Reto Treier

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.

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.

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”.

Gastroesophageal Junction: Structure and Function as Assessed by Using MR Imaging

PURPOSE To develop and validate magnetic resonance (MR) imaging protocols for quantitative assessment of the structural and functional properties of the gastroesophageal junction (GEJ) and to compare MR imaging detection of reflux events against concurrent manometry as a reference method. MATERIALS AND METHODS The local ethics committee approved this study, and written informed consent was obtained. Twelve healthy volunteers were examined. Three-dimensional models of the GEJ and proximal portion of the stomach were reconstructed from high-spatial-resolution anatomic MR images to assess the insertion angle of the esophagus into the stomach and proximal stomach distention before and after ingestion of a large test meal. A linear mixed-effects model was used to detect differences in the insertion angle and proximal stomach distention with respect to the respiratory cycle and gastric filling. Additionally, dynamic MR imaging at high temporal resolution was used to detect reflux events. RESULTS The esophageal insertion angle, given in units of plane angle (radians), was more acute in expiration than in inspiration (0.57 vs 0.73 radian, P = .004) but was not affected by feeding. Progressive distention of the proximal stomach was observed from baseline compared with the postprandial period (0.95 vs 0.65 radian(-1), P < .05). Eighteen reflux events detected by using MR imaging were also detected by using manometry. CONCLUSION MR imaging methods were developed and validated for the assessment of GEJ structure and function (a) to describe the effects of respiration and feeding on the reflux barrier and (b) to detect reflux events in real time. Anatomic and dynamic MR imaging may be useful techniques in the assessment of GEJ physiology and reflux.

Abnormal Structure and Function of the Esophagogastric Junction and Proximal Stomach in Gastroesophageal Reflux Disease

OBJECTIVES:This study applies concurrent magnetic resonance imaging (MRI) and high-resolution manometry (HRM) to test the hypothesis that structural factors involved in reflux protection, in particular, the acute insertion angle of the esophagus into the stomach, are impaired in gastroesophageal reflux disease (GERD) patients.METHODS:A total of 24 healthy volunteers and 24 patients with mild-moderate GERD ingested a test meal. Three-dimensional models of the esophagogastric junction (EGJ) were reconstructed from MRI images. Measurements of the esophagogastric insertion angle, gastric orientation, and volume change were obtained. Esophageal function was assessed by HRM. Number of reflux events and EGJ opening during reflux events were assessed by HRM and cine-MRI. Statistical analysis applied mixed-effects modeling.RESULTS:The esophagogastric insertion angle was wider in GERD patients than in healthy subjects (+7°±3°; P=0.03). EGJ opening during reflux events was greater in GERD patients than in healthy subjects (19.3 mm vs. 16.8 mm; P=0.04). The position of insertion and gastric orientation within the abdomen were also altered (both P<0.05). Median number of reflux events was 3 (95% CI: 2.5–4.6) in GERD and 2 (95% CI: 1.8–3.3) in healthy subjects (P=0.09). Lower esophageal sphincter (LES) pressure was lower (−11±2 mm Hg; P<0.0001) and intra-abdominal LES length was shorter (−1.0±0.3 cm, P<0.0006) in GERD patients.CONCLUSIONS:GERD patients have a wider esophagogastric insertion angle and have altered gastric morphology; structural changes that could compromise reflux protection by the “flap valve” mechanism. In addition, the EGJ opens wider during reflux in GERD patients than in healthy volunteers: an effect that facilitates volume reflux of gastric contents.

Fast and optimized T1 mapping technique for the noninvasive quantification of gastric secretion

To evaluate the noninvasive quantification of gastric secretion volume after administration of a labeled viscous glucose solution by fast T1 mapping.

