Reperfusion differentially induces caspase-3 activation in ischemic core and penumbra after stroke in immature brain

BACKGROUND AND PURPOSE

Different strategies for neuroprotection of neonatal stroke may be required because the developing brain responds differently to hypoxia-ischemia than the mature brain. This study was designed to determine the role of caspase-dependent injury in the pathophysiology of pure focal cerebral ischemia in the immature brain.

METHODS

Postnatal day 7 rats were subjected to permanent or transient middle cerebral artery (MCA) occlusion. Diffusion-weighted MRI was used during occlusion to noninvasively map the evolving ischemic core. The time course of caspase-3 activation in ischemic brain tissue was determined with the use of an Asp-Glu-Val-Asp-aminomethylcoumarin cleavage assay. The anatomy of caspase-3 activation in the ischemic core and penumbra was mapped immunohistochemically with an anti-activated caspase-3 antibody in coronal sections that matched the imaging planes on diffusion-weighted MRI.

RESULTS

A marked increase in caspase-3 activity occurred within 24 hours of reperfusion after transient MCA occlusion. In contrast, caspase-3 activity remained significantly lower within 24 hours of permanent MCA occlusion. Cells with activated caspase-3 were prominent in the penumbra beginning at 3 hours after reperfusion, while a more delayed but marked caspase-3 activation was observed in the ischemic core by 24 hours after reperfusion.

CONCLUSIONS

In the neonate, caspase-3 activation is likely to contribute substantially to cell death not only in the penumbra but also in the core after ischemia with reperfusion. Furthermore, persistent perfusion deficits result in less caspase-3 activation and appear to favor caspase-independent injury.

Carbon dioxide reactivity and pressure autoregulation of brain tissue oxygen

OBJECTIVE

To describe the normal relationships between brain tissue oxygen tension (PbrO2) and physiological parameters of systemic blood pressure and CO2 concentrations.

METHODS

Licox Clark-type oxygen probes (GMS mbH, Kiel, Germany) were inserted in the frontal white matter of 12 swine maintained under general anesthesia with a 1.0 fraction of inspired oxygen (FiO2). In seven swine, alterations in end-tidal carbon dioxide (ET-CO2) concentration (range, 13-72 mm Hg) were induced via hyperventilation or instillation of CO2 into the ventilation circuit. In nine swine, mean arterial pressure (MAP) (range, 33-200 mm Hg) was altered; phenylephrine was used to induce hypertension, and a nitroprusside-esmolol combination or systemic hemorrhage was used for hypotension. Quantitative cerebral blood flow (CBF) was measured in two animals by using a thermal diffusion probe.

RESULTS

Mean baseline PbrO2 was 41.9 +/- 11.3 mm Hg. PbrO2 varied linearly with changes in ET-CO2, ranging from 20 to 60 mm Hg (r2 = 0.70). The minimum PbrO2 with hypocarbia was 5.9 mm Hg, and the maximum PbrO2 with hypercarbia was 132.4 mm Hg. PbrO2 varied with MAP in a sigmoid fashion suggestive of pressure autoregulation between 60 and 150 mm Hg (r2 = 0.72). The minimum PbrO2 with hypotension was 1.4 mm Hg, and the maximum PbrO2 with hypertension was 97.2 mm Hg. In addition, CBF correlated linearly with PbrO2 during CO2 reactivity testing (r2 = 0.84).

CONCLUSION

In the uninjured brain, PbrO2 exhibits CO2 reactivity and pressure autoregulation. The relationship of PbrO2 with ET-CO2 and MAP appears to be similar to those historically established for CBF with ET-CO2 and MAP. This suggests that, under normal conditions, PbrO2 is strongly influenced by factors that regulate CBF.

Mild hypothermia decreases the incidence of transient ADC reduction detected with diffusion MRI and expression of c-fos and hsp70 mRNA during acute focal ischemia in rats

The effects of mild hypothermia on the apparent diffusion coefficient of water (ADC) and expression of c-fos and hsp70 mRNA were examined during acute focal cerebral ischemia. Young adult rats were subjected to 60-min middle cerebral artery occlusion under either normothermia (37.5 degrees C) or hypothermia (33 degrees C). Diffusion-weighted echo-planar magnetic resonance imaging was used to monitor changes in ADC throughout the ischemic period. Perfusion MRI with dysprosium contrast was used at the end of the ischemic period to verify that the occlusion was successful. C-fos and hsp70 mRNA expression were examined with in situ hybridization at the end of the ischemic period. The results indicate that the size of the region that exhibited reduced ADC was smaller during hypothermia than during normothermia. Hypothermia also decreased the frequency of occurrence of transient ADC reductions, especially in dorsal aspects of cortex. Expression of both c-fos and hsp70 mRNA were markedly reduced by hypothermia. Transient ADC reduction and c-fos expression are associated with spreading depression, which is believed to contribute to lesion expansion during acute focal ischemia. The results suggest that part of the neuroprotective effect of hypothermia may be due to a reduced incidence of spreading depression.

Evolution of brain injury after transient middle cerebral artery occlusion in neonatal rats

BACKGROUND AND PURPOSE

Stroke in preterm and term babies is common and results in significant morbidity. The vulnerability and pathophysiological mechanisms of neonatal cerebral ischemia-reperfusion may differ from those in the mature cerebral nervous system because of the immaturity of many receptor systems and differences in metabolism in neonatal brain. This study details the neuropathological sequelae of reperfusion-induced brain injury after transient middle cerebral artery (MCA) occlusion in the postnatal day 7 (P7) rat.

METHODS

P7 rats were subjected to 3 hours of MCA occlusion followed by reperfusion or sham surgery. Diffusion-weighted MRI was performed during MCA occlusion, and maps of the apparent diffusion coefficient (ADC) were constructed. Contrast-enhanced MRI was performed in a subset of animals before and 20 minutes after reperfusion. Triphenyltetrazolium chloride (TTC) staining of the brain was performed 24 hours after reperfusion. Immunohistochemistry to identify astrocytes (glial fibrillary acidic protein), reactive microglia (ED-1), and neurons (microtubule-associated protein 2) and cresyl violet staining were done 4, 8, 24, and 72 hours after reperfusion.

RESULTS

On contrast-enhanced MRI, nearly complete disruption of cerebral blood flow was evident in the vascular territory of the MCA during occlusion. Partial restoration of blood flow occurred after removal of the suture. A significant decrease of the ADC, indicative of early cytotoxic edema, occurred in anatomic regions with a disrupted blood supply. The decline in ADC was associated with TTC- and cresyl violet-determined brain injury in these regions 24 hours later. The ischemic core was rapidly infiltrated with reactive microglia and was surrounded by reactive astroglia.

CONCLUSIONS

In P7 rats, transient MCA occlusion causes acute cytotoxic edema and severe unilateral brain injury. The presence of a prominent inflammatory response suggests that both the ischemic episode and the reperfusion contribute to the neuropathological outcome.

Cerebral oxygenation during hemorrhagic shock: perils of hyperventilation and the therapeutic potential of hypoventilation

OBJECTIVES

Prophylactic hyperventilation of patients with head injuries worsens outcome, presumably by exacerbating tissue hypoxia. Oxygen tension in brain tissue (PbrO2) provides a direct measurement of cerebral metabolic substrate delivery and varies with changing end-tidal carbon dioxide tension (ETCO2) and mean arterial pressure. However, the effects of hyperventilation and hypoventilation on PbrO2 during hemorrhagic shock are not known. The aim of this study was to examine the effects of alteration in ventilation on PbrO2 in hemorrhaged swine.

METHODS

Clark-type polarographic probes were inserted into the brain tissue of seven swine to measure PbrO2 directly. To examine the effects of alterations in ventilation on hemorrhage-induced hypotension, swine were hemorrhaged to 50% estimated blood volume and PbrO2 was monitored during hyperventilation (RR = 30) and hypoventilation (RR = 4).

RESULTS

After the 50% hemorrhage, PbrO2 declined rapidly from 39.8 +/- 4.6 mm Hg to 11.4 +/- 2.2 mm Hg. Hyperventilation resulted in a further 56% mean decrease in PbrO2. Hypoventilation produced a 166% mean increase in PbrO2. These changes were significant (p = 0.001) for absolute and percentage differences from baseline.

CONCLUSION

During hemorrhage, alterations in ventilation significantly changed PbrO2: hyperventilation increased brain-tissue hypoxia whereas hypoventilation alleviated it. This finding suggests that hyperventilation has deleterious effects on brain oxygenation in patients with hemorrhagic shock and those with head trauma. Conversely, hypoventilation with resultant hypercapnia may actually help resolve hemorrhagic shock-induced cerebral hypoxia.

