Detection, characterization and functional assessment of reperfused Q-wave acute myocardial infarction by cine magnetic resonance imaging.

The capability of dynamic gradient-refocused magnetic resonance imaging (cine MRI) to detect, localize and functionally assess acute myocardial infarction (AMI) in 25 patients at a mean time interval of 7 days after AMI was evaluated. Fifteen asymptomatic volunteers were also examined to determine the specificity of the observations. Upon presentation, each patient received intravenous thrombolytic therapy, underwent immediate cardiac catheterization and had percutaneous transluminal coronary angioplasty performed when coronary reperfusion was absent. Twenty-four of the patients had documented coronary reperfusion at a mean interval of 259 +/- 129 minutes. Global ejection fraction and regional wall motion abnormalities were evaluated at 7 days by cine MRI, left ventriculography and radionuclide angiography. Twenty patients with both an absolute decrease in myocardial signal and a matched regional wall motion abnormality had AMI properly identified by cine MRI. In contrast, the finding of both decreased signal intensity and a matched regional wall motion abnormality was absent in the group of asymptomatic volunteers. The ejection fraction by cine MRI correlated better with the ejection fraction by left ventriculography (r = 0.94, standard error of the estimate = 3.6) than did the ejection fraction by radionuclide angiography (r = 0.82, standard error of the estimate = 5.8). The regional wall motion concordance rate in comparison to left ventriculography was similar for both cine MRI (69%) and radionuclide angiography (65%). These findings suggest that cine MRI may play an important role in the future detection and functional characterization of AMI.

Muscle activity localization with 31P spectroscopy and calculated T2-weighted 1H images.

Using 31P spectroscopy and magnetic resonance imaging (MRI), the authors studied changes in muscle phosphorous metabolites and T2 with isometric knee extension to evaluate the potential role of T2 images in coil placement for exercise spectroscopy studies. Increased signal intensity was visible in active muscles on T2 images after exercise. Calculated T2-weighted values were elevated immediately after exercise in the quadriceps (P less than .01). T2 increases for individual quadricep muscles varied, with the largest changes in the rectus femoris and the least in the vastus lateralis. 31P spectroscopy studies demonstrated similar findings: percent change in T2 correlated positively with inorganic phosphorus to phosphocreatine ratio (Pi/PCr) (r = 0.89, P less than .01) and negatively with pH (r = -0.88, P less than .01). The correlations between imaging and spectroscopy suggest that T2 images may allow more precise placement of phosphorous coils in exercise studies. The heterogeneity of T2 changes within the quadriceps with exercise suggests that assumptions about muscle activity may be misleading. T2 images may provide muscle activity verification for exercise studies.

Magnetic resonance imaging of blood flow with a phase subtraction technique. In vitro and in vivo validation.

RATIONALE AND OBJECTIVES: One promising approach to flow quantification uses the velocity-dependent phase change of moving protons. A velocity-encoding phase subtraction technique was used to measure the velocity and flow rate of fluid flow in a phantom and blood flow in volunteers. METHODS: In a model, the authors measured constant flow velocities from 0.1 to 270.0 cm/second with an accuracy (95% confidence intervals) of +/- 12.5 cm/second. There was a linear relationship between the magnetic resonance imaging (MRI) measurement and the actual value (r2 = .99; P = .0001). RESULTS: Measuring mean pulsatile flow from 125 to 1,900 mL/minute, the accuracy of the MRI pulsatile flow measurements (95% confidence intervals) was +/- 70 mL/minute. There was a linear relationship between the MRI pulsatile flow measurement and the actual value (r2 = .99; P = .0001). In 10 normal volunteers, the authors tested the technique in vivo, quantitating flow rates in the pulmonary artery and the aorta. The average difference between the two measurements was 5%. In vivo carotid flow waveforms obtained with MRI agreed well with the shape of corresponding ultrasound Doppler waveforms. CONCLUSIONS: Velocity-encoding phase subtraction MRI bears potential clinical use for the evaluation of blood flow. Potential applications would be in the determination of arterial blood flow to parenchymal organs, the detection and quantification of intra- and extra-cardiac shunts, and the rapid determination of cardiac output and stroke volume.

Skeletal muscle pH assessed by biochemical and 31P-MRS methods during exercise and recovery in men.

The present study was designed to compare evaluation of skeletal muscle metabolism (vastus lateralis) evaluated by 31P-magnetic resonance spectroscopy (MRS) and biochemical analysis. During identical isometric knee extensor exercise protocols to fatigue in eight men, biopsy samples were taken at rest, peak exercise, and 32 s postexercise and 31P-MRS data were collected continuously for phosphocreatine (PCr), pH, ATP, and P(i) at 8- or 32-s intervals. There was no difference in ATP or pH measurements between the two techniques at rest, during peak exercise, or in recovery. Corresponding measurements of pH by the two techniques were closely related (r = 0.88, P < 0.01), and pH measured by 31P-MRS was closely related to muscle lactate accumulation (r = -0.84, P < 0.001). The level of PCr at peak exercise, expressed as a percentage of the baseline value, was not different between the two techniques (42 +/- 15 vs. 46 +/- 15%). The results indicate that, in skeletal muscle in normal subjects, 1) measurements of pH and PCr at rest and during exercise do not differ between the 31P-MRS and biopsy techniques and 2) muscle pH measured by 31P-MRS is closely related to lactate accumulation in men. Our data suggest that direct comparison of results of studies of exercise metabolism using these two techniques is warranted.

