Assessment of intracardiac shunts with gadolinium-enhanced ultrafast MR imaging.

Dye dilutional techniques are widely accepted for the assessment of intracardiac shunts, but current techniques require arterial access or radioisotope injection. Ultrafast (less than 500 msec) magnetic resonance (MR) imaging is ideally suited for the evaluation of an indicator during passage through the heart. Twenty patients were studied, including 13 with shunts. Four-chamber, T1-weighted images were obtained during bolus injection of gadopentetate dimeglumine. A single image was obtained in 420 msec, with repetitive images acquired after each QRS complex. After the contrast material was injected, there was pronounced signal intensity enhancement in the right atrium, followed by the right ventricular cavity, left atrium, left ventricular cavity, and descending aorta. Patients with substantial intracardiac shunts demonstrated early recirculation. First-pass contrast material-enhanced MR imaging is a promising new technique for the rapid assessment of intracardiac shunts. Combined with anatomic and functional MR imaging techniques, it can help provide a comprehensive noninvasive evaluation of suspected intracardiac shunts or provide follow-up in patients with known shunts.

Cerebral blood flow: assessment with dynamic contrast-enhanced T2*-weighted MR imaging at 1.5 T.

The authors assessed regional cerebral blood flow dynamics with magnetic resonance (MR) imaging enhanced with gadolinium diethylenetriaminepentaacetic acid (DTPA). After bolus administration of Gd-DTPA, rapid T2*-weighted gradient-echo images were acquired. Image acquisition time ranged from 2 to 3 seconds. The signal intensity (SI) of brain tissue and blood vessels markedly decreased during the first pass of contrast agent through the brain due to the local field inhomogeneity caused by the concentrated paramagnetic contrast agent. The method was used in 18 subjects with no cerebrovascular disease and 32 patients with stroke, vascular stenosis, arteriovenous malformation, and cerebral neoplasm. Comparison with intracranial angiography was performed in three patients and with single-photon emission computed tomography of blood flow in four. The change in T2* relaxation rate was approximately linearly related to the dose of contrast agent. The SI change increased as the echo time was lengthened. Regions in cerebral infarcts, metastases, and arteriovenous malformations showed different enhancement patterns than those of edema around a lesion and of normal brain tissue. Abnormal circulation times in patients with vascular stenoses were demonstrated. The method provides information about cerebral blood flow dynamics not available from conventional MR imaging and MR angiography.