Andreas Steingoetter

Optimized and combined T1 and B1 mapping technique for fast and accurate T1 quantification in contrast-enhanced abdominal MRI

Fast T1 mapping techniques are a valuable means of quantitatively assessing the distribution and dynamics of intravenously or orally applied paramagnetic contrast agents (CAs) by noninvasive imaging. In this study a fast T1 mapping technique based on the variable flip angle (VFA) approach was optimized for accurate T1 quantification in abdominal contrast‐enhanced (CE) MRI. Optimization methods were developed to maximize the signal‐to‐noise ratio (SNR) and ensure effective RF and gradient spoiling, as well as a steady state, for a defined T1 range of 100–800 ms and a limited acquisition time. We corrected B1 field inhomogeneities by performing an additional measurement using an optimized fast B1 mapping technique. High‐precision in vitro and abdominal in vivo T1 maps were successfully generated at a voxel size of 2.8 × 2.8 × 15 mm3 and a temporal resolution of 2.3 s per T1 map on 1.5T and 3T MRI systems. The application of the proposed fast T1 mapping technique in abdominal CE‐MRI enables noninvasive quantification of abdominal tissue perfusion and vascular permeability, and offers the possibility of quantitatively assessing dilution, distribution, and mixing processes of labeled solutions or drugs in the gastrointestinal tract. Magn Reson Med 57:568–576, 2007.

Magnetic resonance imaging for the assessment of gastrointestinal function

Magnetic resonance imaging (MRI) is a versatile medical imaging tool for which several new applications have been developed in the past decade. Besides its broad clinical use for the detection of a...

Gastric motor function and emptying in the right decubitus and seated body position as assessed by magnetic resonance imaging

To determine the effect of the right decubitus lying body position (RP) on relevant parameters of human gastric motor function in healthy volunteers.

The effect of gastric secretion on gastric physiology and emptying in the fasted and fed state assessed by magnetic resonance imaging

Abstract  Conventional measurement of gastric secretion is invasive and cannot assess the intra‐gastric distribution of gastric contents or the effects of secretion on gastric function. This study assessed the effect of gastric secretion on gastric volume responses and emptying (GE) using a validated fast T1 mapping magnetic resonance imaging (MRI) technique. Twelve healthy participants were studied in the fasted state and after 200 kcal Gadolinium‐DOTA labelled glucose meal during intravenous infusion of pentagastrin or placebo in double‐blind, randomized order. Total gastric volume (TGV) and gastric content volume (GCV) was assessed by MRI volume scans and secretion by fast T1 mapping. Data was described by the κ‐coefficient (volume change after meal ingestion), by GE half time (T50) and maximal GE rate (GERmax) derived all from a GE model. Pentagastrin increased GCV and TGV compared to placebo [κ(GCV):1.6 ± 0.1 vs 0.6 ± 0.1; κ(TGV): 1.6 ± 0.1 vs 0.7 ± 0.1; P < 0.001]. T1 maps revealed a secretion layer above the meal, the volume of which was associated with κ (R2 = 83%, P < 0.001). TGV and GCV change were similar in both conditions (κ; P = ns). T50 was higher for pentagastrin than for placebo (84 ± 7 vs 56 ± 4min, P < 0.001); however, GERmax was similar (5.9 ± 0.6 vs 4.9 ± 0.4 mL min−1, P = ns). This study shows volume and distribution of gastric secretion can be quantified in‐vivo by non‐invasive MRI T1 mapping. Increased GCV drove TGV accommodation without evidence of a direct effect of pentagastrin or excess acid on gastric function. Secretion increases GCV thus prolongs GE as assessed by T50; however, GE rate is unchanged.

