Biomedical subjects
Publications and source records attributed to Jonathan R Lindner.
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Myocardial contrast echocardiography (MCE) is a noninvasive imaging technique that relies on the ultrasound detection of microbubble contrast agents. These agents are confined to the intravascular space thereby producing signal enhancement from the blood pool. This review encompasses many of the key concepts regarding the clinical application of MCE. The first section focuses on the composition, safety, and biokinetics of ultrasound contrast agents. Then we discuss new ultrasound imaging methodology that has been developed to enhance detection of contrast agent and to assess perfusion at the tissue level. Next, the clinical applications of contrast ultrasound are reviewed. These include enhancement of the cardiac chambers for better assessment of cardiac function and masses, myocardial perfusion imaging for the detection of coronary artery disease, and the assessment of myocardial viability and microvascular reflow. Finally, we discuss some of the future applications for MCE, which include molecular imaging of disease and drug/gene delivery. The overall aim of the review is to update the clinician on state-of-the-art MCE and how it can be applied in patients with cardiovascular disease.
We have shown recently that downregulation of the androgen receptor (AR), one of the key players in prostate tumor cells, with short antisense oligodeoxynucleotides (ODNs) results in inhibition of prostate tumor growth. Particularly with regard to an application of these antisense drugs in vivo, we now investigated the usefulness of microbubble-enhanced ultrasound to deliver these ODNs into prostate cancer cells. Our short antisense AR ODNs were loaded onto the lipid surface of cationic gas-filled microbubbles by ion charge binding, and delivered into the cells by bursting the loaded microbubbles with ultrasound. In vitro experiments were initially performed to show that this kind of delivery system works in principle. In fact, transfection of prostate tumor cells with antisense AR ODNs using microbubble-enhanced ultrasound resulted in 49% transfected cells, associated with a decrease in AR expression compared to untreated controls. In vivo, uptake of a digoxigenin-labelled ODN was found in prostate tumour xenografts in nude mice following intratumoral or intravenous injection of loaded microbubbles and subsequent exposure of the tumour to ultrasound, respectively. Our results show that ultrasound seems to be the driving force of this delivery system. Uptake of the ODN was also observed in tumors after treatment with ultrasound alone, with only minor differences compared to the combined use of microbubbles and ultrasound.
Ultrasound contrast microbubbles have been successfully targeted to a number of intravascular disease markers. We hypothesized that targeted delivery could be improved further, by making the microbubbles deformable, leading to increased microbubble-endothelium adhesion contact area and stabilized adhesion. Activated leukocytes utilize such strategy; they deform after binding to inflamed endothelium in the vasculature. Lipid-shell microbubbles were targeted to the endothelial inflammatory protein P-selectin with a monoclonal anti-P-selectin antibody attached to the microbubble shell. Deformable microbubbles were created by controlled pressurization with partial gas loss, which generated an average excess shell surface area of approximately 30% and the formation of outward-projected wrinkles and folds. Targeted microbubble adhesion and deformability were assessed in the parallel plate flow chamber under shear flow. Sustained adhesion of deformable microbubbles at wall shear stresses between 0.4 and 1.35 dyn/cm(2) was consistently better than adhesion of wrinkle-free microbubbles. Over this shear range, targeted wrinkled microbubbles were deformed by shear flow, unlike wrinkle-free microbubbles. In a murine cremaster inflammation model, a significant improvement of deformable microbubble targeting was observed by intravital microscopy. Overall, the mechanical aspects of adhesion, such as particle shape, deformability and surface microstructure, are important in engineering efficient site-targeted particle-based agents for medical imaging and therapy.
The spatial extent of hypoperfusion or viability is important in the treatment of patients with coronary artery disease. We hypothesized that computerized pixel intensity threshold analysis (PITA) could be used for the automated analysis of perfusion defect size during myocardial contrast echocardiography (MCE). For calibration studies, MCE was performed in 6 dogs undergoing ischemia and reperfusion. Infarct size was determined by PITA, which automatically calculates the percentage of pixels within the myocardium that fail to exceed a predetermined threshold of maximum contrast enhancement. A threshold of 10% of maximum yielded infarct sizes that most closely correlated with those determined by histologic staining. For clinical validation, MCE was performed in 30 patients with acute myocardial infarction before primary percutaneous coronary intervention (PCI) for measurement of risk area; and within 5 days and at 4 weeks after PCI to determine infarct size. The defect size by PITA with a 10% threshold value closely correlated with those measured by expert reader planimetry on background-subtracted color-coded image sets (r = 0.95, P < .001). We conclude that automated analysis of perfusion defect size on MCE is possible by PITA. This technique may be useful for rapid and objective analysis of the extent of ischemia and viability, and for clinical experimentation where accurate and sequential analysis of perfusion defect size is imperative.
