Effect of Low-Frequency Therapeutic Ultrasound on Induction of Nitric Oxide in CKD: Potential to Prevent Acute Kidney Injury.

INTRODUCTION: Post-contrast acute kidney injury (PC-AKI) develops in a significant proportion of patients with CKD after invasive cardiology procedures and is strongly associated with adverse outcomes. OBJECTIVE: We sought to determine whether increased intrarenal nitric oxide (NO) would prevent PC-AKI. METHODS: To create a large animal model of CKD, we infused 250 micron particles into the renal arteries in 56 ± 8 kg pigs. We used a low-frequency therapeutic ultrasound device (LOTUS - 29 kHz, 0.4 W/cm2) to induce NO release. NO and laser Doppler probes were used to assess changes in NO content and blood flow. Glomerular filtration rate (GFR) was measured by technetium-diethylene-triamine-pentaacetic acid (Tc-99m-DTPA) radionuclide imaging. PC-AKI was induced by intravenous infusion of 7 cm3/kg diatrizoate. In patients with CKD, we measured GFR at baseline and during LOTUS using Tc-99m-DTPA radionuclide imaging. RESULTS: In the pig model, CKD developed over 4 weeks (serum creatinine [Cr], mg/dL, 1.0 ± 0.2-2.6 ± 0.9, p < 0.01, n = 12). NO and renal blood flow (RBF) increased in cortex and medulla during LOTUS. GFR increased 75 ± 24% (p = 0.016, n = 3). PC-AKI developed following diatrizoate i.v. infusion (Cr 2.6 ± 0.7 baseline to 3.4 ± 0.6 at 24 h, p < 0.01, n = 3). LOTUS (starting 15 min prior to contrast and lasting for 90 min) prevented PC-AKI in the same animals 1 week later (Cr 2.5 ± 0.4 baseline to 2.6 ± 0.7 at 24 h, p = ns, n = 3). In patients with CKD (n = 10), there was an overall 25% increase in GFR in response to LOTUS (p < 0.01). CONCLUSIONS: LOTUS increased intrarenal NO, RBF, and GFR and prevented PC-AKI in a large animal model of CKD, and significantly increased GFR in patients with CKD. This novel approach may provide a noninvasive nonpharmacological means to prevent PC-AKI in high-risk patients.

Automatic TAC extraction from dynamic cardiac PET imaging using iterative correlation from a population template.

This work describes a new iterative method for extracting time-activity curves (TAC) from dynamic imaging studies using a priori information from generic models obtained from TAC templates. Analytical expressions of the TAC templates were derived from TACs obtained by manual segmentation of three (13)NH3 pig studies (gold standard). An iterative method for extracting both ventricular and myocardial TACs using models of the curves obtained as an initial template was then implemented and tested. These TACs were extracted from masked and unmasked images; masking was applied to remove the lungs and surrounding non-relevant structures. The resulting TACs were then compared with TACs obtained manually; the results of kinetic analysis were also compared. Extraction of TACs for each region was sensitive to the presence of other organs (e.g., lungs) in the image. Masking the volume of interest noticeably reduces error. The proposed method yields good results in terms of TAC definition and kinetic parameter estimation, even when the initial TAC templates do not accurately match specific tracer kinetics.

Combining dynamic and ECG-gated 8²Rb-PET for practical implementation in the clinic.

