-FDG PET for viable tumor cannot be improved by application of 18F-FLT.

[Cost considerations for whole-body MRI and PET/CT as part of oncologic staging].

PURPOSE The aim of this study was to evaluate and discuss economic aspects of whole-body MRI and PET/CT in oncologic staging. Considerations from the perspective of the health care system, the radiologist, and the patients are presented. MATERIALS AND METHODS Costs of both whole-body techniques are compared with the conventional radiologic diagnostic recommendations of the AWFM (Arbeitsgemeinschaft Wissenschaftlich Medizinischer Fachgesellschaften) in oncologic staging of the five most frequent tumor entities. Temporal and monetary aspects are calculated. Invasive, endoscopic, and endosonographic techniques are regarded as essential and cannot be replaced by other techniques. Thus only the minimal potential for cost reduction is quantified. RESULTS In the German system there is no cipher to correctly balance whole-body MRI and PET/CT. Using the frequently applied ciphers 5700-5730 and 5378, 5489 (factor 1.0) total costs were 440.45 euros, and adding the cipher for additional series 545.37 euros (60 min examination time) for whole-body MRI and 774.74 euros (879.66 euros) (60/90 min examination time) for whole-body PET/CT. Using the common factor 1.8 costs were 981.66 and 1583.38 euros. On the basis of a simple full cost analysis total costs of whole-body PET/CT were higher than of whole-body MRI by a factor of about 2.0 (about 1123 vs 575 euros). There were substantial monetary and temporal differences between tumor entities. In extended bronchial carcinoma 375.32 euros and 55 min can be saved using whole-body MRI in comparison to conventional recommended techniques and using whole-body PET/CT 88.14 euros and 45 min. In tumor entities of lower stages with thus less essential radiologic diagnostics the potential for cost reduction is substantially lower. CONCLUSION Whole-body imaging techniques make it possible to reduce the number of necessary separate radiologic examinations and thus time in oncologic staging. A substantial reduction of health care costs seems to be possible in many tumor entities but differences between different tumor entities are decisive.

Single and dual energy attenuation correction in PET/CT in the presence of iodine based contrast agents

In present positron emission tomography (PET)/computed tomography (CT) scanners, PET attenuation correction is performed by relying on the information given by a single CT scan. The scaling of the linear attenuation coefficients from CT x-ray energy to PET 511 keV gamma energy is prone to errors especially in the presence of CT contrast agents. Attenuation correction based upon two CT scans at different energies but performed at the same time and patient position should reduce such errors and therefore improve the accuracy of the reconstructed PET images at the cost of introduced additional noise. Such CT scans could be provided by future PET/CT scanners that have either dual source CT or energy sensitive CT. Three different dual energy scaling methods for attenuation correction are introduced and assessed by measurements with a modified NEMA 1994 phantom with different CT contrast agent concentrations. The scaling is achieved by differentiating between (1) Compton and photoelectric effect, (2) atomic number and density, or (3) water-bone and water-iodine scaling schemes. The scaling method (3) is called hybrid dual energy computed tomography attenuation correction (hybrid DECTAC). All three dual energy scaling methods lead to a reduction of contrast agent artifacts with respect to single energy scaling. The hybrid DECTAC method resulted in PET images with the weakest artifacts. Both, the hybrid DECTAC and Compton/photoelectric effect scaling resulted also in images with the lowest PET background variability. Atomic number/density scaling and Compton/photoelectric effect scaling had problems to correctly scale water, hybrid DECTAC scaling and single energy scaling to correctly scale Teflon. Atomic number/density scaling and hybrid DECTAC could be generalized to reduce these problems.

18F-FDG PET/CT for the diagnosis of sarcoidosis in a patient with bilateral inflammatory involvement of the parotid and lacrimal glands (panda sign) and bilateral hilar and mediastinal lymphadenopathy (lambda sign)

A 43-year-old man was hospitalized for Sjogren’s syndrome whichwas suspected to be of paraneoplastic aetiology. F-FDG PET/CT revealed grossly enlarged lacrimal (a) and parotid glands (b) with significantly increased FDG uptake while the submandibular glands (c) were only minimally enlarged with faint FDG uptake. Furthermore, hypermetabolic intrathoracic lymphadenopathy was detected with a typical lambda appearance (h) involving right paratracheal (e), left paratracheal and aortopulmonary window lymph nodes (f) of the mediastinum and bihilar lymph nodes (g). The imaging findings were suggestive of sarcoidosis and bronchoscopic evaluation with biopsy was proposed. Histopathology of a transbronchial biopsy specimen confirmed the diagnosis of sarcoidosis. In 1990 Sulavik et al. first described the panda sign, i.e. normal accumulation of the radionuclide (Ga-citrate) in the nasopharynx combined with increased symmetric accumulation in the parotid and lacrimal glands, giving the impression of the mottled colouring of the giant panda [1–3]. Intrathoracic lymphadenopathy typically manifests as bilateral hilar adenopathy with predominantly right paratracheal adenopathy, referred to as the lambda appearance [4]. With F-FDG PET/CT the appearance of hypermetabolic mediastinal and bihilar lymphadenopathy in sarcoidosis is comparable to the lambda sign of Ga-citrate scintigraphy [5]. Bilateral inflammatory involvement of the parotid and lacrimal glands is also present, resulting in an increased FDG uptake, although the typical panda appearance is partially obscured due to the physiologic FDG avidity of the brain (d). The finding of typical lambda and panda patterns supports the diagnosis of sarcoidosis and reinforces the indication to perform an appropriate biopsy.