MR measurements of mesenteric venous flow: prospective evaluation in healthy volunteers and patients with suspected chronic mesenteric ischemia

Imaging of Bowel Ischemia: An Update, From the AJR Special Series on Emergency Radiology

Acute mesenteric ischemia is a life-threatening condition that results from abrupt reduction in or cessation of blood flow to the bowel. Characterized by nonspecific abdominal symptoms, mesenteric ischemia is infrequently encountered and commonly misdiagnosed, with potentially catastrophic consequences. Prompt clinical diagnosis and early implementation of therapeutic interventions are critical to improving patient outcomes. Because cross-sectional imaging plays a key role in the diagnosis of mesenteric ischemia, radiologists must be familiar with the varied imaging manifestations of intestinal ischemia. Thus, the objectives of this article are to review the various types and common causes of mesenteric ischemia and to describe its spectrum of multimodality imaging findings, with special attention to novel imaging techniques and emerging diagnoses.

Non-invasive assessment of mesenteric hemodynamics in patients with suspected chronic mesenteric ischemia using 4D flow MRI

PURPOSE

Chronic mesenteric ischemia (CMI) is a rare disease with a particularly difficult diagnosis. In this study, 4D flow MRI is used to quantitatively evaluate mesenteric hemodynamics before and after a meal in patients suspected of having CMI and healthy individuals.

METHODS

Nineteen patients suspected of CMI and twenty control subjects were analyzed. Subjects were scanned using a radially undersampled 4D flow MR sequence (PC-VIPR). Flow rates were assessed in the supraceliac (SCAo) and infrarenal aorta, celiac artery, superior mesenteric artery (SMA), left and right renal arteries, superior mesenteric vein (SMV), splenic vein, and portal vein (PV) in a fasting state (preprandial) and 20 min after a 700-kcal meal (postprandial). Patients were subcategorized into positive diagnosis (CMI+, N = 6) and negative diagnosis (CMI-, N = 13) groups based on imaging and clinical findings. Preprandial, postprandial, and percent change in flow rates were compared between subgroups using a Welch t test.

RESULTS

In controls and CMI- patients, SCAo, SMA, SMV, and PV flow increased significantly after meal ingestion. No significant flow increases were observed in CMI+ patients. Percent changes in SMA, SMV, and PV flow were significantly greater in controls compared to CMI+ patients. Additionally, percent changes in flow in the SMV and PV were significantly greater in CMI- patients compared to CMI+ patients.

CONCLUSIONS

4D flow MRI with large volumetric coverage demonstrated significant differences in the redistribution of blood flow in SMA, SMV, and PV in CMI+ patients after a meal challenge. This approach may assist in the challenging diagnosis of CMI.

Colonoscopy-related colonic ischemia

Colonoscopy is a risk factor for colon ischemia. The colon is susceptible to ischemia due to its minor blood flow compared to other abdominal organs; the etiology of colon ischemia after colonoscopy is multifactorial. The causative mechanisms include splanchnic circulation impairment, bowel preparation, drugs used for sedation, bowel wall ischemia due to insufflation/barotrauma, and introduction of the endoscope. Gastroenterologists must be aware of this condition and its risk factors for risk minimization, early diagnosis, and proper treatment.

A pilot study investigating the use of 4D flow MRI for the assessment of splanchnic flow in patients suspected of mesenteric ischaemia

The most common cause of chronic mesenteric ischaemia is atherosclerosis which results in limitation of blood flow to the gastrointestinal tract. This pilot study aimed to evaluate 4D flow MRI as a potential tool for the analysis of blood flow changes post-prandial within the mesenteric vessels. The mesenteric vessels of twelve people were scanned; patients and healthy volunteers. A baseline MRI scan was performed after 6 h of fasting followed by a post-meal scan. Two 4D flow datasets were acquired, over the superior mesenteric artery (SMA) and the main portal venous vessels. Standard 2D time-resolved PC-MRI slices were also obtained across the aorta above the coeliac trunk, superior mesenteric vein, splenic vein and portal vein (PV). In the volunteer cohort there was a marked increase in blood flow post-meal within the PV (p = 0.028), not seen in the patient cohort (p = 0.116). Similarly, there were significant flow changes within the SMA of volunteers (p = 0.028) but not for the patient group (p = 0.116). Our pilot data has shown that there is a significant haemodynamic response to meal challenge in the PV and SMA in normal subjects compared to clinically apparent CMI patients. Therefore, the interrogation of mesenteric venous vessels exclusively is a feasible method to measure post-prandial flow changes in CMI patients.

Influence of Hemodialysis Catheter Insertion on Hemodynamics in the Central Veins

Central venous catheter (CVC) related thrombosis is a major cause of CVC dysfunction in patients under hemodialysis. The aim of our study was to investigate the impact of CVC insertion on hemodynamics in the central veins and to examine the changes in hemodynamic environments that may be related to thrombus formation due to the implantation of CVC. Patient-specific models of the central veins with and without CVC were reconstructed based on computed tomography images. Flow patterns in the veins were numerically simulated to obtain hemodynamic parameters such as time-averaged wall shear stress (TAWSS), oscillating shear index (OSI), relative residence time (RRT), and normalized transverse wall shear stress (transWSS) under pulsatile flow. The non-Newtonian effects of blood flow were also analyzed using the Casson model. The insertion of CVC caused significant changes in the hemodynamic environment in the central veins. A greater disturbance and increase of velocity were observed in the central veins after the insertion of CVC. As a result, TAWSS and transWSS were markedly increased, but most parts of OSI and RRT decreased. Newtonian assumption of blood flow would overestimate the increase in TAWSS after CVC insertion. High wall shear stress (WSS) and flow disturbance, especially the multidirectionality of the flow, induced by the CVC may be a key factor in initiating thrombosis after CVC insertion. Accordingly, approaches to decrease the flow disturbance during CVC insertion may help restrain the occurrence of thrombosis. More case studies with pre-operative and postoperative modeling and clinical follow-up need to be performed to verify these findings. Non-Newtonian blood flow assumption is recommended in computational fluid dynamics (CFD) simulations of veins with CVCs.

