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

Intraluminal Impact of Food: New Insights from MRI

Understanding how the gut responds to food has always been limited by the available investigatory techniques. Previous methods involving intubation and aspiration are largely limited to liquid-only meals. The aim of this review is to describe how MRI has allowed analysis of the processing of complex multiphase meals. This has demonstrated the role of physical factors such as viscosity, fat and fibre content in controlling gastric secretion and motility. It has also allowed the description of changes induced in small bowel water content and the role of osmotic effects of poorly absorbed carbohydrates such as fructose, sorbitol and mannitol. Intestinal secretions can be shown to be stimulated by a range of fruit and vegetables and the effect of this on colonic water content can also be measured. This has been used to demonstrate the mode of action of commonly used laxatives including bran and psyllium. The wealth of data which can be obtained together with its non-invasive nature and safety makes the technique ideal for the serial evaluation of the impact of different nutrients and drugs in both health and disease.

Emulsion Stability Modulates Gastric Secretion and Its Mixing with Emulsified Fat in Healthy Adults in a Randomized Magnetic Resonance Imaging Study

BACKGROUND

Oil-in-water emulsions have recently become of interest to nutritional sciences because of their ability to influence gastrointestinal digestive processes and ultimately benefit human health. MRI offers the potential to noninvasively characterize the interaction between emulsified lipids and gastric secretion within the stomach.

OBJECTIVES

We determined noninvasively how emulsion stability modulates volumes of fat and secretion, layering of fat, and the mixing of emulsified fat with secretion within the stomach. This required the development of MRI technology for quantifying fat and secretion concentrations inside the stomach.

METHODS

Twenty-one healthy adults [13 men, mean ± SD age: 22.5 ± 2.5 y, mean ± SD body mass index (in kg/m²): 22.7 ± 1.8] were analyzed in a single-blind, randomized, parallel design. MRI was used to acquire the distributions of fat and secretion in the stomach after ingestion of 2 emulsions: a stable emulsion (E1) or an unstable emulsion (E4) with 20% fat fraction and ∼0.3 mm droplet sizes. Layer, volume, and mixing variables were fitted to the data and compared between the 2 emulsions.

RESULTS

The intragastric mixing between fat and secretion was better with the E4 than the E1 [increase in content heterogeneity of 17.1% (95% CI: 12.3%, 21.9%)]. The E4 demonstrated a linear relation [slope 1.57 (95% CI: 0.86, 2.29)] between the degree of layering and mixing. In contrast, no such relation was detected for the E1. Accumulated secretion volume in the stomach was lower with the E4 [decrease in volume variable k_s of 2.3 (95% CI: -3.9, -0.7)] and correlated with the degree of layering (r = 0.62, P < 0.001).

CONCLUSIONS

In healthy adults, intragastric fat layering was influenced mainly by the degree of intragastric mixing, rather than the overall dominance of secretion. The E1 triggered a higher accumulation of gastric secretion, which in turn facilitated homogenization of intragastric content in comparison with its unstable counterpart. This trial was registered at clinicaltrials.gov as NCT02602158.

Development and validation of a large, modular test meal with liquid and solid components for assessment of gastric motor and sensory function by non-invasive imaging

BACKGROUND

Current investigations of stomach function are based on small test meals that do not reliably induce symptoms and analysis techniques that rarely detect clinically relevant dysfunction. This study introduces the large 'Nottingham Test Meal' (NTM) for assessment of gastric motor and sensory function by non-invasive imaging.

METHODS

NTM comprises 400 mL liquid nutrient (0.75 kcal/mL) and 12 solid agar-beads (0 kcal) with known breaking strength. Gastric fullness and dyspeptic sensations were documented by 100 mm visual analogue scale (VAS). Gastric emptying (GE) were measured in 24 healthy volunteers (HVs) by gastric scintigraphy (GS) and magnetic resonance imaging (MRI). The contribution of secretion to gastric volume was assessed. Parameters that describe GE were calculated from validated models. Inter-observer agreement and reproducibility were assessed.