OBJECTIVES: For many cardiac clinics, list-mode PET is impractical. Therefore, separate dynamic and ECG-gated acquisitions are needed to detect harmful stenoses, indicate affected coronary arteries, and estimate stenosis severity. However, physicians usually order gated studies only because of dose, time, and cost limitations. These gated studies are limited to detection. In an effort to remove these limitations, we developed a novel curve-fitting algorithm [incomplete data (ICD)] to accurately calculate coronary flow reserve (CFR) from a combined dynamic-ECG protocol of a length equal to a typical gated scan. METHODS: We selected several retrospective dynamic studies to simulate shortened dynamic acquisitions of the combined protocol and compared (a) the accuracy of ICD and a nominal method in extrapolating the complete functional form of arterial input functions (AIFs); and (b) the accuracy of ICD and ICD-AP (ICD with a-posteriori knowledge of complete-data AIFs) in predicting CFRs. RESULTS: According to the Akaike information criterion, AIFs predicted by ICD were more accurate than those predicted by the nominal method in 11 out of 12 studies. CFRs predicted by ICD and ICD-AP were similar to complete-data predictions (PICD=0.94 and PICD-AP=0.91) and had similar average errors (eICD=2.82% and eICD-AP=2.79%). CONCLUSION: According to a nuclear cardiologist and an expert analyst of PET data, both ICD and ICD-AP predicted CFR values with sufficient accuracy for the clinic. Therefore, by using our method, physicians in cardiac clinics would have access to the necessary amount of information to differentiate between single-vessel and triple-vessel disease for treatment decision making.

Pediatric nuclear cardiology.

Echocardiography, magnetic resonance imaging and, more recently, multidetector computed tomography, have led to major advances in noninvasive image assessment of anatomy in pediatric cardiology. The radionuclide methods often lack sufficient resolution to precisely characterize complex morphology in congenital heart lesions. However, these methods provide an accurate and reproduceable quantitative assessment of the physiological consequences of structural heart disease. These unique capabilities will continue to assure ongoing clinical relevance of radionuclide methodology, as is the case in the assessment of heart disease in adult cardiology.

Rodent brain imaging with SPECT/CT.

We evaluated methods of imaging rat models of stroke in vivo using a single photon emission computed tomography (SPECT) system dedicated to small animal imaging (X-SPECT, Gamma Medica-Ideas, Northridge, CA). An animal model of ischemic stroke was developed for in vivo SPECT/CT imaging using the middle cerebral artery occlusion (MCAO) technique. The presence of cerebral ischemia was verified in ex vivo studies using triphenyltetrazolium chloride (TTC) staining. In vivo radionuclide imaging of cerebral blood flow was performed in rats following MCAO using dynamic planar imaging of 99mTc-exametazime with parallel hole collimation. This was followed immediately by in vivo radionuclide imaging of cerebral blood flow with 99mTc-exametazime in the same animals using 1-mm pinhole SPECT. Correlated computed tomography imaging was performed to localize radiopharmaceutical uptake. The animals were allowed to recover and ex vivo autoradiography was performed with separate administration of 99mTc-exametazime. Time activity curve of 99mTc-exametazime showed that the radiopharmaceutical uptake could be maintained for over 9 min. The activity would be expected to be relatively stable for a much longer period, although the data were only obtained for 9 min. TTC staining revealed sizable infarcts by visual observation of inexistence of TTC stain in infracted tissues of MCAO rat brains. In vivo SPECT imaging showed cerebral blood flow deficit in the MCAO model, and the in vivo imaging result was confirmed with ex vivo autoradiography. We have demonstrated a capability of imaging regions of cerebral blood flow deficit in MCAO rat brains in vivo using a pinhole SPECT dedicated to small animal imaging.

Acute neurocardiogenic injury after subarachnoid hemorrhage.