Abdominal Magnetic Resonance Angiography

MR imaging hardware and software improvements have led to new applications for contrast-enhanced and noncontrast-enhanced magnetic resonance angiography in the abdomen and pelvis. Higher magnetic field strength MR imaging scanners have greater signal-to-noise ratio and contrast-to-noise ratio, which is used to improve spatial resolution or temporal resolution for these techniques. New noncontrast-enhanced sequences offer high-resolution magnetic resonance angiography without contrast and provide additional hemodynamic information. Magnetic resonance angiography is particularly well suited to imaging patients with chronic mesenteric ischemia, renal vascular disease, pelvic congestion syndrome, and vascular malformations.

Incorporation of the Time-Varying Postprandial Increase in Splanchnic Blood Flow into a PBPK Model to Predict the Effect of Food on the Pharmacokinetics of Orally Administered High-Extraction Drugs

Following a meal, a transient increase in splanchnic blood flow occurs that can result in increased exposure to orally administered high-extraction drugs. Typically, physiologically based pharmacokinetic (PBPK) models have incorporated this increase in blood flow as a time-invariant fed/fasted ratio, but this approach is unable to explain the extent of increased drug exposure. A model for the time-varying increase in splanchnic blood flow following a moderate- to high-calorie meal (TV-Q _Splanch) was developed to describe the observed data for healthy individuals. This was integrated within a PBPK model and used to predict the contribution of increased splanchnic blood flow to the observed food effect for two orally administered high-extraction drugs, propranolol and ibrutinib. The model predicted geometric mean fed/fasted AUC and C _max ratios of 1.24 and 1.29 for propranolol, which were within the range of published values (within 1.0-1.8-fold of values from eight clinical studies). For ibrutinib, the predicted geometric mean fed/fasted AUC and C _max ratios were 2.0 and 1.84, respectively, which was within 1.1-fold of the reported fed/fasted AUC ratio but underestimated the reported C _max ratio by up to 1.9-fold. For both drugs, the interindividual variability in fed/fasted AUC and C _max ratios was underpredicted. This suggests that the postprandial change in splanchnic blood flow is a major mechanism of the food effect for propranolol and ibrutinib but is insufficient to fully explain the observations. The proposed model is anticipated to improve the prediction of food effect for high-extraction drugs, but should be considered with other mechanisms.

Caval Subtraction 2D Phase-Contrast MRI to Measure Total Liver and Hepatic Arterial Blood Flow: Proof-of-Principle, Correlation With Portal Hypertension Severity and Validation in Patients With Chronic Liver Disease

OBJECTIVES

Caval subtraction phase-contrast magnetic resonance imaging (PCMRI) noninvasive measurements of total liver blood flow (TLBF) and hepatic arterial (HA) flow have been validated in animal models and translated into normal volunteers, but not patients. This study aims to demonstrate its use in patients with liver cirrhosis, evaluate measurement consistency, correlate measurements with portal hypertension severity, and invasively validate TLBF measurements.

MATERIALS AND METHODS

Local research ethics committee approval was obtained. Twelve patients (mean, 50.8 ± 3.1 years; 10 men) with histologically confirmed cirrhosis were recruited prospectively, undergoing 2-dimensional PCMRI of the portal vein (PV) and the infrahepatic and suprahepatic inferior vena cava. Total liver blood flow and HA flow were estimated by subtracting infrahepatic from suprahepatic inferior vena cava flow and PV flow from estimated TLBF, respectively. Invasive hepatic venous pressure gradient (HVPG) and indocyanine green (ICG) clearance TLBF were measured within 7 days of PCMRI. Bland-Altman (BA) analysis of agreement, coefficients of variation, and Pearson correlation coefficients were calculated for comparisons with direct inflow PCMRI, HVPG, and ICG clearance.

RESULTS

The mean difference between caval subtraction TLBF and direct inflow PCMRI was 6.3 ± 4.2 mL/min/100 g (BA 95% limits of agreement, ±28.7 mL/min/100 g). Significant positive correlations were observed between HVPG and caval subtraction HA fraction (r = 0.780, P = 0.014), but not for HA flow (r = 0.625, P = 0.053), PV flow (r = 0.244, P = 0.469), or caval subtraction TLBF (r = 0.473, P = 0.141). Caval subtraction and ICG TLBF agreement was modest (mean difference, -32.6 ± 16.6 mL/min/100 g; BA 95% limits of agreement, ±79.7 mL/min/100 g), but coefficients of variation were not different (65.7% vs 48.1%, P = 0.28).

CONCLUSIONS

In this proof-of-principle study, caval subtraction PCMRI measurements are consistent with direct inflow PCMRI, correlate with portal hypertension severity, and demonstrate modest agreement with invasive TLBF measurements. Larger studies investigating the clinical role of TLBF and HA flow measurement in patients with liver disease are justified.

