Advances in Gastrointestinal Imaging using Ultrasound (Part 2)

Elastic imaging is a basic method of elasticity measurement, caused by tissue deformation under external force. There are three types of elastic imaging: transient elastography, strain elastography, and shear wave elastography (SWE) (Figure 3). Transient elastography relies on mechanical forces applied directly to the skin surface. Although widely used to assess hepatic fibrosis, it lacks direct visualization, and therefore its reliability is lower in the presence of ascites and obesity. However, transient elastography has a fast learning curve and is easy to perform. Strain elastography relies on slight oscillating manual compression, while SWE uses acoustic pulses from a transducer. In each case, a series of measurements is required, with recent literature suggesting that fewer than 10 typical measurements may be sufficient. However, the lack of standardization between manufacturers makes comparisons between manufacturers difficult to interpret.

Elastic imaging is primarily used to assess the degree of fibrosis and the presence of cirrhosis in chronic liver disease. While a useful tool, cutoff values are difficult to determine, and there is significant overlap between grades of fibrosis. Furthermore, significant standardization is needed, including fasting, breath-holding, and depth of measurement. Based on the principle that malignant tissue is difficult to palpate, elastic imaging has been proposed for the assessment of focal liver lesions, but overlap with benign lesions (such as focal nodular hyperplasia, cavernous hemangioma) remains. Therefore, elastic imaging is best used as part of a multiparametric ultrasound assessment.

Splenic ultrasound has been proposed for the assessment of portal hypertension and the risk of variceal bleeding and is frequently used in conjunction with hepatic elastography. Regardless of the technique used, splenic stiffness appears to be closely correlated with portal hypertension, but relies on an adequate B-mode acoustic window for assessment. Limited data on pancreatic elastography suggests that differentiation and grading of chronic pancreatitis may be possible. However, because of the usual requirement for depth and the presence of bowel gas, maximal benefit can be obtained when combined with endoscopy.

In the critical care setting, elastic imaging is currently limited to the assessment of chronic liver disease and as an adjunct to multiparametric ultrasound imaging. However, research is evolving in utilizing this technology to assess interstitial lung disease, a condition that may be more relevant to general critical care.

Ultrasound measurement of gastric antrum cross-sectional area (CSA) is feasible in critically ill patients and directly visualizes gastric contents. On average, a CSA greater than 15–25 cm2 corresponds to a gastric residual volume (GRV) greater than 300 ml.

Normal gastric wall anatomy is divided into five layers (Figure 2):

  (1) Serosa (hyperechoic)

  (2) Muscularis propria (hypoechoic)

  (3) Submucosa (hyperechoic)

  (4) Muscularis mucosae (hypoechoic)

  (5) Mucosa (hyperechoic)

Features of the bowel that can be assessed using ultrasound include:

  (1) Wall thickness

  (2) Diameter and intraluminal contents

  (3) Peristalsis

  (4) Vessels

 Bowel diameter and contents vary according to location, fasting/fed state, and bowel function. In adults, the normal small bowel diameter is less than 30 mm, and the normal large bowel diameter is less than 60 mm. Dilated loops of bowel may show wall thickening (up to 3 mm normally in the small bowel) or valvular thickening (up to 2 mm normally in the large bowel). Exceptions include the duodenal bulb and rectum, where the thickness is less than 3 mm and 4 mm, respectively.

 Assessment of bowel peristalsis is subjective and may be technically difficult but may provide useful information in some circumstances. Increased small bowel peristalsis has been described in abdominal diseases and acute mechanical bowel obstruction. In later stages of obstruction, the lumen may be filled with fluid, the bowel wall thinned and spastic, and there is decreased or absent extra-luminal fluid and peristalsis.

Doppler and Color Doppler Techniques

Doppler imaging is used to assess visceral vascular signals supplying the kidneys, gastrointestinal tract, and smaller intramural vessels. Normal bowel wall perfusion cannot be shown by color or power Doppler, so bowel wall flow is displayed as pathological perfusion (such as hyperemia in segments with active inflammation).

