The authors would like to acknowledge the Center for Molecular and Genomic Imaging at UC Davis and the Biomedical Imaging Department at Genentech. This work was supported by a National Institutes of Health Bioengineering Research Partnership grant number R01 EB00993.

All animal handling and procedures described herein, and according to the Animal Research: Reporting In Vivo Experiments (ARRIVE) guidelines, were performed in accordance with protocols approved by the Association for Assessment of Accreditation of Laboratory Animal Care (AAALAC) International accredited Institutional Animal Care and Use Committee at the University of California, Davis. Proper surgery should not result in signs of any pain or discomfort in the animal, but proper steps should be taken if these signs are o...

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E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=PET+physiological+measurements+using+constant+infusion.">PET physiological measurements using constant infusion.</a> Nuclear Medicine and Biology. 27, 657-660 (2000).</li><li>Greve, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+BOLD+effect.">The BOLD effect.</a> Methods in Molecular Biology. 771, 153-159 (2011).</li><li>Flores, J. 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JM and SW are employees of Genentech.

Simultaneous anatomic MRI, and dynamic DWI-MRI and [18F]FDG PET data were successfully acquired from experimental animals during hypoxic challenge following common carotid artery ligation.&#160; This represents a powerful experimental paradigm for multimodal imaging of the rapidly evolving pathophysiology associated with ischemic insults in the brain and could readily be extended to study other PET radiotracers (for example markers of neuroinflammation) and MRI sequences, as well as the impact of interventiona...

Worldwide, stroke is the second leading cause of death and a major cause of disability 1. The cascade of biochemical and physiological events that occur during and acutely following a stroke event occurs rapidly and with implications for tissue viability and ultimately outcome 2. Cerebral hypoxia-ischemia (H-I), which leads to hypoxic-ischemic encephalopathy (HIE), is estimated to affect up to 0.3% and 4% of full-term and preterm births, respectively 3,4. The mortality rate in infants with HIE is approximately 15% to 20%. In 25% of HIE survivors, permanent complications arise as a result of the injury, including mental retardation, motor deficits, cerebral palsy, and epilepsy 3,4. Past therapeutic interventions have not proven worthy of adoption as standard of care, and consensus has yet to be reached that the most advanced methods, based on hypothermia, are effectively reducing morbidity 3,5. Other issues of contention include method of administration of hypothermia and patient selection 6. Thus, strategies for neuroprotection and neurorestoration are still a fertile area for research7.

Rat models of cerebral H-I have been available since the 1960s, and subsequently were adapted to mice 8,9. Due to the nature of the model and the location of the ligation, there is inherent variability in the outcome due to difference in collateral flow between animals 10. As a result, these models tend to be more variable compared to similar models such as middle cerebral artery occlusion (MCAo). Real time measurement of physiological changes has been demonstrated with laser Doppler flowmetry as well as diffusion-weighted MRI 11. The observed intra-animal variability in cerebral flow blood during and immediately after hypoxia, as well as in acute outcomes such as infarct volume and neurological deficit, suggest that simultaneous acquisition and correlation of multimodal data would be beneficial.

Recent advances in simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) have allowed for new possibilities in preclinical imaging 12-14. The potential advantages of these hybrid, combined systems for preclinical applications have been described in the literature 15,16. While many preclinical questions can be addressed by imaging an individual animal sequentially or by imaging separate animal groups, certain situations &#8211; for example, when each instance of an event such as stroke manifests itself uniquely, with rapidly evolving pathophysiology &#8211; make it desirable and even necessary to use simultaneous measurement. Functional neuroimaging provides one such example, where simultaneous 2-deoxy-2-(18F)fluoro-&#7429;-glucose ([18F]FDG) PET and blood-oxygen-level dependent (BOLD) MRI has recently been demonstrated in rat whisker stimulation studies 14.