Functional testing in the diagnosis of chronic mesenteric ischemia

Chronic mesenteric ischemia (CMI) results from insufficient oxygen delivery or utilization to meet metabolic demand. Two main mechanisms may lead to mesenteric ischemia: occlusion in the arteries or veins of the gastrointestinal tract, or reduced blood flow from shock states or increased intra-abdominal pressure, so-called non-occlusive mesenteric ischemia. Severe stenoses in the three main mesenteric vessels as demonstrated with CT-angiography or MR-angiography are sufficient to proof mesenteric ischemia, for example in patients who present with weight loss, postprandial pain and diarrhea. Still in many clinical situations mesenteric ischemia is only one of many possible explanations. Especially in patients with a single vessel stenosis in the celiac artery or superior mesenteric artery with postprandial pain, mesenteric ischemia remains a diagnosis of probability or assumption without functional proof of actual ischemia. This review is aimed to provide an overview of all past, present and future ways to functionally proof CMI.

Diagnosis and treatment of chronic mesenteric ischemia: An update

Although the prevalence of mesenteric artery stenoses (MAS) is high, symptomatic chronic mesenteric ischemia (CMI) is rare. The collateral network in the mesenteric circulation, a remnant of the extensive embryonal vascular network, serves to prevent most cases of ischemia. This explains the high incidence of MAS and relative rarity of cases of CMI. The number of affected vessels is the major determinant in CMI development. Most subjects with single vessel mesenteric stenosis do not develop ischemic complaints. Our experience is that most subjects with CA and SMA stenoses with abdominal complaints have CMI. A special mention should be made on patients with median arcuate ligament compression (MALS). There is ongoing debate whether the intermittent compression, caused by respiration movement, can cause ischemic complaints. The arguments pro and con treatment of MALS will be discussed. The clinical presentation of CMI consists of postprandial pain, weight loss, and an adapted eating pattern caused by fear of eating. In end-stage disease more continuous pain, diarrhea or a dyspepsia-like presentation can be observed. Workup of patients suspected for CMI consists of three elements: the anamnesis, the vascular anatomy and proof of ischemia. The main modalities to establish mesenteric vessel patency are duplex ultrasound, CT angiography or MR angiography. Assessing actual ischemia is still challenging, with only tonometry and visual light spectroscopy as tested candidates. Treatment consists of limiting metabolic demand, treatment of the atherosclerotic process and endovascular or operative revascularisation. Metabolic demand can be reduced by using smaller and more frequent meals, proton pump inhibition. Treatment of the atherosclerotic process consists of cessation of smoking, treatment of dyslipidemia, hypertension, hyperglycaemia, and medication with trombocyte aggregation inhibitors.

Vascular imaging of the mesenteric vasculature

Imaging of the mesenteric vasculature is crucial in diagnosing vascular disease of the gastro-intestinal tract such as acute or chronic mesenteric ischemia caused by arterial stenosis, embolism or thrombosis, mesenteric vein thrombosis and mesenteric aneurysm or dissection. The reference standard for imaging of the mesenteric vasculature is digital subtraction angiography. However, modalities as duplex ultrasonography, computed tomography angiography and magnetic resonance angiography are developing rapidly and may provide accurate imaging non-invasively. This review provides an up-to-date overview of the anatomic resolution, clinical application, emerging techniques and future perspectives of these four radiological modalities for imaging of the mesenteric vasculature.

Anatomic and radiologic review of chronic mesenteric ischemia and its treatment

Chronic mesenteric ischemia (CMI) is a vascular occlusive disease process that generally affects the elderly population. Clinical presentation occurs when two of the three mesenteric arteries are affected and includes non-specific abdominal pain and weight loss. The most common cause of CMI is atherosclerotic arterial occlusion. The aim of this review is to present the vascular anatomy of the mesenteric arterial circulation including the different collateral pathways. The imaging findings and the different treatment options with a brief review of the literature is presented.

New Insights in Abdominal Pain in Paroxysmal Nocturnal Hemoglobinuria (PNH): A MRI Study

Introduction

Abdominal pain in PNH has never been investigated by in-vivo imaging studies. With MRI, we aimed to assess mesenteric vessels flow and small bowel wall perfusion to investigate the ischemic origin of abdominal pain.

Materials and Methods

Six PNH patients with (AP) and six without (NOP) abdominal pain underwent MRI. In a blinded fashion, mean flow (MF, quantity of blood moving through a vessel within a second, in mL·s^-1) and stroke volume (SV, volume of blood pumped out at each heart contraction, in mL) of Superior Mesenteric Vein (SMV) and Artery (SMA), areas under the curve at 60 (AUC₆₀) and 90 seconds (AUC₉₀) and K^trans were assessed by two operators.

Results

Mean total perfusion and flow parameters were lower in AP than in NOP group. AUC₆₀: 84.81 ± 11.75 vs. 131.73 ± 18.89 (P < 0.001); AUC₉₀: 102.33 ± 14.16 vs. 152.58 ± 22.70 (P < 0.001); K^trans: 0.0346 min^-1 ± 0.0019 vs. 0.0521 ± 0.0015 (P = 0.093 duodenum, 0.009 jejunum/ileum). SMV: MF 4.67 ml/s ± 0.85 vs. 8.32 ± 2.14 (P = 0.002); SV 3.85 ml ± 0.76 vs. 6.55 ± 1.57 (P = 0.02). SMA: MF 6.95 ± 2.61 vs. 11.2 ± 2.32 (P = 0.07); SV 6.52 ± 2.19 vs. 8.78 ± 1.63 (P = 0.07). We found a significant correlation between MF and SV of SMV and AUC₆₀ (MF:ρ = 0.88, P < 0.001; SV: ρ = 0.644, P = 0.024), AUC₉₀ (MF: ρ = 0.874, P < 0.001; SV:ρ = 0.774, P = 0.003) and K^trans (MF:ρ = 0.734, P = 0.007; SV:ρ = 0.581, P = 0.047).

