Acute myocardial ischemia: MR imaging with Mn-TP

Cardiac-gated magnetic resonance (MR) imaging was performed in rats to determine the effects of manganese ethylenediaminetetraphosphonate (TP). Ten normal rats received Mn-TP in a dose of 50 mumol/kg through a tail-vein injection. Spin-echo MR images were obtained before and every 10 minutes after Mn-TP injection for 1 hour. Cardiac signal intensity (SI) increased more than 70% after Mn-TP injection and remained nearly unchanged 1 hour after injection. Myocardial T1 was 517 +/- 49 msec in eight control rats and 282 +/- 61 msec (P less than .001) in six rats 81 +/- 0 minutes after injection. Nine rats underwent occlusion of the left anterior descending coronary artery prior to MR imaging. Images were obtained before and 15, 30, and 60 minutes after Mn-TP injection. In normal myocardium, SI increased up to 82% and remained elevated for 1 hour. In ischemic myocardium, SI rose 11%, leading to a marked contrast between the two tissue zones. T1 was also different in the two regions: In normal tissue, it was 206 msec +/- 54; in ischemic tissue, 338 +/- 82 (P less than .001). With T1-weighted MR imaging, Mn-TP showed a potential for delineating the jeopardized area after acute myocardial ischemia.     

Diffusion-weighted MR imaging of acute stroke: correlation with T2-weighted and magnetic susceptibility-enhanced MR imaging in cats

We evaluated the temporal and anatomic relationships between changes in diffusion-weighted MR image signal intensity, induced by unilateral occlusion of the middle cerebral artery in cats, and tissue perfusion deficits observed in the same animals on T2-weighted MR images after administration of a nonionic intravascular T2 shortening agent. Diffusion-weighted images obtained with strong diffusion-sensitizing gradient strengths (5.6 gauss/cm, corresponding to gradient attenuation factor, b, values of 1413 sec/mm2) displayed increased signal intensity in the ischemic middle cerebral artery territory less than 1 hr after occlusion, whereas T2-weighted images without contrast usually failed to detect injury for 2-3 hr after stroke. After contrast administration (0.5-1.0 mmol/kg by Dy-DTPA-BMA, IV), however, T2-weighted images revealed perfusion deficits (relative hyperintensity) within 1 hr after middle cerebral artery occlusion that corresponded closely to the anatomic regions of ischemic injury shown on diffusion-weighted MR images. Close correlations were also found between early increases in diffusion-weighted MR image signal intensity and disrupted phosphorus-31 and proton metabolite levels evaluated with surface coil MR spectroscopy, as well as with postmortem histopathology. These data indicate that diffusion-weighted MR images more accurately reflect early-onset pathophysiologic changes induced by acute cerebral ischemia than do T2-weighted spin-echo images.     

Magnetic resonance imaging of acute myocardial ischemia using a manganese chelate, Mn-DPDP

Nine adult rats underwent occlusion of the left coronary artery (LCA) to assess the ability of a manganese chelate of N, N'-Bis (pyridoxal-5-phosphate) ethylenediamine-N, N'-diacetic acid (Mn-DPDP), to delineate acute myocardial ischemia. Hemodynamic effects of the contrast medium were tested in the isolated rat heart. Gated transaxial images of the heart at the mid-ventricular level were obtained using a 2 Tesla magnet. A TE of 20 msec and a TR of 1 R-R interval (approximately 250 msec) were used. Images were taken prior to injection of 400 mumol/kg of Mn-DPDP, 2 minutes after injection, and then at 15 minute intervals for one hour. The time between LCA occlusion and injection of contrast averaged 95 minutes. The signal intensity (SI) of normal myocardium was increased by 125 +/- 9% immediately after injection, and did not significantly vary over one hour. SI of ischemic myocardium increased by only 16 +/- 14% immediately after injection, gradually rising to 44 +/- 13% after one hour. Visual discrimination between normal and ischemic myocardium was obtained throughout the study. The percent contrast between normal and ischemic myocardium was 47.2 +/- 6% at 2 minutes after injection, gradually decreasing to a final value of 31.3 +/- 4%. No hemodynamic effects were produced in the isolated heart using concentrations in the perfusate several-fold higher than those expected to be produced by the intravenous injection of 400 mumol/kg Mn-DPDP. Mn-DPDP shows the potential for delineation of the jeopardy area resulting from coronary occlusion for at least one hour after injection of the agent.     