KEY RESULTS

NTM produced moderate fullness (VAS ≥30) but no more than mild dyspeptic symptoms (VAS <30) in 24 HVs. Stable binding of meal components to labels in gastric conditions was confirmed. Distinct early and late-phase GE were detected by both modalities. Liquid GE half-time was median 49 (95% CI: 36-62) min and 68 (57-71) min for GS and MRI, respectively. Differences between GS and MRI measurements were explained by the contribution of gastric secretion. Breaking strength for agar-beads was 0.8 N/m(2) such that median 25 (8-50) % intact agar-beads and 65 (47-74) % solid material remained at 120 min on MRI and GS, respectively. Good reproducibility for liquid GE parameters was present and GE was not altered by agar-beads.

CONCLUSIONS & INFERENCES

The NTM provided an objective assessment of gastric motor and sensory function. The results were reproducible and liquid emptying was not affected by non-nutrient agar-beads. The method is potentially suitable for clinical practice.

On the validity of the (13) C-acetate breath test for comparing gastric emptying of different liquid test meals: a validation study using magnetic resonance imaging

BACKGROUND

(13) C-acetate breath testing (BT) is applied to assess and compare gastric emptying of liquid meals. Gastric half-emptying times (t50 ) from BT show offsets compared to t50 values from γ-scintigraphy and ultrasonography. Linear transformations have been proposed to correct these offsets. This investigation critically validates the BT for the assessment of liquid gastric emptying by using simultaneously recorded meal and total gastric content volume emptying data from magnetic resonance imaging (MRI).

METHODS

Data were collected during a recently published double-blind, randomized, cross-over MRI gastric emptying study of three (13) C-labeled enteral formulas differing in protein sources (PMID: 24699556). Breath testing-derived t50 was computed with the analysis methods commonly applied in gastric emptying research, i.e., the exponential-beta function and the Wagner-Nelson (WN) method, respectively.

KEY RESULTS

Breath testing t50 values from exponential-beta function and WN method showed a positive and negative offset to MRI data, respectively. Linear regression detected low concordance between MRI and both BT methods revealing meal specific and emptying rate-dependent offsets. The WN method showed worse agreement and correlation with MRI emptying data. Breath testing rather reflected meal volume than total gastric content volume emptying.

CONCLUSIONS & INFERENCES

This validation study indicates that the (13) C-acetate breath test may not be applied to compare gastric emptying of arbitrary liquid meals without prior validation by imaging methods. t50 values from BT are biased by (i) the properties of the meal and (ii) the selected method used for (13) CO2 exhalation analysis. No linear transformation common for all meals was applicable to correct the offsets between BT and MRI.

Volume, distribution and acidity of gastric secretion on and off proton pump inhibitor treatment: a randomized double-blind controlled study in patients with gastro-esophageal reflux disease (GERD) and healthy subjects

Background

Postprandial accumulation of gastric secretions in the proximal stomach above the meal adjacent to the esophagogastric junction (EGJ), referred to as the ‘acid pocket’, has been proposed as a pathophysiological factor in gastro-esophageal reflux disease (GERD) and as a target for GERD treatment. This study assessed the effect of proton pump inhibitor (PPI) therapy on the volume, distribution and acidity of gastric secretions in GERD and healthy subjects (HS).

Methods

A randomized, double blind, cross-over study in 12 HS and 12 GERD patients pre-treated with 40 mg pantoprazole (PPI) or placebo b.i.d. was performed. Postprandial secretion volume (SV), formation of a secretion layer and contact between the layer and the EGJ were quantified by Magnetic Resonance Imaging (MRI). Multi-channel pH-monitoring assessed intragastric pH.

