Multislice 2D spin echo imaging using adapted readout gradients for compensation of BO inhomogeneities and gradient nonlinearities

Purpose/Introduction: In whole-body MR/PET, the limitation of an MR-based axial field-of-view (FoV) due to B0 inhomogeneities and gradient nonlinearities [1,2] may cause a truncation of the MR data and has therefore potential impact on the attenuation correction [3]. Furthermore, a large FoV may also be relevant in MR-based interventional applications, e.g. radiotherapy and biopsy. Recently, the feasibility of an axial extension of the MR-based FoV using a gradient field that compensates the B0 inhomogeneities has been shown [4]. However, the optimal gradient strength is space-dependent and therefore does not achieve optimal distortion reduction in all slices. In this work we developed a multislice 2D spin-echo-based sequence that calculates and adapts the readout (RO) gradient for each slice position automatically and offers an extended FoV in multiple slices during one single scan. Subjects and Methods: Distortions due to B0 inhomogeneities and gradient nonlinearities in 2DFT SE frequency encoding can be reduced by using an adapted space-dependent readout gradient: GRO (x,y,z) = - δB0(x,y,z) / c(x,y,z), where δB0 is the B0 inhomogeneity and c is the relative error in the gradient field. Field deviations in the main magnetic field and the gradient field were quantified as described in [4]. A multislice 2D spin-echo-based sequence was developed to calculate and adapt the optimal RO gradient strength and polarity for each slice position automatically (Fig. 1)

MR-based field-of-view extension in MR/PET: B0 homogenization using gradient enhancement (HUGE)

In whole-body MR/PET, the human attenuation correction can be based on the MR data. However, an MR-based field-of-view (FoV) is limited due to physical restrictions such as B0 inhomogeneities and gradient nonlinearities. Therefore, for large patients, the MR image and the attenuation map might be truncated and the attenuation correction might be biased. The aim of this work is to explore extending the MR FoV through B0 homogenization using gradient enhancement in which an optimal readout gradient field is determined to locally compensate B0 inhomogeneities and gradient nonlinearities. A spin-echo-based sequence was developed that computes an optimal gradient for certain regions of interest, for example, the patient's arms. A significant distortion reduction was achieved outside the normal MR-based FoV. This FoV extension was achieved without any hardware modifications. In-plane distortions in a transaxially extended FoV of up to 600 mm were analyzed in phantom studies. In vivo measurements of the patient's arms lying outside the normal specified FoV were compared with and without the use of B0 homogenization using gradient enhancement. In summary, we designed a sequence that provides data for reducing the image distortions due to B0 inhomogeneities and gradient nonlinearities and used the data to extend the MR FoV

MR-basierte Field-of-View-Vergrößerung in der Ganzkörper-MR/PET

In der Ganzkörper-MR/PET können die für die Schwächungskorrektur notwendigen gewebespezifischen Schwächungskoeffizienten aus den MR-Daten bestimmt werden. Dazu ist eine MR-Aufnahme nötig, die die vollständige Anatomie des Patienten in transaxialer Richtung abbildet. Bekannte physikalische Limitationen der Gradientenlinearität und der B0-Feldhomogenität führen zu einer Begrenzung des verzeichnungsfreien Field-of-Views (FoV). Das somit üblicherweise auf 50cm begrenzte FoV deckt insbesondere bei großen Patienten nicht die gesamte Anatomie in Strahlrichtung der Photonenemission ab. Die Arme des Patienten erscheinen im MR-Bild entweder abgeschnitten oder stark verzeichnet. Diese Trunkierung im MR-Bild führt zu einer unvollständigen Schwächungskoeffizienten-Karte und somit zu einer Beeinflussung der Schwächungskorrektur in der PET-Rekonstruktion [1]. Diese Beeinflussung kann durch eine Abschätzung der fehlenden Teilstücke in der PETSchwächungskoeffizienten-Karte mithilfe eines maximum-likelihood a-posteri Algorithmus reduziert werden [2]. Wir werden in unserer Arbeit zeigen, dass jedoch MR-seitig die Ursache der Verzeichnungen kompensiert werden kann und eine starke Verzeichnungsreduzierung außerhalb des normal spezifizierten FoV bis zu 60cm im Durchmesser möglich ist. Nachverarbeitungsalgorithmen zur Verzeichnungskorrektur sind viele bekannt [3,4]. Jedoch treten bei Offcenter-Positionen von bis zu x=±300mm derart starke B0-Feldinhomogenitäten und Gradienten-Nichtlinearitäten auf, dass mehrere Pixel zusammenstauchen können und somit Ortsinformationen für eine erfolgreiche Nachverarbeitung verloren gehen. Die von uns vorgestellte Methode ermittelt, basierend auf B0- und Gradientenfeld-Messungen, für eine zu optimierende Position im Bildraum einen optimalen Gradienten in Ausleserichtung, der zu einer Kompensation der durch B0-Feldinhomogenitäten und durch Nichtlinearitäten des Gradienten verursachten Verzeichnung führt [5]. Diese Methode ermöglicht eine transaxiale FoV-Erweiterung auf bis zu 60cm im Durchmesser und verspricht somit eine mögliche Verbesserung der MR-basierten PETSchwächungskorrektur