BACKGROUND: Left ventricular (LV) systolic dysfunction has been reported in humans with subarachnoid hemorrhage (SAH), and its underlying pathophysiology remains controversial. Possible mechanisms include myocardial ischemia versus excessive catecholamine release from sympathetic nerve terminals. METHODS AND RESULTS: For 38 months, echocardiography and myocardial scintigraphy with technetium sestamibi (MIBI) and meta-[(123)I]iodobenzylguanidine (MIBG) were performed on 42 patients admitted with SAH to assess myocardial perfusion and sympathetic innervation, respectively. A blinded observer interpreted the scintigraphic images. Cardiac troponin I (cTI) was measured to quantify the degree of myocyte necrosis. Blinded observers calculated the LV ejection fraction and graded each LV segment as normal (score=1), hypokinetic (score=2), or akinetic (score=3). A wall-motion score was calculated by averaging the sum of the 16 segments. All subjects with interpretable scans (N=41) had normal MIBI uptake. Twelve subjects had either global (n=9) or regional (n=3) absence of MIBG uptake. In comparison with patients with normal MIBG uptake, those with evidence of functional denervation were more likely to have LV regional wall-motion abnormalities (92% versus 52%, P=0.030) and cTI levels >1 microg/L (58% versus 21%, P=0.029). CONCLUSIONS: LV systolic dysfunction in humans with SAH is associated with normal myocardial perfusion and abnormal sympathetic innervation. These findings may be explained by excessive release of norepinephrine from myocardial sympathetic nerves, which could damage both myocytes and nerve terminals.

Combined effects of mild hypothermia and glycoprotein IIb/IIIa antagonists on platelet-platelet and leukocyte-platelet aggregation.

Endovascular cooling was assessed as a potential treatment for percutaneous coronary intervention in patients with acute myocardial infarction. Here we show that mild hypothermia: (1) inhibits platelet aggregation; (2) augments eptifibatide- and tirofiban- but not abciximab-induced inhibition of platelet aggregation; (3) increases the formation of adenosine diphosphate-induced leukocyte-platelet aggregates; and (4) diminishes the glycoprotein IIb/IIIa antagonist-induced decrease in leukocyte-platelet aggregates.

Hypothermia during reperfusion limits 'no-reflow' injury in a rabbit model of acute myocardial infarction.

OBJECTIVE: Reflow following coronary artery occlusion is an important predictor of clinical outcome. This study tests the effects of regional hypothermia, initiated late during ischemia and maintained for 2 h of reperfusion, on the no-reflow phenomenon. METHODS: Anesthetized, open-chest New Zealand White rabbits received 30 min of coronary artery occlusion and 3 h reperfusion. Regional myocardial hypothermia (H, n=14), starting 10 min before reperfusion and continuing for 2 h of reperfusion, was compared with normothermia (N, n=14). Regional myocardial blood flow (microspheres) was measured during occlusion and at the end of reperfusion. The anatomic zone of no-reflow (thioflavin S in vivo injection) and infarct size were measured in the ischemic risk region at the end of the study. RESULTS: Myocardial temperature in H rabbits was decreased by 5.0+/-0.4 degrees C from baseline (37.1+/-0.2 degrees C) and remained about 32 degrees C during the cooling phase, returning to 36.0+/-0.3 degrees C at 3 h. N hearts remained within 0.2 degrees C of baseline (37.3+/-0.1 degrees C) throughout. Both groups were equally ischemic during occlusion, but at the end of reperfusion reflow to the previously ischemic zone was significantly higher in H, 77+/-5% of normal blood flow versus 36+/-4% in N (P=0.0001). The zone of anatomic no-reflow was significantly smaller in H, 11+/-3% of the ischemic risk zone versus 37+/-3% in N (P=0.0001), and was proportionally smaller when represented as a percent of the necrotic zone 36+/-6% compared with 75+/-5% in N. Infarct size, expressed as a percent of the ischemic risk zone was significantly smaller in H vs. N hearts (27+/-4 and 51+/-5%, P=0.0000). CONCLUSION: This study shows that hypothermic therapy initiated late during ischemia and continuing for several hours of reperfusion significantly improves reflow and reduces macroscopic zones of no-reflow and necrosis in this model. The improvement in reflow was greater than would be expected in the H group compared with N, based on the extent of necrosis. As reflow is a predictor of outcome, this intervention may have clinical implications.

Pinhole single-photon emission computed tomography for myocardial perfusion imaging of mice.