Results

A distinct layer of undiluted acid secretion was present on top of gastric contents in almost all participants on and off high-dose acid suppression. PPI reduced SV (193 ml to 100 ml, in HS, 227 ml to 94 ml in GERD; p < 0.01) and thickness of the acid layer (26 mm to 7 mm, 36 mm to 9 mm respectively, p < 0.01). No differences in secretion volume or layer thickness were observed between groups; however, off treatment, contact time between the secretion layer and EGJ was 2.6 times longer in GERD compared to HS (p = 0.012). This was not the case on PPI.

Conclusions

MRI can visualize and quantify the volume and distribution dynamics of gastric secretions that form a layer in the proximal stomach after ingestion of a liquid meal. The secretion volume and the secretion layer on top of gastric contents is similar in GERD patients and HS; however contact between the layer of undiluted secretion and the EGJ is prolonged in patients. High dose PPI reduced secretion volume by about 50 % and reduced contact time between secretion and EGJ towards normal levels.

Trial registration

NCT01212614.

Electronic supplementary material

The online version of this article (doi:10.1186/s12876-015-0343-x) contains supplementary material, which is available to authorized users.

Measurement of gastric meal and secretion volumes using magnetic resonance imaging

MRI can assess multiple gastric functions without ionizing radiation. However, time consuming image acquisition and analysis of gastric volume data, plus confounding of gastric emptying measurements by gastric secretions mixed with the test meal have limited its use to research centres. This study presents an MRI acquisition protocol and analysis algorithm suitable for the clinical measurement of gastric volume and secretion volume. Reproducibility of gastric volume measurements was assessed using data from 10 healthy volunteers following a liquid test meal with rapid MRI acquisition within one breath-hold and semi-automated analysis. Dilution of the ingested meal with gastric secretion was estimated using a respiratory-triggered T1 mapping protocol. Accuracy of the secretion volume measurements was assessed using data from 24 healthy volunteers following a mixed (liquid/solid) test meal with MRI meal volumes compared to data acquired using gamma scintigraphy (GS) on the same subjects studied on a separate study day. The mean ± SD coefficient of variance between 3 observers for both total gastric contents (including meal, secretions and air) and just the gastric contents (meal and secretion only) was 3  ±  2% at large gastric volumes (>200 ml). Mean ± SD secretion volumes post meal ingestion were 64  ±  51 ml and 110  ±  40 ml at 15 and 75 min, respectively. Comparison with GS meal volumes, showed that MRI meal only volume (after correction for secretion volume) were similar to GS, with a linear regression gradient ± std err of 1.06  ±  0.10 and intercept -11  ±  24 ml. In conclusion, (i) rapid volume acquisition and respiratory triggered T₁ mapping removed the requirement to image during prolonged breath-holds (ii) semi-automatic analysis greatly reduced the time required to derive measurements and (iii) correction for secretion volumes provided accurate assessment of gastric meal volumes and emptying. Together these features provide the scientific basis of a protocol which would be suitable in clinical practice.

Validation of a golden angle radial sequence (GOLD) for abdominal T1 mapping during free breathing: demonstrating clinical feasibility for quantifying gastric secretion and emptying

PURPOSE

To validate a magnetic resonance imaging sequence suitable for quantitative assessment of acid suppression by a proton pump inhibitor (PPI) on gastric secretion and emptying in clinical practice.

METHODS

A golden angle radial sequence (GOLD) was validated in a series of in vitro and in vivo experiments and clinical feasibility was shown in two studies. The impact of free breathing and image plane orientation on T1 values was evaluated in a controlled in vivo experiment. The free-breathing GOLD sequence was compared against a standard breath-hold gradient echo sequence for gastric half emptying time in 23 subjects during a gastric emptying study. Pilot data from five subjects assessed the sensitivity of the GOLD sequence to detect changes in acid secretion volume produced by PPI treatment.