Here, we demonstrate simultaneous PET/MRI imaging during onset of a hypoxic-ischemic stroke in which brain physiology is not at steady state, but instead is rapidly and irreversibly changing during hypoxic challenge. Changes in water diffusion, as measured by MRI and quantified by the apparent diffusion coefficient (ADC) derived from diffusion-weighted imaging (DWI), has been well characterized for stroke in clinical and preclinical data 17,18. In animal models such as MCAo, diffusion of water in affected brain tissue drops rapidly due to the bioenergetic cascade leading to cytotoxic edema 18. These acute changes in ADC are also observed in rodent models of cerebral hypoxia-ischemia 11,19. [18F]FDG PET imaging has been used in stroke patients to assess changes in local glucose metabolism 20, and a small number of in vivo animal studies have also used [18F]FDG 21, including in the cerebral hypoxia-ischemia model 22. In general, these studies show decreased glucose utilization in ischemic regions, although a study using a model with reperfusion found no correlation of these metabolic changes with later infarction development 23. This is in contrast to diffusion changes which have been associated with the irreversibly damaged core 21. Thus, it is important to be able to obtain the complementary information derived from [18F]FDG PET and DWI in a simultaneous manner during the evolution of stroke, as this is likely to yield meaningful information about the progression of injury and the impact of therapeutic interventions. The method we describe here is readily amenable to use with a variety of PET tracers and MRI sequences. For instance, [15O]H2O PET imaging along with DWI and perfusion-weighted images (PWI) from MRI may be used to further explore the development of the ischemic penumbra and validate current techniques within the stroke imaging field.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>Surgery</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Surgical scissors</td>
			<td>Roboz</td>
			<td>RS-5852</td>
			<td></td>
		</tr>
		<tr>
			<td>Forceps</td>
			<td>Roboz</td>
			<td>RS-5237</td>
			<td></td>
		</tr>
		<tr>
			<td>Hartman mosquito forceps</td>
			<td>Miltex</td>
			<td>7-26</td>
			<td></td>
		</tr>
		<tr>
			<td>2x McPherson suturing forceps, 8.5 cm</td>
			<td>Accurate Surgical &#38; Scientific Instruments</td>
			<td>4473</td>
			<td>It is useful to reduce the opening width with a band on the forceps used to hold the carotid artery</td>
		</tr>
		<tr>
			<td>6-0 silicone coated braided silk suture with 3/8 C-1 needle</td>
			<td>Covidien Sofsilk</td>
			<td>S-1172</td>
			<td></td>
		</tr>
		<tr>
			<td>Homeothermic blanket system</td>
			<td>Harvard Apparatus</td>
			<td>507220F</td>
			<td></td>
		</tr>
		<tr>
			<td>Super glue</td>
			<td>(Generic)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Hypoxia</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Flowmeter for O2</td>
			<td>Alicat Scientific</td>
			<td>MC-500SCCM-D</td>
			<td></td>
		</tr>
		<tr>
			<td>Flometer for N2</td>
			<td>Alicat Scientific</td>
			<td>MC-5SLPM-D</td>
			<td></td>
		</tr>
		<tr>
			<td>O2 meter</td>
			<td>MSA</td>
			<td>Altair Pro</td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Imaging</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>7.05 Tesla MRI System</td>
			<td>Bruker</td>
			<td>BioSpec</td>
			<td>20 cm inner bore diameter with gradient set. Paravision 5.1 software.</td>
		</tr>
		<tr>
			<td>Volume Tx/Rx 1H Coil, 35mm ID</td>
			<td>Bruker</td>
			<td>T8100</td>
			<td></td>
		</tr>
		<tr>
			<td>PET system</td>
			<td>(In-house)</td>
			<td></td>
			<td>4x24 LSO-PSAPD detectors, 
			10x10 LSO array per detector, 
			1.2mm crystal pitch and 14mm depth. 14 x 14 mm PSAPD. FOV: 60x35mm. 350-650 keV energy window. 16 ns timing window.</td>
		</tr>
		<tr>
			<td>Vessel cannulation Dumont forceps</td>
			<td>Roboz</td>
			<td>RS-4991</td>
			<td></td>
		</tr>
		<tr>
			<td>PE-10 polyethylene tubing</td>
			<td>BD Intramedic</td>
			<td>427401</td>
			<td></td>
		</tr>
		<tr>
			<td>Infusion pump</td>
			<td>Braintree Scientific</td>
			<td>BS-300</td>
			<td></td>
		</tr>
		<tr>
			<td>Animal monitoring &#38; gating equipment</td>
			<td>Small Animal Instruments Inc.</td>
			<td>Model 1025</td>
			<td>Only respiration monitoring used</td>
		</tr>
		<tr>
			<td>Animal bed with temperature regulation</td>
			<td>(In-house)</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

simultaneous pet/mri imaging during mouse cerebral hypoxia-ischemia

Dynamic changes in tissue water diffusion and glucose metabolism occur during and after hypoxia in cerebral hypoxia-ischemia reflecting a bioenergetics disturbance in affected cells. Diffusion weighted magnetic resonance imaging (MRI) identifies regions that are damaged, potentially irreversibly, by hypoxia-ischemia. Alterations in glucose utilization in the affected tissue may be detectable by positron emission tomography (PET) imaging of 2-deoxy-2-(18F)fluoro-&#7429;-glucose ([18F]FDG) uptake. Due to the rapid and variable nature of injury in this animal model, acquisition of both modes of data must be performed simultaneously in order to meaningfully correlate PET and MRI data. In addition, inter-animal variability in the hypoxic-ischemic injury due to vascular differences limits the ability to analyze multi-modal data and observe changes to a group-wise approach if data is not acquired simultaneously in individual subjects. The method presented here allows one to acquire both diffusion-weighted MRI and [18F]FDG uptake data in the same animal before, during, and after the hypoxic challenge in order to interrogate immediate physiological changes.

Figure 1 demonstrates the result of a proper ligation of the common carotid artery, prior to closing the wound with 6-0 silk suture.
In this method, data obtained from imaging is highly dependent upon the temporal arrangement of the experiment, which in turn dictates and is also dictated by experimental limitations including image acquisition schemes and equipment setup. These and other considerations are further explored in the Discussion section. With the protocol described ...

Watch this Scientific Journal Video about Simultaneous PET/MRI Imaging During Mouse Cerebral Hypoxia-ischemia at JoVE.com

Simultaneous PET/MRI Imaging During Mouse Cerebral Hypoxia-ischemia

The method presented here uses simultaneous positron emission tomography and magnetic resonance imaging. In the cerebral hypoxia-ischemia model, dynamic changes in diffusion and glucose metabolism occur during and after injury. The evolving and irreproducible damage in this model necessitates simultaneous acquisition if meaningful multi-modal imaging data are to be acquired.

Dynamic changes in tissue water diffusion and glucose metabolism occur during and after hypoxia in cerebral hypoxia-ischemia reflecting a bioenergetics ...

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12.2K Views.  University of California, Davis. The method presented here uses simultaneous positron emission tomography and magnetic resonance imaging. In the cerebral hypoxia-ischemia model, dynamic changes in diffusion and glucose metabolism occur during and after injury. The evolving and irreproducible damage in this model necessitates simultaneous acquisition if meaningful multi-modal imaging data are to be acquired.