Differentiation of reversible and irreversible myocardial injury by MR imaging with and without gadolinium-DTPA

The current study evaluated the capability of magnetic resonance (MR) imaging to distinguish myocardium subjected to reversible and irreversible ischemic injury. Nine dogs underwent left anterior descending coronary arterial occlusion for 15 minutes (reversible injury) and nine for 1 hour (irreversible injury), followed by reperfusion for 24 hours in both groups. Six dogs from each group received 0.5 mmol/kg of gadolinium-DTPA intravenously; the remaining dogs received no contrast media. In the dogs with irreversible injury but no contrast media, there were prolonged T1 and T2 of the infarcted myocardium and adequate visualization of the infarct. The percentage of contrast between normal and infarcted myocardium was greatest on T2-weighted images. In the group with irreversible injury and contrast media, Gd-DTPA produced significant T1 shortening of injured myocardium, with resultant high signal intensity of the infarct, and significantly enhanced contrast compared with the group that did not receive Gd-DTPA. In the dogs with reversible injury, there were no regional differences in intensity or relaxation times. MR has the capability to distinguish myocardium with irreversible injury from that with reversible injury. The difference of T1 between normal and reperfused infarcted myocardium is increased by Gd-DTPA; thus, contrast between these two is enhanced on MR images.     

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 SUSCCPtibility contrast material on normal canine myocardium using inversion recovery (IR)- and driven equilibrium @E)-prepared fast gradient-recalled echo (GRE) sequences and (2) to determine the relative value of T1-enhancing and magnetic eusceptibflity 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 NIR 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°. Fast DE-prepared GRE images were obtained using a flip angle of 12° and a DE delay /TR/TE of 60/10.2/4.2 msec. Sequential images were acquired to monitor transit of 0.06 mmol/kg gadodiamide injection and 0.2 and 0.4 mmol/kg sprodiamide injection. On slice-nonselective IR fast GRE images. gadodiamide caused signiflcant 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 <.06) and no signiflcant change in SI from ischemic myocardium. IR- and DE-prepared taet GRE imaging can be used to monitor the transit of Tl-enhancing and magnetic susceptibility contrast material in the heart. respectively. Cardiac image quality was much better when slice-nonselective IR-prepared fast GRE sequences were used rather than DE-prepared fast GRE.     

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 (ΔR2*) 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.     

Reperfused myocardial infarctions on T1- and susceptibility-enhanced MRI: Evidence for loss of compartmentalization of contrast media

The purpose of this study was to characterize the contrast caused by a susceptibility MRI contrast agents, on spin echo T₂-weighted imaging of reperfused myocardial infarction. Our interest in this model focused on the expected requirement that such agents be compartmentalized in the tissue to cause signal loss on spin echo images, a condition which may not be present in reperfused infarcted myocardium. Accordingly, nine rats were subjected to 2 h of left coronary artery occlusion followed by 3 ± 0.5 h of reperfusion prior to administration of contrast media. Three sets of MR images were acquired: (a) baseline axial images at the midventricle, both T₁-weighted (TR/TE = 300/20) and T₂-weighted (TR/TE = 1500/60); (b) T₁-weighted images after administering a T₁-enhancing agent, Gd-DTPA-BMA (0.2 mmol/kg), to document that contrast media is delivered to the reperfused infarction; and (c) T₂-weighted images after administering the susceptibility agent, Dy-DTPA-BMA (1.0 mmol/kg). Gadolinium-enhanced T₁ images depicted reperfused infarction as regions with greatly enhanced signal intensity compared with unin-farcted myocardium, indicating that contrast agent was delivered to the infarcted zone. Dysprosium-enhanced T₁ images depicted the injury as a region of persistent signal intensity relative to depletion of signal in normal myocardium, consistent with failure of the contrast agent to cause signal loss. Similar infarction sizes were observed for unenhanced T₂-weighted images (33 ± 5%), gadolinium-enhanced T₁ weighted images (36 ± 5%) and postmortem staining (30 ± 6%); strong correlations (r > 0.9) were noted in comparisons of these data. Dysprosium-enhanced images exhibited a smaller region of differential signal presumed to be infarction (20 ± 5%, P < 0.05) and weak correlations (r < 0.75) with the other measurements. We conclude that the smaller infarction depicted on dysprosium-enhanced images is a subregion of the true infarction in which myocardial necrosis is sufficiently advanced that the agent is homogeneously distributed throughout all tissue compartments, preventing T₂*-dependent phase loss on spin echo images.     

