Temporal assessment of pancreatic blood flow and perfusion following secretin stimulation using noninvasive MRI

Update on state-of-the-art for arterial spin labeling (ASL) human perfusion imaging outside of the brain

This review article provides an overview of developments for arterial spin labeling (ASL) perfusion imaging in the body (i.e., outside of the brain). It is part of a series of review/recommendation papers from the International Society for Magnetic Resonance in Medicine (ISMRM) Perfusion Study Group. In this review, we focus on specific challenges and developments tailored for ASL in a variety of body locations. After presenting common challenges, organ-specific reviews of challenges and developments are presented, including kidneys, lungs, heart (myocardium), placenta, eye (retina), liver, pancreas, and muscle, which are regions that have seen the most developments outside of the brain. Summaries and recommendations of acquisition parameters (when appropriate) are provided for each organ. We then explore the possibilities for wider adoption of body ASL based on large standardization efforts, as well as the potential opportunities based on recent advances in high/low-field systems and machine-learning. This review seeks to provide an overview of the current state-of-the-art of ASL for applications in the body, highlighting ongoing challenges and solutions that aim to enable more widespread use of the technique in clinical practice.

Pancreatic perfusion modulation following glucose stimulation assessed by noninvasive arterial spin labeling (ASL) MRI

BACKGROUND

More than 100 million adults in the US suffer from prediabetes or type-2 diabetes. Noninvasive imaging of pancreas endocrine function might provide a surrogate marker of β-cell functional integrity loss linked to this disease.

PURPOSE

To noninvasively assess pancreatic blood-flow modulation following a glucose challenge using arterial spin labeling (ASL) MRI.

STUDY TYPE

Prospective.

SUBJECTS

Fourteen adults (30 ± 7 years old, 3M/11F, body mass index [BMI] = 24 ± 3 kg.m^-2 ).

FIELD STRENGTH/SEQUENCE

3T MRI / background-suppressed pseudocontinuous PCASL preparation with single-shot fast-spin-echo (FSE) readout before and after an oral glucose challenge using either fruit juice (n = 7) or over-the-counter glucose gel (n = 7).

ASSESSMENT

Subjects were fasting prior to initiation of oral stimulation, then dynamic perfusion measurements were performed every 2 minutes for 30 minutes. We quantified absolute blood flow at each timepoint.

STATISTICAL TESTS

Repeated-measures analysis of variance (ANOVA) followed by paired t-tests to assess for a significant effect of glucose challenge on measured perfusion.

RESULTS

Measured basal blood flow was 187 ± 53 mL/100g/min. A significant blood flow increase of +38 ± 26% was observed 10 minutes poststimulation (P < 0.05) and continuing until the end of the experiment. The gel stimulation provided the most consistent results, with an early rise followed by an additional later increase consistent with the known pancreatic insulin response to elevated blood glucose. Across-subject variations in blood flow increase were partially attributable to basal flow, with a negative correlation of r = -0.84 between basal and maximal relative flow increase in the gel group.

DATA CONCLUSION

ASL can be used to measure pancreatic flow in response to a glucose challenge, which could be linked to insulin release and secretion. This paradigm might be useful to characterize disorders of glucose regulation.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2020;51:854-860.

CT and MRI assessment of intestinal blood flow

The accuracy of multi-slice computed tomography (CT) in the diagnosis of acute mesenteric ischemia is very high, however, it cannot demonstrate the small embolus of blood vessels and abnormal intestinal blood flow. The intestinal blood flow in chronic mesenteric ischemia decreases whereas there are few morphology changes, which leads to a high misdiagnosis rate of CT and CT angiography. In addition, inflammatory bowel disease, intestinal tumors, and portal hypertension can be diagnosed definitely by conventional CT, but the hemodynamics and microcirculation in these conditions cannot be assessed, which affects the accuracy of clinical staging and the assessment of therapeutic effect. For intestinal diseases, especially mesenteric ischemia, therefore, it is needed not only to make CT morphologic diagnosis but also to further assess the abnormal intestinal blood flow. In recent years, more and more CT and magnetic resonance imaging (MRI)-related new techniques for assessing blood flow have emerged, including CT perfusion, spectral CT imaging, magnetic resonance perfusion imaging, and phase contrast MRI. This paper reviews the clinical application and progress of these techniques for assessing intestinal blood flow.

