Susceptibility-weighted imaging is suitable for evaluating signal strength in different brain regions of a rabbit model of acute hemorrhagic anemia

Acute hemorrhagic anemia can decrease blood flow and oxygen supply to brain, and affect its physiological function. While detecting changes in brain function in patients with acute hemorrhagic anemia is helpful for preventing neurological complications and evaluating therapeutic effects, clinical changes in the nervous systems of these patients have not received much attention. In part, this is because current techniques can only indirectly detect changes in brain function following onset of anemia, which leads to lags between real changes in brain function and their detection. Some methods such as Transcranial Doppler can detect changes in cerebral blood-flow velocity that result from compensatory responses to the deficient oxygen delivery seen in anemia (i.e. higher velocity associated with anemia and lower velocity associated with recovery), but cannot reflect the actual oxygen supply and metabolism in the brain or organic changes such as stroke (Purkayastha and Sorond, 2012). Conventional CT and MRI can detect early cerebral infarction so as to facilitate early treatment, but cannot monitor physiological changes in the brain that occur because of anemia (Allmendinger et al., 2012). Near-infrared spectroscopy is a non-invasive method capable of measuring total blood-oxygen levels in brain (Murkin and Arango, 2009; Navarro et al., 2012; Yamazaki et al., 2013), but is unable to provide fine details concerning alterations in different cerebral tissues. Susceptibility-weighted imaging is a novel, non-invasive method for detecting changes in cerebral oxygen levels that may provide more detailed information regarding cerebral blood flow in patients with hemorrhage (Li et al., 2013). In the present study, we used susceptibility-weighted imaging to detect cerebral changes in an animal model of acute hemorrhagic anemia. Construction of the rabbit model of acute hemorrhagic anemia was based on the method used by Morimoto et al. (2001). A 40-mL blood sample was drawn through an arterial catheter. To compensate for the effects of simple blood-volume loss, the same volume of a 6% hydroxyethyl starch in 0.9% sodium chloride solution was injected through a femoral vein catheter. Following this, a blood sample was drawn again for blood-gas analysis and whole blood tests, and the head of the rabbit was re-scanned by MRI. The bloodletting, fluid infusion, and scanning processes were repeated continuously five times before the animal recovered from anesthesia; on the fifth occasion, the bloodletting volume and fluid-infusion volume were both 50 mL. The investigation conformed to the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH publication No. 85-23, revised 1996), and the protocol was approved by the Institutional Animal Care Committee from Second People's Hospital of Shenzhen City, First Affiliated Hospital of Shenzhen University, China. The blood samples obtained from the femoral artery were used for blood-gas analysis, which determined the PaO2 PaCO2 lactic acid, and pH levels. In addition, mean arterial pressure was measured (Figure 1). A catheter was inserted through the femoral vein to the atrium for infusion of fluid, and red blood-cell count, hemoglobin concentration, hematocrit, and central venous pressure were assessed (Figure 1). There was an approximate halving of the red blood-cell count, hemoglobin concentration, and hematocrit values after the first bloodletting, and these values progressively decreased following each of the four subsequent bloodletting procedures. The values of these parameters after the fifth bloodletting were significantly lower than the pre-bleed values (P 0.05, vs. pre-bleed; Figure 1D). Figure 1 Results of whole blood tests, blood-gas analyses, mean arterial pressure, and central venous pressure after acute hemorrhagic anemia. The rabbits were fixed to a special board in the supine position and scanned by a Siemens Magnetom Avanto 1.5 T MRI Scanner, using a body coil (excitation) and wrap-around surface coil (reception). T2 dual-echo fast spin-echo with fat-suppression (FSE-T2WI/PD) and susceptibility-weighted imaging 3D sequences were used. The scan extended downward from a plane passing through the superior orbital margin to the medulla oblongata of the rabbit. FSE-T2WI/PD acquisition was conducted using the following parameters: repetition time = 2,800 ms, echo time = 33/78 ms, field of view = 12 cm × 12 cm, matrix size = 256 × 256; acquisition time = 3.09 minutes. Susceptibility-weighted imaging acquisition was performed with a three-dimensional gradient echo sequence, as follows: repetition time = 49 ms; echo time = 40 ms; flip angle = 15°; field of view = 15 cm × 15 cm; bandwidth = 80 kHz; IPAT factor = 2. Additional processing was carried out on the phase-corrected susceptibility-weighted imaging sequences. The third ventricle and the olfactory bulb parallel to the corpus callosum were measured. The bilateral frontal cortex, frontal white matter, temporal lobe, and thalamus were selected manually as regions of interest. The control (pre-bleed) susceptibility-weighted imaging-signal value of the frontal white matter was significantly lower than that of the frontal cortex (52.50 ± 20.29 vs. 63.10 ± 22.82; P 0.05). Susceptibility-weighted imaging signals from the frontal cortex, temporal lobe, and thalamus after the second, third, fourth and fifth bloodletting procedures were significantly lower compared with the corresponding control (pre-bleed) values (P < 0.05; Figures ​Figures2,2, ​,33). Figure 2 Representative susceptibility-weighted imaging (SWI) and corresponding T2-weighted images of a rabbit brain that was obtained at the superior aspect of the olfactory bulb (1a, 1b), the border of the olfactory bulb (2a, 2b), the thalamus (3a, 3b), and . Figure 3 Effects of repeated bloodletting on the susceptibility-weighted imaging (SWI) signal values of selected regions of the rabbit brain. We also evaluated the overall cerebral white-gray contrast and vein structure to examine the susceptibility-weighted imaging minimum-intensity projection images. The contrast between cerebral gray and white matter was higher after bloodletting (particularly after the fourth and fifth procedures) than beforehand, and the venous structure was more abundant and clear (Figure 4). Figure 4 Susceptibility-weighted imaging (SWI) minimum-intensity projection images of the rabbit thalamus and cerebral veins obtained by magnetically-sensitive scanning after bloodletting. Hematoxylin and eosin staining showed that degeneration and necrosis of neurons and glial cells were not evident in the brains or cerebellums of our rabbits with acute hemorrhagic anemia (Figure 5). However, spaces had formed around blood vessels and cells, consistent with the occurrence of cerebral edema. Figure 5 Pathological changes in rabbit brain tissue after the fifth bloodletting (hematoxylin-eosin staining, × 100). In conclusion, this study has revealed that susceptibility-weighted imaging is an effective tool for detecting PaO2 and PaCO2-induced changes in cerebral oxygenation of different brain regions after acute hemorrhagic anemia. Therefore, susceptibility-weighted imaging may be a useful technique for monitoring the pathophysiologic changes and related complications associated with acute anemia.