Magnetic resonance lactate and lipid signals in rat brain after middle cerebral artery occlusion model

Abstract

Proton magnetic resonance spectroscopy (1-H MRS) has revealed changes of metabolites in acute cerebral infarction. Although the drastic changes of lactate and N-acetyl-aspartate have been reported to be useful indicators of the ischemic damage in both humans and experimental animals, lipid signals are also detected by the short echo time sequence 1-5 days after ischemia. The objective of this study was to find a novel technique to isolate lactate signals from lipid signals in the ischemic brain. First, MRS was used to study the lipid and lactate components of a spherical phantom in vitro, and parameters were established to separate these components in vitro. Then, MR measurements were obtained from the brains of middle cerebral artery occlusion rats. All MR measurements were performed using a 7-T (300 MHz), 18.3-cm-bore superconducting magnet (Oxford Magnet Technologies) interfaced to a Unity INOVA Imaging System (Varian Technologies). T2-weighted images were obtained from a 1.0-mm-thick coronal section using a 3-cm field of view. It is well known that lipid has a shorter and lactate a longer T2 relaxation time. These distinct magnetic characteristics allowed us to separate the lactate signal from the lipid signal. Thus, adjustment of the echo time is essential to analyze the metabolites in acute cerebral infarction, which may be useful in both the clinic and laboratory.

Fig. 1

Change of signal intensities of lipid (A), lactate (B), and both (C) in the spherical phantom using a STEAM sequence with different TE. Although lipid signals gradually decreased in the longer TE (A), lactate signal showed sign-curve diminution with different TE (B).

Fig. 2

Proton MR spectra from the fat (A, D), the lactate (B, E), and border zone (C, F) in the spherical phantom. Frequency of signal acquired from fat is overlapped with lactate when TE is set as short (20 ms) (C). However, signal of fat disappeared and only lactate signal became obvious when TE was set as longer (563 ms) (F).

Fig. 3

T₂-weighted images (A1–5) and diffusion-weighted images (B1–5) from the brain of a rat from the ischemic lesion at 6, 24, 48, 72 h, and 7 days after MCAO, respectively. MRS spectra were acquired from the high intensity area on the diffusion-weighted images (open box). Scale bar=5 mm.

Fig. 4

In vivo proton MR spectra with short TE (20 ms) of ischemic brain lesions at 6 (A1), 24 (A2), 48 (A3), 72 h (A4), and 7 days (A5) after MCAO. Lactate signal was overlapped with lipid, which was detected 6 h after MCAO, and gradually increased over the time course of 7 days. Lactate signals were isolated with longer TE (563 ms) (B1–5), and were obvious only 6 and 24 h after MCAO (B1 and B2, respectively).

Fig. 5

Summary of lactate signal (open squire) and lipid/creatine ratio (black round) over the time course (n=5).

Fig. 6

Microphotographs of ischemic brain lesion tissue with oil red staining. Lipid droplets were less in the ischemic lesion 6 h after MCAO (A), and were mainly present in the cytoplasm of cells (B). However, a large number of lipid droplets were present both in the cells and extracellular matrix in the lesions 7 days after MCAO (C, D). Photos of higher magnification 6 h and 7 days after MCAO were shown in panels C and D, respectively. Scale bar=20 μm (A, C), 5 μm (B, D).