Transient MRI-detected water apparent diffusion coefficient reduction correlates with c-fos mRNA but not hsp70 mRNA induction during focal cerebral ischemia in rats

Cerebral ischemia induces immediate early genes such as c-fos and stress genes such as hsp70. In this study, the spatial relationships between c-fos and hsp70 mRNA expression and changes detectable with diffusion and perfusion magnetic resonance (MR) imaging were examined. The middle cerebral artery (MCA) of young adult rats was occluded for 30 or 60 min. Diffusion MR (D-MR) images were acquired continuously during the ischemic period and dysprosium-contrast perfusion (P-MR) images were acquired at the end of the ischemic period. C-fos and hsp70 mRNA expression were examined with in situ hybridization. The most significant finding of this work was that for both durations of ischemia, c-fos induction was observed in cortical and sub-cortical regions exhibiting a transient reduction in the apparent diffusion coefficient of water (ADC). Transients which occurred on a time scale of 3 min may have been caused by spreading depression. Those occurring on a 10-min time scale may have been caused by an initial reduction in blood flow with occlusion that was followed by an ischemia-induced increase in collateral blood flow. P-MR imaging showed that perfusion in c-fos positive regions was higher than in regions with persistently reduced ADC. Hsp70 induction did not correlate with transient ADC reduction. It was induced in the MCA territory in regions showing persistent ADC changes, with induction being greatest at the periphery of these regions. It was also induced in regions that exhibited both spontaneous reversal of the diffusion changes and decreased perfusion.

Neonatal reversible focal cerebral ischemia: a new model

While numerous animal models exist for studying neonatal brain injury after cerebral ischemia-hypoxia, an adequate model for assessing reversible focal ischemia in the neonatal rat has not been reported. This paper describes in detail a new surgical procedure for creating a non-hemorrhagic, reperfused focal ischemic lesion in the neonatal, 7-day-old rat pup.

Influence of severity of myocardial injury on distribution of macromolecules: extravascular versus intravascular gadolinium-based magnetic resonance contrast agents

OBJECTIVES

This study sought to 1) compare the distribution of extravascular (573 Da) and intravascular (92 kDa) magnetic resonance (MR) contrast agents in reperfused infarcted myocardium, and 2) investigate the effect of injury severity on these distribution patterns.

BACKGROUND

Myocardial distribution of low and high molecular weight contrast agents depends on vascular permeability, diffusive/convective transport within the interstitium and accessibility of the intracellular compartment (cellular integrity).

METHODS

To vary the severity of myocardial injury, 72 rats were subjected to 20, 30, 45 or 75 min (n = 18, respectively) of coronary artery occlusion. After 2 h of reflow, the animals received either 0.05 mmol/kg of gadolinium-diethylenetriaminepentaacetic acid-bismethylamide (Gd-DTPA-BMA) (n = 24), (Gd-DTPA)30-albumin (n = 24) or saline (control group, n = 24). Three minutes after injection, the hearts were excised and imaged (spin-echo imaging parameters: repetition time 300 ms, echo time 8 ms, 2-tesla system), followed by triphenyltetrazolium chloride staining for infarct detection and sizing.

RESULTS

Histomorphometric and MR infarct size (expressed as percent of slice surface) correlated well: r = 0.96 for Gd-DTPA-BMA; r = 0.95 for (Gd-DTPA)30-albumin. On Gd-DTPA-BMA-enhanced images, reperfused myocardial infarctions were homogeneously enhanced. The ratio of signal intensity of infarcted/ normal myocardium increased with increasing duration of ischemia (overall p < 0.0001, analysis of variance [ANOVA]), indicating an increase in the distribution volume of Gd-DTPA-BMA in postischemic myocardium. On (Gd-DTPA)30-albumin-enhanced images, reperfused infarctions consisted of a bright border zone and a less enhanced central core. The extent of the core increased with increasing duration of ischemia (overall p value < 0.0001, ANOVA).

CONCLUSIONS

At 2 h of reperfusion, the distribution of MR contrast agents in postischemic myocardium is 1) specific for extravascular and intravascular agents, and 2) modulated by the duration of ischemia.

Quantification of area at risk during coronary occlusion and reperfusion by means of MR perfusion imaging

PURPOSE

Considerable clinical interest has focused on the size of ischemic myocardium. Fast MR imaging in conjunction with MR contrast media has the potential to identify hypoperfused and infarcted myocardium. This study used MR perfusion imaging to detect and quantify reperfused ischemic myocardium during a brief coronary occlusion and reperfusion, and to characterize the spatial extent of ischemic and reperfused ischemic myocardium relative to the "true" size of the area at risk as defined in histochemical morphometry at post mortem.

MATERIAL AND METHODS

The left circumflex (LCX) coronary artery in 8 dogs was occluded for 15 min followed by reperfusion in order to produce regional reversible myocardial ischemia. Perivascular Doppler probes were used to measure blood flow in the left anterior descending (LAD) and LCX coronary arteries. Fast inversion recovery-prepared gradient-recalled-echo images were acquired to delineate the ischemic area during occlusion, and the area of reversible ischemic injury at 1 and 30 min of reperfusion. The size of ischemic and reperfused ischemic myocardium were compared with the area at risk as determined by histochemical morphometry at post mortem.

RESULTS

During LCX occlusion, LCX flow decreased from 16+/-1 to 0.2+/-0.1 ml/min. On contrast-enhanced images, ischemic myocardium was evident as a zone of relatively low signal intensity (SI) compared to normal myocardium. The size of the ischemic region was significantly smaller (30+/-2%) than at post mortem (36+/-3%; p<0.05). Immediately after reperfusion, LCX flow increased to 83+/-11 ml/min and the contrast medium caused greater enhancement in the reperfused ischemic region than in the normal myocardium (69+/-3 vs 42+/-3 arbitrary units; p<0.05). The increase in regional SI correlated closely with the increase in regional blood flow (r=0.73). At 1 min of reperfusion, the size of the reperfused ischemic myocardium was larger (48+/-3%, p<0.05) than the area at risk measured at post mortem. At 30 min of reperfusion, when the flow returned to baseline values (16+/-2 ml/min), contrast bolus produced no differential enhancement between the 2 myocardial territories.

CONCLUSION

MR perfusion imaging has the potential to detect and quantify the size of ischemic myocardium and the region of post-occlusive hyperemia in the early reperfusion period. There is a significant direct linear relationship between the regional contrast enhancement of reperfused ischemic myocardium and the blood flow during post-occlusive hyperemia. The difference in the size of the area at risk at MR perfusion imaging and at histochemical morphometry may reflect an influence of coronary collateral circulation.

Quantification of coronary artery volume flow rate using fast velocity-encoded cine MR imaging

OBJECTIVE

Breath-hold velocity-encoded cine (VENC) MR imaging has been proposed as a method for measuring coronary blood flow. However, most studies have measured velocity rather than volume flow rate in the coronary arteries. The purpose of this study was to measure volume flow rate in the coronary artery of dogs using high-speed gradients and to compare MR flow measurements with those obtained with a sonographic flowmeter.

MATERIALS AND METHODS

Fast VENC MR images were obtained with a high-speed-gradient 1.5-T MR system in seven anesthetized dogs before and after administration of dipyridamole. Images were acquired on double oblique planes perpendicular to the left anterior descending arteries with a slice thickness of 5 mm, a field of view of 20 x 10 cm, a velocity window of +/- 1 m/sec, an average imaging time of 21 sec, a TR/TE of 11/5, and a temporal resolution of 44 msec.

RESULTS

Coronary flow measured with VENC MR imaging correlated well with flow measured by the flowmeter (r = .95, slope = 0.97, n = 88). Interobserver variability in measuring coronary flow volume was 8%.

CONCLUSION

Fast VENC MR imaging with high-speed gradients can provide accurate quantification of volume flow rate in coronary arteries.

Measurement of collateral blood flow in a porcine model of aortic coarctation by velocity-encoded cine MRI

The purpose of this study was to investigate the time course of development of collateral blood flow in an animal model of aortic coarctation. A juxtaductal aortic stenosis (model coarctation) was surgically created in five juvenile pigs. MRI was performed preoperatively, 1 to 2 days postoperatively, and 2 to 10 weeks postoperatively. Aortic blood flow was measured by velocity-encoded cine MR (VENC-MR). The percent change in aortic blood flow (delta BF) from proximal to distal descending thoracic aorta was calculated, and a multiple-comparison paired t test used to assess changes in delta BF over time. Invasive flow measurements were obtained in one animal before sacrifice using an ultrasonic probe. delta BF preoperatively was -2 +/- 8% (mean +/- SE). delta BF increased to 32 +/- 7% (mean +/- SE, P = .022) 2 days postoperatively and 55 +/- 19% (P = .032) 2 to 8 weeks postoperatively. Invasive measurements were in qualitative agreement with the VENC-MR data. VENC-MR is an accurate noninvasive method of measuring collateral blood flow in aortic coarctation. Recruitment and development of collateral flow pathways occur rapidly in an animal model.