Pancreatic Blood Flow with Special Emphasis on Blood Perfusion of the Islets of Langerhans

The pancreatic islets are more richly vascularized than the exocrine pancreas, and possess a 5- to 10-fold higher basal and stimulated blood flow, which is separately regulated. This is reflected in the vascular anatomy of the pancreas where islets have separate arterioles. There is also an insulo-acinar portal system, where numerous venules connect each islet to the acinar capillaries. Both islets and acini possess strong metabolic regulation of their blood perfusion. Of particular importance, especially in the islets, is adenosine and ATP/ADP. Basal and stimulated blood flow is modified by local endothelial mediators, the nervous system as well as gastrointestinal hormones. Normally the responses to the nervous system, especially the parasympathetic and sympathetic nerves, are fairly similar in endocrine and exocrine parts. The islets seem to be more sensitive to the effects of endothelial mediators, especially nitric oxide, which is a permissive factor to maintain the high basal islet blood flow. The gastrointestinal hormones with pancreatic effects mainly influence the exocrine pancreatic blood flow, whereas islets are less affected. A notable exception is incretin hormones and adipokines, which preferentially affect islet vasculature. Islet hormones can influence both exocrine and endocrine blood vessels, and these complex effects are discussed. Secondary changes in pancreatic and islet blood flow occur during several conditions. To what extent changes in blood perfusion may affect the pathogenesis of pancreatic diseases is discussed. Both type 2 diabetes mellitus and acute pancreatitis are conditions where we think there is evidence that blood flow may contribute to disease manifestations. © 2019 American Physiological Society. Compr Physiol 9:799-837, 2019.

Advanced MR Imaging Techniques for Pancreas Imaging

Advances in MR imaging with optimization of hardware, software, and techniques have allowed for an increased role of MR in the identification and characterization of pancreatic disorders. Diffusion-weighted imaging improves the detection and staging of pancreatic neoplasms and aides in the evaluation of acute, chronic and autoimmune pancreatitis. The use of secretin-enhanced MR cholangiography improves the detection of morphologic ductal anomalies, and assists in the characterization of pancreatic cystic lesions and evaluation of acute and chronic pancreatitis. Emerging MR techniques such as MR perfusion, T1 mapping/relaxometry, and MR elastography show promise in further evaluating pancreatic diseases.

Robust kidney perfusion mapping in pediatric chronic kidney disease using single-shot 3D-GRASE ASL with optimized retrospective motion correction

PURPOSE

To develop a robust renal arterial spin labeling (ASL) acquisition and processing strategy for mapping renal blood flow (RBF) in a pediatric cohort with severe kidney disease.

METHODS

A single-shot background-suppressed 3D gradient and spin-echo (GRASE) flow-sensitive alternating inversion recovery (FAIR) ASL acquisition method was used to perform 2 studies. First, an evaluation of the feasibility of single-shot 3D-GRASE and retrospective noise reduction methods was performed in healthy volunteers. Second, a pediatric cohort with severe chronic kidney disease underwent single-shot 3D-GRASE FAIR ASL and RBF was quantified following several retrospective motion correction pipelines, including image registration and threshold-free weighted averaging. The effect of motion correction on the fit errors of saturation recovery (SR) images (required for RBF quantification) and on the perfusion-weighted image (PWI) temporal signal-to-noise ratio (tSNR) was evaluated, as well as the intra- and inter-session repeatability of renal longitudinal relaxation time (T₁ ) and RBF.

RESULTS

The mean cortical and/or functional renal parenchyma RBF in healthy volunteers and CKD patients was 295 ± 97 and 95 ± 47 mL/100 g/min, respectively. Motion-correction reduced image artefacts in both T₁ and RBF maps, significantly reduced SR fit errors, significantly increased the PWI tSNR and improved the improved the repeatability of T₁ and RBF in the pediatric patient cohort.

CONCLUSION

Single-shot 3D-GRASE ASL combined with retrospective motion correction enabled repeatable non-invasive RBF mapping in the first pediatric cohort with severe kidney disease undergoing ASL scans.

Dynamic non-invasive ASL perfusion imaging of a normal pancreas with secretin augmented MR imaging

OBJECTIVES

To investigate prospectively the repeatability of pancreatic perfusion measurements using arterial spin labelling (ASL) and to determine the increase in perfusion due to secretin stimulation.

MATERIAL AND METHODS

An (FAIR)-TrueFISP ASL sequence was applied to determine the perfusion of the pancreatic head in a 3T MRI scanner. Ten healthy volunteers (four men, six women: mean age 28.5 ± 4.6 years; age range 25-40 years) were investigated twice within 1 week. The inter-individual variability was calculated using the standard deviation. Intra-individual agreement between the first and second scan was estimated using the Pearson correlation coefficient. A paired Wilcoxon rank-sum test was used to compare perfusion at baseline (BL) and during secretin stimulation.

RESULTS

The mean BL perfusion of the pancreatic head was 285 ± 96 mL/100 g/min with an intra-individual correlation coefficient of 0.67 (strong) for repeated measurements. Secretin stimulation led to a significant increase (by 81%) in perfusion of the pancreatic head to 486 ±156 mL/100 g/min (p=0.002) with an intra-individual correlation of 0.29 (weak). A return to BL values was observed after 239 ± 92 s with a moderate intra-individual correlation coefficient of 0.42 for repeat measurements.

CONCLUSION

Dynamic non-invasive ASL imaging of the pancreas permitted quantification of pancreatic perfusion in a clinically applicable setting.