Conclusions

Perfusion and flow MRI findings suggest that the impairment of small bowel blood supply is significantly associated with abdominal pain in PNH.

Quantitative liver MRI combining phase contrast imaging, elastography, and DWI: assessment of reproducibility and postprandial effect at 3.0 T

PURPOSE

To quantify short-term reproducibility (in fasting conditions) and postprandial changes after a meal in portal vein (PV) flow parameters measured with phase contrast (PC) imaging, liver diffusion parameters measured with multiple b value diffusion-weighted imaging (DWI) and liver stiffness (LS) measured with MR elastography (MRE) in healthy volunteers and patients with liver disease at 3.0 T.

MATERIALS AND METHODS

In this IRB-approved prospective study, 30 subjects (11 healthy volunteers and 19 liver disease patients; 23 males, 7 females; mean age 46.5 y) were enrolled. Imaging included 2D PC imaging, multiple b value DWI and MRE. Subjects were initially scanned twice in fasting state to assess short-term parameter reproducibility, and then scanned 20 min. after a liquid meal. PV flow/velocity, LS, liver true diffusion coefficient (D), pseudodiffusion coefficient (D*), perfusion fraction (PF) and apparent diffusion coefficient (ADC) were measured in fasting and postprandial conditions. Short-term reproducibility was assessed in fasting conditions by measuring coefficients of variation (CV) and Bland-Altman limits of agreement. Differences in MR metrics before and after caloric intake and between healthy volunteers and liver disease patients were assessed.

RESULTS

PV flow parameters, D, ADC and LS showed good to excellent short-term reproducibility in fasting state (CV <16%), while PF and D* showed acceptable and poor reproducibility (CV = 20.4% and 51.6%, respectively). PV flow parameters and LS were significantly higher (p<0.04) in postprandial state while liver diffusion parameters showed no significant change (p>0.2). LS was significantly higher in liver disease patients compared to healthy volunteers both in fasting and postprandial conditions (p<0.001). Changes in LS were significantly correlated with changes in PV flow (Spearman rho = 0.48, p = 0.013).

CONCLUSIONS

Caloric intake had no/minimal/large impact on diffusion/stiffness/portal vein flow, respectively. PC MRI and MRE but not DWI should be performed in controlled fasting state.

Association of splenic MR elastographic findings with gastroesophageal varices in patients with chronic liver disease

PURPOSE

To identify magnetic resonance imaging (MRI)-based parameters associated with gastroesophageal varices (GEVs) in patients with chronic liver disease.

MATERIALS AND METHODS

Ninety-three patients were divided into three groups based on endoscopic findings: group 1 with no GEVs (n = 49), group 2 with mild GEVs (n = 30), and group 3 with severe GEVs (n = 14). We used a multivariate logistic regression analysis to assess liver stiffness, aspartate aminotransferase-to-platelet ratio index, spleen stiffness and volume, portal vein velocity, cross-sectional area, and flow volumes potential independent associators of any (mild and severe) GEVs or severe GEVs.

RESULTS

The analysis showed that spleen and liver stiffness and spleen volume were independently associated with any GEVs (spleen stiffness, odds ratio [95% confidence interval], 1.25 [1.04-1.68], P = 0.018; liver stiffness, 1.52 [1.13-2.17], P = 0.006; spleen volume, 1.01 [1.00-1.01], P = 0.016), whereas spleen stiffness was associated with severe GEVs (1.82 [1.25-2.95]; P = 0.005).

CONCLUSION

Liver and spleen stiffness and spleen volume are associated with GEVs in patients with chronic liver disease. Compared with liver stiffness and spleen volume, spleen stiffness is more strongly associated with severe GEVs.

MR-based measurements of portal vein flow and liver stiffness for predicting gastroesophageal varices

OBJECTIVES

We evaluated flow parameters measured by phase-contrast magnetic resonance (MR) imaging (PC-MRI) of the portal venous system and liver stiffness measured by MR elastography (MRE) to determine the usefulness of these methods in predicting gastroesophageal varices (GEV) in patients with chronic liver disease (CLD).

METHODS

In patients with CLD and controls, we performed PC-MRI on the portal (PV) and superior mesenteric veins; calculated mean velocity (V, cm/s), cross-sectional area (S, mm²), and flow volume (Q, mL/min); and determined markers of liver fibrosis (liver stiffness [kPa]) and aspartate aminotransferase (AST) platelet ratio index [APRI]). We visually assessed GEV and development of collateral pathways of the PV on routine contrast-enhanced dynamic MR imaging and compared patient characteristics, flow parameters, liver stiffness markers, and visual analysis among 3GEV groups, those with mild, severe, or no GEV with reference to endoscopic findings.

RESULTS

Child-Pugh grade, VPV, SPV, liver stiffness, APRI, and visually identified GEV (visible GEV) differed significantly among the 3 groups (P<0.05). We investigated VPV, SPV, liver stiffness, and visible GEV as independent markers to distinguish patients with and without GEV and examined VPV and visible GEV to predict severe GEV. Visible GEV showed low sensitivity (14 to 30%) and high specificity (98%) for predicting GEV in patients with CLD. A subgroup analysis that excluded cases with collateral pathway demonstrated slightly improved diagnostic performance of VPV and liver stiffness.