Transient cerebral ischemia. Association of apoptosis induction with hypoperfusion

Apoptosis is thought to be important in the pathogenesis of cerebral ischemia. The mechanism of apoptosis induction remains unclear but several studies suggest that it is preferentially triggered by mild/moderate microcirculatory disturbances. We examined in cats whether induction of apoptosis after 2.5 h of unilateral middle cerebral artery occlusion plus 10 h of reperfusion is influenced by the degree of cerebral microcirculatory disturbance. Quantitative monitoring over time of the disturbances of cerebral microcirculation in ischemic brain areas and evaluation of cytotoxic edema associated with perfusion deficits was achieved by using two noninvasive magnetic resonance imaging techniques: (a) high-speed echo planar imaging combined with a bolus of magnetic susceptibility contrast agent; and (b) diffusion-weighted imaging. Apoptosis-positive cells were counted in anatomic areas with different severity of ischemic injury characterized by magnetic resonance imaging, triphenyltetrazolium chloride, and hemotoxylin and eosin staining. The number of apoptosis-positive cells was significantly higher in anatomic areas with severe perfusion deficits during occlusion and detectable histologic changes 10 h after reperfusion. In contrast, in areas where perfusion was reduced but maintained during occlusion there were no detectable histological changes and significantly fewer apoptosis-positive cells. A similar number of cells that undergo apoptosis were shown in regions with transient or prolonged subtotal perfusion deficits. These results suggest that the apoptotic process is induced in the ischemic core and contributes significantly in the degeneration of neurons associated with transient ischemia.

Alterations in T1 of normal and reperfused infarcted myocardium after Gd-BOPTA versus GD-DTPA on inversion recovery EPI

This study tested whether Gd-BOPTA/Dimeg or Gd-DTPA exerts greater relaxation enhancement for blood and reperfused infarcted myocardium. Relaxivity of Gd-BOPTA is increased by weak binding to serum albumin. Thirty-six rats were subjected to reperfused infarction before contrast (doses = 0.05, 0.1, and 0.2 mmol/kg). delta R1 was repeatedly measured over 30 min. Gd-BOPTA caused greater delta R1 for blood and myocardium than did Gd-DTPA; clearance of both agents from normal- and infarcted myocardium was similar to blood clearance; plots of delta R1 myocardium/delta R1 blood showed equilibrium phase contrast distribution. Fractional contrast agent distribution volumes were approximately 0.24 for both agents in normal myocardium, 0.98 and 1.6 for Gd-DTPA and Gd-BOPTA, respectively, in reperfused infarction. The high value for Gd-BOPTPA was ascribed to greater relaxivity in infarction versus blood. It was concluded that Gd-BOPTA/Dimeg causes a greater delta R1 than Gd-DTPA in regions which contain serum albumin.

Quantification of the extent of area at risk with fast contrast-enhanced magnetic resonance imaging in experimental coronary artery stenosis

Fast magnetic resonance (MR) imaging techniques have the capability of demonstrating regions of ischemia caused by stenosis. The size of the potentially ischemic area determines the importance of the stenosis. The purpose of this study was to determine the relative values of relaxivity-enhancing and magnetic-susceptibility MR contrast media in detecting and sizing the area at risk in dogs. Eight dogs were subjected to critical left circumflex coronary artery (LCX) stenosis. Sixty sequential inversion-recovery- and driven-equilibrium-prepared fast gradient recalled echo images were acquired during bolus administration of 0.03 mmol/kg gadodiamide or 0.4 mmol/kg sprodiamide in basal and vasodilated (dipyridamole-stress) states. The size of the area at risk was measured and compared with that measured post mortem. In the basal state, gadodiamide and sprodiamide equivalently altered the signal intensities of nonischemic myocardium and the territory of stenosed coronary artery. Dipyridamole produced a significant increase in left anterior descending coronary artery flow with a decrease in LCX flow. The hypoperfused region was observed as a low-and high-signal intensity region after administration of gadodiamide and sprodiamide, respectively. The size of the hypoperfused region was slightly smaller with gadodiamide (37.4% +/- 2.8%) and sprodiamide (34.0% +/- 2.2%) than the true area at risk measured post mortem (41.8% +/- 2.2%; p < 0.05). Dipyridamole perfusion MR imaging with relaxivity or susceptibility contrast media is a noninvasive method to identify and quantify the area at risk in the territory of a stenotic coronary artery. Changes in myocardial signal intensity on fast gradient recalled echo images reflect the augmentation of flow and volume induced with dipyridamole and are consistent with the "steal phenomenon."

Sensitivity of high-speed "perfusion-sensitive" magnetic resonance imaging to mild cerebral ischemia

This study assessed the sensitivity of contrast-enhanced dynamic echo-planar imaging to subtotal stenosis of the middle cerebral artery as a model of mildly compromised cerebral blood supply. Dynamic data was analyzed in terms of the relative cerebral blood volume (rCBV) and bolus peak arrival time (BPAT), and the prognostic utility of these parameters was compared with measurements of the regional apparent diffusion coefficient of water (ADC) with the goal of identifying tissue at risk of future infarct. Dynamic echo-planar MRI in conjunction with bolus administration of a magnetic susceptibility contrast agent was used in a cat model of acute, unilateral cerebral ischemia, induced by partial occlusion (stenosis) of the right middle cerebral artery. The contrast agent transit was analyzed in terms of the regional time of arrival of the peak bolus-induced signal loss as well as the time integral of agent concentration. Pixel-by-pixel maps of cerebrovascular parameters (rCBV, BPAT) were constructed along with spatial maps of the ADC, derived from diffusion-weighted MR images at the same anatomical level. Arterial stenosis was maintained for a 6 h period, after which histological determination of tissue viability was obtained. Maps of BPAT showed sensitivity to mild flow perturbations not detectable from cerebral blood volume estimations from the same bolus injection or from determinations of the apparent diffusion coefficient of water. Of nine animals subjected to subtotal stenosis, BPAT identified compromised tissue in all nine after 1 h of stenosis. No animals had differences in rCBV or ADC at this point. Stenosis was maintained for 6 h in 7 of the cats. After 6 h, two cats had developed identifiable injury on ADC and rCBV maps. Of the remaining five, where rCBV and ADC appeared normal even after 6 h, three exhibited abnormal histological staining, whereas two indeed appeared normal. In the other two cats where initial subtotal stenosis was later made total, the anatomical region identified as "compromised" during stenosis, by the appearance of delayed bolus peak arrival, matched the area of subsequent infarct after total occlusion of the same vessel. Echo planar imaging in conjunction with bolus administration of a magnetic susceptibility contrast agent appears sensitive to mild perturbations to blood supply. These perturbations may not be resolved on synthesized maps of relative cerebral blood volume or apparent diffusion coefficient. Although "compromised" blood supply does not necessarily lead to infarct (over the 6-h course of this study), it may represent tissue particularly at risk of infarct in the event of further insult.

Acute hemodynamic effects of recently developed monomer and dimer magnetic resonance imaging contrast media: a comparative study

RATIONALE AND OBJECTIVES

We evaluated and compared the acute cardiovascular effects of equiosmolar doses of recently developed nonionic monomer and macrocyclic dimer magnetic resonance (MR) imaging contrast media with the clinically available ionic and nonionic MR contrast media.

METHODS

Normotensive adult Sprague-Dawley rats were divided into six groups of seven rats per group. Group 1 received the nonionic monomer Gd-CMPA-BMPA (500 mmol/l solution); group 2 received the nonionic dimer Gd(2)2(O)DO3A (500 mmol/l solution); group 3 also received Gd(2)2(O)DO3A but at a higher concentration (1,000 mmol/l solution); group 4 received gadopentetate dimeglumine (500 mmol/l solution); and group 5 received gadodiamide (500 mmol/l solution). Each rat received a rapid (1-2 sec) bolus intravenous injection of 0.1, 0.25, and 0.5 mmol/kg of each contrast agent. Group 6 was used to test the peak effects of quiosmolar glucose solutions (500, 1,000, and 2,000 mOsm/kg water). Data were acquired at baseline, 20 sec (peak effect) after injection, and 1, 3, 5, and 10 min after injection. Peripheral (systolic, diastolic, and mean) pressure, central venous pressure, left ventricular (LV) pressure (peak systolic and end diastolic) pressure, first derivative of left ventricular pressure (+/-dP/dt), rate pressure product, and heart rate were measured.

RESULTS

Bolus administration (0.1, 0.25, and 0.5 mmol/kg) of Gd-CMPA-BMPA and gadodiamide (500 mmol/l) had no significant effects on the monitored cardiovascular parameters. Bolus injection of 0.25 and 0.5 mmol/kg Gd(2)2(O)DO3A (500 and 1,000 mmol/l) and gadopentetate dimeglumine (500 mmol/l) caused transient cardiovascular depression, including decreased peripheral blood pressure, LV systolic pressure, peak positive and negative dP/dt, and rate pressure product, but an increased LV end diastolic pressure. These cardiovascular effects were slightly less profound than those produced by gadopentetate dimeglumine.