OBJECTIVES: Although transgenic mice have emerged as powerful experimental models of cardiovascular disease, methods for in vivo phenotypic assessment and characterization remain limited, motivating the development of new instruments for biologic measurement. BACKGROUND: We have developed a single-photon emission computed tomography system with a pinhole collimator (pinhole SPECT) for high-resolution cardiovascular imaging of mice. In this study, we describe a protocol for myocardial perfusion imaging of mice using technetium-99m ((99m)Tc)-sestamibi and demonstrate the feasibility for measurement of perfusion defect size from pinhole SPECT images. METHODS: Mice were anesthetized and injected with 370 MBq (10 mCi) of (99m)Tc-sestamibi. Tomographic projection images were acquired by rotating each mouse in a vertical axis in front of a stationary clinical scintillation camera equipped with a pinhole collimator. BALB/c mice (n = 15) were imaged after the permanent ligation of the left anterior descending coronary artery. The resulting defect size was measured from circumferential profiles of short-axis images. After imaging, the hearts were excised and sectioned to obtain ultra-high resolution digital autoradiographs of (99m)Tc-sestamibi, from which the actual infarct size was determined. RESULTS: Reconstructed image quality was equivalent to that obtained for clinical myocardial perfusion imaging. Linear regression analysis produced a correlation coefficient of 0.83 (p < 0.001) between the measured and actual values of the defect size. CONCLUSIONS: These results demonstrate that myocardial perfusion can be characterized qualitatively and quantitatively in mice using pinhole SPECT.

Safety and efficacy of endovascular cooling and rewarming for induction and reversal of hypothermia in human-sized pigs.

BACKGROUND AND PURPOSE: Numerous studies indicate that mild hypothermia provides substantial neuroprotection. However, current systems transfer insufficient heat to rapidly vary core temperature. We thus evaluated the safety and efficacy of endovascular cooling and rewarming for the induction and reversal of hypothermia. METHODS: In 10 anesthetized pigs (weight, 66+/-2 kg), a heat-exchange balloon catheter was inserted into the inferior vena cava and used to cool to a core temperature of 32 degrees C and then rewarm to normothermia. Control animals had 38 degrees C saline infused. Venous blood was sampled before, during, and after cooling. Three animals in each group were killed 1 week later, and the lungs and inferior vena cava were removed for gross and microscopic examination. In 5 additional animals, cardiac output was measured during cooling to 32 degrees C. RESULTS: Body temperature in the hypothermic animals decreased at a rate of 4.5+/-0.4 degrees C/h. Animals were subsequently rewarmed to 36.0+/-0.04 degrees C at 2.5+/-0.2 degrees C/h. There was no difference in heart rate between hypothermic and control animals, whereas systolic pressure decreased during cooling. Cardiac output was well maintained during cooling. There were no thermal effects on blood elements or blood vessels. CONCLUSIONS: The endovascular heat-exchange system effectively cooled and rewarmed pigs with large thermal mass without producing any adverse effects on blood elements, blood vessel integrity, or cardiovascular function.

Induction of mild systemic hypothermia with endovascular cooling during primary percutaneous coronary intervention for acute myocardial infarction.