Magnetic resonance imaging of bowel ischemia induced by ligation of superior mesenteric artery and vein in a cat model

OBJECTIVE: To assess the usefulness of magnetic resonance (MR) imaging for detecting bowel ischemia with strangulation compared with histopathologic findings in an experimental cat model. MATERIALS AND METHODS: Fourteen cats were assigned to the normal control group (n = 3), acute ischemic group (induced by ligation of superior mesenteric vessels for 3 hours, n = 7), and subacute ischemic group (induced by ligation of superior mesenteric vessels for 10 hours, n = 4). Using a 4.7-T MR scanner, contrast-enhanced T1-weighted images were obtained at 0, 10, 20, 30, and 60 minutes after bolus injection of contrast media. T1- and T2-weighted images were obtained from the extracted bowel wall and compared with histopathologic findings. RESULTS: On contrast-enhanced MR images, the target-like bowel wall layers were clearly demonstrated and the submucosal layer showed the most prominent enhancement. At 10 minutes after administration of contrast media, the subacute ischemic group showed significantly lower enhancement of the submucosal layer than the normal or acute ischemic group (P <0.05). On T1-weighted images, there were not significant differences between the normal and ischemic bowel groups (P >0.05). On T2-weighted images, the signal intensity of all layers of acute ischemic bowel wall was significantly higher than that of the normal control or subacute ischemic group (P <0.05). CONCLUSION: Delayed contrast-enhanced MR images and T2-weighted images were helpful for detecting subacute and acute bowel ischemia with strangulation, respectively.     

Multicentre dose-ranging study on the efficacy of USPIO ferumoxtran-10 for liver MR imaging

AIM: A dose ranging multicentre phase-II clinical trial was conducted to evaluate the efficacy of ultrasmall superparamagnetic iron oxide (USPIO) ferumoxtran-10 for magnetic resonance (MR) imaging of focal hepatic lesions. MATERIAL AND METHODS: Ninety-nine patients with focal liver lesions received USPIO at a dose of 0.8 (n = 35), 1.1 (n = 32), or 1.7 (n = 32) mg Fe/kg. Liver MR imaging was performed before and after USPIO with T1-weighted and T2-weighted pulse sequences. Images were analysed by two independent readers for additional information (lesion detection, exclusion, characterization and patient management). Signal intensity (SI) based quantitative measurements were also taken. RESULTS: Post-contrast medium MR imaging showed additional information in 71/97 patients (73%) for reader one and 83/96 patients (86%) for reader two. The results with all three doses were statistically significant (P < 0.05). Signal intensity analysis revealed that all three doses increased liver SI on T1-weighted images and decreased liver SI on T2-weighted images. On T2-weighted images metastases increased in contrast relative to normal hepatic parenchyma whereas haemangiomas decreased in contrast. On T2-weighted images there was statistically improved efficacy at the intermediate dose, which did not improve at the highest dose. CONCLUSION: Ultrasmall superparamagnetic iron oxide was an effective contrast agent for liver MR imaging at all doses and a dose of 1.1 mg Fe/kg was recommended for future clinical trials.     

