Gastric emptying measured by ultrasonography

Abstract

Key Words: gastric emptying; stomach/ultrasonography; stomach/pathophysiology

TEXT

A number of different methods have been used to estimate gastric emptying in humans, and all have their advantages and disadvantages. The method of choice will depend on whether solid or liquid meals are studied, the level of precision required, the degree of invasiveness that the subject or patient will tolerate, ethical considerations, and not at least the facilities available. Scintigraphy, with appropriate labelling of the test meal components and appropriate corrections applied, is considered so far the gold standard for measurement of gastric emptying. However, its application is limited by the need to restrict exposure to ion ising radiation. Other methods are gastric aspiration techniques, radiography, ultrasonography, magnetic resonance imaging, epigastric impedance measurements, applied potential tomography, tracer methods (e.g. paracetamol), and breath tests. Regardless of the method used, the investigator must be aware of the limitations of the method in use, the large inter-individual variability and of the facto rs known to influence gastric emptying.

Ultrasonography is non-invasive, cheap, widely available, and can be repeatedly performed because of its safety. two- dimensional ultrasound has, for many years, been widely used to assess gastric emptying rates[1-5], and good cor relation to radionuclide estimates of emptying rates have been detected[3,6]. In one study, ultrasound measurements of gastric emptying gave comparable sensitivity to scintigraphy in quantifying emptying of both low and high nutrient liquids[7].

Ultrasound imaging of the proximal stomach is usually considered inappropriate due to the presence of gas-pockets and its relative inaccessibility close to the intra-thoracic cavity. However, an ultrasonographic method has been developed to overcome these problems and it demonstrated a moderate day-to day variation and low intra and interobserver error[8]. This method has been applied to study accommodation of the proximal stomach in patients with functional dys pepsia and the effect of different drugs on the stomach[9-12].

In addition to ordinary B-mode imaging, the movements of gastroduodenal contents and velocity curves of transpyloric flow can be synchronously visualised by du plex ultrasound, that is combination of Doppler measurement and B-mode imaging[13-16]. By use of duplex scanning, it was revealed that, in the fed state, a short gush of duodenogastric reflux normally precedes the peristaltic clo sure of the pylorus[14].

One of the latest advances in ultrasound tech nology is three-dimensional (3-D) imaging. An early system for acquisition and processing of 3-D ultrasound data was developed in an attempt to enhance the accuracy of volume computation of th e distal stomach[17]. Using a motor device, the transducer was tilted th rough an angle of 90°. capturing sequential Two-dimensional frames before the data set was transferred to a graphic workstation for final 3-D processing. This 3-D ultrasound system was validated both in vitro and in vivo, and yielded high accuracy and precision in volume estimation of abdominal organs[18,19]. This 3-D scanning system was also used to evaluate patients with fun ctional dyspepsia[20-22]. Despite the significant achievements with resp ect to accuracy in volume estimation and 3-D reconstruction of tissue and organs, this 3-D system could only acquire a 90° fan-like data set from a pre-det ermined, single position of the transducer.

Random or free-hand acquisition of 3-D ultrasound data has been achieved by utilizing mechanical[23,24], acoustic[25-28], or electro magnetic[29,30] devices to locate the exact position and orientation of the tran sducer in space. To enable scanning of a large organ like the fluid-filled stom ach, a commercially available magnetometer-based position and orientation measurement (POM) device was chosen, which is relatively immune to metallic influence and electronic noise from the scanner. This system for magnetic scanhead tracking has been validated with respect to both its precision in locating specific points in space[30] and its accuracy in volume estimation[31,32]. In these studies, the sensor system worked well in scanning human organs, and high precision and accuracy were revealed in point loca-tion and volume estimation.

In one study, 14 male volunteers were examined with 3-D ultrasound after ingest ion of a 500 mL soup meal up to 35 min postcibally[33]. The average half-emptying time of this meal was 22.1 min ± 3.8 min. Intragastric distribut ion of the meal, expressed as proximal/distal volume, varied on average from 3.6 ± 2.1 (5 min postprandially) to 2.7 ± 1.9 (30 min postprandially). This 3-D ultrasound system using magnetic scanhead tracking demonstrated excellent in vitro accuracy, calculated gastric emptying rates more precisely than by two-d imensional ultrasound, and enabled estimation of intragastric distribution of a soup meal in healthy subjects. The same 3-D imaging system was also used to evaluate gastric emptying and duodenogastric reflux stroke volumes using a digital colour Doppler ima ging model[34].

In conclusion, ultrasonography is a reliable and safe method to assess gastric emptying in humans. Ordinary two-dimensional ultrasound imaging can be supplied with Doppler analysis and 3-D scanning to obtain a higher level of information on pathophysiology of the stomach.