RESULTS

The coronal free-breathing GOLD sequence and the axial breath-hold standard gradient echo sequence showed good agreement of the gastric half emptying time (6 ± 3 min, P = 0.053). The GOLD sequence demonstrated sensitivity to reduction of gastric secretion volumes induced by PPI treatment (55 ± 5 mL, P < 0.001).

CONCLUSION

The GOLD sequence allowed for free breathing, multislice, combined imaging and T1 mapping of the stomach content. GOLD presents a promising multipurpose, noninvasive imaging tool for monitoring gastric function in clinical studies.

Gastric acid pocket and gastroesophageal reflux disease

Gastric acid pocket is an area of highly acid that escapes the neutralization of food in the proximal stomach after meals and has been noticed in healthy individuals and gastroesophageal reflux disease (GERD) patients. It was first found by Fletcher et al, who carried out a series of experiments in 2001. After that, other researchers have carried out in-depth studies on it. Hiatus hernia, gastric motility, gastric anatomy and physiology seem to be important elements for the genesis of postprandial proximal gastric acid pocket (PPGAP). Because PPGAP may play an important role in the pathogenesis of GERD, treatments for suppressing the production of gastric acid, antacid agents, prokinetic agents, surgery and other treatments may be used to alter the presence of PPGAP to achieve the purpose of treating GERD. Therefore, the discovery of gastric acid pocket may provide a new idea for the treatment of GERD.

The acid pocket: a target for treatment in reflux disease?

The nadir esophageal pH of reflux observed during pH monitoring in the postprandial period is often more acidic than the concomitant intragastric pH. This paradox prompted the discovery of the "acid pocket", an area of unbuffered gastric acid that accumulates in the proximal stomach after meals and serves as the reservoir for acid reflux in healthy individuals and gastroesophageal reflux disease (GERD) patients. However, there are differentiating features between these populations in the size and position of the acid pocket, with GERD patients predisposed to upward migration of the proximal margin onto the esophageal mucosa, particularly when supine. This upward migration of acid, sometimes referred to as an "acid film", likely contributes to mucosal pathology in the region of the squamocolumnar junction. Furthermore, movement of the acid pocket itself to a supradiaphragmatic location with hiatus hernia increases the propensity for acid reflux by all conventional mechanisms. Consequently, the acid pocket is an attractive target for GERD therapy. It may be targeted in a global way with proton pump inhibitors that attenuate acid pocket development, or with alginate/antacid combinations that colocalize with the acid pocket and displace it distally, thereby demonstrating the potential for selective targeting of the acid pocket in GERD.

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

(13)C-Acetate labeled meals are widely used to determine meal emptying by means of analyzing resulting (13)CO(2) exhalation dynamics. In contrast to the underlying metabolic processes, only few (13)C breath test meal emptying studies have focused on intragastric processes that may alter (13)CO(2) exhalation. This work assessed the effect of enhanced gastric secretion on the reliability of half emptying time (t50) measurements by (13)C-acetate breath test.

METHODS

(13)CO(2) exhalation data were acquired in a double-blind, randomized, cross-over gastric emptying study in 12 healthy volunteers receiving either pentagastrin or placebo intravenously. The standard method proposed by Ghoos et al. was applied to calculate t50 (t50_Ghoos) from (13)CO(2) exhalation data, which were compared and tested for agreement to meal half emptying times (t50_MV) from concurrent recorded MRI (magnetic resonance imaging) volume data. In addition, the accumulated gastric secretion volumes during infusion as detected by MRI (AUC_SV(60)) were correlated with the corresponding cumulative percent (13)C doses recovered (cPDR(60)).

KEY RESULTS

t50_Ghoos and t50_MV showed a linear correlation with a slope of 1.1 ± 0.3 (r(2) = 0.67), however, a positive offset of 136 min for t50_Ghoos. No correlation was detected between AUC_SV(60) and cPDR(60) (r(2) = 0.11). Both, breath test and MRI, revealed a prolonged t50 under pentagastrin infusion with median differences in t50_Ghoos of 45[28-84] min (P = 0.002) and t50_MV of 39[28-52] min (P = 0.002).