Histologic confirmation of microvascular hyperpermeability to macromolecular MR contrast medium in reperfused myocardial infarction

A macromolecular MR contrast medium (MMCM) designed to permit histochemical staining and specific tissue localization, albumin-(biotin)10-(Gd-DTPA)25 (Bio-Alb-Gd), was used in a rat model of reperfused myocardial infarction to confirm the presence and distribution of microvascular hyperpermeability. T1-weighted spin-echo images were acquired before and after administration of Bio-Alb-Gd. An avidin-biotin-complex (ABC) stain, specific for the biotinylated MR contrast medium, was used to define the MMCM distribution and to detect any regional change in micro-vascular permeability related to infarction. Immediately after Bio-Alb-Gd administration, the infarcted region was enhanced, with greatest signal intensity noted at the rim and less at the center. There was a gradual increase in signal intensity of the initially hypointense central region. The steady increase in signal intensity of the central region suggested convection transport of MMCM through the interstitial space and its influx into cellular compartment after leakage from the vascular compartment. Histologic findings confirmed regional microvascular hyperpermeability corresponding to the site of infarction and a predominant rim distribution of the MMCM. Bio-Alb-Gd was identified at high microscopic power in the intravascular, interstitial, and intracellular spaces at the periphery of reperfused infarcted myocardium. Bio-Alb-Gd can be used as an MR contrast medium in reperfused infarcted myocardium to confirm the existence and to localize altered microvascular permeability to macromolecules. Bio-Alb-Gd contrast technique removes all the ambiguity between the distribution of the MR or other imaging contrast agent and the distribution of the substrate for histochemical staining.     

Acute myocardial infarction: MR evaluation in 29 patients

This study evaluated the ability of MR to identify and characterize the region of myocardial infarction in humans. Twenty-nine patients, all with ECG and enzyme rises consistent with an acute myocardial infarction, were studied by MR 3-17 days from the onset of acute chest pain. Four patients were excluded because of inability to acquire adequate MR studies. For comparison, 20 normal subjects were studied who also had gated MR examinations. The site of infarction was visualized in 23 patients as an area of high signal intensity in relation to the normal myocardium, a contrast that increased on the second-echo image. The regions of abnormal signal intensity corresponded to the anatomic site of infarction as defined by the ECG changes. The mean T2 relaxation time of the infarcted myocardium (79 +/- 22 msec) was significantly prolonged in comparison with the mean T2 (43.9 +/- 9 msec) of normal myocardium (p less than .01). The mean percentage of contrast (intensity difference) between normal and infarcted myocardium was much greater on the second-echo images (65.6 +/- 34.0%) than the first-echo images (27.5 +/- 18.7%). In the normal subjects there was no difference in T2 between the anterolateral (40.3 +/- 5.7 msec) and septal (39.5 +/- 7.4 msec) regions, and percentages of contrast between these two regions of myocardium on the first-echo (9.1 +/- 7.4%) and second-echo (15.0 +/- 13.3%) images were similar. Thus, MR can be used to directly visualize acute infarcts. However, it has several pitfalls, including the necessity to differentiate signal from slowly flowing blood in the ventricle, from increased signal from a region of infarction and artifactual variation of signal intensity in the myocardium due to respiratory motion or residual cardiac motion.     