CONCLUSION

Gd-CMPA-BMPA and gadodiamide have no adverse cardiovascular effects. Gd(2)2(O)DO3A and gadopentetate dimeglumine cause vasodilation and reduced cardiac performance. Therefore, presuming similar effects, if Gd(2)2(O)DO3A and gadopentetate dimeglumine are to be used at high doses for the MR quantification of blood volume or as a bolus for perfusion study, appropriate consideration should be given to possible adverse physiologic changes.

MR imaging characterization of postischemic myocardial dysfunction ("stunned myocardium"): relationship between functional and perfusion abnormalities

Stunned myocardium has been detected in patients treated successfully with thrombolytic agents. The hypothesis of this study was that fast gradient echo (GRE) imaging could be used to characterize the regional functional and perfusion abnormalities that are indicative of myocardial stunning. This study was designed to monitor and correlate the extent of wall thickness and perfusion abnormalities as determined by fast (segmented k space) cine and contrast enhanced GRE imaging, respectively. Dogs were subjected to left circumflex (LCX) coronary artery occlusion (15 min) followed by 30-minute reperfusion (n = 8). Perivascular flow probes were used to continuously measure flow in left anterior descending (LAD) and LCX coronary arteries. Short-axis inversion recovery prepared fast GRE and cine images were acquired at baseline, at occlusion, and at 1, 10, and 30 minutes of reflow. Regional signal intensity and percent systolic wall thickening were determined at 26 equally spaced circumferential positions to compare the extent of functional and perfusion abnormalities. During occlusion and reperfusion, the ischemic region was demonstrated on contrast-enhanced images as a hypointense and hyperintense region, respectively. During occlusion, the extent of the perfusion defect (32% +/- 2% of the circumference of the equatorial slice) correlated closely (r = .74) with the extent of contractile dysfunction (35% +/- 2%). After reperfusion, there was transient recovery in the percent wall thickening (26% +/- 4% vs 36% +/- 4% normal), coinciding with the reactive hyperemic response, but this was followed by a significant decline in wall thickening at 10 minutes (19% +/- 4%) and 30 minutes (12% +/- 2%). Fast MR imaging may be useful to monitor postischemic myocardial abnormalities after thrombolytic therapy and the response to pharmacologic interventions.

Evaluation of the effects of intravascular MR contrast media (gadolinium dendrimer) on 3D time of flight magnetic resonance angiography of the body

The aims of this preliminary study were to establish the efficacy and minimum effective dose of TG(5)(FdDO3A)(52) gadolinium dendrimer for contrast-enhanced, three-dimensional (3D) time of flight (TOF) magnetic resonance angiography (MRA) of the body. In a dose ranging study in eight rabbits (Group A), each of two animals received 0.03; 0.02; 0.01; or 0.005 mmol/kg of the agent for 3D-TOF MRA of the pelvic circulation in the axial and coronal planes. An additional nine animals (Group B) received a dose of 0.02 mmol/kg for 3D-TOF MRA of the mediastinum, abdomen or of the lower limbs. Quantitative and qualitative analyses of the images from Group A demonstrated a dose-related reduction in saturation effects and improved visualization of vascular structures, with maximal augmentation of the contrast-to-noise ratio (CNR) at 0.03 mmol/kg. The dose of 0.02 mmol/kg was found to be the minimal effective dose at the three vascular regions.

Verapamil reduces the size of reperfused ischemically injured myocardium in hypertrophied rat hearts as assessed by magnetic resonance imaging

Contrast-enhanced magnetic resonance (MR) imaging was used to detect and quantify the extent of myocardial injury after a brief coronary occlusion and reperfusion in response to verapamil treatment in a rat model of left ventricular hypertrophy (LVH). Two groups of rats were prepared by banding the abdominal aorta for 7 to 8 weeks to produce LVH. Group 1 (n = 13) received oral verapamil for 3 days, whereas group 2 (n = 13) received no therapy. Before MR examination was performed, each rat was subjected to 25 min of coronary artery occlusion followed by 1 hour of reperfusion. T1-weighted spin echo images were acquired before and after 0.3 mmol/kg gadoteridol was injected. Three images were acquired at contiguous levels of the LV and used to estimate the size of the myocardial injury. The size of the infarcted region was demarcated at postmortem examination by using triphenyltetrazolium chloride dye (TTC). Before contrast medium was administered, no significant difference in signal intensity was seen between nonischemic and reperfused ischemically injured myocardium. After gadoteridol was injected, a hyperintense zone indicative of myocardial injury was observed in 8 of 13 rats treated with verapamil and in all untreated animals. The size of the injury was significantly larger in untreated hearts than in hearts treated with verapamil as defined on MR images (25% +/- 5% vs 18% +/- 5%, p < 0.05) and TTC staining (12% +/- 4% and 4% +/- 1%, p < 0.05). Good correlation (r = 0.91) was found between the two measurements. No significant difference in the size of jeopardy area was seen between the two groups as (defined by blue dye infusion). In conclusion, contrast-enhanced MR imaging is a suitable technique to evaluate the effects of therapies applied to reduce myocardial injury. Verapamil can cause reduction in the extent of ischemic injury after reperfusion of hypertrophied myocardium.

Evaluation of recombinant human basic fibroblast growth factor (rhbFGF) as a cerebroprotective agent using high speed MR imaging

The potential cerebroprotective effects of recombinant human basic fibroblast growth factor (rhbFGF) were evaluated in a feline model of acute cerebral ischemia using high-speed magnetic resonance imaging (MRI) in conjunction with immunohistology. The neuroprotective efficacy of three doses of rhbFGF was evaluated in a unilateral middle cerebral artery (MCA) occlusion/reperfusion model. Ten h following a 2 h period of MCA occlusion in control (vehicle-treated) animals, cerebral perfusion in the ischemic hemisphere was 58 +/- 17% of the contralateral normal hemisphere. Corresponding ischemic/normal perfusion ratios in rhbFGF-treated groups were not significantly different: 54 +/- 16% (14 micrograms/kg/h dose), 40 +/- 19% (42 micrograms/kg/h dose) and 75 +/- 8% (125 micrograms/kg/h dose). Triphenyltetrazolium chloride histopathological assessment demonstrated brain damage in vehicle-treated animals comprising 31 +/- 15% of the hemisphere; in rhbFGF-treated groups injury was not significantly different: 19 +/- 4% (14 micrograms/kg/h rhbFGF), 24 +/- 6% (42 micrograms/kg/h rhbFGF) and 16 +/- 10% (125 micrograms/kg/h rhbFGF). Immunohistochemical analysis of brain sections using glial fibrillary acidic protein (GFAP) revealed that in animals that showed marked perfusion deficits throughout the entire experiment (regardless of treatment), GFAP staining was elevated contralateral to the occlusion and absent ipsilaterally. While some tendency towards protection is found, particularly at higher doses of rhbFGF, it must be concluded that in the chosen stroke model and time interval, the doses used did not significantly improve reperfusion or confer significant cerebroprotective benefit. Non-invasive high-speed MRI was found to be useful for evaluation of putative cerebroprotective agents.

Multislice measurement of first-pass transit of gadobenate dimeglumine in normal and ischemic myocardium in dogs

RATIONALE AND OBJECTIVES

We monitored the differences in the first passage of gadobenate dimeglumine through normal and ischemic myocardium with left anterior descending (LAD) coronary artery occlusion in dogs.

METHODS

Dynamic multislice images of the heart were taken on a 1.5-T magnetic resonance (MR) imager. In six normal dogs, inversion recovery (IR)-prepared fast gradient-recalled echo (GRE) images were acquired at five doses of gadobenate dimeglumine (0.005-0.1 mmol/kg). First passage of the contrast medium through normal and acutely ischemic myocardium were monitored in seven dogs subjected to LAD coronary artery occlusion.

RESULTS

IR-prepared GRE images showed a dose-dependent increase in the signal intensity (SI) of the myocardium. In dogs with LAD coronary artery occlusion, there was a significant increase in the SI of normal myocardium (p < .01) than in ischemic myocardium after injection of 0.025 mmol/kg gadobenate dimeglumine.

CONCLUSION

The first-pass dynamics of gadobenate dimeglumine through normal and ischemic myocardium can be monitored with a multislice acquisition using a clinical MR imager and differentiated between normal and ischemic myocardium in dogs.

A new model for occluding the left coronary artery in the rabbit without thoracotomy

A small, nonmagnetic coil was fluoroscopically guided into the left circumflex coronary artery (LCXA) at the midventricular level via the left femoral artery in seven rabbits. Coronary artery occlusion was confirmed by intracoronary contrast injection, electrocardiographic changes, akinesis of the posterolateral wall distal to the occluded area, and histopathological staining of the myocardium. This new model of a closed-chest coronary artery occlusion in the rabbit may be suitable for acute ischemic myocardium studies that do not require reperfusion.