KEY POINTS

• ASL imaging of the pancreas permitted quantification of pancreatic perfusion • Secretin stimulation led to a significant increase in pancreatic perfusion • The intra-individual correlation coefficient for baseline perfusion was strong for repeated measurements.

Non-invasive assessment of portal hypertension using quantitative magnetic resonance imaging

BACKGROUND & AIMS

Hepatic venous pressure gradient (HVPG) measurement is currently the only validated technique to accurately evaluate changes in portal pressure. In this study, we evaluate the use of non-contrast quantitative magnetic resonance imaging (MRI) as a surrogate measure of portal pressure.

METHODS

Thirty patients undergoing HVPG measurement were prospectively recruited. MR parameters of longitudinal relaxation time (T₁), perfusion of the liver and spleen (by arterial spin labelling), and blood flow in the portal, splanchnic and collateral circulation (by phase contrast MRI) were assessed. We estimated the liver stiffness measurement (LSM) and enhanced liver fibrosis (ELF) score. The correlation of all non-invasive parameters with HVPG was evaluated.

RESULTS

The mean (range) HVPG of the patients was 9.8 (1-22) mmHg, and 14 patients (48%) had clinically significant portal hypertension (CSPH, HVPG ⩾10mmHg). Liver T₁ relaxation time, splenic artery and superior mesenteric artery velocity correlated significantly with HVPG. Using multiple linear regression, liver T₁ and splenic artery velocity remained as the two parameters in the multivariate model significantly associated with HVPG (R=0.90, p<0.001). This correlation was maintained in patients with CSPH (R=0.85, p<0.001). A validation cohort (n=10) showed this linear model provided a good prediction of HVPG. LSM and ELF score correlated significantly with HVPG in the whole population but the correlation was absent in CSPH.

CONCLUSIONS

MR parameters related to both hepatic architecture and splanchnic haemodynamics correlate significantly with HVPG. This proposed model, confirmed in a validation cohort, could replace the invasive HVPG measurement.

LAY SUMMARY

In patients with cirrhosis, the development and progression of portal hypertension is related to worse outcomes. However, the standard technique of assessing portal pressure is invasive and not widely used in clinical practice. Here, we have studied the use of non-invasive MRI in evaluating portal pressure. The MRI measures of liver architecture and blood flow in the splenic artery correlated well with portal pressure. Therefore, this non-invasive method can potentially be used to assess portal pressure in clinical trials and monitoring treatment in practice.

Pancreatic perfusion and subsequent response to glucose in healthy individuals and patients with type 1 diabetes

AIMS/HYPOTHESIS

The aim of this study was to investigate pancreatic perfusion and its response to a glucose load in patients with type 1 diabetes mellitus compared with non-diabetic ('healthy') individuals.

METHODS

Eight individuals with longstanding type 1 diabetes and ten sex-, age- and BMI-matched healthy controls underwent dynamic positron emission tomography scanning with (15)O-labelled water before and after intravenous administration of glucose. Perfusion in the pancreas was measured. Portal and arterial hepatic perfusion were recorded as references.

RESULTS

Under fasting conditions, total pancreatic perfusion was on average 23% lower in the individuals with diabetes compared with healthy individuals. Glucose increased total pancreatic and portal hepatic blood perfusion in healthy individuals by 48% and 38%, respectively. In individuals with diabetes there was no significant increase in either total pancreatic or portal hepatic perfusion.

CONCLUSIONS/INTERPRETATION

Individuals with type 1 diabetes have reduced basal pancreatic perfusion and a severely impaired pancreatic and splanchnic perfusion response to intravenous glucose stimulation.

Survival of encapsulated islets: More than a membrane story

At present, proven clinical treatments but no cures are available for diabetes, a global epidemic with a huge economic burden. Transplantation of islets of Langerhans by their infusion into vascularized organs is an experimental clinical protocol, the first approach to attain cure. However, it is associated with lifelong use of immunosuppressants. To overcome the need for immunosuppression, islets are encapsulated and separated from the host immune system by a permselective membrane. The lead material for this application is alginate which was tested in many animal models and a few clinical trials. This review discusses all aspects related to the function of transplanted encapsulated islets such as the basic requirements from a permselective membrane (e.g., allowable hydrodynamic radii, implications of the thickness of the membrane and relative electrical charge). Another aspect involves adequate oxygen supply, which is essential for survival/performance of transplanted islets, especially when using large retrievable macro-capsules implanted in poorly oxygenated sites like the subcutis. Notably, islets can survive under low oxygen tension and are physiologically active at > 40 Torr. Surprisingly, when densely crowded, islets are fully functional under hyperoxic pressure of up to 500 Torr (> 300% of atmospheric oxygen tension). The review also addresses an additional category of requirements for optimal performance of transplanted islets, named auxiliary technologies. These include control of inflammation, apoptosis, angiogenesis, and the intra-capsular environment. The review highlights that curing diabetes with a functional bio-artificial pancreas requires optimizing all of these aspects, and that significant advances have already been made in many of them.