Lymph nodes: microstructural anatomy at MR imaging

High-resolution microscopic magnetic resonance (MR) images of rodent lymph nodes were directly correlated with sections obtained for histologic study to determine the microstructural anatomy of lymph nodes seen at MR imaging and to evaluate signal intensity changes induced by a novel intravenous lymphotropic MR contrast agent (ultrasmall superparamagnetic iron oxide [USPIO]). High-resolution T2-weighted images of unenhanced lymph nodes demonstrated medullary sinus as regions of low signal intensity and follicles as high-intensity structures. After a single intravenous administration of USPIO (160 mumol/kg), both T1-weighted and T2-weighted images showed areas of focal signal intensity loss in medullary sinuses corresponding to the distribution of uptake by macrophages. Lymph follicles appeared unchanged in signal intensity, as they are largely devoid of macrophages. This model of microscopic MR imaging should provide the basis for (a) understanding differences between patterns of contrast-enhanced normal lymph nodes and those of diseased ones and (b) guiding the development of targeting strategies for novel pharmaceuticals at the cellular level.     

Reversible and irreversible injury in the reperfused myocardium: differentiation with contrast material-enhanced MR imaging

Thrombolytic agents are being used with increasing frequency to produce reperfusion of acute myocardial infarction in humans. This study was designed to assess the potential of a magnetic resonance (MR) contrast agent, the manganese chelate of N,N'-bis(pyridoxal-5-phosphate) ethylenediamine-N,N'-diacetic acid (DPDP), in differentiating between reversible and irreversible myocardial injury during reperfusion. Ischemia was produced in rats by occlusion of the left coronary artery for either 15 minutes (n = 10) or 2 hours (n = 10), followed in both cases by 30 minutes of reperfusion. Signal intensity (SI) was measured before and after (every 15 minutes for 1 hour) the administration of Mn-DPDP. Prior to intravenous injection of the contrast agent, no significant difference in SI was observed between normal and reversibly or irreversibly injured myocardium. Mn-DPDP produced greater enhancement of SI of the irreversibly injured region compared with normal myocardium. There was no differential enhancement of the reversibly injured region compared with normal myocardium. Thus, Mn-DPDP is useful in discriminating reversible from irreversible injury in reperfused myocardium.     

Characterization of focal hepatic lesions with ferumoxides-enhanced T2-weighted MR imaging

OBJECTIVE: The purpose of this study was to evaluate whether ferumoxides-enhanced MR imaging of focal hepatic lesions provides distinctive signal intensity and lesion-to-liver contrast changes for benign and malignant lesions, helping to further characterize and differentiate these lesions. MATERIALS AND METHODS: Data analysis was performed on 70 patients, with previously identified focal hepatic lesions, who underwent MR imaging of the liver before and after IV administration of ferumoxides (10 micromol Fe/kg). Lesions analyzed with pathologically proven diagnoses included metastases (n = 40), hepatocellular carcinoma (n = 11), cholangiocarcinoma (n = 6), hemangioma (n = 4), focal nodular hyperplasia (n = 6), and hepatocellular adenoma (n = 3). Response variables measured and statistically compared included the percentage of signal-intensity change and lesion-to-liver contrast. RESULTS: Focal nodular hyperplasia showed significant signal intensity loss on ferumoxides-enhanced T2-weighted images (mean, -43%+/-6.7%, p < 0.01). All other lesion groups showed no statistically significant change in signal intensity on ferumoxides-enhanced T2-weighted images, although signal intensity loss was seen in some individual hepatocellular adenomas (mean, -6.6%+/-24.0%) and hepatocellular carcinomas (mean, -3.3%+/-10.3%). All lesions, with the exception of hepatocellular carcinoma, had a marked increase in lesion-to-liver contrast on ferumoxides-enhanced T2-weighted images, which was statistically significant for metastases and hemangioma (p < 0.02). CONCLUSION: Focal nodular hyperplasia shows significant decrease in signal intensity on ferumoxides-enhanced T2-weighted images, which may aid in the differentiation of focal nodular hyperplasia from other focal hepatic lesions. Other lesions, namely, hepatocellular adenoma and carcinoma, can have reticuloendothelial uptake, but usually to a lesser degree than that of focal nodular hyperplasia.     

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.     