Magnetic Resonance Imaging for the in Vivo Evaluation of Gastric-Retentive Tablets

AbstractPurpose. To develop a magnetic resonance imaging (MRI) technique for assessing in vivo properties of orally ingested gastric-retentive tablets under physiologic conditions. Methods. Tablets with different floating characteristics (tablet A-C) were marked with superparamagnetic Fe3O4 particles to analyze intragastric tablet position and residence time in human volunteers. Optimal Fe3O4 concentration was determined in vitro. Intragastric release characteristic of one slow-release tablet (tablet D) was analyzed by embedding gadolinium chelates (Gd-DOTA) as a drug model into the tablet. All volunteers underwent MRI in the sitting position. Tablet performance was analyzed in terms of relative position of tablet to intragastric meal level (with 100% at meal surface), intragastric residence time (min) and Gd-DOTA distribution volume (% of meal volume). Results. Intragastric tablet floating performance and residence time of tablets (tablet A-D) as well as the intragastric Gd-DOTA distribution of tablet D could be monitored using MRI. Tablet floating performance was different between the tablets (A, 93%(95 − 9%); B, 80%(80 − 68%); C, 38%(63 − 32%); p < 0.05). The intragastric distribution volume of Gd-DOTA was 19.9% proximally and 35.5% distally. Conclusions. The use of MRI allows the assessment of galenic properties of orally ingested tablets in humans in seated position.

Effects of posture on the physiology of gastric emptying: A magnetic resonance imaging study

Objective. Gastric contents empty from the stomach despite frequent changes in body position. The mechanism that maintains gastric emptying independent of position is poorly understood. The aim of this study was to determine the effects of body position on gastric emptying and motor function. Material and methods. Twelve volunteers were investigated in seated position (SP) and upside-down position (UDP) after ingestion of 300 ml water. Magnetic resonance imaging provided a non-invasive assessment of gastric emptying and volumes, intragastric distribution and peristaltic function. Results. A marked difference in distal/proximal intragastric distribution between UDP and SP was present (7% versus 40%; p < 0.01). Gastric-emptying time was similar but emptying pattern was linear in UDP and exponential in SP. Peristalsis was slower in UDP than SP (2.75 versus 2.96 min−1; p < 0.01), but no correlation was found between peristaltic frequency and the rate of gastric emptying in either position. Postprandial volume response (gastric relaxation) was greater in UDP than SP (280 versus 250 ml; p < 0.05). A correlation was found between gastric relaxation and gastric-emptying time in SP (r2=0.46) but not in UDP. Conclusions. The stomach maintains the rate of gastric emptying despite radical changes in body position and intragastric distribution of gastric contents. In SP, hydrostatic pressure (modulated by gastric tone) dictates the gastric emptying. In UDP, gastric emptying also appears to be mediated by continuous adaptation of gastric tone. These findings provide support for the hypothesis that the mechanism of gastric emptying resembles a “pressure pump” rather than a “peristaltic pump”.

Intersubject and intrasubject variability of gastric volumes in response to isocaloric liquid meals in functional dyspepsia and health

Abstract  Gastric emptying (GE) has a considerable variability, but data on reproducibility of gastric volume measurements are sparse. We aimed to study the reproducibility of postprandial gastric volume responses and GE using magnetic resonance imaging (MRI) in healthy controls (HC) and patients with functional dyspepsia (FD). Eight HC and eight FD patients underwent a MRI study on two occasions. MR images were acquired in seated position before and up to 120 min after liquid meal administration (200 mL, 300 kcal). Fasting (V0), initial postprandial stomach volumes (V1), volume changes (V1 − V0) and meal emptying half‐times () were determined. Intersubject and intrasubject coefficients of variation (CVinter, CVintra) and Pearsons correlation coefficients (r) were calculated. on both occasions were (mean ± SD) 113 ± 28 and 121 ± 30 min in HC (ns) and 127 ± 31 and 128 ± 37 min in FD (ns), respectively. In HC, CVinter, CVintra, r were 31%, 23%, 0.49 for V0; 13%, 7%, 0.68 for V1; 10%, 4%, 0.71 for V1 − V0 and 25%, 7%, 0.90 for . In FD these parameters were for V0: 42%, 41%, −0.06; for V1: 18%, 10%, 0.40; for V1 − V0: 20%, 14%, 0.74 and for : 26%, 10%, 0.84. The stomach accommodates to a given meal volume, resulting in similar and reproducible postprandial volumes within‐ and between‐subjects. MRI provides reproducible measurements of gastric volume responses in health and disease.