MR-based FoV Extension in Whole-Body MR/PET Using Continuous Table Move

In whole-body MR/PET attenuation correction, an MR-based FoV extension is of emerging interest. However, gradient nonlinearities and B0 inhomogeneities often hamper an accurate spatial encoding at the edges of large FoVs. Recently, we proposed a method to axially extend the FoV by determining an optimal readout gradient field which locally compensates B0 inhomogeneities and gradient nonlinearities. In this work a combination of the mentioned axial FoV extension and Continuous Table Movement is presented. In experiments on volunteers a significant distortion reduction has been achieved at off-center positions of up to 300 mm off from the iso-center

MR-based FoV Extension of Human Attenuation Correction in Whole-Body MR/PET Hybrid Imaging

In whole-body MR/PET, the human tissue attenuation correction (AC) of PET data can be based on the MR data. However, the MR FoV is limited due to B0inhomogeneities and gradient nonlinearities. Therefore, the human AC map may be truncated and thus the PET reconstruction may be biased. In this work we explored extending the MR-based AC map using a purely MR-based FoV extension. We applied this method to whole-body MR/PET examinations on patients and evaluated the impact on the PET reconstruction. The reported bias was reduced and the PET reconstruction was in good agreement with a PET/CT reference measurement

MR-Based Field-Of-View Extension: Compensation of Field Imperfections

Recently, the potential impact of a limited MR-based field-of-view (FoV) in whole-body MR/PET attenuation correction has been shown. In MR/PET the tissue attenuation map can be calculated from the MR images. However, the FoV restriction may cause a truncation of the MR data for bigger patients and therefore can bias the PET data reconstruction. In this work, we will show exemplarily for 2DFT spin-echo sequences a method that offers an extended FoV in the transversal plane of up to 600mm using a gradient field that compensates B0 inhomogeneities

Effect of intravenous fructose on the P-31 MR spectrum of the liver: dose response in healthy volunteers

Dynamic phosphorus-31 magnetic resonance (MR) spectroscopy of the liver after intravenous administration of fructose has been suggested as a test of liver function. To establish dose-response curves of the phosphorus metabolites in the normal human liver, each of four healthy volunteers was given two to four different fructose doses on separate days: 62.5, 125, 250, 375, or 500 mg per kilogram of body weight. P-31 MR spectra of the liver were acquired with a 2-T whole-body magnetic, both before and after fructose administration, at 2.5-minute intervals over at least 30 minutes. The fructose load caused a significant, linearly dose-dependent accumulation of phosphomonoesters (r = .72, P less than .01) and a decrease in inorganic phosphate (r = .78, P less than .005) and adenosine triphosphate (r = .73, P less than .01). On the basis of these experiments, dynamic P-31 MR spectroscopy seems promising in the assessment of liver function

Field of view extension and truncation correction for MR-based human attenuation correction in simultaneous MR/PET imaging

Purpose: In quantitative PET imaging, it is critical to accurately measure and compensate for the attenuation of the photons absorbed in the tissue. While in PET/CT the linear attenuation coefficients can be easily determined from a low-dose CT-based transmission scan, in whole-body MR/PET the computation of the linear attenuation coefficients is based on the MR data. However, a constraint of the MR-based attenuation correction (AC) is the MR-inherent field-of-view (FoV) limitation due to static magnetic field (B0) inhomogeneities and gradient nonlinearities. Therefore, the MR-based human AC map may be truncated or geometrically distorted toward the edges of the FoV and, consequently, the PET reconstruction with MR-based AC may be biased. This is especially of impact laterally where the patient arms rest beside the body and are not fully considered. Methods: A method is proposed to extend the MR FoV by determining an optimal readout gradient field which locally compensates B0 inhomogeneities and gradient nonlinearities. This technique was used to reduce truncation in AC maps of 12 patients, and the impact on the PET quantification was analyzed and compared to truncated data without applying the FoV extension and additionally to an established approach of PET-based FoV extension. Results: The truncation artifacts in the MR-based AC maps were successfully reduced in all patients, and the mean body volume was thereby increased by 5.4%. In some cases large patient-dependent changes in SUV of up to 30% were observed in individual lesions when compared to the standard truncated attenuation map. Conclusions: The proposed technique successfully extends the MR FoV in MR-based attenuation correction and shows an improvement of PET quantification in whole-body MR/PET hybrid imaging. In comparison to the PET-based completion of the truncated body contour, the proposed method is also applicable to specialized PET tracers with little uptake in the arms and might reduce the computation time by obviating the need for iterative calculations of the PET emission data beyond those required for reconstructing images

Welches Verständnis von Ökonomie haben Medizinstudierende?