First-pass contrast-enhanced inversion recovery and driven equilibrium fast GRE imaging studies: detection of acute myocardial ischemia

The purpose of this study was (1) to monitor the dynamic effects of T1-enhancing and magnetic susceptibility contrast material on normal canine myocardium using inversion recovery (IR)- and driven equilibrium (DE)-prepared fast gradient-recalled echo (GRE) sequences and (2) to determine the relative value of T1-enhancing and magnetic susceptibility contrast material in detecting regions of ischemia in the same animal. Normal dogs (n = 5) and dogs with acute occlusion of the left anterior descending (LAD) coronary artery (n = 11) were studied using a 1.5-T MR imager. ECG-gated fast IR-prepared GRE images were acquired using TI/TR/TE of 700/7.0/2.9 msec and a flip angle of 7 degrees. Fast DE-prepared GRE images were obtained using a flip angle of 12 degrees and a DE delay/TR/TE of 60/10.2/4.2 msec. Sequential images were acquired to monitor transit of 0.05 mmol/kg gadodiamide injection and 0.2 and 0.4 mmol/kg sprodiamide injection. On slice-nonselective IR fast GRE images, gadodiamide caused significant enhancement of the normal myocardium and the left ventricular (LV) chamber blood. In dogs with LAD occlusion, the ischemic region was defined as an area of low signal intensity (SI). On DE-prepared GRE sequences, administration of sprodiamide resulted in a substantial decrease in signal from normal myocardium and LV chamber blood in normal dogs. In animals subjected to LAD occlusion, this contrast medium produced a transient decrease in SI from normal myocardium (P < .05) and no significant change in SI from ischemic myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary artery stenosis: detection with contrast-enhanced MR imaging in dogs

PURPOSE

To monitor with fast gradient-echo magnetic resonance (MR) imaging the dynamics of gadolinium benzyloxypropionictetraacetate (gadobenate) dimeglumine on myocardial signal intensity in dogs with critical left circumflex coronary artery stenosis.

MATERIALS AND METHODS

Fast gradient-echo MR images were acquired in a short axis of the left ventricle. Two bolus injections of 0.05 mmol/kg gadobenate dimeglumine were administered in the basal state after stenosis and after infusion of 0.5 mg/kg dipyridamole.

RESULTS

In the basal state, there was an equivalent increase in signal intensity of normal and hypoperfused myocardium during the first pass. Dipyridamole increased left anterior descending flow (287% +/- 36; P < .05) and decreased left circumflex flow (65% +/- 14; P < .05). The magnitude of signal intensity increase during the second bolus in the hypoperfused region was less than that of normal myocardium (P < .05). Contrast-enhanced images showed the hypoperfused region as smaller than the postmortem measurement (43.8% +/- 3.3; P < .05).

CONCLUSION

Contrast-enhanced fast MR imaging in the vasodilated state allows detection of hypoperfused myocardium in the presence of critical coronary stenosis.

AUR Memorial Award. Identification of myocardial cell death in reperfused myocardial injury using dual mechanisms of contrast-enhanced magnetic resonance imaging

RATIONALE AND OBJECTIVES

Because the magnitude of dysprosium-induced signal loss depends on the microheterogeneity of its distribution (exclusion from intracellular space), we proposed that loss of myocardial cell integrity would be reflected by decreased potency of dysprosium in the injured compared with normal myocardium. We measured the effect of dysprosium on magnetic resonance (MR) imaging signal intensity of reperfused infarcted and nonischemic myocardium and related it to tissue concentration of the contrast media.

METHODS

Rats were subjected to 1 hr coronary artery occlusion followed by 1 hr reperfusion. After 45 min of reflow, group 1 (n = 9) received 1.0 and 0.2 mmol/kg dysprosium diethylenetriamine pentaacetic acid-bismethylamide (Dy-DTPA-BMA) and gadodiamide (Gd-DTPA-BMA), respectively. Group 2 (n = 7) received no contrast agents. Excised hearts were imaged with spin-echo T1- and T2-weighted sequences. After imaging, hearts were stained (triphenyltetrazolium chloride) to define the injured zones. Concentrations of Dy-DTPA-BMA and Gd-DPTA-BMA in regional myocardial tissue were determined by induction coupled plasma-atomic emission spectrometry. Separate groups received one or the other contrast medium alone to control for potential error from the mixed effects of the two agents.

RESULTS

Gd-DTPA-BMA delineated reperfused infarcted myocardium as a bright zone on T1-weighted images, thus indicating delivery of the agent and reperfusion at the tissue level. Dy-DTPA-BMA delineated the reperfused infarction as a bright region by decreasing the signal intensity of nonischemic myocardium significantly more than that of injured myocardium, despite being present in greater concentration (by 2.46-fold) in the injured myocardium.

CONCLUSION

These findings are consistent with the hypothesis that the failure of myocardial cells to exclude the dysprosium compound is responsible for the diminished potency of dysprosium to cause MR imaging signal intensity loss in reperfused myocardial infarction. The combination of the two contrast media may define reperfusion of the myocardium at the tissue level (Gadolinium distribution) and the presence and extent of myocardial necrosis (diminished dysprosium effect) in reperfused myocardial infarctions.

Inversion recovery EPI of bolus transit in rat myocardium using intravascular and extravascular gadolinium-based MR contrast media: dose effects on peak signal enhancement

Inversion recovery gradient recalled echo planar imaging (TI/TR/TE = 700/2000/10 ms) was used to dynamically monitor the first pass of an intravascular (GdDOTA-polylysine) and an extravascular (GdDTPA-BMA) contrast agent through normal rat myocardium. It was found that myocardial enhancement increased with dose of the intravascular agent to a limiting value of approximately 50% of fully relaxed intensity, consistent with enhancement of 40% of myocardial water content during the first pass. Larger doses produced no further increase in peak response. On the other hand, the extravascular agent caused incrementally increased enhancement throughout the dose range examined to a final value of 68 +/- 2% of fully relaxed intensity. The profile of dose dependence for both agents was inconsistent with monoexponential T1 relaxation. It was concluded that: (a) compartmentalization of myocardial water combined with restricted myocardial water diffusion limits the peak response during bolus transit; (b) extraction of the extravascular agent during transit elevates the peak response over that obtained from agent confined to the vascular volume; and (c) models that assume simple monoexponential T1 relaxation to derive time-density curves do not adequately describe the relationship between changes in signal intensity, R1 and contrast concentration.

Identification of myocardial reperfusion with echo planar magnetic resonance imaging. Discrimination between occlusive and reperfused infarctions

BACKGROUND

The current treatment of many cases of acute myocardial infarction involves the use of thrombolytic agents. Evaluation of this therapy requires determination of the success of reperfusion and assessment of the presence and extent of infarction in the reperfused territory. The present study was designed to simulate in rat models several possible outcomes of reperfusion therapy: (1) successful reperfusion and absence of myocardial infarction, (2) successful reperfusion and presence of myocardial infarction, and (3) unsuccessful reperfusion. The usefulness of contrast-enhanced fast magnetic resonance (MR) imaging in defining the success of reperfusion was investigated. The dynamic effects were examined of low and high doses of gadolinium-BOPTA/dimeglumine (Gd-BOPTA/dimeg) on myocardial signal using MR inversion recovery echo planar imaging (IR-EPI) and gradient recalled echo planar imaging (GR-EPI), respectively.

METHODS AND RESULTS

Rats were subjected to one of the following regimens: reperfused reversible myocardial injury (n = 9), reperfused irreversible myocardial injury (n = 9), and occlusive infarction (n = 9). MR echo planar images were acquired every 1 or 2 seconds before, during, and after administration of Gd-BOPTA/dimeg. In all groups, normal myocardial signal was sharply increased on IR-EPI and decreased on GR-EPI at the peak of the bolus, followed by a gradual decline to baseline. In animals subjected to reperfused reversible myocardial injury, normal and previously ischemic regions were indistinguishable during and after the passage of Gd-BOPTA/dimeg. On the other hand, enhancement of reperfused irreversibly injured myocardium was delayed but increased steadily to a higher level than normal myocardium on IR-EPI. The reperfused irreversibly injured myocardium was identified on IR-EPI as a zone of high signal (hot spot). On GR-EPI, signal loss in reperfused irreversibly injured myocardium was significantly less compared with normally perfused myocardium. In animals with occlusive infarctions, there was no change in signal intensity over the ischemic region on either IR-EPI or GR-EPI. Occlusive infarction was identified as zones of either low (cold spot) or high (hot spot) signal compared with normal myocardium, depending on MR pulse sequence and dose of the contrast medium.

CONCLUSIONS

The transit of Gd-BOPTA/dimeg monitored by fast MR imaging techniques can be used to distinguish between reperfused reversibly and reperfused irreversibly injured myocardium and between occlusive and reperfused infarctions.

