Introduction
Atherosclerosis commonly involves the mesenteric vessels, reported in up to 20% of patients 65 years or older {Wilson, 2006 #57}. Mesenteric Ischemia occurs when perfusion to the visceral organs doesn’t meet its metabolic demands. It is categorized into two clinical scenarios: acute or chronic mesenteric ischemia. Acute mesenteric ischemia is a life-threatening medical condition that requires emergent restoration of perfusion so ultrasound has a limited role in diagnosis.
Chronic mesenteric ischemia (CMI), on the other hand, is typically an indolent disease characterized by post-prandial abdominal pain and progressive weight loss. The pain often occurs 15-30 minutes after a meal, when the organs supplied by the mesenteric vessels have increased their metabolic needs for digestion. Atherosclerosis of these vessels limits the compensatory increase in perfusion to the visceral organs, resulting in visceral ischemia. The patient experiences abdominal pain as a result, termed intestinal angina. Intestinal angina leads to weight loss as patients begin to fear food as a result of the predictable abdominal pain.
CMI is one of many causes of abdominal pain and as a result may be clinically difficult to diagnose by history and physical exam alone. Angiography remains the “gold standard” for diagnosis of CMI, however it is invasive and time consuming. CTA and MRA may be helpful, but are limited by the extensive calcifications seen in these calcified lesions “spilling over” from the aorta to the visceral orifices. As a result, mesenteric duplex scanning has great appeal as a non-invasive method of diagnosing patients with possible CMI. The appeal of mesenteric duplex scanning is balanced however by the technical challenge and complexity of this examination. To best apply this testing modality, a thorough understanding of mesenteric anatomy, physiology and application and interpretation of the examination is critical.
Normal Anatomy and Physiology
The mesenteric vessels include the celiac artery (CA), superior mesenteric artery (SMA) and the inferior mesenteric artery (IMA). The celiac artery supplies blood to the solid visceral organs (liver, pancreas and spleen) as well as the stomach and proximal small intestine. The SMA supplies a majority of the small intestine and over half of the large intestine. The IMA supplies the distal large intestine and rectum. There is a rich collateral network between the mesenteric vessels. As a result, typically at least two of the three vessels must be diseased (>70% stenosis) to cause CMI symptoms.
The mesenteric vessels are unique vascular beds because of their ability to be either low or high-resistance vascular beds depending on their metabolic needs. The body has a variety of autoregulatory mechanisms that mediate vasoconstriction or vasodilation depending on a fasting or post-prandial state. This autoregulation leads to significant changes in the mesenteric blood flow, from 10% of the cardiac output during fasting to 35% after a meal {Rosenblum, 1997 #58}. As a result, at rest or during fasting, duplex examination of the SMA reveals a high resistance waveform with a brisk systolic upstroke and little diastolic flow (Figure 1).
Figure 1: SMA waveform in a fasting state.
After a meal, the vessels to the visceral organs dilate and decrease their resistance, increasing flow to the visceral organs. The SMA now exhibits a low-resistance waveform with increased diastolic flow after a meal (Figure 2).
Figure 2: Waveform differences seen the SMA before and after a meal.
Because the celiac artery provides perfusion to the liver, which is a large, low-resistance organ that necessitates higher diastolic flow at rest than the SMA, spectral waveforms in the CA show less variation after a meal. Unique to the celiac artery is the relationship with the diaphragm and the ligament between both sides of the diaphragm, called the median arcuate ligament. During normal expiration the median arcuate ligament moves downward compressing the origin of the celiac artery. This may lead to changes seen in the celiac artery between inspiration or expiration phases of breathing (Figure 3a & 3b).
Figure 3: Celiac artery Doppler, shown during normal respiration (top) and during deep inspiration (bottom).
Examination Technique
Evaluation of the visceral vessels begins by examining the proximal abdominal aorta and then systematically evaluating the mesenteric vessels, focusing on the ostium and proximal portion of the vessels. The distal portion of the mesenteric vessels is difficult to identify and has little clinical relevance as most atherosclerosis occurs in the proximal portion of the vessel in a majority of cases.
Examination of the mesenteric vessels is often technically challenging for several reasons. Bowel gas is a major limitation of this study and can lead to significant scatter or attenuation of the Doppler signal. Further, the hemodynamics of the visceral vessels are dynamic and vary depending on fasting or non-fasting state. To minimize this effect, patients should fast at least 12 hours prior to the study, decreasing these limitations and standardizing the interpretation of velocities. The examination is begun in the supine position with a curvilinear low frequency (2-5 mHz) transducer because of the depth needed to visualize the origins of the vessels. If the visualization is difficult, due to bowel gas or patient’s body habitus, laying the patient in different positions such as decubitus or oblique positions can be helpful. Further, utilizing the liver or the spleen as the acoustic window can minimize the scatter from bowel gas.
The aorta is identified below the xiphoid process and anterior to the spine on the left side. A diameter and velocity are identified in both the transverse and sagittal planes to identify aneurysmal degeneration or identify problems with inflow (dissection, heart failure or proximal atherosclerosis). Visualization of the celiac artery is best found in the sagittal plane and a transverse view classically displays the “seagull sign” or the T-shaped bifurcation of the proximal celiac branches (Figure 4).
Figure 4: Celiac artery in the transverse view showing the “seagull sign”.
The SMA is also found in the sagittal view as a parallel structure to the aorta near its origin (Figure 5).