Heterogeneous signal intensity in magnetic resonance images of hypertrophied left ventricular myocardium.

Left ventricular hypertrophy is associated with decreased longevity and often leads to congestive heart failure. An exploratory study of magnetic resonance imaging in human left ventricular hypertrophy was performed. First, 13 patients with left ventricular hypertrophy and 7 controls of similar ages were studied using electrocardiogramgated end-diastolic images. Visual inspection suggested that low-intensity zones were frequently found within the hypertrophied myocardium. To verify this observation, the images were processed with semi-automatic edge detection and a derivative-based tissue characterization algorithm, yielding tissue heterogeneity indices (THI-A and THI-V) which objectively measured the low-intensity zones. THI-A and THI-V were both significantly greater in left ventricular hypertrophy patients than in controls (THI-A: 0.111 vs 0.038, p = 0.009). THI was also significantly correlated with duration of disease and electrocardiographic abnormalities. To validate these initial findings prospectively, the same quantitative analysis was applied to magnetic resonance images of an additional 20 left ventricular hypertrophy patients and 12 controls from two institutions, using different imaging systems and different acquisition parameters. Again, THI was significantly greater in patients than in controls. Analysis of end-systolic images yielded similar results. In four dogs with left ventricular hypertrophy induced by aortic banding, THI showed a statistically significant increase as left ventricular hypertrophy developed. Hypertrophied myocardium thus shows reproducible differences from normal tissue with magnetic resonance imaging; hence, quantitative magnetic resonance tissue characterization may be useful in assessing pathologic changes in LVH.

Evaluation of BA1112 rhabdomyosarcoma oxygenation with microelectrodes, optical spectrophotometry, radiosensitivity, and magnetic resonance spectroscopy.

We studied tumor tissue oxygenation in the BA1112 rhabdomyosarcoma using micro-electrode pO2 measurements, optical spectrophotometry, analyses of cell survival after irradiation, and 31P magnetic resonance spectroscopy (MRS). Studies were carried out in WAG/Rij/Y rats breathing normoxic, hypoxic, and hyperoxic gas mixtures with and without iv administration of perfluorochemical. Significant changes in tissue oxygenation and metabolic status were found when pO2 values, optical measurements of hemoglobin saturation and cytochrome a, a3 reduction-oxidation, radiation cell survival determinations, and MRS measurements of phosphometabolite ratios were obtained in rats breathing different gas mixtures. Inhalation of 100% O2 caused increases in tumor pO2, hemoglobin saturation, cytochrome a, a3 oxidation, tumor radiosensitivity, and PCr/Pi, NTP/Pi, and PDE/Pi ratios. Such changes were augmented by pretreatment with iv perfluorochemicals. Inhalation of hypoxic gas mixtures resulted in reductions in the above parameters. These results indicate that tissue oxygenation can be manipulated reproducibly in the BA1112 rhabdomyosarcoma and suggest that 31P MRS can be used to monitor changes in tumor oxygenation in this model system.

MR characterization of blood flow in native and grafted internal mammary arteries.

In the postoperative patient with anginal symptoms, differentiation between bypass graft compromise and nonischemic causes has until now been accomplished only by means of x-ray angiography. A non-invasive test is clearly desirable. The authors used a cine phase-contrast (PC) magnetic resonance (MR) imaging technique to characterize blood flow in native and grafted internal mammary arteries (IMAs). Ten volunteers and 15 patients who had recently undergone IMA coronary artery bypass grafting were imaged. Cine PC MR imaging was performed in the transaxial plane at the level of the pulmonary artery bifurcation. Flow in both IMAs was quantified and expressed as a percentage of cardiac output measured in the ascending aorta. In the 15 patients, flow analysis was performed in both the native and grafted IMAs. In the volunteers, IMA blood flow ranged from 2.1% to 4.3% of cardiac output on the left (mean, 3.5%) and 2.1% to 5.1% (mean, 3.5%) on the right. There was considerable intersubject variability, with coefficients of variation of 10.7% for the left and 12.3% for the right IMA. Intrasubject variability was limited, with estimated common standard deviations of 0.45% of cardiac output (range, 0.2%-1.1%) for the left and 0.39% (range, 0.1%-0.6%) for the right IMA. Flow in grafted IMAs was identified in 13 of 15 patients. In one of two patients without demonstrable IMA graft flow, cardiac catheterization confirmed lack of flow. IMA graft flow varied from 28 to 164 mL/min (mean, 80.3 mL/min). This study shows the feasibility of using cine PC MR imaging as a quantitative method of evaluating blood flow in IMA coronary artery bypass grafts.