BACKGROUND & AIMS: Inflammatory bowel disease (IBD) is the second most common chronic inflammatory disorder worldwide; however, a noninvasive means of accurately assessing the severity and extent of intestinal inflammation is currently not available. The aim of the present study was to develop a noninvasive imaging modality to detect and evaluate ileitis in SAMP1/YitFc (SAMP) mice. METHODS: An image-enhancing ultrasound (US) contrast agent, consisting of encapsulated gaseous microbubbles (MB), was developed specifically to bind mucosal addressin cellular adhesion molecule-1 (MAdCAM-1), a mucosal-restricted addressin up-regulated during gut inflammation. MAdCAM-1-targeted MB (MB(M)) were tested for binding specificity on MAdCAM-1 protein and tumor necrosis factor (TNF)-stimulated SVEC4-10 endothelial cells using an in vitro flow chamber assay and for their ability to detect and quantify ileitis by intravital microscopy and transabdominal US. RESULTS: Under in vitro flow conditions, a 100-fold increase in MB(M) binding was observed on MAdCAM-1 protein compared with nonspecific MB (P < .001). TNF-stimulated endothelial cells bound significantly more MB(M) vs nonspecific MB (P < .001), which was abrogated after preincubation with anti-MAdCAM-1 antibodies (P < .001). In vivo, MB(M) specifically accumulated in focal areas of ileal inflammation and produced stronger acoustic echoes, measured by average video intensity, in SAMP vs uninflamed AKR mice (P < .001) or SAMP given nonspecific MB (P < .001). MB(M)-specific video intensity showed a strong positive correlation with total ileal inflammatory scores (R2 = 0.92). CONCLUSIONS: We have developed a novel intravascular US contrast agent targeting MAdCAM-1 that specifically detects and quantifies intestinal inflammation in experimental ileitis, providing the potential for a reliable, noninvasive means to diagnose and monitor disease in patients with IBD.
OBJECTIVES: Molecular imaging with contrast-enhanced ultrasound (CEU) relies on the detection of microbubbles retained in regions of disease. The aim of this study was to determine whether microbubble attachment to cells influences their acoustic signal generation and stability. MATERIALS AND METHODS: Biotinylated microbubbles were attached to streptavidin-coated plates to derive density versus intensity relations during low- and high-power imaging. To assess damping from microbubble attachment to solid or cell surfaces, in vitro imaging was performed for microbubbles charge-coupled to methacrylate spheres and for vascular cell adhesion molecule-1-targeted microbubbles attached to endothelial cells. RESULTS: Signal enhancement on plates increased according to acoustic power and microbubble site density up to 300 mm. Microbubble signal was reduced by attachment to solid spheres during high- and low-power imaging but was minimally reduced by attachment to endothelial cells and only at low power. CONCLUSION: Attachment of targeted microbubbles to rigid surfaces results in damping and a reduction of their acoustic signal, which is not seen when microbubbles are attached to cells. A reliable concentration versus intensity relationship can be expected from microbubble attachment to 2-dimensional surfaces until a very high site density is reached.
Strategies for improvement of blood flow by promoting new vessel growth in ischemic tissue are being developed. Recently, contrast-enhanced ultrasound (CEU) imaging has been used to assess tissue perfusion in models of ischemia-related angiogenesis, growth-factor mediated angiogenesis, and tumor angiogenesis. In these studies, microvascular flow is measured in order to assess the total impact of adaptations at different vascular levels. High-resolution methods for imaging larger vessels have been developed in order to derive "angiograms" of arteries, veins, and medium to large microvessels. We describe a novel method of vascular bed (microvessel and arterial) characterization of vessel anatomy and flow simultaneously, using serial measurement of the fractal dimension (FD) of a temporal sequence of CEU images. This method is proposed as an experimental methodology to distinguish ischemic from nonischemic tissue. Moreover, an improved approach for extracting the FD unique to this application is introduced.