CONCLUSIONS

Portal vein flow parameters and liver stiffness can be useful markers for predicting GEV in patients with CLD.

Effects of postprandial state and mesenteric blood flow on the repeatability of MR elastography in asymptomatic subjects

PURPOSE

To determine the repeatability of stiffness measurements in the liver using MR elastography (MRE) during the fasted and fed states. MRE has gained increased recognition as a noninvasive method to quantify fibrotic changes in the liver. It is well known that eating increases splanchnic blood flow, and fasting status of patients has been recognized as a factor that may affect hepatic stiffness measured with MRE.

MATERIALS AND METHODS

Hepatic MRE stiffness and flow through the superior mesenteric vein (SMV) were measured in 12 healthy subjects in fasted and fed states, and measurements were repeated 5 weeks later. A linear mixed effects model was used to estimate the sources of variability in the data, which included day (exams on different days) and subject. Sources were combined to calculate the overall standard deviation of a single MRE measurement.

RESULTS

The total within-subject standard deviation of an MRE exam is 8.5% (standard error [SE] = 1.7%) or 9.0% (SE = 1.8%) for fasted and fed states, respectively. No significant differences between fasted/fed state stiffness and no correlation between hepatic stiffness and SMV flow were observed.

CONCLUSION

As seen in this smaller population, healthy subjects scanned in a known fasted or fed state provide repeatable stiffness estimates with no relationship to SMV flow.

Acute mesenteric ischemia: diagnostic approach and surgical treatment

Mortality related to acute mesenteric arterial occlusion remains very high. Patient survival is dependent on prompt recognition and revascularization before ischemia progresses to intestinal gangrene. Biphasic computed tomography angiography has surpassed angiography as the diagnostic test of choice due to its ability to define the arterial anatomy and to evaluate secondary signs of mesenteric ischemia. Unlike chronic mesenteric ischemia, the treatment of acute arterial mesenteric ischemia, either embolic or thrombotic, remains largely surgical. This is due to the emergent need for revascularization combined with a careful evaluation of the intestines. Endovascular techniques remain useful, however, and can save precious time in the treatment of these challenging patients if integrated into a treatment pathway combined with definitive surgical treatment. A new hybrid endovascular-surgical treatment for the treatment of acute mesenteric thrombosis is described.

Abdominal angina

Abdominal angina is an underrecognized cause of postprandial abdominal pain and weight loss. Diagnosis is often delayed and requires both a careful exclusion of more common causes and a high degree of clinical suspicion, based on the patient's age, the coexistence of multiple risk factors for atherosclerosis, and the presence of vasculopathy in other districts. Appropriate investigations include duplex ultrasound, traditional angiography, magnetic resonance angiography, computed tomography angiography, and tonometry. The purpose of this review is to discuss the pathophysiology and clinical presentation of chronic mesenteric ischemia and to suggest a diagnostic flowchart for this complex condition.

Dynamic contrast-enhanced MR imaging of the liver: current status and future directions

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MR imaging) is emerging as a tool that can quantify changes in liver perfusion that occur in both diffuse and focal liver diseases. Recent data show promise for DCE-MR imaging of the liver in diagnosing fibrosis and cirrhosis before morphologic changes can be detected. It may also be valuable in the assessment of hepatocellular carcinoma and liver metastases. Acquisition parameters, postprocessing methods, applications, and recent results of DCE-MR imaging of the liver are also described. Finally, it reviews the limitations and future directions of DCE-MR imaging for liver applications.

The role of noninvasive vascular imaging in splanchnic and mesenteric pathology

Traditionally, catheter angiography (CA) has been the mainstay of diagnosis for mesenteric arterial diseases. However, CA is invasive and is associated with complications that result from the procedure itself, depending on the experience of the operators, site of vascular access, ionized radiation that could be significant when combined with interventional procedures, and administered contrast material. During the past 2 decades, technical improvements in computed tomography (CT) and magnetic resonance hardware and methods have contributed new, noninvasive tools, specifically CT angiography (CTA) and 3-dimensional gadolinium-enhanced magnetic resonance angiography (3D Gd-MRA). This article outlines the current applications, strengths, and weaknesses of CTA and 3D Gd-MRA in imaging of the mesenteric vessels.

Diagnosis and management of splanchnic ischemia

Splanchnic or gastrointestinal ischemia is rare and randomized studies are absent. This review focuses on new developments in clinical presentation, diagnostic approaches, and treatments. Splanchnic ischemia can be caused by occlusions of arteries or veins and by physiological vasoconstriction during low-flow states. The prevalence of significant splanchnic arterial stenoses is high, but it remains mostly asymptomatic due to abundant collateral circulation. This is known as chronic splanchnic disease (CSD). Chronic splanchnic syndrome (CSS) occurs when ischemic symptoms develop. Ischemic symptoms are characterized by postprandial pain, fear of eating and weight loss. CSS is diagnosed by a test for actual ischemia. Recently, gastro-intestinal tonometry has been validated as a diagnostic test to detect splanchnic ischemia and to guide treatment. In single-vessel CSD, the complication rate is very low, but some patients have ischemic complaints, and can be treated successfully. In multi-vessel stenoses, the complication rate is considerable, while most have CSS and treatment should be strongly considered. CT and MR-based angiographic reconstruction techniques have emerged as alternatives for digital subtraction angiography for imaging of splanchnic vessels. Duplex ultrasound is still the first choice for screening purposes. The strengths and weaknesses of each modality will be discussed. CSS may be treated by minimally invasive endoscopic treatment of the celiac axis compression syndrome, endovascular antegrade stenting, or laparotomy-assisted retrograde endovascular recanalization and stenting. The treatment plan is highly individualized and is mainly based on precise vessel anatomy, body weight, co-morbidity and severity of ischemia.