Acute myocardial ischemia and reperfusion: MR imaging with albumin-Gd- DTPA

The utility of a macromolecular, intravascular contrast agent, albumin-gadolinium diethylenetriaminepentaacetic acid (DTPA), for the differentiation of acutely ischemic and reperfused myocardium on magnetic resonance (MR) images was investigated. Regional, reversible myocardial ischemia was produced in rats and confirmed. After reperfusion, flow to the compromised myocardial segment returned to baseline. Normal myocardium could not be differentiated from ischemic myocardium on nonenhanced MR images (n = 12). After 5 minutes of myocardial ischemia and after administration of albumin-Gd-DTPA, the ischemic zone involving the free wall of the left ventricle was characterized by the absence of significant enhancement. Normal myocardium appeared homogeneously enhanced (by 145%). This pattern persisted for up to 1 hour of myocardial ischemia. In six rats that underwent myocardial reperfusion after 5 minutes of ischemia, the normal and reperfused myocardium became isointense. Radiotracer studies with albumin-Gd-153-DTPA confirmed the decreased distribution of contrast agent to the ischemic myocardium, possibly due to decreased blood pool or a blocked primary delivery system in the ischemic myocardium.     

Low-field-strength MR imaging of failed hip arthroplasty: association of femoral periprosthetic signal intensity with radiographic, surgical, and pathologic findings

PURPOSE: To investigate the association between periprosthetic signal intensity at low-field-strength magnetic resonance (MR) imaging after failed hip arthroplasty and radiographic, surgical, and pathologic findings. MATERIALS AND METHODS: The study group comprised 22 consecutive women who underwent hip arthroplasty (mean age, 62 years; age range, 35-74 years). All patients underwent MR imaging prior to revision surgery. Coronal fast short inversion time inversion-recovery (STIR) images and spin-echo T1-weighted images were obtained with a 0.5-T MR imaging unit before and after administration of contrast material. The periprosthetic region was divided into the seven femoral Gruen zones. Two observers retrospectively analyzed signal intensity patterns. Association of signal intensity patterns with radiographic, surgical, and pathologic findings was determined with chi2 analysis and generalized estimating equations. RESULTS: Diagnostic-quality images were obtained for 150 zones. Periprosthetic signal intensity was greater than that of bone marrow in the distal femur on the fast STIR images, and no contrast enhancement was seen on the T1-weighted images (type I signal intensity pattern) in 11 zones. Signal intensity was greater than that of bone marrow on the fast STIR images, and contrast enhancement was seen on the T1-weighted images (type II signal intensity pattern) in 45 zones. Signal intensity was less than or equal to that of bone marrow on the fast STIR images, and no contrast enhancement was seen on the T1-weighted images (type III signal intensity pattern) in 94 zones. Type I and II patterns were associated with focal or nonfocal lucency, an unstable stem, and fibrosis or granuloma. A type III pattern was associated with a normal radiographic appearance, a stable stem, and normal bone tissue. Significant association was demonstrated between periprosthetic signal intensity and radiographic (P <.001, chi2 test and generalized estimating equations), surgical (P <.05, Mantel-Haenszel chi2 test and generalized estimating equations), and pathologic findings (P <.05, chi2 test). CONCLUSION: Low-field-strength MR imaging depicted periprosthetic tissue signal intensity that was significantly associated with radiographic, surgical, and pathologic findings.     

Normal and infarcted myocardium: differentiation with cellular uptake of manganese at MR imaging in a rat model

