A new phase-encoding scheme for gradient- and spin-echo (GRASE) imaging giving a short effective TE is described. Unlike previous orders, phase encoding is centric rather than sequential. The sequence is a development of k-banded GRASE that uses different time segments of the echo train to encode different bands of k space. This phase-encoding order has been implemented in single-shot sequences on an imager with high performance gradients. Approximately 144 phase-encoding lines can be acquired in an echo train time of 390 ms. With centric phase encoding, the effective TE is 8 ms, compared with 75 ms for sequential encoding, and signal-to-noise ratios (SNRs) in brain tissue are 50 to 70% higher. The sequence can be employed in, for example, diffusion and velocity imaging.

Artifacts↗

PubMedDate not supplied

Segmentation analysis in functional MRI: activation sensitivity and gray-matter specificity of RARE and FLASH.

Brain activation is accompanied by local decreases in vascular deoxyhemoglobin. Theoretically, gradient-echo and spin-echo sequences show similar sensitivity to capillary deoxyhemoglobin, but spin-echo sequences should be less sensitive to venous deoxyhemoglobin. This is an important distinction in the context of cortical localization. We report herein a direct experimental comparison of a gradient-echo sequence (fast low-angle shot [FLASH]) with a spin-echo sequence (rapid acquisition with relaxation enhancement [RARE]) for functional MRI (fMRI) in seven subjects undergoing visual stimulation. A Student t test analysis was used to locate areas of significant activation, and then computerized image segmentation was performed to determine the type of activated tissue. Contrary to previous reports, both sequences proved equally sensitive to overall activation. RARE activation, however, was more specific for gray matter, as suggested by prior theoretical models.

Adult↗

PubMedDate not supplied

An evaluation of central laboratories in three VA cooperative studies.

We compared central laboratory with local determinations of key clinical measurements in three VA Cooperative Studies. Electrocardiographic evidence of new myocardial infarction was assessed in the study of Aspirin Therapy and Unstable Angina, ejection fraction measurement in the Coronary Artery Bypass Surgery Trial and lesion size in the Angioplasty Compared with Medicine (ACME) Trial. The findings in the Aspirin Trial indicated that central coding of all serial electrocardiograms in 1266 patients to detect new acute myocardial infarction by computer algorithm was not cost-effective when compared with the local investigator's diagnosis on the basis of a central Electrocardiographic Committee as the reference standard. In the other two trials, the contribution of the central laboratories was important because the assessments of the local investigators generally underestimated the degree of abnormality in the Bypass Trial and overestimated it in the ACME Trial. The VA results have clearly demonstrated two cases in which the decision for central evaluation was prudent, but one case in which it was not cost-effective. These equivocal findings indicate the need to evaluate the contribution of central laboratories when used as an adjunct to local determinations. Such evaluations may provide guidelines for decision-making in the design of future trials.

Angioplasty, Balloon, Coronary↗

PubMedDate not supplied

Myocardial technetium 99m-labeled teboroxime clearance derived from canine scans differentiates severity of stenosis after dipyridamole.

BACKGROUND: The purposes of this study were to determine (1) whether perfusion defects and differential teboroxime clearance observed on serial postdipyridamole planar gamma camera images have utility in differentiating severity of coronary stenosis in canine myocardium and (2) whether single-zone (99mTc activity in one zone over time) or dual-zone (99mTc activity ratio in two zones over time) quantitative analysis provides the most useful characterization of teboroxime clearance kinetics. METHODS AND RESULTS: Accordingly, eight dogs received a mild coronary stenosis (microsphere-determined, post-dipyridamole flow > or = 0.4 ml/min/gm) and 10 dogs received a severe coronary stenosis (flow < or = 0.3 ml/min/gm). In three control dogs there was no coronary stenosis. Teboroxime (5 mCi) was injected after dipyridamole (0.8 mg/kg/min), and gamma camera imaging was begun immediately and continued for 60 minutes. Quantitative analysis of single-zone clearance curves indicated that teboroxime retention for both zones of stenosis (mild = 66.3% +/- 2.8%, SEM; severe = 69.5% +/- 3.7%) was significantly greater than control zone retention (54.5% +/- 3.0%; p < 0.05) at 10 minutes. Retention in mild (37.2% +/- 1.9%, SEM) and severe (42.3% +/- 1.5%) zones of stenosis was significantly different from each other at 60 minutes (p < 0.05). There was a significant direct, linear correlation of flow with teboroxime clearance at 20 minutes (r = +0.74, normal and ischemic zones). Quantitative analysis with dual-zone count ratios (stenosed/normal) demonstrated significant differences among control (0.93 +/- 0.04, SEM), mild stenoses (0.54 +/- 0.04), and severe stenoses (0.39 +/- 0.03) as early as 2 minutes after administration of 99mTc-labeled teboroxime. The correlation coefficient for flow ratio versus dual-zone counts/pixel ratio at 20 minutes was r = +0.74. CONCLUSION: Thus in this canine stenosis model with dipyridamole, gamma camera imaging could distinguish mild from severe coronary stenoses by either single- or dual-zone analysis.

Animals↗

PubMedDate not supplied

Antidepressant effect of lithium.

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Adult↗