Measuring the interaction of meal and gastric secretion: a combined quantitative magnetic resonance imaging and pharmacokinetic modeling approach

Background  The stimulation and intragastric accumulation of gastric secretion has been recognized as an important factor in gastroesophageal reflux disease. However, the interaction of gastric secretion and meal emptying has not been fully understood. Current methods to assess gastric secretion are either invasive or unable to provide information on its volume, distribution and dynamics. The aim of this study was to quantify the interaction between meal emptying and meal induced gastric secretion by using quantitative magnetic resonance imaging (MRI) and pharmacokinetic analysis.

Characterization of gastric volume responses and liquid emptying in functional dyspepsia and health by MRI or barostat and simultaneous 13C-acetate breath test

Abstract  The assessment of gastric accommodation and emptying by different methodologies provides inconsistent results. We aimed to compare magnetic resonance imaging (MRI), barostat and 13C‐acetate breath test (BT) for the assessment of gastric volume responses and emptying in healthy controls (HC) and patients with functional dyspepsia (FD). Eight HC and eight FD patients underwent: (i) continuous BT with simultaneous MRI in the upright position after ingestion of isocaloric, 300 kcal, 200 and 800 mL meals, both labelled with 100 mg of 13C‐acetate; and (ii) BT with gastric barostat after ingestion of the 200 mL meal. MRI measured total gastric volume and gastric content volume (GCV) at baseline, after filling and during emptying. Meal emptying half‐times (T½) for MRI and BT were calculated (mean ± SD). We found: (i) Initial GCV was lower in FD than in HC (762 ± 22 vs 810 ± 52 mL, P < 0.04) after the 800 mL meal but not the 200 mL meal. T½MRI was shorter for the 800 mL than the 200 mL meal (P < 0.001), but similar in HC and FD (200 mL: HC 117 ± 30 min vs FD 138 ± 42 min, ns; 800 mL: HC 71 ± 16 min vs FD 78 ± 27 min, ns). In contrast, T½BT was similar between meals and groups (200 mL: HC 111 ± 11 min vs FD 116 ± 19 min; 800 mL: HC 114 ± 14 min vs FD: 113 ± 17 min). (ii) Barostat measurements showed similar postprandial volume increases between groups. We conclude that direct measurements by MRI provide a sensitive, non‐invasive assessment of gastric accommodation and emptying after a meal. In contrast to MRI, BT did not detect faster emptying of high‐volume compared to low‐volume liquid nutrient meals in HC or FD.

Validation of preclinical multiparametric imaging for prediction of necrosis in hepatocellular carcinoma after embolization

BACKGROUND & AIMS The hepatocellular carcinoma (HCC) exhibits varying degrees of vascularization with more poorly differentiated carcinoma commonly exhibiting high amounts of vascularization. Transcatheter arterial embolization (TAE) of HCC tumor nodules results in varying amounts of tumor necrosis. Reliable quantification of necrosis after TAE, would aid in treatment planning and testing of novel combinatorial treatment regimen. The aim of this work was to validate different imaging parameters as individual or combined predictors of tumor necrosis after TAE in an orthotopic rat HCC tumor model. METHODS Unifocal rat HCC was imaged by T(2)-weighted MRI, quantitative dynamic contrast enhanced (DCE) MRI, diffusion weighted MRI (DWI) and [(18)F]-FDG PET imaging before (day-1) and after (days 1 and 3) TAE. Univariate and multivariate regression analyses were carried out to analyze the ability of each imaging parameter to predict the percent residual vital tumor (vtu) and vital tissue (vti) as determined by quantitative histopathology. RESULTS TAE induced a wide range of tumor necrosis. Tumor volume was the only parameter showing a correlation with vti (r(2) = 0.63) before TAE. After TAE, moderate correlations were found for FDG tracer uptake (r(2) = 0.56) and plasma tissue transfer constant (r(2) = 0.55). Correlations were higher for the extravascular extracellular volume fraction (v(e), r(2) = 0.68) and highest for the apparent diffusion coefficient (ADC, r(2) = 0.86). Multivariate analyses confirmed highest correlation of ADC and v(e) with vtu and vti. CONCLUSIONS DWI and DCE-MRI with the respective parameters ADC (day 3) and v(e) (day 1) were identified as the most promising imaging techniques for the prediction of necrosis. This study validates a preclinical platform allowing for the improved tumor stratification after TAE and thus the testing of novel combinatorial therapy approaches in HCC.