Figure 5: Celiac and SMA shown on B-mode imaging in the sagittal view.
In the transverse plane, the SMA is surrounded by soft tissue, separating it from the pancreas. Following the aorta caudally, the left renal vein is found crossing over the aorta at the level of the renal arteries. The IMA is found just beyond the renal arteries, in a similar fashion to the SMA. Vascular anomalies are common and can make the examination more difficult. The most common anomaly is aberrant branches of the celiac artery arising directly from the aorta.
As with all vascular ultrasound studies, optimization of the grey-scale and color Doppler is essential to proper imaging and correct interpretation. The color Doppler gain, pulse repetition frequency and wall filter need to be adjusted for every patient so that laminar flow in a normal vessel shows a homogenous color flow pattern. Pulsed Doppler waveforms must be obtained with a small sample volume (1.5-3mm) to ensure the velocity is from the vessel of interest. The Doppler angle should not exceed 60 degrees, as it can overestimate the velocities. This cannot be overstated, as an increase from 60 to 80 degrees produces a 120% false increase in the velocity (Rizzo et al.). The Intersocietal Accreditation Commission-Vascular Testing (IAC) has specific standards for performance of mesenteric artery duplex scanning. (Table 1).
Table 1: IAC Mesenteric arterial system recommendations (IAC reference).
1- Greyscale and/or color Doppler images must be documented and must include at a minimum
- Adjacent aorta to celiac or SMA;
- Celiac artery;
- Superior mesenteric artery;
- Inferior mesenteric artery.
2- Spectral Doppler waveforms and velocity measurements must be documented and must include at a minimum:
- Adjacent aorta;
- Celiac artery origin;
- Hepatic artery (does not require velocity measurements);
- Superior mesenteric artery origin;
- Proximal superior mesenteric artery (beyond the origin);
- Inferior mesenteric artery.
Diagnostic Criteria
Interpretation of mesenteric duplexes is aimed at identifying hemodynamically significant lesions. Decreased pressure and/or flow is seen beyond occlusive lesions of greater than 70%. At the site of the stenosis, there is elevation in the peak systolic and end-diastolic velocities. Immediately distal to the stenosis there is turbulent flow and eventually a dampened waveform. There have been many proposed criteria for evaluating the clinical significance of Doppler findings on mesenteric vessels. There are no true consensus criteria however the original description by Moneta et al. is the most widely accepted (Moneta). A greater than 70% stenosis on angiogram was correlated with a peak systolic velocity (PSV) of greater than 200 cm/sec in the celiac and 275 cm/sec in the SMA (Figure 6).
Figure 6: SMA Doppler with evidence of greater than 70% stenosis.
These criteria have since been validated by other institutions (Zwolek) and a prospective study (Moneta). Further research identified an end-diastolic velocity (EDV) of 45 cm/sec in the SMA or 55cm/sec in the celiac artery as the most sensitive predictor of a hemodynamic stenosis (>50% diameter reduction)(Lim) (Table 2). There are no established criteria for the IMA, due to the “two-vessel rule” highlighted earlier. However, visualization of patency and flow still remains a critical portion of a mesenteric vascular examination. It is often valuable to include a velocity ratio for each vessel, much like is done for other vascular beds. This is done by performing a ratio of the PSV of the vessel of interest by the PSV of the adjacent aorta (Mesenteric to Aortic Ratio, MAR). In general a MAR of around 1.0 is considered normal while a MAR of greater than 3.0 is considered significant (Patel). The MAR is particularly helpful in patients with either high baseline velocities (Hypertensive patients) or low baseline velocities (Congestive heart failure). Interpretation can also be complicated due to the rich collateral network of the mesenteric vessels. For example, in a patient with an occluded or high-grade stenosis of the celiac will have compensatory increased flow through their SMA. This may lead to over-estimation of mesenteric disease so clinical correlation is critical.
Table 2: Mesenteric velocity criteria
Celiac Artery
- >50% stenosis = end-diastolic velocity > 55 cm/sec
- >70% stenosis = peak systolic velocity > 200 cm/sec
Superior Mesenteric Artery
- >50% stenosis = end-diastolic velocity > 45 cm/sec
- >70% stenosis = peak systolic velocity > 275 cm/sec
*Performed in fasting patients(Moneta 2)
Conclusion
Duplex canning is a valuable tool in the evaluation of patients for mesenteric ischemia. When performed and interpreted properly, a mesenteric ultrasound can accurately diagnose mesenteric occlusive disease and aid in the treatment.
References
Rizzo RJ, et al. Mesenteric flow velocity variation as a function of angle of insonation. J VASC SURG 11:688:694, 1991.
IAC Standards and Guidelines for Vascular Testing Accreditation. w.... visited 6.5.2015.
Moneta GL, et al. Duplex ultrasound criteria for diagnosis of splanchnic artery stenosis or occlusion. J Vasc Surg. 14: 511-520, 1991.
Moneta GL et al. Mesenteric duplex scanning: A blinded prospective study. J Vasc Surg. 17:79-86, 1993.
Lim HK, et al. Splanchinic arterial stenosis or occlusion: Diagnosis at Doppler US, Radiology 211:405-410, 1999.
Zwolak RM, et al. Mesenteric and celiac duplex scanning: A validation study. J VASC Surg 26:288-293, 1997.
Pellerito JS et al. Doppler sonographic criteria for the diagnosis of inferior mesenteric artery stenosis. J Ultraound Med 28:641-650, 2009.