 Common measurements that can be performed include

  (1) Systolic, diastolic, and mean velocities

  (2) Pulsatility index (systolic peak velocity – diastolic minimum velocity)/(mean velocity)

  (3) Resistance index (systolic peak velocity – end-diastolic velocity)/systolic peak velocity)

  (4) Blood flow

 Gastrointestinal Blood Flow

Doppler imaging can detect stenosis, embolism, and thrombosis of the visible portions of the abdominal aorta, superior mesenteric artery (SMA), and inferior mesenteric artery (IMA).

 Systolic velocities greater than 250-300 cm/s are sensitive indicators of severe mesenteric artery stenosis. Spectral analysis of Doppler signals from the splanchnic arteries and veins can be used to estimate intestinal perfusion. Both transverse and longitudinal assessments should be performed. Low flow states are due to the presence of spontaneous contrast and turbulence in large vessels.

 Hepatic Blood Flow

Normal portal venous flow in the main portal vein (MPV) is characterized by a slow undulating waveform with peak systolic velocities between 20-40 cm/s. Low velocities of less than 16 cm/s as well as MPV diameter are diagnostic features of portal hypertension. Further deterioration in portal hypertension leads to a reciprocating flow pattern, with nearly stagnant blood columns in the portal vein moving in and out of the liver with the respiratory cycle. In advanced stages, stagnation of the column can lead to thrombosis or progress to flow reversal or negative portal venous flow.

 Normal hepatic venous flow is triphasic, with two phases related to atrioventricular diastole. Fibrotic or inflammatory changes can produce a monophasic flow pattern. Early changes in waveforms in cirrhosis include widening and attenuation of the normal spectrum, with a reversed presystolic waveform in the hepatic vein. Subsequently, the normal triphasic waveform may be attenuated or replaced by a monophasic waveform, indicating relatively higher portal pressure.

 Renal Blood Flow

Similarly, Doppler techniques can be used to assess renal perfusion. A normal resistance index is approximately 0.58 ± 0.10, with values above 0.70 considered abnormal. Renal Doppler resistance index can detect early renal dysfunction or predict short-term reversibility of acute kidney injury (AKI) in critically ill patients. A recent meta-analysis showed that the resistance index may be a predictor of persistent AKI in critically ill patients, with sensitivity and specificity of 0.83 (95% CI, 0.77-0.88) and 0.84 (95% CI, 0.79-0.88), respectively.

 Elevated renal resistance index is an independent predictor of worsening cardiovascular and renal prognosis, especially when glomerular filtration rate (GFR) is reduced, thus providing a useful diagnostic adjunct for renal function assessment in these patients. High renal resistance index is also associated with hypertension and atherosclerotic organ damage. Values greater than 0.80 have been reported to predict all-cause mortality in CKD patients.

Multiparametric Approach

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Gastrointestinal Function

  (1) Function: Peristalsis, bowel motility, bowel obstruction

  (2) Size: Luminal dilatation, bowel wall edema

  (3) Content: In-luminal content, free fluid

  (4) Perfusion: Local, organ-specific blood flow

Combining anatomical, functional, and blood flow assessments provides a detailed, holistic view of the gastrointestinal tract.

A structured and stepwise approach, graded according to the European consensus definition (Table 2), may provide an improved, practical management approach for adult critically ill patients with acute gastrointestinal injury (AGI).

 

Hemodynamics

Hemodynamics and fluid management (along with oxygen therapy) are cornerstones of critical care. The use of ultrasound to assess cardiac and pulmonary function is becoming standard practice. Our understanding of the hemodynamics and therapeutic strategies in critical illness has been further refined with the appreciation of the importance of stressed versus unstressed capacity and the role of the venous circulation. Previously relying solely on central venous pressure, assessment of the venous system has experienced a renaissance with the introduction of ultrasound technology.

 Assessment of venous congestion using CCUS can be incorporated as part of the assessment of volume status. This is particularly important in patients with septic shock, congestive heart failure, and acute kidney injury as it provides additional data points to support clinical decisions regarding initiation/cessation of fluids, diuretics, or vasopressor selection.

The VExUS ultrasound score is a relatively novel multimodal approach that assesses evidence of venous congestion in the liver, gut, and kidneys to help predict early signs of organ end-organ damage and optimize fluid management strategies (Figure 5).

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