Effects of postgastric 13C-acetate processing on measurement of gastric emptying: a systematic investigation in health

Abstract  Uniform postgastric processing of the gastric emptying (GE) marker 13C‐acetate (Ac) is an unverified assumption behind its widespread application to measure GE. This study assessed the postgastric processing of Ac administered by intraduodenal (i.d.) infusion simulating different physiological conditions. 13CO2 in breath was assessed in three groups of six volunteers after i.d. administration of A: Different caloric densities (0.75/1.5/3 kcal min−1) in a 200 mL meal at constant 1 mg Ac min−1 simulating a physiological range of nutrient delivery rates; B: different tracer delivery rates (0.5/1.0/2.5 mg Ac min−1) simulating delayed, normal and increased GE; C1: a 500 mL meal resulting in same marker and caloric delivery compared to protocol A; C2: 50 mL water bolus injections of 12.5/25/50/100 mg Ac and C3 bolus injections of 50 mg Ac in 50/100/200 mL water in randomized order. A: 13CO2 excretion was independent of caloric load (P = 0.59). B: The dynamic of 13CO2 excretion was modulated by tracer elimination which was in turn dependent on the speed of tracer delivery, i.e. with faster deliveries resulting in lower 13CO2 recovery during infusion (P < 0.001). C: Increasing Ac doses resulted in decreased 13CO2 recovery (P < 0.001) over the first hour. 13CO2 recovery kinetics was independent of the volume delivered. This study shows 13C‐acetate absorption and metabolism is independent of the volume and caloric delivery of test meals. The ‘lag’ in estimates of GE derived from 13CO2 breath tests is due to a postgastric, dose‐dependent delay to 13CO2 elimination. This can be corrected for in analytical derivations of GE parameters based on 13C‐acetate breath test measurements.

Gastric secretion does not affect the reliability of the 13C‐acetate breath test: A validation of the 13C‐acetate breath test by magnetic resonance imaging

Background  13C‐Acetate labeled meals are widely used to determine meal emptying by means of analyzing resulting 13CO2 exhalation dynamics. In contrast to the underlying metabolic processes, only few 13C breath test meal emptying studies have focused on intragastric processes that may alter 13CO2 exhalation. This work assessed the effect of enhanced gastric secretion on the reliability of half emptying time (t50) measurements by 13C‐acetate breath test.

96 The Structure and Function of the Gastro-Esophageal Junction in Health and Reflux Disease Assessed By Magnetic Resonance Imaging and High Resolution Manometry

nitrate, and during acid reflux, generation of NO is shifted to the distal esophagus. We have demonstrated that high concentrations of luminal NO can impair the gastric mucosal barrier function by disrupting the tight junction (Gut 2008). We hypothesized that NO generated luminally during acid reflux could disrupt the esophageal barrier function and provoke DIS. Aim: To investigate the direct effects of luminal NO on the esophageal barrier function using an ex vivo chamber model. Design and Setting: A chamber model in which the rat esophageal mucosal membrane was mounted between the two halves of a chamber was designed to simulate the microenvironment of the lumen and the adjacent mucosa of the esophagus. On the mucosal side of the chamber, NO was generated by the acidification of physiologic concentrations of sodium nitrite. The epithelial barrier function was evaluated by electrophysiological transmembrane resistance (R) and membrane permeability with 3H-mannitol flux in four groups; Krebs buffer, acid alone (pH 1.5), acid + sodium nitrite 5.0 mM, and acid + sodium nitrite 1.0 mM. Intercellular space diameters were measured on transmission electron microscopy photomicrographs. Results: In all groups except for Krebs buffer, the R decreased rapidly within the initial 15 minutes, followed by gradual decrease thereafter. At 180 minutes, the R decreased by 34% in acid alone group, by 39% in acid + nitrite 1.0 mM group, by 45% in acid + nitrite 5.0 mM group by 55%. Consequently, the administration of acidified nitrite (1.0 mM or 5.0 mM) to the mucosal side significantly decreased the R compared with that of the acid alone (p<0.01). While epithelial permeability with 3Hmannitol slightly increased at 180 minutes in acid alone group, it remarkably increased in acid + nitrite 5.0 mM group. Thus, the administration of acidified nitrite to the mucosal side significantly increased epithelial permeability compared with the acid alone group (p<0.05). DIS was observed in the nitrite groups but not in the acid alone group. Conclusions: The NO generated luminally via acidification of nitrite disrupted the barrier function of the esophageal epithelium and provoked DIS, suggesting that NO generated luminally contributes to DIS observed in GERD patients and plays an important role in the pathophysiology of GERD.