Evaluation grundlegender Abhängigkeiten und Optimierung der kombinierten Diffusions- und Perfusions-Magnetresonanzbildgebung mittels Intravoxel-Incoherent-Motion in Abdomen und Unterschenkel

Intravoxel Incoherent Motion Imaging (IVIM imaging) allows the simultaneous measurement of tissue diffusion and perfusion properties in different organs. In this thesis, the originally introduced biexponential model as well as a less known triexponential model for the application in the liver and kidney will be discussed. Regardless of the used model, the obtained IVIM parameters are diffcult to reproduce, and therefore most studies are not comparable. For this reason, the aim of this work was to increase the reproducibility of the determined IVIM parameters. First of all, the magnetic field dependence of the IVIM parameters of both models was investigated using two magnetic field strengths. This showed a dependence of the diffusion coeffcient and the pseudo-diffusion coefficient of the biexponential model on the used magnetic fieled srength. In this work, the found dependency was attributed to a different signal composition of the investigated volume, since the T2-times of the signal components change with the used magnetic field strength. As a further step to increase the reproducibility of the triexponential IVIM parameters, the choice of measurement points for sampling the triexponential signal curve was optimized for application in the liver. Starting from an optimized distribution consisting of six b-values, another optimal b-value was searched for and added to the existing distribution until an optimized b-value distribution consisting of 100 b-values was achieved. This distribution was compared with distributions already used in the literature by simulations and in vivo measurements. In both cases the IVIM parameters could be determined more precisely by using the optimized b-value distribution. Due to the gained precision, it was possible for the first time to generate triexponential IVIM parameter maps of the liver using the in vivo data sets acquired by the optimized b-value distribution. For the special case of the liver, the diffusion coefficient is also difficult to reproduce, since, due to its position in relation to the heart, the pulsation of the heart leads to a reduced signal in the area of the left liver lobe. In a clinical study it was tested whether the so-called pulsation artifact can be reduced by a breathhold in expiration or inspiration. There was no difference in the expression of the artifact in either of the two breathing techniques used. In images taken during inspiration, a fat artifact was found in the area of the right kidney. For this reason, and due to the increased patient comfort in expiration, the acquisition of diffusion-weighted data in expiration is recommended in this study. In addition to imaging in the liver, another clinical study investigated the influence of external pressure in the form of a compression stocking, as well as the intensive strain on the calf muscles, on the biexponential IVIM parameters of the complete lower leg muscles. For this purpose, the lower legs of healthy volunteers were measured at six different points in time. The results of the study show no influence of compression, at all points in time, on the specic IVIM parameters. The increase in blood flow during muscle activity as well as the edema induced by strain could be visualized with IVIM imaging.Die Intravoxel-Incoherent-Motion-Bildgebung (IVIM-Bildgebung) ermöglicht die gleichzeitige Messung von Diffusions- und Perfusionsparametern in unterschiedlichen Organen. In dieser Arbeit wird speziell auf das ursprünglich eingeführte biexponentielle Modell sowie ein weniger bekanntes triexponentielles Modell für die Anwendung in Leber und Niere eingegangen. Unabhängig vom verwendeten Modell, sind die ermittelten IVIMParameter schwer reproduzierbar, wodurch die meisten Studien untereinander nicht vergleichbar sind. Aus diesem Grund war das Ziel dieser Arbeit, die Reproduzierbarkeit der ermittelten IVIM-Parameter zu erhöhen. Hierfür wurde unter anderem, die Magnetfeldabhängigkeit der IVIM-Parameter beider Modelle mithilfe von zwei Magnetfeldstärken untersucht. Hierbei zeigte sich eine Abhängigkeit des Diffusionskoeffizienten sowie der Pseudo-Diffusionskonstante des biexponentiellen Modells, von der verwendeten Magnetfeldstärke. Die gefundene Abhängigkeit wurde in dieser Arbeit auf eine unterschiedliche Signalzusammensetzung des untersuchten Volumens zurückgeführt, da sich die T2-Zeiten der Signalkomponenten mit der verwendeten Magnetfeldstärke verändern. Als ein weiterer Schritt, um die Reproduzierbarkeit der triexponentiellen IVIM-Parameter zu erhöhen, wurde die Wahl der Messpunkte zur Abtastung der triexponentiellen Signalkurve für die Anwendung in der Leber optimiert. Ausgehend von einer optimierten Verteilung bestehend aus sechs b-Werten wurde jeweils ein weiterer optimaler b-Wert gesucht und dieser zur bestehenden Verteilung hinzugefügt, bis eine optimierte b-Wert- Verteilung mit 100 b-Werten erreicht wurde. Diese wurde mit bereits in der Literatur verwendeten Verteilungen durch Simulationen und in vivo Messungen verglichen. In beiden Fällen konnten die IVIM-Parameter durch die optimierte b-Wert-Verteilung präziser ermittelt werden. Aufgrund der zugewonnenen Präzision war es erstmals möglich, triexponentielle IVIM-Parameterkarten der Leber mithilfe der durch die optimierte b-Wert- Verteilung aufgenommenen in vivo Datensets zu erzeugen. Für den speziellen Fall der Leber ist der Diffusionskoeffzient ebenfalls schwer reproduzierbar, da aufgrund der Lage zum Herzen dessen Pulsation zu einem reduzierten Signal im Bereich des linken Leberlappens führt. Im Rahmen einer klinischen Studie wurde getestet, ob das sogenannte Pulsationsartefakt durch eine Atemanhalte in Exspiration oder Inspiration reduziert werden kann. Es zeigte sich kein Unterschied in der Ausprägung des Artefaktes in einer der beiden verwendeten Atemtechniken. In Bildern, welche während Inspiration aufgenommen wurden, wurde ein Fettartefakt im Bereich der rechten Niere gefunden. Aus diesem Grund sowie aufgrund des erhöhten Patientenkomforts in Exspiration wird im Rahmen dieser Arbeit, die Aufnahme von diffusionsgewichteten Daten in Exspiration empfohlen. Neben der Bildgebung in der Leber, wurde in einer weiteren klinischen Studie der Einfluss von externem Druck in Form eines Kompressionsstrumpfes, sowie die intensive Belastung der Wadenmuskulatur auf die biexponentiellen IVIM-Parameter der kompletten Unterschenkelmuskulatur untersucht. Hierfür wurden die Unterschenkel von gesunden Probanden zu sechs verschiedenen Zeitpunkten gemessen. Das Ergebnis der Studie zeigt keinen Einfluss der Kompression, zu allen Zeitpunkten, auf die bestimmten IVIMParameter. Der Anstieg der Durchblutung während der Muskelaktivität sowie das durch die Belastung induzierte Ödem, konnte mit der IVIM-Bildgebung dargestellt werden