Intense exercise and insulin each increases total limb blood flow and recruits muscle capillaries, presumably to facilitate nutrient exchange. Whether mixed meals or light exercise likewise recruits capillaries is unknown. We fed 18 (9 M, 9 F) healthy volunteers a 480-kcal liquid mixed meal. Plasma glucose, insulin, brachial artery flow, and forearm muscle microvascular blood volume were measured before and after the meal. Brachial artery flow and microvascular volume were also examined with light (25% max), moderate (50%), and heavy (80%) forearm contraction every 20 s in 5 (4 M, 1 F) healthy adults. After the meal, glucose and insulin rose modestly (to approximately 7 mM and approximately 270 pM) and peaked by 30 min, whereas brachial artery blood flow (P < 0.05) and the microvascular volume (P < 0.01) each increased significantly by 60 min, and microvascular flow velocity did not change. For exercise, both 50 and 80%, but not 25% maximal handgrip, increased average forearm and brachial artery blood flow (P < 0.01). Flow increased immediately after each contraction and declined toward basal over 15 s. Exercise at 25% max increased microvascular volume threefold (P < 0.01) without affecting microvascular flow velocity or total forearm blood flow. Forearm exercise at 80% maximal grip increased both microvascular volume and microvascular flow velocity (P < 0.05 each). We conclude that light exercise and simple meals each markedly increases muscle microvascular volume, thereby expanding the endothelial surface for nutrient exchange, and that capillary recruitment is an important physiological response to facilitate nutrient/hormone delivery in healthy humans.
We have previously shown that skeletal muscle capillaries are rapidly recruited by physiological doses of insulin in both humans and animals. This facilitates glucose and insulin delivery to muscle, thus augmenting glucose uptake. In obese rats, both insulin-mediated microvascular recruitment and glucose uptake are diminished; however, this action of insulin has not been studied in obese humans. Here we used contrast ultrasound to measure microvascular blood volume (MBV) (an index of microvascular recruitment) in the forearm flexor muscles of lean and obese adults before and after a 120-min euglycemic-hyperinsulinemic (1 mU . min(-1) . kg(-1)) clamp. We also measured brachial artery flow, fasting lipid profile, and anthropomorphic variables. Fasting plasma glucose (5.4 +/- 0.1 vs. 5.1 +/- 0.1 mmol/l, P = 0.05), insulin (79 +/- 11 vs. 38 +/- 6 pmol/l, P = 0.003), and percent body fat (44 +/- 2 vs. 25 +/- 2%, P = 0.001) were higher in the obese than the lean adults. After 2 h of insulin infusion, whole-body glucose infusion rate was significantly lower in the obese versus lean group (19.3 +/- 3.2 and 37.4 +/- 2.6 mumol . min(-1) . kg(-1) respectively, P < 0.001). Compared with baseline, insulin increased MBV in the lean (18.7 +/- 3.3 to 25.0 +/- 4.1, P = 0.019) but not in the obese group (20.4 +/- 3.6 to 18.8 +/- 3.8, NS). Insulin increased brachial artery diameter and flow in the lean but not in the obese group. We observed a significant, negative correlation between DeltaMBV and BMI (R = -0.482, P = 0.027) in response to insulin. In conclusion, obesity eliminated the insulin-stimulated muscle microvascular recruitment and increased brachial artery blood flow seen in lean individuals.
OBJECT: The aims of this study were to determine whether contrast-enhanced ultrasonography (CEU) could be used for noninvasive evaluation of cerebral perfusion in patients with traumatic brain injury (TBI) and to assess the effect of decompressive surgery on cerebral perfusion as measured by CEU. METHODS: Contrast-enhanced ultrasonography with intravenous administration of a microbubble contrast agent was performed in six patients with TBI undergoing decompressive craniectomy. Contrast-enhanced ultrasonography was performed through a bur hole before craniectomy and through the calvarial defect immediately after craniectomy and on postoperative Days 1 and 2. For the latter two studies, patients were placed in the recumbent position and at a 35 degrees incline to investigate changes in perfusion produced by modulation of intracranial pressure (ICP). Cerebral microvascular blood flow increased by almost threefold immediately after craniectomy, from a mean of 7.5 +/- 6.9 (standard deviation [SD]) to 20.9 +/- 11.6 (p < 0.05), and further improved on postoperative Day 1 (mean 37.1 +/- 13.9 [SD], p < 0.05, compared with postcraniectomy microvascular blood flow) without subsequent change on Day 2. The change in microvascular perfusion correlated inversely with the initial ICP (p < 0.01), indicating less recovery of flow when preoperative ICP was markedly elevated. On postoperative Days 1 and 2, head-of-bed elevation produced an increase in microvascular perfusion on CEU (mean 37 +/- 11 compared with 51 +/- 20, p < 0.05) and a small decrease in ICP (mean 16 +/- 5 mm Hg compared with 12 +/- 4 mm Hg, p < 0.05). In patients with parenchymal hematoma, CEU provided spatial information on perfusion abnormalities in the hemorrhagic core and surrounding tissues. CONCLUSIONS: Contrast-enhanced ultrasonography has potential for the intraoperative and bedside assessment of cerebral perfusion in patients with TBI. The technique may be appropriate for evaluating responses to therapies aimed at preventing secondary ischemia and for assessing regional perfusion abnormalities.