Dynamic measurements of total hepatic blood flow with Phase Contrast MRI

BACKGROUND/AIMS

To measure total hepatic blood flow including portal and proper hepatic artery flows as well as the temporal evolution of the vessel's section during a cardiac cycle.

METHODS

Twenty healthy subjects, with a mean age of 26 years, were explored. Magnetic resonance imaging blood flow measurements were carried out in the portal vein and the proper hepatic artery. MR studies were performed using a 1.5T imager (General Electric Medical Systems). Gradient-echo 2D Fast Cine Phase Contrast sequences were used with both cardiac and respiratory gatings. Data analysis was performed using a semi-automatic software built in our laboratory.

RESULTS

The total hepatic flow rate measured was 1.35+/-0.18L/min or 19.7+/-4.6mL/(minkg). The proper hepatic artery provided 19.1% of the total hepatic blood flow entering the liver. Those measurements were in agreement with earlier studies using direct measurements. Mean and maximum velocities were also assessed and a discrepancy between our values and the literature's Doppler data was found. Measurements of the portal vein area have shown a mean variation, defined as a "pulsatility" index of 18% over a cardiac cycle.

CONCLUSIONS

We report here proper hepatic artery blood flow rate measurements using MRI. Associated with portal flow measurements, we have shown the feasibility of total hepatic flowmetry using a non-invasive and harmless technique.

Advanced liver fibrosis: diagnosis with 3D whole-liver perfusion MR imaging--initial experience

Institutional review board approval and informed consent were obtained for this HIPAA-compliant study. The purpose of this study was to prospectively evaluate sensitivity and specificity of various estimated perfusion parameters at three-dimensional (3D) perfusion magnetic resonance (MR) imaging of the liver in the diagnosis of advanced liver fibrosis (stage >or= 3), with histologic analysis, liver function tests, or MR imaging as the reference standard. Whole-liver 3D perfusion MR imaging was performed in 27 patients (17 men, 10 women; mean age, 55 years) after dynamic injection of 8-10 mL of gadopentetate dimeglumine. The following estimated perfusion parameters were measured with a dual-input single-compartment model: absolute arterial blood flow (F(a)), absolute portal venous blood flow (F(p)), absolute total liver blood flow (F(t)) (F(t) = F(a) + F(p)), arterial fraction (ART), portal venous fraction (PV), distribution volume (DV), and mean transit time (MTT) of gadopentetate dimeglumine. Patients were assigned to two groups (those with fibrosis stage <or= 2 and those with fibrosis stage >or= 3), and the nonparametric Mann-Whitney test was used to compare F(a), F(p), F(t), ART, PV, DV, and MTT between groups. Receiver operating characteristic curve analysis was used to assess the utility of perfusion estimates as predictors of advanced liver fibrosis. There were significant differences for all perfusion MR imaging-estimated parameters except F(p) and F(t). There was an increase in F(a), ART, DV, and MTT and a decrease in PV in patients with advanced fibrosis compared with those without advanced fibrosis. DV had the best performance, with an area under the receiver operating characteristic curve of 0.824, a sensitivity of 76.9% (95% confidence interval: 46.2%, 94.7%), and a specificity of 78.5% (95% confidence interval: 49.2%, 95.1%) in the prediction of advanced fibrosis.

Acute and chronic mesenteric ischemia

This article briefly reviews the various etiologies, presentation, and diagnosis of different types of mesenteric ischemia. Operative management techniques and the applicability of percutaneous endovascular intervention are discussed. Finally, the authors explore emerging technologies that have the potential to further improve diagnosis and treatment of this frequently lethal disease process.

Review article: diagnosis and management of mesenteric ischaemia with an emphasis on pharmacotherapy

Mesenteric ischaemia results from decreased blood flow to the bowel, causing cellular injury from lack of oxygen and nutrients. Acute mesenteric ischaemia (AMI) is an uncommon disorder with high morbidity and mortality, but outcomes are improved with prompt recognition and aggressive treatment. Five subgroups of AMI have been identified, with superior mesenteric artery embolism (SMAE) the most common. Older age and cardiovascular disease are common risk factors for AMI, excepting acute mesenteric venous thrombosis (AMVT), which affects younger patients with hypercoaguable states. AMI is characterized by sudden onset of abdominal pain; a benign abdominal exam may be observed prior to bowel infarction. Conventional angiography and more recently, computed tomography angiography, are the cornerstones of diagnosis. Correction of predisposing conditions, volume resuscitation and antibiotic treatment are standard treatments for AMI, and surgery is mandated in the setting of peritoneal signs. Intra-arterial vasodilators are used routinely in the treatment of non-occlusive mesenteric ischaemia (NOMI) and also are advocated in the treatment of occlusive AMI to decrease associated vasospasm. Thrombolytics have been used on a limited basis to treat occlusive AMI. A variety of agents have been studied in animal models to treat reperfusion injury, which sometimes can be more harmful than ischaemic injury. Chronic mesenteric ischaemia (CMI) usually is caused by severe obstructive atherosclerotic disease of two or more splanchnic vessels, presents with post-prandial pain and weight loss, and is treated by either surgical revascularization or percutaneous angioplasty and stenting.