Introduction: Economic topics appear in the medical studies curriculum at different times. Despite socio-political relevance, there is hardly any information about the degree of understanding that medical students have of "economics in medicine". The present study addresses the questions: What understanding of "economics in medicine" do medical students have before the start of the Practical Year? To what extent is economic teaching content understood as "economization" from outside the profession?Method: Magdeburg medical students in the 5th year of study, who participated in preparatory seminars for the Practical Year (PY) in 2014 and 2015 (60 participants each), assessed the relevance of various seminar topics four months prior to the start of the semester. On the basis of a three-stage qualitative-reconstructive partial evaluation, students' economic understanding is explored through secondary analysis: deductive derivation of the analysis units; integrative basic method ("segmentation", "micro-linguistic detailed analysis", "central theme"); development of a theoretical model by placing the central themes in context following Grounded Theory. Results: Based on the theory, 19 free-text answers with economic reference were identified from the total of all free-text answers. Each answer was assigned to at least one of a total of six themes of the students' understanding of economics: de-professionalizing economization, deciding and working economically, ambivalent requirements for efficiency and equity, the doctor as an entrepreneur, economics as relevant learning content, PY as a conflict-laden setting for economized working and learning. The theoretical model contains social, praxeological and professional references, which can themselves be ambivalent and conflicting.Conclusion: Despite their critical attitude, the surveyed medical students are neither hostile to economics nor do they regard economics in medicine as a taboo subject. Economic learning content is recognized as relevant. Educational formats that tackle the tension between patient and system orientation in a problem-oriented manner can be a productive setting for economic reflection.Einleitung: Ökonomische Themen sind im Medizinstudium zu verschiedenen Zeitpunkten curricular verankert. Trotz gesellschaftspolitischer Relevanz bestehen kaum Kenntnisse darüber, welches Verständnis Medizinstudierende von 'Ökonomie in der Medizin' haben. Die vorliegende Untersuchung geht den Fragen nach: Welches Verständnis von 'Ökonomie in der Medizin' haben Medizinstudierende vor dem Beginn des Praktischen Jahres? Inwiefern werden ökonomische Lerninhalte als professionsfremde "Ökonomisierung" verstanden?Methode: Magdeburger Medizinstudierende im 5. Studienjahr, die 2014 und 2015 an Vorbereitungsseminaren für das Praktische Jahr (PJ) teilnahmen (je 60 Teilnehmende), schätzten vier Monate vor Seminarbeginn teilstandardisiert-fragebogengestützt die Relevanz verschiedener Seminarinhalte ein. Anhand einer dreistufigen qualitativ-rekonstruktiven Teilauswertung werden studentische Ökonomieverständnisse sekundäranalytisch exploriert: Deduktive Ableitung der Analyseeinheiten; Integratives Basisverfahren ("Segmentierung", "mikrosprachliche Feinanalyse", "zentrales Motiv"); Inbezugsetzung der zentralen Motive als theoretisches Modell nach Grounded Theory. Ergebnisse: 19 Freitextantworten mit ökonomischem Bezug wurden theoriegeleitet aus dem Gesamt aller Freitextantworten identifiziert. Jeder Antwort wurde mindestens eine von insgesamt sechs Motiven studentischen Ökonomieverständnisses zugeordnet: De-professionalisierende Ökonomisierung, Wirtschaftlich entscheiden und arbeiten, Ambivalente Effizienz- und Gerechtigkeitsanforderungen, ÄrztIn als UnternehmerIn, Ökonomie als relevanter Lerninhalt, PJ als konflikthaftes ökonomisiertes Arbeits- und Lernsetting. Das theoretische Modell beinhaltet gesellschaftliche, praxeologische und professionelle Bezüge, die in sich ambivalent und zueinander konflikthaft sein können.Schlussfolgerung: Die befragten Medizinstudierenden sind trotz kritischer Haltung weder pauschal ökonomiefeindlich noch tabuisieren sie Ökonomie in der Medizin. Ökonomische Lerninhalte sind als relevant anerkannt. Lehrformate, die problemorientiert das Spannungsfeld zwischen Patienten- und Systemorientierung aufgreifen, können ein produktives Setting für ökonomische Reflexion sein