There is growing interest in the availability of methods for imaging disease at the level of the cellular and/or molecular mediators. Techniques for imaging molecular alterations have been develop for essentially all non-invasive cardiac imaging modalities. Molecular imaging with contrast-enhanced ultrasound relies on the detection of novel site-targeted contrast agents. These microbubbles or nanoparticles are retained within regions of a specific disease process, thereby allowing phenotypic characterization of tissue. Since most of these tracers remain within the intravascular space, the disease processes assessed must be characterized by antigens that are expressed within the vascular compartment. Accordingly, the pathologic states that have been targeted include inflammation, ischemia-and tumor-related angiogenesis, and thrombus formation; all of which are mediated in part by molecular events within the vascular space. This review describes: 10 different strategies that have been employed to target ultrasound contrast agents to regions of disease, 2) the unique challenges for imaging targeted ultrasound contrast agents, and 3) some of the early experience imaging molecular events in animal models of disease.
The aim of this study was to use myocardial contrast echocardiography to evaluate the effect of intravenous adenosine on microvascular reflow in patients with acute myocardial infarction who underwent primary coronary stenting (PCS). Thirty patients who underwent primary PCS for acute myocardial infarction were randomized to intravenous adenosine (50 to 70 mug/kg/min) or vehicle for 3 hours. Myocardial contrast echocardiography was performed before and sequentially after PCS to determine the risk area during coronary occlusion and infarct size. The risk area was similar in the adenosine- and placebo-treated patients. The infarct size as a ratio to the risk area was smaller in patients treated with adenosine when measured at 3 to 5 days (0.37 +/- 0.29 vs 0.68 +/- 0.25, p <0.01) and at 4 weeks (0.34 +/- 0.26 vs 0.60 +/- 0.21, p <0.01) after PCS. This effect was greatest when patency was achieved <4 hours after symptom onset (0.18 +/- 0.18 vs 0.74 +/- 0.31, p <0.05), with little effect after 4 hours. The relative microvascular blood volume in the risk area at 4 weeks was higher in patients receiving adenosine than in those receiving placebo (0.73 +/- 0.22 vs 0.57 +/- 0.20, p <0.01), and was highest when patency was achieved in <4 hours. In conclusion, the adjunctive use of intravenous adenosine after PCS reduces the infarct size relative to the risk area. This beneficial effect occurs primarily in those undergoing early intervention.
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BACKGROUND: We hypothesized that molecular imaging with contrast-enhanced ultrasound (CEU) and microbubbles targeted to endothelial integrins could be used to noninvasively assess early angiogenic responses to ischemia and growth factor therapy. METHODS AND RESULTS: Hindlimb ischemia was produced in 48 rats by ligation of an iliac artery. Half of the animals received intramuscular sustained-release fibroblast growth factor-2 (FGF-2). Immediately after ligation and at subsequent intervals from 4 to 28 days, blood flow and oxygen tension in the proximal adductor muscles were measured by CEU perfusion imaging and phosphor quenching, respectively. Targeted CEU imaging of alpha(v)- and alpha5beta1-integrin expression was performed with microbubbles bearing the disintegrin echistatin. Iliac artery ligation produced a 65% to 70% reduction in blood flow and oxygen tension. In untreated ischemic muscle, muscle flow and oxygen tension partially recovered by days 14 to 28. In these animals, signal from integrin-targeted microbubbles was intense and peaked before flow increase (days 4 to 7). In comparison to untreated animals, FGF-2-treated muscle had a greater rate and extent of blood flow recovery and greater signal intensity from integrin-targeted microbubbles, which peaked before maximal recovery of flow. On immunohistology, arteriolar but not capillary density increased in the ischemic limb after ligation, the rate and degree of which were greater in FGF-2-treated rats. Immunofluorescence demonstrated intense staining for alpha(v) in arterioles, the temporal course of which correlated with targeted imaging. CONCLUSIONS: Targeted CEU can be used to assess endogenous and therapeutic arteriogenesis before recovery of tissue perfusion. These results suggest that molecular imaging of integrin expression may be useful for evaluating proangiogenic therapies.
