Gd-enhanced Susceptibility Weighted Imaging in Neonatal Rats
Yu-Chieh Jill Kao1,2, Chia-Feng Lu1,2,3, Hua-Shan Liu4,5, Fei-Ting Hsu4, Ping-Huei Tsai2,4, Li-Chun Hsieh4, Pen-Yuan Liao4, and Cheng-Yu Chen2,4

1Translational Research Imaging Center, College of Medicine, Taipei Medical University, Taipei, Taiwan, 2Department of Radiology, School of Medicine, Taipei Medical University, Taipei, Taiwan, 3Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming University, Taipei, Taiwan, 4Department of Medical Imaging, Taipei Medical University Hospital, Taipei, Taiwan, 5Graduate Institute of Clinical Medicine, Taipei Medical University, Taipei, Taiwan

Synopsis

Gd-enhanced susceptibility weighted imaging in neonatal rats, which highlights the penetrating vessels in the neonatal brain, may provide a new imaging protocol to investigate pediatric neurological disorders.

Introduction

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.

Methods

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.

Results & Discussion

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.

Acknowledgements

This study was supported by the Taipei Medical University (TMU103-AE1-B27) and the Ministry of Science and Technology, Taipei, Taiwan, Republic of China (MOST 104-2314-B-038-028).

References

1. Mittal S, Wu Z, Neelavalli J, et al., Susceptibility-weighted imaging: technical aspects and clinical applications, part 2. AJNR Am J Neuroradiol, 2009. 30(2): 232-52.

2. El-Koussy M, Schenk P, Kiefer C, et al., Susceptibility-weighted imaging of the brain: does gadolinium administration matter? Eur J Radiol, 2012. 81(2): 272-6.

3. Meoded A, Poretti A, Northington F J, et al., Susceptibility weighted imaging of the neonatal brain. Clin Radiol, 2012. 67(8): 793-801.

4. Haacke E M, Mittal S, Wu Z, et al., Susceptibility-weighted imaging: technical aspects and clinical applications, part 1. AJNR Am J Neuroradiol, 2009. 30(1): 19-30.

5. Derugin N, Wendland M, Muramatsu K, et al., Evolution of brain injury after transient middle cerebral artery occlusion in neonatal rats. Stroke, 2000. 31(7): 1752-61.

6. Zoetis T and Hurtt M E, Species comparison of anatomical and functional renal development. Birth Defects Res B Dev Reprod Toxicol, 2003. 68(2): 111-20.

7. Watson R E, Desesso J M, Hurtt M E, et al., Postnatal growth and morphological development of the brain: a species comparison. Birth Defects Res B Dev Reprod Toxicol, 2006. 77(5): 471-84.

Figures

Figure 1. SWI images before and after Gd injection and the corresponding T2-weighted images from the representative neonatal rat (P15). Uniform low signal intensity was shown in the penetrating vessels in the cortex, subcortical nuclei, and ventricles after Gd injection.

Figure 2. BOLD signal change during the Gd injection. Although with similar intensity change, a delay about 200 ms was found in neonatal rats. Error bars represent SEM. .

Figure 3. Masks denoting phase changes from the processed masks (HP-filtered phase image with phase multiplication of 4) before and after Gd injection in neonatal (P15) and adult rats. Significant larger phase information were highlighted after Gd injection in neonatal rats. .

Figure 4. Pixel number (A) and intensity (B) with phase difference before and after Gd injection in neonatal and adult rats. Significant larger number of pixels was enhanced by Gd injection in neonatal rats. Lower signal intensity was observed in phase images after Gd injection denoting Gd-induced T2* change in neonatal and adult rats. * and ** indicates significant difference before and after Gd injection (P<0.05 and P<0.005, respectively). Error bars represent SEM. .



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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