The use of contrast-enhanced magnetic resonance imaging to define ischemic injury after reperfusion. Comparison in normal and hypertrophied hearts

RATIONALE AND OBJECTIVES

Magnetic resonance imaging (MRI) was used to demonstrate the infarction size in reperfused ischemic myocardium of normal and hypertrophied hearts, and to test the hypothesis that hypertrophied hearts manifest greater susceptibility to ischemia.

METHODS

Normal rats (n = 11) and rats subjected to left ventricular hypertrophy (LVH) by aortic banding (n = 13) were studied. After 7 weeks, the left coronary artery was occluded for 25 minutes and reperfused for 1 hour before MRI. Electrocardiogram-gated spin-echo images were acquired before and after administration of 0.3 mmol/kg gadoteridol. To quantify the hyperintense area demarcated by gadoteridol, 3 transaxial images were acquired at different levels. Jeopardy and infarcted areas were measured in the same three slices postmortem using blue dye and triphenyltetrazolium chloride (TTC) stain, respectively.

RESULTS

Before administration, there was no significant difference in signal intensity between nonischemic (0.42 +/- 0.03 arbitrary units) and ischemic (0.41 +/- 0.03) myocardium in either group. After gadoteridol injection, signal intensity of the reperfused injured region was higher than that of nonischemic myocardium (1.48 +/- 0.16 vs. 0.72 +/- 0.06, P < .05). Magnetic resonance delineation of the hyperintense area persisted for at least 30 minutes. The size of the hyperintense area was larger in LVH than in control hearts (25 +/- 5% vs. 7 +/- 3% of LV surface area, P < .05) and did relate closely to the area of myocardial infarction (r = .97), but not with the jeopardy area (r = .42). On TTC staining, the infarction size also was significantly greater in LVH than in normal group (18 +/- 5% vs. 5 +/- 2% of LV surface area, P < .05). The jeopardy areas of normal and LVH hearts showed no significant difference (46 +/- 2% vs. 47 +/- 3%).

CONCLUSION

Magnetic resonance imaging confirms the concept that reperfused myocardial injury is larger in LVH than normal hearts after brief coronary occlusion. Contrast-enhanced MRI can define the size of reperfused myocardial injury. Thus, MRI is a suitable technique to assess conditions accentuating ischemic injury.

New and reproducible technique for experimentally induced middle cerebral artery stenosis

A reproducible technique has been developed to create reversible middle cerebral artery (MCA) stenosis in the cat. Under pentobarbital anesthesia, the right eye was enucleated, MCA dissected and a suture looped around the vessel. A short segment of thicker suture was set horizontally on the MCA. The looped suture was then tied and the segment suture removed, resulting in partial stenosis. The knot could be easily removed, restoring blood flow. Perfusion and diffusion-sensitive magnetic resonance (MR) images were obtained over the next 6 hours to document cerebral blood flow abnormalities and developing ischemic changes, respectively. Laser-Doppler was performed to further document reduced flow through the ipsilateral hemisphere following MCA stenosis. Histopathologic sections were assessed for ischemic damage. Our results show this to be a reliable model of MCA stenosis and reperfusion, which is suitable for the evaluation of acute cerebral perfusion abnormalities resulting from arterial stenosis.

Mechanisms of brain injury associated with partial and complete occlusion of the MCA in cat

High-speed MR diffusion/perfusion imaging was performed to assess variable degree stenosis of the MCA and the formation of cytotoxic edema in a cat model of acute ischemia. Sodium transport was estimated in synaptosomes isolated from moderately perfused or non-perfused brain tissue. Complete MCA occlusion for 50-75 min produced a major disruption of brain sodium transport, whereas continued preservation of ion homeostasis and the activation of adaptive cell volume regulatory systems was associated with longer duration of moderate severity of ischemia. Preservation of neuronal ion homeostasis might be one of the main mechanisms contributing to the relative tolerance of the brain to moderate reductions in cerebral blood flow.

Comparison of T1-enhancing and magnetic susceptibility magnetic resonance contrast agents for demarcation of the jeopardy area in experimental myocardial infarction

RATIONALE AND OBJECTIVES

This study compared the areas demarcated by a T1-enhancing agent, Gd-DTPA-BMA, and a magnetic susceptibility agent, Dy-DTPA-BMA, with 201thallium autoradiography (indicator of perfusion) and postmortem histochemical staining with triphenyltetrazolium chloride (TTC)(indicator of infarction).

METHODS

Thirteen rats were subjected to coronary artery occlusion for 3 to 4 hours before acquisition of four sets of electrocardiogram-gated spin-echo magnetic resonance (MR) images: T1-weighted images before and after 0.2 mmol/kg Gd-DTPA-BMA; and T2-weighted images before and after 0.3 mmol/kg Dy-DTPA-BMA. After MR imaging, intravenous 201thallium delineated the area of decreased myocardial perfusion. At autopsy, TTC staining delineated the area of myocardial infarction.

RESULTS

A myocardial region in the distribution of the occluded artery was delinated as a hyperintense area ("hot-spot") by Dy-DTPA-BMA and as a hypointense area ("cold-spot") by Gd-DTPA-BMA. The hyperintense area demarcated by Dy-DTPA-BMA (51 +/- 3% of the area of the midequitorial slice of the left ventricle) showed a closer relationship to the area of decreased myocardial perfusion (jeopardized area) (46 +/- 3%), determined by 201thallium autoradiography, than the area of myocardial infarction (36 +/- 4%), determined by histochemical staining. However, the hypointense area demarcated by Gd-DTPA-BMA (29 +/- 2%) did not relate as closely to the area of decreased myocardial perfusion (slope = 0.54) or the area of myocardial infarction (r = 0.46).

CONCLUSIONS

The abnormal myocardial area delineated by the magnetic susceptibility agent showed a closer relationship to the area of deficient myocardial perfusion (jeopardy area) after coronary occlusion than that defined by T1-enhancing contrast media.

High-speed MR imaging of ischemic brain injury following stenosis of the middle cerebral artery

Magnetic susceptibility contrast-enhanced and diffusion-weighted echo planar magnetic resonance (MR) imaging was performed using a cat model of acute regional cerebral ischemia induced by partial stenosis of the right middle cerebral artery (MCA). The imaging data were correlated with triphenyltetrazolium chloride (TTC)-stained histopathologic coronal brain sections to determine the prognostic efficacy of high-speed MR imaging techniques in differentiating mild, moderate, and severe cerebral hypoperfusion. Brains of animals without cortical injury on TTC staining were found to have a reduction in peak contrast enhancement of 32 +/- 6% (mean +/- SD) below control values with no significant change in the apparent diffusion coefficient (ADC), determined from the diffusion-weighted MR images. In cases where moderate ischemic injury was observed in the TTC-stained sections, a 10-20% drop in the ADC was found over the 6-h study period, accompanied by a much wider variation in peak contrast enhancement. Finally, where TTC staining showed severe ischemic brain damage, a 40-50% drop in ADC and a reduction in peak contrast enhancement effect of > 95% were observed as early as 1 h following MCA stenosis. The significant correlation between imaging observations and histologically confirmed cerebral ischemia indicates that magnetic susceptibility contrast-enhanced echo planar MR imaging is sensitive to slight reductions in cerebral perfusion that fall below the threshold for reliably detectable ischemia-induced alterations in ADC. First-pass perfusion-sensitive imaging may thus be diagnostically useful in differentiating severely hypoperfused permanently injured tissue from the mildly hypoperfused ischemic penumbra.

First pass of an MR susceptibility contrast agent through normal and ischemic heart: gradient-recalled echo-planar imaging

Gradient-recalled echo-planar magnetic resonance (MR) imaging was used to monitor the first pass of a magnetic susceptibility contrast agent through the heart of normal rats and rats subjected to 60-minute occlusion of the anterior branch of the left main coronary artery. Each animal (six normal and six ischemic) received four doses (0.05, 0.1, 0.15, and 0.2 mmol/kg) of Dy-DTPA-BMA [diethylenetriaminepentaacetic acid-bis(methylamide)] administered as a bolus volume of 1.0 mL/kg. In both normal and ischemic rats, signal intensity in nonischemic myocardium was reduced by the contrast agent in a dose-dependent manner. Signal intensity in the ischemic zone was reduced much less, so that at a contrast agent dose of 0.1 mmol/kg or greater the ischemic zone was clearly defined as a high-intensity zone on echo-planar images. Plots of the change in the apparent T2* relaxation rate (delta R2*) during the peak bolus effect versus injected dose were well fit by straight lines for normal, nonischemic, and ischemic myocardium but not for blood in the left ventricle. No difference was seen between myocardial response in normal animals and in nonischemic regions in animals with coronary artery occlusion. These findings suggest that the contrast agent-induced changes in tissue T2* are monoexponential and support the idea that data derived from contrast transit studies may be useful for calculation of myocardial blood flow.