Echo time dependence of biexponential and triexponential intravoxel incoherent motion parameters in the liver

Purpose: Intravoxel incoherent motion (IVIM) studies are performed with different acquisition protocols. Comparing them requires knowledge of echo time (TE) dependencies. The TE-dependence of the biexponential perfusion fraction f is well-documented, unlike that of its triexponential counterparts f1 and f2 and the biexponential and triexponential pseudodiffusion coefficients D*, (Formula presented.), and (Formula presented.). The purpose was to investigate the TE-dependence of these parameters and to check whether the triexponential pseudodiffusion compartments are associated with arterial and venous blood. Methods: Fifteen healthy volunteers (19-58 y; mean: 24.7 y) underwent diffusion-weighted imaging of the abdomen with 24 b-values (0.2-800 s/mm2) at TEs of 45, 60, 75, and 90 ms. Regions of interest (ROIs) were manually drawn in the liver. One set of bi- and triexponential IVIM parameters per volunteer and TE was determined. The TE-dependence was assessed with the Kruskal-Wallis test. Results: TE-dependence was observed for f (P <.001), f1 (P =.001), and f2 (P <.001). Their median values at the four measured TEs were: f: 0.198/0.240/0.274/0.359, f1: 0.113/0.139/0.146/0.205, f2: 0.115/0.155/0.182/0.194. D, D*, (Formula presented.), and (Formula presented.) showed no significant TE-dependence. Their values were: diffusion coefficient D (10−4 mm2/s): 9.45/9.63/9.75/9.41, biexponential D* (10−2 mm2/s): 5.26/5.52/6.13/5.82, triexponential (Formula presented.) (10−2 mm2/s): 1.73/2.91/2.25/2.51, triexponential (Formula presented.) (mm2/s): 0.478/1.385/0.616/0.846. Conclusion: f1 and f2 show similar TE-dependence as f, ie, increase with rising TE; an effect that must be accounted for when comparing different studies. The diffusion and pseudodiffusion coefficients might be compared without TE correction. Because of the similar TE-dependence of f1 and f2, the triexponential pseudodiffusion compartments are most probably not associated to venous and arterial blood

On the dependence of the cardiac motion artifact on the breathing cycle in liver diffusion-weighted imaging

Purpose The purpose of this study was to investigate whether the cardiac motion artifact that regularly appears in diffusion-weighted imaging of the left liver lobe might be reduced by acquiring images in inspiration, when the coupling between heart and liver might be minimal. Materials and methods 43 patients with known or suspected focal liver lesions were examined at 1.5 T with breath hold acquisition, once in inspiration and once in expiration. Data were acquired with a diffusion-weighted echo planar imaging sequence and two b-values (b50 = 50 s/mm² and b800 = 800 s/mm²). The severity of the cardiac motion artifact in the left liver lobe was rated by two experienced radiologists for both b-values with a 5 point Likert scale. Additionally, the normalized signal S(b800)/S(b50) in the left liver lobe was computed. The Wilcoxon signed-rank test was used comparing the scores of the two readers obtained in inspiration and expiration, and to compare the normalized signal in inspiration and expiration. Results The normalized signal in inspiration was slightly higher than in expiration (0.349±0.077 vs 0.336±0.058), which would indicate a slight reduction of the cardiac motion artifact, but this difference was not significant (p = 0.24). In the qualitative evaluation, the readers did not observe a significant difference for b50 (reader 1: p = 0.61; reader 2: p = 0.18). For b800, reader 1 observed a significant difference of small effect size favouring expiration (p = 0.03 with a difference of mean Likert scores of 0.27), while reader 2 observed no significant difference (p = 0.62). Conclusion Acquiring the data in inspiration does not lead to a markedly reduced cardiac motion artifact in diffusion-weighted imaging of the left liver lobe and is in this regard not to be preferred over acquiring the data in expiration