MR imaging of apparent small-bowel perfusion for diagnosing mesenteric ischemia: feasibility study

The purpose of this study, which was approved by the institutional review board, was to assess the differentiation of individuals with from those without mesenteric ischemia. All subjects provided written informed consent. Six healthy volunteers and six patients with documented chronic mesenteric ischemia underwent magnetic resonance (MR) imaging with and without oral caloric stimulation. After intravenous administration of paramagnetic contrast material, signal intensity values of the small-bowel wall were measured up to 130 seconds after contrast material injection. Volunteers and patients, respectively, had maximum enhancement of the bowel wall between 70 and 85 seconds after contrast material administration that amounted to 269% and 267% without and 425% and 333% with caloric stimulation. MR imaging assessment of small-bowel perfusion is possible and seems feasible for differentiating individuals with from those without mesenteric ischemia.

Parameters affecting bolus geometry in CTA: a review

CT angiography (CTA) is based on acquisition of data during the arterial phase of contrast material passage. CTA needs timing of the contrast bolus, which should be based on accurate knowledge of bolus geometry. Experimental and human studies on bolus geometry and bolus timing in CTA were reviewed. Important parameters of bolus geometry and methods of bolus timing (test bolus and bolus tracking) are described. Recommendations are given for an optimal CTA protocol.

MR angiography of the mesenteric vasculature

MRA has evolved from a research tool to a robust clinical diagnostic modality. In many centers worldwide, it is the technique of choice for evaluating patients with suspected CMI, assessing operability of patients with pancreatic cancer, and investigating the portal system. Evolving indications include the assessment of liver transplant patients before and after transplant and of living related liver transplant donors. The search for the bleeding source in patients with gastrointestinal hemorrhage may be an indication in the future, once intravascular contrast agents become available.

Noninvasive assessment of portomesenteric venous thrombosis: current concepts and imaging strategies

Rapid, noninvasive imaging strategies, especially multidetector spiral CT and CT angiography (CTA) as well as gadolinium-enhanced MR angiography (MRA), have facilitated early diagnosis of splanchnic venous thrombosis, a potentially lethal cause of intestinal ischemia. Single breath-hold volumetric acquisitions permit superior temporal and contrast resolution while eliminating motion artifact and suppressing respiratory misregistration. Increased spatial resolution is aided by thinner slice collimation. These cross-sectional imaging techniques are becoming a preferred noninvasive alternative to conventional selective mesenteric angiography with delayed imaging for venous evaluation and should be considered the primary diagnostic modalities for evaluating patients with high clinical suspicion of nonsurgical mesenteric ischemia.

Magnetic resonance imaging of inflammatory bowel disease: evaluation in a rabbit model

PURPOSE

To correlate intestinal contrast enhancement and wall thickening with the degree of inflammation in an experimental model of inflammatory bowel disease.

MATERIALS AND METHODS

Inflammatory bowel disease was elicited in 39 New Zealand White rabbits by rectal instillation of 2,4,6-trinitrobenzene sulphonic acid (TNBA). Magnetic resonance imaging (MRI) was used to determine bowel wall thickness and intestinal contrast enhancement after the administration of 0.1 mmol/kg of gadodiamide intravenously. MR measurements were compared with the complete histopathologic analysis.

RESULTS

MR measurements of bowel wall thickness correlated well with histopathologic measurements in vitro (r = 0.85, P < 0.0001) and with histopathologic evidence of chronic inflammatory bowel disease (P < 0.02). Chronic inflammation was characterized by increased intestinal contrast enhancement (137 +/- 25%) when compared to normal bowel (86 +/- 7%, P = 0.04).

CONCLUSION

Contrast-enhanced MRI accurately reflects inflammatory bowel disease in the rabbit model.

Ischemic bowel disease in the elderly

The ischemic bowel diseases are a heterogeneous group of disorders usually seen in elderly individuals. They represent ischemic damage to different portions [figure: see text] of the bowel and produce a variety of clinical syndromes and outcomes. Colonic ischemia is the commonest of these disorders and has a favorable prognosis in most cases. In contrast, acute mesenteric ischemia, most commonly caused by a superior mesenteric artery embolus, is a disease with a poor prognosis. Acute mesenteric ischemia secondary to nonocclusive mesenteric ischemia usually is a [figure: see text] catastrophic complication of other severe medical illnesses, most notably atherosclerosis. Proper diagnosis and management of patients with ischemic bowel disease requires vigilance on the part of the physician and a willingness to embark on an aggressive plan of diagnosis and management in the appropriate setting.

Mesenteric ischemia

Mesenteric ischemia is an important clinical condition becoming more prevalent with aging of the population. Mesenteric ischemia may be manifest in an acute presentation, usually secondary to thromboembolism or cardiac insufficiency. Patients have abdominal pain, lactic acidosis, benign abdominal examination, and, often, coexistent multisystem organ dysfunction. Chronic mesenteric ischemia is secondary to proximal arterial stenosis or occlusions inadequately compensated by collateral flow. Clinical presentation may simulate occult malignancy. In this review article, the role of Doppler sonography and other diagnostic imaging tests in suspected acute mesenteric ischemia and mesenteric arterial insufficiency are evaluated with emphasis on diagnostic criteria and appropriate use in each clinical context.