Echo-planar perfusion-sensitive MR imaging of acute cerebral ischemia

T2*-sensitive echo-planar magnetic resonance imaging was used with first-pass magnetic susceptibility contrast enhancement in a cat model of acute regional stroke to evaluate the relationship between cerebral hypoperfusion and ischemic brain damage. In normal brain, dose-dependent decreases in signal intensity were observed after intravenous injection of 0.15-0.50 mmol/kg dysprosium-diethylenetriaminepentaacetic acid bismethylamide or gadodiamide injection. Shortly after unilateral occlusion of the middle cerebral artery, foci of signal hyperintensity on diffusion-weighted images were observed in the ipsilateral basal ganglia. Sixty minutes after occlusion, perfusion deficits in the ipsilateral parietal and temporal cortical gray matter were observed to be spatially correlated with areas of hyperintensity on diffusion-weighted images. When reflow was attempted after 60 minutes, delayed contrast agent transit suggestive of partial ischemic tissue injury was demonstrated. Attempts to produce reflow after 2 hours did not restore normal brain perfusion and resulted in image hyperintensity and histopathologic brain damage. Six-hour occlusion was associated with pronounced perfusion deficits in the ischemic territory.

Effect of lidocaine on acute regional myocardial ischemia and reperfusion in the cat. An in-vivo 31P magnetic resonance spectroscopy study

RATIONALE AND OBJECTIVES

The authors examined the relationship between myocardial infarction, high-energy phosphate compounds, and regional contractility after myocardial ischemia and reperfusion in cats.

METHODS

Hemodynamic measurements, high-energy phosphate levels, and segmental shortening were measured every 30 minutes in two groups of cats subjected to 2 hours of occlusion of the left anterior descending coronary artery and 4 hours reperfusion. Group 1 (n = 10) animals were infused with a low level of lidocaine, 0.05 mg/kg/hr, while group 2 (n = 10) received a higher dose, 7.5 mg/kg/hr. The infarcted region was measured postmortem.

RESULTS

Group 1 animals had larger infarcts (39 +/- 6 vs. 12 +/- 5% of jeopardy, P < .05) and less phosphocreatine recovery during reflow (52 +/- 7% vs. 73 +/- 2% of control, P < .01) than did group 2. Group 2 showed recovery of percentage systolic shortening during reflow (1.4 +/- 2% at 30 minutes vs. 7.1 +/- 2.3% at 4 hours, P < .05), whereas group 1 exhibited no improvement. A significant correlation was found between infarct size under the surface coil and phosphocreatine content during reflow, but not between contractile function and infarction size or metabolite levels during reflow.

CONCLUSIONS

Lidocaine infusion enhanced recovery of myocardial contractility during reperfusion and decreased infarct size. Greater recovery of phosphocreatine during reperfusion was predictive of greater myocardial salvage during reperfusion.

Rapid MR imaging of a vascular challenge to focal ischemia in cat brain

Deoxygenated blood was effectively used as a magnetic resonance (MR) susceptibility contrast agent to distinguish perfused and nonperfused (ischemic) regions in a focal ischemia model in cat brain at 2T. Modulation of cerebral blood oxygenation levels in response to apnea was followed in real time with T2*-weighted (gradient-recalled) echo-planar MR imaging. Signal loss in the T2*-weighted images occurred only in perfused tissues as blood became globally deoxygenated. These data complemented information from diffusion-weighted and contrast agent bolus-tracking images. In addition, observation of the signal recovery behavior on reventilation in both normal and ischemic brain offered potentially useful information about the state of the cerebral autoregulatory mechanism.

Echo-planar MR imaging of normal and ischemic myocardium with gadodiamide injection

Rapid echo-planar (EP) magnetic resonance (MR) imaging was used to monitor the first pass of a bolus of gadodiamide injection in the hearts of normal rats and rats subjected to left coronary artery occlusion. Inversion-recovery EP imaging combined with a low dose (0.05 mmol/kg) of the contrast agent caused signal enhancement of normal myocardium from 19% +/- 4 to 63% +/- 5 (mean +/- 1 standard error of the mean) of fully relaxed intensity at the peak of the bolus but only slight increase in signal intensity of the ischemic zone. Thus, ischemic myocardium was demarcated as a hypointense zone (cold spot) during passage of the bolus. A higher dose (0.20 mmol/kg) of the same agent caused signal loss of normal myocardium from 100% to 39% +/- 7 of control at the peak of the bolus on gradient-recalled echo EP images, and ischemic myocardium was visualized as a hyperintense zone (hot spot). With either method of monitoring bolus transit, myocardial signal intensity recovered slowly following the peak bolus effect, consistent with substantial extraction of the agent during the first pass through the heart. Use of gadodiamide injection can allow discrimination between ischemic and nonischemic myocardium on both T1- and susceptibility-weighted EP images during bolus transit.

Dual mechanisms for change in myocardial signal intensity by means of a single MR contrast medium: dependence on concentration and pulse sequence

To determine whether gadodiamide injection can provide sufficient enhancement on both T1- and T2-weighted spin-echo magnetic resonance (MR) images of the heart and skeletal muscles, anesthetized rats were divided into five groups. Groups 1-3 received 0.1 (n = 9), 0.3 (n = 8), or 0.5 (n = 8) mmol/kg gadodiamide injection, respectively, and T1-weighted images were obtained. Groups 4 and 5 received 0.3 or 0.5 mmol/kg gadodiamide injection, respectively, and T2-weighted images were obtained. Gadolinium concentration was measured in myocardium by means of inductively coupled plasma-atomic emission spectroscopy. On T1-weighted images, the 0.1 and 0.3 mmol/kg doses produced a dose-dependent increase in myocardial signal intensity proportional to gadolinium concentration. A dose of 0.5 mmol/kg, which correlated with higher gadolinium concentration and did not further increase myocardial signal intensity, prolonged the imaging window. On T2-weighted images, the 0.3 mmol/kg dose caused a transient decrease in myocardial signal intensity; the 0.5 mmol/kg dose produced greater and persistent loss of signal intensity. In conclusion, the changes in signal intensity induced by gadodiamide injection depend on the dose, pulse sequence, and type of tissue.

Real-time dynamics of an extravascular magnetic resonance contrast medium in acutely infarcted myocardium using inversion recovery and gradient-recalled echo-planar imaging

RATIONALE AND OBJECTIVES

The purposes of this study are to evaluate the first-pass profile of gadolinium-BOPTA/Dimeg (Gd-BOPTA/Dimeg) during its transit through hearts subjected to acute myocardial infarction, and to delineate these infarcted regions by the use of ultrafast magnetic resonance imaging (MRI).

METHODS

Regional ischemia was induced in anesthetized rats by occluding the left coronary artery. Imaging parameters for single shot EPI included TE, 10 mseconds; AT, 33 mseconds; and 64 x 64-pixel matrix. Consecutive images were obtained every 1 to 2 seconds over a 30-second period. After approximately two images, Gd-BOPTA/Dimeg was injected intravenously (0.05 and 0.25 mmol/kg).

RESULTS

Gd-BOPTA/Dimeg (0.05 mmol/kg), with inversion recovery EPI, produced a substantial increase in signal intensity of right and then left ventricular blood. Normally perfused myocardium also was enhanced, but not the acutely infarcted region. Clear delineation of the infarcted region as negatively enhanced "cold spots" persisted for at least 20 seconds. Gd-BOPTA/Dimeg (0.25 mmol/kg) with standard gradient-recalled EPI produced a different profile of signal intensity changes. Signal intensities of ventricular blood and normal myocardium were greatly reduced, leaving the infarcted zone as a positively enhanced "hot spot." Delineation of the infarcted region persisted for 6 to 8 seconds. The infarcted zone detected with MRI corresponded to that observed at autopsy.

CONCLUSIONS

Regions of acute myocardial infarction can be detected as negatively enhanced "cold spots" or positively enhanced "hot spots" by studying the first-pass dynamics of Gd-BOPTA/Dimeg through hearts with regional ischemia by use of single shot EPI.

Real-time observation of transient focal ischemia and hyperemia in cat brain

Gradient-recalled echo-planar (T2*-weighted) imaging was used to noninvasively monitor regional blood oxygenation state changes in real time during transient episodes of focal ischemia in cat brain. Varying ischemic intervals (12 s to 30 min) were caused by middle cerebral artery occlusion. A rapid signal drop was noted upon occlusion, due to deoxygenation of static blood in the ischemic tissues. Upon successful reperfusion, the signal intensity recovered immediately and increased above (overshot) the baseline level before slowly returning to normal. The "overshoot" response was strongly dependent on the duration of the ischemic interval and is thought to reflect reactive hyperemia.

