Quantitation of normal canine hippocampus formation volume: correlation of MRI with gross histology.

The hippocampal formation possesses an important role in the development and maintenance of short-term memory. In this study, magnetic resonance imaging (MRI) and gross histology were used to quantify the volume of the hippocampal formation in canines. High resolution MRI, using 1 mm thick slices and an intraplanar resolution of 0.35 mm was performed at 2.0 T both in vivo and in vitro following in situ fixation. The volumes of the hippocampal formations were determined from MR images and compared to those obtained from one mm thick gross histologic sections. The average volume of the canine hippocampal formation, measured from in vivo and in vitro MR images was 476.0 +/- 79.5 and 467.3 +/- 53.7 mm3, respectively. Determined from gross histology, the volume of the hippocampal formation was 463.6 +/- 24.1 mm3. Quantitation of the canine hippocampal formation using in vivo MRI showed good correlation with in vitro MRI and histology, verifying the reliability and reproducibility of in vivo MRI measurements. High resolution MRI using 1 mm thick slices through the whole canine hippocampal formation is necessary for accurate volume determination of a structure of this size.

Direct in vivo observation of transventricular absorption in the hydrocephalic dog using magnetic resonance imaging.

RATIONALE AND OBJECTIVES: A model of chronic noncommunicating hydrocephalus in canines was developed, and gadolinium-DTPA (Gd-DTPA)-enhanced magnetic resonance imaging, physiologic and morphologic studies were performed to investigate transventricular absorption of cerebrospinal fluid. METHODS: Chronic hydrocephalus was induced in 12 mongrel dogs by injection of a silastic mixture into the prepontine cisterns. Ventricular pressure was measured during the development of hydrocephalus, and lateral ventriculo-ventricular perfusions with Gd-DTPA were performed under controlled conditions during serial magnetic resonance imaging studies. RESULTS: Hydrocephalus developed over an average of 129 +/- 24 days after induction, and the intraventricular pressure increased from an initial level of 14 +/- 4 cm H2O to a stabilized plateau of 25 +/- 5 cm H2O. Increased signal intensity in the brain matter, as seen on magnetic resonance images of chronic hydrocephalic dogs perfused with Gd-DTPA in the lateral ventricles, was consistent with the presence of the contrast agent in the periventricular extracellular space. This increased signal intensity was not observed in control animals. CONCLUSIONS: These results provide direct evidence of transventricular absorption in chronic hydrocephalus.

MR imaging of acute intracranial hemorrhage: findings on sequential spin-echo and gradient-echo images in a dog model.

Seven intraparenchymal hematomas (four venous and three arterial) were placed in the brains of six dogs in order to study the MR appearance of acute hemorrhage and to evaluate the effects of several variables on the signal intensity of the hematoma. MR imaging at 0.6 and 1.5 T was performed by using standard short and long TR spin-echo and low-flip-angle gradient-echo sequences. Sequential examinations were performed during the first week following hematoma creation. MR findings were compared with CT and postmortem examinations. Three patterns of signal intensity were observed, which varied according to the size (small vs large) and location (parenchymal vs intraventricular) of the hematomas. The small parenchymal hematomas did not undergo evolutionary changes. On short TR scans they were isointense at both field strengths, and therefore not detectable; on long TR scans these hematomas were of variable intensity at 1.5 T and were hyperintense at 0.6 T. On gradient-echo scans, they were hypointense at all times at both field strengths. The large parenchymal hematomas underwent evolutionary changes typical of those seen in clinical imaging. On short TR scans they were initially isointense and became hyperintense 1-3 days later. Long TR scans demonstrated initial hyperintensity, followed by the development of hypointensity within 12 hr in the venous hematomas and within 60 hr in the arterial hematoma. The intensity changes on long TR scans were seen at both 0.6 and 1.5 T, but occurred sooner and to a greater degree at 1.5 T. Gradient-echo imaging of these large lesions demonstrated hypointensity at all times at both field strengths. The intraventricular hemorrhages demonstrated more rapid development of hyperintensity on short TR scans and slower and less pronounced development of hypointensity on long TR scans compared with the parenchymal clots in the same animal. Gradient-echo imaging of the intraventricular hemorrhages demonstrated hypointensity at all times at both field strengths. A multifactorial hypothesis is proposed to explain the differences in intensity between venous, arterial, and intraventricular blood. Gradient-echo sequences should prove to be highly useful in detecting and delineating hemorrhages and are recommended for the MR protocol of patients with acute neurologic ictus and suspected hemorrhage.