Susceptibility weighted imaging (SWI), taking advantages of phase shift effect due to different susceptibility information among tissues, enhances the contrast of intravascular deoxygenated blood, non-haem iron and calcium deposition, and has been widely performed in clinical practice.¹ As no significant signal alteration after Gd-injection was demonstrated in humans, it is feasible to perform SWI after contrast medium injection.² SWI has also been applied to aid in evaluation of pediatric neurological disorders including stroke, brain tumor, traumatic brain injury, etc.³ Neonatal rats were the most widely used animals for building up pediatric disease models. However, due to small animal size, unstable physiological condition and hardware constraints, in vivo MR studies in neonatal rats, especially with contrast-medium injection protocols, were relative challenging. To date, SWI in neonatal rats has not been fully implemented. In this study, we employed postnatal day 10-21 rats (P10-P21) to examine the feasibility of SWI and also performed tail vein cannulation to evaluate the stability of doing Gd-injection. Our findings shows that, after Gd-administration in neonatal rats, larger number of penetrating vessels which were nearly invisible before injection were enhanced in the cortex and subcortical nuclei. We postulated that the conventional Gd contrast medium may serve as blood pool agent in neonatal rats because of extended circulation time. Gd-enhanced SWI, thus, provides a new diagnostic protocol for pediatric neurovascular disease.Neonatal rats (P10-P24, n=9) were under anesthesia with 0.5-0.75 % isoflurane for tail vein cannulation. During MRI, animals were anesthetized with 0.75-1% isoflurane and maintained on a circulating hot water bladder (respiratory rate: ~40 bpm). MRI in neonatal rats was performed on a Bruker 7 T PharmaScan with a volume coil (i.d. = 72 mm) as the transmitter and mouse surface coil as the receiver. Pre- and post-Gd SWI was measured by flow-compensated gradient-echo sequence with bandwidth=30 kHz, TR/TE=600/18 ms, flip angle=40°, matrix=192x192, FOV=1.6x1.6 cm², slice thickness=0.75 mm, and avg=8. Adult rats (male, 250-350 gw, n=5) were scanned with similar set-up except using a rat surface coil as the receiver, and underwent the same protocol with FOV=2.0x2.0 cm² and slice thickness=1 mm. Gd injection (0.3 mmol/kg) was performed during gradient-echo EPI scans (bandwidth=200 kHz, TR/TE=1000/20 ms, same geometry as SWI, scan time=5 min) to monitor Gd-induced signal reduction. The high-pass filtered SWI phase images were automatically reconstructed, and SWI phase images before and after Gd injection were spatially co-registered. The phase mask with the value range between 0 and 1 was processed by 4 times of the reconstructed phase image.⁴ The pixel with value > 0.85 and the discontinuous signal drop < 10 pixels in the phase masks were recognized as no phase characteristics and noise, respectively, and removed from the processed mask. The number of pixels in the processed masks, and the signal intensity before and after Gd injection masking by the post-Gd processed masks were tabulated. Paired t-test was used to compare the effect of Gd injection, with a threshold of P<0.05. Large number of penetrating vessels in the cortex, thalamus and striatum, and the additional volume of ventricles show uniform low intensity signals in post-Gd SWI in neonatal rats (P10-21) (Fig. A). Reasonable Gd dosage for neonatal rats⁵ was substantiated by comparable BOLD signal change in neonatal (44.60±16.31%) and adult (45.62±28.42%) rats (Fig. B). However, Gd dynamics in neonatal rats with a delay about 200 ms implied the slow excretion of Gd due to immature renal function.⁶ From the processed masks, penetrating vessels enhanced by Gd was observed in neonatal but not in adult rats (Fig. C). Significant larger number of pixels was depicted after Gd injection in neonatal rats (P< 0.005, Fig. 4A) are most likely due to the permeable blood-brain barrier (BBB) in early first few weeks after birth.⁷ Hypointensity signal after Gd injection in the brain regions was observed in both neonatal (P< 0.005) and adult (P< 0.05) rats (Fig. 4B), suggesting the general T2* effect of Gd in animals regardless of brain development. Here, we demonstrated the feasibility of performing Gd injection in rats on postnatal day of 10 (P10), and showed the Gd-enhanced SWI in neonatal rats up to postnatal day of 21 (P21). Our data suggested that difference in excretion, metabolism, and the maturation of BBB in neonatal rats may change the elimination and distribution of Gd in the brain and, thus, affect SWI signal. This is the first demonstration of Gd-enhanced SWI showing large amount of penetrating vessels in neonatal brains, which will be useful to explore the development and disease of cerebral vasculature in newborns.