OBJECTIVE: We tested the hypothesis that simultaneous inhibition of the endothelial integrin alpha(v)beta(3) and the platelet glycoprotein IIb/IIIa receptor will substantially reduce infarct size in a model of acute coronary thrombosis and primary angioplasty. METHODS: Dogs were subjected to thrombus formation in the left anterior descending coronary artery followed by primary angioplasty. Prior to angioplasty, they were randomized into 3 treatment groups. Group 1 (n=7) received saline; Group 2 (n=9) received MK-383 that inhibits only IIb/IIIa; and Group 3 (n=9) received CP-4715, that inhibits both IIb/IIIa and alpha(v)beta(3). RESULTS: There was a 59% reduction in infarct size in dogs receiving CP-4715 compared to controls (p=0.002) and a 37% reduction compared to the dogs receiving MK-383 (p=0.04). Myocardium microthrombi were seen to be reduced similarly with both drugs on post-mortem (99m)Tc-DMP444 autoradiography that reflects in vivo IIb/IIIa receptor activity. In vivo imaging using echistatin-conjugated and leukocyte-targeted microbubbles revealed significant alpha(v)beta(3) inhibition and reduction in active leukocyte recruitment only in Group 3 dogs. Myocardial blood flow and regional function after reperfusion were also significantly better in this group. CONCLUSION: Simultaneous inhibition of IIb/IIIa and alpha(v)beta(3) causes a marked reduction in infarct size in a model of acute coronary thrombosis and primary PTCA that is associated with reduced myocardial microthrombi and inflammation, as well as improved myocardial blood flow and regional function. These results may have important implications in the treatment of acute coronary syndromes.
OBJECTIVES: We sought to determine whether the severity of peripheral arterial disease (PAD) can be assessed by measuring blood flow reserve in limb skeletal muscle with contrast-enhanced ultrasound (CEU). BACKGROUND: Noninvasive imaging of distal limb perfusion could improve management of patients with PAD by evaluating the impact of large and small vessel disease, and collateral flow. METHODS: In 12 dogs, blood flow in the quadriceps femoris was measured by CEU at rest and during either electrostimulated contractile exercise or adenosine infusion. Femoral artery blood flow was measured by Doppler ultrasound. Studies were performed in the absence and presence of either moderate or severe stenosis (pressure gradient of 10 to 20 mm Hg and >20 mm Hg, respectively). RESULTS: Resting femoral artery blood flow progressively decreased with stenosis severity, while resting skeletal muscle flow was reduced only with severe stenosis (52 +/- 21% of baseline, p < 0.05), indicating the presence of collateral flow. Skeletal muscle flow reserve during contractile exercise or adenosine decreased incrementally with increasing stenosis severity (p < 0.01). The stenotic pressure gradient correlated with skeletal muscle flow reserve for exercise and adenosine (r = 0.70 for both, p < 0.01). CONCLUSIONS: Contrast-enhanced ultrasound of limb skeletal muscle can be used to assess the severity of PAD by measuring muscle flow reserve during either contractile exercise or pharmacologic vasodilation. Unlike currently used methods, this technique may provide a measure of the physiologic effects of large- and small-vessel PAD, and the influence of collateral perfusion.
BACKGROUND: The clinical response to biventricular pacing is unpredictable, especially in patients with ischemic cardiomyopathy. OBJECTIVES: The purpose of this study was to prospectively examine the relationship between the extent of myocardial viability and the response to cardiac resynchronization therapy. METHODS: Twenty-one patients with ischemic left ventricular (LV) dysfunction (left ventricular ejection fraction [LVEF] 21 +/- 5%), New York Heart Association (NYHA) functional class III-IV, and QRS >120 ms received biventricular devices. Myocardial viability was assessed by myocardial contrast echocardiography, and a perfusion score index (PSI) was calculated from summed segmental perfusion scores. LV performance was assessed by echocardiography on the day after implantation and at 6 months. RESULTS: PSI was closely correlated with acute improvement in LVEF (P = .003, r = 0.65), stroke volume (P = .02, r = 0.54), and end-systolic volume (P = .05, r = -0.49). PSI also correlated with early diastolic LV relaxation (E', P < .05, r = 0.50) and global myocardial performance or Tei index (P = .003, r = 0.63). By multiple linear regression analysis, PSI provided incremental predictive value to the degree of dyssynchrony, measured by tissue Doppler imaging, for predicting improvement in LVEF. At 6 months, PSI remained positively correlated with improvement in ventricular performance and with reduction in LV end-diastolic dimension (P = .003, r = -0.68). PSI also influenced the clinical variables of NYHA class, 6-minute walk distance, quality-of-life score, and number of hospitalizations for heart failure. CONCLUSION: In patients with ischemic cardiomyopathy, the extent of myocardial viability predicts acute and long-term improvement in LV performance, exercise tolerance, and reduction in LV end-diastolic dimension with biventricular pacing.