On the Field Strength Dependence of Bi- and Triexponential Intravoxel Incoherent Motion (IVIM) Parameters in the Liver

Background: Studies on intravoxel incoherent motion (IVIM) imaging are carried out with different acquisition protocols. Purpose: To investigate the dependence of IVIM parameters on the B0 field strength when using a bi- or triexponential model. Study Type: Prospective. Study Population: 20 healthy volunteers (age: 19–28 years). Field Strength/Sequence: Volunteers were examined at two field strengths (1.5 and 3T). Diffusion-weighted images of the abdomen were acquired at 24 b-values ranging from 0.2 to 500 s/mm2. Assessment: ROIs were manually drawn in the liver. Data were fitted with a bi- and a triexponential IVIM model. The resulting parameters were compared between both field strengths. Statistical Tests: One-way analysis of variance (ANOVA) and Kruskal–Wallis test were used to test the obtained IVIM parameters for a significant field strength dependency. Results: At b-values below 6 s/mm2, the triexponential model provided better agreement with the data than the biexponential model. The average tissue diffusivity was D = 1.22/1.00 μm2/msec at 1.5/3T. The average pseudodiffusion coefficients for the biexponential model were D* = 308/260 μm2/msec at 1.5/3T; and for the triexponential model D* = 81.3/65.9 μm2/msec, D* 2 = 2453/2333 μm2/msec at 1.5/3T. The average perfusion fractions for the biexponential model were f = 0.286/0.303 at 1.5/3T; and for the triexponential model f1 = 0.161/0.174 and f2 = 0.152/0.159 at 1.5/3T. A significant B0 dependence was only found for the biexponential pseudodiffusion coefficient (ANOVA/KW P = 0.037/0.0453) and tissue diffusivity (ANOVA/KW: P < 0.001). Data Conclusion: Our experimental results suggest that triexponential pseudodiffusion coefficients and perfusion fractions obtained at different field strengths could be compared across different studies using different B0. However, it is recommended to take the field strength into account when comparing tissue diffusivities or using the biexponential IVIM model. Considering published values for oxygenation-dependent transversal relaxation times of blood, it is unlikely that the two blood compartments of the triexponential model represent venous and arterial blood. Level of Evidence: 1. Technical Efficacy Stage: 2. J. Magn. Reson. Imaging 2019;50:1883–1892

On the field strength dependence of Bi- and Triexponential Intravoxel Incoherent Motion (IVIM) parameters in the liver

Background Studies on intravoxel incoherent motion (IVIM) imaging are carried out with different acquisition protocols. Purpose To investigate the dependence of IVIM parameters on the B0 field strength when using a bi‐ or triexponential model. Study Type Prospective. Study Population 20 healthy volunteers (age: 19–28 years). Field Strength/Sequence Volunteers were examined at two field strengths (1.5 and 3T). Diffusion‐weighted images of the abdomen were acquired at 24 b‐values ranging from 0.2 to 500 s/mm2. Assessment ROIs were manually drawn in the liver. Data were fitted with a bi‐ and a triexponential IVIM model. The resulting parameters were compared between both field strengths. Statistical Tests One‐way analysis of variance (ANOVA) and Kruskal–Wallis test were used to test the obtained IVIM parameters for a significant field strength dependency. Results At b‐values below 6 s/mm2, the triexponential model provided better agreement with the data than the biexponential model. The average tissue diffusivity was D = 1.22/1.00 μm2/msec at 1.5/3T. The average pseudodiffusion coefficients for the biexponential model were D* = 308/260 μm2/msec at 1.5/3T; and for the triexponential model urn:x-wiley:10531807:media:jmri26730:jmri26730-math-0001 = 81.3/65.9 μm2/msec, urn:x-wiley:10531807:media:jmri26730:jmri26730-math-0002 = 2453/2333 μm2/msec at 1.5/3T. The average perfusion fractions for the biexponential model were f = 0.286/0.303 at 1.5/3T; and for the triexponential model f1 = 0.161/0.174 and f2 = 0.152/0.159 at 1.5/3T. A significant B0 dependence was only found for the biexponential pseudodiffusion coefficient (ANOVA/KW P = 0.037/0.0453) and tissue diffusivity (ANOVA/KW: P < 0.001). Data Conclusion Our experimental results suggest that triexponential pseudodiffusion coefficients and perfusion fractions obtained at different field strengths could be compared across different studies using different B0. However, it is recommended to take the field strength into account when comparing tissue diffusivities or using the biexponential IVIM model. Considering published values for oxygenation‐dependent transversal relaxation times of blood, it is unlikely that the two blood compartments of the triexponential model represent venous and arterial blood