OBJECTIVES: The purpose of this study was to evaluate the safety and feasibility of endovascular cooling during primary percutaneous coronary intervention (PCI) for acute myocardial infarction (AMI). BACKGROUND: In experimental models of AMI, mild systemic hypothermia has been shown to reduce metabolic demand and limit infarct size. METHODS: In a multi-center study, 42 patients with AMI (<6 h from symptom onset) were randomized to primary PCI with or without endovascular cooling (target core temperature 33 degrees C). Cooling was maintained for 3 h after reperfusion. Skin warming, oral buspirone, and intravenous meperidine were used to reduce the shivering threshold. The primary end point was major adverse cardiac events at 30 days. Infarct size at 30 days was measured using (99m)Tc-sestamibi SPECT imaging. RESULTS: Endovascular cooling was performed successfully in 20 patients (95%). All achieved a core temperature below 34 degrees C (mean target temperature 33.2 +/- 0.9 degrees C). The mean temperature at reperfusion was 34.7 +/- 0.9 degrees C. Cooling was well tolerated, with no hemodynamic instability or increase in arrhythmia. Nine patients experienced mild episodic shivering. Major adverse cardiac events occurred in 0% vs. 10% (p = NS) of treated versus control patients. The median infarct size was non-significantly smaller in patients who received cooling compared with the control group (2% vs. 8% of the left ventricle, p = 0.80). CONCLUSIONS: Endovascular cooling can be performed safely as an adjunct to primary PCI for AMI. Further clinical trials are required to determine whether induction of mild systemic hypothermia with endovascular cooling will limit infarct size in patients undergoing reperfusion therapy.

Effect of endovascular cooling on myocardial temperature, infarct size, and cardiac output in human-sized pigs.

Mild hypothermia reduces myocardial infarct size in small animals; however, the extent of myocardial protection in large animals with greater thermal mass remains unknown. We evaluated the effects of mild endovascular cooling on myocardial temperature, infarct size, and cardiac output in 60- to 80-kg isoflurane-anesthetized pigs. We occluded the left anterior descending coronary artery for 60 min, followed by reperfusion for 3 h. An endovascular heat-exchange catheter was used to either lower core body temperature to 34 degrees C (n = 11) or maintain temperature at 38 degrees C (n = 11). Additional studies assessed myocardial viability and microvascular perfusion with (99m)Tc-sestamibi autoradiography. Endovascular cooling reduced infarct size compared with normothermia (9 +/- 6% vs. 45 +/- 8% of the area at risk; P < 0.001), whereas the area at risk was comparable (19 +/- 3% vs. 20 +/- 7%; P = 0.65). Salvaged myocardium showed normal sestamibi uptake, confirming intact microvascular flow and myocyte viability. Cardiac output was maintained in hypothermic hearts because of an increase in stroke volume, despite a decrease in heart rate. Mild endovascular cooling to 34 degrees C lowers myocardial temperature sufficiently in human-sized hearts to cause a substantial cardioprotective effect, preserve microvascular flow, and maintain cardiac output.

Performance evaluation of a pinhole SPECT system for myocardial perfusion imaging of mice.

The increasing use of transgenic mice as models of human physiology and disease has motivated the development of dedicated in vivo imaging systems for anatomic and functional characterization of mice as an adjunct to or a replacement for established ex vivo techniques. We have developed a pinhole single photon emission computed tomography (SPECT) system for high resolution imaging of mice with cardiovascular imaging as the primary application. In this work, we characterize the system performance through phantom studies. The spatial resolution and sensitivity were measured from images of a line source and point source, respectively, and were reported for a range of object-to-pinhole distances and pinhole diameters. Tomographic images of a uniform cylindrical phantom, Defrise phantom, and grid phantom were used to characterize the image uniformity and spatial linearity. The uniform phantom image did not contain any ring or reconstruction artifacts, but blurring in the axial direction was evident in the Defrise phantom images. The grid phantom images demonstrated excellent spatial linearity. A novel phantom modeling perfusion of the left ventricle of a mouse was designed and built with perfusion defects of varying sizes to evaluate the system performance for myocardial perfusion imaging of mice. The defect volumes were measured from the pinhole SPECT images and correlated to the actual defect volumes calculated according to geometric formulas. Linear regression analysis produced a correlation coefficient of r = 0.995 (p < 0.001), demonstrating the feasibility for measurement of perfusion defect size in mice using pinhole SPECT. We have performed phantom studies to characterize the spatial resolution, sensitivity, image uniformity, and spatial linearity of the pinhole SPECT system. Measurement of the perfusion defect size is a valuable phenotypic assessment and will be useful for hypothesis testing in murine models of cardiovascular disease.