Chronic splanchnic ischaemia

Chronic splanchnic ischaemia is a relatively unusual clinical entity consisting of pain and/or weight loss and caused by chronic splanchnic disease (i.e. stenosis and/or occlusion of the coeliac and superior mesenteric artery). The occlusive disease is usually caused by atherosclerosis and is in itself not rare in older individuals. Extensive collateral circulation can develop between the three splanchnic arteries and may compensate for the decreased splanchnic perfusion over time. The pathophysiology of chronic splanchnic ischaemia has still not been completely elucidated.A reliable diagnosis of chronic splanchnic ischaemia, based on a proven causal relationship between the occlusive disease and the symptoms, can be very difficult. Traditionally, tests for evaluating the haemodynamic consequences of the vascular stenoses were not available. Important improvements in establishing a more reliable diagnosis have been achieved with duplex ultrasound and magnetic resonance evaluation of the splanchnic circulation. Tonometry is another promising functional test that may prove useful not only for gaining greater insight into the pathophysiology of chronic splanchnic ischaemia but also for the clinical evaluation of this syndrome. The natural history of chronic splanchnic disease suggests that progressive disease may result in acute mesenteric ischaemia. Surgical reconstruction of the coeliac and/or the superior mesenteric artery is the therapeutic standard with excellent short and long-term results. Satisfactory early results using angioplasty with or without stent suggest that this type of intervention may relieve symptoms in selected patients with a higher surgical risk.

AGA technical review on intestinal ischemia. American Gastrointestinal Association

This literature review and the recommendations therein were prepared for the American Gastroenterological Association Clinical Practice and Practice Economics Committee. The paper was approved by the committee on September 25, 1999, and by the AGA Governing Board on November 25, 1999.

Contrast-enhanced three-dimensional magnetic resonance angiography of the mesenteric vasculature

The diagnosis and treatment of mesenteric occlusive disease is important due to the high morbidity and mortality associated with ischemia of the bowel. This article describes the application of magnetic resonance angiography (MRA) as a minimally invasive tool for diagnosing mesenteric and portal vascular disease. The techniques for three-dimensional (3D) contrast-enhanced MRA and flow measurement of the mesenteric circulation are described. Excellent image quality is obtained using sagittal contrast-enhanced 3D MRA of the aorta and proximal mesenteric vessels. Delineation of the small distal mesenteric branch vessels is still limited due to the finite spatial resolution of MRA. The application of MRA techniques to the diagnosis of mesenteric ischemia, revascularization, transplantation, and portal hypertension are demonstrated. Finally, the merits of various acquisition techniques and future contrast agents are discussed. MR angiography using intravenously administered contrast agents provides the clinician with a powerful, minimally invasive method for diagnosing mesenteric vascular disease.

Contrast-enhanced three-dimensional magnetic resonance angiography of the splanchnic vasculature before and after caloric stimulation. Original investigation

RATIONALE AND OBJECTIVES

To develop a comprehensive noninvasive magnetic resonance angiography (MRA) strategy for the morphologic and functional assessment of the splanchnic arteries, based on a combination of breath-held contrast-enhanced 3D MRA and segmented k-space 2D phase-contrast acquisitions acquired before and after caloric stimulation.

METHODS

Ten healthy volunteers were examined twice: once in the fasting state (6 hours with no food intake) and a second time following caloric stimulation with a standard 475-kcal meal. Flow in the superior mesenteric artery (SMA) and vein (SMV) was quantitated using a 2D breath-held, segmented k-space phase-contrast (PC) acquisition in a plane perpendicular to the axis of the vessels, while vascular morphology was displayed with a contrast-enhanced 3D MRA acquisition consisting of 44 contiguous 2-mm sections, acquired in apnea (28 seconds). For comparative analysis, the splanchnic vasculature was divided into 11 segments and evaluated on a 2-point scale (cannot exclude pathology, can exclude pathology).

RESULTS

Flow volume in the SMA increased from 2.3 ml/min/kg (+/- 0.9 ml/min kg) to 7.3 ml/min kg (+/- 4.7 ml/min kg) following caloric stimulation (P < 0.05). Flow in the SMV exceeded flow in the SMA and increased from 3.4 ml/min/kg (+/- 0.3 ml/min kg) to 9.1 ml/min/kg (+/- 4.8 ml/min/kg) following stimulation. Flow volume of SMV correlated better with SMA flow after stimulation. Caloric stimulation significantly improved visualization of the splanchnic arterial vasculature (P < 0.05). Only 5 of 110 evaluated arterial segments (4.5%) remained inadequately seen to exclude vascular pathology.

CONCLUSION

Magnetic resonance imaging offers a comprehensive assessment of the splanchnic arterial vasculature based on 3D display of vessel morphology and analysis of flow function. While the most relevant proximal vessel segments are visible even under fasting conditions, caloric stimulation enhances visualization of small vessels.

Magnetic resonance angiography of mesenteric arteries. A review

There has been continued development of MRI techniques for evaluating mesenteric vascular disease. Contrast-enhanced magnetic resonance angiography (MRA) can provide reproducible high resolution, high contrast images of the arterial and venous mesenteric vasculature and may allow detection of segmental ischemia by detection of segmental delayed mesenteric or bowel wall enhancement. Cine phase-contrast MRA can provide additional information about the rate and volume of flow within the major mesenteric arteries and veins. Real-time MRI can provide interactive visualization of the mesenteric vessels in any plane, and with suitable bowel contrast, it can be used to monitor global and segmental small bowel motility. With in vivo MR oximetry, flow independent measurements of the T2 relaxation of blood allow the oxygen saturation of the mesenteric circulation to be determined. These MR techniques can be combined for evaluating both anatomic and functional aspects of the mesenteric circulation.