CONCLUSIONS & INFERENCES

This study suggests that (13)CO(2) exhalation after ingestion of a (13) C-labeled liquid test meal is not affected by stimulated gastric secretion, but is rather reflecting the dynamics of meal or caloric emptying from the stomach.

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.

METHODS

A chocolate test meal was developed which is secretion stimulating and MRI compatible. Meal emptying and gastric secretion were assessed in fourteen healthy volunteers using a validated quantitative MRI technique. A population based pharmacokinetic model was developed and applied to the extracted volume data, assessing the meal emptying rate, rate of secretion and their interaction.

KEY RESULTS

The test meal continuously induced gastric secretion in all subjects, which partly accumulated at the meal-air interface, forming a 'secretion layer' in the proximal stomach. Traditional fitting detected a significant correlation between meal emptying rate and rate of secretion. The pharmacokinetic model quantified this interaction and estimated a 2.3 ± 1 fold higher effect of meal on secretion than vice versa. The efficacy of the emptied meal to produce gastric secretion was 61%.

CONCLUSIONS & INFERENCES

The combined quantitative MRI and pharmacokinetic model approach allows for the quantification of gastric secretion volume and its interaction on meal emptying. The observed secretion layer might explain previous findings postulating the presence of an intragastric 'acid pocket'.

Assessment of gastrointestinal motor functions by MRI: a comprehensive review

BACKGROUND

Magnetic resonance imaging (MRI) is a useful technique for evaluating gastrointestinal (GI) motor functions because it provides multi-planar imaging capability, high imaging speed, good spatial resolution and excellent soft tissue contrast. Moreover, multiple parameters of GI function can be assessed non-invasively during a single study.

PURPOSE

This critical review of the literature discusses the strengths and limitations of MRI for evaluating GI motor functions.

The use of contrast agents with fast field-cycling magnetic resonance imaging

Fast field-cycling (FFC) MRI allows switching of the magnetic field during an imaging scan. FFC-MRI takes advantage of the T(1) dispersion properties of contrast agents to improve contrast, thus enabling more sensitive detection of the agent. A new contrast agent designed specifically for use with FFC was imaged using both a homebuilt FFC-MRI system and a 3 T Philips clinical MRI scanner. T(1) dispersion curves were obtained using a commercial relaxometer which showed large changes in relaxation rate between fields. A model of magnetization behaviour was used to predict optimum evolution times for the maximum T(1) contrast between samples at each field. Images were processed and analysed to create maps of R(1) values using a set of images at each field. The R(1) maps produced at two different fields were then subtracted from each other in order to create a map of ΔR(1) in which pixel values depend on the change in R(1) of the sample between the two fields. The dispersion properties of the agent resulted in higher contrast in a ΔR(1) image compared with a standard T(1)-weighted image.

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

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 (13)C-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 (T(1/2)) 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. T(1/2)(MRI) 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, T(1/2)(BT) 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.

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

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 T(1) 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 T(1) mapping. Data was described by the kappa-coefficient (volume change after meal ingestion), by GE half time (T(50)) and maximal GE rate (GER(max)) derived all from a GE model. Pentagastrin increased GCV and TGV compared to placebo [kappa(GCV):1.6 +/- 0.1 vs 0.6 +/- 0.1; kappa(TGV): 1.6 +/- 0.1 vs 0.7 +/- 0.1; P < 0.001]. T(1) maps revealed a secretion layer above the meal, the volume of which was associated with kappa (R(2) = 83%, P < 0.001). TGV and GCV change were similar in both conditions (kappa; P = ns). T(50) was higher for pentagastrin than for placebo (84 +/- 7 vs 56 +/- 4min, P < 0.001); however, GER(max) 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 T(1) 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 T(50); however, GE rate is unchanged.