Effect of cilazapril on regional left ventricular wall thickness and chamber dimension following acute myocardial infarction: in vivo assessment using MRI

The primary goal of the current study was to assess in situ, using magnetic resonance imaging, the effect of a new angiotensin-converting enzyme inhibitor, cilazapril, in reducing left ventricular remodeling after acute myocardial infarction. Three groups of animals were investigated: (1) sham-operated rats (n = 19); (2) infarcted rats receiving no treatment (n = 23); and (3) infarcted rats receiving cilazapril (100 mg/L drinking water, n = 20). Treatment with cilazapril began on the third day postocclusion and continued for 3 to 4 months. Myocardial infarction was produced by ligation of the left coronary artery, and electrocardiographic (ECG)-gated short-axis images were acquired 3 to 4 months later. Sham-operated animals were subjected to the same procedure but the left coronary artery was not ligated. From the image acquired in the middle of the left ventricle (equatorial slice), left ventricular wall thicknesses, chamber diameters, and surface area measurements of the cavities were determined. At autopsy examination, infarct size and tissue water content were determined. The results demonstrate that magnetic resonance imaging has the potential to assess in situ the alterations of left ventricular dimensions and mass after acute myocardial infarction and can be used to document the influence of therapeutic interventions. Cilazapril provided protection against the deleterious remodeling changes such as ventricular dilation and wall thinning consequent to acute myocardial infarction.

Preservation of high-energy phosphate reserves in a cat model of post-ischemic myocardial dysfunction

Brief episodes of myocardial ischemia are known to cause reversible depression of regional myocardial contraction after reperfusion. One of the mechanisms of this persistent regional dysfunction has been proposed to be depletion of high-energy phosphate compounds. Eight cats were prepared with a reversible snare occluder around the left anterior descending artery (LAD); a surface coil sutured to the epicardial surface over the LAD territory for measurement of 31-phosphorus (31P) magnetic resonance spectroscopy (MRS) spectra; and a pair of ultrasonic crystals implanted in the mid-myocardium for measurement of regional segment length shortening. The baseline value of percent segment length shortening (%SS) was 12.8 +/- 1.4%. Increased afterload did not significantly alter high-energy phosphate levels or %SS. All animals exhibited passive systolic bulging during occlusion (-8.4 +/- 3.6% systolic shortening) as well as reduced phosphocreatine (PRc, 30 +/- 3% of control) and increased inorganic phosphorus (Pi) (239 +/- 18%), but there was no change in adenosine triphosphate (ATP). During reflow, %SS did not completely recover (4.0 +/- 2.9%, P less than .05 versus baseline). PCr and Pi returned to control levels during the first 30 minutes of reperfusion. Increased afterload had no significant effect on high-energy phosphates or %SS in stunned hearts. These findings indicate a lack of correlation between recovery of high-energy phosphate stores and regional myocardial contractility in stunned myocardium. High-energy phosphate reserves are preserved in stunned myocardium and are unlikely to be a direct cause of myocardial dysfunction.

Hypercarbia-induced changes in cerebral blood volume in the cat: a 1H MRI and intravascular contrast agent study

Cerebral blood volume changes with arterial carbon dioxide were monitored by proton T1-weighted MR images following administration of the intravascular contrast agent Gd-DTPA labeled with human serum albumin. Without MR contrast, no significant image intensity changes were observed with PaCO2. Following contrast, regional brain image intensities increased significantly over control (0% inspired CO2) in cortical gray, white, and basal ganglia regions with increasing PaCO2 and returned to control intensities upon return to 0% inspired CO2. Imaging of through-plane and in-plane phantoms was performed to assess flow effects. Signal losses of 2 and 6% (relative to no flow) were observed for bulk velocities of 5 mm/s at TE values of 15 ms. An intravascular contrast agent may be useful for MRI monitoring of local cerebral blood volume changes during cerebral perturbations.

Myocardial infarction: assessment with an intravascular MR contrast medium. Work in progress

The effect of a new intravascular magnetic resonance (MR) contrast medium (gadolinium diethylenetriaminepentaacetic acid [DTPA] polylysine) was evaluated in acute, subacute, and chronic myocardial infarctions in rats. Signal intensity (SI) was measured before and after intravenous administration of Gd-DTPA polylysine. Before administration of contrast material, chronic infarctions had lower SI than normal myocardium. With Gd-DTPA polylysine, three zones were identified in acute and subacute stages of myocardial infarction, but in the chronic stage, images demonstrated two zones. In acute and subacute infarctions, Gd-DTPA polylysine produced greater enhancement (over 60 minutes) in the peri-infarction zone than in the normal or infarcted myocardium. In chronic infarctions, Gd-DTPA polylysine had no discernible effect on the SI of the central infarction zone. Overall, it caused no significant hemodynamic effects. MR imaging with Gd-DTPA polylysine produced differential tissue enhancement in myocardial infarctions, which varied according to the age of the infarction.

Ischemic brain damage: reduction by sodium-calcium ion channel modulator RS-87476

A novel sodium-calcium ion channel modulator, RS-87476, reduced cerebral infarct size in cats subjected to permanent unilateral occlusion of the middle cerebral artery. Cerebral injury was assessed in vivo with a combination of magnetic resonance (MR) imaging and spectroscopy for 5-12 hours after occlusion and was compared with the area of histochemically ischemic brain tissue. Compared with infarcts in placebo-treated animals, infarcts in cats given RS-87476 were reduced by an average of 70% at the lowest dose, 75% at the intermediate dose, and 88% at the highest dose. Tissue edema, observed as areas of signal hyperintensity on diffusion- and T2-weighted spin-echo images, was confined to small regions of the parietal cortex and basal ganglia in drug-treated animals. Mean plasma levels of RS-87476 at the lowest dose were 13 ng/mL initially, falling to maintenance levels of 3-5 ng/mL; at the intermediate and highest doses, plasma levels of drug were approximately five- and 20-fold greater. The drug was only slightly hypotensive. At least part of the potent cerebroprotective effects of RS-87476 result from its ability to stabilize metabolic energy reserves, reduce lactate formation in ischemic tissues, and attenuate intracerebral edema.

Hyperglycemia augments ischemic brain injury: in vivo MR imaging/spectroscopic study with nicardipine in cats with occluded middle cerebral arteries

Hyperglycemia is often associated with an increased frequency of cerebrovascular disease and exacerbation of neuronal injury in focal ischemic cerebral infarction. We used a combination of high-field proton MR imaging and 1H and 31P MR spectroscopy to investigate whether hyperglycemia would adversely influence cerebral metabolism and eventual infarct size following unilateral occlusion of the middle cerebral artery (MCA) of cats pretreated with the calcium channel blocker nicardipine. Normoglycemic animals injected with 10 micrograms/kg of nicardipine (8 micrograms.kg-1.hr-1 maintenance dose) manifested only mild disturbances in phosphorus metabolism and cerebral pH regulation compared with untreated controls, and showed a significant reduction in infarct size 7 hr after MCA occlusion. By comparison, hyperglycemic cats (plasma glucose, 200-300 mg/dl) had significantly reduced cerebral high-energy phosphates, elevated lactic acid, and larger ischemic lesions in the occluded MCA territory, irrespective of whether they were treated with nicardipine. These results indicate that moderate hyperglycemia can exaggerate ischemic brain damage by enhancing formation of tissue lactic acid and impairing normal phosphorus metabolism. One implication of this study is that dextrose should not be provided to patients with acute ischemic stroke.

Effects of nicardipine, a calcium antagonist, on myocardial salvage and high energy phosphate stores in reperfused myocardial injury

The current study determined the effectiveness of nicardipine, a 1,4-dihydropyridine calcium antagonist, in preserving reperfused myocardium in a cat model of temporary coronary occlusion and ascertained if replenishment of myocardial phosphate stores during reperfusion as defined by phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy was indicative of salvage. Twenty open chest, anesthetized cats were studied with use of a snare ligature around the proximal left anterior descending coronary artery, with a coil sutured to the epicardial surface overlying the distribution of the artery. Peak areas of phosphocreatine, inorganic phosphate and adenosine triphosphate (ATP) NMR signals were measured during 1 h of occlusion followed by 1.5 h of reperfusion. Infarct size and jeopardy area were determined in vitro by simultaneous infusion of phthalocyanine blue dye and triphenyltetrazolium chloride into the aorta and the left anterior descending coronary artery, respectively, after 5 h of myocardial reperfusion. Nicardipine-treated and control groups had similar jeopardy area values (41.2 +/- 1.6% versus 47.4 +/- 3.1% of the left ventricle), but infarct area was significantly reduced in the nicardipine-treated group (3.2 +/- 1.1% versus 24.9 +/- 7.5% of jeopardy area, p less than 0.01). High energy phosphate compounds remained markedly altered during reperfusion in both groups. No significant improvement in phosphocreatine or inorganic phosphate recovery was observed in animals pretreated with nicardipine despite an 87% reduction in infarct size. Myocardial ATP was greater during reperfusion in the nicardipine-treated compared with the control group (average over initial 90 min of reperfusion 58 +/- 6% versus 46 +/- 3% of baseline values, p less than 0.05), suggesting improved recovery of ATP. However, the measured levels of high energy phosphate compounds during reperfusion and their ratios did not correlate with infarct size and thus were not predictive of myocardial salvage.