PURPOSE: To assess whether normal myocardium can be distinguished from infarction at magnetic resonance (MR) imaging with low doses of manganese dipyridoxyl diphosphate (Mn-DPDP). MATERIALS AND METHODS: After 1-hour coronary arterial occlusion and 2-hour reperfusion, three groups of eight rats each were injected with 25, 50, or 100 micromol of Mn-DPDP per kilogram of body weight. The longitudinal relaxation rate (R1) in normal myocardium, reperfused infarction, and blood was repeatedly measured at inversion-recovery echo-planar imaging before and for 1 hour after the administration of contrast material. Afterward, several animals from each group were examined at high-spatial-resolution inversion-recovery spin-echo (SE) MR imaging. RESULTS: Manganese accumulated in normal myocardium but was cleared from reperfused infarction and blood. One hour after the administration of Mn-DPDP, R1 in normal myocardium (1.53 sec(-1) +/- 0.03, 1.73 sec(-1) +/- 0.03, and 1.94 sec(-1) +/- 0.02, respectively, for 25, 50, and 100 micromol/kg) was significantly (P <.05) faster than that of reperfused infarction (0.99 sec(-1) +/- 0.03, 1.11 sec(-1) +/- 0.03, and 1.48 sec(-1) +/- 0.06). Normal myocardium appeared hyperintense on T1-weighted inversion-recovery SE MR images and was clearly distinguishable from reperfused infarction. CONCLUSION: Mn-DPDP-enhanced inversion-recovery echo-planar and SE MR images demonstrated retention of manganese in normal myocardium and clearance of manganese from infarction. Mn-DPDP has characteristics similar to those of widely used thallium and may be useful in the assessment of myocardial viability at MR imaging.     

An improved MR imaging technique for the visualization of myocardial infarction

PURPOSE: To design a segmented inversion-recovery turbo fast low-angle shot (turboFLASH) magnetic resonance (MR) imaging pulse sequence for the visualization of myocardial infarction, compare this technique with other MR imaging approaches in a canine model of ischemic injury, and evaluate its utility in patients with coronary artery disease. MATERIALS AND METHODS: Six dogs and 18 patients were examined. In dogs, infarction was produced and images were acquired by using 10 different pulse sequences. In patients, the segmented turboFLASH technique was used to acquire contrast material-enhanced images 19 days +/- 7 (SD) after myocardial infarction. RESULTS: Myocardial regions of increased signal intensity were observed in all animals and patients at imaging. With the postcontrast segmented turboFLASH sequence, the signal intensity of the infarcted myocardium was 1,080% +/- 214 higher than that of the normal myocardium in dogs-nearly twice that of the next best sequence tested and approximately 10-fold greater than that in previous reports. All 18 patients with myocardial infarction demonstrated high signal intensity at imaging. On average, the signal intensity of the high-signal-intensity regions in patients was 485% +/- 43 higher than that of the normal myocardium. CONCLUSION: The segmented inversion-recovery turboFLASH sequence produced the greatest differences in regional myocardial signal intensity in animals. Application of this technique in patients with infarction substantially improved differentiation between injured and normal regions.     

Occlusive myocardial infarction: investigation of bis-gadolinium mesoporphyrins-enhanced T1-weighted MR imaging in a cat model

PURPOSE: To test whether bis-gadolinium mesoporphyrins-enhanced magnetic resonance (MR) imaging can accurately depict irreversibly damaged myocardium in occlusive myocardial infarction. MATERIALS AND METHODS: Ten cats were subjected to 90 minutes of occlusion of the left anterior descending coronary artery. Bis-gadolinium mesoporphyrins-enhanced T1-weighted MR imaging was performed in the cats for 6 hours. Histopathologic examinations with 2'3'5-triphenyl tetrazolium chloride (TTC) staining and electron microscopy were performed on the resected specimens. The time course and pattern of signal intensity enhancement were evaluated. The size of the infarcted myocardium was estimated on the MR images by measuring the size of the signal intensity-enhanced area. RESULTS: In eight of 10 cats, it was impossible to distinguish infarcted myocardium from normal myocardium at visual inspection of T1-weighted MR images. The contrast ratio between infarcted and normal myocardium did not increase significantly over time. In one of the two remaining cats, a doughnut pattern of signal intensity enhancement was noted. The other cat showed intensely homogeneous enhancement of infarcted myocardium at MR imaging. The size of the area of signal intensity enhancement at MR imaging in these two cats was accurately mapped to that of the infarction on the TTC-stained specimens. CONCLUSION: Occlusive myocardial infarction cannot be accurately detected at bis-gadolinium mesoporphyrins-enhanced MR imaging.